perlop
PERLOP(1) Perl Programmers Reference Guide PERLOP(1)
NAME
perlop - Perl operators and precedence
DESCRIPTION
In Perl, the operator determines what operation is performed,
independent of the type of the operands. For example "$x+$y" is always
a numeric addition, and if $x or $y do not contain numbers, an attempt
is made to convert them to numbers first.
This is in contrast to many other dynamic languages, where the
operation is determined by the type of the first argument. It also
means that Perl has two versions of some operators, one for numeric and
one for string comparison. For example "$x==$y" compares two numbers
for equality, and "$xeq$y" compares two strings.
There are a few exceptions though: "x" can be either string repetition
or list repetition, depending on the type of the left operand, and "&",
"|", "^" and "~" can be either string or numeric bit operations.
Operator Precedence and Associativity
Operator precedence and associativity work in Perl more or less like
they do in mathematics.
Operator precedence means some operators group more tightly than
others. For example, in "2 + 4 * 5", the multiplication has higher
precedence, so "4 * 5" is grouped together as the right-hand operand of
the addition, rather than "2 + 4" being grouped together as the left-
hand operand of the multiplication. It is as if the expression were
written "2 + (4 * 5)", not "(2 + 4) * 5". So the expression yields "2 +
20 == 22", rather than "6 * 5 == 30".
Operator associativity defines what happens if a sequence of the same
operators is used one after another: whether they will be grouped at
the left or the right. For example, in "9 - 3 - 2", subtraction is left
associative, so "9 - 3" is grouped together as the left-hand operand of
the second subtraction, rather than "3 - 2" being grouped together as
the right-hand operand of the first subtraction. It is as if the
expression were written "(9 - 3) - 2", not "9 - (3 - 2)". So the
expression yields "6 - 2 == 4", rather than "9 - 1 == 8".
For simple operators that evaluate all their operands and then combine
the values in some way, precedence and associativity (and parentheses)
imply some ordering requirements on those combining operations. For
example, in "2 + 4 * 5", the grouping implied by precedence means that
the multiplication of 4 and 5 must be performed before the addition of
2 and 20, simply because the result of that multiplication is required
as one of the operands of the addition. But the order of operations is
not fully determined by this: in "2 * 2 + 4 * 5" both multiplications
must be performed before the addition, but the grouping does not say
anything about the order in which the two multiplications are
performed. In fact Perl has a general rule that the operands of an
operator are evaluated in left-to-right order. A few operators such as
"&&=" have special evaluation rules that can result in an operand not
being evaluated at all; in general, the top-level operator in an
expression has control of operand evaluation.
Perl operators have the following associativity and precedence, listed
from highest precedence to lowest. Operators borrowed from C keep the
same precedence relationship with each other, even where C's precedence
is slightly screwy. (This makes learning Perl easier for C folks.)
With very few exceptions, these all operate on scalar values only, not
array values.
left terms and list operators (leftward)
left ->
nonassoc ++ --
right **
right ! ~ \ and unary + and -
left =~ !~
left * / % x
left + - .
left << >>
nonassoc named unary operators
nonassoc < > <= >= lt gt le ge
nonassoc == != <=> eq ne cmp ~~
left &
left | ^
left &&
left || //
nonassoc .. ...
right ?:
right = += -= *= etc. goto last next redo dump
left , =>
nonassoc list operators (rightward)
right not
left and
left or xor
In the following sections, these operators are covered in detail, in
the same order in which they appear in the table above.
Many operators can be overloaded for objects. See overload.
Terms and List Operators (Leftward)
A TERM has the highest precedence in Perl. They include variables,
quote and quote-like operators, any expression in parentheses, and any
function whose arguments are parenthesized. Actually, there aren't
really functions in this sense, just list operators and unary operators
behaving as functions because you put parentheses around the arguments.
These are all documented in perlfunc.
If any list operator ("print()", etc.) or any unary operator
("chdir()", etc.) is followed by a left parenthesis as the next token,
the operator and arguments within parentheses are taken to be of
highest precedence, just like a normal function call.
In the absence of parentheses, the precedence of list operators such as
"print", "sort", or "chmod" is either very high or very low depending
on whether you are looking at the left side or the right side of the
operator. For example, in
@ary = (1, 3, sort 4, 2);
print @ary; # prints 1324
the commas on the right of the "sort" are evaluated before the "sort",
but the commas on the left are evaluated after. In other words, list
operators tend to gobble up all arguments that follow, and then act
like a simple TERM with regard to the preceding expression. Be careful
with parentheses:
# These evaluate exit before doing the print:
print($foo, exit); # Obviously not what you want.
print $foo, exit; # Nor is this.
# These do the print before evaluating exit:
(print $foo), exit; # This is what you want.
print($foo), exit; # Or this.
print ($foo), exit; # Or even this.
Also note that
print ($foo & 255) + 1, "\n";
probably doesn't do what you expect at first glance. The parentheses
enclose the argument list for "print" which is evaluated (printing the
result of "$foo&255"). Then one is added to the return value of
"print" (usually 1). The result is something like this:
1 + 1, "\n"; # Obviously not what you meant.
To do what you meant properly, you must write:
print(($foo & 255) + 1, "\n");
See "Named Unary Operators" for more discussion of this.
Also parsed as terms are the "do{}" and "eval{}" constructs, as well as
subroutine and method calls, and the anonymous constructors "[]" and
"{}".
See also "Quote and Quote-like Operators" toward the end of this
section, as well as "I/O Operators".
The Arrow Operator
""->"" is an infix dereference operator, just as it is in C and C++.
If the right side is either a "[...]", "{...}", or a "(...)" subscript,
then the left side must be either a hard or symbolic reference to an
array, a hash, or a subroutine respectively. (Or technically speaking,
a location capable of holding a hard reference, if it's an array or
hash reference being used for assignment.) See perlreftut and perlref.
Otherwise, the right side is a method name or a simple scalar variable
containing either the method name or a subroutine reference, and the
left side must be either an object (a blessed reference) or a class
name (that is, a package name). See perlobj.
The dereferencing cases (as opposed to method-calling cases) are
somewhat extended by the "postderef" feature. For the details of that
feature, consult "Postfix Dereference Syntax" in perlref.
Auto-increment and Auto-decrement
"++" and "--" work as in C. That is, if placed before a variable, they
increment or decrement the variable by one before returning the value,
and if placed after, increment or decrement after returning the value.
$i = 0; $j = 0;
print $i++; # prints 0
print ++$j; # prints 1
Note that just as in C, Perl doesn't define when the variable is
incremented or decremented. You just know it will be done sometime
before or after the value is returned. This also means that modifying
a variable twice in the same statement will lead to undefined behavior.
Avoid statements like:
$i = $i ++;
print ++ $i + $i ++;
Perl will not guarantee what the result of the above statements is.
The auto-increment operator has a little extra builtin magic to it. If
you increment a variable that is numeric, or that has ever been used in
a numeric context, you get a normal increment. If, however, the
variable has been used in only string contexts since it was set, and
has a value that is not the empty string and matches the pattern
"/^[a-zA-Z]*[0-9]*\z/", the increment is done as a string, preserving
each character within its range, with carry:
print ++($foo = "99"); # prints "100"
print ++($foo = "a0"); # prints "a1"
print ++($foo = "Az"); # prints "Ba"
print ++($foo = "zz"); # prints "aaa"
"undef" is always treated as numeric, and in particular is changed to 0
before incrementing (so that a post-increment of an undef value will
return 0 rather than "undef").
The auto-decrement operator is not magical.
Exponentiation
Binary "**" is the exponentiation operator. It binds even more tightly
than unary minus, so "-2**4" is "-(2**4)", not "(-2)**4". (This is
implemented using C's pow(3) function, which actually works on doubles
internally.)
Note that certain exponentiation expressions are ill-defined: these
include "0**0", "1**Inf", and "Inf**0". Do not expect any particular
results from these special cases, the results are platform-dependent.
Symbolic Unary Operators
Unary "!" performs logical negation, that is, "not". See also "not"
for a lower precedence version of this.
Unary "-" performs arithmetic negation if the operand is numeric,
including any string that looks like a number. If the operand is an
identifier, a string consisting of a minus sign concatenated with the
identifier is returned. Otherwise, if the string starts with a plus or
minus, a string starting with the opposite sign is returned. One
effect of these rules is that "-bareword" is equivalent to the string
"-bareword". If, however, the string begins with a non-alphabetic
character (excluding "+" or "-"), Perl will attempt to convert the
string to a numeric, and the arithmetic negation is performed. If the
string cannot be cleanly converted to a numeric, Perl will give the
warning Argument "the string" isn't numeric in negation (-) at ....
Unary "~" performs bitwise negation, that is, 1's complement. For
example, "0666&~027" is 0640. (See also "Integer Arithmetic" and
"Bitwise String Operators".) Note that the width of the result is
platform-dependent: "~0" is 32 bits wide on a 32-bit platform, but 64
bits wide on a 64-bit platform, so if you are expecting a certain bit
width, remember to use the "&" operator to mask off the excess bits.
Starting in Perl 5.28, it is a fatal error to try to complement a
string containing a character with an ordinal value above 255.
If the "bitwise" feature is enabled via "usefeature'bitwise'" or "use
v5.28", then unary "~" always treats its argument as a number, and an
alternate form of the operator, "~.", always treats its argument as a
string. So "~0" and "~"0"" will both give 2**32-1 on 32-bit platforms,
whereas "~.0" and "~."0"" will both yield "\xff". Until Perl 5.28,
this feature produced a warning in the "experimental::bitwise"
category.
Unary "+" has no effect whatsoever, even on strings. It is useful
syntactically for separating a function name from a parenthesized
expression that would otherwise be interpreted as the complete list of
function arguments. (See examples above under "Terms and List
Operators (Leftward)".)
Unary "\" creates references. If its operand is a single sigilled
thing, it creates a reference to that object. If its operand is a
parenthesised list, then it creates references to the things mentioned
in the list. Otherwise it puts its operand in list context, and
creates a list of references to the scalars in the list provided by the
operand. See perlreftut and perlref. Do not confuse this behavior
with the behavior of backslash within a string, although both forms do
convey the notion of protecting the next thing from interpolation.
Binding Operators
Binary "=~" binds a scalar expression to a pattern match. Certain
operations search or modify the string $_ by default. This operator
makes that kind of operation work on some other string. The right
argument is a search pattern, substitution, or transliteration. The
left argument is what is supposed to be searched, substituted, or
transliterated instead of the default $_. When used in scalar context,
the return value generally indicates the success of the operation. The
exceptions are substitution ("s///") and transliteration ("y///") with
the "/r" (non-destructive) option, which cause the return value to be
the result of the substitution. Behavior in list context depends on
the particular operator. See "Regexp Quote-Like Operators" for details
and perlretut for examples using these operators.
If the right argument is an expression rather than a search pattern,
substitution, or transliteration, it is interpreted as a search pattern
at run time. Note that this means that its contents will be
interpolated twice, so
'\\' =~ q'\\';
is not ok, as the regex engine will end up trying to compile the
pattern "\", which it will consider a syntax error.
Binary "!~" is just like "=~" except the return value is negated in the
logical sense.
Binary "!~" with a non-destructive substitution ("s///r") or
transliteration ("y///r") is a syntax error.
Multiplicative Operators
Binary "*" multiplies two numbers.
Binary "/" divides two numbers.
Binary "%" is the modulo operator, which computes the division
remainder of its first argument with respect to its second argument.
Given integer operands $m and $n: If $n is positive, then "$m%$n" is $m
minus the largest multiple of $n less than or equal to $m. If $n is
negative, then "$m%$n" is $m minus the smallest multiple of $n that is
not less than $m (that is, the result will be less than or equal to
zero). If the operands $m and $n are floating point values and the
absolute value of $n (that is "abs($n)") is less than "(UV_MAX+1)",
only the integer portion of $m and $n will be used in the operation
(Note: here "UV_MAX" means the maximum of the unsigned integer type).
If the absolute value of the right operand ("abs($n)") is greater than
or equal to "(UV_MAX+1)", "%" computes the floating-point remainder $r
in the equation "($r=$m-$i*$n)" where $i is a certain integer that
makes $r have the same sign as the right operand $n (not as the left
operand $m like C function "fmod()") and the absolute value less than
that of $n. Note that when "useinteger" is in scope, "%" gives you
direct access to the modulo operator as implemented by your C compiler.
This operator is not as well defined for negative operands, but it will
execute faster.
Binary "x" is the repetition operator. In scalar context, or if the
left operand is neither enclosed in parentheses nor a "qw//" list, it
performs a string repetition. In that case it supplies scalar context
to the left operand, and returns a string consisting of the left
operand string repeated the number of times specified by the right
operand. If the "x" is in list context, and the left operand is either
enclosed in parentheses or a "qw//" list, it performs a list
repetition. In that case it supplies list context to the left operand,
and returns a list consisting of the left operand list repeated the
number of times specified by the right operand. If the right operand
is zero or negative (raising a warning on negative), it returns an
empty string or an empty list, depending on the context.
print '-' x 80; # print row of dashes
print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
@ones = (1) x 80; # a list of 80 1's
@ones = (5) x @ones; # set all elements to 5
Additive Operators
Binary "+" returns the sum of two numbers.
Binary "-" returns the difference of two numbers.
Binary "." concatenates two strings.
Shift Operators
Binary "<<" returns the value of its left argument shifted left by the
number of bits specified by the right argument. Arguments should be
integers. (See also "Integer Arithmetic".)
Binary ">>" returns the value of its left argument shifted right by the
number of bits specified by the right argument. Arguments should be
integers. (See also "Integer Arithmetic".)
If "useinteger" (see "Integer Arithmetic") is in force then signed C
integers are used (arithmetic shift), otherwise unsigned C integers are
used (logical shift), even for negative shiftees. In arithmetic right
shift the sign bit is replicated on the left, in logical shift zero
bits come in from the left.
Either way, the implementation isn't going to generate results larger
than the size of the integer type Perl was built with (32 bits or 64
bits).
Shifting by negative number of bits means the reverse shift: left shift
becomes right shift, right shift becomes left shift. This is unlike in
C, where negative shift is undefined.
Shifting by more bits than the size of the integers means most of the
time zero (all bits fall off), except that under "useinteger" right
overshifting a negative shiftee results in -1. This is unlike in C,
where shifting by too many bits is undefined. A common C behavior is
"shift by modulo wordbits", so that for example
1 >> 64 == 1 >> (64 % 64) == 1 >> 0 == 1 # Common C behavior.
but that is completely accidental.
