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 (with the literal control character
18 form deprecated). These names are all reserved for
19 special uses by Perl; for example, the all-digits names are used
20 to hold data captured by backreferences after a regular expression
21 match. Perl has a special syntax for the single-control-character
22 names: It understands C<^X> (caret C<X>) to mean the control-C<X>
23 character. For example, the notation C<$^W> (dollar-sign caret
24 C<W>) is the scalar variable whose name is the single character
25 control-C<W>. This is better than typing a literal control-C<W>
28 Since Perl v5.6.0, Perl variable names may be alphanumeric
29 strings that begin with control characters (or better yet, a caret).
30 These variables must be written in the form C<${^Foo}>; the braces
31 are not optional. C<${^Foo}> denotes the scalar variable whose
32 name is a control-C<F> followed by two C<o>'s. These variables are
33 reserved for future special uses by Perl, except for the ones that
34 begin with C<^_> (control-underscore or caret-underscore). No
35 control-character name that begins with C<^_> will acquire a special
36 meaning in any future version of Perl; such names may therefore be
37 used safely in programs. C<$^_> itself, however, I<is> reserved.
39 Perl identifiers that begin with digits, control characters, or
40 punctuation characters are exempt from the effects of the C<package>
41 declaration and are always forced to be in package C<main>; they are
42 also exempt from C<strict 'vars'> errors. A few other names are also
51 In particular, the special C<${^_XYZ}> variables are always taken
52 to be in package C<main>, regardless of any C<package> declarations
55 =head1 SPECIAL VARIABLES
57 The following names have special meaning to Perl. Most punctuation
58 names have reasonable mnemonics, or analogs in the shells.
59 Nevertheless, if you wish to use long variable names, you need only say:
63 at the top of your program. This aliases all the short names to the long
64 names in the current package. Some even have medium names, generally
65 borrowed from B<awk>. For more info, please see L<English>.
67 Before you continue, note the sort order for variables. In general, we
68 first list the variables in case-insensitive, almost-lexigraphical
69 order (ignoring the C<{> or C<^> preceding words, as in C<${^UNICODE}>
70 or C<$^T>), although C<$_> and C<@_> move up to the top of the pile.
71 For variables with the same identifier, we list it in order of scalar,
72 array, hash, and bareword.
74 =head2 General Variables
83 The default input and pattern-searching space. The following pairs are
86 while (<>) {...} # equivalent only in while!
87 while (defined($_ = <>)) {...}
98 Here are the places where Perl will assume C<$_> even if you don't use it:
104 The following functions use C<$_> as a default argument:
106 abs, alarm, chomp, chop, chr, chroot,
107 cos, defined, eval, evalbytes, exp, fc, glob, hex, int, lc,
108 lcfirst, length, log, lstat, mkdir, oct, ord, pos, print, printf,
109 quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only),
110 rmdir, say, sin, split (for its second
111 argument), sqrt, stat, study, uc, ucfirst,
116 All file tests (C<-f>, C<-d>) except for C<-t>, which defaults to STDIN.
121 The pattern matching operations C<m//>, C<s///> and C<tr///> (aka C<y///>)
122 when used without an C<=~> operator.
126 The default iterator variable in a C<foreach> loop if no other
127 variable is supplied.
131 The implicit iterator variable in the C<grep()> and C<map()> functions.
135 The implicit variable of C<given()>.
139 The default place to put the next value or input record
140 when a C<< <FH> >>, C<readline>, C<readdir> or C<each>
141 operation's result is tested by itself as the sole criterion of a C<while>
142 test. Outside a C<while> test, this will not happen.
146 C<$_> is by default a global variable. However, as
147 of perl v5.10.0, you can use a lexical version of
148 C<$_> by declaring it in a file or in a block with C<my>. Moreover,
149 declaring C<our $_> restores the global C<$_> in the current scope. Though
150 this seemed like a good idea at the time it was introduced, lexical C<$_>
151 actually causes more problems than it solves. If you call a function that
152 expects to be passed information via C<$_>, it may or may not work,
153 depending on how the function is written, there not being any easy way to
154 solve this. Just avoid lexical C<$_>, unless you are feeling particularly
155 masochistic. For this reason lexical C<$_> is still experimental and will
156 produce a warning unless warnings have been disabled. As with other
157 experimental features, the behavior of lexical C<$_> is subject to change
158 without notice, including change into a fatal error.
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($;, $x, $y, $z)}
382 @foo{$x,$y,$z} # a slice--note the @
386 ($foo{$x},$foo{$y},$foo{$z})
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.
414 As of v5.18.0, both keys and values stored in C<%ENV> are stringified.
418 if( ref $ENV{'bar'} ) {
419 say "Pre 5.18.0 Behaviour";
421 say "Post 5.18.0 Behaviour";
424 Previously, only child processes received stringified values:
429 # Always printed 'non ref'
431 q/print ( ref $ENV{'bar'} ? 'ref' : 'non ref' ) /);
433 This happens because you can't really share arbitrary data structures with
439 X<$^F> X<$SYSTEM_FD_MAX>
441 The maximum system file descriptor, ordinarily 2. System file
442 descriptors are passed to C<exec()>ed processes, while higher file
443 descriptors are not. Also, during an
444 C<open()>, system file descriptors are
445 preserved even if the C<open()> fails (ordinary file descriptors are
446 closed before the C<open()> is attempted). The close-on-exec
447 status of a file descriptor will be decided according to the value of
448 C<$^F> when the corresponding file, pipe, or socket was opened, not the
449 time of the C<exec()>.
454 The array C<@F> contains the fields of each line read in when autosplit
455 mode is turned on. See L<perlrun> for the B<-a> switch. This array
456 is package-specific, and must be declared or given a full package name
457 if not in package main when running under C<strict 'vars'>.
462 The array C<@INC> contains the list of places that the C<do EXPR>,
463 C<require>, or C<use> constructs look for their library files. It
464 initially consists of the arguments to any B<-I> command-line
465 switches, followed by the default Perl library, probably
466 F</usr/local/lib/perl>, followed by ".", to represent the current
467 directory. ("." will not be appended if taint checks are enabled,
468 either by C<-T> or by C<-t>.) If you need to modify this at runtime,
469 you should use the C<use lib> pragma to get the machine-dependent
470 library properly loaded also:
472 use lib '/mypath/libdir/';
475 You can also insert hooks into the file inclusion system by putting Perl
476 code directly into C<@INC>. Those hooks may be subroutine references,
477 array references or blessed objects. See L<perlfunc/require> for details.
482 The hash C<%INC> contains entries for each filename included via the
483 C<do>, C<require>, or C<use> operators. The key is the filename
484 you specified (with module names converted to pathnames), and the
485 value is the location of the file found. The C<require>
486 operator uses this hash to determine whether a particular file has
487 already been included.
489 If the file was loaded via a hook (e.g. a subroutine reference, see
490 L<perlfunc/require> for a description of these hooks), this hook is
491 by default inserted into C<%INC> in place of a filename. Note, however,
492 that the hook may have set the C<%INC> entry by itself to provide some more
498 X<$^I> X<$INPLACE_EDIT>
500 The current value of the inplace-edit extension. Use C<undef> to disable
503 Mnemonic: value of B<-i> switch.
508 By default, running out of memory is an untrappable, fatal error.
509 However, if suitably built, Perl can use the contents of C<$^M>
510 as an emergency memory pool after C<die()>ing. Suppose that your Perl
511 were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
514 $^M = 'a' x (1 << 16);
516 would allocate a 64K buffer for use in an emergency. See the
517 F<INSTALL> file in the Perl distribution for information on how to
518 add custom C compilation flags when compiling perl. To discourage casual
519 use of this advanced feature, there is no L<English|English> long name for
522 This variable was added in Perl 5.004.
529 The name of the operating system under which this copy of Perl was
530 built, as determined during the configuration process. For examples
531 see L<perlport/PLATFORMS>.
