perlop


DESCRIPTION
   Operator Precedence and Associativity
       Operator precedence and associativity work in Perl more or less like
       they do in mathematics.

       Operator precedence means some operators are evaluated before others.
       For example, in "2 + 4 * 5", the multiplication has higher precedence
       so "4 * 5" is evaluated first yielding "2 + 20 == 22" and not "6 * 5 ==
       30".

       Operator associativity defines what happens if a sequence of the same
       operators is used one after another: whether the evaluator will
       evaluate the left operations first or the right.  For example, in "8 -
       4 - 2", subtraction is left associative so Perl evaluates the
       expression left to right.  "8 - 4" is evaluated first making the
       expression "4 - 2 == 2" and not "8 - 2 == 6".

       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.
           left        , =>
           nonassoc    list operators (rightward)
           right       not
           left        and
           left        or xor

       In the following sections, these operators are covered in precedence
       order.

       Many operators can be overloaded for objects.  See overload.
       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,

           $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.)

   Symbolic Unary Operators
       Unary "!" performs logical negation, i.e., "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
       characters will be in either 32- or 64-bit complements, depending on
       your architecture.  So for example, "~"\x{3B1}"" is "\x{FFFF_FC4E}" on
       32-bit machines and "\x{FFFF_FFFF_FFFF_FC4E}" on 64-bit machines.

       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 a reference to whatever follows it.  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 $a and $b: If $b is positive, then "$a % $b" is
       $a minus the largest multiple of $b less than or equal to $a.  If $b is
       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 not enclosed in parentheses, it returns a string
       consisting of the left operand repeated the number of times specified
       by the right operand.  In list context, if the left operand is enclosed
       in parentheses or is a list formed by "qw/STRING/", it repeats the
       list.  If the right operand is zero or 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".)

       Note that both "<<" and ">>" in Perl are implemented directly using
       "<<" and ">>" in C.  If "use integer" (see "Integer Arithmetic") is in
       force then signed C integers are used, else unsigned C integers are
       used.  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).

       The result of overflowing the range of the integers is undefined
       because it is undefined also in C.  In other words, using 32-bit
       integers, "1 << 32" is undefined.  Shifting by a negative number of
       bits is also undefined.

   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
           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
       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 NaNs (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 != anything else. If your platform doesn't support NaNs
       then NaN is just a string with numeric value 0.

       Binary "~~" does a smart match between its arguments. Smart matching is
       described in "Smart matching in detail" in perlsyn.

       "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order
       specified by the current locale if "use locale" is in effect.  See
       perllocale.

   Bitwise And
       Binary "&" returns its operands ANDed together bit by bit.  (See also
       "Integer Arithmetic" and "Bitwise String Operators".)

       Note that "&" has lower priority than relational operators, so for
       example the brackets are essential in a test like

               print "Even\n" if ($x & 1) == 0;

   Bitwise Or and Exclusive Or
       Binary "|" returns its operands ORed together bit by bit.  (See also
       "Integer Arithmetic" and "Bitwise String Operators".)

       Binary "^" returns its operands XORed together bit by bit.  (See also
       "Integer Arithmetic" and "Bitwise String Operators".)

       Note that "|" and "^" have lower priority than relational operators, so
       for example the brackets are essential in a test like

               print "false\n" if (8 | 2) != 10;

   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.

   C-style 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, "$a // $b" is similar to "defined($a) || $b" (except that
       it returns the value of $a rather than the value of "defined($a)") and
       yields the same result as "defined($a) ? $a : $b" (except that the
       ternary-operator form can be used as a lvalue, while "$a // $b"
       cannot).  This is very useful for providing default values for
       variables.  If you actually want to test if at least one of $a and $b
       is defined, use "defined($a // $b)".

       The "||", "//" and "&&" operators return the last value evaluated
           @a = scalar(@b) || @c;      # really meant this
           @a = @b ? @b : @c;          # this works fine, though

       As more readable 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);

       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 &&.
       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:


       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.

       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 would use the pattern "/(?:(?=\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.