If you get tired of being subject to your platform's native integers,
the "usebigint" pragma neatly sidesteps the issue altogether:
print 20 << 20; # 20971520
print 20 << 40; # 5120 on 32-bit machines,
# 21990232555520 on 64-bit machines
use bigint;
print 20 << 100; # 25353012004564588029934064107520
Named Unary Operators
The various named unary operators are treated as functions with one
argument, with optional parentheses.
If any list operator ("print()", etc.) or any unary operator
("chdir()", etc.) is followed by a left parenthesis as the next token,
the operator and arguments within parentheses are taken to be of
highest precedence, just like a normal function call. For example,
because named unary operators are higher precedence than "||":
chdir $foo || die; # (chdir $foo) || die
chdir($foo) || die; # (chdir $foo) || die
chdir ($foo) || die; # (chdir $foo) || die
chdir +($foo) || die; # (chdir $foo) || die
but, because "*" is higher precedence than named operators:
chdir $foo * 20; # chdir ($foo * 20)
chdir($foo) * 20; # (chdir $foo) * 20
chdir ($foo) * 20; # (chdir $foo) * 20
chdir +($foo) * 20; # chdir ($foo * 20)
rand 10 * 20; # rand (10 * 20)
rand(10) * 20; # (rand 10) * 20
rand (10) * 20; # (rand 10) * 20
rand +(10) * 20; # rand (10 * 20)
Regarding precedence, the filetest operators, like "-f", "-M", etc. are
treated like named unary operators, but they don't follow this
functional parenthesis rule. That means, for example, that
"-f($file).".bak"" is equivalent to "-f"$file.bak"".
See also "Terms and List Operators (Leftward)".
Relational Operators
Perl operators that return true or false generally return values that
can be safely used as numbers. For example, the relational operators
in this section and the equality operators in the next one return 1 for
true and a special version of the defined empty string, "", which
counts as a zero but is exempt from warnings about improper numeric
conversions, just as "0buttrue" is.
Binary "<" returns true if the left argument is numerically less than
the right argument.
Binary ">" returns true if the left argument is numerically greater
than the right argument.
Binary "<=" returns true if the left argument is numerically less than
or equal to the right argument.
Binary ">=" returns true if the left argument is numerically greater
than or equal to the right argument.
Binary "lt" returns true if the left argument is stringwise less than
the right argument.
Binary "gt" returns true if the left argument is stringwise greater
than the right argument.
Binary "le" returns true if the left argument is stringwise less than
or equal to the right argument.
Binary "ge" returns true if the left argument is stringwise greater
than or equal to the right argument.
Equality Operators
Binary "==" returns true if the left argument is numerically equal to
the right argument.
Binary "!=" returns true if the left argument is numerically not equal
to the right argument.
Binary "<=>" returns -1, 0, or 1 depending on whether the left argument
is numerically less than, equal to, or greater than the right argument.
If your platform supports "NaN"'s (not-a-numbers) as numeric values,
using them with "<=>" returns undef. "NaN" is not "<", "==", ">", "<="
or ">=" anything (even "NaN"), so those 5 return false. "NaN!=NaN"
returns true, as does "NaN!="anythingelse. If your platform doesn't
support "NaN"'s then "NaN" is just a string with numeric value 0.
$ perl -le '$x = "NaN"; print "No NaN support here" if $x == $x'
$ perl -le '$x = "NaN"; print "NaN support here" if $x != $x'
(Note that the bigint, bigrat, and bignum pragmas all support "NaN".)
Binary "eq" returns true if the left argument is stringwise equal to
the right argument.
Binary "ne" returns true if the left argument is stringwise not equal
to the right argument.
Binary "cmp" returns -1, 0, or 1 depending on whether the left argument
is stringwise less than, equal to, or greater than the right argument.
Binary "~~" does a smartmatch between its arguments. Smart matching is
described in the next section.
"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order
specified by the current "LC_COLLATE" locale if a "uselocale" form that
includes collation is in effect. See perllocale. Do not mix these
with Unicode, only use them with legacy 8-bit locale encodings. The
standard "Unicode::Collate" and "Unicode::Collate::Locale" modules
offer much more powerful solutions to collation issues.
For case-insensitive comparisons, look at the "fc" in perlfunc case-
folding function, available in Perl v5.16 or later:
if ( fc($x) eq fc($y) ) { ... }
Smartmatch Operator
First available in Perl 5.10.1 (the 5.10.0 version behaved
differently), binary "~~" does a "smartmatch" between its arguments.
This is mostly used implicitly in the "when" construct described in
perlsyn, although not all "when" clauses call the smartmatch operator.
Unique among all of Perl's operators, the smartmatch operator can
recurse. The smartmatch operator is experimental and its behavior is
subject to change.
It is also unique in that all other Perl operators impose a context
(usually string or numeric context) on their operands, autoconverting
those operands to those imposed contexts. In contrast, smartmatch
infers contexts from the actual types of its operands and uses that
type information to select a suitable comparison mechanism.
The "~~" operator compares its operands "polymorphically", determining
how to compare them according to their actual types (numeric, string,
array, hash, etc.). Like the equality operators with which it shares
the same precedence, "~~" returns 1 for true and "" for false. It is
often best read aloud as "in", "inside of", or "is contained in",
because the left operand is often looked for inside the right operand.
That makes the order of the operands to the smartmatch operand often
opposite that of the regular match operator. In other words, the
"smaller" thing is usually placed in the left operand and the larger
one in the right.
The behavior of a smartmatch depends on what type of things its
arguments are, as determined by the following table. The first row of
the table whose types apply determines the smartmatch behavior.
Because what actually happens is mostly determined by the type of the
second operand, the table is sorted on the right operand instead of on
the left.
Left Right Description and pseudocode
===============================================================
Any undef check whether Any is undefined
like: !defined Any
Any Object invoke ~~ overloading on Object, or die
Right operand is an ARRAY:
Left Right Description and pseudocode
===============================================================
ARRAY1 ARRAY2 recurse on paired elements of ARRAY1 and ARRAY2[2]
like: (ARRAY1[0] ~~ ARRAY2[0])
&& (ARRAY1[1] ~~ ARRAY2[1]) && ...
HASH ARRAY any ARRAY elements exist as HASH keys
like: grep { exists HASH->{$_} } ARRAY
Regexp ARRAY any ARRAY elements pattern match Regexp
like: grep { /Regexp/ } ARRAY
undef ARRAY undef in ARRAY
like: grep { !defined } ARRAY
Any ARRAY smartmatch each ARRAY element[3]
like: grep { Any ~~ $_ } ARRAY
Right operand is a HASH:
Left Right Description and pseudocode
===============================================================
HASH1 HASH2 all same keys in both HASHes
like: keys HASH1 ==
grep { exists HASH2->{$_} } keys HASH1
ARRAY HASH any ARRAY elements exist as HASH keys
like: grep { exists HASH->{$_} } ARRAY
Regexp HASH any HASH keys pattern match Regexp
like: grep { /Regexp/ } keys HASH
undef HASH always false (undef can't be a key)
like: 0 == 1
Any HASH HASH key existence
like: exists HASH->{Any}
Right operand is CODE:
Left Right Description and pseudocode
===============================================================
ARRAY CODE sub returns true on all ARRAY elements[1]
like: !grep { !CODE->($_) } ARRAY
HASH CODE sub returns true on all HASH keys[1]
like: !grep { !CODE->($_) } keys HASH
Any CODE sub passed Any returns true
like: CODE->(Any)
Right operand is a Regexp:
Left Right Description and pseudocode
===============================================================
ARRAY Regexp any ARRAY elements match Regexp
like: grep { /Regexp/ } ARRAY
HASH Regexp any HASH keys match Regexp
like: grep { /Regexp/ } keys HASH
Any Regexp pattern match
like: Any =~ /Regexp/
Other:
Left Right Description and pseudocode
===============================================================
Object Any invoke ~~ overloading on Object,
or fall back to...
Any Num numeric equality
like: Any == Num
Num nummy[4] numeric equality
like: Num == nummy
undef Any check whether undefined
like: !defined(Any)
Any Any string equality
like: Any eq Any
Notes:
1. Empty hashes or arrays match.
2. That is, each element smartmatches the element of the same index in
the other array.[3]
3. If a circular reference is found, fall back to referential equality.
4. Either an actual number, or a string that looks like one.
The smartmatch implicitly dereferences any non-blessed hash or array
reference, so the "HASH" and "ARRAY" entries apply in those cases. For
blessed references, the "Object" entries apply. Smartmatches involving
hashes only consider hash keys, never hash values.
The "like" code entry is not always an exact rendition. For example,
the smartmatch operator short-circuits whenever possible, but "grep"
does not. Also, "grep" in scalar context returns the number of
matches, but "~~" returns only true or false.
Unlike most operators, the smartmatch operator knows to treat "undef"
specially:
use v5.10.1;
@array = (1, 2, 3, undef, 4, 5);
say "some elements undefined" if undef ~~ @array;
Each operand is considered in a modified scalar context, the
modification being that array and hash variables are passed by
reference to the operator, which implicitly dereferences them. Both
elements of each pair are the same:
use v5.10.1;
my %hash = (red => 1, blue => 2, green => 3,
orange => 4, yellow => 5, purple => 6,
black => 7, grey => 8, white => 9);
my @array = qw(red blue green);
say "some array elements in hash keys" if @array ~~ %hash;
say "some array elements in hash keys" if \@array ~~ \%hash;
say "red in array" if "red" ~~ @array;
say "red in array" if "red" ~~ \@array;
say "some keys end in e" if /e$/ ~~ %hash;
say "some keys end in e" if /e$/ ~~ \%hash;
Two arrays smartmatch if each element in the first array smartmatches
(that is, is "in") the corresponding element in the second array,
recursively.
use v5.10.1;
my @little = qw(red blue green);
my @bigger = ("red", "blue", [ "orange", "green" ] );
if (@little ~~ @bigger) { # true!
say "little is contained in bigger";
}
Because the smartmatch operator recurses on nested arrays, this will
still report that "red" is in the array.
use v5.10.1;
my @array = qw(red blue green);
my $nested_array = [[[[[[[ @array ]]]]]]];
say "red in array" if "red" ~~ $nested_array;
If two arrays smartmatch each other, then they are deep copies of each
others' values, as this example reports:
use v5.12.0;
my @a = (0, 1, 2, [3, [4, 5], 6], 7);
my @b = (0, 1, 2, [3, [4, 5], 6], 7);
if (@a ~~ @b && @b ~~ @a) {
say "a and b are deep copies of each other";
}
elsif (@a ~~ @b) {
say "a smartmatches in b";
}
elsif (@b ~~ @a) {
say "b smartmatches in a";
}
else {
say "a and b don't smartmatch each other at all";
}
If you were to set "$b[3]=4", then instead of reporting that "a and b
are deep copies of each other", it now reports that "b smartmatches in
a". That's because the corresponding position in @a contains an array
that (eventually) has a 4 in it.
Smartmatching one hash against another reports whether both contain the
same keys, no more and no less. This could be used to see whether two
records have the same field names, without caring what values those
fields might have. For example:
use v5.10.1;
sub make_dogtag {
state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
my ($class, $init_fields) = @_;
die "Must supply (only) name, rank, and serial number"
unless $init_fields ~~ $REQUIRED_FIELDS;
...
}
However, this only does what you mean if $init_fields is indeed a hash
reference. The condition "$init_fields ~~ $REQUIRED_FIELDS" also allows
the strings "name", "rank", "serial_num" as well as any array reference
that contains "name" or "rank" or "serial_num" anywhere to pass
through.
The smartmatch operator is most often used as the implicit operator of
a "when" clause. See the section on "Switch Statements" in perlsyn.
Smartmatching of Objects
To avoid relying on an object's underlying representation, if the
smartmatch's right operand is an object that doesn't overload "~~", it
raises the exception ""Smartmatching a non-overloaded object breaks
encapsulation"". That's because one has no business digging around to
see whether something is "in" an object. These are all illegal on
objects without a "~~" overload:
%hash ~~ $object
42 ~~ $object
"fred" ~~ $object
However, you can change the way an object is smartmatched by
overloading the "~~" operator. This is allowed to extend the usual
smartmatch semantics. For objects that do have an "~~" overload, see
overload.
Using an object as the left operand is allowed, although not very
useful. Smartmatching rules take precedence over overloading, so even
if the object in the left operand has smartmatch overloading, this will
be ignored. A left operand that is a non-overloaded object falls back
on a string or numeric comparison of whatever the "ref" operator
returns. That means that
$object ~~ X
does not invoke the overload method with "X" as an argument. Instead
the above table is consulted as normal, and based on the type of "X",
overloading may or may not be invoked. For simple strings or numbers,
"in" becomes equivalent to this:
$object ~~ $number ref($object) == $number
$object ~~ $string ref($object) eq $string
For example, this reports that the handle smells IOish (but please
don't really do this!):
use IO::Handle;
my $fh = IO::Handle->new();
if ($fh ~~ /\bIO\b/) {
say "handle smells IOish";
}
That's because it treats $fh as a string like
"IO::Handle=GLOB(0x8039e0)", then pattern matches against that.
Bitwise And
Binary "&" returns its operands ANDed together bit by bit. Although no
warning is currently raised, the result is not well defined when this
operation is performed on operands that aren't either numbers (see
"Integer Arithmetic") nor bitstrings (see "Bitwise String Operators").
Note that "&" has lower priority than relational operators, so for
example the parentheses are essential in a test like
print "Even\n" if ($x & 1) == 0;
If the "bitwise" feature is enabled via "usefeature'bitwise'" or "use
v5.28", then this operator always treats its operands as numbers.
Before Perl 5.28 this feature produced a warning in the
"experimental::bitwise" category.
Bitwise Or and Exclusive Or
Binary "|" returns its operands ORed together bit by bit.
Binary "^" returns its operands XORed together bit by bit.
Although no warning is currently raised, the results are not well
defined when these operations are performed on operands that aren't
either numbers (see "Integer Arithmetic") nor bitstrings (see "Bitwise
String Operators").
Note that "|" and "^" have lower priority than relational operators, so
for example the parentheses are essential in a test like
print "false\n" if (8 | 2) != 10;
If the "bitwise" feature is enabled via "usefeature'bitwise'" or "use
v5.28", then this operator always treats its operands as numbers.
Before Perl 5.28. this feature produced a warning in the
"experimental::bitwise" category.
C-style Logical And
Binary "&&" performs a short-circuit logical AND operation. That is,
if the left operand is false, the right operand is not even evaluated.