533 The value is identical to C<$Config{'osname'}>. See also L<Config>
534 and the B<-V> command-line switch documented in L<perlrun>.
536 In Windows platforms, C<$^O> is not very helpful: since it is always
537 C<MSWin32>, it doesn't tell the difference between
538 95/98/ME/NT/2000/XP/CE/.NET. Use C<Win32::GetOSName()> or
539 Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
540 between the variants.
542 This variable was added in Perl 5.003.
547 The hash C<%SIG> contains signal handlers for signals. For example:
549 sub handler { # 1st argument is signal name
551 print "Caught a SIG$sig--shutting down\n";
556 $SIG{'INT'} = \&handler;
557 $SIG{'QUIT'} = \&handler;
559 $SIG{'INT'} = 'DEFAULT'; # restore default action
560 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
562 Using a value of C<'IGNORE'> usually has the effect of ignoring the
563 signal, except for the C<CHLD> signal. See L<perlipc> for more about
566 Here are some other examples:
568 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not
570 $SIG{"PIPE"} = \&Plumber; # just fine; assume current
572 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
573 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber()
576 Be sure not to use a bareword as the name of a signal handler,
577 lest you inadvertently call it.
579 If your system has the C<sigaction()> function then signal handlers
580 are installed using it. This means you get reliable signal handling.
582 The default delivery policy of signals changed in Perl v5.8.0 from
583 immediate (also known as "unsafe") to deferred, also known as "safe
584 signals". See L<perlipc> for more information.
586 Certain internal hooks can be also set using the C<%SIG> hash. The
587 routine indicated by C<$SIG{__WARN__}> is called when a warning
588 message is about to be printed. The warning message is passed as the
589 first argument. The presence of a C<__WARN__> hook causes the
590 ordinary printing of warnings to C<STDERR> to be suppressed. You can
591 use this to save warnings in a variable, or turn warnings into fatal
594 local $SIG{__WARN__} = sub { die $_[0] };
597 As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
598 disable warnings using the empty subroutine:
600 local $SIG{__WARN__} = sub {};
602 The routine indicated by C<$SIG{__DIE__}> is called when a fatal
603 exception is about to be thrown. The error message is passed as the
604 first argument. When a C<__DIE__> hook routine returns, the exception
605 processing continues as it would have in the absence of the hook,
606 unless the hook routine itself exits via a C<goto &sub>, a loop exit,
607 or a C<die()>. The C<__DIE__> handler is explicitly disabled during
608 the call, so that you can die from a C<__DIE__> handler. Similarly
611 Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
612 even inside an C<eval()>. Do not use this to rewrite a pending
613 exception in C<$@>, or as a bizarre substitute for overriding
614 C<CORE::GLOBAL::die()>. This strange action at a distance may be fixed
615 in a future release so that C<$SIG{__DIE__}> is only called if your
616 program is about to exit, as was the original intent. Any other use is
619 C<__DIE__>/C<__WARN__> handlers are very special in one respect: they
620 may be called to report (probable) errors found by the parser. In such
621 a case the parser may be in inconsistent state, so any attempt to
622 evaluate Perl code from such a handler will probably result in a
623 segfault. This means that warnings or errors that result from parsing
624 Perl should be used with extreme caution, like this:
626 require Carp if defined $^S;
627 Carp::confess("Something wrong") if defined &Carp::confess;
628 die "Something wrong, but could not load Carp to give "
630 . "To see backtrace try starting Perl with -MCarp switch";
632 Here the first line will load C<Carp> I<unless> it is the parser who
633 called the handler. The second line will print backtrace and die if
634 C<Carp> was available. The third line will be executed only if C<Carp> was
637 Having to even think about the C<$^S> variable in your exception
638 handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
639 invites grievous and difficult to track down errors. Avoid it
640 and use an C<END{}> or CORE::GLOBAL::die override instead.
642 See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
643 L<warnings> for additional information.
650 The time at which the program began running, in seconds since the
651 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
652 and B<-C> filetests are based on this value.
657 X<$^V> X<$PERL_VERSION>
659 The revision, version, and subversion of the Perl interpreter,
660 represented as a C<version> object.
662 This variable first appeared in perl v5.6.0; earlier versions of perl
663 will see an undefined value. Before perl v5.10.0 C<$^V> was represented
666 C<$^V> can be used to determine whether the Perl interpreter executing
667 a script is in the right range of versions. For example:
669 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
671 To convert C<$^V> into its string representation use C<sprintf()>'s
674 printf "version is v%vd\n", $^V; # Perl's version
676 See the documentation of C<use VERSION> and C<require VERSION>
677 for a convenient way to fail if the running Perl interpreter is too old.
679 See also C<$]> for an older representation of the Perl version.
681 This variable was added in Perl v5.6.0.
683 Mnemonic: use ^V for Version Control.
685 =item ${^WIN32_SLOPPY_STAT}
686 X<${^WIN32_SLOPPY_STAT}> X<sitecustomize> X<sitecustomize.pl>
688 If this variable is set to a true value, then C<stat()> on Windows will
689 not try to open the file. This means that the link count cannot be
690 determined and file attributes may be out of date if additional
691 hardlinks to the file exist. On the other hand, not opening the file
692 is considerably faster, especially for files on network drives.
694 This variable could be set in the F<sitecustomize.pl> file to
695 configure the local Perl installation to use "sloppy" C<stat()> by
696 default. See the documentation for B<-f> in
697 L<perlrun|perlrun/"Command Switches"> for more information about site
700 This variable was added in Perl v5.10.0.
702 =item $EXECUTABLE_NAME
705 X<$^X> X<$EXECUTABLE_NAME>
707 The name used to execute the current copy of Perl, from C's
708 C<argv[0]> or (where supported) F</proc/self/exe>.
710 Depending on the host operating system, the value of C<$^X> may be
711 a relative or absolute pathname of the perl program file, or may
712 be the string used to invoke perl but not the pathname of the
713 perl program file. Also, most operating systems permit invoking
714 programs that are not in the PATH environment variable, so there
715 is no guarantee that the value of C<$^X> is in PATH. For VMS, the
716 value may or may not include a version number.
718 You usually can use the value of C<$^X> to re-invoke an independent
719 copy of the same perl that is currently running, e.g.,
721 @first_run = `$^X -le "print int rand 100 for 1..100"`;
723 But recall that not all operating systems support forking or
724 capturing of the output of commands, so this complex statement
727 It is not safe to use the value of C<$^X> as a path name of a file,
728 as some operating systems that have a mandatory suffix on
729 executable files do not require use of the suffix when invoking
730 a command. To convert the value of C<$^X> to a path name, use the
731 following statements:
733 # Build up a set of file names (not command names).
737 $this_perl .= $Config{_exe}
738 unless $this_perl =~ m/$Config{_exe}$/i;
741 Because many operating systems permit anyone with read access to
742 the Perl program file to make a copy of it, patch the copy, and
743 then execute the copy, the security-conscious Perl programmer
744 should take care to invoke the installed copy of perl, not the
745 copy referenced by C<$^X>. The following statements accomplish
746 this goal, and produce a pathname that can be invoked as a
747 command or referenced as a file.
