perlvar
PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)
NAME
perlvar - Perl predefined variables
DESCRIPTION
The Syntax of Variable Names
Variable names in Perl can have several formats. Usually, they must
begin with a letter or underscore, in which case they can be
arbitrarily long (up to an internal limit of 251 characters) and may
contain letters, digits, underscores, or the special sequence "::" or
"'". In this case, the part before the last "::" or "'" is taken to be
a package qualifier; see perlmod. A Unicode letter that is not ASCII
is not considered to be a letter unless "useutf8" is in effect, and
somewhat more complicated rules apply; see "Identifier parsing" in
perldata for details.
Perl variable names may also be a sequence of digits, a single
punctuation character, or the two-character sequence: "^" (caret or
CIRCUMFLEX ACCENT) followed by any one of the characters "[][A-Z^_?\]".
These names are all reserved for special uses by Perl; for example, the
all-digits names are used to hold data captured by backreferences after
a regular expression match.
Since Perl v5.6.0, Perl variable names may also be alphanumeric strings
preceded by a caret. These must all be written in the form "${^Foo}";
the braces are not optional. "${^Foo}" denotes the scalar variable
whose name is considered to be a control-"F" followed by two "o"'s.
These variables are reserved for future special uses by Perl, except
for the ones that begin with "^_" (caret-underscore). No name that
begins with "^_" will acquire a special meaning in any future version
of Perl; such names may therefore be used safely in programs. $^_
itself, however, is reserved.
Perl identifiers that begin with digits or punctuation characters are
exempt from the effects of the "package" declaration and are always
forced to be in package "main"; they are also exempt from "strict
'vars'" errors. A few other names are also exempt in these ways:
ENV STDIN
INC STDOUT
ARGV STDERR
ARGVOUT
SIG
In particular, the special "${^_XYZ}" variables are always taken to be
in package "main", regardless of any "package" declarations presently
in scope.
SPECIAL VARIABLES
The following names have special meaning to Perl. Most punctuation
names have reasonable mnemonics, or analogs in the shells.
Nevertheless, if you wish to use long variable names, you need only
say:
use English;
at the top of your program. This aliases all the short names to the
long names in the current package. Some even have medium names,
generally borrowed from awk. For more info, please see English.
Before you continue, note the sort order for variables. In general, we
first list the variables in case-insensitive, almost-lexigraphical
order (ignoring the "{" or "^" preceding words, as in "${^UNICODE}" or
$^T), although $_ and @_ move up to the top of the pile. For variables
with the same identifier, we list it in order of scalar, array, hash,
and bareword.
General Variables
$ARG
$_ The default input and pattern-searching space. The following
pairs are equivalent:
while (<>) {...} # equivalent only in while!
while (defined($_ = <>)) {...}
/^Subject:/
$_ =~ /^Subject:/
tr/a-z/A-Z/
$_ =~ tr/a-z/A-Z/
chomp
chomp($_)
Here are the places where Perl will assume $_ even if you don't
use it:
o The following functions use $_ as a default argument:
abs, alarm, chomp, chop, chr, chroot, cos, defined, eval,
evalbytes, exp, fc, glob, hex, int, lc, lcfirst, length,
log, lstat, mkdir, oct, ord, pos, print, printf, quotemeta,
readlink, readpipe, ref, require, reverse (in scalar context
only), rmdir, say, sin, split (for its second argument),
sqrt, stat, study, uc, ucfirst, unlink, unpack.
o All file tests ("-f", "-d") except for "-t", which defaults
to STDIN. See "-X" in perlfunc
o The pattern matching operations "m//", "s///" and "tr///"
(aka "y///") when used without an "=~" operator.
o The default iterator variable in a "foreach" loop if no
other variable is supplied.
o The implicit iterator variable in the "grep()" and "map()"
functions.
o The implicit variable of "given()".
o The default place to put the next value or input record when
a "<FH>", "readline", "readdir" or "each" operation's result
is tested by itself as the sole criterion of a "while" test.
Outside a "while" test, this will not happen.
$_ is a global variable.
However, between perl v5.10.0 and v5.24.0, it could be used
lexically by writing "my $_". Making $_ refer to the global $_
in the same scope was then possible with "our $_". This
experimental feature was removed and is now a fatal error, but
you may encounter it in older code.
Mnemonic: underline is understood in certain operations.
@ARG
@_ Within a subroutine the array @_ contains the parameters passed
to that subroutine. Inside a subroutine, @_ is the default
array for the array operators "pop" and "shift".
See perlsub.
$LIST_SEPARATOR
$" When an array or an array slice is interpolated into a double-
quoted string or a similar context such as "/.../", its
elements are separated by this value. Default is a space. For
example, this:
print "The array is: @array\n";
is equivalent to this:
print "The array is: " . join($", @array) . "\n";
Mnemonic: works in double-quoted context.
$PROCESS_ID
$PID
$$ The process number of the Perl running this script. Though you
can set this variable, doing so is generally discouraged,
although it can be invaluable for some testing purposes. It
will be reset automatically across "fork()" calls.
Note for Linux and Debian GNU/kFreeBSD users: Before Perl
v5.16.0 perl would emulate POSIX semantics on Linux systems
using LinuxThreads, a partial implementation of POSIX Threads
that has since been superseded by the Native POSIX Thread
Library (NPTL).
LinuxThreads is now obsolete on Linux, and caching "getpid()"
like this made embedding perl unnecessarily complex (since
you'd have to manually update the value of $$), so now $$ and
"getppid()" will always return the same values as the
underlying C library.
Debian GNU/kFreeBSD systems also used LinuxThreads up until and
including the 6.0 release, but after that moved to FreeBSD
thread semantics, which are POSIX-like.
To see if your system is affected by this discrepancy check if
"getconf GNU_LIBPTHREAD_VERSION | grep -q NPTL" returns a false
value. NTPL threads preserve the POSIX semantics.
Mnemonic: same as shells.
$PROGRAM_NAME
$0 Contains the name of the program being executed.
On some (but not all) operating systems assigning to $0
modifies the argument area that the "ps" program sees. On some
platforms you may have to use special "ps" options or a
different "ps" to see the changes. Modifying the $0 is more
useful as a way of indicating the current program state than it
is for hiding the program you're running.
Note that there are platform-specific limitations on the
maximum length of $0. In the most extreme case it may be
limited to the space occupied by the original $0.
In some platforms there may be arbitrary amount of padding, for
example space characters, after the modified name as shown by
"ps". In some platforms this padding may extend all the way to
the original length of the argument area, no matter what you do
(this is the case for example with Linux 2.2).
Note for BSD users: setting $0 does not completely remove
"perl" from the ps(1) output. For example, setting $0 to
"foobar" may result in "perl: foobar (perl)" (whether both the
"perl: " prefix and the " (perl)" suffix are shown depends on
your exact BSD variant and version). This is an operating
system feature, Perl cannot help it.
In multithreaded scripts Perl coordinates the threads so that
any thread may modify its copy of the $0 and the change becomes
visible to ps(1) (assuming the operating system plays along).
Note that the view of $0 the other threads have will not change
since they have their own copies of it.
If the program has been given to perl via the switches "-e" or
"-E", $0 will contain the string "-e".
On Linux as of perl v5.14.0 the legacy process name will be set
with prctl(2), in addition to altering the POSIX name via
"argv[0]" as perl has done since version 4.000. Now system
utilities that read the legacy process name such as ps, top and
killall will recognize the name you set when assigning to $0.
The string you supply will be cut off at 16 bytes, this is a
limitation imposed by Linux.
Mnemonic: same as sh and ksh.
$REAL_GROUP_ID
$GID
$( The real gid of this process. If you are on a machine that
supports membership in multiple groups simultaneously, gives a
space separated list of groups you are in. The first number is
the one returned by "getgid()", and the subsequent ones by
"getgroups()", one of which may be the same as the first
number.
However, a value assigned to $( must be a single number used to
set the real gid. So the value given by $( should not be
assigned back to $( without being forced numeric, such as by
adding zero. Note that this is different to the effective gid
($)) which does take a list.
You can change both the real gid and the effective gid at the
same time by using "POSIX::setgid()". Changes to $( require a
check to $! to detect any possible errors after an attempted
change.
Mnemonic: parentheses are used to group things. The real gid
is the group you left, if you're running setgid.