Scalar or list context propagates down to the right operand if it is
evaluated.
C-style Logical Or
Binary "||" performs a short-circuit logical OR operation. That is, if
the left operand is true, the right operand is not even evaluated.
Scalar or list context propagates down to the right operand if it is
evaluated.
Logical Defined-Or
Although it has no direct equivalent in C, Perl's "//" operator is
related to its C-style "or". In fact, it's exactly the same as "||",
except that it tests the left hand side's definedness instead of its
truth. Thus, "EXPR1//EXPR2" returns the value of "EXPR1" if it's
defined, otherwise, the value of "EXPR2" is returned. ("EXPR1" is
evaluated in scalar context, "EXPR2" in the context of "//" itself).
Usually, this is the same result as "defined(EXPR1)?EXPR1:EXPR2"
(except that the ternary-operator form can be used as a lvalue, while
"EXPR1//EXPR2" cannot). This is very useful for providing default
values for variables. If you actually want to test if at least one of
$x and $y is defined, use "defined($x//$y)".
The "||", "//" and "&&" operators return the last value evaluated
(unlike C's "||" and "&&", which return 0 or 1). Thus, a reasonably
portable way to find out the home directory might be:
$home = $ENV{HOME}
// $ENV{LOGDIR}
// (getpwuid($<))[7]
// die "You're homeless!\n";
In particular, this means that you shouldn't use this for selecting
between two aggregates for assignment:
@a = @b || @c; # This doesn't do the right thing
@a = scalar(@b) || @c; # because it really means this.
@a = @b ? @b : @c; # This works fine, though.
As alternatives to "&&" and "||" when used for control flow, Perl
provides the "and" and "or" operators (see below). The short-circuit
behavior is identical. The precedence of "and" and "or" is much lower,
however, so that you can safely use them after a list operator without
the need for parentheses:
unlink "alpha", "beta", "gamma"
or gripe(), next LINE;
With the C-style operators that would have been written like this:
unlink("alpha", "beta", "gamma")
|| (gripe(), next LINE);
It would be even more readable to write that this way:
unless(unlink("alpha", "beta", "gamma")) {
gripe();
next LINE;
}
Using "or" for assignment is unlikely to do what you want; see below.
Range Operators
Binary ".." is the range operator, which is really two different
operators depending on the context. In list context, it returns a list
of values counting (up by ones) from the left value to the right value.
If the left value is greater than the right value then it returns the
empty list. The range operator is useful for writing "foreach(1..10)"
loops and for doing slice operations on arrays. In the current
implementation, no temporary array is created when the range operator
is used as the expression in "foreach" loops, but older versions of
Perl might burn a lot of memory when you write something like this:
for (1 .. 1_000_000) {
# code
}
The range operator also works on strings, using the magical auto-
increment, see below.
In scalar context, ".." returns a boolean value. The operator is
bistable, like a flip-flop, and emulates the line-range (comma)
operator of sed, awk, and various editors. Each ".." operator
maintains its own boolean state, even across calls to a subroutine that
contains it. It is false as long as its left operand is false. Once
the left operand is true, the range operator stays true until the right
operand is true, AFTER which the range operator becomes false again.
It doesn't become false till the next time the range operator is
evaluated. It can test the right operand and become false on the same
evaluation it became true (as in awk), but it still returns true once.
If you don't want it to test the right operand until the next
evaluation, as in sed, just use three dots ("...") instead of two. In
all other regards, "..." behaves just like ".." does.
The right operand is not evaluated while the operator is in the "false"
state, and the left operand is not evaluated while the operator is in
the "true" state. The precedence is a little lower than || and &&.
The value returned is either the empty string for false, or a sequence
number (beginning with 1) for true. The sequence number is reset for
each range encountered. The final sequence number in a range has the
string "E0" appended to it, which doesn't affect its numeric value, but
gives you something to search for if you want to exclude the endpoint.
You can exclude the beginning point by waiting for the sequence number
to be greater than 1.
If either operand of scalar ".." is a constant expression, that operand
is considered true if it is equal ("==") to the current input line
number (the $. variable).
To be pedantic, the comparison is actually "int(EXPR)==int(EXPR)", but
that is only an issue if you use a floating point expression; when
implicitly using $. as described in the previous paragraph, the
comparison is "int(EXPR)==int($.)" which is only an issue when $. is
set to a floating point value and you are not reading from a file.
Furthermore, "span".."spat" or "2.18..3.14" will not do what you want
in scalar context because each of the operands are evaluated using
their integer representation.
Examples:
As a scalar operator:
if (101 .. 200) { print; } # print 2nd hundred lines, short for
# if ($. == 101 .. $. == 200) { print; }
next LINE if (1 .. /^$/); # skip header lines, short for
# next LINE if ($. == 1 .. /^$/);
# (typically in a loop labeled LINE)
s/^/> / if (/^$/ .. eof()); # quote body
# parse mail messages
while (<>) {
$in_header = 1 .. /^$/;
$in_body = /^$/ .. eof;
if ($in_header) {
# do something
} else { # in body
# do something else
}
} continue {
close ARGV if eof; # reset $. each file
}
Here's a simple example to illustrate the difference between the two
range operators:
@lines = (" - Foo",
"01 - Bar",
"1 - Baz",
" - Quux");
foreach (@lines) {
if (/0/ .. /1/) {
print "$_\n";
}
}
This program will print only the line containing "Bar". If the range
operator is changed to "...", it will also print the "Baz" line.
And now some examples as a list operator:
for (101 .. 200) { print } # print $_ 100 times
@foo = @foo[0 .. $#foo]; # an expensive no-op
@foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
The range operator (in list context) makes use of the magical auto-
increment algorithm if the operands are strings. You can say
@alphabet = ("A" .. "Z");
to get all normal letters of the English alphabet, or
$hexdigit = (0 .. 9, "a" .. "f")[$num & 15];
to get a hexadecimal digit, or
@z2 = ("01" .. "31");
print $z2[$mday];
to get dates with leading zeros.
If the final value specified is not in the sequence that the magical
increment would produce, the sequence goes until the next value would
be longer than the final value specified.
As of Perl 5.26, the list-context range operator on strings works as
expected in the scope of "usefeature'unicode_strings". In previous
versions, and outside the scope of that feature, it exhibits "The
"Unicode Bug"" in perlunicode: its behavior depends on the internal
encoding of the range endpoint.
If the initial value specified isn't part of a magical increment
sequence (that is, a non-empty string matching "/^[a-zA-Z]*[0-9]*\z/"),
only the initial value will be returned. So the following will only
return an alpha:
use charnames "greek";
my @greek_small = ("\N{alpha}" .. "\N{omega}");
To get the 25 traditional lowercase Greek letters, including both
sigmas, you could use this instead:
use charnames "greek";
my @greek_small = map { chr } ( ord("\N{alpha}")
..
ord("\N{omega}")
);
However, because there are many other lowercase Greek characters than
just those, to match lowercase Greek characters in a regular
expression, you could use the pattern "/(?:(?=\p{Greek})\p{Lower})+/"
(or the experimental feature "/(?[\p{Greek}&\p{Lower}])+/").
Because each operand is evaluated in integer form, "2.18..3.14" will
return two elements in list context.
@list = (2.18 .. 3.14); # same as @list = (2 .. 3);
Conditional Operator
Ternary "?:" is the conditional operator, just as in C. It works much
like an if-then-else. If the argument before the "?" is true, the
argument before the ":" is returned, otherwise the argument after the
":" is returned. For example:
printf "I have %d dog%s.\n", $n,
($n == 1) ? "" : "s";
Scalar or list context propagates downward into the 2nd or 3rd
argument, whichever is selected.
$x = $ok ? $y : $z; # get a scalar
@x = $ok ? @y : @z; # get an array
$x = $ok ? @y : @z; # oops, that's just a count!
The operator may be assigned to if both the 2nd and 3rd arguments are
legal lvalues (meaning that you can assign to them):
($x_or_y ? $x : $y) = $z;
Because this operator produces an assignable result, using assignments
without parentheses will get you in trouble. For example, this:
$x % 2 ? $x += 10 : $x += 2
Really means this:
(($x % 2) ? ($x += 10) : $x) += 2
Rather than this:
($x % 2) ? ($x += 10) : ($x += 2)
That should probably be written more simply as:
$x += ($x % 2) ? 10 : 2;
Assignment Operators
"=" is the ordinary assignment operator.
Assignment operators work as in C. That is,
$x += 2;
is equivalent to
$x = $x + 2;
although without duplicating any side effects that dereferencing the
lvalue might trigger, such as from "tie()". Other assignment operators
work similarly. The following are recognized:
**= += *= &= &.= <<= &&=
-= /= |= |.= >>= ||=
.= %= ^= ^.= //=
x=
Although these are grouped by family, they all have the precedence of
assignment. These combined assignment operators can only operate on
scalars, whereas the ordinary assignment operator can assign to arrays,
hashes, lists and even references. (See "Context" and "List value
constructors" in perldata, and "Assigning to References" in perlref.)
Unlike in C, the scalar assignment operator produces a valid lvalue.
Modifying an assignment is equivalent to doing the assignment and then
modifying the variable that was assigned to. This is useful for
modifying a copy of something, like this:
($tmp = $global) =~ tr/13579/24680/;
Although as of 5.14, that can be also be accomplished this way:
use v5.14;
$tmp = ($global =~ tr/13579/24680/r);
Likewise,
($x += 2) *= 3;
is equivalent to
$x += 2;
$x *= 3;
Similarly, a list assignment in list context produces the list of
lvalues assigned to, and a list assignment in scalar context returns
the number of elements produced by the expression on the right hand
side of the assignment.
The three dotted bitwise assignment operators ("&.=" "|.=" "^.=") are
new in Perl 5.22. See "Bitwise String Operators".
Comma Operator
Binary "," is the comma operator. In scalar context it evaluates its
left argument, throws that value away, then evaluates its right
argument and returns that value. This is just like C's comma operator.
In list context, it's just the list argument separator, and inserts
both its arguments into the list. These arguments are also evaluated
from left to right.
The "=>" operator (sometimes pronounced "fat comma") is a synonym for
the comma except that it causes a word on its left to be interpreted as
a string if it begins with a letter or underscore and is composed only
of letters, digits and underscores. This includes operands that might
otherwise be interpreted as operators, constants, single number
v-strings or function calls. If in doubt about this behavior, the left
operand can be quoted explicitly.
Otherwise, the "=>" operator behaves exactly as the comma operator or
list argument separator, according to context.
For example:
use constant FOO => "something";
my %h = ( FOO => 23 );
is equivalent to:
my %h = ("FOO", 23);
It is NOT:
my %h = ("something", 23);
The "=>" operator is helpful in documenting the correspondence between
keys and values in hashes, and other paired elements in lists.
%hash = ( $key => $value );
login( $username => $password );
The special quoting behavior ignores precedence, and hence may apply to
part of the left operand:
print time.shift => "bbb";
That example prints something like "1314363215shiftbbb", because the
"=>" implicitly quotes the "shift" immediately on its left, ignoring
the fact that "time.shift" is the entire left operand.
List Operators (Rightward)
On the right side of a list operator, the comma has very low
precedence, such that it controls all comma-separated expressions found
there. The only operators with lower precedence are the logical
operators "and", "or", and "not", which may be used to evaluate calls
to list operators without the need for parentheses:
open HANDLE, "< :encoding(UTF-8)", "filename"
or die "Can't open: $!\n";
However, some people find that code harder to read than writing it with
parentheses:
open(HANDLE, "< :encoding(UTF-8)", "filename")
or die "Can't open: $!\n";
in which case you might as well just use the more customary "||"
operator:
open(HANDLE, "< :encoding(UTF-8)", "filename")
|| die "Can't open: $!\n";
See also discussion of list operators in "Terms and List Operators
(Leftward)".
Logical Not
Unary "not" returns the logical negation of the expression to its
right. It's the equivalent of "!" except for the very low precedence.
Logical And
Binary "and" returns the logical conjunction of the two surrounding
expressions. It's equivalent to "&&" except for the very low
precedence. This means that it short-circuits: the right expression is
evaluated only if the left expression is true.
Logical or and Exclusive Or
Binary "or" returns the logical disjunction of the two surrounding
expressions. It's equivalent to "||" except for the very low
precedence. This makes it useful for control flow:
print FH $data or die "Can't write to FH: $!";
This means that it short-circuits: the right expression is evaluated
only if the left expression is false. Due to its precedence, you must
be careful to avoid using it as replacement for the "||" operator. It
usually works out better for flow control than in assignments:
$x = $y or $z; # bug: this is wrong
($x = $y) or $z; # really means this
$x = $y || $z; # better written this way
However, when it's a list-context assignment and you're trying to use
"||" for control flow, you probably need "or" so that the assignment
takes higher precedence.
@info = stat($file) || die; # oops, scalar sense of stat!
@info = stat($file) or die; # better, now @info gets its due
Then again, you could always use parentheses.
Binary "xor" returns the exclusive-OR of the two surrounding
expressions. It cannot short-circuit (of course).
There is no low precedence operator for defined-OR.
C Operators Missing From Perl
Here is what C has that Perl doesn't:
unary & Address-of operator. (But see the "\" operator for taking a
reference.)
unary * Dereference-address operator. (Perl's prefix dereferencing
operators are typed: "$", "@", "%", and "&".)
(TYPE) Type-casting operator.
Quote and Quote-like Operators
While we usually think of quotes as literal values, in Perl they
function as operators, providing various kinds of interpolating and
pattern matching capabilities. Perl provides customary quote
characters for these behaviors, but also provides a way for you to
choose your quote character for any of them. In the following table, a
"{}" represents any pair of delimiters you choose.
Customary Generic Meaning Interpolates
'' q{} Literal no
"" qq{} Literal yes
`` qx{} Command yes*
qw{} Word list no
// m{} Pattern match yes*
qr{} Pattern yes*
s{}{} Substitution yes*
tr{}{} Transliteration no (but see below)
y{}{} Transliteration no (but see below)
<<EOF here-doc yes*
* unless the delimiter is ''.
Non-bracketing delimiters use the same character fore and aft, but the
four sorts of ASCII brackets (round, angle, square, curly) all nest,
which means that
q{foo{bar}baz}
is the same as
'foo{bar}baz'
Note, however, that this does not always work for quoting Perl code:
$s = q{ if($x eq "}") ... }; # WRONG
is a syntax error. The "Text::Balanced" module (standard as of v5.8,
and from CPAN before then) is able to do this properly.