750 my $secure_perl_path = $Config{perlpath};
752 $secure_perl_path .= $Config{_exe}
753 unless $secure_perl_path =~ m/$Config{_exe}$/i;
758 =head2 Variables related to regular expressions
760 Most of the special variables related to regular expressions are side
761 effects. Perl sets these variables when it has a successful match, so
762 you should check the match result before using them. For instance:
764 if( /P(A)TT(ER)N/ ) {
765 print "I found $1 and $2\n";
768 These variables are read-only and dynamically-scoped, unless we note
771 The dynamic nature of the regular expression variables means that
772 their value is limited to the block that they are in, as demonstrated
775 my $outer = 'Wallace and Grommit';
776 my $inner = 'Mutt and Jeff';
778 my $pattern = qr/(\S+) and (\S+)/;
780 sub show_n { print "\$1 is $1; \$2 is $2\n" }
784 show_n() if $outer =~ m/$pattern/;
787 show_n() if $inner =~ m/$pattern/;
793 The output shows that while in the C<OUTER> block, the values of C<$1>
794 and C<$2> are from the match against C<$outer>. Inside the C<INNER>
795 block, the values of C<$1> and C<$2> are from the match against
796 C<$inner>, but only until the end of the block (i.e. the dynamic
797 scope). After the C<INNER> block completes, the values of C<$1> and
798 C<$2> return to the values for the match against C<$outer> even though
799 we have not made another match:
801 $1 is Wallace; $2 is Grommit
802 $1 is Mutt; $2 is Jeff
803 $1 is Wallace; $2 is Grommit
805 =head3 Performance issues
807 Traditionally in Perl, any use of any of the three variables C<$`>, C<$&>
808 or C<$'> (or their C<use English> equivalents) anywhere in the code, caused
809 all subsequent successful pattern matches to make a copy of the matched
810 string, in case the code might subsequently access one of those variables.
811 This imposed a considerable performance penalty across the whole program,
812 so generally the use of these variables has been discouraged.
814 In Perl 5.6.0 the C<@-> and C<@+> dynamic arrays were introduced that
815 supply the indices of successful matches. So you could for example do
820 print $`, $&, $'; # bad: perfomance hit
822 print # good: no perfomance hit
823 substr($str, 0, $-[0]),
824 substr($str, $-[0], $+[0]-$-[0]),
827 In Perl 5.10.0 the C</p> match operator flag and the C<${^PREMATCH}>,
828 C<${^MATCH}>, and C<${^POSTMATCH}> variables were introduced, that allowed
829 you to suffer the penalties only on patterns marked with C</p>.
831 In Perl 5.18.0 onwards, perl started noting the presence of each of the
832 three variables separately, and only copied that part of the string
835 $`; $&; "abcdefgh" =~ /d/
837 perl would only copy the "abcd" part of the string. That could make a big
838 difference in something like
840 $str = 'x' x 1_000_000;
842 $str =~ /x/g # one char copied a million times, not a million chars
844 In Perl 5.20.0 a new copy-on-write system was enabled by default, which
845 finally fixes all performance issues with these three variables, and makes
846 them safe to use anywhere.
848 The C<Devel::NYTProf> and C<Devel::FindAmpersand> modules can help you
849 find uses of these problematic match variables in your code.
853 =item $<I<digits>> ($1, $2, ...)
856 Contains the subpattern from the corresponding set of capturing
857 parentheses from the last successful pattern match, not counting patterns
858 matched in nested blocks that have been exited already.
860 These variables are read-only and dynamically-scoped.
862 Mnemonic: like \digits.
869 The string matched by the last successful pattern match (not counting
870 any matches hidden within a BLOCK or C<eval()> enclosed by the current
873 See L</Performance issues> above for the serious performance implications
874 of using this variable (even once) in your code.
876 This variable is read-only and dynamically-scoped.
878 Mnemonic: like C<&> in some editors.
883 This is similar to C<$&> (C<$MATCH>) except that it does not incur the
884 performance penalty associated with that variable.
886 See L</Performance issues> above.
888 In Perl v5.18 and earlier, it is only guaranteed
889 to return a defined value when the pattern was compiled or executed with
890 the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
891 C<${^MATCH}> does the same thing as C<$MATCH>.
893 This variable was added in Perl v5.10.0.
895 This variable is read-only and dynamically-scoped.
900 X<$`> X<$PREMATCH> X<${^PREMATCH}>
902 The string preceding whatever was matched by the last successful
903 pattern match, not counting any matches hidden within a BLOCK or C<eval>
904 enclosed by the current BLOCK.
906 See L</Performance issues> above for the serious performance implications
907 of using this variable (even once) in your code.
909 This variable is read-only and dynamically-scoped.
911 Mnemonic: C<`> often precedes a quoted string.
914 X<$`> X<${^PREMATCH}>
916 This is similar to C<$`> ($PREMATCH) except that it does not incur the
917 performance penalty associated with that variable.
919 See L</Performance issues> above.
921 In Perl v5.18 and earlier, it is only guaranteed
922 to return a defined value when the pattern was compiled or executed with
923 the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
924 C<${^PREMATCH}> does the same thing as C<$PREMATCH>.
926 This variable was added in Perl v5.10.0.
928 This variable is read-only and dynamically-scoped.
933 X<$'> X<$POSTMATCH> X<${^POSTMATCH}> X<@->
935 The string following whatever was matched by the last successful
936 pattern match (not counting any matches hidden within a BLOCK or C<eval()>
937 enclosed by the current BLOCK). Example:
939 local $_ = 'abcdefghi';
941 print "$`:$&:$'\n"; # prints abc:def:ghi
943 See L</Performance issues> above for the serious performance implications
944 of using this variable (even once) in your code.
946 This variable is read-only and dynamically-scoped.
948 Mnemonic: C<'> often follows a quoted string.
951 X<${^POSTMATCH}> X<$'> X<$POSTMATCH>
953 This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
954 performance penalty associated with that variable.
956 See L</Performance issues> above.
958 In Perl v5.18 and earlier, it is only guaranteed
959 to return a defined value when the pattern was compiled or executed with
960 the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
961 C<${^POSTMATCH}> does the same thing as C<$POSTMATCH>.
963 This variable was added in Perl v5.10.0.
965 This variable is read-only and dynamically-scoped.
967 =item $LAST_PAREN_MATCH
970 X<$+> X<$LAST_PAREN_MATCH>
972 The text matched by the last bracket of the last successful search pattern.
973 This is useful if you don't know which one of a set of alternative patterns
974 matched. For example:
976 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
978 This variable is read-only and dynamically-scoped.
980 Mnemonic: be positive and forward looking.
982 =item $LAST_SUBMATCH_RESULT
985 X<$^N> X<$LAST_SUBMATCH_RESULT>
987 The text matched by the used group most-recently closed (i.e. the group
988 with the rightmost closing parenthesis) of the last successful search
991 This is primarily used inside C<(?{...})> blocks for examining text
992 recently matched. For example, to effectively capture text to a variable
993 (in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
995 (?:(...)(?{ $var = $^N }))
997 By setting and then using C<$var> in this way relieves you from having to
998 worry about exactly which numbered set of parentheses they are.
1000 This variable was added in Perl v5.8.0.
1002 Mnemonic: the (possibly) Nested parenthesis that most recently closed.
1004 =item @LAST_MATCH_END
1007 X<@+> X<@LAST_MATCH_END>
1009 This array holds the offsets of the ends of the last successful
1010 submatches in the currently active dynamic scope. C<$+[0]> is
1011 the offset into the string of the end of the entire match. This
1012 is the same value as what the C<pos> function returns when called
1013 on the variable that was matched against. The I<n>th element
1014 of this array holds the offset of the I<n>th submatch, so
1015 C<$+[1]> is the offset past where C<$1> ends, C<$+[2]> the offset
1016 past where C<$2> ends, and so on. You can use C<$#+> to determine
1017 how many subgroups were in the last successful match. See the
1018 examples given for the C<@-> variable.
