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<${^_XYZ}> variables are always taken
51 to be in package C<main>, regardless of any C<package> declarations
54 =head1 SPECIAL VARIABLES
56 The following names have special meaning to Perl. Most punctuation
57 names have reasonable mnemonics, or analogs in the shells.
58 Nevertheless, if you wish to use long variable names, you need only say:
62 at the top of your program. This aliases all the short names to the long
63 names in the current package. Some even have medium names, generally
64 borrowed from B<awk>. For more info, please see L<English>.
66 Before you continue, note the sort order for variables. In general, we
67 first list the variables in case-insensitive, almost-lexigraphical
68 order (ignoring the C<{> or C<^> preceding words, as in C<${^UNICODE}>
69 or C<$^T>), although C<$_> and C<@_> move up to the top of the pile.
70 For variables with the same identifier, we list it in order of scalar,
71 array, hash, and bareword.
73 =head2 General Variables
82 The default input and pattern-searching space. The following pairs are
85 while (<>) {...} # equivalent only in while!
86 while (defined($_ = <>)) {...}
97 Here are the places where Perl will assume C<$_> even if you don't use it:
103 The following functions use C<$_> as a default argument:
105 abs, alarm, chomp, chop, chr, chroot,
106 cos, defined, eval, evalbytes, exp, fc, glob, hex, int, lc,
107 lcfirst, length, log, lstat, mkdir, oct, ord, pos, print, printf,
108 quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only),
109 rmdir, say, sin, split (for its second
110 argument), sqrt, stat, study, uc, ucfirst,
115 All file tests (C<-f>, C<-d>) except for C<-t>, which defaults to STDIN.
120 The pattern matching operations C<m//>, C<s///> and C<tr///> (aka C<y///>)
121 when used without an C<=~> operator.
125 The default iterator variable in a C<foreach> loop if no other
126 variable is supplied.
130 The implicit iterator variable in the C<grep()> and C<map()> functions.
134 The implicit variable of C<given()>.
138 The default place to put the next value or input record
139 when a C<< <FH> >>, C<readline>, C<readdir> or C<each>
140 operation's result is tested by itself as the sole criterion of a C<while>
141 test. Outside a C<while> test, this will not happen.
145 C<$_> is by default a global variable. However, as
146 of perl v5.10.0, you can use a lexical version of
147 C<$_> by declaring it in a file or in a block with C<my>. Moreover,
148 declaring C<our $_> restores the global C<$_> in the current scope. Though
149 this seemed like a good idea at the time it was introduced, lexical C<$_>
150 actually causes more problems than it solves. If you call a function that
151 expects to be passed information via C<$_>, it may or may not work,
152 depending on how the function is written, there not being any easy way to
153 solve this. Just avoid lexical C<$_>, unless you are feeling particularly
154 masochistic. For this reason lexical C<$_> is still experimental and will
155 produce a warning unless warnings have been disabled. As with other
156 experimental features, the behavior of lexical C<$_> is subject to change
157 without notice, including change into a fatal error.
159 Mnemonic: underline is understood in certain operations.
166 Within a subroutine the array C<@_> contains the parameters passed to
167 that subroutine. Inside a subroutine, C<@_> is the default array for
168 the array operators C<push>, C<pop>, C<shift>, and C<unshift>.
172 =item $LIST_SEPARATOR
175 X<$"> X<$LIST_SEPARATOR>
177 When an array or an array slice is interpolated into a double-quoted
178 string or a similar context such as C</.../>, its elements are
179 separated by this value. Default is a space. For example, this:
181 print "The array is: @array\n";
183 is equivalent to this:
185 print "The array is: " . join($", @array) . "\n";
187 Mnemonic: works in double-quoted context.
194 X<$$> X<$PID> X<$PROCESS_ID>
196 The process number of the Perl running this script. Though you I<can> set
197 this variable, doing so is generally discouraged, although it can be
198 invaluable for some testing purposes. It will be reset automatically
199 across C<fork()> calls.
201 Note for Linux and Debian GNU/kFreeBSD users: Before Perl v5.16.0 perl
202 would emulate POSIX semantics on Linux systems using LinuxThreads, a
203 partial implementation of POSIX Threads that has since been superseded
204 by the Native POSIX Thread Library (NPTL).
206 LinuxThreads is now obsolete on Linux, and caching C<getpid()>
207 like this made embedding perl unnecessarily complex (since you'd have
208 to manually update the value of $$), so now C<$$> and C<getppid()>
209 will always return the same values as the underlying C library.
211 Debian GNU/kFreeBSD systems also used LinuxThreads up until and
212 including the 6.0 release, but after that moved to FreeBSD thread
213 semantics, which are POSIX-like.
215 To see if your system is affected by this discrepancy check if
216 C<getconf GNU_LIBPTHREAD_VERSION | grep -q NPTL> returns a false
217 value. NTPL threads preserve the POSIX semantics.
219 Mnemonic: same as shells.
224 X<$0> X<$PROGRAM_NAME>
226 Contains the name of the program being executed.
228 On some (but not all) operating systems assigning to C<$0> modifies
229 the argument area that the C<ps> program sees. On some platforms you
230 may have to use special C<ps> options or a different C<ps> to see the
231 changes. Modifying the C<$0> is more useful as a way of indicating the
232 current program state than it is for hiding the program you're
235 Note that there are platform-specific limitations on the maximum
236 length of C<$0>. In the most extreme case it may be limited to the
237 space occupied by the original C<$0>.
239 In some platforms there may be arbitrary amount of padding, for
240 example space characters, after the modified name as shown by C<ps>.
241 In some platforms this padding may extend all the way to the original
242 length of the argument area, no matter what you do (this is the case
243 for example with Linux 2.2).
245 Note for BSD users: setting C<$0> does not completely remove "perl"
246 from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
247 result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
248 and the " (perl)" suffix are shown depends on your exact BSD variant
249 and version). This is an operating system feature, Perl cannot help it.
251 In multithreaded scripts Perl coordinates the threads so that any
252 thread may modify its copy of the C<$0> and the change becomes visible
253 to ps(1) (assuming the operating system plays along). Note that
254 the view of C<$0> the other threads have will not change since they
255 have their own copies of it.
257 If the program has been given to perl via the switches C<-e> or C<-E>,
258 C<$0> will contain the string C<"-e">.
260 On Linux as of perl v5.14.0 the legacy process name will be set with
261 C<prctl(2)>, in addition to altering the POSIX name via C<argv[0]> as
262 perl has done since version 4.000. Now system utilities that read the
263 legacy process name such as ps, top and killall will recognize the
264 name you set when assigning to C<$0>. The string you supply will be
265 cut off at 16 bytes, this is a limitation imposed by Linux.
267 Mnemonic: same as B<sh> and B<ksh>.
274 X<$(> X<$GID> X<$REAL_GROUP_ID>
276 The real gid of this process. If you are on a machine that supports
277 membership in multiple groups simultaneously, gives a space separated
278 list of groups you are in. The first number is the one returned by
279 C<getgid()>, and the subsequent ones by C<getgroups()>, one of which may be
280 the same as the first number.
282 However, a value assigned to C<$(> must be a single number used to
283 set the real gid. So the value given by C<$(> should I<not> be assigned
284 back to C<$(> without being forced numeric, such as by adding zero. Note
285 that this is different to the effective gid (C<$)>) which does take a
288 You can change both the real gid and the effective gid at the same
289 time by using C<POSIX::setgid()>. Changes
290 to C<$(> require a check to C<$!>
291 to detect any possible errors after an attempted change.
293 Mnemonic: parentheses are used to I<group> things. The real gid is the
294 group you I<left>, if you're running setgid.
296 =item $EFFECTIVE_GROUP_ID
301 X<$)> X<$EGID> X<$EFFECTIVE_GROUP_ID>
303 The effective gid of this process. If you are on a machine that
304 supports membership in multiple groups simultaneously, gives a space
305 separated list of groups you are in. The first number is the one
306 returned by C<getegid()>, and the subsequent ones by C<getgroups()>,
307 one of which may be the same as the first number.
309 Similarly, a value assigned to C<$)> must also be a space-separated
310 list of numbers. The first number sets the effective gid, and
311 the rest (if any) are passed to C<setgroups()>. To get the effect of an
312 empty list for C<setgroups()>, just repeat the new effective gid; that is,
313 to force an effective gid of 5 and an effectively empty C<setgroups()>
314 list, say C< $) = "5 5" >.
