3 perlvar - Perl predefined variables
7 =head2 The Syntax of Variable Names
9 Variable names in Perl can have several formats. Usually, they
10 must begin with a letter or underscore, in which case they can be
11 arbitrarily long (up to an internal limit of 251 characters) and
12 may contain letters, digits, underscores, or the special sequence
13 C<::> or C<'>. In this case, the part before the last C<::> or
14 C<'> is taken to be a I<package qualifier>; see L<perlmod>.
16 Perl variable names may also be a sequence of digits or a single
17 punctuation or control character. These names are all reserved for
18 special uses by Perl; for example, the all-digits names are used
19 to hold data captured by backreferences after a regular expression
20 match. Perl has a special syntax for the single-control-character
21 names: It understands C<^X> (caret C<X>) to mean the control-C<X>
22 character. For example, the notation C<$^W> (dollar-sign caret
23 C<W>) is the scalar variable whose name is the single character
24 control-C<W>. This is better than typing a literal control-C<W>
27 Since Perl v5.6.0, Perl variable names may be alphanumeric
28 strings that begin with control characters (or better yet, a caret).
29 These variables must be written in the form C<${^Foo}>; the braces
30 are not optional. C<${^Foo}> denotes the scalar variable whose
31 name is a control-C<F> followed by two C<o>'s. These variables are
32 reserved for future special uses by Perl, except for the ones that
33 begin with C<^_> (control-underscore or caret-underscore). No
34 control-character name that begins with C<^_> will acquire a special
35 meaning in any future version of Perl; such names may therefore be
36 used safely in programs. C<$^_> itself, however, I<is> reserved.
38 Perl identifiers that begin with digits, control characters, or
39 punctuation characters are exempt from the effects of the C<package>
40 declaration and are always forced to be in package C<main>; they are
41 also exempt from C<strict 'vars'> errors. A few other names are also
50 In particular, the special C<${^_XYZ}> variables are always taken
51 to be in package C<main>, regardless of any C<package> declarations
54 =head1 SPECIAL VARIABLES
56 The following names have special meaning to Perl. Most punctuation
57 names have reasonable mnemonics, or analogs in the shells.
58 Nevertheless, if you wish to use long variable names, you need only say:
62 at the top of your program. This aliases all the short names to the long
63 names in the current package. Some even have medium names, generally
64 borrowed from B<awk>. To avoid a performance hit, if you don't need the
65 C<$PREMATCH>, C<$MATCH>, or C<$POSTMATCH> it's best to use the C<English>
68 use English '-no_match_vars';
70 Before you continue, note the sort order for variables. In general, we
71 first list the variables in case-insensitive, almost-lexigraphical
72 order (ignoring the C<{> or C<^> preceding words, as in C<${^UNICODE}>
73 or C<$^T>), although C<$_> and C<@_> move up to the top of the pile.
74 For variables with the same identifier, we list it in order of scalar,
75 array, hash, and bareword.
77 =head2 General Variables
86 The default input and pattern-searching space. The following pairs are
89 while (<>) {...} # equivalent only in while!
90 while (defined($_ = <>)) {...}
101 Here are the places where Perl will assume C<$_> even if you don't use it:
107 The following functions use C<$_> as a default argument:
109 abs, alarm, chomp, chop, chr, chroot,
110 cos, defined, eval, evalbytes, exp, fc, glob, hex, int, lc,
111 lcfirst, length, log, lstat, mkdir, oct, ord, pos, print, printf,
112 quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only),
113 rmdir, say, sin, split (for its second
114 argument), sqrt, stat, study, uc, ucfirst,
119 All file tests (C<-f>, C<-d>) except for C<-t>, which defaults to STDIN.
124 The pattern matching operations C<m//>, C<s///> and C<tr///> (aka C<y///>)
125 when used without an C<=~> operator.
129 The default iterator variable in a C<foreach> loop if no other
130 variable is supplied.
134 The implicit iterator variable in the C<grep()> and C<map()> functions.
138 The implicit variable of C<given()>.
142 The default place to put the next value or input record
143 when a C<< <FH> >>, C<readline>, C<readdir> or C<each>
144 operation's result is tested by itself as the sole criterion of a C<while>
145 test. Outside a C<while> test, this will not happen.
149 C<$_> is by default a global variable. However, as
150 of perl v5.10.0, you can use a lexical version of
151 C<$_> by declaring it in a file or in a block with C<my>. Moreover,
152 declaring C<our $_> restores the global C<$_> in the current scope. Though
153 this seemed like a good idea at the time it was introduced, lexical C<$_>
154 actually causes more problems than it solves. If you call a function that
155 expects to be passed information via C<$_>, it may or may not work,
156 depending on how the function is written, there not being any easy way to
157 solve this. Just avoid lexical C<$_>, unless you are feeling particularly
160 Mnemonic: underline is understood in certain operations.
167 Within a subroutine the array C<@_> contains the parameters passed to
168 that subroutine. Inside a subroutine, C<@_> is the default array for
169 the array operators C<push>, C<pop>, C<shift>, and C<unshift>.
173 =item $LIST_SEPARATOR
176 X<$"> X<$LIST_SEPARATOR>
178 When an array or an array slice is interpolated into a double-quoted
179 string or a similar context such as C</.../>, its elements are
180 separated by this value. Default is a space. For example, this:
182 print "The array is: @array\n";
184 is equivalent to this:
186 print "The array is: " . join($", @array) . "\n";
188 Mnemonic: works in double-quoted context.
195 X<$$> X<$PID> X<$PROCESS_ID>
197 The process number of the Perl running this script. Though you I<can> set
198 this variable, doing so is generally discouraged, although it can be
199 invaluable for some testing purposes. It will be reset automatically
200 across C<fork()> calls.
202 Note for Linux and Debian GNU/kFreeBSD users: Before Perl v5.16.0 perl
203 would emulate POSIX semantics on Linux systems using LinuxThreads, a
204 partial implementation of POSIX Threads that has since been superseded
205 by the Native POSIX Thread Library (NPTL).
207 LinuxThreads is now obsolete on Linux, and caching C<getpid()>
208 like this made embedding perl unnecessarily complex (since you'd have
209 to manually update the value of $$), so now C<$$> and C<getppid()>
210 will always return the same values as the underlying C library.
212 Debian GNU/kFreeBSD systems also used LinuxThreads up until and
213 including the 6.0 release, but after that moved to FreeBSD thread
214 semantics, which are POSIX-like.
216 To see if your system is affected by this discrepancy check if
217 C<getconf GNU_LIBPTHREAD_VERSION | grep -q NPTL> returns a false
218 value. NTPL threads preserve the POSIX semantics.
220 Mnemonic: same as shells.
225 X<$0> X<$PROGRAM_NAME>
227 Contains the name of the program being executed.
229 On some (but not all) operating systems assigning to C<$0> modifies
230 the argument area that the C<ps> program sees. On some platforms you
231 may have to use special C<ps> options or a different C<ps> to see the
232 changes. Modifying the C<$0> is more useful as a way of indicating the
233 current program state than it is for hiding the program you're
236 Note that there are platform-specific limitations on the maximum
237 length of C<$0>. In the most extreme case it may be limited to the
238 space occupied by the original C<$0>.
240 In some platforms there may be arbitrary amount of padding, for
241 example space characters, after the modified name as shown by C<ps>.
242 In some platforms this padding may extend all the way to the original
243 length of the argument area, no matter what you do (this is the case
244 for example with Linux 2.2).
246 Note for BSD users: setting C<$0> does not completely remove "perl"
247 from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
248 result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
249 and the " (perl)" suffix are shown depends on your exact BSD variant
250 and version). This is an operating system feature, Perl cannot help it.
252 In multithreaded scripts Perl coordinates the threads so that any
253 thread may modify its copy of the C<$0> and the change becomes visible
254 to ps(1) (assuming the operating system plays along). Note that
255 the view of C<$0> the other threads have will not change since they
256 have their own copies of it.
258 If the program has been given to perl via the switches C<-e> or C<-E>,
259 C<$0> will contain the string C<"-e">.
261 On Linux as of perl v5.14.0 the legacy process name will be set with
262 C<prctl(2)>, in addition to altering the POSIX name via C<argv[0]> as
263 perl has done since version 4.000. Now system utilities that read the
264 legacy process name such as ps, top and killall will recognize the
265 name you set when assigning to C<$0>. The string you supply will be
266 cut off at 16 bytes, this is a limitation imposed by Linux.