1020 This variable was added in Perl v5.6.0.
1022 =item %LAST_PAREN_MATCH
1025 X<%+> X<%LAST_PAREN_MATCH>
1027 Similar to C<@+>, the C<%+> hash allows access to the named capture
1028 buffers, should they exist, in the last successful match in the
1029 currently active dynamic scope.
1031 For example, C<$+{foo}> is equivalent to C<$1> after the following match:
1033 'foo' =~ /(?<foo>foo)/;
1035 The keys of the C<%+> hash list only the names of buffers that have
1036 captured (and that are thus associated to defined values).
1038 The underlying behaviour of C<%+> is provided by the
1039 L<Tie::Hash::NamedCapture> module.
1041 B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1042 associated with the last successful regular expression. Therefore mixing
1043 iterative access to them via C<each> may have unpredictable results.
1044 Likewise, if the last successful match changes, then the results may be
1047 This variable was added in Perl v5.10.0.
1049 This variable is read-only and dynamically-scoped.
1051 =item @LAST_MATCH_START
1054 X<@-> X<@LAST_MATCH_START>
1056 C<$-[0]> is the offset of the start of the last successful match.
1057 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
1058 I<n>-th subpattern, or undef if the subpattern did not match.
1060 Thus, after a match against C<$_>, C<$&> coincides with C<substr $_, $-[0],
1061 $+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
1062 $+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
1063 C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the
1064 last matched subgroup in the last successful match. Contrast with
1065 C<$#+>, the number of subgroups in the regular expression. Compare
1068 This array holds the offsets of the beginnings of the last
1069 successful submatches in the currently active dynamic scope.
1070 C<$-[0]> is the offset into the string of the beginning of the
1071 entire match. The I<n>th element of this array holds the offset
1072 of the I<n>th submatch, so C<$-[1]> is the offset where C<$1>
1073 begins, C<$-[2]> the offset where C<$2> begins, and so on.
1075 After a match against some variable C<$var>:
1079 =item C<$`> is the same as C<substr($var, 0, $-[0])>
1081 =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
1083 =item C<$'> is the same as C<substr($var, $+[0])>
1085 =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
1087 =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
1089 =item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
1093 This variable was added in Perl v5.6.0.
1095 =item %LAST_MATCH_START
1098 X<%-> X<%LAST_MATCH_START>
1100 Similar to C<%+>, this variable allows access to the named capture groups
1101 in the last successful match in the currently active dynamic scope. To
1102 each capture group name found in the regular expression, it associates a
1103 reference to an array containing the list of values captured by all
1104 buffers with that name (should there be several of them), in the order
1109 if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
1110 foreach my $bufname (sort keys %-) {
1111 my $ary = $-{$bufname};
1112 foreach my $idx (0..$#$ary) {
1113 print "\$-{$bufname}[$idx] : ",
1114 (defined($ary->[$idx])
1129 The keys of the C<%-> hash correspond to all buffer names found in
1130 the regular expression.
1132 The behaviour of C<%-> is implemented via the
1133 L<Tie::Hash::NamedCapture> module.
1135 B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1136 associated with the last successful regular expression. Therefore mixing
1137 iterative access to them via C<each> may have unpredictable results.
1138 Likewise, if the last successful match changes, then the results may be
1141 This variable was added in Perl v5.10.0.
1143 This variable is read-only and dynamically-scoped.
1145 =item $LAST_REGEXP_CODE_RESULT
1148 X<$^R> X<$LAST_REGEXP_CODE_RESULT>
1150 The result of evaluation of the last successful C<(?{ code })>
1151 regular expression assertion (see L<perlre>). May be written to.
1153 This variable was added in Perl 5.005.
1155 =item ${^RE_DEBUG_FLAGS}
1156 X<${^RE_DEBUG_FLAGS}>
1158 The current value of the regex debugging flags. Set to 0 for no debug output
1159 even when the C<re 'debug'> module is loaded. See L<re> for details.
1161 This variable was added in Perl v5.10.0.
1163 =item ${^RE_TRIE_MAXBUF}
1164 X<${^RE_TRIE_MAXBUF}>
1166 Controls how certain regex optimisations are applied and how much memory they
1167 utilize. This value by default is 65536 which corresponds to a 512kB
1168 temporary cache. Set this to a higher value to trade
1169 memory for speed when matching large alternations. Set
1170 it to a lower value if you want the optimisations to
1171 be as conservative of memory as possible but still occur, and set it to a
1172 negative value to prevent the optimisation and conserve the most memory.
1173 Under normal situations this variable should be of no interest to you.
1175 This variable was added in Perl v5.10.0.
1179 =head2 Variables related to filehandles
1181 Variables that depend on the currently selected filehandle may be set
1182 by calling an appropriate object method on the C<IO::Handle> object,
1183 although this is less efficient than using the regular built-in
1184 variables. (Summary lines below for this contain the word HANDLE.)
1189 after which you may use either
1195 HANDLE->method(EXPR)
1197 Each method returns the old value of the C<IO::Handle> attribute. The
1198 methods each take an optional EXPR, which, if supplied, specifies the
1199 new value for the C<IO::Handle> attribute in question. If not
1200 supplied, most methods do nothing to the current value--except for
1201 C<autoflush()>, which will assume a 1 for you, just to be different.
1203 Because loading in the C<IO::Handle> class is an expensive operation,
1204 you should learn how to use the regular built-in variables.
1206 A few of these variables are considered "read-only". This means that
1207 if you try to assign to this variable, either directly or indirectly
1208 through a reference, you'll raise a run-time exception.
1210 You should be very careful when modifying the default values of most
1211 special variables described in this document. In most cases you want
1212 to localize these variables before changing them, since if you don't,
1213 the change may affect other modules which rely on the default values
1214 of the special variables that you have changed. This is one of the
1215 correct ways to read the whole file at once:
1217 open my $fh, "<", "foo" or die $!;
1218 local $/; # enable localized slurp mode
1219 my $content = <$fh>;
1222 But the following code is quite bad:
1224 open my $fh, "<", "foo" or die $!;
1225 undef $/; # enable slurp mode
1226 my $content = <$fh>;
1229 since some other module, may want to read data from some file in the
1230 default "line mode", so if the code we have just presented has been
1231 executed, the global value of C<$/> is now changed for any other code
1232 running inside the same Perl interpreter.
1234 Usually when a variable is localized you want to make sure that this
1235 change affects the shortest scope possible. So unless you are already
1236 inside some short C<{}> block, you should create one yourself. For
1240 open my $fh, "<", "foo" or die $!;
1247 Here is an example of how your own code can go broken:
1257 # do something with $_
1260 You probably expect this code to print the equivalent of
1264 but instead you get:
1268 Why? Because C<nasty_break()> modifies C<$\> without localizing it
1269 first. The value you set in C<nasty_break()> is still there when you
1270 return. The fix is to add C<local()> so the value doesn't leak out of
1275 It's easy to notice the problem in such a short example, but in more
1276 complicated code you are looking for trouble if you don't localize
1277 changes to the special variables.
1284 Contains the name of the current file when reading from C<< <> >>.
1289 The array C<@ARGV> contains the command-line arguments intended for
1290 the script. C<$#ARGV> is generally the number of arguments minus
1291 one, because C<$ARGV[0]> is the first argument, I<not> the program's
1292 command name itself. See L</$0> for the command name.