316 You can change both the effective gid and the real gid at the same
317 time by using C<POSIX::setgid()> (use only a single numeric argument).
318 Changes to C<$)> require a check to C<$!> to detect any possible errors
319 after an attempted change.
321 C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
322 machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
323 and C<$)> can be swapped only on machines supporting C<setregid()>.
325 Mnemonic: parentheses are used to I<group> things. The effective gid
326 is the group that's I<right> for you, if you're running setgid.
333 X<< $< >> X<$UID> X<$REAL_USER_ID>
335 The real uid of this process. You can change both the real uid and the
336 effective uid at the same time by using C<POSIX::setuid()>. Since
337 changes to C<< $< >> require a system call, check C<$!> after a change
338 attempt to detect any possible errors.
340 Mnemonic: it's the uid you came I<from>, if you're running setuid.
342 =item $EFFECTIVE_USER_ID
347 X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID>
349 The effective uid of this process. For example:
351 $< = $>; # set real to effective uid
352 ($<,$>) = ($>,$<); # swap real and effective uids
354 You can change both the effective uid and the real uid at the same
355 time by using C<POSIX::setuid()>. Changes to C<< $> >> require a check
356 to C<$!> to detect any possible errors after an attempted change.
358 C<< $< >> and C<< $> >> can be swapped only on machines
359 supporting C<setreuid()>.
361 Mnemonic: it's the uid you went I<to>, if you're running setuid.
363 =item $SUBSCRIPT_SEPARATOR
368 X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR>
370 The subscript separator for multidimensional array emulation. If you
371 refer to a hash element as
377 $foo{join($;, $a, $b, $c)}
381 @foo{$a,$b,$c} # a slice--note the @
385 ($foo{$a},$foo{$b},$foo{$c})
387 Default is "\034", the same as SUBSEP in B<awk>. If your keys contain
388 binary data there might not be any safe value for C<$;>.
390 Consider using "real" multidimensional arrays as described
393 Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
400 Special package variables when using C<sort()>, see L<perlfunc/sort>.
401 Because of this specialness C<$a> and C<$b> don't need to be declared
402 (using C<use vars>, or C<our()>) even when using the C<strict 'vars'>
403 pragma. Don't lexicalize them with C<my $a> or C<my $b> if you want to
404 be able to use them in the C<sort()> comparison block or function.
409 The hash C<%ENV> contains your current environment. Setting a
410 value in C<ENV> changes the environment for any child processes
411 you subsequently C<fork()> off.
413 As of v5.18.0, both keys and values stored in C<%ENV> are stringified.
417 if( ref $ENV{'bar'} ) {
418 say "Pre 5.18.0 Behaviour";
420 say "Post 5.18.0 Behaviour";
423 Previously, only child processes received stringified values:
428 # Always printed 'non ref'
430 q/print ( ref $ENV{'bar'} ? 'ref' : 'non ref' ) /);
432 This happens because you can't really share arbitrary data structures with
438 X<$^F> X<$SYSTEM_FD_MAX>
440 The maximum system file descriptor, ordinarily 2. System file
441 descriptors are passed to C<exec()>ed processes, while higher file
442 descriptors are not. Also, during an
443 C<open()>, system file descriptors are
444 preserved even if the C<open()> fails (ordinary file descriptors are
445 closed before the C<open()> is attempted). The close-on-exec
446 status of a file descriptor will be decided according to the value of
447 C<$^F> when the corresponding file, pipe, or socket was opened, not the
448 time of the C<exec()>.
453 The array C<@F> contains the fields of each line read in when autosplit
454 mode is turned on. See L<perlrun> for the B<-a> switch. This array
455 is package-specific, and must be declared or given a full package name
456 if not in package main when running under C<strict 'vars'>.
461 The array C<@INC> contains the list of places that the C<do EXPR>,
462 C<require>, or C<use> constructs look for their library files. It
463 initially consists of the arguments to any B<-I> command-line
464 switches, followed by the default Perl library, probably
465 F</usr/local/lib/perl>, followed by ".", to represent the current
466 directory. ("." will not be appended if taint checks are enabled,
467 either by C<-T> or by C<-t>.) If you need to modify this at runtime,
468 you should use the C<use lib> pragma to get the machine-dependent
469 library properly loaded also:
471 use lib '/mypath/libdir/';
474 You can also insert hooks into the file inclusion system by putting Perl
475 code directly into C<@INC>. Those hooks may be subroutine references,
476 array references or blessed objects. See L<perlfunc/require> for details.
481 The hash C<%INC> contains entries for each filename included via the
482 C<do>, C<require>, or C<use> operators. The key is the filename
483 you specified (with module names converted to pathnames), and the
484 value is the location of the file found. The C<require>
485 operator uses this hash to determine whether a particular file has
486 already been included.
488 If the file was loaded via a hook (e.g. a subroutine reference, see
489 L<perlfunc/require> for a description of these hooks), this hook is
490 by default inserted into C<%INC> in place of a filename. Note, however,
491 that the hook may have set the C<%INC> entry by itself to provide some more
497 X<$^I> X<$INPLACE_EDIT>
499 The current value of the inplace-edit extension. Use C<undef> to disable
502 Mnemonic: value of B<-i> switch.
507 By default, running out of memory is an untrappable, fatal error.
508 However, if suitably built, Perl can use the contents of C<$^M>
509 as an emergency memory pool after C<die()>ing. Suppose that your Perl
510 were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
513 $^M = 'a' x (1 << 16);
515 would allocate a 64K buffer for use in an emergency. See the
516 F<INSTALL> file in the Perl distribution for information on how to
517 add custom C compilation flags when compiling perl. To discourage casual
518 use of this advanced feature, there is no L<English|English> long name for
521 This variable was added in Perl 5.004.
528 The name of the operating system under which this copy of Perl was
529 built, as determined during the configuration process. For examples
530 see L<perlport/PLATFORMS>.
532 The value is identical to C<$Config{'osname'}>. See also L<Config>
533 and the B<-V> command-line switch documented in L<perlrun>.
535 In Windows platforms, C<$^O> is not very helpful: since it is always
536 C<MSWin32>, it doesn't tell the difference between
537 95/98/ME/NT/2000/XP/CE/.NET. Use C<Win32::GetOSName()> or
538 Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
539 between the variants.
541 This variable was added in Perl 5.003.
546 The hash C<%SIG> contains signal handlers for signals. For example:
548 sub handler { # 1st argument is signal name
550 print "Caught a SIG$sig--shutting down\n";
555 $SIG{'INT'} = \&handler;
556 $SIG{'QUIT'} = \&handler;
558 $SIG{'INT'} = 'DEFAULT'; # restore default action
559 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
561 Using a value of C<'IGNORE'> usually has the effect of ignoring the
562 signal, except for the C<CHLD> signal. See L<perlipc> for more about
565 Here are some other examples:
567 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not
569 $SIG{"PIPE"} = \&Plumber; # just fine; assume current
571 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
572 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber()
575 Be sure not to use a bareword as the name of a signal handler,
576 lest you inadvertently call it.
578 If your system has the C<sigaction()> function then signal handlers
579 are installed using it. This means you get reliable signal handling.
581 The default delivery policy of signals changed in Perl v5.8.0 from
582 immediate (also known as "unsafe") to deferred, also known as "safe
583 signals". See L<perlipc> for more information.
