| PERLVAR(1) | Perl Programmers Reference Guide | PERLVAR(1) |
perlvar - Perl predefined variables
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 "use utf8" 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 using the demarcated variable form using curly braces such as "${^Foo}"; the braces are not optional. "${^Foo}" denotes the scalar variable whose name is considered to be a control-"F" followed by two "o"'s. (See "Demarcated variable names using braces" in perldata for more information on this form of spelling a variable name or specifying access to an element of an array or a hash). 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. $^_ itself, however, is reserved.
Note that you also must use the demarcated form to access subscripts of variables of this type when interpolating, for instance to access the first element of the "@{^CAPTURE}" variable inside of a double quoted string you would write "${^CAPTURE[0]}" and NOT "${^CAPTURE}[0]" which would mean to reference a scalar variable named "${^CAPTURE}" and not index 0 of the magic "@{^CAPTURE}" array which is populated by the regex engine.
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 "${^_XYZ}" variables are always taken to be in package "main", regardless of any "package" declarations presently in scope.
The following names have special meaning to Perl. Most punctuation names have reasonable mnemonics, or analogs in the shells. Nevertheless, if you wish to use long variable names, you need only say:
use English;
at the top of your program. This aliases all the short names to the long names in the current package. Some even have medium names, generally borrowed from awk. For more info, please see English.
Before you continue, note the sort order for variables. In general, we first list the variables in case-insensitive, almost-lexigraphical order (ignoring the "{" or "^" preceding words, as in "${^UNICODE}" or $^T), although $_ and @_ move up to the top of the pile. For variables with the same identifier, we list it in order of scalar, array, hash, and bareword.
while (<>) {...} # equivalent only in while!
while (defined($_ = <>)) {...}
/^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:
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.
$_ 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 $_". This experimental feature was removed and is now a fatal error, but you may encounter it in older code.
Mnemonic: underline is understood in certain operations.
See perlsub.
print "The array is: @array\n";
is equivalent to this:
print "The array is: " . join($", @array) . "\n";
Mnemonic: works in double-quoted context.
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.
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.
Wide characters are invalid in $0 values. For historical reasons, though, Perl accepts them and encodes them to UTF-8. When this happens a wide-character warning is triggered.
Mnemonic: same as sh and ksh.
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.
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.
Mnemonic: it's the uid you came from, if you're running setuid.
$< = $>; # set real to effective uid
($<,$>) = ($>,$<); # 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.
$foo{$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.
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 $ENV{'bar'} ? 'ref' : 'non ref' ) /);
This happens because you can't really share arbitrary data structures with foreign processes.
This variable can be used to determine whether the Perl interpreter executing a script is in the right range of versions:
warn "No PerlIO!\n" if "$]" < 5.008;
When comparing $], numeric comparison operators should be used, but the variable should be stringified first to avoid issues where its original numeric value is inaccurate.
See also the documentation of "use VERSION" and "require VERSION" for a convenient way to fail if the running Perl interpreter is too old.
See "$^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 are subject to the binary floating point representation; it's good for numeric 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?
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.
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.
Mnemonic: value of -i switch.
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 = '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.
The value is identical to $Config{'osname'}. See also Config and the -V command-line switch documented in perlrun.
In Windows platforms, $^O is not very helpful: since it is always "MSWin32", it doesn't tell the difference between 95/98/ME/NT/2000/XP/CE/.NET. Use "Win32::GetOSName()" or Win32::GetOSVersion() (see Win32 and perlport) to distinguish between the variants.
This variable was added in Perl 5.003.
sub handler { # 1st argument is signal name
my($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. Using an empty string or "undef" as the value has the same effect as 'DEFAULT'.
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.
Using a string that doesn't correspond to any existing function or a glob that doesn't contain a code slot is equivalent to 'IGNORE', but a warning is emitted when the handler is being called (the warning is not emitted for the internal hooks described below).
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__", its effect is the same as using 'DEFAULT'. 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.
The $SIG{__DIE__} doesn't support 'IGNORE'; it has the same effect as 'DEFAULT'.
"__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.
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.
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;
}
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.
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.
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, $2, ...)", "%{^CAPTURE}" and "%{^CAPTURE_ALL}".
Note that unlike most other regex magic variables there is no single letter equivalent to "@{^CAPTURE}". Also be aware that when interpolating subscripts of this array you must use the demarcated variable form, for instance
print "${^CAPTURE[0]}"
see "Demarcated variable names using braces" in perldata for more information on this form and its uses.
This variable was added in 5.25.7
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.
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 $MATCH.
This variable was added in Perl v5.10.0.
This variable is read-only and dynamically-scoped.
See "Performance issues" above for the serious performance implications of using this variable (even once) in your code.
This variable is read-only and dynamically-scoped.
Mnemonic: "`" often precedes a quoted string.
See "Performance issues" above.