1297 The special filehandle that iterates over command-line filenames in
1298 C<@ARGV>. Usually written as the null filehandle in the angle operator
1299 C<< <> >>. Note that currently C<ARGV> only has its magical effect
1300 within the C<< <> >> operator; elsewhere it is just a plain filehandle
1301 corresponding to the last file opened by C<< <> >>. In particular,
1302 passing C<\*ARGV> as a parameter to a function that expects a filehandle
1303 may not cause your function to automatically read the contents of all the
1309 The special filehandle that points to the currently open output file
1310 when doing edit-in-place processing with B<-i>. Useful when you have
1311 to do a lot of inserting and don't want to keep modifying C<$_>. See
1312 L<perlrun> for the B<-i> switch.
1314 =item IO::Handle->output_field_separator( EXPR )
1316 =item $OUTPUT_FIELD_SEPARATOR
1321 X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>
1323 The output field separator for the print operator. If defined, this
1324 value is printed between each of print's arguments. Default is C<undef>.
1326 You cannot call C<output_field_separator()> on a handle, only as a
1327 static method. See L<IO::Handle|IO::Handle>.
1329 Mnemonic: what is printed when there is a "," in your print statement.
1331 =item HANDLE->input_line_number( EXPR )
1333 =item $INPUT_LINE_NUMBER
1338 X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
1340 Current line number for the last filehandle accessed.
1342 Each filehandle in Perl counts the number of lines that have been read
1343 from it. (Depending on the value of C<$/>, Perl's idea of what
1344 constitutes a line may not match yours.) When a line is read from a
1345 filehandle (via C<readline()> or C<< <> >>), or when C<tell()> or
1346 C<seek()> is called on it, C<$.> becomes an alias to the line counter
1347 for that filehandle.
1349 You can adjust the counter by assigning to C<$.>, but this will not
1350 actually move the seek pointer. I<Localizing C<$.> will not localize
1351 the filehandle's line count>. Instead, it will localize perl's notion
1352 of which filehandle C<$.> is currently aliased to.
1354 C<$.> is reset when the filehandle is closed, but B<not> when an open
1355 filehandle is reopened without an intervening C<close()>. For more
1356 details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
1357 an explicit close, line numbers increase across C<ARGV> files (but see
1358 examples in L<perlfunc/eof>).
1360 You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
1361 line counter for a given filehandle without having to worry about
1362 which handle you last accessed.
1364 Mnemonic: many programs use "." to mean the current line number.
1366 =item IO::Handle->input_record_separator( EXPR )
1368 =item $INPUT_RECORD_SEPARATOR
1373 X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR>
1375 The input record separator, newline by default. This influences Perl's
1376 idea of what a "line" is. Works like B<awk>'s RS variable, including
1377 treating empty lines as a terminator if set to the null string (an
1378 empty line cannot contain any spaces or tabs). You may set it to a
1379 multi-character string to match a multi-character terminator, or to
1380 C<undef> to read through the end of file. Setting it to C<"\n\n">
1381 means something slightly different than setting to C<"">, if the file
1382 contains consecutive empty lines. Setting to C<""> will treat two or
1383 more consecutive empty lines as a single empty line. Setting to
1384 C<"\n\n"> will blindly assume that the next input character belongs to
1385 the next paragraph, even if it's a newline.
1387 local $/; # enable "slurp" mode
1388 local $_ = <FH>; # whole file now here
1391 Remember: the value of C<$/> is a string, not a regex. B<awk> has to
1392 be better for something. :-)
1394 Setting C<$/> to a reference to an integer, scalar containing an
1395 integer, or scalar that's convertible to an integer will attempt to
1396 read records instead of lines, with the maximum record size being the
1397 referenced integer number of characters. So this:
1399 local $/ = \32768; # or \"32768", or \$var_containing_32768
1400 open my $fh, "<", $myfile or die $!;
1403 will read a record of no more than 32768 characters from $fh. If you're
1404 not reading from a record-oriented file (or your OS doesn't have
1405 record-oriented files), then you'll likely get a full chunk of data
1406 with every read. If a record is larger than the record size you've
1407 set, you'll get the record back in pieces. Trying to set the record
1408 size to zero or less is deprecated and will cause $/ to have the value
1409 of "undef", which will cause reading in the (rest of the) whole file.
1411 As of 5.19.9 setting C<$/> to any other form of reference will throw a
1412 fatal exception. This is in preparation for supporting new ways to set
1413 C<$/> in the future.
1415 On VMS only, record reads bypass PerlIO layers and any associated
1416 buffering, so you must not mix record and non-record reads on the
1417 same filehandle. Record mode mixes with line mode only when the
1418 same buffering layer is in use for both modes.
1420 You cannot call C<input_record_separator()> on a handle, only as a
1421 static method. See L<IO::Handle|IO::Handle>.
1423 See also L<perlport/"Newlines">. Also see L</$.>.
1425 Mnemonic: / delimits line boundaries when quoting poetry.
1427 =item IO::Handle->output_record_separator( EXPR )
1429 =item $OUTPUT_RECORD_SEPARATOR
1434 X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR>
1436 The output record separator for the print operator. If defined, this
1437 value is printed after the last of print's arguments. Default is C<undef>.
1439 You cannot call C<output_record_separator()> on a handle, only as a
1440 static method. See L<IO::Handle|IO::Handle>.
1442 Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
1443 Also, it's just like C<$/>, but it's what you get "back" from Perl.
1445 =item HANDLE->autoflush( EXPR )
1447 =item $OUTPUT_AUTOFLUSH
1450 X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
1452 If set to nonzero, forces a flush right away and after every write or
1453 print on the currently selected output channel. Default is 0
1454 (regardless of whether the channel is really buffered by the system or
1455 not; C<$|> tells you only whether you've asked Perl explicitly to
1456 flush after each write). STDOUT will typically be line buffered if
1457 output is to the terminal and block buffered otherwise. Setting this
1458 variable is useful primarily when you are outputting to a pipe or
1459 socket, such as when you are running a Perl program under B<rsh> and
1460 want to see the output as it's happening. This has no effect on input
1461 buffering. See L<perlfunc/getc> for that. See L<perlfunc/select> on
1462 how to select the output channel. See also L<IO::Handle>.
1464 Mnemonic: when you want your pipes to be piping hot.
1469 This read-only variable contains a reference to the last-read filehandle.
1470 This is set by C<< <HANDLE> >>, C<readline>, C<tell>, C<eof> and C<seek>.
1471 This is the same handle that C<$.> and C<tell> and C<eof> without arguments
1472 use. It is also the handle used when Perl appends ", <STDIN> line 1" to
1473 an error or warning message.
1475 This variable was added in Perl v5.18.0.
1479 =head3 Variables related to formats
1481 The special variables for formats are a subset of those for
1482 filehandles. See L<perlform> for more information about Perl's
1490 X<$^A> X<$ACCUMULATOR>
1492 The current value of the C<write()> accumulator for C<format()> lines.
1493 A format contains C<formline()> calls that put their result into
1494 C<$^A>. After calling its format, C<write()> prints out the contents
1495 of C<$^A> and empties. So you never really see the contents of C<$^A>
1496 unless you call C<formline()> yourself and then look at it. See
1497 L<perlform> and L<perlfunc/"formline PICTURE,LIST">.
1499 =item IO::Handle->format_formfeed(EXPR)
1501 =item $FORMAT_FORMFEED
1504 X<$^L> X<$FORMAT_FORMFEED>
1506 What formats output as a form feed. The default is C<\f>.
1508 You cannot call C<format_formfeed()> on a handle, only as a static
1509 method. See L<IO::Handle|IO::Handle>.