585 Certain internal hooks can be also set using the C<%SIG> hash. The
586 routine indicated by C<$SIG{__WARN__}> is called when a warning
587 message is about to be printed. The warning message is passed as the
588 first argument. The presence of a C<__WARN__> hook causes the
589 ordinary printing of warnings to C<STDERR> to be suppressed. You can
590 use this to save warnings in a variable, or turn warnings into fatal
593 local $SIG{__WARN__} = sub { die $_[0] };
596 As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
597 disable warnings using the empty subroutine:
599 local $SIG{__WARN__} = sub {};
601 The routine indicated by C<$SIG{__DIE__}> is called when a fatal
602 exception is about to be thrown. The error message is passed as the
603 first argument. When a C<__DIE__> hook routine returns, the exception
604 processing continues as it would have in the absence of the hook,
605 unless the hook routine itself exits via a C<goto &sub>, a loop exit,
606 or a C<die()>. The C<__DIE__> handler is explicitly disabled during
607 the call, so that you can die from a C<__DIE__> handler. Similarly
610 Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
611 even inside an C<eval()>. Do not use this to rewrite a pending
612 exception in C<$@>, or as a bizarre substitute for overriding
613 C<CORE::GLOBAL::die()>. This strange action at a distance may be fixed
614 in a future release so that C<$SIG{__DIE__}> is only called if your
615 program is about to exit, as was the original intent. Any other use is
618 C<__DIE__>/C<__WARN__> handlers are very special in one respect: they
619 may be called to report (probable) errors found by the parser. In such
620 a case the parser may be in inconsistent state, so any attempt to
621 evaluate Perl code from such a handler will probably result in a
622 segfault. This means that warnings or errors that result from parsing
623 Perl should be used with extreme caution, like this:
625 require Carp if defined $^S;
626 Carp::confess("Something wrong") if defined &Carp::confess;
627 die "Something wrong, but could not load Carp to give "
629 . "To see backtrace try starting Perl with -MCarp switch";
631 Here the first line will load C<Carp> I<unless> it is the parser who
632 called the handler. The second line will print backtrace and die if
633 C<Carp> was available. The third line will be executed only if C<Carp> was
636 Having to even think about the C<$^S> variable in your exception
637 handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
638 invites grievous and difficult to track down errors. Avoid it
639 and use an C<END{}> or CORE::GLOBAL::die override instead.
641 See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
642 L<warnings> for additional information.
649 The time at which the program began running, in seconds since the
650 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
651 and B<-C> filetests are based on this value.
656 X<$^V> X<$PERL_VERSION>
658 The revision, version, and subversion of the Perl interpreter,
659 represented as a C<version> object.
661 This variable first appeared in perl v5.6.0; earlier versions of perl
662 will see an undefined value. Before perl v5.10.0 C<$^V> was represented
665 C<$^V> can be used to determine whether the Perl interpreter executing
666 a script is in the right range of versions. For example:
668 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
670 To convert C<$^V> into its string representation use C<sprintf()>'s
673 printf "version is v%vd\n", $^V; # Perl's version
675 See the documentation of C<use VERSION> and C<require VERSION>
676 for a convenient way to fail if the running Perl interpreter is too old.
678 See also C<$]> for an older representation of the Perl version.
680 This variable was added in Perl v5.6.0.
682 Mnemonic: use ^V for Version Control.
684 =item ${^WIN32_SLOPPY_STAT}
685 X<${^WIN32_SLOPPY_STAT}> X<sitecustomize> X<sitecustomize.pl>
687 If this variable is set to a true value, then C<stat()> on Windows will
688 not try to open the file. This means that the link count cannot be
689 determined and file attributes may be out of date if additional
690 hardlinks to the file exist. On the other hand, not opening the file
691 is considerably faster, especially for files on network drives.
693 This variable could be set in the F<sitecustomize.pl> file to
694 configure the local Perl installation to use "sloppy" C<stat()> by
695 default. See the documentation for B<-f> in
696 L<perlrun|perlrun/"Command Switches"> for more information about site
699 This variable was added in Perl v5.10.0.
701 =item $EXECUTABLE_NAME
704 X<$^X> X<$EXECUTABLE_NAME>
706 The name used to execute the current copy of Perl, from C's
707 C<argv[0]> or (where supported) F</proc/self/exe>.
709 Depending on the host operating system, the value of C<$^X> may be
710 a relative or absolute pathname of the perl program file, or may
711 be the string used to invoke perl but not the pathname of the
712 perl program file. Also, most operating systems permit invoking
713 programs that are not in the PATH environment variable, so there
714 is no guarantee that the value of C<$^X> is in PATH. For VMS, the
715 value may or may not include a version number.
717 You usually can use the value of C<$^X> to re-invoke an independent
718 copy of the same perl that is currently running, e.g.,
720 @first_run = `$^X -le "print int rand 100 for 1..100"`;
722 But recall that not all operating systems support forking or
723 capturing of the output of commands, so this complex statement
726 It is not safe to use the value of C<$^X> as a path name of a file,
727 as some operating systems that have a mandatory suffix on
728 executable files do not require use of the suffix when invoking
729 a command. To convert the value of C<$^X> to a path name, use the
730 following statements:
732 # Build up a set of file names (not command names).
736 $this_perl .= $Config{_exe}
737 unless $this_perl =~ m/$Config{_exe}$/i;
740 Because many operating systems permit anyone with read access to
741 the Perl program file to make a copy of it, patch the copy, and
742 then execute the copy, the security-conscious Perl programmer
743 should take care to invoke the installed copy of perl, not the
744 copy referenced by C<$^X>. The following statements accomplish
745 this goal, and produce a pathname that can be invoked as a
746 command or referenced as a file.
749 my $secure_perl_path = $Config{perlpath};
751 $secure_perl_path .= $Config{_exe}
752 unless $secure_perl_path =~ m/$Config{_exe}$/i;
757 =head2 Variables related to regular expressions
759 Most of the special variables related to regular expressions are side
760 effects. Perl sets these variables when it has a successful match, so
761 you should check the match result before using them. For instance:
763 if( /P(A)TT(ER)N/ ) {
764 print "I found $1 and $2\n";
767 These variables are read-only and dynamically-scoped, unless we note
770 The dynamic nature of the regular expression variables means that
771 their value is limited to the block that they are in, as demonstrated
774 my $outer = 'Wallace and Grommit';
775 my $inner = 'Mutt and Jeff';
777 my $pattern = qr/(\S+) and (\S+)/;
779 sub show_n { print "\$1 is $1; \$2 is $2\n" }
783 show_n() if $outer =~ m/$pattern/;
786 show_n() if $inner =~ m/$pattern/;
792 The output shows that while in the C<OUTER> block, the values of C<$1>
793 and C<$2> are from the match against C<$outer>. Inside the C<INNER>
794 block, the values of C<$1> and C<$2> are from the match against
795 C<$inner>, but only until the end of the block (i.e. the dynamic
796 scope). After the C<INNER> block completes, the values of C<$1> and
797 C<$2> return to the values for the match against C<$outer> even though
798 we have not made another match:
800 $1 is Wallace; $2 is Grommit
801 $1 is Mutt; $2 is Jeff
802 $1 is Wallace; $2 is Grommit
804 =head3 Performance issues
806 Traditionally in Perl, any use of any of the three variables C<$`>, C<$&>
807 or C<$'> (or their C<use English> equivalents) anywhere in the code, caused
808 all subsequent successful pattern matches to make a copy of the matched
809 string, in case the code might subsequently access one of those variables.
810 This imposed a considerable performance penalty across the whole program,
811 so generally the use of these variables has been discouraged.
813 In Perl 5.6.0 the C<@-> and C<@+> dynamic arrays were introduced that
814 supply the indices of successful matches. So you could for example do
819 print $`, $&, $'; # bad: perfomance hit
821 print # good: no perfomance hit
822 substr($str, 0, $-[0]),
823 substr($str, $-[0], $+[0]-$-[0]),
826 In Perl 5.10.0 the C</p> match operator flag and the C<${^PREMATCH}>,
827 C<${^MATCH}>, and C<${^POSTMATCH}> variables were introduced, that allowed
828 you to suffer the penalties only on patterns marked with C</p>.
830 In Perl 5.18.0 onwards, perl started noting the presence of each of the
831 three variables separately, and only copied that part of the string
834 $`; $&; "abcdefgh" =~ /d/
836 perl would only copy the "abcd" part of the string. That could make a big
837 difference in something like
839 $str = 'x' x 1_000_000;
841 $str =~ /x/g # one char copied a million times, not a million chars
843 In Perl 5.20.0 a new copy-on-write system was enabled by default, which
844 finally fixes all performance issues with these three variables, and makes
845 them safe to use anywhere.
847 The C<Devel::NYTProf> and C<Devel::FindAmpersand> modules can help you
848 find uses of these problematic match variables in your code.
852 =item $<I<digits>> ($1, $2, ...)
855 Contains the subpattern from the corresponding set of capturing
856 parentheses from the last successful pattern match, not counting patterns
857 matched in nested blocks that have been exited already.