There can (and in some cases, must) be whitespace between the operator
and the quoting characters, except when "#" is being used as the
quoting character. "q#foo#" is parsed as the string "foo", while
"q#foo#" is the operator "q" followed by a comment. Its argument will
be taken from the next line. This allows you to write:
s {foo} # Replace foo
{bar} # with bar.
The cases where whitespace must be used are when the quoting character
is a word character (meaning it matches "/\w/"):
q XfooX # Works: means the string 'foo'
qXfooX # WRONG!
The following escape sequences are available in constructs that
interpolate, and in transliterations whose delimiters aren't single
quotes ("'").
Sequence Note Description
\t tab (HT, TAB)
\n newline (NL)
\r return (CR)
\f form feed (FF)
\b backspace (BS)
\a alarm (bell) (BEL)
\e escape (ESC)
\x{263A} [1,8] hex char (example: SMILEY)
\x1b [2,8] restricted range hex char (example: ESC)
\N{name} [3] named Unicode character or character sequence
\N{U+263D} [4,8] Unicode character (example: FIRST QUARTER MOON)
\c[ [5] control char (example: chr(27))
\o{23072} [6,8] octal char (example: SMILEY)
\033 [7,8] restricted range octal char (example: ESC)
[1] The result is the character specified by the hexadecimal number
between the braces. See "[8]" below for details on which
character.
Only hexadecimal digits are valid between the braces. If an
invalid character is encountered, a warning will be issued and the
invalid character and all subsequent characters (valid or invalid)
within the braces will be discarded.
If there are no valid digits between the braces, the generated
character is the NULL character ("\x{00}"). However, an explicit
empty brace ("\x{}") will not cause a warning (currently).
[2] The result is the character specified by the hexadecimal number in
the range 0x00 to 0xFF. See "[8]" below for details on which
character.
Only hexadecimal digits are valid following "\x". When "\x" is
followed by fewer than two valid digits, any valid digits will be
zero-padded. This means that "\x7" will be interpreted as "\x07",
and a lone "\x" will be interpreted as "\x00". Except at the end
of a string, having fewer than two valid digits will result in a
warning. Note that although the warning says the illegal character
is ignored, it is only ignored as part of the escape and will still
be used as the subsequent character in the string. For example:
Original Result Warns?
"\x7" "\x07" no
"\x" "\x00" no
"\x7q" "\x07q" yes
"\xq" "\x00q" yes
[3] The result is the Unicode character or character sequence given by
name. See charnames.
[4] "\N{U+hexadecimalnumber}" means the Unicode character whose Unicode
code point is hexadecimal number.
[5] The character following "\c" is mapped to some other character as
shown in the table:
Sequence Value
\c@ chr(0)
\cA chr(1)
\ca chr(1)
\cB chr(2)
\cb chr(2)
...
\cZ chr(26)
\cz chr(26)
\c[ chr(27)
# See below for chr(28)
\c] chr(29)
\c^ chr(30)
\c_ chr(31)
\c? chr(127) # (on ASCII platforms; see below for link to
# EBCDIC discussion)
In other words, it's the character whose code point has had 64
xor'd with its uppercase. "\c?" is DELETE on ASCII platforms
because "ord("?")^64" is 127, and "\c@" is NULL because the ord of
"@" is 64, so xor'ing 64 itself produces 0.
Also, "\c\X" yields "chr(28)."X"" for any X, but cannot come at the
end of a string, because the backslash would be parsed as escaping
the end quote.
On ASCII platforms, the resulting characters from the list above
are the complete set of ASCII controls. This isn't the case on
EBCDIC platforms; see "OPERATOR DIFFERENCES" in perlebcdic for a
full discussion of the differences between these for ASCII versus
EBCDIC platforms.
Use of any other character following the "c" besides those listed
above is discouraged, and as of Perl v5.20, the only characters
actually allowed are the printable ASCII ones, minus the left brace
"{". What happens for any of the allowed other characters is that
the value is derived by xor'ing with the seventh bit, which is 64,
and a warning raised if enabled. Using the non-allowed characters
generates a fatal error.
To get platform independent controls, you can use "\N{...}".
[6] The result is the character specified by the octal number between
the braces. See "[8]" below for details on which character.
If a character that isn't an octal digit is encountered, a warning
is raised, and the value is based on the octal digits before it,
discarding it and all following characters up to the closing brace.
It is a fatal error if there are no octal digits at all.
[7] The result is the character specified by the three-digit octal
number in the range 000 to 777 (but best to not use above 077, see
next paragraph). See "[8]" below for details on which character.
Some contexts allow 2 or even 1 digit, but any usage without
exactly three digits, the first being a zero, may give unintended
results. (For example, in a regular expression it may be confused
with a backreference; see "Octal escapes" in perlrebackslash.)
Starting in Perl 5.14, you may use "\o{}" instead, which avoids all
these problems. Otherwise, it is best to use this construct only
for ordinals "\077" and below, remembering to pad to the left with
zeros to make three digits. For larger ordinals, either use
"\o{}", or convert to something else, such as to hex and use
"\N{U+}" (which is portable between platforms with different
character sets) or "\x{}" instead.
[8] Several constructs above specify a character by a number. That
number gives the character's position in the character set encoding
(indexed from 0). This is called synonymously its ordinal, code
position, or code point. Perl works on platforms that have a
native encoding currently of either ASCII/Latin1 or EBCDIC, each of
which allow specification of 256 characters. In general, if the
number is 255 (0xFF, 0377) or below, Perl interprets this in the
platform's native encoding. If the number is 256 (0x100, 0400) or
above, Perl interprets it as a Unicode code point and the result is
the corresponding Unicode character. For example "\x{50}" and
"\o{120}" both are the number 80 in decimal, which is less than
256, so the number is interpreted in the native character set
encoding. In ASCII the character in the 80th position (indexed
from 0) is the letter "P", and in EBCDIC it is the ampersand symbol
"&". "\x{100}" and "\o{400}" are both 256 in decimal, so the
number is interpreted as a Unicode code point no matter what the
native encoding is. The name of the character in the 256th
position (indexed by 0) in Unicode is "LATIN CAPITAL LETTER A WITH
MACRON".
An exception to the above rule is that "\N{U+hexnumber}" is always
interpreted as a Unicode code point, so that "\N{U+0050}" is "P"
even on EBCDIC platforms.
NOTE: Unlike C and other languages, Perl has no "\v" escape sequence
for the vertical tab (VT, which is 11 in both ASCII and EBCDIC), but
you may use "\N{VT}", "\ck", "\N{U+0b}", or "\x0b". ("\v" does have
meaning in regular expression patterns in Perl, see perlre.)
The following escape sequences are available in constructs that
interpolate, but not in transliterations.
\l lowercase next character only
\u titlecase (not uppercase!) next character only
\L lowercase all characters till \E or end of string
\U uppercase all characters till \E or end of string
\F foldcase all characters till \E or end of string
\Q quote (disable) pattern metacharacters till \E or
end of string
\E end either case modification or quoted section
(whichever was last seen)
See "quotemeta" in perlfunc for the exact definition of characters that
are quoted by "\Q".
"\L", "\U", "\F", and "\Q" can stack, in which case you need one "\E"
for each. For example:
say"This \Qquoting \ubusiness \Uhere isn't quite\E done yet,\E is it?";
This quoting\ Business\ HERE\ ISN\'T\ QUITE\ done\ yet\, is it?
If a "uselocale" form that includes "LC_CTYPE" is in effect (see
perllocale), the case map used by "\l", "\L", "\u", and "\U" is taken
from the current locale. If Unicode (for example, "\N{}" or code
points of 0x100 or beyond) is being used, the case map used by "\l",
"\L", "\u", and "\U" is as defined by Unicode. That means that case-
mapping a single character can sometimes produce a sequence of several
characters. Under "uselocale", "\F" produces the same results as "\L"
for all locales but a UTF-8 one, where it instead uses the Unicode
definition.
All systems use the virtual "\n" to represent a line terminator, called
a "newline". There is no such thing as an unvarying, physical newline
character. It is only an illusion that the operating system, device
drivers, C libraries, and Perl all conspire to preserve. Not all
systems read "\r" as ASCII CR and "\n" as ASCII LF. For example, on
the ancient Macs (pre-MacOS X) of yesteryear, these used to be
reversed, and on systems without a line terminator, printing "\n" might
emit no actual data. In general, use "\n" when you mean a "newline"
for your system, but use the literal ASCII when you need an exact
character. For example, most networking protocols expect and prefer a
CR+LF ("\015\012" or "\cM\cJ") for line terminators, and although they
often accept just "\012", they seldom tolerate just "\015". If you get
in the habit of using "\n" for networking, you may be burned some day.
For constructs that do interpolate, variables beginning with ""$"" or
""@"" are interpolated. Subscripted variables such as $a[3] or
"$href->{key}[0]" are also interpolated, as are array and hash slices.
But method calls such as "$obj->meth" are not.
Interpolating an array or slice interpolates the elements in order,
separated by the value of $", so is equivalent to interpolating
"join$",@array". "Punctuation" arrays such as "@*" are usually
interpolated only if the name is enclosed in braces "@{*}", but the
arrays @_, "@+", and "@-" are interpolated even without braces.
For double-quoted strings, the quoting from "\Q" is applied after
interpolation and escapes are processed.
"abc\Qfoo\tbar$s\Exyz"
is equivalent to
"abc" . quotemeta("foo\tbar$s") . "xyz"
For the pattern of regex operators ("qr//", "m//" and "s///"), the
quoting from "\Q" is applied after interpolation is processed, but
before escapes are processed. This allows the pattern to match
literally (except for "$" and "@"). For example, the following
matches:
'\s\t' =~ /\Q\s\t/
Because "$" or "@" trigger interpolation, you'll need to use something
like "/\Quser\E\@\Qhost/" to match them literally.
Patterns are subject to an additional level of interpretation as a
regular expression. This is done as a second pass, after variables are
interpolated, so that regular expressions may be incorporated into the
pattern from the variables. If this is not what you want, use "\Q" to
interpolate a variable literally.
Apart from the behavior described above, Perl does not expand multiple
levels of interpolation. In particular, contrary to the expectations
of shell programmers, back-quotes do NOT interpolate within double
quotes, nor do single quotes impede evaluation of variables when used
within double quotes.
Regexp Quote-Like Operators
Here are the quote-like operators that apply to pattern matching and
related activities.
"qr/STRING/msixpodualn"
This operator quotes (and possibly compiles) its STRING as a
regular expression. STRING is interpolated the same way as
PATTERN in "m/PATTERN/". If "'" is used as the delimiter, no
variable interpolation is done. Returns a Perl value which may
be used instead of the corresponding "/STRING/msixpodualn"
expression. The returned value is a normalized version of the
original pattern. It magically differs from a string
containing the same characters: "ref(qr/x/)" returns "Regexp";
however, dereferencing it is not well defined (you currently
get the normalized version of the original pattern, but this
may change).
For example,
$rex = qr/my.STRING/is;
print $rex; # prints (?si-xm:my.STRING)
s/$rex/foo/;
is equivalent to
s/my.STRING/foo/is;
The result may be used as a subpattern in a match:
$re = qr/$pattern/;
$string =~ /foo${re}bar/; # can be interpolated in other
# patterns
$string =~ $re; # or used standalone
$string =~ /$re/; # or this way
Since Perl may compile the pattern at the moment of execution
of the "qr()" operator, using "qr()" may have speed advantages
in some situations, notably if the result of "qr()" is used
standalone:
sub match {
my $patterns = shift;
my @compiled = map qr/$_/i, @$patterns;
grep {
my $success = 0;
foreach my $pat (@compiled) {
$success = 1, last if /$pat/;
}
$success;
} @_;
}
Precompilation of the pattern into an internal representation
at the moment of "qr()" avoids the need to recompile the
pattern every time a match "/$pat/" is attempted. (Perl has
many other internal optimizations, but none would be triggered
in the above example if we did not use "qr()" operator.)
Options (specified by the following modifiers) are:
m Treat string as multiple lines.
s Treat string as single line. (Make . match a newline)
i Do case-insensitive pattern matching.
x Use extended regular expressions; specifying two
x's means \t and the SPACE character are ignored within
square-bracketed character classes
p When matching preserve a copy of the matched string so
that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be
defined (ignored starting in v5.20) as these are always
defined starting in that release
o Compile pattern only once.
a ASCII-restrict: Use ASCII for \d, \s, \w and [[:posix:]]
character classes; specifying two a's adds the further
restriction that no ASCII character will match a
non-ASCII one under /i.
l Use the current run-time locale's rules.
u Use Unicode rules.
d Use Unicode or native charset, as in 5.12 and earlier.
n Non-capture mode. Don't let () fill in $1, $2, etc...
If a precompiled pattern is embedded in a larger pattern then
the effect of "msixpluadn" will be propagated appropriately.
The effect that the "/o" modifier has is not propagated, being
restricted to those patterns explicitly using it.
The "/a", "/d", "/l", and "/u" modifiers (added in Perl 5.14)
control the character set rules, but "/a" is the only one you
are likely to want to specify explicitly; the other three are
selected automatically by various pragmas.
See perlre for additional information on valid syntax for
STRING, and for a detailed look at the semantics of regular
expressions. In particular, all modifiers except the largely
obsolete "/o" are further explained in "Modifiers" in perlre.
"/o" is described in the next section.
"m/PATTERN/msixpodualngc"
"/PATTERN/msixpodualngc"
Searches a string for a pattern match, and in scalar context
returns true if it succeeds, false if it fails. If no string
is specified via the "=~" or "!~" operator, the $_ string is
searched. (The string specified with "=~" need not be an
lvalue--it may be the result of an expression evaluation, but
remember the "=~" binds rather tightly.) See also perlre.
Options are as described in "qr//" above; in addition, the
following match process modifiers are available:
g Match globally, i.e., find all occurrences.
c Do not reset search position on a failed match when /g is
in effect.
If "/" is the delimiter then the initial "m" is optional. With
the "m" you can use any pair of non-whitespace (ASCII)
characters as delimiters. This is particularly useful for
matching path names that contain "/", to avoid LTS (leaning
toothpick syndrome). If "?" is the delimiter, then a match-
only-once rule applies, described in "m?PATTERN?" below. If
"'" (single quote) is the delimiter, no variable interpolation
is performed on the PATTERN. When using a delimiter character
valid in an identifier, whitespace is required after the "m".