268 Mnemonic: same as B<sh> and B<ksh>.
275 X<$(> X<$GID> X<$REAL_GROUP_ID>
277 The real gid of this process. If you are on a machine that supports
278 membership in multiple groups simultaneously, gives a space separated
279 list of groups you are in. The first number is the one returned by
280 C<getgid()>, and the subsequent ones by C<getgroups()>, one of which may be
281 the same as the first number.
283 However, a value assigned to C<$(> must be a single number used to
284 set the real gid. So the value given by C<$(> should I<not> be assigned
285 back to C<$(> without being forced numeric, such as by adding zero. Note
286 that this is different to the effective gid (C<$)>) which does take a
289 You can change both the real gid and the effective gid at the same
290 time by using C<POSIX::setgid()>. Changes
291 to C<$(> require a check to C<$!>
292 to detect any possible errors after an attempted change.
294 Mnemonic: parentheses are used to I<group> things. The real gid is the
295 group you I<left>, if you're running setgid.
297 =item $EFFECTIVE_GROUP_ID
302 X<$)> X<$EGID> X<$EFFECTIVE_GROUP_ID>
304 The effective gid of this process. If you are on a machine that
305 supports membership in multiple groups simultaneously, gives a space
306 separated list of groups you are in. The first number is the one
307 returned by C<getegid()>, and the subsequent ones by C<getgroups()>,
308 one of which may be the same as the first number.
310 Similarly, a value assigned to C<$)> must also be a space-separated
311 list of numbers. The first number sets the effective gid, and
312 the rest (if any) are passed to C<setgroups()>. To get the effect of an
313 empty list for C<setgroups()>, just repeat the new effective gid; that is,
314 to force an effective gid of 5 and an effectively empty C<setgroups()>
315 list, say C< $) = "5 5" >.
317 You can change both the effective gid and the real gid at the same
318 time by using C<POSIX::setgid()> (use only a single numeric argument).
319 Changes to C<$)> require a check to C<$!> to detect any possible errors
320 after an attempted change.
322 C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
323 machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
324 and C<$)> can be swapped only on machines supporting C<setregid()>.
326 Mnemonic: parentheses are used to I<group> things. The effective gid
327 is the group that's I<right> for you, if you're running setgid.
334 X<< $< >> X<$UID> X<$REAL_USER_ID>
336 The real uid of this process. You can change both the real uid and the
337 effective uid at the same time by using C<POSIX::setuid()>. Since
338 changes to C<< $< >> require a system call, check C<$!> after a change
339 attempt to detect any possible errors.
341 Mnemonic: it's the uid you came I<from>, if you're running setuid.
343 =item $EFFECTIVE_USER_ID
348 X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID>
350 The effective uid of this process. For example:
352 $< = $>; # set real to effective uid
353 ($<,$>) = ($>,$<); # swap real and effective uids
355 You can change both the effective uid and the real uid at the same
356 time by using C<POSIX::setuid()>. Changes to C<< $> >> require a check
357 to C<$!> to detect any possible errors after an attempted change.
359 C<< $< >> and C<< $> >> can be swapped only on machines
360 supporting C<setreuid()>.
362 Mnemonic: it's the uid you went I<to>, if you're running setuid.
364 =item $SUBSCRIPT_SEPARATOR
369 X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR>
371 The subscript separator for multidimensional array emulation. If you
372 refer to a hash element as
378 $foo{join($;, $a, $b, $c)}
382 @foo{$a,$b,$c} # a slice--note the @
386 ($foo{$a},$foo{$b},$foo{$c})
388 Default is "\034", the same as SUBSEP in B<awk>. If your keys contain
389 binary data there might not be any safe value for C<$;>.
391 Consider using "real" multidimensional arrays as described
394 Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
401 Special package variables when using C<sort()>, see L<perlfunc/sort>.
402 Because of this specialness C<$a> and C<$b> don't need to be declared
403 (using C<use vars>, or C<our()>) even when using the C<strict 'vars'>
404 pragma. Don't lexicalize them with C<my $a> or C<my $b> if you want to
405 be able to use them in the C<sort()> comparison block or function.
410 The hash C<%ENV> contains your current environment. Setting a
411 value in C<ENV> changes the environment for any child processes
412 you subsequently C<fork()> off.
417 X<$^F> X<$SYSTEM_FD_MAX>
419 The maximum system file descriptor, ordinarily 2. System file
420 descriptors are passed to C<exec()>ed processes, while higher file
421 descriptors are not. Also, during an
422 C<open()>, system file descriptors are
423 preserved even if the C<open()> fails (ordinary file descriptors are
424 closed before the C<open()> is attempted). The close-on-exec
425 status of a file descriptor will be decided according to the value of
426 C<$^F> when the corresponding file, pipe, or socket was opened, not the
427 time of the C<exec()>.
432 The array C<@F> contains the fields of each line read in when autosplit
433 mode is turned on. See L<perlrun> for the B<-a> switch. This array
434 is package-specific, and must be declared or given a full package name
435 if not in package main when running under C<strict 'vars'>.
440 The array C<@INC> contains the list of places that the C<do EXPR>,
441 C<require>, or C<use> constructs look for their library files. It
442 initially consists of the arguments to any B<-I> command-line
443 switches, followed by the default Perl library, probably
444 F</usr/local/lib/perl>, followed by ".", to represent the current
445 directory. ("." will not be appended if taint checks are enabled,
446 either by C<-T> or by C<-t>.) If you need to modify this at runtime,
447 you should use the C<use lib> pragma to get the machine-dependent
448 library properly loaded also:
450 use lib '/mypath/libdir/';
453 You can also insert hooks into the file inclusion system by putting Perl
454 code directly into C<@INC>. Those hooks may be subroutine references,
455 array references or blessed objects. See L<perlfunc/require> for details.
460 The hash C<%INC> contains entries for each filename included via the
461 C<do>, C<require>, or C<use> operators. The key is the filename
462 you specified (with module names converted to pathnames), and the
463 value is the location of the file found. The C<require>
464 operator uses this hash to determine whether a particular file has
465 already been included.
467 If the file was loaded via a hook (e.g. a subroutine reference, see
468 L<perlfunc/require> for a description of these hooks), this hook is
469 by default inserted into C<%INC> in place of a filename. Note, however,
470 that the hook may have set the C<%INC> entry by itself to provide some more
476 X<$^I> X<$INPLACE_EDIT>
478 The current value of the inplace-edit extension. Use C<undef> to disable
481 Mnemonic: value of B<-i> switch.
486 By default, running out of memory is an untrappable, fatal error.
487 However, if suitably built, Perl can use the contents of C<$^M>
488 as an emergency memory pool after C<die()>ing. Suppose that your Perl
489 were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
492 $^M = 'a' x (1 << 16);
494 would allocate a 64K buffer for use in an emergency. See the
495 F<INSTALL> file in the Perl distribution for information on how to
496 add custom C compilation flags when compiling perl. To discourage casual
497 use of this advanced feature, there is no L<English|English> long name for
500 This variable was added in Perl 5.004.
507 The name of the operating system under which this copy of Perl was
508 built, as determined during the configuration process. For examples
509 see L<perlport/PLATFORMS>.
511 The value is identical to C<$Config{'osname'}>. See also L<Config>
512 and the B<-V> command-line switch documented in L<perlrun>.
514 In Windows platforms, C<$^O> is not very helpful: since it is always
515 C<MSWin32>, it doesn't tell the difference between
516 95/98/ME/NT/2000/XP/CE/.NET. Use C<Win32::GetOSName()> or
517 Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
518 between the variants.
520 This variable was added in Perl 5.003.
525 The hash C<%SIG> contains signal handlers for signals. For example:
527 sub handler { # 1st argument is signal name
529 print "Caught a SIG$sig--shutting down\n";
534 $SIG{'INT'} = \&handler;
535 $SIG{'QUIT'} = \&handler;
537 $SIG{'INT'} = 'DEFAULT'; # restore default action
538 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
540 Using a value of C<'IGNORE'> usually has the effect of ignoring the
541 signal, except for the C<CHLD> signal. See L<perlipc> for more about
544 Here are some other examples:
546 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not
548 $SIG{"PIPE"} = \&Plumber; # just fine; assume current
550 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
551 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber()
554 Be sure not to use a bareword as the name of a signal handler,
555 lest you inadvertently call it.
557 If your system has the C<sigaction()> function then signal handlers
558 are installed using it. This means you get reliable signal handling.