859 These variables are read-only and dynamically-scoped.
861 Mnemonic: like \digits.
868 The string matched by the last successful pattern match (not counting
869 any matches hidden within a BLOCK or C<eval()> enclosed by the current
872 See L</Performance issues> above for the serious performance implications
873 of using this variable (even once) in your code.
875 This variable is read-only and dynamically-scoped.
877 Mnemonic: like C<&> in some editors.
882 This is similar to C<$&> (C<$MATCH>) except that it does not incur the
883 performance penalty associated with that variable.
885 See L</Performance issues> above.
887 In Perl v5.18 and earlier, it is only guaranteed
888 to return a defined value when the pattern was compiled or executed with
889 the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
890 C<${^MATCH}> does the same thing as C<$MATCH>.
892 This variable was added in Perl v5.10.0.
894 This variable is read-only and dynamically-scoped.
899 X<$`> X<$PREMATCH> X<${^PREMATCH}>
901 The string preceding whatever was matched by the last successful
902 pattern match, not counting any matches hidden within a BLOCK or C<eval>
903 enclosed by the current BLOCK.
905 See L</Performance issues> above for the serious performance implications
906 of using this variable (even once) in your code.
908 This variable is read-only and dynamically-scoped.
910 Mnemonic: C<`> often precedes a quoted string.
913 X<$`> X<${^PREMATCH}>
915 This is similar to C<$`> ($PREMATCH) except that it does not incur the
916 performance penalty associated with that variable.
918 See L</Performance issues> above.
920 In Perl v5.18 and earlier, it is only guaranteed
921 to return a defined value when the pattern was compiled or executed with
922 the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
923 C<${^PREMATCH}> does the same thing as C<$PREMATCH>.
925 This variable was added in Perl v5.10.0
927 This variable is read-only and dynamically-scoped.
932 X<$'> X<$POSTMATCH> X<${^POSTMATCH}> X<@->
934 The string following whatever was matched by the last successful
935 pattern match (not counting any matches hidden within a BLOCK or C<eval()>
936 enclosed by the current BLOCK). Example:
938 local $_ = 'abcdefghi';
940 print "$`:$&:$'\n"; # prints abc:def:ghi
942 See L</Performance issues> above for the serious performance implications
943 of using this variable (even once) in your code.
945 This variable is read-only and dynamically-scoped.
947 Mnemonic: C<'> often follows a quoted string.
950 X<${^POSTMATCH}> X<$'> X<$POSTMATCH>
952 This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
953 performance penalty associated with that variable.
955 See L</Performance issues> above.
957 In Perl v5.18 and earlier, it is only guaranteed
958 to return a defined value when the pattern was compiled or executed with
959 the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
960 C<${^POSTMATCH}> does the same thing as C<$POSTMATCH>.
962 This variable was added in Perl v5.10.0.
964 This variable is read-only and dynamically-scoped.
966 =item $LAST_PAREN_MATCH
969 X<$+> X<$LAST_PAREN_MATCH>
971 The text matched by the last bracket of the last successful search pattern.
972 This is useful if you don't know which one of a set of alternative patterns
973 matched. For example:
975 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
977 This variable is read-only and dynamically-scoped.
979 Mnemonic: be positive and forward looking.
981 =item $LAST_SUBMATCH_RESULT
984 X<$^N> X<$LAST_SUBMATCH_RESULT>
986 The text matched by the used group most-recently closed (i.e. the group
987 with the rightmost closing parenthesis) of the last successful search
990 This is primarily used inside C<(?{...})> blocks for examining text
991 recently matched. For example, to effectively capture text to a variable
992 (in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
994 (?:(...)(?{ $var = $^N }))
996 By setting and then using C<$var> in this way relieves you from having to
997 worry about exactly which numbered set of parentheses they are.
999 This variable was added in Perl v5.8.0.
1001 Mnemonic: the (possibly) Nested parenthesis that most recently closed.
1003 =item @LAST_MATCH_END
1006 X<@+> X<@LAST_MATCH_END>
1008 This array holds the offsets of the ends of the last successful
1009 submatches in the currently active dynamic scope. C<$+[0]> is
1010 the offset into the string of the end of the entire match. This
1011 is the same value as what the C<pos> function returns when called
1012 on the variable that was matched against. The I<n>th element
1013 of this array holds the offset of the I<n>th submatch, so
1014 C<$+[1]> is the offset past where C<$1> ends, C<$+[2]> the offset
1015 past where C<$2> ends, and so on. You can use C<$#+> to determine
1016 how many subgroups were in the last successful match. See the
1017 examples given for the C<@-> variable.
1019 This variable was added in Perl v5.6.0.
1021 =item %LAST_PAREN_MATCH
1024 X<%+> X<%LAST_PAREN_MATCH>
1026 Similar to C<@+>, the C<%+> hash allows access to the named capture
1027 buffers, should they exist, in the last successful match in the
1028 currently active dynamic scope.
1030 For example, C<$+{foo}> is equivalent to C<$1> after the following match:
1032 'foo' =~ /(?<foo>foo)/;
1034 The keys of the C<%+> hash list only the names of buffers that have
1035 captured (and that are thus associated to defined values).
1037 The underlying behaviour of C<%+> is provided by the
1038 L<Tie::Hash::NamedCapture> module.
1040 B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1041 associated with the last successful regular expression. Therefore mixing
1042 iterative access to them via C<each> may have unpredictable results.
1043 Likewise, if the last successful match changes, then the results may be
1046 This variable was added in Perl v5.10.0.
1048 This variable is read-only and dynamically-scoped.
1050 =item @LAST_MATCH_START
1053 X<@-> X<@LAST_MATCH_START>
1055 C<$-[0]> is the offset of the start of the last successful match.
1056 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
1057 I<n>-th subpattern, or undef if the subpattern did not match.
1059 Thus, after a match against C<$_>, C<$&> coincides with C<substr $_, $-[0],
1060 $+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
1061 $+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
1062 C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the
1063 last matched subgroup in the last successful match. Contrast with
1064 C<$#+>, the number of subgroups in the regular expression. Compare
1067 This array holds the offsets of the beginnings of the last
1068 successful submatches in the currently active dynamic scope.
1069 C<$-[0]> is the offset into the string of the beginning of the
1070 entire match. The I<n>th element of this array holds the offset
1071 of the I<n>th submatch, so C<$-[1]> is the offset where C<$1>
1072 begins, C<$-[2]> the offset where C<$2> begins, and so on.
1074 After a match against some variable C<$var>:
1078 =item C<$`> is the same as C<substr($var, 0, $-[0])>
1080 =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
1082 =item C<$'> is the same as C<substr($var, $+[0])>
1084 =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
1086 =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
1088 =item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
1092 This variable was added in Perl v5.6.0.
1094 =item %LAST_MATCH_START
1097 X<%-> X<%LAST_MATCH_START>
1099 Similar to C<%+>, this variable allows access to the named capture groups
1100 in the last successful match in the currently active dynamic scope. To
1101 each capture group name found in the regular expression, it associates a
1102 reference to an array containing the list of values captured by all
1103 buffers with that name (should there be several of them), in the order
1108 if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
1109 foreach my $bufname (sort keys %-) {
1110 my $ary = $-{$bufname};
1111 foreach my $idx (0..$#$ary) {
1112 print "\$-{$bufname}[$idx] : ",
1113 (defined($ary->[$idx])
1128 The keys of the C<%-> hash correspond to all buffer names found in
1129 the regular expression.
1131 The behaviour of C<%-> is implemented via the
1132 L<Tie::Hash::NamedCapture> module.
1134 B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1135 associated with the last successful regular expression. Therefore mixing
1136 iterative access to them via C<each> may have unpredictable results.
1137 Likewise, if the last successful match changes, then the results may be
1140 This variable was added in Perl v5.10.0.
1142 This variable is read-only and dynamically-scoped.
1144 =item $LAST_REGEXP_CODE_RESULT
1147 X<$^R> X<$LAST_REGEXP_CODE_RESULT>
1149 The result of evaluation of the last successful C<(?{ code })>
1150 regular expression assertion (see L<perlre>). May be written to.