           $a = $ok ? $b : $c;  # get a scalar
           @a = $ok ? @b : @c;  # get an array
           $a = $ok ? @b : @c;  # 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):


           ($a % 2) ? ($a += 10) : ($a += 2)

       That should probably be written more simply as:

           $a += ($a % 2) ? 10 : 2;

   Assignment Operators
       "=" is the ordinary assignment operator.

       Assignment operators work as in C.  That is,

           $a += 2;

       is equivalent to

           $a = $a + 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.

       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 [0-9] [a-j];

       Likewise,

           ($a += 2) *= 3;

       is equivalent to

           $a += 2;
           $a *= 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 Triple-Dot Operator
       The triple-dot operator, "...", sometimes called the "whatever
       operator", the "yada-yada operator", or the "et cetera" operator, is a
       statement.  These examples of the triple-dot work:

           { ... }

           sub foo { ... }

           ...;

           eval { ... };

           sub foo {
               my ($self) = shift;
               ...;
           }

           do {
               my $variable;
               ...;
               say "Hurrah!";
           } while $cheering;

       The yada-yada--or whatever--cannot stand in for an expression that is
       part of a larger statement since the "..." is also the three-dot
       version of the binary range operator (see "Range Operators").  These
       examples of the whatever operator are still syntax errors:

           print ...;

           open(PASSWD, ">", "/dev/passwd") or ...;

           if ($condition && ...) { say "Hello" }

       There are some cases where Perl can't immediately tell the difference
       between an expression and a statement. For instance, the syntax for a
       block and an anonymous hash reference constructor look the same unless
       there's something in the braces that give Perl a hint. The whatever is
       a syntax error if Perl doesn't guess that the "{ ... }" is a block. In
       that case, it doesn't think the "..." is the whatever because it's
       expecting an expression instead of a statement:

           my @transformed = map { ... } @input;  # syntax error

       You can use a ";" inside your block to denote that the "{ ... }" is a
       block and not a hash reference constructor. Now the whatever works:

           my @transformed = map {; ... } @input; # ; disambiguates

           my @transformed = map { ...; } @input; # ; disambiguates

   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.

       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 );

   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 extra parentheses:

           open HANDLE, "< $file"
               or die "Can't open $file: $!\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, Defined 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
           @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).

   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:

           $s = q{ if($a eq "}") ... }; # WRONG

       The following escape sequences are available in constructs that
       interpolate, and in transliterations:

           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.

              ...
              \cZ      chr(26)
              \cz      chr(26)
              \c[      chr(27)
              \c]      chr(29)
              \c^      chr(30)
              \c?      chr(127)

           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 the
           complete list of what these sequences mean on both ASCII and EBCDIC
           platforms.

           Use of any other character following the "c" besides those listed
           above is discouraged, and some are deprecated with the intention of
           removing those in Perl 5.16.  What happens for any of these other
           characters currently though, is that the value is derived by
           inverting the 7th bit (0x40).

           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, 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 "\x{}"
           instead.

           Having fewer than 3 digits may lead to a misleading warning message
           that says that what follows is ignored.  For example, "\128" in the
           ASCII character set is equivalent to the two characters "\n8", but
           the warning "Illegal octal digit '8' ignored" will be thrown.  To
           avoid this warning, make sure to pad your octal number with 0's:
           "\0128".
           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 100th
           position (indexed by 0) in Unicode is "LATIN CAPITAL LETTER A WITH
           MACRON".

           There are a couple of exceptions to the above rule.  "\N{U+hex
           number}" is always interpreted as a Unicode code point, so that
           "\N{U+0050}" is "P" even on EBCDIC platforms.  And if
           "use encoding" is in effect, the number is considered to be in that
           encoding, and is translated from that into the platform's native
           encoding if there is a corresponding native character; otherwise to
           Unicode.

       NOTE: Unlike C and other languages, Perl has no "\v" escape sequence
       for the vertical tab (VT - ASCII 11), but you may use "\ck" 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 seen
           \U          uppercase all characters till \E seen
           \Q          quote non-word characters till \E
           \E          end either case modification or quoted section
                       (whichever was last seen)

       "\L", "\U", 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 "use locale" is in effect, the case map used by "\l", "\L", "\u",
       and "\U" is taken from the current locale.  See perllocale.  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 several characters.