1511 =item HANDLE->format_page_number(EXPR)
1513 =item $FORMAT_PAGE_NUMBER
1516 X<$%> X<$FORMAT_PAGE_NUMBER>
1518 The current page number of the currently selected output channel.
1520 Mnemonic: C<%> is page number in B<nroff>.
1522 =item HANDLE->format_lines_left(EXPR)
1524 =item $FORMAT_LINES_LEFT
1527 X<$-> X<$FORMAT_LINES_LEFT>
1529 The number of lines left on the page of the currently selected output
1532 Mnemonic: lines_on_page - lines_printed.
1534 =item IO::Handle->format_line_break_characters EXPR
1536 =item $FORMAT_LINE_BREAK_CHARACTERS
1539 X<$:> X<FORMAT_LINE_BREAK_CHARACTERS>
1541 The current set of characters after which a string may be broken to
1542 fill continuation fields (starting with C<^>) in a format. The default is
1543 S<" \n-">, to break on a space, newline, or a hyphen.
1545 You cannot call C<format_line_break_characters()> on a handle, only as
1546 a static method. See L<IO::Handle|IO::Handle>.
1548 Mnemonic: a "colon" in poetry is a part of a line.
1550 =item HANDLE->format_lines_per_page(EXPR)
1552 =item $FORMAT_LINES_PER_PAGE
1555 X<$=> X<$FORMAT_LINES_PER_PAGE>
1557 The current page length (printable lines) of the currently selected
1558 output channel. The default is 60.
1560 Mnemonic: = has horizontal lines.
1562 =item HANDLE->format_top_name(EXPR)
1564 =item $FORMAT_TOP_NAME
1567 X<$^> X<$FORMAT_TOP_NAME>
1569 The name of the current top-of-page format for the currently selected
1570 output channel. The default is the name of the filehandle with C<_TOP>
1571 appended. For example, the default format top name for the C<STDOUT>
1572 filehandle is C<STDOUT_TOP>.
1574 Mnemonic: points to top of page.
1576 =item HANDLE->format_name(EXPR)
1581 X<$~> X<$FORMAT_NAME>
1583 The name of the current report format for the currently selected
1584 output channel. The default format name is the same as the filehandle
1585 name. For example, the default format name for the C<STDOUT>
1586 filehandle is just C<STDOUT>.
1588 Mnemonic: brother to C<$^>.
1592 =head2 Error Variables
1593 X<error> X<exception>
1595 The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1596 about different types of error conditions that may appear during
1597 execution of a Perl program. The variables are shown ordered by
1598 the "distance" between the subsystem which reported the error and
1599 the Perl process. They correspond to errors detected by the Perl
1600 interpreter, C library, operating system, or an external program,
1603 To illustrate the differences between these variables, consider the
1604 following Perl expression, which uses a single-quoted string. After
1605 execution of this statement, perl may have set all four special error
1609 open my $pipe, "/cdrom/install |" or die $!;
1611 close $pipe or die "bad pipe: $?, $!";
1614 When perl executes the C<eval()> expression, it translates the
1615 C<open()>, C<< <PIPE> >>, and C<close> calls in the C run-time library
1616 and thence to the operating system kernel. perl sets C<$!> to
1617 the C library's C<errno> if one of these calls fails.
1619 C<$@> is set if the string to be C<eval>-ed did not compile (this may
1620 happen if C<open> or C<close> were imported with bad prototypes), or
1621 if Perl code executed during evaluation C<die()>d. In these cases the
1622 value of C<$@> is the compile error, or the argument to C<die> (which
1623 will interpolate C<$!> and C<$?>). (See also L<Fatal>, though.)
1625 Under a few operating systems, C<$^E> may contain a more verbose error
1626 indicator, such as in this case, "CDROM tray not closed." Systems that
1627 do not support extended error messages leave C<$^E> the same as C<$!>.
1629 Finally, C<$?> may be set to non-0 value if the external program
1630 F</cdrom/install> fails. The upper eight bits reflect specific error
1631 conditions encountered by the program (the program's C<exit()> value).
1632 The lower eight bits reflect mode of failure, like signal death and
1633 core dump information. See L<wait(2)> for details. In contrast to
1634 C<$!> and C<$^E>, which are set only if error condition is detected,
1635 the variable C<$?> is set on each C<wait> or pipe C<close>,
1636 overwriting the old value. This is more like C<$@>, which on every
1637 C<eval()> is always set on failure and cleared on success.
1639 For more details, see the individual descriptions at C<$@>, C<$!>,
1644 =item ${^CHILD_ERROR_NATIVE}
1645 X<$^CHILD_ERROR_NATIVE>
1647 The native status returned by the last pipe close, backtick (C<``>)
1648 command, successful call to C<wait()> or C<waitpid()>, or from the
1649 C<system()> operator. On POSIX-like systems this value can be decoded
1650 with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED,
1651 WSTOPSIG and WIFCONTINUED functions provided by the L<POSIX> module.
1653 Under VMS this reflects the actual VMS exit status; i.e. it is the
1654 same as C<$?> when the pragma C<use vmsish 'status'> is in effect.
1656 This variable was added in Perl v5.10.0.
1658 =item $EXTENDED_OS_ERROR
1661 X<$^E> X<$EXTENDED_OS_ERROR>
1663 Error information specific to the current operating system. At the
1664 moment, this differs from C<$!> under only VMS, OS/2, and Win32 (and
1665 for MacPerl). On all other platforms, C<$^E> is always just the same
1668 Under VMS, C<$^E> provides the VMS status value from the last system
1669 error. This is more specific information about the last system error
1670 than that provided by C<$!>. This is particularly important when C<$!>
1671 is set to B<EVMSERR>.
1673 Under OS/2, C<$^E> is set to the error code of the last call to OS/2
1674 API either via CRT, or directly from perl.
1676 Under Win32, C<$^E> always returns the last error information reported
1677 by the Win32 call C<GetLastError()> which describes the last error
1678 from within the Win32 API. Most Win32-specific code will report errors
1679 via C<$^E>. ANSI C and Unix-like calls set C<errno> and so most
1680 portable Perl code will report errors via C<$!>.
1682 Caveats mentioned in the description of C<$!> generally apply to
1685 This variable was added in Perl 5.003.
1687 Mnemonic: Extra error explanation.
1689 =item $EXCEPTIONS_BEING_CAUGHT
1692 X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>
1694 Current state of the interpreter.
1697 --------- -------------------------------------
1698 undef Parsing module, eval, or main program
1699 true (1) Executing an eval
1702 The first state may happen in C<$SIG{__DIE__}> and C<$SIG{__WARN__}>
1705 The English name $EXCEPTIONS_BEING_CAUGHT is slightly misleading, because
1706 the C<undef> value does not indicate whether exceptions are being caught,
1707 since compilation of the main program does not catch exceptions.
1709 This variable was added in Perl 5.004.
1716 The current value of the warning switch, initially true if B<-w> was
1717 used, false otherwise, but directly modifiable.
1719 See also L<warnings>.
1721 Mnemonic: related to the B<-w> switch.
1723 =item ${^WARNING_BITS}
1726 The current set of warning checks enabled by the C<use warnings> pragma.
1727 It has the same scoping as the C<$^H> and C<%^H> variables. The exact
1728 values are considered internal to the L<warnings> pragma and may change
1729 between versions of Perl.