1152 This variable was added in Perl 5.005.
1154 =item ${^RE_DEBUG_FLAGS}
1155 X<${^RE_DEBUG_FLAGS}>
1157 The current value of the regex debugging flags. Set to 0 for no debug output
1158 even when the C<re 'debug'> module is loaded. See L<re> for details.
1160 This variable was added in Perl v5.10.0.
1162 =item ${^RE_TRIE_MAXBUF}
1163 X<${^RE_TRIE_MAXBUF}>
1165 Controls how certain regex optimisations are applied and how much memory they
1166 utilize. This value by default is 65536 which corresponds to a 512kB
1167 temporary cache. Set this to a higher value to trade
1168 memory for speed when matching large alternations. Set
1169 it to a lower value if you want the optimisations to
1170 be as conservative of memory as possible but still occur, and set it to a
1171 negative value to prevent the optimisation and conserve the most memory.
1172 Under normal situations this variable should be of no interest to you.
1174 This variable was added in Perl v5.10.0.
1178 =head2 Variables related to filehandles
1180 Variables that depend on the currently selected filehandle may be set
1181 by calling an appropriate object method on the C<IO::Handle> object,
1182 although this is less efficient than using the regular built-in
1183 variables. (Summary lines below for this contain the word HANDLE.)
1188 after which you may use either
1194 HANDLE->method(EXPR)
1196 Each method returns the old value of the C<IO::Handle> attribute. The
1197 methods each take an optional EXPR, which, if supplied, specifies the
1198 new value for the C<IO::Handle> attribute in question. If not
1199 supplied, most methods do nothing to the current value--except for
1200 C<autoflush()>, which will assume a 1 for you, just to be different.
1202 Because loading in the C<IO::Handle> class is an expensive operation,
1203 you should learn how to use the regular built-in variables.
1205 A few of these variables are considered "read-only". This means that
1206 if you try to assign to this variable, either directly or indirectly
1207 through a reference, you'll raise a run-time exception.
1209 You should be very careful when modifying the default values of most
1210 special variables described in this document. In most cases you want
1211 to localize these variables before changing them, since if you don't,
1212 the change may affect other modules which rely on the default values
1213 of the special variables that you have changed. This is one of the
1214 correct ways to read the whole file at once:
1216 open my $fh, "<", "foo" or die $!;
1217 local $/; # enable localized slurp mode
1218 my $content = <$fh>;
1221 But the following code is quite bad:
1223 open my $fh, "<", "foo" or die $!;
1224 undef $/; # enable slurp mode
1225 my $content = <$fh>;
1228 since some other module, may want to read data from some file in the
1229 default "line mode", so if the code we have just presented has been
1230 executed, the global value of C<$/> is now changed for any other code
1231 running inside the same Perl interpreter.
1233 Usually when a variable is localized you want to make sure that this
1234 change affects the shortest scope possible. So unless you are already
1235 inside some short C<{}> block, you should create one yourself. For
1239 open my $fh, "<", "foo" or die $!;
1246 Here is an example of how your own code can go broken:
1256 # do something with $_
1259 You probably expect this code to print the equivalent of
1263 but instead you get:
1267 Why? Because C<nasty_break()> modifies C<$\> without localizing it
1268 first. The value you set in C<nasty_break()> is still there when you
1269 return. The fix is to add C<local()> so the value doesn't leak out of
1274 It's easy to notice the problem in such a short example, but in more
1275 complicated code you are looking for trouble if you don't localize
1276 changes to the special variables.
1283 Contains the name of the current file when reading from C<< <> >>.
1288 The array C<@ARGV> contains the command-line arguments intended for
1289 the script. C<$#ARGV> is generally the number of arguments minus
1290 one, because C<$ARGV[0]> is the first argument, I<not> the program's
1291 command name itself. See L</$0> for the command name.
1296 The special filehandle that iterates over command-line filenames in
1297 C<@ARGV>. Usually written as the null filehandle in the angle operator
1298 C<< <> >>. Note that currently C<ARGV> only has its magical effect
1299 within the C<< <> >> operator; elsewhere it is just a plain filehandle
1300 corresponding to the last file opened by C<< <> >>. In particular,
1301 passing C<\*ARGV> as a parameter to a function that expects a filehandle
1302 may not cause your function to automatically read the contents of all the
1308 The special filehandle that points to the currently open output file
1309 when doing edit-in-place processing with B<-i>. Useful when you have
1310 to do a lot of inserting and don't want to keep modifying C<$_>. See
1311 L<perlrun> for the B<-i> switch.
1313 =item IO::Handle->output_field_separator( EXPR )
1315 =item $OUTPUT_FIELD_SEPARATOR
1320 X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>
1322 The output field separator for the print operator. If defined, this
1323 value is printed between each of print's arguments. Default is C<undef>.
1325 You cannot call C<output_field_separator()> on a handle, only as a
1326 static method. See L<IO::Handle|IO::Handle>.
1328 Mnemonic: what is printed when there is a "," in your print statement.
1330 =item HANDLE->input_line_number( EXPR )
1332 =item $INPUT_LINE_NUMBER
1337 X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
1339 Current line number for the last filehandle accessed.
1341 Each filehandle in Perl counts the number of lines that have been read
1342 from it. (Depending on the value of C<$/>, Perl's idea of what
1343 constitutes a line may not match yours.) When a line is read from a
1344 filehandle (via C<readline()> or C<< <> >>), or when C<tell()> or
1345 C<seek()> is called on it, C<$.> becomes an alias to the line counter
1346 for that filehandle.
1348 You can adjust the counter by assigning to C<$.>, but this will not
1349 actually move the seek pointer. I<Localizing C<$.> will not localize
1350 the filehandle's line count>. Instead, it will localize perl's notion
1351 of which filehandle C<$.> is currently aliased to.
1353 C<$.> is reset when the filehandle is closed, but B<not> when an open
1354 filehandle is reopened without an intervening C<close()>. For more
1355 details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
1356 an explicit close, line numbers increase across C<ARGV> files (but see
1357 examples in L<perlfunc/eof>).
1359 You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
1360 line counter for a given filehandle without having to worry about
1361 which handle you last accessed.
1363 Mnemonic: many programs use "." to mean the current line number.
1365 =item IO::Handle->input_record_separator( EXPR )
1367 =item $INPUT_RECORD_SEPARATOR
1372 X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR>
1374 The input record separator, newline by default. This influences Perl's
1375 idea of what a "line" is. Works like B<awk>'s RS variable, including
1376 treating empty lines as a terminator if set to the null string (an
1377 empty line cannot contain any spaces or tabs). You may set it to a
1378 multi-character string to match a multi-character terminator, or to
1379 C<undef> to read through the end of file. Setting it to C<"\n\n">
1380 means something slightly different than setting to C<"">, if the file
1381 contains consecutive empty lines. Setting to C<""> will treat two or
1382 more consecutive empty lines as a single empty line. Setting to
1383 C<"\n\n"> will blindly assume that the next input character belongs to
1384 the next paragraph, even if it's a newline.
1386 local $/; # enable "slurp" mode
1387 local $_ = <FH>; # whole file now here
1390 Remember: the value of C<$/> is a string, not a regex. B<awk> has to
1391 be better for something. :-)
1393 Setting C<$/> to a reference to an integer, scalar containing an
1394 integer, or scalar that's convertible to an integer will attempt to
1395 read records instead of lines, with the maximum record size being the
1396 referenced integer number of characters. So this:
1398 local $/ = \32768; # or \"32768", or \$var_containing_32768
1399 open my $fh, "<", $myfile or die $!;
1402 will read a record of no more than 32768 characters from $fh. If you're
1403 not reading from a record-oriented file (or your OS doesn't have
1404 record-oriented files), then you'll likely get a full chunk of data
1405 with every read. If a record is larger than the record size you've
1406 set, you'll get the record back in pieces. Trying to set the record
1407 size to zero or less is deprecated and will cause $/ to have the value
1408 of "undef", which will cause reading in the (rest of the) whole file.
1410 As of 5.19.9 setting C<$/> to any other form of reference will throw a
1411 fatal exception. This is in preparation for supporting new ways to set
1412 C<$/> in the future.
1414 On VMS only, record reads bypass PerlIO layers and any associated
1415 buffering, so you must not mix record and non-record reads on the
1416 same filehandle. Record mode mixes with line mode only when the
1417 same buffering layer is in use for both modes.