PATTERN may contain variables, which will be interpolated every
time the pattern search is evaluated, except for when the
delimiter is a single quote. (Note that $(, $), and $| are not
interpolated because they look like end-of-string tests.) Perl
will not recompile the pattern unless an interpolated variable
that it contains changes. You can force Perl to skip the test
and never recompile by adding a "/o" (which stands for "once")
after the trailing delimiter. Once upon a time, Perl would
recompile regular expressions unnecessarily, and this modifier
was useful to tell it not to do so, in the interests of speed.
But now, the only reasons to use "/o" are one of:
1. The variables are thousands of characters long and you know
that they don't change, and you need to wring out the last
little bit of speed by having Perl skip testing for that.
(There is a maintenance penalty for doing this, as
mentioning "/o" constitutes a promise that you won't change
the variables in the pattern. If you do change them, Perl
won't even notice.)
2. you want the pattern to use the initial values of the
variables regardless of whether they change or not. (But
there are saner ways of accomplishing this than using
"/o".)
3. If the pattern contains embedded code, such as
use re 'eval';
$code = 'foo(?{ $x })';
/$code/
then perl will recompile each time, even though the pattern
string hasn't changed, to ensure that the current value of
$x is seen each time. Use "/o" if you want to avoid this.
The bottom line is that using "/o" is almost never a good idea.
The empty pattern "//"
If the PATTERN evaluates to the empty string, the last
successfully matched regular expression is used instead. In
this case, only the "g" and "c" flags on the empty pattern are
honored; the other flags are taken from the original pattern.
If no match has previously succeeded, this will (silently) act
instead as a genuine empty pattern (which will always match).
Note that it's possible to confuse Perl into thinking "//" (the
empty regex) is really "//" (the defined-or operator). Perl is
usually pretty good about this, but some pathological cases
might trigger this, such as "$x///" (is that "($x)/(//)" or
"$x///"?) and "print$fh//" ("print$fh(//" or "print($fh//"?).
In all of these examples, Perl will assume you meant defined-
or. If you meant the empty regex, just use parentheses or
spaces to disambiguate, or even prefix the empty regex with an
"m" (so "//" becomes "m//").
Matching in list context
If the "/g" option is not used, "m//" in list context returns a
list consisting of the subexpressions matched by the
parentheses in the pattern, that is, ($1, $2, $3...) (Note
that here $1 etc. are also set). When there are no parentheses
in the pattern, the return value is the list "(1)" for success.
With or without parentheses, an empty list is returned upon
failure.
Examples:
open(TTY, "+</dev/tty")
|| die "can't access /dev/tty: $!";
<TTY> =~ /^y/i && foo(); # do foo if desired
if (/Version: *([0-9.]*)/) { $version = $1; }
next if m#^/usr/spool/uucp#;
# poor man's grep
$arg = shift;
while (<>) {
print if /$arg/o; # compile only once (no longer needed!)
}
if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
This last example splits $foo into the first two words and the
remainder of the line, and assigns those three fields to $F1,
$F2, and $Etc. The conditional is true if any variables were
assigned; that is, if the pattern matched.
The "/g" modifier specifies global pattern matching--that is,
matching as many times as possible within the string. How it
behaves depends on the context. In list context, it returns a
list of the substrings matched by any capturing parentheses in
the regular expression. If there are no parentheses, it
returns a list of all the matched strings, as if there were
parentheses around the whole pattern.
In scalar context, each execution of "m//g" finds the next
match, returning true if it matches, and false if there is no
further match. The position after the last match can be read
or set using the "pos()" function; see "pos" in perlfunc. A
failed match normally resets the search position to the
beginning of the string, but you can avoid that by adding the
"/c" modifier (for example, "m//gc"). Modifying the target
string also resets the search position.
"\G assertion"
You can intermix "m//g" matches with "m/\G.../g", where "\G" is
a zero-width assertion that matches the exact position where
the previous "m//g", if any, left off. Without the "/g"
modifier, the "\G" assertion still anchors at "pos()" as it was
at the start of the operation (see "pos" in perlfunc), but the
match is of course only attempted once. Using "\G" without
"/g" on a target string that has not previously had a "/g"
match applied to it is the same as using the "\A" assertion to
match the beginning of the string. Note also that, currently,
"\G" is only properly supported when anchored at the very
beginning of the pattern.
Examples:
# list context
($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
# scalar context
local $/ = "";
while ($paragraph = <>) {
while ($paragraph =~ /\p{Ll}['")]*[.!?]+['")]*\s/g) {
$sentences++;
}
}
say $sentences;
Here's another way to check for sentences in a paragraph:
my $sentence_rx = qr{
(?: (?<= ^ ) | (?<= \s ) ) # after start-of-string or
# whitespace
\p{Lu} # capital letter
.*? # a bunch of anything
(?<= \S ) # that ends in non-
# whitespace
(?<! \b [DMS]r ) # but isn't a common abbr.
(?<! \b Mrs )
(?<! \b Sra )
(?<! \b St )
[.?!] # followed by a sentence
# ender
(?= $ | \s ) # in front of end-of-string
# or whitespace
}sx;
local $/ = "";
while (my $paragraph = <>) {
say "NEW PARAGRAPH";
my $count = 0;
while ($paragraph =~ /($sentence_rx)/g) {
printf "\tgot sentence %d: <%s>\n", ++$count, $1;
}
}
Here's how to use "m//gc" with "\G":
$_ = "ppooqppqq";
while ($i++ < 2) {
print "1: '";
print $1 while /(o)/gc; print "', pos=", pos, "\n";
print "2: '";
print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
print "3: '";
print $1 while /(p)/gc; print "', pos=", pos, "\n";
}
print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
The last example should print:
1: 'oo', pos=4
2: 'q', pos=5
3: 'pp', pos=7
1: '', pos=7
2: 'q', pos=8
3: '', pos=8
Final: 'q', pos=8
Notice that the final match matched "q" instead of "p", which a
match without the "\G" anchor would have done. Also note that
the final match did not update "pos". "pos" is only updated on
a "/g" match. If the final match did indeed match "p", it's a
good bet that you're running an ancient (pre-5.6.0) version of
Perl.
A useful idiom for "lex"-like scanners is "/\G.../gc". You can
combine several regexps like this to process a string part-by-
part, doing different actions depending on which regexp
matched. Each regexp tries to match where the previous one
leaves off.
$_ = <<'EOL';
$url = URI::URL->new( "http://example.com/" );
die if $url eq "xXx";
EOL
LOOP: {
print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
print(" lowercase"), redo LOOP
if /\G\p{Ll}+\b[,.;]?\s*/gc;
print(" UPPERCASE"), redo LOOP
if /\G\p{Lu}+\b[,.;]?\s*/gc;
print(" Capitalized"), redo LOOP
if /\G\p{Lu}\p{Ll}+\b[,.;]?\s*/gc;
print(" MiXeD"), redo LOOP if /\G\pL+\b[,.;]?\s*/gc;
print(" alphanumeric"), redo LOOP
if /\G[\p{Alpha}\pN]+\b[,.;]?\s*/gc;
print(" line-noise"), redo LOOP if /\G\W+/gc;
print ". That's all!\n";
}
Here is the output (split into several lines):
line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE
line-noise lowercase line-noise lowercase line-noise lowercase
lowercase line-noise lowercase lowercase line-noise lowercase
lowercase line-noise MiXeD line-noise. That's all!
"m?PATTERN?msixpodualngc"
This is just like the "m/PATTERN/" search, except that it
matches only once between calls to the "reset()" operator.
This is a useful optimization when you want to see only the
first occurrence of something in each file of a set of files,
for instance. Only "m??" patterns local to the current
package are reset.
while (<>) {
if (m?^$?) {
# blank line between header and body
}
} continue {
reset if eof; # clear m?? status for next file
}
Another example switched the first "latin1" encoding it finds
to "utf8" in a pod file:
s//utf8/ if m? ^ =encoding \h+ \K latin1 ?x;
The match-once behavior is controlled by the match delimiter
being "?"; with any other delimiter this is the normal "m//"
operator.
In the past, the leading "m" in "m?PATTERN?" was optional, but
omitting it would produce a deprecation warning. As of
v5.22.0, omitting it produces a syntax error. If you encounter
this construct in older code, you can just add "m".
"s/PATTERN/REPLACEMENT/msixpodualngcer"
Searches a string for a pattern, and if found, replaces that
pattern with the replacement text and returns the number of
substitutions made. Otherwise it returns false (a value that
is both an empty string ("") and numeric zero (0) as described
in "Relational Operators").
If the "/r" (non-destructive) option is used then it runs the
substitution on a copy of the string and instead of returning
the number of substitutions, it returns the copy whether or not
a substitution occurred. The original string is never changed
when "/r" is used. The copy will always be a plain string,
even if the input is an object or a tied variable.
If no string is specified via the "=~" or "!~" operator, the $_
variable is searched and modified. Unless the "/r" option is
used, the string specified must be a scalar variable, an array
element, a hash element, or an assignment to one of those; that
is, some sort of scalar lvalue.
If the delimiter chosen is a single quote, no variable
interpolation is done on either the PATTERN or the REPLACEMENT.
Otherwise, if the PATTERN contains a "$" that looks like a
variable rather than an end-of-string test, the variable will
be interpolated into the pattern at run-time. If you want the
pattern compiled only once the first time the variable is
interpolated, use the "/o" option. If the pattern evaluates to
the empty string, the last successfully executed regular
expression is used instead. See perlre for further explanation
on these.
Options are as with "m//" with the addition of the following
replacement specific options:
e Evaluate the right side as an expression.
ee Evaluate the right side as a string then eval the
result.
r Return substitution and leave the original string
untouched.
Any non-whitespace delimiter may replace the slashes. Add
space after the "s" when using a character allowed in
identifiers. If single quotes are used, no interpretation is
done on the replacement string (the "/e" modifier overrides
this, however). Note that Perl treats backticks as normal
delimiters; the replacement text is not evaluated as a command.
If the PATTERN is delimited by bracketing quotes, the
REPLACEMENT has its own pair of quotes, which may or may not be
bracketing quotes, for example, "s(foo)(bar)" or "s<foo>/bar/".
A "/e" will cause the replacement portion to be treated as a
full-fledged Perl expression and evaluated right then and
there. It is, however, syntax checked at compile-time. A
second "e" modifier will cause the replacement portion to be
"eval"ed before being run as a Perl expression.
Examples:
s/\bgreen\b/mauve/g; # don't change wintergreen
$path =~ s|/usr/bin|/usr/local/bin|;
s/Login: $foo/Login: $bar/; # run-time pattern
($foo = $bar) =~ s/this/that/; # copy first, then
# change
($foo = "$bar") =~ s/this/that/; # convert to string,
# copy, then change
$foo = $bar =~ s/this/that/r; # Same as above using /r
$foo = $bar =~ s/this/that/r
=~ s/that/the other/r; # Chained substitutes
# using /r
@foo = map { s/this/that/r } @bar # /r is very useful in
# maps
$count = ($paragraph =~ s/Mister\b/Mr./g); # get change-cnt
$_ = 'abc123xyz';
s/\d+/$&*2/e; # yields 'abc246xyz'
s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
s/%(.)/$percent{$1}/g; # change percent escapes; no /e
s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
s/^=(\w+)/pod($1)/ge; # use function call
$_ = 'abc123xyz';
$x = s/abc/def/r; # $x is 'def123xyz' and
# $_ remains 'abc123xyz'.
# expand variables in $_, but dynamics only, using
# symbolic dereferencing
s/\$(\w+)/${$1}/g;
# Add one to the value of any numbers in the string
s/(\d+)/1 + $1/eg;
# Titlecase words in the last 30 characters only
substr($str, -30) =~ s/\b(\p{Alpha}+)\b/\u\L$1/g;
# This will expand any embedded scalar variable
# (including lexicals) in $_ : First $1 is interpolated
# to the variable name, and then evaluated
s/(\$\w+)/$1/eeg;
# Delete (most) C comments.
$program =~ s {
/\* # Match the opening delimiter.
.*? # Match a minimal number of characters.
\*/ # Match the closing delimiter.
} []gsx;
s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_,
# expensively
for ($variable) { # trim whitespace in $variable,
# cheap
s/^\s+//;
s/\s+$//;
}
s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
$foo !~ s/A/a/g; # Lowercase all A's in $foo; return
# 0 if any were found and changed;
# otherwise return 1
Note the use of "$" instead of "\" in the last example. Unlike
sed, we use the \<digit> form only in the left hand side.
Anywhere else it's $<digit>.
Occasionally, you can't use just a "/g" to get all the changes
to occur that you might want. Here are two common cases:
# put commas in the right places in an integer
1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
# expand tabs to 8-column spacing
1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
Quote-Like Operators
"q/STRING/"
'STRING'
A single-quoted, literal string. A backslash represents a
backslash unless followed by the delimiter or another backslash, in
which case the delimiter or backslash is interpolated.
$foo = q!I said, "You said, 'She said it.'"!;
$bar = q('This is it.');
$baz = '\n'; # a two-character string
"qq/STRING/"
"STRING"
A double-quoted, interpolated string.
$_ .= qq
(*** The previous line contains the naughty word "$1".\n)
if /\b(tcl|java|python)\b/i; # :-)
$baz = "\n"; # a one-character string
"qx/STRING/"
"`STRING`"
A string which is (possibly) interpolated and then executed as a
system command with /bin/sh or its equivalent. Shell wildcards,
pipes, and redirections will be honored. The collected standard
output of the command is returned; standard error is unaffected.
In scalar context, it comes back as a single (potentially multi-
line) string, or "undef" if the command failed. In list context,
returns a list of lines (however you've defined lines with $/ or
$INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
Because backticks do not affect standard error, use shell file
descriptor syntax (assuming the shell supports this) if you care to
address this. To capture a command's STDERR and STDOUT together:
$output = `cmd 2>&1`;
To capture a command's STDOUT but discard its STDERR:
$output = `cmd 2>/dev/null`;
To capture a command's STDERR but discard its STDOUT (ordering is
important here):
$output = `cmd 2>&1 1>/dev/null`;
To exchange a command's STDOUT and STDERR in order to capture the
STDERR but leave its STDOUT to come out the old STDERR:
$output = `cmd 3>&1 1>&2 2>&3 3>&-`;
To read both a command's STDOUT and its STDERR separately, it's
easiest to redirect them separately to files, and then read from
those files when the program is done:
system("program args 1>program.stdout 2>program.stderr");
The STDIN filehandle used by the command is inherited from Perl's
STDIN. For example:
open(SPLAT, "stuff") || die "can't open stuff: $!";
open(STDIN, "<&SPLAT") || die "can't dupe SPLAT: $!";
print STDOUT `sort`;
will print the sorted contents of the file named "stuff".