$EFFECTIVE_GROUP_ID
$EGID
$) The effective gid of this process. If you are on a machine
that supports membership in multiple groups simultaneously,
gives a space separated list of groups you are in. The first
number is the one returned by "getegid()", and the subsequent
ones by "getgroups()", one of which may be the same as the
first number.
Similarly, a value assigned to $) must also be a space-
separated list of numbers. The first number sets the effective
gid, and the rest (if any) are passed to "setgroups()". To get
the effect of an empty list for "setgroups()", just repeat the
new effective gid; that is, to force an effective gid of 5 and
an effectively empty "setgroups()" list, say " $) = "5 5" ".
You can change both the effective gid and the real gid at the
same time by using "POSIX::setgid()" (use only a single numeric
argument). Changes to $) require a check to $! to detect any
possible errors after an attempted change.
$<, $>, $( and $) can be set only on machines that support the
corresponding set[re][ug]id() routine. $( and $) can be
swapped only on machines supporting "setregid()".
Mnemonic: parentheses are used to group things. The effective
gid is the group that's right for you, if you're running
setgid.
$REAL_USER_ID
$UID
$< The real uid of this process. You can change both the real uid
and the effective uid at the same time by using
"POSIX::setuid()". Since changes to $< require a system call,
check $! after a change attempt to detect any possible errors.
Mnemonic: it's the uid you came from, if you're running setuid.
$EFFECTIVE_USER_ID
$EUID
$> The effective uid of this process. For example:
$< = $>; # set real to effective uid
($<,$>) = ($>,$<); # swap real and effective uids
You can change both the effective uid and the real uid at the
same time by using "POSIX::setuid()". Changes to $> require a
check to $! to detect any possible errors after an attempted
change.
$< and $> can be swapped only on machines supporting
"setreuid()".
Mnemonic: it's the uid you went to, if you're running setuid.
$SUBSCRIPT_SEPARATOR
$SUBSEP
$; The subscript separator for multidimensional array emulation.
If you refer to a hash element as
$foo{$x,$y,$z}
it really means
$foo{join($;, $x, $y, $z)}
But don't put
@foo{$x,$y,$z} # a slice--note the @
which means
($foo{$x},$foo{$y},$foo{$z})
Default is "\034", the same as SUBSEP in awk. If your keys
contain binary data there might not be any safe value for $;.
Consider using "real" multidimensional arrays as described in
perllol.
Mnemonic: comma (the syntactic subscript separator) is a semi-
semicolon.
$a
$b Special package variables when using "sort()", see "sort" in
perlfunc. Because of this specialness $a and $b don't need to
be declared (using "use vars", or "our()") even when using the
"strict 'vars'" pragma. Don't lexicalize them with "my $a" or
"my $b" if you want to be able to use them in the "sort()"
comparison block or function.
%ENV The hash %ENV contains your current environment. Setting a
value in "ENV" changes the environment for any child processes
you subsequently "fork()" off.
As of v5.18.0, both keys and values stored in %ENV are
stringified.
my $foo = 1;
$ENV{'bar'} = \$foo;
if( ref $ENV{'bar'} ) {
say "Pre 5.18.0 Behaviour";
} else {
say "Post 5.18.0 Behaviour";
}
Previously, only child processes received stringified values:
my $foo = 1;
$ENV{'bar'} = \$foo;
# Always printed 'non ref'
system($^X, '-e',
q/print ( ref $ENV{'bar'} ? 'ref' : 'non ref' ) /);
This happens because you can't really share arbitrary data
structures with foreign processes.
$OLD_PERL_VERSION
$] The revision, version, and subversion of the Perl interpreter,
represented as a decimal of the form 5.XXXYYY, where XXX is the
version / 1e3 and YYY is the subversion / 1e6. For example,
Perl v5.10.1 would be "5.010001".
This variable can be used to determine whether the Perl
interpreter executing a script is in the right range of
versions:
warn "No PerlIO!\n" if $] lt '5.008';
When comparing $], string comparison operators are highly
recommended. The inherent limitations of binary floating point
representation can sometimes lead to incorrect comparisons for
some numbers on some architectures.
See also the documentation of "use VERSION" and "require
VERSION" for a convenient way to fail if the running Perl
interpreter is too old.
See "$^V" for a representation of the Perl version as a version
object, which allows more flexible string comparisons.
The main advantage of $] over $^V is that it works the same on
any version of Perl. The disadvantages are that it can't
easily be compared to versions in other formats (e.g. literal
v-strings, "v1.2.3" or version objects) and numeric comparisons
can occasionally fail; it's good for string literal version
checks and bad for comparing to a variable that hasn't been
sanity-checked.
The $OLD_PERL_VERSION form was added in Perl v5.20.0 for
historical reasons but its use is discouraged. (If your reason
to use $] is to run code on old perls then referring to it as
$OLD_PERL_VERSION would be self-defeating.)
Mnemonic: Is this version of perl in the right bracket?
$SYSTEM_FD_MAX
$^F The maximum system file descriptor, ordinarily 2. System file
descriptors are passed to "exec()"ed processes, while higher
file descriptors are not. Also, during an "open()", system
file descriptors are preserved even if the "open()" fails
(ordinary file descriptors are closed before the "open()" is
attempted). The close-on-exec status of a file descriptor will
be decided according to the value of $^F when the corresponding
file, pipe, or socket was opened, not the time of the "exec()".
@F The array @F contains the fields of each line read in when
autosplit mode is turned on. See perlrun for the -a switch.
This array is package-specific, and must be declared or given a
full package name if not in package main when running under
"strict 'vars'".
@INC The array @INC contains the list of places that the "do EXPR",
"require", or "use" constructs look for their library files.
It initially consists of the arguments to any -I command-line
switches, followed by the default Perl library, probably
/usr/local/lib/perl. Prior to Perl 5.26, "." -which represents
the current directory, was included in @INC; it has been
removed. This change in behavior is documented in
"PERL_USE_UNSAFE_INC" and it is not recommended that "." be re-
added to @INC. If you need to modify @INC at runtime, you
should use the "use lib" pragma to get the machine-dependent
library properly loaded as well:
use lib '/mypath/libdir/';
use SomeMod;
You can also insert hooks into the file inclusion system by
putting Perl code directly into @INC. Those hooks may be
subroutine references, array references or blessed objects.
See "require" in perlfunc for details.
%INC The hash %INC contains entries for each filename included via
the "do", "require", or "use" operators. The key is the
filename you specified (with module names converted to
pathnames), and the value is the location of the file found.
The "require" operator uses this hash to determine whether a
particular file has already been included.
If the file was loaded via a hook (e.g. a subroutine reference,
see "require" in perlfunc for a description of these hooks),
this hook is by default inserted into %INC in place of a
filename. Note, however, that the hook may have set the %INC
entry by itself to provide some more specific info.
$INPLACE_EDIT
$^I The current value of the inplace-edit extension. Use "undef"
to disable inplace editing.
Mnemonic: value of -i switch.
@ISA Each package contains a special array called @ISA which
contains a list of that class's parent classes, if any. This
array is simply a list of scalars, each of which is a string
that corresponds to a package name. The array is examined when
Perl does method resolution, which is covered in perlobj.
To load packages while adding them to @ISA, see the parent
pragma. The discouraged base pragma does this as well, but
should not be used except when compatibility with the
discouraged fields pragma is required.
$^M By default, running out of memory is an untrappable, fatal
error. However, if suitably built, Perl can use the contents
of $^M as an emergency memory pool after "die()"ing. Suppose
that your Perl were compiled with "-DPERL_EMERGENCY_SBRK" and
used Perl's malloc. Then
$^M = 'a' x (1 << 16);
would allocate a 64K buffer for use in an emergency. See the
INSTALL file in the Perl distribution for information on how to
add custom C compilation flags when compiling perl. To
discourage casual use of this advanced feature, there is no
English long name for this variable.
This variable was added in Perl 5.004.
$OSNAME
$^O The name of the operating system under which this copy of Perl
was built, as determined during the configuration process. For
examples see "PLATFORMS" in perlport.
The value is identical to $Config{'osname'}. See also Config
and the -V command-line switch documented in perlrun.
In Windows platforms, $^O is not very helpful: since it is
always "MSWin32", it doesn't tell the difference between
95/98/ME/NT/2000/XP/CE/.NET. Use "Win32::GetOSName()" or
Win32::GetOSVersion() (see Win32 and perlport) to distinguish
between the variants.
This variable was added in Perl 5.003.