1419 You cannot call C<input_record_separator()> on a handle, only as a
1420 static method. See L<IO::Handle|IO::Handle>.
1422 See also L<perlport/"Newlines">. Also see L</$.>.
1424 Mnemonic: / delimits line boundaries when quoting poetry.
1426 =item IO::Handle->output_record_separator( EXPR )
1428 =item $OUTPUT_RECORD_SEPARATOR
1433 X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR>
1435 The output record separator for the print operator. If defined, this
1436 value is printed after the last of print's arguments. Default is C<undef>.
1438 You cannot call C<output_record_separator()> on a handle, only as a
1439 static method. See L<IO::Handle|IO::Handle>.
1441 Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
1442 Also, it's just like C<$/>, but it's what you get "back" from Perl.
1444 =item HANDLE->autoflush( EXPR )
1446 =item $OUTPUT_AUTOFLUSH
1449 X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
1451 If set to nonzero, forces a flush right away and after every write or
1452 print on the currently selected output channel. Default is 0
1453 (regardless of whether the channel is really buffered by the system or
1454 not; C<$|> tells you only whether you've asked Perl explicitly to
1455 flush after each write). STDOUT will typically be line buffered if
1456 output is to the terminal and block buffered otherwise. Setting this
1457 variable is useful primarily when you are outputting to a pipe or
1458 socket, such as when you are running a Perl program under B<rsh> and
1459 want to see the output as it's happening. This has no effect on input
1460 buffering. See L<perlfunc/getc> for that. See L<perlfunc/select> on
1461 how to select the output channel. See also L<IO::Handle>.
1463 Mnemonic: when you want your pipes to be piping hot.
1468 This read-only variable contains a reference to the last-read filehandle.
1469 This is set by C<< <HANDLE> >>, C<readline>, C<tell>, C<eof> and C<seek>.
1470 This is the same handle that C<$.> and C<tell> and C<eof> without arguments
1471 use. It is also the handle used when Perl appends ", <STDIN> line 1" to
1472 an error or warning message.
1474 This variable was added in Perl v5.18.0.
1478 =head3 Variables related to formats
1480 The special variables for formats are a subset of those for
1481 filehandles. See L<perlform> for more information about Perl's
1489 X<$^A> X<$ACCUMULATOR>
1491 The current value of the C<write()> accumulator for C<format()> lines.
1492 A format contains C<formline()> calls that put their result into
1493 C<$^A>. After calling its format, C<write()> prints out the contents
1494 of C<$^A> and empties. So you never really see the contents of C<$^A>
1495 unless you call C<formline()> yourself and then look at it. See
1496 L<perlform> and L<perlfunc/"formline PICTURE,LIST">.
1498 =item IO::Handle->format_formfeed(EXPR)
1500 =item $FORMAT_FORMFEED
1503 X<$^L> X<$FORMAT_FORMFEED>
1505 What formats output as a form feed. The default is C<\f>.
1507 You cannot call C<format_formfeed()> on a handle, only as a static
1508 method. See L<IO::Handle|IO::Handle>.
1510 =item HANDLE->format_page_number(EXPR)
1512 =item $FORMAT_PAGE_NUMBER
1515 X<$%> X<$FORMAT_PAGE_NUMBER>
1517 The current page number of the currently selected output channel.
1519 Mnemonic: C<%> is page number in B<nroff>.
1521 =item HANDLE->format_lines_left(EXPR)
1523 =item $FORMAT_LINES_LEFT
1526 X<$-> X<$FORMAT_LINES_LEFT>
1528 The number of lines left on the page of the currently selected output
1531 Mnemonic: lines_on_page - lines_printed.
1533 =item IO::Handle->format_line_break_characters EXPR
1535 =item $FORMAT_LINE_BREAK_CHARACTERS
1538 X<$:> X<FORMAT_LINE_BREAK_CHARACTERS>
1540 The current set of characters after which a string may be broken to
1541 fill continuation fields (starting with C<^>) in a format. The default is
1542 S<" \n-">, to break on a space, newline, or a hyphen.
1544 You cannot call C<format_line_break_characters()> on a handle, only as
1545 a static method. See L<IO::Handle|IO::Handle>.
1547 Mnemonic: a "colon" in poetry is a part of a line.
1549 =item HANDLE->format_lines_per_page(EXPR)
1551 =item $FORMAT_LINES_PER_PAGE
1554 X<$=> X<$FORMAT_LINES_PER_PAGE>
1556 The current page length (printable lines) of the currently selected
1557 output channel. The default is 60.
1559 Mnemonic: = has horizontal lines.
1561 =item HANDLE->format_top_name(EXPR)
1563 =item $FORMAT_TOP_NAME
1566 X<$^> X<$FORMAT_TOP_NAME>
1568 The name of the current top-of-page format for the currently selected
1569 output channel. The default is the name of the filehandle with C<_TOP>
1570 appended. For example, the default format top name for the C<STDOUT>
1571 filehandle is C<STDOUT_TOP>.
1573 Mnemonic: points to top of page.
1575 =item HANDLE->format_name(EXPR)
1580 X<$~> X<$FORMAT_NAME>
1582 The name of the current report format for the currently selected
1583 output channel. The default format name is the same as the filehandle
1584 name. For example, the default format name for the C<STDOUT>
1585 filehandle is just C<STDOUT>.
1587 Mnemonic: brother to C<$^>.
1591 =head2 Error Variables
1592 X<error> X<exception>
1594 The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1595 about different types of error conditions that may appear during
1596 execution of a Perl program. The variables are shown ordered by
1597 the "distance" between the subsystem which reported the error and
1598 the Perl process. They correspond to errors detected by the Perl
1599 interpreter, C library, operating system, or an external program,
1602 To illustrate the differences between these variables, consider the
1603 following Perl expression, which uses a single-quoted string. After
1604 execution of this statement, perl may have set all four special error
1608 open my $pipe, "/cdrom/install |" or die $!;
1610 close $pipe or die "bad pipe: $?, $!";
1613 When perl executes the C<eval()> expression, it translates the
1614 C<open()>, C<< <PIPE> >>, and C<close> calls in the C run-time library
1615 and thence to the operating system kernel. perl sets C<$!> to
1616 the C library's C<errno> if one of these calls fails.
1618 C<$@> is set if the string to be C<eval>-ed did not compile (this may
1619 happen if C<open> or C<close> were imported with bad prototypes), or
1620 if Perl code executed during evaluation C<die()>d. In these cases the
1621 value of C<$@> is the compile error, or the argument to C<die> (which
1622 will interpolate C<$!> and C<$?>). (See also L<Fatal>, though.)
1624 Under a few operating systems, C<$^E> may contain a more verbose error
1625 indicator, such as in this case, "CDROM tray not closed." Systems that
1626 do not support extended error messages leave C<$^E> the same as C<$!>.
1628 Finally, C<$?> may be set to non-0 value if the external program
1629 F</cdrom/install> fails. The upper eight bits reflect specific error
1630 conditions encountered by the program (the program's C<exit()> value).
1631 The lower eight bits reflect mode of failure, like signal death and
1632 core dump information. See L<wait(2)> for details. In contrast to
1633 C<$!> and C<$^E>, which are set only if error condition is detected,
1634 the variable C<$?> is set on each C<wait> or pipe C<close>,
1635 overwriting the old value. This is more like C<$@>, which on every
1636 C<eval()> is always set on failure and cleared on success.
1638 For more details, see the individual descriptions at C<$@>, C<$!>,
1643 =item ${^CHILD_ERROR_NATIVE}
1644 X<$^CHILD_ERROR_NATIVE>
1646 The native status returned by the last pipe close, backtick (C<``>)
1647 command, successful call to C<wait()> or C<waitpid()>, or from the
1648 C<system()> operator. On POSIX-like systems this value can be decoded
1649 with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED,
1650 WSTOPSIG and WIFCONTINUED functions provided by the L<POSIX> module.
1652 Under VMS this reflects the actual VMS exit status; i.e. it is the
1653 same as C<$?> when the pragma C<use vmsish 'status'> is in effect.