560 The default delivery policy of signals changed in Perl v5.8.0 from
561 immediate (also known as "unsafe") to deferred, also known as "safe
562 signals". See L<perlipc> for more information.
564 Certain internal hooks can be also set using the C<%SIG> hash. The
565 routine indicated by C<$SIG{__WARN__}> is called when a warning
566 message is about to be printed. The warning message is passed as the
567 first argument. The presence of a C<__WARN__> hook causes the
568 ordinary printing of warnings to C<STDERR> to be suppressed. You can
569 use this to save warnings in a variable, or turn warnings into fatal
572 local $SIG{__WARN__} = sub { die $_[0] };
575 As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
576 disable warnings using the empty subroutine:
578 local $SIG{__WARN__} = sub {};
580 The routine indicated by C<$SIG{__DIE__}> is called when a fatal
581 exception is about to be thrown. The error message is passed as the
582 first argument. When a C<__DIE__> hook routine returns, the exception
583 processing continues as it would have in the absence of the hook,
584 unless the hook routine itself exits via a C<goto &sub>, a loop exit,
585 or a C<die()>. The C<__DIE__> handler is explicitly disabled during
586 the call, so that you can die from a C<__DIE__> handler. Similarly
589 Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
590 even inside an C<eval()>. Do not use this to rewrite a pending
591 exception in C<$@>, or as a bizarre substitute for overriding
592 C<CORE::GLOBAL::die()>. This strange action at a distance may be fixed
593 in a future release so that C<$SIG{__DIE__}> is only called if your
594 program is about to exit, as was the original intent. Any other use is
597 C<__DIE__>/C<__WARN__> handlers are very special in one respect: they
598 may be called to report (probable) errors found by the parser. In such
599 a case the parser may be in inconsistent state, so any attempt to
600 evaluate Perl code from such a handler will probably result in a
601 segfault. This means that warnings or errors that result from parsing
602 Perl should be used with extreme caution, like this:
604 require Carp if defined $^S;
605 Carp::confess("Something wrong") if defined &Carp::confess;
606 die "Something wrong, but could not load Carp to give "
608 . "To see backtrace try starting Perl with -MCarp switch";
610 Here the first line will load C<Carp> I<unless> it is the parser who
611 called the handler. The second line will print backtrace and die if
612 C<Carp> was available. The third line will be executed only if C<Carp> was
615 Having to even think about the C<$^S> variable in your exception
616 handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
617 invites grievous and difficult to track down errors. Avoid it
618 and use an C<END{}> or CORE::GLOBAL::die override instead.
620 See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
621 L<warnings> for additional information.
628 The time at which the program began running, in seconds since the
629 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
630 and B<-C> filetests are based on this value.
635 X<$^V> X<$PERL_VERSION>
637 The revision, version, and subversion of the Perl interpreter,
638 represented as a C<version> object.
640 This variable first appeared in perl v5.6.0; earlier versions of perl
641 will see an undefined value. Before perl v5.10.0 C<$^V> was represented
644 C<$^V> can be used to determine whether the Perl interpreter executing
645 a script is in the right range of versions. For example:
647 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
649 To convert C<$^V> into its string representation use C<sprintf()>'s
652 printf "version is v%vd\n", $^V; # Perl's version
654 See the documentation of C<use VERSION> and C<require VERSION>
655 for a convenient way to fail if the running Perl interpreter is too old.
657 See also C<$]> for an older representation of the Perl version.
659 This variable was added in Perl v5.6.0.
661 Mnemonic: use ^V for Version Control.
663 =item ${^WIN32_SLOPPY_STAT}
664 X<${^WIN32_SLOPPY_STAT}> X<sitecustomize> X<sitecustomize.pl>
666 If this variable is set to a true value, then C<stat()> on Windows will
667 not try to open the file. This means that the link count cannot be
668 determined and file attributes may be out of date if additional
669 hardlinks to the file exist. On the other hand, not opening the file
670 is considerably faster, especially for files on network drives.
672 This variable could be set in the F<sitecustomize.pl> file to
673 configure the local Perl installation to use "sloppy" C<stat()> by
674 default. See the documentation for B<-f> in
675 L<perlrun|perlrun/"Command Switches"> for more information about site
678 This variable was added in Perl v5.10.0.
680 =item $EXECUTABLE_NAME
683 X<$^X> X<$EXECUTABLE_NAME>
685 The name used to execute the current copy of Perl, from C's
686 C<argv[0]> or (where supported) F</proc/self/exe>.
688 Depending on the host operating system, the value of C<$^X> may be
689 a relative or absolute pathname of the perl program file, or may
690 be the string used to invoke perl but not the pathname of the
691 perl program file. Also, most operating systems permit invoking
692 programs that are not in the PATH environment variable, so there
693 is no guarantee that the value of C<$^X> is in PATH. For VMS, the
694 value may or may not include a version number.
696 You usually can use the value of C<$^X> to re-invoke an independent
697 copy of the same perl that is currently running, e.g.,
699 @first_run = `$^X -le "print int rand 100 for 1..100"`;
701 But recall that not all operating systems support forking or
702 capturing of the output of commands, so this complex statement
705 It is not safe to use the value of C<$^X> as a path name of a file,
706 as some operating systems that have a mandatory suffix on
707 executable files do not require use of the suffix when invoking
708 a command. To convert the value of C<$^X> to a path name, use the
709 following statements:
711 # Build up a set of file names (not command names).
715 $this_perl .= $Config{_exe}
716 unless $this_perl =~ m/$Config{_exe}$/i;
719 Because many operating systems permit anyone with read access to
720 the Perl program file to make a copy of it, patch the copy, and
721 then execute the copy, the security-conscious Perl programmer
722 should take care to invoke the installed copy of perl, not the
723 copy referenced by C<$^X>. The following statements accomplish
724 this goal, and produce a pathname that can be invoked as a
725 command or referenced as a file.
728 my $secure_perl_path = $Config{perlpath};
730 $secure_perl_path .= $Config{_exe}
731 unless $secure_perl_path =~ m/$Config{_exe}$/i;
736 =head2 Variables related to regular expressions
738 Most of the special variables related to regular expressions are side
739 effects. Perl sets these variables when it has a successful match, so
740 you should check the match result before using them. For instance:
742 if( /P(A)TT(ER)N/ ) {
743 print "I found $1 and $2\n";
746 These variables are read-only and dynamically-scoped, unless we note
749 The dynamic nature of the regular expression variables means that
750 their value is limited to the block that they are in, as demonstrated
753 my $outer = 'Wallace and Grommit';
754 my $inner = 'Mutt and Jeff';
756 my $pattern = qr/(\S+) and (\S+)/;
758 sub show_n { print "\$1 is $1; \$2 is $2\n" }
762 show_n() if $outer =~ m/$pattern/;
765 show_n() if $inner =~ m/$pattern/;
771 The output shows that while in the C<OUTER> block, the values of C<$1>
772 and C<$2> are from the match against C<$outer>. Inside the C<INNER>
773 block, the values of C<$1> and C<$2> are from the match against
774 C<$inner>, but only until the end of the block (i.e. the dynamic
775 scope). After the C<INNER> block completes, the values of C<$1> and
776 C<$2> return to the values for the match against C<$outer> even though
777 we have not made another match:
779 $1 is Wallace; $2 is Grommit
780 $1 is Mutt; $2 is Jeff
781 $1 is Wallace; $2 is Grommit
783 If you are using Perl v5.16 or earlier, note that C<use
784 English> imposes a considerable performance penalty on all regular
785 expression matches in a program because it uses the C<$`>, C<$&>, and
786 C<$'>, regardless of whether they occur in the scope of C<use
787 English>. For that reason, saying C<use English> in libraries is
788 strongly discouraged unless you import it without the match variables:
790 use English '-no_match_vars'
792 The C<Devel::NYTProf> and C<Devel::FindAmpersand>
793 modules can help you find uses of these
794 problematic match variables in your code.
796 Since Perl v5.10.0, you can use the C</p> match operator flag and the
797 C<${^PREMATCH}>, C<${^MATCH}>, and C<${^POSTMATCH}> variables instead
798 so you only suffer the performance penalties.
800 If you are using Perl v5.18.0 or higher, you do not need to worry about
801 this, as the three naughty variables are no longer naughty.
805 =item $<I<digits>> ($1, $2, ...)
808 Contains the subpattern from the corresponding set of capturing
809 parentheses from the last successful pattern match, not counting patterns
810 matched in nested blocks that have been exited already.