Using single-quote as a delimiter protects the command from Perl's
double-quote interpolation, passing it on to the shell instead:
$perl_info = qx(ps $$); # that's Perl's $$
$shell_info = qx'ps $$'; # that's the new shell's $$
How that string gets evaluated is entirely subject to the command
interpreter on your system. On most platforms, you will have to
protect shell metacharacters if you want them treated literally.
This is in practice difficult to do, as it's unclear how to escape
which characters. See perlsec for a clean and safe example of a
manual "fork()" and "exec()" to emulate backticks safely.
On some platforms (notably DOS-like ones), the shell may not be
capable of dealing with multiline commands, so putting newlines in
the string may not get you what you want. You may be able to
evaluate multiple commands in a single line by separating them with
the command separator character, if your shell supports that (for
example, ";" on many Unix shells and "&" on the Windows NT "cmd"
shell).
Perl will attempt to flush all files opened for output before
starting the child process, but this may not be supported on some
platforms (see perlport). To be safe, you may need to set $|
($AUTOFLUSH in "English") or call the "autoflush()" method of
"IO::Handle" on any open handles.
Beware that some command shells may place restrictions on the
length of the command line. You must ensure your strings don't
exceed this limit after any necessary interpolations. See the
platform-specific release notes for more details about your
particular environment.
Using this operator can lead to programs that are difficult to
port, because the shell commands called vary between systems, and
may in fact not be present at all. As one example, the "type"
command under the POSIX shell is very different from the "type"
command under DOS. That doesn't mean you should go out of your way
to avoid backticks when they're the right way to get something
done. Perl was made to be a glue language, and one of the things
it glues together is commands. Just understand what you're getting
yourself into.
Like "system", backticks put the child process exit code in $?. If
you'd like to manually inspect failure, you can check all possible
failure modes by inspecting $? like this:
if ($? == -1) {
print "failed to execute: $!\n";
}
elsif ($? & 127) {
printf "child died with signal %d, %s coredump\n",
($? & 127), ($? & 128) ? 'with' : 'without';
}
else {
printf "child exited with value %d\n", $? >> 8;
}
Use the open pragma to control the I/O layers used when reading the
output of the command, for example:
use open IN => ":encoding(UTF-8)";
my $x = `cmd-producing-utf-8`;
"qx//" can also be called like a function with "readpipe" in
perlfunc.
See "I/O Operators" for more discussion.
"qw/STRING/"
Evaluates to a list of the words extracted out of STRING, using
embedded whitespace as the word delimiters. It can be understood
as being roughly equivalent to:
split(" ", q/STRING/);
the differences being that it only splits on ASCII whitespace,
generates a real list at compile time, and in scalar context it
returns the last element in the list. So this expression:
qw(foo bar baz)
is semantically equivalent to the list:
"foo", "bar", "baz"
Some frequently seen examples:
use POSIX qw( setlocale localeconv )
@EXPORT = qw( foo bar baz );
A common mistake is to try to separate the words with commas or to
put comments into a multi-line "qw"-string. For this reason, the
"usewarnings" pragma and the -w switch (that is, the $^W variable)
produces warnings if the STRING contains the "," or the "#"
character.
"tr/SEARCHLIST/REPLACEMENTLIST/cdsr"
"y/SEARCHLIST/REPLACEMENTLIST/cdsr"
Transliterates all occurrences of the characters found (or not
found if the "/c" modifier is specified) in the search list with
the positionally corresponding character in the replacement list,
possibly deleting some, depending on the modifiers specified. It
returns the number of characters replaced or deleted. If no string
is specified via the "=~" or "!~" operator, the $_ string is
transliterated.
For sed devotees, "y" is provided as a synonym for "tr".
If the "/r" (non-destructive) option is present, a new copy of the
string is made and its characters transliterated, and this copy is
returned no matter whether it was modified or not: the original
string is always left unchanged. The new copy is always a plain
string, even if the input string is an object or a tied variable.
Unless the "/r" option is used, the string specified with "=~" must
be a scalar variable, an array element, a hash element, or an
assignment to one of those; in other words, an lvalue.
If the characters delimiting SEARCHLIST and REPLACEMENTLIST are
single quotes ("tr'SEARCHLIST'REPLACEMENTLIST'"), the only
interpolation is removal of "\" from pairs of "\\".
Otherwise, a character range may be specified with a hyphen, so
"tr/A-J/0-9/" does the same replacement as
"tr/ACEGIBDFHJ/0246813579/".
If the SEARCHLIST is delimited by bracketing quotes, the
REPLACEMENTLIST must have its own pair of quotes, which may or may
not be bracketing quotes; for example, "tr[aeiouy][yuoiea]" or
"tr(+\-*/)/ABCD/".
Characters may be literals, or (if the delimiters aren't single
quotes) any of the escape sequences accepted in double-quoted
strings. But there is never any variable interpolation, so "$" and
"@" are always treated as literals. A hyphen at the beginning or
end, or preceded by a backslash is also always considered a
literal. Escape sequence details are in the table near the
beginning of this section.
Note that "tr" does not do regular expression character classes
such as "\d" or "\pL". The "tr" operator is not equivalent to the
tr(1) utility. "tr[a-z][A-Z]" will uppercase the 26 letters "a"
through "z", but for case changing not confined to ASCII, use "lc",
"uc", "lcfirst", "ucfirst" (all documented in perlfunc), or the
substitution operator "s/PATTERN/REPLACEMENT/" (with "\U", "\u",
"\L", and "\l" string-interpolation escapes in the REPLACEMENT
portion).
Most ranges are unportable between character sets, but certain ones
signal Perl to do special handling to make them portable. There
are two classes of portable ranges. The first are any subsets of
the ranges "A-Z", "a-z", and "0-9", when expressed as literal
characters.
tr/h-k/H-K/
capitalizes the letters "h", "i", "j", and "k" and nothing else, no
matter what the platform's character set is. In contrast, all of
tr/\x68-\x6B/\x48-\x4B/
tr/h-\x6B/H-\x4B/
tr/\x68-k/\x48-K/
do the same capitalizations as the previous example when run on
ASCII platforms, but something completely different on EBCDIC ones.
The second class of portable ranges is invoked when one or both of
the range's end points are expressed as "\N{...}"
$string =~ tr/\N{U+20}-\N{U+7E}//d;
removes from $string all the platform's characters which are
equivalent to any of Unicode U+0020, U+0021, ... U+007D, U+007E.
This is a portable range, and has the same effect on every platform
it is run on. In this example, these are the ASCII printable
characters. So after this is run, $string has only controls and
characters which have no ASCII equivalents.
But, even for portable ranges, it is not generally obvious what is
included without having to look things up in the manual. A sound
principle is to use only ranges that both begin from, and end at,
either ASCII alphabetics of equal case ("b-e", "B-E"), or digits
("1-4"). Anything else is unclear (and unportable unless "\N{...}"
is used). If in doubt, spell out the character sets in full.
Options:
c Complement the SEARCHLIST.
d Delete found but unreplaced characters.
r Return the modified string and leave the original string
untouched.
s Squash duplicate replaced characters.
If the "/d" modifier is specified, any characters specified by
SEARCHLIST not found in REPLACEMENTLIST are deleted. (Note that
this is slightly more flexible than the behavior of some tr
programs, which delete anything they find in the SEARCHLIST,
period.)
If the "/s" modifier is specified, sequences of characters, all in
a row, that were transliterated to the same character are squashed
down to a single instance of that character.
my $a = "aaaba"
$a =~ tr/a/a/s # $a now is "aba"
If the "/d" modifier is used, the REPLACEMENTLIST is always
interpreted exactly as specified. Otherwise, if the
REPLACEMENTLIST is shorter than the SEARCHLIST, the final
character, if any, is replicated until it is long enough. There
won't be a final character if and only if the REPLACEMENTLIST is
empty, in which case REPLACEMENTLIST is copied from SEARCHLIST.
An empty REPLACEMENTLIST is useful for counting characters in a
class, or for squashing character sequences in a class.
tr/abcd// tr/abcd/abcd/
tr/abcd/AB/ tr/abcd/ABBB/
tr/abcd//d s/[abcd]//g
tr/abcd/AB/d (tr/ab/AB/ + s/[cd]//g) - but run together
If the "/c" modifier is specified, the characters to be
transliterated are the ones NOT in SEARCHLIST, that is, it is
complemented. If "/d" and/or "/s" are also specified, they apply
to the complemented SEARCHLIST. Recall, that if REPLACEMENTLIST is
empty (except under "/d") a copy of SEARCHLIST is used instead.
That copy is made after complementing under "/c". SEARCHLIST is
sorted by code point order after complementing, and any
REPLACEMENTLIST is applied to that sorted result. This means that
under "/c", the order of the characters specified in SEARCHLIST is
irrelevant. This can lead to different results on EBCDIC systems
if REPLACEMENTLIST contains more than one character, hence it is
generally non-portable to use "/c" with such a REPLACEMENTLIST.
Another way of describing the operation is this: If "/c" is
specified, the SEARCHLIST is sorted by code point order, then
complemented. If REPLACEMENTLIST is empty and "/d" is not
specified, REPLACEMENTLIST is replaced by a copy of SEARCHLIST (as
modified under "/c"), and these potentially modified lists are used
as the basis for what follows. Any character in the target string
that isn't in SEARCHLIST is passed through unchanged. Every other
character in the target string is replaced by the character in
REPLACEMENTLIST that positionally corresponds to its mate in
SEARCHLIST, except that under "/s", the 2nd and following
characters are squeezed out in a sequence of characters in a row
that all translate to the same character. If SEARCHLIST is longer
than REPLACEMENTLIST, characters in the target string that match a
character in SEARCHLIST that doesn't have a correspondence in
REPLACEMENTLIST are either deleted from the target string if "/d"
is specified; or replaced by the final character in REPLACEMENTLIST
if "/d" isn't specified.
Some examples:
$ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case ASCII
$cnt = tr/*/*/; # count the stars in $_
$cnt = tr/*//; # same thing
$cnt = $sky =~ tr/*/*/; # count the stars in $sky
$cnt = $sky =~ tr/*//; # same thing
$cnt = $sky =~ tr/*//c; # count all the non-stars in $sky
$cnt = $sky =~ tr/*/*/c; # same, but transliterate each non-star
# into a star, leaving the already-stars
# alone. Afterwards, everything in $sky
# is a star.
$cnt = tr/0-9//; # count the ASCII digits in $_
tr/a-zA-Z//s; # bookkeeper -> bokeper
tr/o/o/s; # bookkeeper -> bokkeeper
tr/oe/oe/s; # bookkeeper -> bokkeper
tr/oe//s; # bookkeeper -> bokkeper
tr/oe/o/s; # bookkeeper -> bokkopor
($HOST = $host) =~ tr/a-z/A-Z/;
$HOST = $host =~ tr/a-z/A-Z/r; # same thing
$HOST = $host =~ tr/a-z/A-Z/r # chained with s///r
=~ s/:/ -p/r;
tr/a-zA-Z/ /cs; # change non-alphas to single space
@stripped = map tr/a-zA-Z/ /csr, @original;
# /r with map
tr [\200-\377]
[\000-\177]; # wickedly delete 8th bit
$foo !~ tr/A/a/ # transliterate all the A's in $foo to 'a',
# return 0 if any were found and changed.
# Otherwise return 1
If multiple transliterations are given for a character, only the
first one is used:
tr/AAA/XYZ/
will transliterate any A to X.
Because the transliteration table is built at compile time, neither
the SEARCHLIST nor the REPLACEMENTLIST are subjected to double
quote interpolation. That means that if you want to use variables,
you must use an "eval()":
eval "tr/$oldlist/$newlist/";
die $@ if $@;
eval "tr/$oldlist/$newlist/, 1" or die $@;
"<<EOF"
A line-oriented form of quoting is based on the shell "here-
document" syntax. Following a "<<" you specify a string to
terminate the quoted material, and all lines following the current
line down to the terminating string are the value of the item.
Prefixing the terminating string with a "~" specifies that you want
to use "Indented Here-docs" (see below).
The terminating string may be either an identifier (a word), or
some quoted text. An unquoted identifier works like double quotes.
There may not be a space between the "<<" and the identifier,
unless the identifier is explicitly quoted. The terminating string
must appear by itself (unquoted and with no surrounding whitespace)
on the terminating line.
If the terminating string is quoted, the type of quotes used
determine the treatment of the text.
Double Quotes
Double quotes indicate that the text will be interpolated using
exactly the same rules as normal double quoted strings.
print <<EOF;
The price is $Price.
EOF
print << "EOF"; # same as above
The price is $Price.
EOF
Single Quotes
Single quotes indicate the text is to be treated literally with
no interpolation of its content. This is similar to single
quoted strings except that backslashes have no special meaning,
with "\\" being treated as two backslashes and not one as they
would in every other quoting construct.
Just as in the shell, a backslashed bareword following the "<<"
means the same thing as a single-quoted string does:
$cost = <<'VISTA'; # hasta la ...
That'll be $10 please, ma'am.
VISTA
$cost = <<\VISTA; # Same thing!
That'll be $10 please, ma'am.
VISTA
This is the only form of quoting in perl where there is no need
to worry about escaping content, something that code generators
can and do make good use of.
Backticks
The content of the here doc is treated just as it would be if
the string were embedded in backticks. Thus the content is
interpolated as though it were double quoted and then executed
via the shell, with the results of the execution returned.
print << `EOC`; # execute command and get results
echo hi there
EOC
Indented Here-docs
The here-doc modifier "~" allows you to indent your here-docs
to make the code more readable:
if ($some_var) {
print <<~EOF;
This is a here-doc
EOF
}
This will print...
This is a here-doc
...with no leading whitespace.
The delimiter is used to determine the exact whitespace to
remove from the beginning of each line. All lines must have at
least the same starting whitespace (except lines only
containing a newline) or perl will croak. Tabs and spaces can
be mixed, but are matched exactly. One tab will not be equal
to 8 spaces!
Additional beginning whitespace (beyond what preceded the
delimiter) will be preserved:
print <<~EOF;
This text is not indented
This text is indented with two spaces
This text is indented with two tabs
EOF
Finally, the modifier may be used with all of the forms
mentioned above:
<<~\EOF;
<<~'EOF'
<<~"EOF"
<<~`EOF`
And whitespace may be used between the "~" and quoted
delimiters:
<<~ 'EOF'; # ... "EOF", `EOF`
It is possible to stack multiple here-docs in a row:
print <<"foo", <<"bar"; # you can stack them
I said foo.
foo
I said bar.
bar
myfunc(<< "THIS", 23, <<'THAT');
Here's a line
or two.