1655 This variable was added in Perl v5.10.0.
1657 =item $EXTENDED_OS_ERROR
1660 X<$^E> X<$EXTENDED_OS_ERROR>
1662 Error information specific to the current operating system. At the
1663 moment, this differs from C<$!> under only VMS, OS/2, and Win32 (and
1664 for MacPerl). On all other platforms, C<$^E> is always just the same
1667 Under VMS, C<$^E> provides the VMS status value from the last system
1668 error. This is more specific information about the last system error
1669 than that provided by C<$!>. This is particularly important when C<$!>
1670 is set to B<EVMSERR>.
1672 Under OS/2, C<$^E> is set to the error code of the last call to OS/2
1673 API either via CRT, or directly from perl.
1675 Under Win32, C<$^E> always returns the last error information reported
1676 by the Win32 call C<GetLastError()> which describes the last error
1677 from within the Win32 API. Most Win32-specific code will report errors
1678 via C<$^E>. ANSI C and Unix-like calls set C<errno> and so most
1679 portable Perl code will report errors via C<$!>.
1681 Caveats mentioned in the description of C<$!> generally apply to
1684 This variable was added in Perl 5.003.
1686 Mnemonic: Extra error explanation.
1688 =item $EXCEPTIONS_BEING_CAUGHT
1691 X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>
1693 Current state of the interpreter.
1696 --------- -------------------------------------
1697 undef Parsing module, eval, or main program
1698 true (1) Executing an eval
1701 The first state may happen in C<$SIG{__DIE__}> and C<$SIG{__WARN__}>
1704 The English name $EXCEPTIONS_BEING_CAUGHT is slightly misleading, because
1705 the C<undef> value does not indicate whether exceptions are being caught,
1706 since compilation of the main program does not catch exceptions.
1708 This variable was added in Perl 5.004.
1715 The current value of the warning switch, initially true if B<-w> was
1716 used, false otherwise, but directly modifiable.
1718 See also L<warnings>.
1720 Mnemonic: related to the B<-w> switch.
1722 =item ${^WARNING_BITS}
1725 The current set of warning checks enabled by the C<use warnings> pragma.
1726 It has the same scoping as the C<$^H> and C<%^H> variables. The exact
1727 values are considered internal to the L<warnings> pragma and may change
1728 between versions of Perl.
1730 This variable was added in Perl v5.6.0.
1737 X<$!> X<$ERRNO> X<$OS_ERROR>
1739 When referenced, C<$!> retrieves the current value
1740 of the C C<errno> integer variable.
1741 If C<$!> is assigned a numerical value, that value is stored in C<errno>.
1742 When referenced as a string, C<$!> yields the system error string
1743 corresponding to C<errno>.
1745 Many system or library calls set C<errno> if they fail,
1746 to indicate the cause of failure. They usually do B<not>
1747 set C<errno> to zero if they succeed. This means C<errno>,
1748 hence C<$!>, is meaningful only I<immediately> after a B<failure>:
1750 if (open my $fh, "<", $filename) {
1751 # Here $! is meaningless.
1755 # ONLY here is $! meaningful.
1757 # Already here $! might be meaningless.
1759 # Since here we might have either success or failure,
1760 # $! is meaningless.
1762 Here, I<meaningless> means that C<$!> may be unrelated to the outcome
1763 of the C<open()> operator. Assignment to C<$!> is similarly ephemeral.
1764 It can be used immediately before invoking the C<die()> operator,
1765 to set the exit value, or to inspect the system error string
1766 corresponding to error I<n>, or to restore C<$!> to a meaningful state.
1768 Note that when stringified, the text is always returned as if both
1769 S<L<C<"use locale">|perllocale>> and S<L<C<"use bytes">|bytes>> are in
1770 effect. This is likely to change in v5.22.
1772 Mnemonic: What just went bang?
1779 X<%!> X<%OS_ERROR> X<%ERRNO>
1781 Each element of C<%!> has a true value only if C<$!> is set to that
1782 value. For example, C<$!{ENOENT}> is true if and only if the current
1783 value of C<$!> is C<ENOENT>; that is, if the most recent error was "No
1784 such file or directory" (or its moral equivalent: not all operating
1785 systems give that exact error, and certainly not all languages). To
1786 check if a particular key is meaningful on your system, use C<exists
1787 $!{the_key}>; for a list of legal keys, use C<keys %!>. See L<Errno>
1788 for more information, and also see L</$!>.
1790 This variable was added in Perl 5.005.
1795 X<$?> X<$CHILD_ERROR>
1797 The status returned by the last pipe close, backtick (C<``>) command,
1798 successful call to C<wait()> or C<waitpid()>, or from the C<system()>
1799 operator. This is just the 16-bit status word returned by the
1800 traditional Unix C<wait()> system call (or else is made up to look
1801 like it). Thus, the exit value of the subprocess is really (C<<< $? >>
1802 8 >>>), and C<$? & 127> gives which signal, if any, the process died
1803 from, and C<$? & 128> reports whether there was a core dump.
1805 Additionally, if the C<h_errno> variable is supported in C, its value
1806 is returned via C<$?> if any C<gethost*()> function fails.
1808 If you have installed a signal handler for C<SIGCHLD>, the
1809 value of C<$?> will usually be wrong outside that handler.
1811 Inside an C<END> subroutine C<$?> contains the value that is going to be
1812 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
1813 change the exit status of your program. For example:
1816 $? = 1 if $? == 255; # die would make it 255
1819 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
1820 actual VMS exit status, instead of the default emulation of POSIX
1821 status; see L<perlvms/$?> for details.
1823 Mnemonic: similar to B<sh> and B<ksh>.
1828 X<$@> X<$EVAL_ERROR>
1830 The Perl syntax error message from the
1831 last C<eval()> operator. If C<$@> is
1832 the null string, the last C<eval()> parsed and executed correctly
1833 (although the operations you invoked may have failed in the normal
1836 Warning messages are not collected in this variable. You can, however,
1837 set up a routine to process warnings by setting C<$SIG{__WARN__}> as
1838 described in L</%SIG>.
1840 Mnemonic: Where was the syntax error "at"?
1844 =head2 Variables related to the interpreter state
1846 These variables provide information about the current interpreter state.
1853 X<$^C> X<$COMPILING>
1855 The current value of the flag associated with the B<-c> switch.
1856 Mainly of use with B<-MO=...> to allow code to alter its behavior
1857 when being compiled, such as for example to C<AUTOLOAD> at compile
1858 time rather than normal, deferred loading. Setting
1859 C<$^C = 1> is similar to calling C<B::minus_c>.
1861 This variable was added in Perl v5.6.0.
1866 X<$^D> X<$DEBUGGING>
1868 The current value of the debugging flags. May be read or set. Like its
1869 command-line equivalent, you can use numeric or symbolic values, eg
1870 C<$^D = 10> or C<$^D = "st">.
1872 Mnemonic: value of B<-D> switch.
1877 The I<object reference> to the C<Encode> object that is used to convert
1878 the source code to Unicode. Thanks to this variable your Perl script
1879 does not have to be written in UTF-8. Default is I<undef>. The direct
1880 manipulation of this variable is highly discouraged.
1882 This variable was added in Perl 5.8.2.
1884 =item ${^GLOBAL_PHASE}
1887 The current phase of the perl interpreter.
1889 Possible values are:
1895 The C<PerlInterpreter*> is being constructed via C<perl_construct>. This
1896 value is mostly there for completeness and for use via the
1897 underlying C variable C<PL_phase>. It's not really possible for Perl
1898 code to be executed unless construction of the interpreter is
1903 This is the global compile-time. That includes, basically, every
1904 C<BEGIN> block executed directly or indirectly from during the
1905 compile-time of the top-level program.
1907 This phase is not called "BEGIN" to avoid confusion with
1908 C<BEGIN>-blocks, as those are executed during compile-time of any
1909 compilation unit, not just the top-level program. A new, localised
1910 compile-time entered at run-time, for example by constructs as
1911 C<eval "use SomeModule"> are not global interpreter phases, and
1912 therefore aren't reflected by C<${^GLOBAL_PHASE}>.
1916 Execution of any C<CHECK> blocks.
1920 Similar to "CHECK", but for C<INIT>-blocks, not C<CHECK> blocks.
1924 The main run-time, i.e. the execution of C<PL_main_root>.
1928 Execution of any C<END> blocks.
1936 Also note that there's no value for UNITCHECK-blocks. That's because
1937 those are run for each compilation unit individually, and therefore is
1938 not a global interpreter phase.