812 These variables are read-only and dynamically-scoped.
814 Mnemonic: like \digits.
821 The string matched by the last successful pattern match (not counting
822 any matches hidden within a BLOCK or C<eval()> enclosed by the current
825 In Perl v5.16 and earlier, the use of this variable
826 anywhere in a program imposes a considerable
827 performance penalty on all regular expression matches. To avoid this
828 penalty, you can extract the same substring by using L</@->. Starting
829 with Perl v5.10.0, you can use the C</p> match flag and the C<${^MATCH}>
830 variable to do the same thing for particular match operations.
832 This variable is read-only and dynamically-scoped.
834 Mnemonic: like C<&> in some editors.
839 This is similar to C<$&> (C<$MATCH>) except that it does not incur the
840 performance penalty associated with that variable.
841 In Perl v5.16 and earlier, it is only guaranteed
842 to return a defined value when the pattern was compiled or executed with
843 the C</p> modifier. In Perl v5.18, the C</p> modifier does nothing, so
844 C<${^MATCH}> does the same thing as C<$MATCH>.
846 This variable was added in Perl v5.10.0.
848 This variable is read-only and dynamically-scoped.
853 X<$`> X<$PREMATCH> X<${^PREMATCH}>
855 The string preceding whatever was matched by the last successful
856 pattern match, not counting any matches hidden within a BLOCK or C<eval>
857 enclosed by the current BLOCK.
859 In Perl v5.16 and earlier, the use of this variable
860 anywhere in a program imposes a considerable
861 performance penalty on all regular expression matches. To avoid this
862 penalty, you can extract the same substring by using L</@->. Starting
863 with Perl v5.10.0, you can use the C</p> match flag and the
864 C<${^PREMATCH}> variable to do the same thing for particular match
867 This variable is read-only and dynamically-scoped.
869 Mnemonic: C<`> often precedes a quoted string.
872 X<$`> X<${^PREMATCH}>
874 This is similar to C<$`> ($PREMATCH) except that it does not incur the
875 performance penalty associated with that variable.
876 In Perl v5.16 and earlier, it is only guaranteed
877 to return a defined value when the pattern was compiled or executed with
878 the C</p> modifier. In Perl v5.18, the C</p> modifier does nothing, so
879 C<${^PREMATCH}> does the same thing as C<$PREMATCH>.
881 This variable was added in Perl v5.10.0
883 This variable is read-only and dynamically-scoped.
888 X<$'> X<$POSTMATCH> X<${^POSTMATCH}> X<@->
890 The string following whatever was matched by the last successful
891 pattern match (not counting any matches hidden within a BLOCK or C<eval()>
892 enclosed by the current BLOCK). Example:
894 local $_ = 'abcdefghi';
896 print "$`:$&:$'\n"; # prints abc:def:ghi
898 In Perl v5.16 and earlier, the use of this variable
899 anywhere in a program imposes a considerable
900 performance penalty on all regular expression matches.
901 To avoid this penalty, you can extract the same substring by
902 using L</@->. Starting with Perl v5.10.0, you can use the C</p> match flag
903 and the C<${^POSTMATCH}> variable to do the same thing for particular
906 This variable is read-only and dynamically-scoped.
908 Mnemonic: C<'> often follows a quoted string.
911 X<${^POSTMATCH}> X<$'> X<$POSTMATCH>
913 This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
914 performance penalty associated with that variable.
915 In Perl v5.16 and earlier, it is only guaranteed
916 to return a defined value when the pattern was compiled or executed with
917 the C</p> modifier. In Perl v5.18, the C</p> modifier does nothing, so
918 C<${^POSTMATCH}> does the same thing as C<$POSTMATCH>.
920 This variable was added in Perl v5.10.0.
922 This variable is read-only and dynamically-scoped.
924 =item $LAST_PAREN_MATCH
927 X<$+> X<$LAST_PAREN_MATCH>
929 The text matched by the last bracket of the last successful search pattern.
930 This is useful if you don't know which one of a set of alternative patterns
931 matched. For example:
933 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
935 This variable is read-only and dynamically-scoped.
937 Mnemonic: be positive and forward looking.
939 =item $LAST_SUBMATCH_RESULT
942 X<$^N> X<$LAST_SUBMATCH_RESULT>
944 The text matched by the used group most-recently closed (i.e. the group
945 with the rightmost closing parenthesis) of the last successful search
948 This is primarily used inside C<(?{...})> blocks for examining text
949 recently matched. For example, to effectively capture text to a variable
950 (in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
952 (?:(...)(?{ $var = $^N }))
954 By setting and then using C<$var> in this way relieves you from having to
955 worry about exactly which numbered set of parentheses they are.
957 This variable was added in Perl v5.8.0.
959 Mnemonic: the (possibly) Nested parenthesis that most recently closed.
961 =item @LAST_MATCH_END
964 X<@+> X<@LAST_MATCH_END>
966 This array holds the offsets of the ends of the last successful
967 submatches in the currently active dynamic scope. C<$+[0]> is
968 the offset into the string of the end of the entire match. This
969 is the same value as what the C<pos> function returns when called
970 on the variable that was matched against. The I<n>th element
971 of this array holds the offset of the I<n>th submatch, so
972 C<$+[1]> is the offset past where C<$1> ends, C<$+[2]> the offset
973 past where C<$2> ends, and so on. You can use C<$#+> to determine
974 how many subgroups were in the last successful match. See the
975 examples given for the C<@-> variable.
977 This variable was added in Perl v5.6.0.
979 =item %LAST_PAREN_MATCH
982 X<%+> X<%LAST_PAREN_MATCH>
984 Similar to C<@+>, the C<%+> hash allows access to the named capture
985 buffers, should they exist, in the last successful match in the
986 currently active dynamic scope.
988 For example, C<$+{foo}> is equivalent to C<$1> after the following match:
990 'foo' =~ /(?<foo>foo)/;
992 The keys of the C<%+> hash list only the names of buffers that have
993 captured (and that are thus associated to defined values).
995 The underlying behaviour of C<%+> is provided by the
996 L<Tie::Hash::NamedCapture> module.
998 B<Note:> C<%-> and C<%+> are tied views into a common internal hash
999 associated with the last successful regular expression. Therefore mixing
1000 iterative access to them via C<each> may have unpredictable results.
1001 Likewise, if the last successful match changes, then the results may be
1004 This variable was added in Perl v5.10.0.
1006 This variable is read-only and dynamically-scoped.
1008 =item @LAST_MATCH_START
1011 X<@-> X<@LAST_MATCH_START>
1013 C<$-[0]> is the offset of the start of the last successful match.
1014 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
1015 I<n>-th subpattern, or undef if the subpattern did not match.
1017 Thus, after a match against C<$_>, C<$&> coincides with C<substr $_, $-[0],
1018 $+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
1019 $+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
1020 C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the
1021 last matched subgroup in the last successful match. Contrast with
1022 C<$#+>, the number of subgroups in the regular expression. Compare
1025 This array holds the offsets of the beginnings of the last
1026 successful submatches in the currently active dynamic scope.
1027 C<$-[0]> is the offset into the string of the beginning of the
1028 entire match. The I<n>th element of this array holds the offset
1029 of the I<n>th submatch, so C<$-[1]> is the offset where C<$1>
1030 begins, C<$-[2]> the offset where C<$2> begins, and so on.
1032 After a match against some variable C<$var>:
1036 =item C<$`> is the same as C<substr($var, 0, $-[0])>
1038 =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
1040 =item C<$'> is the same as C<substr($var, $+[0])>
1042 =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
1044 =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
1046 =item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
1050 This variable was added in Perl v5.6.0.
1052 =item %LAST_MATCH_START
1055 X<%-> X<%LAST_MATCH_START>
1057 Similar to C<%+>, this variable allows access to the named capture groups
1058 in the last successful match in the currently active dynamic scope. To
1059 each capture group name found in the regular expression, it associates a
1060 reference to an array containing the list of values captured by all
1061 buffers with that name (should there be several of them), in the order
1066 if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
1067 foreach my $bufname (sort keys %-) {
1068 my $ary = $-{$bufname};
1069 foreach my $idx (0..$#$ary) {
1070 print "\$-{$bufname}[$idx] : ",
1071 (defined($ary->[$idx])
1086 The keys of the C<%-> hash correspond to all buffer names found in
1087 the regular expression.