%SIG The hash %SIG contains signal handlers for signals. For
example:
sub handler { # 1st argument is signal name
my($sig) = @_;
print "Caught a SIG$sig--shutting down\n";
close(LOG);
exit(0);
}
$SIG{'INT'} = \&handler;
$SIG{'QUIT'} = \&handler;
...
$SIG{'INT'} = 'DEFAULT'; # restore default action
$SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
Using a value of 'IGNORE' usually has the effect of ignoring
the signal, except for the "CHLD" signal. See perlipc for more
about this special case.
Here are some other examples:
$SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not
# recommended)
$SIG{"PIPE"} = \&Plumber; # just fine; assume current
# Plumber
$SIG{"PIPE"} = *Plumber; # somewhat esoteric
$SIG{"PIPE"} = Plumber(); # oops, what did Plumber()
# return??
Be sure not to use a bareword as the name of a signal handler,
lest you inadvertently call it.
If your system has the "sigaction()" function then signal
handlers are installed using it. This means you get reliable
signal handling.
The default delivery policy of signals changed in Perl v5.8.0
from immediate (also known as "unsafe") to deferred, also known
as "safe signals". See perlipc for more information.
Certain internal hooks can be also set using the %SIG hash.
The routine indicated by $SIG{__WARN__} is called when a
warning message is about to be printed. The warning message is
passed as the first argument. The presence of a "__WARN__"
hook causes the ordinary printing of warnings to "STDERR" to be
suppressed. You can use this to save warnings in a variable,
or turn warnings into fatal errors, like this:
local $SIG{__WARN__} = sub { die $_[0] };
eval $proggie;
As the 'IGNORE' hook is not supported by "__WARN__", you can
disable warnings using the empty subroutine:
local $SIG{__WARN__} = sub {};
The routine indicated by $SIG{__DIE__} is called when a fatal
exception is about to be thrown. The error message is passed
as the first argument. When a "__DIE__" hook routine returns,
the exception processing continues as it would have in the
absence of the hook, unless the hook routine itself exits via a
"goto &sub", a loop exit, or a "die()". The "__DIE__" handler
is explicitly disabled during the call, so that you can die
from a "__DIE__" handler. Similarly for "__WARN__".
The $SIG{__DIE__} hook is called even inside an "eval()". It
was never intended to happen this way, but an implementation
glitch made this possible. This used to be deprecated, as it
allowed strange action at a distance like rewriting a pending
exception in $@. Plans to rectify this have been scrapped, as
users found that rewriting a pending exception is actually a
useful feature, and not a bug.
"__DIE__"/"__WARN__" handlers are very special in one respect:
they may be called to report (probable) errors found by the
parser. In such a case the parser may be in inconsistent
state, so any attempt to evaluate Perl code from such a handler
will probably result in a segfault. This means that warnings
or errors that result from parsing Perl should be used with
extreme caution, like this:
require Carp if defined $^S;
Carp::confess("Something wrong") if defined &Carp::confess;
die "Something wrong, but could not load Carp to give "
. "backtrace...\n\t"
. "To see backtrace try starting Perl with -MCarp switch";
Here the first line will load "Carp" unless it is the parser
who called the handler. The second line will print backtrace
and die if "Carp" was available. The third line will be
executed only if "Carp" was not available.
Having to even think about the $^S variable in your exception
handlers is simply wrong. $SIG{__DIE__} as currently
implemented invites grievous and difficult to track down
errors. Avoid it and use an "END{}" or CORE::GLOBAL::die
override instead.
See "die" in perlfunc, "warn" in perlfunc, "eval" in perlfunc,
and warnings for additional information.
$BASETIME
$^T The time at which the program began running, in seconds since
the epoch (beginning of 1970). The values returned by the -M,
-A, and -C filetests are based on this value.
$PERL_VERSION
$^V The revision, version, and subversion of the Perl interpreter,
represented as a version object.
This variable first appeared in perl v5.6.0; earlier versions
of perl will see an undefined value. Before perl v5.10.0 $^V
was represented as a v-string rather than a version object.
$^V can be used to determine whether the Perl interpreter
executing a script is in the right range of versions. For
example:
warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
While version objects overload stringification, to portably
convert $^V into its string representation, use "sprintf()"'s
"%vd" conversion, which works for both v-strings or version
objects:
printf "version is v%vd\n", $^V; # Perl's version
See the documentation of "use VERSION" and "require VERSION"
for a convenient way to fail if the running Perl interpreter is
too old.
See also "$]" for a decimal representation of the Perl version.
The main advantage of $^V over $] is that, for Perl v5.10.0 or
later, it overloads operators, allowing easy comparison against
other version representations (e.g. decimal, literal v-string,
"v1.2.3", or objects). The disadvantage is that prior to
v5.10.0, it was only a literal v-string, which can't be easily
printed or compared, whereas the behavior of $] is unchanged on
all versions of Perl.
Mnemonic: use ^V for a version object.
${^WIN32_SLOPPY_STAT}
If this variable is set to a true value, then "stat()" on
Windows will not try to open the file. This means that the
link count cannot be determined and file attributes may be out
of date if additional hardlinks to the file exist. On the
other hand, not opening the file is considerably faster,
especially for files on network drives.
This variable could be set in the sitecustomize.pl file to
configure the local Perl installation to use "sloppy" "stat()"
by default. See the documentation for -f in perlrun for more
information about site customization.
This variable was added in Perl v5.10.0.
$EXECUTABLE_NAME
$^X The name used to execute the current copy of Perl, from C's
"argv[0]" or (where supported) /proc/self/exe.
Depending on the host operating system, the value of $^X may be
a relative or absolute pathname of the perl program file, or
may be the string used to invoke perl but not the pathname of
the perl program file. Also, most operating systems permit
invoking programs that are not in the PATH environment
variable, so there is no guarantee that the value of $^X is in
PATH. For VMS, the value may or may not include a version
number.
You usually can use the value of $^X to re-invoke an
independent copy of the same perl that is currently running,
e.g.,
@first_run = `$^X -le "print int rand 100 for 1..100"`;
But recall that not all operating systems support forking or
capturing of the output of commands, so this complex statement
may not be portable.
It is not safe to use the value of $^X as a path name of a
file, as some operating systems that have a mandatory suffix on
executable files do not require use of the suffix when invoking
a command. To convert the value of $^X to a path name, use the
following statements:
# Build up a set of file names (not command names).
use Config;
my $this_perl = $^X;
if ($^O ne 'VMS') {
$this_perl .= $Config{_exe}
unless $this_perl =~ m/$Config{_exe}$/i;
}
Because many operating systems permit anyone with read access
to the Perl program file to make a copy of it, patch the copy,
and then execute the copy, the security-conscious Perl
programmer should take care to invoke the installed copy of
perl, not the copy referenced by $^X. The following statements
accomplish this goal, and produce a pathname that can be
invoked as a command or referenced as a file.
use Config;
my $secure_perl_path = $Config{perlpath};
if ($^O ne 'VMS') {
$secure_perl_path .= $Config{_exe}
unless $secure_perl_path =~ m/$Config{_exe}$/i;
}
Variables related to regular expressions
Most of the special variables related to regular expressions are side
effects. Perl sets these variables when it has a successful match, so
you should check the match result before using them. For instance:
if( /P(A)TT(ER)N/ ) {
print "I found $1 and $2\n";
}
These variables are read-only and dynamically-scoped, unless we note
otherwise.
The dynamic nature of the regular expression variables means that their
value is limited to the block that they are in, as demonstrated by this
bit of code:
my $outer = 'Wallace and Grommit';
my $inner = 'Mutt and Jeff';
my $pattern = qr/(\S+) and (\S+)/;
sub show_n { print "\$1 is $1; \$2 is $2\n" }
{
OUTER:
show_n() if $outer =~ m/$pattern/;
INNER: {
show_n() if $inner =~ m/$pattern/;
}
show_n();
}
The output shows that while in the "OUTER" block, the values of $1 and
$2 are from the match against $outer. Inside the "INNER" block, the
values of $1 and $2 are from the match against $inner, but only until
the end of the block (i.e. the dynamic scope). After the "INNER" block
completes, the values of $1 and $2 return to the values for the match
against $outer even though we have not made another match:
$1 is Wallace; $2 is Grommit
$1 is Mutt; $2 is Jeff
$1 is Wallace; $2 is Grommit
Performance issues
Traditionally in Perl, any use of any of the three variables "$`", $&
or "$'" (or their "use English" equivalents) anywhere in the code,
caused all subsequent successful pattern matches to make a copy of the
matched string, in case the code might subsequently access one of those
variables. This imposed a considerable performance penalty across the
whole program, so generally the use of these variables has been
discouraged.