       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 line terminator, printing "\n" might
       emit no actual data.  In general, use "\n" when you mean a "newline"
       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/msixpodual
               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
               interpolation is done.  Returns a Perl value which may be used
               instead of the corresponding "/STRING/msixpodual" 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", even though
               dereferencing the result returns undef.

               For example,

                   $rex = qr/my.STRING/is;
                   $string =~ /$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 a 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.
                   p   When matching preserve a copy of the matched string so
                       that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
                   o   Compile pattern only once.
                   l   Use the locale
                   u   Use Unicode rules
                   a   Use ASCII for \d, \s, \w; specifying two a's further restricts
                       /i matching so that no ASCII character will match a non-ASCII
                       one
                   d   Use Unicode or native charset, as in 5.12 and earlier

               If a precompiled pattern is embedded in a larger pattern then
               the effect of "msixpluad" will be propagated appropriately.
               The effect the "o" modifier has is not propagated, being
               restricted to those patterns explicitly using it.

               The last four modifiers listed above, added in Perl 5.14,
               control the character set semantics.

               See perlre for additional information on valid syntax for
               STRING, and for a detailed look at the semantics of regular
               expressions.  In particular, all the modifiers execpt "/o" are
               further explained in "Modifiers" in perlre.  "/o" is described
               in the next section.

                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
               "'" is the delimiter, no interpolation is performed on the
               PATTERN.  When using a 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 either:

               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 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".)

       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 "$a///" (is that "($a) / (//)" or
               "$a // /"?) 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
               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 (e.g. "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.

                   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 abbreviation
                       (?<! \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 a very old (pre-5.6.0) version of
               Perl.
                    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?msixpodualgc
       ?PATTERN?msixpodualgc
               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.

               For historical reasons, the leading "m" in "m?PATTERN?" is
               optional, but the resulting "?PATTERN?" syntax is deprecated,
               will warn on usage and might be removed from a future stable
               release of Perl (without further notice!).

       s/PATTERN/REPLACEMENT/msixpodualgcer
               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 (specifically,
               the empty string).


               If the delimiter chosen is a single quote, no 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).  Unlike Perl 4, Perl 5 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, e.g., "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-count

                   $_ = 'abc123xyz';
                   s/\d+/$&*2/e;               # yields 'abc246xyz'
                   s/\d+/sprintf("%5d",$&)/e;  # yields 'abc  246xyz'
                   s/\w/$& x 2/eg;             # yields 'aabbcc  224466xxyyzz'

                   # 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

               Note the use of $ instead of \ in the last example.  Unlike
               sed, we use the \<digit> form in only 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;

               "s///le" is treated as a substitution followed by the "le"
               operator, not the "/le" flags.  This may change in a future
               version of Perl.  It produces a warning if warnings are
               enabled.  To disambiguate, use a space or change the order of
               the flags:

                   s/foo/bar/ le 5;  # "le" infix operator
                   s/foo/bar/el;     # "e" and "l" flags

   Quote-Like Operators
       q/STRING/
       'STRING'
           A single-quoted, literal string.  A backslash represents a
                (*** 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 $$);            # that's Perl's $$

           evaluate multiple commands in a single line by separating them with
           the command separator character, if your shell supports that (e.g.
           ";" on many Unix shells; "&" on the Windows NT "cmd" shell).

           Beginning with v5.6.0, 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.

           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 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 comma or to
           put comments into a multi-line "qw"-string.  For this reason, the
           "use warnings" pragma and the -w switch (that is, the $^W variable)
           produces warnings if the STRING contains the "," or the "#"
           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.

           A character range may be specified with a hyphen, so "tr/A-J/0-9/"
           does the same replacement as "tr/ACEGIBDFHJ/0246813579/".  For sed
           devotees, "y" is provided as a synonym for "tr".  If the SEARCHLIST
           is delimited by bracketing quotes, the REPLACEMENTLIST has its own
           pair of quotes, which may or may not be bracketing quotes; for
           example, "tr[aeiouy][yuoiea]" or "tr(+\-*/)/ABCD/".

           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.  If you want to map strings between lower/upper
           cases, see "lc" in perlfunc and "uc" in perlfunc, and in general
           consider using the "s" operator if you need regular expressions.
           The "\U", "\u", "\L", and "\l" string-interpolation escapes on the
           right side of a substitution operator will perform correct case-
           mappings, but "tr[a-z][A-Z]" will not (except sometimes on legacy
           7-bit data).