1089 The behaviour of C<%-> is implemented via the
1090 L<Tie::Hash::NamedCapture> module.
1092 B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1093 associated with the last successful regular expression. Therefore mixing
1094 iterative access to them via C<each> may have unpredictable results.
1095 Likewise, if the last successful match changes, then the results may be
1098 This variable was added in Perl v5.10.0.
1100 This variable is read-only and dynamically-scoped.
1102 =item $LAST_REGEXP_CODE_RESULT
1105 X<$^R> X<$LAST_REGEXP_CODE_RESULT>
1107 The result of evaluation of the last successful C<(?{ code })>
1108 regular expression assertion (see L<perlre>). May be written to.
1110 This variable was added in Perl 5.005.
1112 =item ${^RE_DEBUG_FLAGS}
1113 X<${^RE_DEBUG_FLAGS}>
1115 The current value of the regex debugging flags. Set to 0 for no debug output
1116 even when the C<re 'debug'> module is loaded. See L<re> for details.
1118 This variable was added in Perl v5.10.0.
1120 =item ${^RE_TRIE_MAXBUF}
1121 X<${^RE_TRIE_MAXBUF}>
1123 Controls how certain regex optimisations are applied and how much memory they
1124 utilize. This value by default is 65536 which corresponds to a 512kB
1125 temporary cache. Set this to a higher value to trade
1126 memory for speed when matching large alternations. Set
1127 it to a lower value if you want the optimisations to
1128 be as conservative of memory as possible but still occur, and set it to a
1129 negative value to prevent the optimisation and conserve the most memory.
1130 Under normal situations this variable should be of no interest to you.
1132 This variable was added in Perl v5.10.0.
1136 =head2 Variables related to filehandles
1138 Variables that depend on the currently selected filehandle may be set
1139 by calling an appropriate object method on the C<IO::Handle> object,
1140 although this is less efficient than using the regular built-in
1141 variables. (Summary lines below for this contain the word HANDLE.)
1146 after which you may use either
1152 HANDLE->method(EXPR)
1154 Each method returns the old value of the C<IO::Handle> attribute. The
1155 methods each take an optional EXPR, which, if supplied, specifies the
1156 new value for the C<IO::Handle> attribute in question. If not
1157 supplied, most methods do nothing to the current value--except for
1158 C<autoflush()>, which will assume a 1 for you, just to be different.
1160 Because loading in the C<IO::Handle> class is an expensive operation,
1161 you should learn how to use the regular built-in variables.
1163 A few of these variables are considered "read-only". This means that
1164 if you try to assign to this variable, either directly or indirectly
1165 through a reference, you'll raise a run-time exception.
1167 You should be very careful when modifying the default values of most
1168 special variables described in this document. In most cases you want
1169 to localize these variables before changing them, since if you don't,
1170 the change may affect other modules which rely on the default values
1171 of the special variables that you have changed. This is one of the
1172 correct ways to read the whole file at once:
1174 open my $fh, "<", "foo" or die $!;
1175 local $/; # enable localized slurp mode
1176 my $content = <$fh>;
1179 But the following code is quite bad:
1181 open my $fh, "<", "foo" or die $!;
1182 undef $/; # enable slurp mode
1183 my $content = <$fh>;
1186 since some other module, may want to read data from some file in the
1187 default "line mode", so if the code we have just presented has been
1188 executed, the global value of C<$/> is now changed for any other code
1189 running inside the same Perl interpreter.
1191 Usually when a variable is localized you want to make sure that this
1192 change affects the shortest scope possible. So unless you are already
1193 inside some short C<{}> block, you should create one yourself. For
1197 open my $fh, "<", "foo" or die $!;
1204 Here is an example of how your own code can go broken:
1214 # do something with $_
1217 You probably expect this code to print the equivalent of
1221 but instead you get:
1225 Why? Because C<nasty_break()> modifies C<$\> without localizing it
1226 first. The value you set in C<nasty_break()> is still there when you
1227 return. The fix is to add C<local()> so the value doesn't leak out of
1232 It's easy to notice the problem in such a short example, but in more
1233 complicated code you are looking for trouble if you don't localize
1234 changes to the special variables.
1241 Contains the name of the current file when reading from C<< <> >>.
1246 The array C<@ARGV> contains the command-line arguments intended for
1247 the script. C<$#ARGV> is generally the number of arguments minus
1248 one, because C<$ARGV[0]> is the first argument, I<not> the program's
1249 command name itself. See L</$0> for the command name.
1254 The special filehandle that iterates over command-line filenames in
1255 C<@ARGV>. Usually written as the null filehandle in the angle operator
1256 C<< <> >>. Note that currently C<ARGV> only has its magical effect
1257 within the C<< <> >> operator; elsewhere it is just a plain filehandle
1258 corresponding to the last file opened by C<< <> >>. In particular,
1259 passing C<\*ARGV> as a parameter to a function that expects a filehandle
1260 may not cause your function to automatically read the contents of all the
1266 The special filehandle that points to the currently open output file
1267 when doing edit-in-place processing with B<-i>. Useful when you have
1268 to do a lot of inserting and don't want to keep modifying C<$_>. See
1269 L<perlrun> for the B<-i> switch.
1271 =item IO::Handle->output_field_separator( EXPR )
1273 =item $OUTPUT_FIELD_SEPARATOR
1278 X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>
1280 The output field separator for the print operator. If defined, this
1281 value is printed between each of print's arguments. Default is C<undef>.
1283 You cannot call C<output_field_separator()> on a handle, only as a
1284 static method. See L<IO::Handle|IO::Handle>.
1286 Mnemonic: what is printed when there is a "," in your print statement.
1288 =item HANDLE->input_line_number( EXPR )
1290 =item $INPUT_LINE_NUMBER
1295 X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
1297 Current line number for the last filehandle accessed.
1299 Each filehandle in Perl counts the number of lines that have been read
1300 from it. (Depending on the value of C<$/>, Perl's idea of what
1301 constitutes a line may not match yours.) When a line is read from a
1302 filehandle (via C<readline()> or C<< <> >>), or when C<tell()> or
1303 C<seek()> is called on it, C<$.> becomes an alias to the line counter
1304 for that filehandle.
1306 You can adjust the counter by assigning to C<$.>, but this will not
1307 actually move the seek pointer. I<Localizing C<$.> will not localize
1308 the filehandle's line count>. Instead, it will localize perl's notion
1309 of which filehandle C<$.> is currently aliased to.
1311 C<$.> is reset when the filehandle is closed, but B<not> when an open
1312 filehandle is reopened without an intervening C<close()>. For more
1313 details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
1314 an explicit close, line numbers increase across C<ARGV> files (but see
1315 examples in L<perlfunc/eof>).
1317 You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
1318 line counter for a given filehandle without having to worry about
1319 which handle you last accessed.
1321 Mnemonic: many programs use "." to mean the current line number.
1323 =item IO::Handle->input_record_separator( EXPR )
1325 =item $INPUT_RECORD_SEPARATOR
1330 X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR>
1332 The input record separator, newline by default. This influences Perl's
1333 idea of what a "line" is. Works like B<awk>'s RS variable, including
1334 treating empty lines as a terminator if set to the null string (an
1335 empty line cannot contain any spaces or tabs). You may set it to a
1336 multi-character string to match a multi-character terminator, or to
1337 C<undef> to read through the end of file. Setting it to C<"\n\n">
1338 means something slightly different than setting to C<"">, if the file
1339 contains consecutive empty lines. Setting to C<""> will treat two or
1340 more consecutive empty lines as a single empty line. Setting to
1341 C<"\n\n"> will blindly assume that the next input character belongs to
1342 the next paragraph, even if it's a newline.
1344 local $/; # enable "slurp" mode
1345 local $_ = <FH>; # whole file now here
1348 Remember: the value of C<$/> is a string, not a regex. B<awk> has to
1349 be better for something. :-)
1351 Setting C<$/> to a reference to an integer, scalar containing an
1352 integer, or scalar that's convertible to an integer will attempt to
1353 read records instead of lines, with the maximum record size being the
1354 referenced integer. So this:
1356 local $/ = \32768; # or \"32768", or \$var_containing_32768
1357 open my $fh, "<", $myfile or die $!;
1360 will read a record of no more than 32768 bytes from $fh. If you're
1361 not reading from a record-oriented file (or your OS doesn't have
1362 record-oriented files), then you'll likely get a full chunk of data
1363 with every read. If a record is larger than the record size you've
1364 set, you'll get the record back in pieces. Trying to set the record
1365 size to zero or less will cause reading in the (rest of the) whole file.