In Perl 5.6.0 the "@-" and "@+" dynamic arrays were introduced that
supply the indices of successful matches. So you could for example do
this:
$str =~ /pattern/;
print $`, $&, $'; # bad: performance hit
print # good: no performance hit
substr($str, 0, $-[0]),
substr($str, $-[0], $+[0]-$-[0]),
substr($str, $+[0]);
In Perl 5.10.0 the "/p" match operator flag and the "${^PREMATCH}",
"${^MATCH}", and "${^POSTMATCH}" variables were introduced, that
allowed you to suffer the penalties only on patterns marked with "/p".
In Perl 5.18.0 onwards, perl started noting the presence of each of the
three variables separately, and only copied that part of the string
required; so in
$`; $&; "abcdefgh" =~ /d/
perl would only copy the "abcd" part of the string. That could make a
big difference in something like
$str = 'x' x 1_000_000;
$&; # whoops
$str =~ /x/g # one char copied a million times, not a million chars
In Perl 5.20.0 a new copy-on-write system was enabled by default, which
finally fixes all performance issues with these three variables, and
makes them safe to use anywhere.
The "Devel::NYTProf" and "Devel::FindAmpersand" modules can help you
find uses of these problematic match variables in your code.
$<digits> ($1, $2, ...)
Contains the subpattern from the corresponding set of capturing
parentheses from the last successful pattern match, not
counting patterns matched in nested blocks that have been
exited already.
Note there is a distinction between a capture buffer which
matches the empty string a capture buffer which is optional.
Eg, "(x?)" and "(x)?" The latter may be undef, the former not.
These variables are read-only and dynamically-scoped.
Mnemonic: like \digits.
@{^CAPTURE}
An array which exposes the contents of the capture buffers, if
any, of the last successful pattern match, not counting
patterns matched in nested blocks that have been exited
already.
Note that the 0 index of @{^CAPTURE} is equivalent to $1, the 1
index is equivalent to $2, etc.
if ("foal"=~/(.)(.)(.)(.)/) {
print join "-", @{^CAPTURE};
}
should output "f-o-a-l".
See also "$<digits> ($1, $2, ...)", "%{^CAPTURE}" and
"%{^CAPTURE_ALL}".
Note that unlike most other regex magic variables there is no
single letter equivalent to "@{^CAPTURE}".
This variable was added in 5.25.7
$MATCH
$& The string matched by the last successful pattern match (not
counting any matches hidden within a BLOCK or "eval()" enclosed
by the current BLOCK).
See "Performance issues" above for the serious performance
implications of using this variable (even once) in your code.
This variable is read-only and dynamically-scoped.
Mnemonic: like "&" in some editors.
${^MATCH}
This is similar to $& ($MATCH) except that it does not incur
the performance penalty associated with that variable.
See "Performance issues" above.
In Perl v5.18 and earlier, it is only guaranteed to return a
defined value when the pattern was compiled or executed with
the "/p" modifier. In Perl v5.20, the "/p" modifier does
nothing, so "${^MATCH}" does the same thing as $MATCH.
This variable was added in Perl v5.10.0.
This variable is read-only and dynamically-scoped.
$PREMATCH
$` The string preceding whatever was matched by the last
successful pattern match, not counting any matches hidden
within a BLOCK or "eval" enclosed by the current BLOCK.
See "Performance issues" above for the serious performance
implications of using this variable (even once) in your code.
This variable is read-only and dynamically-scoped.
Mnemonic: "`" often precedes a quoted string.
${^PREMATCH}
This is similar to "$`" ($PREMATCH) except that it does not
incur the performance penalty associated with that variable.
See "Performance issues" above.
In Perl v5.18 and earlier, it is only guaranteed to return a
defined value when the pattern was compiled or executed with
the "/p" modifier. In Perl v5.20, the "/p" modifier does
nothing, so "${^PREMATCH}" does the same thing as $PREMATCH.
This variable was added in Perl v5.10.0.
This variable is read-only and dynamically-scoped.
$POSTMATCH
$' The string following whatever was matched by the last
successful pattern match (not counting any matches hidden
within a BLOCK or "eval()" enclosed by the current BLOCK).
Example:
local $_ = 'abcdefghi';
/def/;
print "$`:$&:$'\n"; # prints abc:def:ghi
See "Performance issues" above for the serious performance
implications of using this variable (even once) in your code.
This variable is read-only and dynamically-scoped.
Mnemonic: "'" often follows a quoted string.
${^POSTMATCH}
This is similar to "$'" ($POSTMATCH) except that it does not
incur the performance penalty associated with that variable.
See "Performance issues" above.
In Perl v5.18 and earlier, it is only guaranteed to return a
defined value when the pattern was compiled or executed with
the "/p" modifier. In Perl v5.20, the "/p" modifier does
nothing, so "${^POSTMATCH}" does the same thing as $POSTMATCH.
This variable was added in Perl v5.10.0.
This variable is read-only and dynamically-scoped.
$LAST_PAREN_MATCH
$+ The text matched by the highest used capture group of the last
successful search pattern. It is logically equivalent to the
highest numbered capture variable ($1, $2, ...) which has a
defined value.
This is useful if you don't know which one of a set of
alternative patterns matched. For example:
/Version: (.*)|Revision: (.*)/ && ($rev = $+);
This variable is read-only and dynamically-scoped.
Mnemonic: be positive and forward looking.
$LAST_SUBMATCH_RESULT
$^N The text matched by the used group most-recently closed (i.e.
the group with the rightmost closing parenthesis) of the last
successful search pattern. This is subtly different from $+.
For example in
"ab" =~ /^((.)(.))$/
we have
$1,$^N have the value "ab"
$2 has the value "a"
$3,$+ have the value "b"
This is primarily used inside "(?{...})" blocks for examining
text recently matched. For example, to effectively capture
text to a variable (in addition to $1, $2, etc.), replace
"(...)" with
(?:(...)(?{ $var = $^N }))
By setting and then using $var in this way relieves you from
having to worry about exactly which numbered set of parentheses
they are.
This variable was added in Perl v5.8.0.
Mnemonic: the (possibly) Nested parenthesis that most recently
closed.
@LAST_MATCH_END
@+ This array holds the offsets of the ends of the last successful
submatches in the currently active dynamic scope. $+[0] is the
offset into the string of the end of the entire match. This is
the same value as what the "pos" function returns when called
on the variable that was matched against. The nth element of
this array holds the offset of the nth submatch, so $+[1] is
the offset past where $1 ends, $+[2] the offset past where $2
ends, and so on. You can use $#+ to determine how many
subgroups were in the last successful match. See the examples
given for the "@-" variable.
This variable was added in Perl v5.6.0.
%{^CAPTURE}
%LAST_PAREN_MATCH
%+ Similar to "@+", the "%+" hash allows access to the named
capture buffers, should they exist, in the last successful
match in the currently active dynamic scope.
For example, $+{foo} is equivalent to $1 after the following
match:
'foo' =~ /(?<foo>foo)/;
The keys of the "%+" hash list only the names of buffers that
have captured (and that are thus associated to defined values).
If multiple distinct capture groups have the same name, then
$+{NAME} will refer to the leftmost defined group in the match.
The underlying behaviour of "%+" is provided by the
Tie::Hash::NamedCapture module.
Note: "%-" and "%+" are tied views into a common internal hash
associated with the last successful regular expression.
Therefore mixing iterative access to them via "each" may have
unpredictable results. Likewise, if the last successful match
changes, then the results may be surprising.
This variable was added in Perl v5.10.0. The "%{^CAPTURE}"
alias was added in 5.25.7.
This variable is read-only and dynamically-scoped.
@LAST_MATCH_START
@- "$-[0]" is the offset of the start of the last successful
match. "$-[n]" is the offset of the start of the substring
matched by n-th subpattern, or undef if the subpattern did not
match.
Thus, after a match against $_, $& coincides with "substr $_,
$-[0], $+[0] - $-[0]". Similarly, $n coincides with "substr
$_, $-[n], $+[n] - $-[n]" if "$-[n]" is defined, and $+
coincides with "substr $_, $-[$#-], $+[$#-] - $-[$#-]". One
can use "$#-" to find the last matched subgroup in the last
successful match. Contrast with $#+, the number of subgroups
in the regular expression. Compare with "@+".