THIS
and here's another.
THAT
Just don't forget that you have to put a semicolon on the end to
finish the statement, as Perl doesn't know you're not going to try
to do this:
print <<ABC
179231
ABC
+ 20;
If you want to remove the line terminator from your here-docs, use
"chomp()".
chomp($string = <<'END');
This is a string.
END
If you want your here-docs to be indented with the rest of the
code, use the "<<~FOO" construct described under "Indented Here-
docs":
$quote = <<~'FINIS';
The Road goes ever on and on,
down from the door where it began.
FINIS
If you use a here-doc within a delimited construct, such as in
"s///eg", the quoted material must still come on the line following
the "<<FOO" marker, which means it may be inside the delimited
construct:
s/this/<<E . 'that'
the other
E
. 'more '/eg;
It works this way as of Perl 5.18. Historically, it was
inconsistent, and you would have to write
s/this/<<E . 'that'
. 'more '/eg;
the other
E
outside of string evals.
Additionally, quoting rules for the end-of-string identifier are
unrelated to Perl's quoting rules. "q()", "qq()", and the like are
not supported in place of '' and "", and the only interpolation is
for backslashing the quoting character:
print << "abc\"def";
testing...
abc"def
Finally, quoted strings cannot span multiple lines. The general
rule is that the identifier must be a string literal. Stick with
that, and you should be safe.
Gory details of parsing quoted constructs
When presented with something that might have several different
interpretations, Perl uses the DWIM (that's "Do What I Mean") principle
to pick the most probable interpretation. This strategy is so
successful that Perl programmers often do not suspect the ambivalence
of what they write. But from time to time, Perl's notions differ
substantially from what the author honestly meant.
This section hopes to clarify how Perl handles quoted constructs.
Although the most common reason to learn this is to unravel
labyrinthine regular expressions, because the initial steps of parsing
are the same for all quoting operators, they are all discussed
together.
The most important Perl parsing rule is the first one discussed below:
when processing a quoted construct, Perl first finds the end of that
construct, then interprets its contents. If you understand this rule,
you may skip the rest of this section on the first reading. The other
rules are likely to contradict the user's expectations much less
frequently than this first one.
Some passes discussed below are performed concurrently, but because
their results are the same, we consider them individually. For
different quoting constructs, Perl performs different numbers of
passes, from one to four, but these passes are always performed in the
same order.
Finding the end
The first pass is finding the end of the quoted construct. This
results in saving to a safe location a copy of the text (between
the starting and ending delimiters), normalized as necessary to
avoid needing to know what the original delimiters were.
If the construct is a here-doc, the ending delimiter is a line that
has a terminating string as the content. Therefore "<<EOF" is
terminated by "EOF" immediately followed by "\n" and starting from
the first column of the terminating line. When searching for the
terminating line of a here-doc, nothing is skipped. In other
words, lines after the here-doc syntax are compared with the
terminating string line by line.
For the constructs except here-docs, single characters are used as
starting and ending delimiters. If the starting delimiter is an
opening punctuation (that is "(", "[", "{", or "<"), the ending
delimiter is the corresponding closing punctuation (that is ")",
"]", "}", or ">"). If the starting delimiter is an unpaired
character like "/" or a closing punctuation, the ending delimiter
is the same as the starting delimiter. Therefore a "/" terminates
a "qq//" construct, while a "]" terminates both "qq[]" and "qq]]"
constructs.
When searching for single-character delimiters, escaped delimiters
and "\\" are skipped. For example, while searching for terminating
"/", combinations of "\\" and "\/" are skipped. If the delimiters
are bracketing, nested pairs are also skipped. For example, while
searching for a closing "]" paired with the opening "[",
combinations of "\\", "\]", and "\[" are all skipped, and nested
"[" and "]" are skipped as well. However, when backslashes are
used as the delimiters (like "qq\\" and "tr\\\"), nothing is
skipped. During the search for the end, backslashes that escape
delimiters or other backslashes are removed (exactly speaking, they
are not copied to the safe location).
For constructs with three-part delimiters ("s///", "y///", and
"tr///"), the search is repeated once more. If the first delimiter
is not an opening punctuation, the three delimiters must be the
same, such as "s!!!" and "tr)))", in which case the second
delimiter terminates the left part and starts the right part at
once. If the left part is delimited by bracketing punctuation
(that is "()", "[]", "{}", or "<>"), the right part needs another
pair of delimiters such as "s(){}" and "tr[]//". In these cases,
whitespace and comments are allowed between the two parts, although
the comment must follow at least one whitespace character;
otherwise a character expected as the start of the comment may be
regarded as the starting delimiter of the right part.
During this search no attention is paid to the semantics of the
construct. Thus:
"$hash{"$foo/$bar"}"
or:
m/
bar # NOT a comment, this slash / terminated m//!
/x
do not form legal quoted expressions. The quoted part ends on the
first """ and "/", and the rest happens to be a syntax error.
Because the slash that terminated "m//" was followed by a "SPACE",
the example above is not "m//x", but rather "m//" with no "/x"
modifier. So the embedded "#" is interpreted as a literal "#".
Also no attention is paid to "\c\" (multichar control char syntax)
during this search. Thus the second "\" in "qq/\c\/" is
interpreted as a part of "\/", and the following "/" is not
recognized as a delimiter. Instead, use "\034" or "\x1c" at the
end of quoted constructs.
Interpolation
The next step is interpolation in the text obtained, which is now
delimiter-independent. There are multiple cases.
"<<'EOF'"
No interpolation is performed. Note that the combination "\\"
is left intact, since escaped delimiters are not available for
here-docs.
"m''", the pattern of "s'''"
No interpolation is performed at this stage. Any backslashed
sequences including "\\" are treated at the stage to "parsing
regular expressions".
'', "q//", "tr'''", "y'''", the replacement of "s'''"
The only interpolation is removal of "\" from pairs of "\\".
Therefore "-" in "tr'''" and "y'''" is treated literally as a
hyphen and no character range is available. "\1" in the
replacement of "s'''" does not work as $1.
"tr///", "y///"
No variable interpolation occurs. String modifying
combinations for case and quoting such as "\Q", "\U", and "\E"
are not recognized. The other escape sequences such as "\200"
and "\t" and backslashed characters such as "\\" and "\-" are
converted to appropriate literals. The character "-" is
treated specially and therefore "\-" is treated as a literal
"-".
"", "``", "qq//", "qx//", "<file*glob>", "<<"EOF""
"\Q", "\U", "\u", "\L", "\l", "\F" (possibly paired with "\E")
are converted to corresponding Perl constructs. Thus,
"$foo\Qbaz$bar" is converted to "$foo.(quotemeta("baz".$bar))"
internally. The other escape sequences such as "\200" and "\t"
and backslashed characters such as "\\" and "\-" are replaced
with appropriate expansions.
Let it be stressed that whatever falls between "\Q" and "\E" is
interpolated in the usual way. Something like "\Q\\E" has no
"\E" inside. Instead, it has "\Q", "\\", and "E", so the
result is the same as for "\\\\E". As a general rule,
backslashes between "\Q" and "\E" may lead to counterintuitive
results. So, "\Q\t\E" is converted to "quotemeta("\t")", which
is the same as "\\\t" (since TAB is not alphanumeric). Note
also that:
$str = '\t';
return "\Q$str";
may be closer to the conjectural intention of the writer of
"\Q\t\E".
Interpolated scalars and arrays are converted internally to the
"join" and "." catenation operations. Thus, "$fooXXX'@arr'"
becomes:
$foo . " XXX '" . (join $", @arr) . "'";
All operations above are performed simultaneously, left to
right.
Because the result of "\QSTRING\E" has all metacharacters
quoted, there is no way to insert a literal "$" or "@" inside a
"\Q\E" pair. If protected by "\", "$" will be quoted to become
"\\\$"; if not, it is interpreted as the start of an
interpolated scalar.
Note also that the interpolation code needs to make a decision
on where the interpolated scalar ends. For instance, whether
"a$x->{c}" really means:
"a " . $x . " -> {c}";
or:
"a " . $x -> {c};
Most of the time, the longest possible text that does not
include spaces between components and which contains matching
braces or brackets. because the outcome may be determined by
voting based on heuristic estimators, the result is not
strictly predictable. Fortunately, it's usually correct for
ambiguous cases.
the replacement of "s///"
Processing of "\Q", "\U", "\u", "\L", "\l", "\F" and
interpolation happens as with "qq//" constructs.
It is at this step that "\1" is begrudgingly converted to $1 in
the replacement text of "s///", in order to correct the
incorrigible sed hackers who haven't picked up the saner idiom
yet. A warning is emitted if the "usewarnings" pragma or the
-w command-line flag (that is, the $^W variable) was set.
"RE" in "m?RE?", "/RE/", "m/RE/", "s/RE/foo/",
Processing of "\Q", "\U", "\u", "\L", "\l", "\F", "\E", and
interpolation happens (almost) as with "qq//" constructs.
Processing of "\N{...}" is also done here, and compiled into an
intermediate form for the regex compiler. (This is because, as
mentioned below, the regex compilation may be done at execution
time, and "\N{...}" is a compile-time construct.)
However any other combinations of "\" followed by a character
are not substituted but only skipped, in order to parse them as
regular expressions at the following step. As "\c" is skipped
at this step, "@" of "\c@" in RE is possibly treated as an
array symbol (for example @foo), even though the same text in
"qq//" gives interpolation of "\c@".
Code blocks such as "(?{BLOCK})" are handled by temporarily
passing control back to the perl parser, in a similar way that
an interpolated array subscript expression such as
"foo$array[1+f("[xyz")]bar" would be.
Moreover, inside "(?{BLOCK})", "(?#comment)", and a "#"-comment
in a "/x"-regular expression, no processing is performed
whatsoever. This is the first step at which the presence of
the "/x" modifier is relevant.
Interpolation in patterns has several quirks: $|, $(, $), "@+"
and "@-" are not interpolated, and constructs $var[SOMETHING]
are voted (by several different estimators) to be either an
array element or $var followed by an RE alternative. This is
where the notation "${arr[$bar]}" comes handy: "/${arr[0-9]}/"
is interpreted as array element "-9", not as a regular
expression from the variable $arr followed by a digit, which
would be the interpretation of "/$arr[0-9]/". Since voting
among different estimators may occur, the result is not
predictable.
The lack of processing of "\\" creates specific restrictions on
the post-processed text. If the delimiter is "/", one cannot
get the combination "\/" into the result of this step. "/"
will finish the regular expression, "\/" will be stripped to
"/" on the previous step, and "\\/" will be left as is.
Because "/" is equivalent to "\/" inside a regular expression,
this does not matter unless the delimiter happens to be
character special to the RE engine, such as in "s*foo*bar*",
"m[foo]", or "m?foo?"; or an alphanumeric char, as in:
m m ^ a \s* b mmx;
In the RE above, which is intentionally obfuscated for
illustration, the delimiter is "m", the modifier is "mx", and
after delimiter-removal the RE is the same as for
"m/^a\s*b/mx". There's more than one reason you're encouraged
to restrict your delimiters to non-alphanumeric, non-whitespace
choices.
This step is the last one for all constructs except regular
expressions, which are processed further.
parsing regular expressions
Previous steps were performed during the compilation of Perl code,
but this one happens at run time, although it may be optimized to
be calculated at compile time if appropriate. After preprocessing
described above, and possibly after evaluation if concatenation,
joining, casing translation, or metaquoting are involved, the
resulting string is passed to the RE engine for compilation.
Whatever happens in the RE engine might be better discussed in
perlre, but for the sake of continuity, we shall do so here.
This is another step where the presence of the "/x" modifier is
relevant. The RE engine scans the string from left to right and
converts it into a finite automaton.
Backslashed characters are either replaced with corresponding
literal strings (as with "\{"), or else they generate special nodes
in the finite automaton (as with "\b"). Characters special to the
RE engine (such as "|") generate corresponding nodes or groups of
nodes. "(?#...)" comments are ignored. All the rest is either
converted to literal strings to match, or else is ignored (as is
whitespace and "#"-style comments if "/x" is present).
Parsing of the bracketed character class construct, "[...]", is
rather different than the rule used for the rest of the pattern.
The terminator of this construct is found using the same rules as
for finding the terminator of a "{}"-delimited construct, the only
exception being that "]" immediately following "[" is treated as
though preceded by a backslash.
The terminator of runtime "(?{...})" is found by temporarily
switching control to the perl parser, which should stop at the
point where the logically balancing terminating "}" is found.
It is possible to inspect both the string given to RE engine and
the resulting finite automaton. See the arguments
"debug"/"debugcolor" in the "usere" pragma, as well as Perl's -Dr
command-line switch documented in "Command Switches" in perlrun.
Optimization of regular expressions
This step is listed for completeness only. Since it does not
change semantics, details of this step are not documented and are
subject to change without notice. This step is performed over the
finite automaton that was generated during the previous pass.
It is at this stage that "split()" silently optimizes "/^/" to mean
"/^/m".
I/O Operators
There are several I/O operators you should know about.
A string enclosed by backticks (grave accents) first undergoes double-
quote interpolation. It is then interpreted as an external command,
and the output of that command is the value of the backtick string,
like in a shell. In scalar context, a single string consisting of all
output is returned. In list context, a list of values is returned, one
per line of output. (You can set $/ to use a different line
terminator.) The command is executed each time the pseudo-literal is
evaluated. The status value of the command is returned in $? (see
perlvar for the interpretation of $?). Unlike in csh, no translation
is done on the return data--newlines remain newlines. Unlike in any of
the shells, single quotes do not hide variable names in the command
from interpretation. To pass a literal dollar-sign through to the
shell you need to hide it with a backslash. The generalized form of
backticks is "qx//", or you can call the "readpipe" in perlfunc
function. (Because backticks always undergo shell expansion as well,
see perlsec for security concerns.)
In scalar context, evaluating a filehandle in angle brackets yields the
next line from that file (the newline, if any, included), or "undef" at
end-of-file or on error. When $/ is set to "undef" (sometimes known as
file-slurp mode) and the file is empty, it returns '' the first time,
followed by "undef" subsequently.