1367 On VMS only, record reads bypass PerlIO layers and any associated
1368 buffering,so you must not mix record and non-record reads on the
1369 same filehandle. Record mode mixes with line mode only when the
1370 same buffering layer is in use for both modes.
1372 If you perform a record read on a FILE with an encoding layer such as
1373 C<:encoding(latin1)> or C<:utf8>, you may get an invalid string as a
1374 result, may leave the FILE positioned between characters in the stream
1375 and may not be reading the number of bytes from the underlying file
1376 that you specified. This behaviour may change without warning in a
1377 future version of perl.
1379 You cannot call C<input_record_separator()> on a handle, only as a
1380 static method. See L<IO::Handle|IO::Handle>.
1382 See also L<perlport/"Newlines">. Also see L</$.>.
1384 Mnemonic: / delimits line boundaries when quoting poetry.
1386 =item IO::Handle->output_record_separator( EXPR )
1388 =item $OUTPUT_RECORD_SEPARATOR
1393 X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR>
1395 The output record separator for the print operator. If defined, this
1396 value is printed after the last of print's arguments. Default is C<undef>.
1398 You cannot call C<output_record_separator()> on a handle, only as a
1399 static method. See L<IO::Handle|IO::Handle>.
1401 Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
1402 Also, it's just like C<$/>, but it's what you get "back" from Perl.
1404 =item HANDLE->autoflush( EXPR )
1406 =item $OUTPUT_AUTOFLUSH
1409 X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
1411 If set to nonzero, forces a flush right away and after every write or
1412 print on the currently selected output channel. Default is 0
1413 (regardless of whether the channel is really buffered by the system or
1414 not; C<$|> tells you only whether you've asked Perl explicitly to
1415 flush after each write). STDOUT will typically be line buffered if
1416 output is to the terminal and block buffered otherwise. Setting this
1417 variable is useful primarily when you are outputting to a pipe or
1418 socket, such as when you are running a Perl program under B<rsh> and
1419 want to see the output as it's happening. This has no effect on input
1420 buffering. See L<perlfunc/getc> for that. See L<perlfunc/select> on
1421 how to select the output channel. See also L<IO::Handle>.
1423 Mnemonic: when you want your pipes to be piping hot.
1428 This read-only variable contains a reference to the last-read filehandle.
1429 This is set by C<< <HANDLE> >>, C<readline>, C<tell>, C<eof> and C<seek>.
1430 This is the same handle that C<$.> and C<tell> and C<eof> without arguments
1431 use. It is also the handle used when Perl appends ", <STDIN> line 1" to
1432 an error or warning message.
1434 This variable was added in Perl v5.18.0.
1438 =head3 Variables related to formats
1440 The special variables for formats are a subset of those for
1441 filehandles. See L<perlform> for more information about Perl's
1449 X<$^A> X<$ACCUMULATOR>
1451 The current value of the C<write()> accumulator for C<format()> lines.
1452 A format contains C<formline()> calls that put their result into
1453 C<$^A>. After calling its format, C<write()> prints out the contents
1454 of C<$^A> and empties. So you never really see the contents of C<$^A>
1455 unless you call C<formline()> yourself and then look at it. See
1456 L<perlform> and L<perlfunc/"formline PICTURE,LIST">.
1458 =item IO::Handle->format_formfeed(EXPR)
1460 =item $FORMAT_FORMFEED
1463 X<$^L> X<$FORMAT_FORMFEED>
1465 What formats output as a form feed. The default is C<\f>.
1467 You cannot call C<format_formfeed()> on a handle, only as a static
1468 method. See L<IO::Handle|IO::Handle>.
1470 =item HANDLE->format_page_number(EXPR)
1472 =item $FORMAT_PAGE_NUMBER
1475 X<$%> X<$FORMAT_PAGE_NUMBER>
1477 The current page number of the currently selected output channel.
1479 Mnemonic: C<%> is page number in B<nroff>.
1481 =item HANDLE->format_lines_left(EXPR)
1483 =item $FORMAT_LINES_LEFT
1486 X<$-> X<$FORMAT_LINES_LEFT>
1488 The number of lines left on the page of the currently selected output
1491 Mnemonic: lines_on_page - lines_printed.
1493 =item IO::Handle->format_line_break_characters EXPR
1495 =item $FORMAT_LINE_BREAK_CHARACTERS
1498 X<$:> X<FORMAT_LINE_BREAK_CHARACTERS>
1500 The current set of characters after which a string may be broken to
1501 fill continuation fields (starting with C<^>) in a format. The default is
1502 S<" \n-">, to break on a space, newline, or a hyphen.
1504 You cannot call C<format_line_break_characters()> on a handle, only as
1505 a static method. See L<IO::Handle|IO::Handle>.
1507 Mnemonic: a "colon" in poetry is a part of a line.
1509 =item HANDLE->format_lines_per_page(EXPR)
1511 =item $FORMAT_LINES_PER_PAGE
1514 X<$=> X<$FORMAT_LINES_PER_PAGE>
1516 The current page length (printable lines) of the currently selected
1517 output channel. The default is 60.
1519 Mnemonic: = has horizontal lines.
1521 =item HANDLE->format_top_name(EXPR)
1523 =item $FORMAT_TOP_NAME
1526 X<$^> X<$FORMAT_TOP_NAME>
1528 The name of the current top-of-page format for the currently selected
1529 output channel. The default is the name of the filehandle with C<_TOP>
1530 appended. For example, the default format top name for the C<STDOUT>
1531 filehandle is C<STDOUT_TOP>.
1533 Mnemonic: points to top of page.
1535 =item HANDLE->format_name(EXPR)
1540 X<$~> X<$FORMAT_NAME>
1542 The name of the current report format for the currently selected
1543 output channel. The default format name is the same as the filehandle
1544 name. For example, the default format name for the C<STDOUT>
1545 filehandle is just C<STDOUT>.
1547 Mnemonic: brother to C<$^>.
1551 =head2 Error Variables
1552 X<error> X<exception>
1554 The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1555 about different types of error conditions that may appear during
1556 execution of a Perl program. The variables are shown ordered by
1557 the "distance" between the subsystem which reported the error and
1558 the Perl process. They correspond to errors detected by the Perl
1559 interpreter, C library, operating system, or an external program,
1562 To illustrate the differences between these variables, consider the
1563 following Perl expression, which uses a single-quoted string. After
1564 execution of this statement, perl may have set all four special error
1568 open my $pipe, "/cdrom/install |" or die $!;
1570 close $pipe or die "bad pipe: $?, $!";
1573 When perl executes the C<eval()> expression, it translates the
1574 C<open()>, C<< <PIPE> >>, and C<close> calls in the C run-time library
1575 and thence to the operating system kernel. perl sets C<$!> to
1576 the C library's C<errno> if one of these calls fails.
1578 C<$@> is set if the string to be C<eval>-ed did not compile (this may
1579 happen if C<open> or C<close> were imported with bad prototypes), or
1580 if Perl code executed during evaluation C<die()>d. In these cases the
1581 value of C<$@> is the compile error, or the argument to C<die> (which
1582 will interpolate C<$!> and C<$?>). (See also L<Fatal>, though.)
1584 Under a few operating systems, C<$^E> may contain a more verbose error
1585 indicator, such as in this case, "CDROM tray not closed." Systems that
1586 do not support extended error messages leave C<$^E> the same as C<$!>.
1588 Finally, C<$?> may be set to non-0 value if the external program
1589 F</cdrom/install> fails. The upper eight bits reflect specific error
1590 conditions encountered by the program (the program's C<exit()> value).
1591 The lower eight bits reflect mode of failure, like signal death and
1592 core dump information. See L<wait(2)> for details. In contrast to
1593 C<$!> and C<$^E>, which are set only if error condition is detected,
1594 the variable C<$?> is set on each C<wait> or pipe C<close>,
1595 overwriting the old value. This is more like C<$@>, which on every
1596 C<eval()> is always set on failure and cleared on success.
1598 For more details, see the individual descriptions at C<$@>, C<$!>,
1603 =item ${^CHILD_ERROR_NATIVE}
1604 X<$^CHILD_ERROR_NATIVE>
1606 The native status returned by the last pipe close, backtick (C<``>)
1607 command, successful call to C<wait()> or C<waitpid()>, or from the
1608 C<system()> operator. On POSIX-like systems this value can be decoded
1609 with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED,
1610 WSTOPSIG and WIFCONTINUED functions provided by the L<POSIX> module.
1612 Under VMS this reflects the actual VMS exit status; i.e. it is the
1613 same as C<$?> when the pragma C<use vmsish 'status'> is in effect.