This array holds the offsets of the beginnings of the last
successful submatches in the currently active dynamic scope.
"$-[0]" is the offset into the string of the beginning of the
entire match. The nth element of this array holds the offset
of the nth submatch, so "$-[1]" is the offset where $1 begins,
"$-[2]" the offset where $2 begins, and so on.
After a match against some variable $var:
"$`" is the same as "substr($var, 0, $-[0])"
$& is the same as "substr($var, $-[0], $+[0] - $-[0])"
"$'" is the same as "substr($var, $+[0])"
$1 is the same as "substr($var, $-[1], $+[1] - $-[1])"
$2 is the same as "substr($var, $-[2], $+[2] - $-[2])"
$3 is the same as "substr($var, $-[3], $+[3] - $-[3])"
This variable was added in Perl v5.6.0.
%{^CAPTURE_ALL}
%- Similar to "%+", this variable allows access to the named
capture groups in the last successful match in the currently
active dynamic scope. To each capture group name found in the
regular expression, it associates a reference to an array
containing the list of values captured by all buffers with that
name (should there be several of them), in the order where they
appear.
Here's an example:
if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
foreach my $bufname (sort keys %-) {
my $ary = $-{$bufname};
foreach my $idx (0..$#$ary) {
print "\$-{$bufname}[$idx] : ",
(defined($ary->[$idx])
? "'$ary->[$idx]'"
: "undef"),
"\n";
}
}
}
would print out:
$-{A}[0] : '1'
$-{A}[1] : '3'
$-{B}[0] : '2'
$-{B}[1] : '4'
The keys of the "%-" hash correspond to all buffer names found
in the regular expression.
The behaviour of "%-" is implemented via the
Tie::Hash::NamedCapture module.
Note: "%-" and "%+" are tied views into a common internal hash
associated with the last successful regular expression.
Therefore mixing iterative access to them via "each" may have
unpredictable results. Likewise, if the last successful match
changes, then the results may be surprising.
This variable was added in Perl v5.10.0. The "%{^CAPTURE_ALL}"
alias was added in 5.25.7.
This variable is read-only and dynamically-scoped.
$LAST_REGEXP_CODE_RESULT
$^R The result of evaluation of the last successful "(?{ code })"
regular expression assertion (see perlre). May be written to.
This variable was added in Perl 5.005.
${^RE_COMPILE_RECURSION_LIMIT}
The current value giving the maximum number of open but
unclosed parenthetical groups there may be at any point during
a regular expression compilation. The default is currently
1000 nested groups. You may adjust it depending on your needs
and the amount of memory available.
This variable was added in Perl v5.30.0.
${^RE_DEBUG_FLAGS}
The current value of the regex debugging flags. Set to 0 for
no debug output even when the "re 'debug'" module is loaded.
See re for details.
This variable was added in Perl v5.10.0.
${^RE_TRIE_MAXBUF}
Controls how certain regex optimisations are applied and how
much memory they utilize. This value by default is 65536 which
corresponds to a 512kB temporary cache. Set this to a higher
value to trade memory for speed when matching large
alternations. Set it to a lower value if you want the
optimisations to be as conservative of memory as possible but
still occur, and set it to a negative value to prevent the
optimisation and conserve the most memory. Under normal
situations this variable should be of no interest to you.
This variable was added in Perl v5.10.0.
Variables related to filehandles
Variables that depend on the currently selected filehandle may be set
by calling an appropriate object method on the "IO::Handle" object,
although this is less efficient than using the regular built-in
variables. (Summary lines below for this contain the word HANDLE.)
First you must say
use IO::Handle;
after which you may use either
method HANDLE EXPR
or more safely,
HANDLE->method(EXPR)
Each method returns the old value of the "IO::Handle" attribute. The
methods each take an optional EXPR, which, if supplied, specifies the
new value for the "IO::Handle" attribute in question. If not supplied,
most methods do nothing to the current value--except for "autoflush()",
which will assume a 1 for you, just to be different.
Because loading in the "IO::Handle" class is an expensive operation,
you should learn how to use the regular built-in variables.
A few of these variables are considered "read-only". This means that
if you try to assign to this variable, either directly or indirectly
through a reference, you'll raise a run-time exception.
You should be very careful when modifying the default values of most
special variables described in this document. In most cases you want
to localize these variables before changing them, since if you don't,
the change may affect other modules which rely on the default values of
the special variables that you have changed. This is one of the
correct ways to read the whole file at once:
open my $fh, "<", "foo" or die $!;
local $/; # enable localized slurp mode
my $content = <$fh>;
close $fh;
But the following code is quite bad:
open my $fh, "<", "foo" or die $!;
undef $/; # enable slurp mode
my $content = <$fh>;
close $fh;
since some other module, may want to read data from some file in the
default "line mode", so if the code we have just presented has been
executed, the global value of $/ is now changed for any other code
running inside the same Perl interpreter.
Usually when a variable is localized you want to make sure that this
change affects the shortest scope possible. So unless you are already
inside some short "{}" block, you should create one yourself. For
example:
my $content = '';
open my $fh, "<", "foo" or die $!;
{
local $/;
$content = <$fh>;
}
close $fh;
Here is an example of how your own code can go broken:
for ( 1..3 ){
$\ = "\r\n";
nasty_break();
print "$_";
}
sub nasty_break {
$\ = "\f";
# do something with $_
}
You probably expect this code to print the equivalent of
"1\r\n2\r\n3\r\n"
but instead you get:
"1\f2\f3\f"
Why? Because "nasty_break()" modifies "$\" without localizing it first.
The value you set in "nasty_break()" is still there when you return.
The fix is to add "local()" so the value doesn't leak out of
"nasty_break()":
local $\ = "\f";
It's easy to notice the problem in such a short example, but in more
complicated code you are looking for trouble if you don't localize
changes to the special variables.
$ARGV Contains the name of the current file when reading from "<>".
@ARGV The array @ARGV contains the command-line arguments intended
for the script. $#ARGV is generally the number of arguments
minus one, because $ARGV[0] is the first argument, not the
program's command name itself. See "$0" for the command name.
ARGV The special filehandle that iterates over command-line
filenames in @ARGV. Usually written as the null filehandle in
the angle operator "<>". Note that currently "ARGV" only has
its magical effect within the "<>" operator; elsewhere it is
just a plain filehandle corresponding to the last file opened
by "<>". In particular, passing "\*ARGV" as a parameter to a
function that expects a filehandle may not cause your function
to automatically read the contents of all the files in @ARGV.
ARGVOUT The special filehandle that points to the currently open output
file when doing edit-in-place processing with -i. Useful when
you have to do a lot of inserting and don't want to keep
modifying $_. See perlrun for the -i switch.
IO::Handle->output_field_separator( EXPR )
$OUTPUT_FIELD_SEPARATOR
$OFS
$, The output field separator for the print operator. If defined,
this value is printed between each of print's arguments.
Default is "undef".
You cannot call "output_field_separator()" on a handle, only as
a static method. See IO::Handle.
Mnemonic: what is printed when there is a "," in your print
statement.
HANDLE->input_line_number( EXPR )
$INPUT_LINE_NUMBER
$NR
$. Current line number for the last filehandle accessed.
Each filehandle in Perl counts the number of lines that have
been read from it. (Depending on the value of $/, Perl's idea
of what constitutes a line may not match yours.) When a line
is read from a filehandle (via "readline()" or "<>"), or when
"tell()" or "seek()" is called on it, $. becomes an alias to
the line counter for that filehandle.
You can adjust the counter by assigning to $., but this will
not actually move the seek pointer. Localizing $. will not
localize the filehandle's line count. Instead, it will
localize perl's notion of which filehandle $. is currently
aliased to.
$. is reset when the filehandle is closed, but not when an open
filehandle is reopened without an intervening "close()". For
more details, see "I/O Operators" in perlop. Because "<>"
never does an explicit close, line numbers increase across
"ARGV" files (but see examples in "eof" in perlfunc).
You can also use "HANDLE->input_line_number(EXPR)" to access
the line counter for a given filehandle without having to worry
about which handle you last accessed.
Mnemonic: many programs use "." to mean the current line
number.