1940 Not every program has to go through each of the possible phases, but
1941 transition from one phase to another can only happen in the order
1942 described in the above list.
1944 An example of all of the phases Perl code can see:
1946 BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }
1948 INIT { print "init-time: ${^GLOBAL_PHASE}\n" }
1950 CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }
1953 package Print::Phase;
1956 my ($class, $time) = @_;
1957 return bless \$time, $class;
1962 print "$$self: ${^GLOBAL_PHASE}\n";
1966 print "run-time: ${^GLOBAL_PHASE}\n";
1968 my $runtime = Print::Phase->new(
1969 "lexical variables are garbage collected before END"
1972 END { print "end-time: ${^GLOBAL_PHASE}\n" }
1974 our $destruct = Print::Phase->new(
1975 "package variables are garbage collected after END"
1984 lexical variables are garbage collected before END: RUN
1986 package variables are garbage collected after END: DESTRUCT
1988 This variable was added in Perl 5.14.0.
1993 WARNING: This variable is strictly for
1994 internal use only. Its availability,
1995 behavior, and contents are subject to change without notice.
1997 This variable contains compile-time hints for the Perl interpreter. At the
1998 end of compilation of a BLOCK the value of this variable is restored to the
1999 value when the interpreter started to compile the BLOCK.
2001 When perl begins to parse any block construct that provides a lexical scope
2002 (e.g., eval body, required file, subroutine body, loop body, or conditional
2003 block), the existing value of C<$^H> is saved, but its value is left unchanged.
2004 When the compilation of the block is completed, it regains the saved value.
2005 Between the points where its value is saved and restored, code that
2006 executes within BEGIN blocks is free to change the value of C<$^H>.
2008 This behavior provides the semantic of lexical scoping, and is used in,
2009 for instance, the C<use strict> pragma.
2011 The contents should be an integer; different bits of it are used for
2012 different pragmatic flags. Here's an example:
2014 sub add_100 { $^H |= 0x100 }
2021 Consider what happens during execution of the BEGIN block. At this point
2022 the BEGIN block has already been compiled, but the body of C<foo()> is still
2023 being compiled. The new value of C<$^H>
2024 will therefore be visible only while
2025 the body of C<foo()> is being compiled.
2027 Substitution of C<BEGIN { add_100() }> block with:
2029 BEGIN { require strict; strict->import('vars') }
2031 demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
2032 version of the same lexical pragma:
2035 require strict; strict->import('vars') if $condition
2038 This variable was added in Perl 5.003.
2043 The C<%^H> hash provides the same scoping semantic as C<$^H>. This makes
2044 it useful for implementation of lexically scoped pragmas. See
2047 When putting items into C<%^H>, in order to avoid conflicting with other
2048 users of the hash there is a convention regarding which keys to use.
2049 A module should use only keys that begin with the module's name (the
2050 name of its main package) and a "/" character. For example, a module
2051 C<Foo::Bar> should use keys such as C<Foo::Bar/baz>.
2053 This variable was added in Perl v5.6.0.
2058 An internal variable used by PerlIO. A string in two parts, separated
2059 by a C<\0> byte, the first part describes the input layers, the second
2060 part describes the output layers.
2062 This variable was added in Perl v5.8.0.
2069 The internal variable for debugging support. The meanings of the
2070 various bits are subject to change, but currently indicate:
2076 Debug subroutine enter/exit.
2080 Line-by-line debugging. Causes C<DB::DB()> subroutine to be called for
2081 each statement executed. Also causes saving source code lines (like
2086 Switch off optimizations.
2090 Preserve more data for future interactive inspections.
2094 Keep info about source lines on which a subroutine is defined.
2098 Start with single-step on.
2102 Use subroutine address instead of name when reporting.
2106 Report C<goto &subroutine> as well.
2110 Provide informative "file" names for evals based on the place they were compiled.
2114 Provide informative names to anonymous subroutines based on the place they
2119 Save source code lines into C<@{"_<$filename"}>.
2123 When saving source, include evals that generate no subroutines.
2127 When saving source, include source that did not compile.
2131 Some bits may be relevant at compile-time only, some at
2132 run-time only. This is a new mechanism and the details may change.
2133 See also L<perldebguts>.
2138 Reflects if taint mode is on or off. 1 for on (the program was run with
2139 B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
2142 This variable is read-only.
2144 This variable was added in Perl v5.8.0.
2149 Reflects certain Unicode settings of Perl. See L<perlrun>
2150 documentation for the C<-C> switch for more information about
2151 the possible values.
2153 This variable is set during Perl startup and is thereafter read-only.
2155 This variable was added in Perl v5.8.2.
2160 This variable controls the state of the internal UTF-8 offset caching code.
2161 1 for on (the default), 0 for off, -1 to debug the caching code by checking
2162 all its results against linear scans, and panicking on any discrepancy.
2164 This variable was added in Perl v5.8.9. It is subject to change or
2165 removal without notice, but is currently used to avoid recalculating the
2166 boundaries of multi-byte UTF-8-encoded characters.
2168 =item ${^UTF8LOCALE}
2171 This variable indicates whether a UTF-8 locale was detected by perl at
2172 startup. This information is used by perl when it's in
2173 adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
2174 switch); see L<perlrun> for more info on this.
2176 This variable was added in Perl v5.8.8.
2180 =head2 Deprecated and removed variables
2182 Deprecating a variable announces the intent of the perl maintainers to
2183 eventually remove the variable from the language. It may still be
2184 available despite its status. Using a deprecated variable triggers
2187 Once a variable is removed, its use triggers an error telling you
2188 the variable is unsupported.
2190 See L<perldiag> for details about error messages.
2197 C<$#> was a variable that could be used to format printed numbers.
2198 After a deprecation cycle, its magic was removed in Perl v5.10.0 and
2199 using it now triggers a warning: C<$# is no longer supported>.
2201 This is not the sigil you use in front of an array name to get the
2202 last index, like C<$#array>. That's still how you get the last index
2203 of an array in Perl. The two have nothing to do with each other.
2205 Deprecated in Perl 5.
2207 Removed in Perl v5.10.0.
2212 C<$*> was a variable that you could use to enable multiline matching.
2213 After a deprecation cycle, its magic was removed in Perl v5.10.0.
2214 Using it now triggers a warning: C<$* is no longer supported>.
2215 You should use the C</s> and C</m> regexp modifiers instead.
2217 Deprecated in Perl 5.
2219 Removed in Perl v5.10.0.
2224 This variable stores the index of the first element in an array, and
2225 of the first character in a substring. The default is 0, but you could
2226 theoretically set it to 1 to make Perl behave more like B<awk> (or Fortran)
2227 when subscripting and when evaluating the index() and substr() functions.
2229 As of release 5 of Perl, assignment to C<$[> is treated as a compiler
2230 directive, and cannot influence the behavior of any other file.
2231 (That's why you can only assign compile-time constants to it.)
2232 Its use is highly discouraged.
2234 Prior to Perl v5.10.0, assignment to C<$[> could be seen from outer lexical
2235 scopes in the same file, unlike other compile-time directives (such as
2236 L<strict>). Using local() on it would bind its value strictly to a lexical
2237 block. Now it is always lexically scoped.
2239 As of Perl v5.16.0, it is implemented by the L<arybase> module. See
2240 L<arybase> for more details on its behaviour.
2242 Under C<use v5.16>, or C<no feature "array_base">, C<$[> no longer has any
2243 effect, and always contains 0. Assigning 0 to it is permitted, but any
2244 other value will produce an error.
2246 Mnemonic: [ begins subscripts.
2248 Deprecated in Perl v5.12.0.
2253 See L</$^V> for a more modern representation of the Perl version that allows
2254 accurate string comparisons.
2256 The version + patchlevel / 1000 of the Perl interpreter. This variable
2257 can be used to determine whether the Perl interpreter executing a
2258 script is in the right range of versions:
2260 warn "No PerlIO!\n" if $] lt '5.008';
2262 The floating point representation can sometimes lead to inaccurate
2263 numeric comparisons, so string comparisons are recommended.
2265 See also the documentation of C<use VERSION> and C<require VERSION>
2266 for a convenient way to fail if the running Perl interpreter is too old.
2268 Mnemonic: Is this version of perl in the right bracket?