1615 This variable was added in Perl v5.10.0.
1617 =item $EXTENDED_OS_ERROR
1620 X<$^E> X<$EXTENDED_OS_ERROR>
1622 Error information specific to the current operating system. At the
1623 moment, this differs from C<$!> under only VMS, OS/2, and Win32 (and
1624 for MacPerl). On all other platforms, C<$^E> is always just the same
1627 Under VMS, C<$^E> provides the VMS status value from the last system
1628 error. This is more specific information about the last system error
1629 than that provided by C<$!>. This is particularly important when C<$!>
1630 is set to B<EVMSERR>.
1632 Under OS/2, C<$^E> is set to the error code of the last call to OS/2
1633 API either via CRT, or directly from perl.
1635 Under Win32, C<$^E> always returns the last error information reported
1636 by the Win32 call C<GetLastError()> which describes the last error
1637 from within the Win32 API. Most Win32-specific code will report errors
1638 via C<$^E>. ANSI C and Unix-like calls set C<errno> and so most
1639 portable Perl code will report errors via C<$!>.
1641 Caveats mentioned in the description of C<$!> generally apply to
1644 This variable was added in Perl 5.003.
1646 Mnemonic: Extra error explanation.
1648 =item $EXCEPTIONS_BEING_CAUGHT
1651 X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>
1653 Current state of the interpreter.
1656 --------- -------------------------------------
1657 undef Parsing module, eval, or main program
1658 true (1) Executing an eval
1661 The first state may happen in C<$SIG{__DIE__}> and C<$SIG{__WARN__}>
1664 The English name $EXCEPTIONS_BEING_CAUGHT is slightly misleading, because
1665 the C<undef> value does not indicate whether exceptions are being caught,
1666 since compilation of the main program does not catch exceptions.
1668 This variable was added in Perl 5.004.
1675 The current value of the warning switch, initially true if B<-w> was
1676 used, false otherwise, but directly modifiable.
1678 See also L<warnings>.
1680 Mnemonic: related to the B<-w> switch.
1682 =item ${^WARNING_BITS}
1685 The current set of warning checks enabled by the C<use warnings> pragma.
1686 It has the same scoping as the C<$^H> and C<%^H> variables. The exact
1687 values are considered internal to the L<warnings> pragma and may change
1688 between versions of Perl.
1690 This variable was added in Perl v5.6.0.
1697 X<$!> X<$ERRNO> X<$OS_ERROR>
1699 When referenced, C<$!> retrieves the current value
1700 of the C C<errno> integer variable.
1701 If C<$!> is assigned a numerical value, that value is stored in C<errno>.
1702 When referenced as a string, C<$!> yields the system error string
1703 corresponding to C<errno>.
1705 Many system or library calls set C<errno> if they fail,
1706 to indicate the cause of failure. They usually do B<not>
1707 set C<errno> to zero if they succeed. This means C<errno>,
1708 hence C<$!>, is meaningful only I<immediately> after a B<failure>:
1710 if (open my $fh, "<", $filename) {
1711 # Here $! is meaningless.
1715 # ONLY here is $! meaningful.
1717 # Already here $! might be meaningless.
1719 # Since here we might have either success or failure,
1720 # $! is meaningless.
1722 Here, I<meaningless> means that C<$!> may be unrelated to the outcome
1723 of the C<open()> operator. Assignment to C<$!> is similarly ephemeral.
1724 It can be used immediately before invoking the C<die()> operator,
1725 to set the exit value, or to inspect the system error string
1726 corresponding to error I<n>, or to restore C<$!> to a meaningful state.
1728 Mnemonic: What just went bang?
1735 X<%!> X<%OS_ERROR> X<%ERRNO>
1737 Each element of C<%!> has a true value only if C<$!> is set to that
1738 value. For example, C<$!{ENOENT}> is true if and only if the current
1739 value of C<$!> is C<ENOENT>; that is, if the most recent error was "No
1740 such file or directory" (or its moral equivalent: not all operating
1741 systems give that exact error, and certainly not all languages). To
1742 check if a particular key is meaningful on your system, use C<exists
1743 $!{the_key}>; for a list of legal keys, use C<keys %!>. See L<Errno>
1744 for more information, and also see L</$!>.
1746 This variable was added in Perl 5.005.
1751 X<$?> X<$CHILD_ERROR>
1753 The status returned by the last pipe close, backtick (C<``>) command,
1754 successful call to C<wait()> or C<waitpid()>, or from the C<system()>
1755 operator. This is just the 16-bit status word returned by the
1756 traditional Unix C<wait()> system call (or else is made up to look
1757 like it). Thus, the exit value of the subprocess is really (C<<< $? >>
1758 8 >>>), and C<$? & 127> gives which signal, if any, the process died
1759 from, and C<$? & 128> reports whether there was a core dump.
1761 Additionally, if the C<h_errno> variable is supported in C, its value
1762 is returned via C<$?> if any C<gethost*()> function fails.
1764 If you have installed a signal handler for C<SIGCHLD>, the
1765 value of C<$?> will usually be wrong outside that handler.
1767 Inside an C<END> subroutine C<$?> contains the value that is going to be
1768 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
1769 change the exit status of your program. For example:
1772 $? = 1 if $? == 255; # die would make it 255
1775 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
1776 actual VMS exit status, instead of the default emulation of POSIX
1777 status; see L<perlvms/$?> for details.
1779 Mnemonic: similar to B<sh> and B<ksh>.
1784 X<$@> X<$EVAL_ERROR>
1786 The Perl syntax error message from the
1787 last C<eval()> operator. If C<$@> is
1788 the null string, the last C<eval()> parsed and executed correctly
1789 (although the operations you invoked may have failed in the normal
1792 Warning messages are not collected in this variable. You can, however,
1793 set up a routine to process warnings by setting C<$SIG{__WARN__}> as
1794 described in L</%SIG>.
1796 Mnemonic: Where was the syntax error "at"?
1800 =head2 Variables related to the interpreter state
1802 These variables provide information about the current interpreter state.
1809 X<$^C> X<$COMPILING>
1811 The current value of the flag associated with the B<-c> switch.
1812 Mainly of use with B<-MO=...> to allow code to alter its behavior
1813 when being compiled, such as for example to C<AUTOLOAD> at compile
1814 time rather than normal, deferred loading. Setting
1815 C<$^C = 1> is similar to calling C<B::minus_c>.
1817 This variable was added in Perl v5.6.0.
1822 X<$^D> X<$DEBUGGING>
1824 The current value of the debugging flags. May be read or set. Like its
1825 command-line equivalent, you can use numeric or symbolic values, eg
1826 C<$^D = 10> or C<$^D = "st">.
1828 Mnemonic: value of B<-D> switch.
1833 The I<object reference> to the C<Encode> object that is used to convert
1834 the source code to Unicode. Thanks to this variable your Perl script
1835 does not have to be written in UTF-8. Default is I<undef>. The direct
1836 manipulation of this variable is highly discouraged.
1838 This variable was added in Perl 5.8.2.
1840 =item ${^GLOBAL_PHASE}
1843 The current phase of the perl interpreter.
1845 Possible values are:
1851 The C<PerlInterpreter*> is being constructed via C<perl_construct>. This
1852 value is mostly there for completeness and for use via the
1853 underlying C variable C<PL_phase>. It's not really possible for Perl
1854 code to be executed unless construction of the interpreter is
1859 This is the global compile-time. That includes, basically, every
1860 C<BEGIN> block executed directly or indirectly from during the
1861 compile-time of the top-level program.
1863 This phase is not called "BEGIN" to avoid confusion with
1864 C<BEGIN>-blocks, as those are executed during compile-time of any
1865 compilation unit, not just the top-level program. A new, localised
1866 compile-time entered at run-time, for example by constructs as
1867 C<eval "use SomeModule"> are not global interpreter phases, and
1868 therefore aren't reflected by C<${^GLOBAL_PHASE}>.
1872 Execution of any C<CHECK> blocks.
1876 Similar to "CHECK", but for C<INIT>-blocks, not C<CHECK> blocks.
1880 The main run-time, i.e. the execution of C<PL_main_root>.
1884 Execution of any C<END> blocks.
1892 Also note that there's no value for UNITCHECK-blocks. That's because
1893 those are run for each compilation unit individually, and therefore is
1894 not a global interpreter phase.