IO::Handle->input_record_separator( EXPR )
$INPUT_RECORD_SEPARATOR
$RS
$/ The input record separator, newline by default. This
influences Perl's idea of what a "line" is. Works like awk's
RS variable, including treating empty lines as a terminator if
set to the null string (an empty line cannot contain any spaces
or tabs). You may set it to a multi-character string to match
a multi-character terminator, or to "undef" to read through the
end of file. Setting it to "\n\n" means something slightly
different than setting to "", if the file contains consecutive
empty lines. Setting to "" will treat two or more consecutive
empty lines as a single empty line. Setting to "\n\n" will
blindly assume that the next input character belongs to the
next paragraph, even if it's a newline.
local $/; # enable "slurp" mode
local $_ = <FH>; # whole file now here
s/\n[ \t]+/ /g;
Remember: the value of $/ is a string, not a regex. awk has to
be better for something. :-)
Setting $/ to an empty string -- the so-called paragraph mode
-- merits special attention. When $/ is set to "" and the
entire file is read in with that setting, any sequence of
consecutive newlines "\n\n" at the beginning of the file is
discarded. With the exception of the final record in the file,
each sequence of characters ending in two or more newlines is
treated as one record and is read in to end in exactly two
newlines. If the last record in the file ends in zero or one
consecutive newlines, that record is read in with that number
of newlines. If the last record ends in two or more
consecutive newlines, it is read in with two newlines like all
preceding records.
Suppose we wrote the following string to a file:
my $string = "\n\n\n";
$string .= "alpha beta\ngamma delta\n\n\n";
$string .= "epsilon zeta eta\n\n";
$string .= "theta\n";
my $file = 'simple_file.txt';
open my $OUT, '>', $file or die;
print $OUT $string;
close $OUT or die;
Now we read that file in paragraph mode:
local $/ = ""; # paragraph mode
open my $IN, '<', $file or die;
my @records = <$IN>;
close $IN or die;
@records will consist of these 3 strings:
(
"alpha beta\ngamma delta\n\n",
"epsilon zeta eta\n\n",
"theta\n",
)
Setting $/ to a reference to an integer, scalar containing an
integer, or scalar that's convertible to an integer will
attempt to read records instead of lines, with the maximum
record size being the referenced integer number of characters.
So this:
local $/ = \32768; # or \"32768", or \$var_containing_32768
open my $fh, "<", $myfile or die $!;
local $_ = <$fh>;
will read a record of no more than 32768 characters from $fh.
If you're not reading from a record-oriented file (or your OS
doesn't have record-oriented files), then you'll likely get a
full chunk of data with every read. If a record is larger than
the record size you've set, you'll get the record back in
pieces. Trying to set the record size to zero or less is
deprecated and will cause $/ to have the value of "undef",
which will cause reading in the (rest of the) whole file.
As of 5.19.9 setting $/ to any other form of reference will
throw a fatal exception. This is in preparation for supporting
new ways to set $/ in the future.
On VMS only, record reads bypass PerlIO layers and any
associated buffering, so you must not mix record and non-record
reads on the same filehandle. Record mode mixes with line mode
only when the same buffering layer is in use for both modes.
You cannot call "input_record_separator()" on a handle, only as
a static method. See IO::Handle.
See also "Newlines" in perlport. Also see "$.".
Mnemonic: / delimits line boundaries when quoting poetry.
IO::Handle->output_record_separator( EXPR )
$OUTPUT_RECORD_SEPARATOR
$ORS
$\ The output record separator for the print operator. If
defined, this value is printed after the last of print's
arguments. Default is "undef".
You cannot call "output_record_separator()" on a handle, only
as a static method. See IO::Handle.
Mnemonic: you set "$\" instead of adding "\n" at the end of the
print. Also, it's just like $/, but it's what you get "back"
from Perl.
HANDLE->autoflush( EXPR )
$OUTPUT_AUTOFLUSH
$| If set to nonzero, forces a flush right away and after every
write or print on the currently selected output channel.
Default is 0 (regardless of whether the channel is really
buffered by the system or not; $| tells you only whether you've
asked Perl explicitly to flush after each write). STDOUT will
typically be line buffered if output is to the terminal and
block buffered otherwise. Setting this variable is useful
primarily when you are outputting to a pipe or socket, such as
when you are running a Perl program under rsh and want to see
the output as it's happening. This has no effect on input
buffering. See "getc" in perlfunc for that. See "select" in
perlfunc on how to select the output channel. See also
IO::Handle.
Mnemonic: when you want your pipes to be piping hot.
${^LAST_FH}
This read-only variable contains a reference to the last-read
filehandle. This is set by "<HANDLE>", "readline", "tell",
"eof" and "seek". This is the same handle that $. and "tell"
and "eof" without arguments use. It is also the handle used
when Perl appends ", <STDIN> line 1" to an error or warning
message.
This variable was added in Perl v5.18.0.
Variables related to formats
The special variables for formats are a subset of those for
filehandles. See perlform for more information about Perl's formats.
$ACCUMULATOR
$^A The current value of the "write()" accumulator for "format()"
lines. A format contains "formline()" calls that put their
result into $^A. After calling its format, "write()" prints
out the contents of $^A and empties. So you never really see
the contents of $^A unless you call "formline()" yourself and
then look at it. See perlform and "formline PICTURE,LIST" in
perlfunc.
IO::Handle->format_formfeed(EXPR)
$FORMAT_FORMFEED
$^L What formats output as a form feed. The default is "\f".
You cannot call "format_formfeed()" on a handle, only as a
static method. See IO::Handle.
HANDLE->format_page_number(EXPR)
$FORMAT_PAGE_NUMBER
$% The current page number of the currently selected output
channel.
Mnemonic: "%" is page number in nroff.
HANDLE->format_lines_left(EXPR)
$FORMAT_LINES_LEFT
$- The number of lines left on the page of the currently selected
output channel.
Mnemonic: lines_on_page - lines_printed.
IO::Handle->format_line_break_characters EXPR
$FORMAT_LINE_BREAK_CHARACTERS
$: The current set of characters after which a string may be
broken to fill continuation fields (starting with "^") in a
format. The default is "\n-", to break on a space, newline, or
a hyphen.
You cannot call "format_line_break_characters()" on a handle,
only as a static method. See IO::Handle.
Mnemonic: a "colon" in poetry is a part of a line.
HANDLE->format_lines_per_page(EXPR)
$FORMAT_LINES_PER_PAGE
$= The current page length (printable lines) of the currently
selected output channel. The default is 60.
Mnemonic: = has horizontal lines.
HANDLE->format_top_name(EXPR)
$FORMAT_TOP_NAME
$^ The name of the current top-of-page format for the currently
selected output channel. The default is the name of the
filehandle with "_TOP" appended. For example, the default
format top name for the "STDOUT" filehandle is "STDOUT_TOP".
Mnemonic: points to top of page.
HANDLE->format_name(EXPR)
$FORMAT_NAME
$~ The name of the current report format for the currently
selected output channel. The default format name is the same
as the filehandle name. For example, the default format name
for the "STDOUT" filehandle is just "STDOUT".
Mnemonic: brother to $^.
Error Variables
The variables $@, $!, $^E, and $? contain information about different
types of error conditions that may appear during execution of a Perl
program. The variables are shown ordered by the "distance" between the
subsystem which reported the error and the Perl process. They
correspond to errors detected by the Perl interpreter, C library,
operating system, or an external program, respectively.
To illustrate the differences between these variables, consider the
following Perl expression, which uses a single-quoted string. After
execution of this statement, perl may have set all four special error
variables:
eval q{
open my $pipe, "/cdrom/install |" or die $!;
my @res = <$pipe>;
close $pipe or die "bad pipe: $?, $!";
};
When perl executes the "eval()" expression, it translates the "open()",
"<PIPE>", and "close" calls in the C run-time library and thence to the
operating system kernel. perl sets $! to the C library's "errno" if
one of these calls fails.
$@ is set if the string to be "eval"-ed did not compile (this may
happen if "open" or "close" were imported with bad prototypes), or if
Perl code executed during evaluation "die()"d. In these cases the
value of $@ is the compile error, or the argument to "die" (which will
interpolate $! and $?). (See also Fatal, though.)
Under a few operating systems, $^E may contain a more verbose error
indicator, such as in this case, "CDROM tray not closed." Systems that
do not support extended error messages leave $^E the same as $!.