Ordinarily you must assign the returned value to a variable, but there
is one situation where an automatic assignment happens. If and only if
the input symbol is the only thing inside the conditional of a "while"
statement (even if disguised as a "for(;;)" loop), the value is
automatically assigned to the global variable $_, destroying whatever
was there previously. (This may seem like an odd thing to you, but
you'll use the construct in almost every Perl script you write.) The
$_ variable is not implicitly localized. You'll have to put a
"local$_;" before the loop if you want that to happen. Furthermore, if
the input symbol or an explicit assignment of the input symbol to a
scalar is used as a "while"/"for" condition, then the condition
actually tests for definedness of the expression's value, not for its
regular truth value.
Thus the following lines are equivalent:
while (defined($_ = <STDIN>)) { print; }
while ($_ = <STDIN>) { print; }
while (<STDIN>) { print; }
for (;<STDIN>;) { print; }
print while defined($_ = <STDIN>);
print while ($_ = <STDIN>);
print while <STDIN>;
This also behaves similarly, but assigns to a lexical variable instead
of to $_:
while (my $line = <STDIN>) { print $line }
In these loop constructs, the assigned value (whether assignment is
automatic or explicit) is then tested to see whether it is defined.
The defined test avoids problems where the line has a string value that
would be treated as false by Perl; for example a "" or a "0" with no
trailing newline. If you really mean for such values to terminate the
loop, they should be tested for explicitly:
while (($_ = <STDIN>) ne '0') { ... }
while (<STDIN>) { last unless $_; ... }
In other boolean contexts, "<FILEHANDLE>" without an explicit "defined"
test or comparison elicits a warning if the "usewarnings" pragma or the
-w command-line switch (the $^W variable) is in effect.
The filehandles STDIN, STDOUT, and STDERR are predefined. (The
filehandles "stdin", "stdout", and "stderr" will also work except in
packages, where they would be interpreted as local identifiers rather
than global.) Additional filehandles may be created with the "open()"
function, amongst others. See perlopentut and "open" in perlfunc for
details on this.
If a "<FILEHANDLE>" is used in a context that is looking for a list, a
list comprising all input lines is returned, one line per list element.
It's easy to grow to a rather large data space this way, so use with
care.
"<FILEHANDLE>" may also be spelled "readline(*FILEHANDLE)". See
"readline" in perlfunc.
The null filehandle "<>" is special: it can be used to emulate the
behavior of sed and awk, and any other Unix filter program that takes a
list of filenames, doing the same to each line of input from all of
them. Input from "<>" comes either from standard input, or from each
file listed on the command line. Here's how it works: the first time
"<>" is evaluated, the @ARGV array is checked, and if it is empty,
$ARGV[0] is set to "-", which when opened gives you standard input.
The @ARGV array is then processed as a list of filenames. The loop
while (<>) {
... # code for each line
}
is equivalent to the following Perl-like pseudo code:
unshift(@ARGV, '-') unless @ARGV;
while ($ARGV = shift) {
open(ARGV, $ARGV);
while (<ARGV>) {
... # code for each line
}
}
except that it isn't so cumbersome to say, and will actually work. It
really does shift the @ARGV array and put the current filename into the
$ARGV variable. It also uses filehandle ARGV internally. "<>" is just
a synonym for "<ARGV>", which is magical. (The pseudo code above
doesn't work because it treats "<ARGV>" as non-magical.)
Since the null filehandle uses the two argument form of "open" in
perlfunc it interprets special characters, so if you have a script like
this:
while (<>) {
print;
}
and call it with "perldangerous.pl'rm-rfv*|'", it actually opens a
pipe, executes the "rm" command and reads "rm"'s output from that pipe.
If you want all items in @ARGV to be interpreted as file names, you can
use the module "ARGV::readonly" from CPAN, or use the double bracket:
while (<<>>) {
print;
}
Using double angle brackets inside of a while causes the open to use
the three argument form (with the second argument being "<"), so all
arguments in "ARGV" are treated as literal filenames (including "-").
(Note that for convenience, if you use "<<>>" and if @ARGV is empty, it
will still read from the standard input.)
You can modify @ARGV before the first "<>" as long as the array ends up
containing the list of filenames you really want. Line numbers ($.)
continue as though the input were one big happy file. See the example
in "eof" in perlfunc for how to reset line numbers on each file.
If you want to set @ARGV to your own list of files, go right ahead.
This sets @ARGV to all plain text files if no @ARGV was given:
@ARGV = grep { -f && -T } glob('*') unless @ARGV;
You can even set them to pipe commands. For example, this
automatically filters compressed arguments through gzip:
@ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
If you want to pass switches into your script, you can use one of the
"Getopts" modules or put a loop on the front like this:
while ($_ = $ARGV[0], /^-/) {
shift;
last if /^--$/;
if (/^-D(.*)/) { $debug = $1 }
if (/^-v/) { $verbose++ }
# ... # other switches
}
while (<>) {
# ... # code for each line
}
The "<>" symbol will return "undef" for end-of-file only once. If you
call it again after this, it will assume you are processing another
@ARGV list, and if you haven't set @ARGV, will read input from STDIN.
If what the angle brackets contain is a simple scalar variable (for
example, $foo), then that variable contains the name of the filehandle
to input from, or its typeglob, or a reference to the same. For
example:
$fh = \*STDIN;
$line = <$fh>;
If what's within the angle brackets is neither a filehandle nor a
simple scalar variable containing a filehandle name, typeglob, or
typeglob reference, it is interpreted as a filename pattern to be
globbed, and either a list of filenames or the next filename in the
list is returned, depending on context. This distinction is determined
on syntactic grounds alone. That means "<$x>" is always a "readline()"
from an indirect handle, but "<$hash{key}>" is always a "glob()".
That's because $x is a simple scalar variable, but $hash{key} is
not--it's a hash element. Even "<$x >" (note the extra space) is
treated as "glob("$x ")", not "readline($x)".
One level of double-quote interpretation is done first, but you can't
say "<$foo>" because that's an indirect filehandle as explained in the
previous paragraph. (In older versions of Perl, programmers would
insert curly brackets to force interpretation as a filename glob:
"<${foo}>". These days, it's considered cleaner to call the internal
function directly as "glob($foo)", which is probably the right way to
have done it in the first place.) For example:
while (<*.c>) {
chmod 0644, $_;
}
is roughly equivalent to:
open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
while (<FOO>) {
chomp;
chmod 0644, $_;
}
except that the globbing is actually done internally using the standard
"File::Glob" extension. Of course, the shortest way to do the above
is:
chmod 0644, <*.c>;
A (file)glob evaluates its (embedded) argument only when it is starting
a new list. All values must be read before it will start over. In
list context, this isn't important because you automatically get them
all anyway. However, in scalar context the operator returns the next
value each time it's called, or "undef" when the list has run out. As
with filehandle reads, an automatic "defined" is generated when the
glob occurs in the test part of a "while", because legal glob returns
(for example, a file called 0) would otherwise terminate the loop.
Again, "undef" is returned only once. So if you're expecting a single
value from a glob, it is much better to say
($file) = <blurch*>;
than
$file = <blurch*>;
because the latter will alternate between returning a filename and
returning false.
If you're trying to do variable interpolation, it's definitely better
to use the "glob()" function, because the older notation can cause
people to become confused with the indirect filehandle notation.
@files = glob("$dir/*.[ch]");
@files = glob($files[$i]);
If an angle-bracket-based globbing expression is used as the condition
of a "while" or "for" loop, then it will be implicitly assigned to $_.
If either a globbing expression or an explicit assignment of a globbing
expression to a scalar is used as a "while"/"for" condition, then the
condition actually tests for definedness of the expression's value, not
for its regular truth value.
Constant Folding
Like C, Perl does a certain amount of expression evaluation at compile
time whenever it determines that all arguments to an operator are
static and have no side effects. In particular, string concatenation
happens at compile time between literals that don't do variable
substitution. Backslash interpolation also happens at compile time.
You can say
'Now is the time for all'
. "\n"
. 'good men to come to.'
and this all reduces to one string internally. Likewise, if you say
foreach $file (@filenames) {
if (-s $file > 5 + 100 * 2**16) { }
}
the compiler precomputes the number which that expression represents so
that the interpreter won't have to.
No-ops
Perl doesn't officially have a no-op operator, but the bare constants 0
and 1 are special-cased not to produce a warning in void context, so
you can for example safely do
1 while foo();
Bitwise String Operators
Bitstrings of any size may be manipulated by the bitwise operators ("~
| & ^").
If the operands to a binary bitwise op are strings of different sizes,
| and ^ ops act as though the shorter operand had additional zero bits
on the right, while the & op acts as though the longer operand were
truncated to the length of the shorter. The granularity for such
extension or truncation is one or more bytes.
# ASCII-based examples
print "j p \n" ^ " a h"; # prints "JAPH\n"
print "JA" | " ph\n"; # prints "japh\n"
print "japh\nJunk" & '_____'; # prints "JAPH\n";
print 'p N$' ^ " E<H\n"; # prints "Perl\n";
If you are intending to manipulate bitstrings, be certain that you're
supplying bitstrings: If an operand is a number, that will imply a
numeric bitwise operation. You may explicitly show which type of
operation you intend by using "" or "0+", as in the examples below.
$foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
$foo = '150' | 105; # yields 255
$foo = 150 | '105'; # yields 255
$foo = '150' | '105'; # yields string '155' (under ASCII)
$baz = 0+$foo & 0+$bar; # both ops explicitly numeric
$biz = "$foo" ^ "$bar"; # both ops explicitly stringy
This somewhat unpredictable behavior can be avoided with the "bitwise"
feature, new in Perl 5.22. You can enable it via "usefeature'bitwise'"
or "use v5.28". Before Perl 5.28, it used to emit a warning in the
"experimental::bitwise" category. Under this feature, the four
standard bitwise operators ("~ | & ^") are always numeric. Adding a
dot after each operator ("~. |. &. ^.") forces it to treat its operands
as strings:
use feature "bitwise";
$foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
$foo = '150' | 105; # yields 255
$foo = 150 | '105'; # yields 255
$foo = '150' | '105'; # yields 255
$foo = 150 |. 105; # yields string '155'
$foo = '150' |. 105; # yields string '155'
$foo = 150 |.'105'; # yields string '155'
$foo = '150' |.'105'; # yields string '155'
$baz = $foo & $bar; # both operands numeric
$biz = $foo ^. $bar; # both operands stringy
The assignment variants of these operators ("&= |= ^= &.= |.= ^.=")
behave likewise under the feature.
It is a fatal error if an operand contains a character whose ordinal
value is above 0xFF, and hence not expressible except in UTF-8. The
operation is performed on a non-UTF-8 copy for other operands encoded
in UTF-8. See "Byte and Character Semantics" in perlunicode.
See "vec" in perlfunc for information on how to manipulate individual
bits in a bit vector.
Integer Arithmetic
By default, Perl assumes that it must do most of its arithmetic in
floating point. But by saying
use integer;
you may tell the compiler to use integer operations (see integer for a
detailed explanation) from here to the end of the enclosing BLOCK. An
inner BLOCK may countermand this by saying
no integer;
which lasts until the end of that BLOCK. Note that this doesn't mean
everything is an integer, merely that Perl will use integer operations
for arithmetic, comparison, and bitwise operators. For example, even
under "useinteger", if you take the sqrt(2), you'll still get
1.4142135623731 or so.
Used on numbers, the bitwise operators ("&" "|" "^" "~" "<<" ">>")
always produce integral results. (But see also "Bitwise String
Operators".) However, "useinteger" still has meaning for them. By
default, their results are interpreted as unsigned integers, but if
"useinteger" is in effect, their results are interpreted as signed
integers. For example, "~0" usually evaluates to a large integral
value. However, "useinteger;~0" is "-1" on two's-complement machines.
Floating-point Arithmetic
While "useinteger" provides integer-only arithmetic, there is no
analogous mechanism to provide automatic rounding or truncation to a
certain number of decimal places. For rounding to a certain number of
digits, "sprintf()" or "printf()" is usually the easiest route. See
perlfaq4.
Floating-point numbers are only approximations to what a mathematician
would call real numbers. There are infinitely more reals than floats,
so some corners must be cut. For example:
printf "%.20g\n", 123456789123456789;
# produces 123456789123456784
Testing for exact floating-point equality or inequality is not a good
idea. Here's a (relatively expensive) work-around to compare whether
two floating-point numbers are equal to a particular number of decimal
places. See Knuth, volume II, for a more robust treatment of this
topic.
sub fp_equal {
my ($X, $Y, $POINTS) = @_;
my ($tX, $tY);
$tX = sprintf("%.${POINTS}g", $X);
$tY = sprintf("%.${POINTS}g", $Y);
return $tX eq $tY;
}
The POSIX module (part of the standard perl distribution) implements
"ceil()", "floor()", and other mathematical and trigonometric
functions. The "Math::Complex" module (part of the standard perl
distribution) defines mathematical functions that work on both the
reals and the imaginary numbers. "Math::Complex" is not as efficient
as POSIX, but POSIX can't work with complex numbers.
Rounding in financial applications can have serious implications, and
the rounding method used should be specified precisely. In these
cases, it probably pays not to trust whichever system rounding is being
used by Perl, but to instead implement the rounding function you need
yourself.
Bigger Numbers
The standard "Math::BigInt", "Math::BigRat", and "Math::BigFloat"
modules, along with the "bignum", "bigint", and "bigrat" pragmas,
provide variable-precision arithmetic and overloaded operators,
although they're currently pretty slow. At the cost of some space and
considerable speed, they avoid the normal pitfalls associated with
limited-precision representations.
use 5.010;
use bigint; # easy interface to Math::BigInt
$x = 123456789123456789;
say $x * $x;
+15241578780673678515622620750190521
Or with rationals:
use 5.010;
use bigrat;
$x = 3/22;
$y = 4/6;
say "x/y is ", $x/$y;
say "x*y is ", $x*$y;
x/y is 9/44
x*y is 1/11
Several modules let you calculate with unlimited or fixed precision
(bound only by memory and CPU time). There are also some non-standard
modules that provide faster implementations via external C libraries.
Here is a short, but incomplete summary:
Math::String treat string sequences like numbers
Math::FixedPrecision calculate with a fixed precision
Math::Currency for currency calculations
Bit::Vector manipulate bit vectors fast (uses C)
Math::BigIntFast Bit::Vector wrapper for big numbers
Math::Pari provides access to the Pari C library
Math::Cephes uses the external Cephes C library (no
big numbers)
Math::Cephes::Fraction fractions via the Cephes library
Math::GMP another one using an external C library
Math::GMPz an alternative interface to libgmp's big ints
Math::GMPq an interface to libgmp's fraction numbers
Math::GMPf an interface to libgmp's floating point numbers
Choose wisely.
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