           Note also that the whole range idea is rather unportable between
           character sets--and even within character sets they may cause
           results you probably didn't expect.  A sound principle is to use
           only ranges that begin from and end at either alphabets of equal
           case (a-e, A-E), or digits (0-4).  Anything else is unsafe.  If in
           doubt, spell out the character sets in full.

           Options:

               c   Complement the SEARCHLIST.
               d   Delete found but unreplaced characters.
               s   Squash duplicate replaced characters.
               r   Return the modified string and leave the original string
                   untouched.

           If the "/c" modifier is specified, the SEARCHLIST character set is
           complemented.  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
           that were transliterated to the same character are squashed down to
           a single instance of the character.

           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
           is replicated till it is long enough.  If the REPLACEMENTLIST is
           empty, the SEARCHLIST is replicated.  This latter is useful for

               tr/a-zA-Z//s;               # bookkeeper -> bokeper

               ($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

           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.

           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.  (If you put a space it
           will be treated as a null identifier, which is valid, and matches
           the first empty line.)  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
               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

           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;

                  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 come on the lines following the
           final delimiter.  So instead of

               s/this/<<E . 'that'
               the other
               E
                . 'more '/eg;

           you have to write

               s/this/<<E . 'that'
                . 'more '/eg;
               the other
               E

           If the terminating identifier is on the last line of the program,
           you must be sure there is a newline after it; otherwise, Perl will
           give the warning Can't find string terminator "END" anywhere before
           EOF....

           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
           The first pass is finding the end of the quoted construct, where
           the information about the delimiters is used in parsing.  During
           this search, text between the starting and ending delimiters is
           copied to a safe location. The text copied gets delimiter-
           independent.

           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 same as the starting delimiter.  Therefore a "/" terminates a
           "qq//" construct, while a "]" terminates "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 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 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, three delimiters must be 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 both parts, though 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"}"

           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" (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";

               Because the result of "\Q STRING \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 became
               "\\\$"; 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 $b -> {c}" really means:

                 "a " . $b . " -> {c}";

               or:

                 "a " . $b -> {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", 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 "use warnings" pragma or the
               -w command-line flag (that is, the $^W variable) was set.

           "RE" in "?RE?", "/RE/", "m/RE/", "s/RE/foo/",
               Processing of "\Q", "\U", "\u", "\L", "\l", "\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@".

               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.
               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 "?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 to 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.  Similarly, the terminator of
           "(?{...})" is found using the same rules as for finding the
           terminator of a "{}"-delimited construct.

           "/^/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//".  (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.

       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 avoids $_ :

           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 line has a string value that
       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.  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 "perl dangerous.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.
       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 (e.g.,
       <$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:

       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
       (e.g. 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]);

   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 will precompute 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 to not produce a warning in a void context, so
       you can for example safely do

           1 while foo();
           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

       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 "use integer", if you take the sqrt(2), you'll still get
       1.4142135623731 or so.

       Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<", and
       ">>") always produce integral results.  (But see also "Bitwise String
       Operators".)  However, "use integer" still has meaning for them.  By
       default, their results are interpreted as unsigned integers, but if
       "use integer" is in effect, their results are interpreted as signed
       integers.  For example, "~0" usually evaluates to a large integral
       value.  However, "use integer; ~0" is "-1" on two's-complement
       machines.

   Floating-point Arithmetic
       While "use integer" 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.
       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 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 "bigint", "bigrat", and "bitfloat" 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;
               $a = 3/22;
               $b = 4/6;
               say "a/b is ", $a/$b;
               say "a*b is ", $a*$b;
           a/b is 9/44
           a*b is 1/11

       Several modules let you calculate with (bound only by memory and CPU
       time) unlimited or fixed precision. There are also some non-standard
       modules that provide faster implementations via external C libraries.

       Here is a short, but incomplete summary:

       Choose wisely.



perl v5.14.2                      2011-09-26                         PERLOP(1)
Man Pages Copyright Respective Owners. Site Copyright (C) 1994 - 2017 Hurricane Electric. All Rights Reserved.