Finally, $? may be set to a non-0 value if the external program
/cdrom/install fails. The upper eight bits reflect specific error
conditions encountered by the program (the program's "exit()" value).
The lower eight bits reflect mode of failure, like signal death and
core dump information. See wait(2) for details. In contrast to $! and
$^E, which are set only if an error condition is detected, the variable
$? is set on each "wait" or pipe "close", overwriting the old value.
This is more like $@, which on every "eval()" is always set on failure
and cleared on success.
For more details, see the individual descriptions at $@, $!, $^E, and
$?.
${^CHILD_ERROR_NATIVE}
The native status returned by the last pipe close, backtick
("``") command, successful call to "wait()" or "waitpid()", or
from the "system()" operator. On POSIX-like systems this value
can be decoded with the WIFEXITED, WEXITSTATUS, WIFSIGNALED,
WTERMSIG, WIFSTOPPED, WSTOPSIG and WIFCONTINUED functions
provided by the POSIX module.
Under VMS this reflects the actual VMS exit status; i.e. it is
the same as $? when the pragma "use vmsish 'status'" is in
effect.
This variable was added in Perl v5.10.0.
$EXTENDED_OS_ERROR
$^E Error information specific to the current operating system. At
the moment, this differs from "$!" under only VMS, OS/2, and
Win32 (and for MacPerl). On all other platforms, $^E is always
just the same as $!.
Under VMS, $^E provides the VMS status value from the last
system error. This is more specific information about the last
system error than that provided by $!. This is particularly
important when $! is set to EVMSERR.
Under OS/2, $^E is set to the error code of the last call to
OS/2 API either via CRT, or directly from perl.
Under Win32, $^E always returns the last error information
reported by the Win32 call "GetLastError()" which describes the
last error from within the Win32 API. Most Win32-specific code
will report errors via $^E. ANSI C and Unix-like calls set
"errno" and so most portable Perl code will report errors via
$!.
Caveats mentioned in the description of "$!" generally apply to
$^E, also.
This variable was added in Perl 5.003.
Mnemonic: Extra error explanation.
$EXCEPTIONS_BEING_CAUGHT
$^S Current state of the interpreter.
$^S State
--------- -------------------------------------
undef Parsing module, eval, or main program
true (1) Executing an eval
false (0) Otherwise
The first state may happen in $SIG{__DIE__} and $SIG{__WARN__}
handlers.
The English name $EXCEPTIONS_BEING_CAUGHT is slightly
misleading, because the "undef" value does not indicate whether
exceptions are being caught, since compilation of the main
program does not catch exceptions.
This variable was added in Perl 5.004.
$WARNING
$^W The current value of the warning switch, initially true if -w
was used, false otherwise, but directly modifiable.
See also warnings.
Mnemonic: related to the -w switch.
${^WARNING_BITS}
The current set of warning checks enabled by the "use warnings"
pragma. It has the same scoping as the $^H and "%^H"
variables. The exact values are considered internal to the
warnings pragma and may change between versions of Perl.
This variable was added in Perl v5.6.0.
$OS_ERROR
$ERRNO
$! When referenced, $! retrieves the current value of the C
"errno" integer variable. If $! is assigned a numerical value,
that value is stored in "errno". When referenced as a string,
$! yields the system error string corresponding to "errno".
Many system or library calls set "errno" if they fail, to
indicate the cause of failure. They usually do not set "errno"
to zero if they succeed. This means "errno", hence $!, is
meaningful only immediately after a failure:
if (open my $fh, "<", $filename) {
# Here $! is meaningless.
...
}
else {
# ONLY here is $! meaningful.
...
# Already here $! might be meaningless.
}
# Since here we might have either success or failure,
# $! is meaningless.
Here, meaningless means that $! may be unrelated to the outcome
of the "open()" operator. Assignment to $! is similarly
ephemeral. It can be used immediately before invoking the
"die()" operator, to set the exit value, or to inspect the
system error string corresponding to error n, or to restore $!
to a meaningful state.
Mnemonic: What just went bang?
%OS_ERROR
%ERRNO
%! Each element of "%!" has a true value only if $! is set to that
value. For example, $!{ENOENT} is true if and only if the
current value of $! is "ENOENT"; that is, if the most recent
error was "No such file or directory" (or its moral equivalent:
not all operating systems give that exact error, and certainly
not all languages). The specific true value is not guaranteed,
but in the past has generally been the numeric value of $!. To
check if a particular key is meaningful on your system, use
"exists $!{the_key}"; for a list of legal keys, use "keys %!".
See Errno for more information, and also see "$!".
This variable was added in Perl 5.005.
$CHILD_ERROR
$? The status returned by the last pipe close, backtick ("``")
command, successful call to "wait()" or "waitpid()", or from
the "system()" operator. This is just the 16-bit status word
returned by the traditional Unix "wait()" system call (or else
is made up to look like it). Thus, the exit value of the
subprocess is really ("$? >> 8"), and "$? & 127" gives which
signal, if any, the process died from, and "$? & 128" reports
whether there was a core dump.
Additionally, if the "h_errno" variable is supported in C, its
value is returned via $? if any "gethost*()" function fails.
If you have installed a signal handler for "SIGCHLD", the value
of $? will usually be wrong outside that handler.
Inside an "END" subroutine $? contains the value that is going
to be given to "exit()". You can modify $? in an "END"
subroutine to change the exit status of your program. For
example:
END {
$? = 1 if $? == 255; # die would make it 255
}
Under VMS, the pragma "use vmsish 'status'" makes $? reflect
the actual VMS exit status, instead of the default emulation of
POSIX status; see "$?" in perlvms for details.
Mnemonic: similar to sh and ksh.
$EVAL_ERROR
$@ The Perl error from the last "eval" operator, i.e. the last
exception that was caught. For "eval BLOCK", this is either a
runtime error message or the string or reference "die" was
called with. The "eval STRING" form also catches syntax errors
and other compile time exceptions.
If no error occurs, "eval" sets $@ to the empty string.
Warning messages are not collected in this variable. You can,
however, set up a routine to process warnings by setting
$SIG{__WARN__} as described in "%SIG".
Mnemonic: Where was the error "at"?
Variables related to the interpreter state
These variables provide information about the current interpreter
state.
$COMPILING
$^C The current value of the flag associated with the -c switch.
Mainly of use with -MO=... to allow code to alter its behavior
when being compiled, such as for example to "AUTOLOAD" at
compile time rather than normal, deferred loading. Setting
"$^C = 1" is similar to calling "B::minus_c".
This variable was added in Perl v5.6.0.
$DEBUGGING
$^D The current value of the debugging flags. May be read or set.
Like its command-line equivalent, you can use numeric or
symbolic values, e.g. "$^D = 10" or "$^D = "st"". See
"-Dnumber" in perlrun. The contents of this variable also
affects the debugger operation. See "Debugger Internals" in
perldebguts.
Mnemonic: value of -D switch.
${^ENCODING}
This variable is no longer supported.
It used to hold the object reference to the "Encode" object
that was used to convert the source code to Unicode.
Its purpose was to allow your non-ASCII Perl scripts not to
have to be written in UTF-8; this was useful before editors
that worked on UTF-8 encoded text were common, but that was
long ago. It caused problems, such as affecting the operation
of other modules that weren't expecting it, causing general
mayhem.
If you need something like this functionality, it is
recommended that use you a simple source filter, such as
Filter::Encoding.
If you are coming here because code of yours is being adversely
affected by someone's use of this variable, you can usually
work around it by doing this:
local ${^ENCODING};
near the beginning of the functions that are getting broken.
This undefines the variable during the scope of execution of
the including function.
This variable was added in Perl 5.8.2 and removed in 5.26.0.
Setting it to anything other than "undef" was made fatal in
Perl 5.28.0.
${^GLOBAL_PHASE}
The current phase of the perl interpreter.
Possible values are:
CONSTRUCT
The "PerlInterpreter*" is being constructed via
"perl_construct". This value is mostly there for
completeness and for use via the underlying C variable
"PL_phase". It's not really possible for Perl code to
be executed unless construction of the interpreter is
finished.
START This is the global compile-time. That includes,
basically, every "BEGIN" block executed directly or
indirectly from during the compile-time of the top-
level program.
This phase is not called "BEGIN" to avoid confusion
with "BEGIN"-blocks, as those are executed during
compile-time of any compilation unit, not just the top-
level program. A new, localised compile-time entered
at run-time, for example by constructs as "eval "use
SomeModule"" are not global interpreter phases, and
therefore aren't reflected by "${^GLOBAL_PHASE}".