In Perl v5.18 and earlier, it is only guaranteed to return a defined value when the pattern was compiled or executed with the "/p" modifier. In Perl v5.20, the "/p" modifier does nothing, so "${^PREMATCH}" does the same thing as $PREMATCH.
This variable was added in Perl v5.10.0.
This variable is read-only and dynamically-scoped.
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.
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 $POSTMATCH.
This variable was added in Perl v5.10.0.
This variable is read-only and dynamically-scoped.
This is useful if you don't know which one of a set of alternative patterns matched. For example:
/Version: (.*)|Revision: (.*)/ && ($rev = $+);
This variable is read-only and dynamically-scoped.
Mnemonic: be positive and forward looking.
"ab" =~ /^((.)(.))$/
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.
This variable was added in Perl v5.6.0.
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.
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:
This variable was added in Perl v5.6.0.
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.
This variable was added in Perl 5.005.
This variable was added in Perl v5.30.0.
This variable was added in Perl v5.10.0.
This variable was added in Perl v5.10.0.
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.
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.
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.
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 one or more consecutive newlines 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.
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.
Mnemonic: when you want your pipes to be piping hot.
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.
You cannot call "format_formfeed()" on a handle, only as a static method. See IO::Handle.
Mnemonic: "%" is page number in nroff.
Mnemonic: lines_on_page - lines_printed.
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.
Mnemonic: = has horizontal lines.
Mnemonic: points to top of page.
Mnemonic: brother to $^.
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 $?.
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.
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.
$^S State
--------- -------------------------------------
undef Parsing module, eval, or main program
true (1) Executing an eval or try block
false (0) Otherwise
The first state may happen in $SIG{__DIE__} and $SIG{__WARN__} handlers.
The English name $EXCEPTIONS_BEING_CAUGHT is slightly misleading, because the "undef" value does not indicate whether exceptions are being caught, since compilation of the main program does not catch exceptions.
This variable was added in Perl 5.004.
See also warnings.
Mnemonic: related to the -w switch.
Each time a statement completes being compiled, the current value of "${^WARNING_BITS}" is stored with that statement, and can later be retrieved via "(caller($level))[9]".
This variable was added in Perl v5.6.0.
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 and may set "errno" to a non-zero value on success. 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.
Perl itself may set "errno" to a non-zero on failure even if no system call is performed.
Mnemonic: What just went bang?
This variable was added in Perl 5.005.
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.
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 $SIG{__WARN__} as described in "%SIG".
Mnemonic: Where was the error "at"?
These variables provide information about the current interpreter state.
This variable was added in Perl v5.6.0.
Mnemonic: value of -D switch.
Possible values are:
This phase is not called "BEGIN" to avoid confusion with "BEGIN"-blocks, as those are executed during compile-time of any compilation unit, not just the top-level program. A new, localised compile-time entered at run-time, for example by constructs as "eval "use SomeModule"" are not global interpreter phases, and therefore aren't reflected by "${^GLOBAL_PHASE}".
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.
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.
Each time a statement completes being compiled, the current value of $^H is stored with that statement, and can later be retrieved via "(caller($level))[8]".
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 $condition
}
This variable was added in Perl 5.003.
Each time a statement completes being compiled, the current value of "%^H" is stored with that statement, and can later be retrieved via "(caller($level))[10]".
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.
This is the mechanism that applies the lexical effects of the open pragma, and the main program scope effects of the "io" or "D" options for the -C command-line switch and PERL_UNICODE environment variable.
The functions "accept()", "open()", "pipe()", "readpipe()" (as well as the related "qx" and "`STRING`" operators), "socket()", "socketpair()", and "sysopen()" are affected by the lexical value of this variable. The implicit "ARGV" handle opened by "readline()" (or the related "<>" and "<<>>" operators) on passed filenames is also affected (but not if it opens "STDIN"). If this variable is not set, these functions will set the default layers as described in "Defaults and how to override them" in PerlIO.
"open()" ignores this variable (and the default layers) when called with 3 arguments and explicit layers are specified. Indirect calls to these functions via modules like IO::Handle are not affected as they occur in a different lexical scope. Directory handles such as opened by "opendir()" are not currently affected.
This variable was added in Perl v5.8.0.
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.
Note: if your perl was built without taint support (see perlsec), then "${^TAINT}" will always be 0, even if the program was run with -T).
This variable is read-only.
This variable was added in Perl v5.8.0.
This variable is read-only.
This variable was added in Perl v5.28.0.
This variable is set during Perl startup and is thereafter read-only.
This variable was added in Perl v5.8.2.
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.
This variable was added in Perl v5.8.8.
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.
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.
Deprecated in Perl 5.
Removed in Perl v5.10.0.
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.
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.
This variable was added in Perl v5.10.0 and removed in Perl v5.34.0.
| 2023-11-25 | perl v5.36.0 |