1731 This variable was added in Perl v5.6.0.
1738 X<$!> X<$ERRNO> X<$OS_ERROR>
1740 When referenced, C<$!> retrieves the current value
1741 of the C C<errno> integer variable.
1742 If C<$!> is assigned a numerical value, that value is stored in C<errno>.
1743 When referenced as a string, C<$!> yields the system error string
1744 corresponding to C<errno>.
1746 Many system or library calls set C<errno> if they fail,
1747 to indicate the cause of failure. They usually do B<not>
1748 set C<errno> to zero if they succeed. This means C<errno>,
1749 hence C<$!>, is meaningful only I<immediately> after a B<failure>:
1751 if (open my $fh, "<", $filename) {
1752 # Here $! is meaningless.
1756 # ONLY here is $! meaningful.
1758 # Already here $! might be meaningless.
1760 # Since here we might have either success or failure,
1761 # $! is meaningless.
1763 Here, I<meaningless> means that C<$!> may be unrelated to the outcome
1764 of the C<open()> operator. Assignment to C<$!> is similarly ephemeral.
1765 It can be used immediately before invoking the C<die()> operator,
1766 to set the exit value, or to inspect the system error string
1767 corresponding to error I<n>, or to restore C<$!> to a meaningful state.
1769 Mnemonic: What just went bang?
1776 X<%!> X<%OS_ERROR> X<%ERRNO>
1778 Each element of C<%!> has a true value only if C<$!> is set to that
1779 value. For example, C<$!{ENOENT}> is true if and only if the current
1780 value of C<$!> is C<ENOENT>; that is, if the most recent error was "No
1781 such file or directory" (or its moral equivalent: not all operating
1782 systems give that exact error, and certainly not all languages). To
1783 check if a particular key is meaningful on your system, use C<exists
1784 $!{the_key}>; for a list of legal keys, use C<keys %!>. See L<Errno>
1785 for more information, and also see L</$!>.
1787 This variable was added in Perl 5.005.
1792 X<$?> X<$CHILD_ERROR>
1794 The status returned by the last pipe close, backtick (C<``>) command,
1795 successful call to C<wait()> or C<waitpid()>, or from the C<system()>
1796 operator. This is just the 16-bit status word returned by the
1797 traditional Unix C<wait()> system call (or else is made up to look
1798 like it). Thus, the exit value of the subprocess is really (C<<< $? >>
1799 8 >>>), and C<$? & 127> gives which signal, if any, the process died
1800 from, and C<$? & 128> reports whether there was a core dump.
1802 Additionally, if the C<h_errno> variable is supported in C, its value
1803 is returned via C<$?> if any C<gethost*()> function fails.
1805 If you have installed a signal handler for C<SIGCHLD>, the
1806 value of C<$?> will usually be wrong outside that handler.
1808 Inside an C<END> subroutine C<$?> contains the value that is going to be
1809 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
1810 change the exit status of your program. For example:
1813 $? = 1 if $? == 255; # die would make it 255
1816 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
1817 actual VMS exit status, instead of the default emulation of POSIX
1818 status; see L<perlvms/$?> for details.
1820 Mnemonic: similar to B<sh> and B<ksh>.
1825 X<$@> X<$EVAL_ERROR>
1827 The Perl syntax error message from the
1828 last C<eval()> operator. If C<$@> is
1829 the null string, the last C<eval()> parsed and executed correctly
1830 (although the operations you invoked may have failed in the normal
1833 Warning messages are not collected in this variable. You can, however,
1834 set up a routine to process warnings by setting C<$SIG{__WARN__}> as
1835 described in L</%SIG>.
1837 Mnemonic: Where was the syntax error "at"?
1841 =head2 Variables related to the interpreter state
1843 These variables provide information about the current interpreter state.
1850 X<$^C> X<$COMPILING>
1852 The current value of the flag associated with the B<-c> switch.
1853 Mainly of use with B<-MO=...> to allow code to alter its behavior
1854 when being compiled, such as for example to C<AUTOLOAD> at compile
1855 time rather than normal, deferred loading. Setting
1856 C<$^C = 1> is similar to calling C<B::minus_c>.
1858 This variable was added in Perl v5.6.0.
1863 X<$^D> X<$DEBUGGING>
1865 The current value of the debugging flags. May be read or set. Like its
1866 command-line equivalent, you can use numeric or symbolic values, eg
1867 C<$^D = 10> or C<$^D = "st">.
1869 Mnemonic: value of B<-D> switch.
1876 The I<object reference> to the C<Encode> object that is used to convert
1877 the source code to Unicode. Thanks to this variable your Perl script
1878 does not have to be written in UTF-8. Default is C<undef>.
1880 Setting this variable to any other value than C<undef> is deprecated due
1881 to fundamental defects in its design and implementation. It is planned
1882 to remove it from a future Perl version. Its purpose was to allow your
1883 non-ASCII Perl scripts to not have to be written in UTF-8; this was
1884 useful before editors that worked on UTF-8 encoded text were common, but
1885 that was long ago. It causes problems, such as affecting the operation
1886 of other modules that aren't expecting it, causing general mayhem. Its
1887 use can lead to segfaults.
1889 If you need something like this functionality, you should use the
1890 L<encoding> pragma, which is also deprecated, but has fewer nasty side
1893 If you are coming here because code of yours is being adversely affected
1894 by someone's use of this variable, you can usually work around it by
1899 near the beginning of the functions that are getting broken. This
1900 undefines the variable during the scope of execution of the including
1903 This variable was added in Perl 5.8.2.
1905 =item ${^GLOBAL_PHASE}
1908 The current phase of the perl interpreter.
1910 Possible values are:
1916 The C<PerlInterpreter*> is being constructed via C<perl_construct>. This
1917 value is mostly there for completeness and for use via the
1918 underlying C variable C<PL_phase>. It's not really possible for Perl
1919 code to be executed unless construction of the interpreter is
1924 This is the global compile-time. That includes, basically, every
1925 C<BEGIN> block executed directly or indirectly from during the
1926 compile-time of the top-level program.
1928 This phase is not called "BEGIN" to avoid confusion with
1929 C<BEGIN>-blocks, as those are executed during compile-time of any
1930 compilation unit, not just the top-level program. A new, localised
1931 compile-time entered at run-time, for example by constructs as
1932 C<eval "use SomeModule"> are not global interpreter phases, and
1933 therefore aren't reflected by C<${^GLOBAL_PHASE}>.
1937 Execution of any C<CHECK> blocks.
1941 Similar to "CHECK", but for C<INIT>-blocks, not C<CHECK> blocks.
1945 The main run-time, i.e. the execution of C<PL_main_root>.
1949 Execution of any C<END> blocks.
1957 Also note that there's no value for UNITCHECK-blocks. That's because
1958 those are run for each compilation unit individually, and therefore is
1959 not a global interpreter phase.
1961 Not every program has to go through each of the possible phases, but
1962 transition from one phase to another can only happen in the order
1963 described in the above list.
1965 An example of all of the phases Perl code can see:
1967 BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }
1969 INIT { print "init-time: ${^GLOBAL_PHASE}\n" }
1971 CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }
1974 package Print::Phase;
1977 my ($class, $time) = @_;
1978 return bless \$time, $class;
1983 print "$$self: ${^GLOBAL_PHASE}\n";
1987 print "run-time: ${^GLOBAL_PHASE}\n";
1989 my $runtime = Print::Phase->new(
1990 "lexical variables are garbage collected before END"
1993 END { print "end-time: ${^GLOBAL_PHASE}\n" }
1995 our $destruct = Print::Phase->new(
1996 "package variables are garbage collected after END"
2005 lexical variables are garbage collected before END: RUN
2007 package variables are garbage collected after END: DESTRUCT
2009 This variable was added in Perl 5.14.0.
2014 WARNING: This variable is strictly for
2015 internal use only. Its availability,
2016 behavior, and contents are subject to change without notice.