CHECK Execution of any "CHECK" blocks.
INIT Similar to "CHECK", but for "INIT"-blocks, not "CHECK"
blocks.
RUN The main run-time, i.e. the execution of
"PL_main_root".
END Execution of any "END" blocks.
DESTRUCT
Global destruction.
Also note that there's no value for UNITCHECK-blocks. That's
because those are run for each compilation unit individually,
and therefore is not a global interpreter phase.
Not every program has to go through each of the possible
phases, but transition from one phase to another can only
happen in the order described in the above list.
An example of all of the phases Perl code can see:
BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }
INIT { print "init-time: ${^GLOBAL_PHASE}\n" }
CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }
{
package Print::Phase;
sub new {
my ($class, $time) = @_;
return bless \$time, $class;
}
sub DESTROY {
my $self = shift;
print "$$self: ${^GLOBAL_PHASE}\n";
}
}
print "run-time: ${^GLOBAL_PHASE}\n";
my $runtime = Print::Phase->new(
"lexical variables are garbage collected before END"
);
END { print "end-time: ${^GLOBAL_PHASE}\n" }
our $destruct = Print::Phase->new(
"package variables are garbage collected after END"
);
This will print out
compile-time: START
check-time: CHECK
init-time: INIT
run-time: RUN
lexical variables are garbage collected before END: RUN
end-time: END
package variables are garbage collected after END: DESTRUCT
This variable was added in Perl 5.14.0.
$^H WARNING: This variable is strictly for internal use only. Its
availability, behavior, and contents are subject to change
without notice.
This variable contains compile-time hints for the Perl
interpreter. At the end of compilation of a BLOCK the value of
this variable is restored to the value when the interpreter
started to compile the BLOCK.
When perl begins to parse any block construct that provides a
lexical scope (e.g., eval body, required file, subroutine body,
loop body, or conditional block), the existing value of $^H is
saved, but its value is left unchanged. When the compilation
of the block is completed, it regains the saved value. Between
the points where its value is saved and restored, code that
executes within BEGIN blocks is free to change the value of
$^H.
This behavior provides the semantic of lexical scoping, and is
used in, for instance, the "use strict" pragma.
The contents should be an integer; different bits of it are
used for different pragmatic flags. Here's an example:
sub add_100 { $^H |= 0x100 }
sub foo {
BEGIN { add_100() }
bar->baz($boon);
}
Consider what happens during execution of the BEGIN block. At
this point the BEGIN block has already been compiled, but the
body of "foo()" is still being compiled. The new value of $^H
will therefore be visible only while the body of "foo()" is
being compiled.
Substitution of "BEGIN { add_100() }" block with:
BEGIN { require strict; strict->import('vars') }
demonstrates how "use strict 'vars'" is implemented. Here's a
conditional version of the same lexical pragma:
BEGIN {
require strict; strict->import('vars') if $condition
}
This variable was added in Perl 5.003.
%^H The "%^H" hash provides the same scoping semantic as $^H. This
makes it useful for implementation of lexically scoped pragmas.
See perlpragma. All the entries are stringified when accessed
at runtime, so only simple values can be accommodated. This
means no pointers to objects, for example.
When putting items into "%^H", in order to avoid conflicting
with other users of the hash there is a convention regarding
which keys to use. A module should use only keys that begin
with the module's name (the name of its main package) and a "/"
character. For example, a module "Foo::Bar" should use keys
such as "Foo::Bar/baz".
This variable was added in Perl v5.6.0.
${^OPEN}
An internal variable used by PerlIO. A string in two parts,
separated by a "\0" byte, the first part describes the input
layers, the second part describes the output layers.
This variable was added in Perl v5.8.0.
$PERLDB
$^P The internal variable for debugging support. The meanings of
the various bits are subject to change, but currently indicate:
0x01 Debug subroutine enter/exit.
0x02 Line-by-line debugging. Causes "DB::DB()" subroutine to
be called for each statement executed. Also causes
saving source code lines (like 0x400).
0x04 Switch off optimizations.
0x08 Preserve more data for future interactive inspections.
0x10 Keep info about source lines on which a subroutine is
defined.
0x20 Start with single-step on.
0x40 Use subroutine address instead of name when reporting.
0x80 Report "goto &subroutine" as well.
0x100 Provide informative "file" names for evals based on the
place they were compiled.
0x200 Provide informative names to anonymous subroutines based
on the place they were compiled.
0x400 Save source code lines into "@{"_<$filename"}".
0x800 When saving source, include evals that generate no
subroutines.
0x1000
When saving source, include source that did not compile.
Some bits may be relevant at compile-time only, some at run-
time only. This is a new mechanism and the details may change.
See also perldebguts.
${^TAINT}
Reflects if taint mode is on or off. 1 for on (the program was
run with -T), 0 for off, -1 when only taint warnings are
enabled (i.e. with -t or -TU).
This variable is read-only.
This variable was added in Perl v5.8.0.
${^SAFE_LOCALES}
Reflects if safe locale operations are available to this perl
(when the value is 1) or not (the value is 0). This variable
is always 1 if the perl has been compiled without threads. It
is also 1 if this perl is using thread-safe locale operations.
Note that an individual thread may choose to use the global
locale (generally unsafe) by calling "switch_to_global_locale"
in perlapi. This variable currently is still set to 1 in such
threads.
This variable is read-only.
This variable was added in Perl v5.28.0.
${^UNICODE}
Reflects certain Unicode settings of Perl. See perlrun
documentation for the "-C" switch for more information about
the possible values.
This variable is set during Perl startup and is thereafter
read-only.
This variable was added in Perl v5.8.2.
${^UTF8CACHE}
This variable controls the state of the internal UTF-8 offset
caching code. 1 for on (the default), 0 for off, -1 to debug
the caching code by checking all its results against linear
scans, and panicking on any discrepancy.
This variable was added in Perl v5.8.9. It is subject to
change or removal without notice, but is currently used to
avoid recalculating the boundaries of multi-byte UTF-8-encoded
characters.
${^UTF8LOCALE}
This variable indicates whether a UTF-8 locale was detected by
perl at startup. This information is used by perl when it's in
adjust-utf8ness-to-locale mode (as when run with the "-CL"
command-line switch); see perlrun for more info on this.
This variable was added in Perl v5.8.8.
Deprecated and removed variables
Deprecating a variable announces the intent of the perl maintainers to
eventually remove the variable from the language. It may still be
available despite its status. Using a deprecated variable triggers a
warning.
Once a variable is removed, its use triggers an error telling you the
variable is unsupported.
See perldiag for details about error messages.
$# $# was a variable that could be used to format printed numbers.
After a deprecation cycle, its magic was removed in Perl
v5.10.0 and using it now triggers a warning: "$# is no longer
supported".
This is not the sigil you use in front of an array name to get
the last index, like $#array. That's still how you get the
last index of an array in Perl. The two have nothing to do
with each other.
Deprecated in Perl 5.
Removed in Perl v5.10.0.
$* $* was a variable that you could use to enable multiline
matching. After a deprecation cycle, its magic was removed in
Perl v5.10.0. Using it now triggers a warning: "$* is no
longer supported". You should use the "/s" and "/m" regexp
modifiers instead.
Deprecated in Perl 5.
Removed in Perl v5.10.0.
$[ This variable stores the index of the first element in an
array, and of the first character in a substring. The default
is 0, but you could theoretically set it to 1 to make Perl
behave more like awk (or Fortran) when subscripting and when
evaluating the index() and substr() functions.
As of release 5 of Perl, assignment to $[ is treated as a
compiler directive, and cannot influence the behavior of any
other file. (That's why you can only assign compile-time
constants to it.) Its use is highly discouraged.
Prior to Perl v5.10.0, assignment to $[ could be seen from
outer lexical scopes in the same file, unlike other compile-
time directives (such as strict). Using local() on it would
bind its value strictly to a lexical block. Now it is always
lexically scoped.
As of Perl v5.16.0, it is implemented by the arybase module.
As of Perl v5.30.0, or under "use v5.16", or "no feature
"array_base"", $[ no longer has any effect, and always contains
0. Assigning 0 to it is permitted, but any other value will
produce an error.
Mnemonic: [ begins subscripts.
Deprecated in Perl v5.12.0.
perl v5.30.0 2023-11-23 PERLVAR(1)
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