1896 Not every program has to go through each of the possible phases, but
1897 transition from one phase to another can only happen in the order
1898 described in the above list.
1900 An example of all of the phases Perl code can see:
1902 BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }
1904 INIT { print "init-time: ${^GLOBAL_PHASE}\n" }
1906 CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }
1909 package Print::Phase;
1912 my ($class, $time) = @_;
1913 return bless \$time, $class;
1918 print "$$self: ${^GLOBAL_PHASE}\n";
1922 print "run-time: ${^GLOBAL_PHASE}\n";
1924 my $runtime = Print::Phase->new(
1925 "lexical variables are garbage collected before END"
1928 END { print "end-time: ${^GLOBAL_PHASE}\n" }
1930 our $destruct = Print::Phase->new(
1931 "package variables are garbage collected after END"
1940 lexical variables are garbage collected before END: RUN
1942 package variables are garbage collected after END: DESTRUCT
1944 This variable was added in Perl 5.14.0.
1949 WARNING: This variable is strictly for
1950 internal use only. Its availability,
1951 behavior, and contents are subject to change without notice.
1953 This variable contains compile-time hints for the Perl interpreter. At the
1954 end of compilation of a BLOCK the value of this variable is restored to the
1955 value when the interpreter started to compile the BLOCK.
1957 When perl begins to parse any block construct that provides a lexical scope
1958 (e.g., eval body, required file, subroutine body, loop body, or conditional
1959 block), the existing value of C<$^H> is saved, but its value is left unchanged.
1960 When the compilation of the block is completed, it regains the saved value.
1961 Between the points where its value is saved and restored, code that
1962 executes within BEGIN blocks is free to change the value of C<$^H>.
1964 This behavior provides the semantic of lexical scoping, and is used in,
1965 for instance, the C<use strict> pragma.
1967 The contents should be an integer; different bits of it are used for
1968 different pragmatic flags. Here's an example:
1970 sub add_100 { $^H |= 0x100 }
1977 Consider what happens during execution of the BEGIN block. At this point
1978 the BEGIN block has already been compiled, but the body of C<foo()> is still
1979 being compiled. The new value of C<$^H>
1980 will therefore be visible only while
1981 the body of C<foo()> is being compiled.
1983 Substitution of C<BEGIN { add_100() }> block with:
1985 BEGIN { require strict; strict->import('vars') }
1987 demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
1988 version of the same lexical pragma:
1991 require strict; strict->import('vars') if $condition
1994 This variable was added in Perl 5.003.
1999 The C<%^H> hash provides the same scoping semantic as C<$^H>. This makes
2000 it useful for implementation of lexically scoped pragmas. See
2003 When putting items into C<%^H>, in order to avoid conflicting with other
2004 users of the hash there is a convention regarding which keys to use.
2005 A module should use only keys that begin with the module's name (the
2006 name of its main package) and a "/" character. For example, a module
2007 C<Foo::Bar> should use keys such as C<Foo::Bar/baz>.
2009 This variable was added in Perl v5.6.0.
2014 An internal variable used by PerlIO. A string in two parts, separated
2015 by a C<\0> byte, the first part describes the input layers, the second
2016 part describes the output layers.
2018 This variable was added in Perl v5.8.0.
2025 The internal variable for debugging support. The meanings of the
2026 various bits are subject to change, but currently indicate:
2032 Debug subroutine enter/exit.
2036 Line-by-line debugging. Causes C<DB::DB()> subroutine to be called for
2037 each statement executed. Also causes saving source code lines (like
2042 Switch off optimizations.
2046 Preserve more data for future interactive inspections.
2050 Keep info about source lines on which a subroutine is defined.
2054 Start with single-step on.
2058 Use subroutine address instead of name when reporting.
2062 Report C<goto &subroutine> as well.
2066 Provide informative "file" names for evals based on the place they were compiled.
2070 Provide informative names to anonymous subroutines based on the place they
2075 Save source code lines into C<@{"_<$filename"}>.
2079 Some bits may be relevant at compile-time only, some at
2080 run-time only. This is a new mechanism and the details may change.
2081 See also L<perldebguts>.
2086 Reflects if taint mode is on or off. 1 for on (the program was run with
2087 B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
2090 This variable is read-only.
2092 This variable was added in Perl v5.8.0.
2097 Reflects certain Unicode settings of Perl. See L<perlrun>
2098 documentation for the C<-C> switch for more information about
2099 the possible values.
2101 This variable is set during Perl startup and is thereafter read-only.
2103 This variable was added in Perl v5.8.2.
2108 This variable controls the state of the internal UTF-8 offset caching code.
2109 1 for on (the default), 0 for off, -1 to debug the caching code by checking
2110 all its results against linear scans, and panicking on any discrepancy.
2112 This variable was added in Perl v5.8.9. It is subject to change or
2113 removal without notice, but is currently used to avoid recalculating the
2114 boundaries of multi-byte UTF-8-encoded characters.
2116 =item ${^UTF8LOCALE}
2119 This variable indicates whether a UTF-8 locale was detected by perl at
2120 startup. This information is used by perl when it's in
2121 adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
2122 switch); see L<perlrun> for more info on this.
2124 This variable was added in Perl v5.8.8.
2128 =head2 Deprecated and removed variables
2130 Deprecating a variable announces the intent of the perl maintainers to
2131 eventually remove the variable from the language. It may still be
2132 available despite its status. Using a deprecated variable triggers
2135 Once a variable is removed, its use triggers an error telling you
2136 the variable is unsupported.
2138 See L<perldiag> for details about error messages.
2147 C<$#> was a variable that could be used to format printed numbers.
2148 After a deprecation cycle, its magic was removed in Perl v5.10.0 and
2149 using it now triggers a warning: C<$# is no longer supported>.
2151 This is not the sigil you use in front of an array name to get the
2152 last index, like C<$#array>. That's still how you get the last index
2153 of an array in Perl. The two have nothing to do with each other.
2155 Deprecated in Perl 5.
2157 Removed in Perl v5.10.0.
2162 C<$*> was a variable that you could use to enable multiline matching.
2163 After a deprecation cycle, its magic was removed in Perl v5.10.0.
2164 Using it now triggers a warning: C<$* is no longer supported>.
2165 You should use the C</s> and C</m> regexp modifiers instead.
2167 Deprecated in Perl 5.
2169 Removed in Perl v5.10.0.
2174 X<$[> X<$ARRAY_BASE>
2176 This variable stores the index of the first element in an array, and
2177 of the first character in a substring. The default is 0, but you could
2178 theoretically set it to 1 to make Perl behave more like B<awk> (or Fortran)
2179 when subscripting and when evaluating the index() and substr() functions.
2181 As of release 5 of Perl, assignment to C<$[> is treated as a compiler
2182 directive, and cannot influence the behavior of any other file.
2183 (That's why you can only assign compile-time constants to it.)
2184 Its use is highly discouraged.
2186 Prior to Perl v5.10.0, assignment to C<$[> could be seen from outer lexical
2187 scopes in the same file, unlike other compile-time directives (such as
2188 L<strict>). Using local() on it would bind its value strictly to a lexical
2189 block. Now it is always lexically scoped.
2191 As of Perl v5.16.0, it is implemented by the L<arybase> module. See
2192 L<arybase> for more details on its behaviour.
2194 Under C<use v5.16>, or C<no feature "array_base">, C<$[> no longer has any
2195 effect, and always contains 0. Assigning 0 to it is permitted, but any
2196 other value will produce an error.
2198 Mnemonic: [ begins subscripts.
2200 Deprecated in Perl v5.12.0.
2202 =item $OLD_PERL_VERSION
2205 X<$]> X<$OLD_PERL_VERSION>
2207 See L</$^V> for a more modern representation of the Perl version that allows
2208 accurate string comparisons.
2210 The version + patchlevel / 1000 of the Perl interpreter. This variable
2211 can be used to determine whether the Perl interpreter executing a
2212 script is in the right range of versions:
2214 warn "No checksumming!\n" if $] < 3.019;
2216 The floating point representation can sometimes lead to inaccurate
2217 numeric comparisons.
2219 See also the documentation of C<use VERSION> and C<require VERSION>
2220 for a convenient way to fail if the running Perl interpreter is too old.
2222 Mnemonic: Is this version of perl in the right bracket?