2018 This variable contains compile-time hints for the Perl interpreter. At the
2019 end of compilation of a BLOCK the value of this variable is restored to the
2020 value when the interpreter started to compile the BLOCK.
2022 When perl begins to parse any block construct that provides a lexical scope
2023 (e.g., eval body, required file, subroutine body, loop body, or conditional
2024 block), the existing value of C<$^H> is saved, but its value is left unchanged.
2025 When the compilation of the block is completed, it regains the saved value.
2026 Between the points where its value is saved and restored, code that
2027 executes within BEGIN blocks is free to change the value of C<$^H>.
2029 This behavior provides the semantic of lexical scoping, and is used in,
2030 for instance, the C<use strict> pragma.
2032 The contents should be an integer; different bits of it are used for
2033 different pragmatic flags. Here's an example:
2035 sub add_100 { $^H |= 0x100 }
2042 Consider what happens during execution of the BEGIN block. At this point
2043 the BEGIN block has already been compiled, but the body of C<foo()> is still
2044 being compiled. The new value of C<$^H>
2045 will therefore be visible only while
2046 the body of C<foo()> is being compiled.
2048 Substitution of C<BEGIN { add_100() }> block with:
2050 BEGIN { require strict; strict->import('vars') }
2052 demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
2053 version of the same lexical pragma:
2056 require strict; strict->import('vars') if $condition
2059 This variable was added in Perl 5.003.
2064 The C<%^H> hash provides the same scoping semantic as C<$^H>. This makes
2065 it useful for implementation of lexically scoped pragmas. See
2066 L<perlpragma>. All the entries are stringified when accessed at
2067 runtime, so only simple values can be accommodated. This means no
2068 pointers to objects, for example.
2070 When putting items into C<%^H>, in order to avoid conflicting with other
2071 users of the hash there is a convention regarding which keys to use.
2072 A module should use only keys that begin with the module's name (the
2073 name of its main package) and a "/" character. For example, a module
2074 C<Foo::Bar> should use keys such as C<Foo::Bar/baz>.
2076 This variable was added in Perl v5.6.0.
2081 An internal variable used by PerlIO. A string in two parts, separated
2082 by a C<\0> byte, the first part describes the input layers, the second
2083 part describes the output layers.
2085 This variable was added in Perl v5.8.0.
2092 The internal variable for debugging support. The meanings of the
2093 various bits are subject to change, but currently indicate:
2099 Debug subroutine enter/exit.
2103 Line-by-line debugging. Causes C<DB::DB()> subroutine to be called for
2104 each statement executed. Also causes saving source code lines (like
2109 Switch off optimizations.
2113 Preserve more data for future interactive inspections.
2117 Keep info about source lines on which a subroutine is defined.
2121 Start with single-step on.
2125 Use subroutine address instead of name when reporting.
2129 Report C<goto &subroutine> as well.
2133 Provide informative "file" names for evals based on the place they were compiled.
2137 Provide informative names to anonymous subroutines based on the place they
2142 Save source code lines into C<@{"_<$filename"}>.
2146 When saving source, include evals that generate no subroutines.
2150 When saving source, include source that did not compile.
2154 Some bits may be relevant at compile-time only, some at
2155 run-time only. This is a new mechanism and the details may change.
2156 See also L<perldebguts>.
2161 Reflects if taint mode is on or off. 1 for on (the program was run with
2162 B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
2165 This variable is read-only.
2167 This variable was added in Perl v5.8.0.
2172 Reflects certain Unicode settings of Perl. See L<perlrun>
2173 documentation for the C<-C> switch for more information about
2174 the possible values.
2176 This variable is set during Perl startup and is thereafter read-only.
2178 This variable was added in Perl v5.8.2.
2183 This variable controls the state of the internal UTF-8 offset caching code.
2184 1 for on (the default), 0 for off, -1 to debug the caching code by checking
2185 all its results against linear scans, and panicking on any discrepancy.
2187 This variable was added in Perl v5.8.9. It is subject to change or
2188 removal without notice, but is currently used to avoid recalculating the
2189 boundaries of multi-byte UTF-8-encoded characters.
2191 =item ${^UTF8LOCALE}
2194 This variable indicates whether a UTF-8 locale was detected by perl at
2195 startup. This information is used by perl when it's in
2196 adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
2197 switch); see L<perlrun> for more info on this.
2199 This variable was added in Perl v5.8.8.
2203 =head2 Deprecated and removed variables
2205 Deprecating a variable announces the intent of the perl maintainers to
2206 eventually remove the variable from the language. It may still be
2207 available despite its status. Using a deprecated variable triggers
2210 Once a variable is removed, its use triggers an error telling you
2211 the variable is unsupported.
2213 See L<perldiag> for details about error messages.
2220 C<$#> was a variable that could be used to format printed numbers.
2221 After a deprecation cycle, its magic was removed in Perl v5.10.0 and
2222 using it now triggers a warning: C<$# is no longer supported>.
2224 This is not the sigil you use in front of an array name to get the
2225 last index, like C<$#array>. That's still how you get the last index
2226 of an array in Perl. The two have nothing to do with each other.
2228 Deprecated in Perl 5.
2230 Removed in Perl v5.10.0.
2235 C<$*> was a variable that you could use to enable multiline matching.
2236 After a deprecation cycle, its magic was removed in Perl v5.10.0.
2237 Using it now triggers a warning: C<$* is no longer supported>.
2238 You should use the C</s> and C</m> regexp modifiers instead.
2240 Deprecated in Perl 5.
2242 Removed in Perl v5.10.0.
2247 This variable stores the index of the first element in an array, and
2248 of the first character in a substring. The default is 0, but you could
2249 theoretically set it to 1 to make Perl behave more like B<awk> (or Fortran)
2250 when subscripting and when evaluating the index() and substr() functions.
2252 As of release 5 of Perl, assignment to C<$[> is treated as a compiler
2253 directive, and cannot influence the behavior of any other file.
2254 (That's why you can only assign compile-time constants to it.)
2255 Its use is highly discouraged.
2257 Prior to Perl v5.10.0, assignment to C<$[> could be seen from outer lexical
2258 scopes in the same file, unlike other compile-time directives (such as
2259 L<strict>). Using local() on it would bind its value strictly to a lexical
2260 block. Now it is always lexically scoped.
2262 As of Perl v5.16.0, it is implemented by the L<arybase> module. See
2263 L<arybase> for more details on its behaviour.
2265 Under C<use v5.16>, or C<no feature "array_base">, C<$[> no longer has any
2266 effect, and always contains 0. Assigning 0 to it is permitted, but any
2267 other value will produce an error.
2269 Mnemonic: [ begins subscripts.
2271 Deprecated in Perl v5.12.0.
2276 See L</$^V> for a more modern representation of the Perl version that allows
2277 accurate string comparisons.
2279 The version + patchlevel / 1000 of the Perl interpreter. This variable
2280 can be used to determine whether the Perl interpreter executing a
2281 script is in the right range of versions:
2283 warn "No PerlIO!\n" if $] lt '5.008';
2285 The floating point representation can sometimes lead to inaccurate
2286 numeric comparisons, so string comparisons are recommended.
2288 See also the documentation of C<use VERSION> and C<require VERSION>
2289 for a convenient way to fail if the running Perl interpreter is too old.
2291 Mnemonic: Is this version of perl in the right bracket?