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<$^W> (dollar-sign caret
23 C<W>) is the scalar variable whose name is the single character
24 control-C<W>. This is better than typing a literal control-C<W>
27 Since Perl 5.6, Perl variable names may be alphanumeric
28 strings that begin with control characters (or better yet, a caret).
29 These variables must be written in the form C<${^Foo}>; the braces
30 are not optional. C<${^Foo}> denotes the scalar variable whose
31 name is a control-C<F> followed by two C<o>'s. These variables are
32 reserved for future special uses by Perl, except for the ones that
33 begin with C<^_> (control-underscore or caret-underscore). No
34 control-character name that begins with C<^_> will acquire a special
35 meaning in any future version of Perl; such names may therefore be
36 used safely in programs. C<$^_> itself, however, I<is> reserved.
38 Perl identifiers that begin with digits, control characters, or
39 punctuation characters are exempt from the effects of the C<package>
40 declaration and are always forced to be in package C<main>; they are
41 also exempt from C<strict 'vars'> errors. A few other names are also
50 In particular, the special C<${^_XYZ}> variables are always taken
51 to be in package C<main>, regardless of any C<package> declarations
54 =head1 SPECIAL VARIABLES
56 The following names have special meaning to Perl. Most punctuation
57 names have reasonable mnemonics, or analogs in the shells.
58 Nevertheless, if you wish to use long variable names, you need only say:
62 at the top of your program. This aliases all the short names to the long
63 names in the current package. Some even have medium names, generally
64 borrowed from B<awk>. To avoid a performance hit, if you don't need the
65 C<$PREMATCH>, C<$MATCH>, or C<$POSTMATCH> it's best to use the C<English>
68 use English '-no_match_vars';
70 Before you continue, note the sort order for variables. In general, we
71 first list the variables in case-insensitive, almost-lexigraphical
72 order (ignoring the C<{> or C<^> preceding words, as in C<${^UNICODE}>
73 or C<$^T>), although C<$_> and C<@_> move up to the top of the pile.
74 For variables with the same identifier, we list it in order of scalar,
75 array, hash, and bareword.
77 =head2 General Variables
86 The default input and pattern-searching space. The following pairs are
89 while (<>) {...} # equivalent only in while!
90 while (defined($_ = <>)) {...}
101 Here are the places where Perl will assume C<$_> even if you don't use it:
107 The following functions use C<$_> as a default argument:
109 abs, alarm, chomp, chop, chr, chroot, cos, defined, eval, exp, glob,
110 hex, int, lc, lcfirst, length, log, lstat, mkdir, oct, ord, pos, print,
111 quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only),
112 rmdir, sin, split (on its second argument), sqrt, stat, study, uc, ucfirst,
117 All file tests (C<-f>, C<-d>) except for C<-t>, which defaults to STDIN.
122 The pattern matching operations C<m//>, C<s///> and C<tr///> (aka C<y///>)
123 when used without an C<=~> operator.
127 The default iterator variable in a C<foreach> loop if no other
128 variable is supplied.
132 The implicit iterator variable in the C<grep()> and C<map()> functions.
136 The implicit variable of C<given()>.
140 The default place to put an input record when a C<< <FH> >>
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 As C<$_> is a global variable, this may lead in some cases to unwanted
147 side-effects. As of perl 5.9.1, you can now use a lexical version of
148 C<$_> 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.
151 Mnemonic: underline is understood in certain operations.
158 Within a subroutine the array C<@_> contains the parameters passed to
159 that subroutine. Inside a subroutine, C<@_> is the default array for
160 the array operators C<push>, C<pop>, C<shift>, and C<unshift>.
164 =item $LIST_SEPARATOR
167 X<$"> X<$LIST_SEPARATOR>
169 When an array or an array slice is interpolated into a double-quoted
170 string or a similar context such as C</.../>, its elements are
171 separated by this value. Default is a space. For example, this:
173 print "The array is: @array\n";
175 is equivalent to this:
177 print "The array is: " . join($", @array) . "\n";
179 Mnemonic: works in double-quoted context.
186 X<$$> X<$PID> X<$PROCESS_ID>
188 The process number of the Perl running this script. Though you I<can> set
189 this variable, doing so is generally discouraged, although it can be
190 invaluable for some testing purposes. It will be reset automatically
191 across C<fork()> calls.
193 Note for Linux users: on Linux, the C functions C<getpid()> and
194 C<getppid()> return different values from different threads. In order to
195 be portable, this behavior is not reflected by C<$$>, whose value remains
196 consistent across threads. If you want to call the underlying C<getpid()>,
197 you may use the CPAN module C<Linux::Pid>.
199 Mnemonic: same as shells.
204 X<$0> X<$PROGRAM_NAME>
206 Contains the name of the program being executed.
208 On some (but not all) operating systems assigning to C<$0> modifies
209 the argument area that the C<ps> program sees. On some platforms you
210 may have to use special C<ps> options or a different C<ps> to see the
211 changes. Modifying the C<$0> is more useful as a way of indicating the
212 current program state than it is for hiding the program you're
215 Note that there are platform-specific limitations on the maximum
216 length of C<$0>. In the most extreme case it may be limited to the
217 space occupied by the original C<$0>.
219 In some platforms there may be arbitrary amount of padding, for
220 example space characters, after the modified name as shown by C<ps>.
221 In some platforms this padding may extend all the way to the original
222 length of the argument area, no matter what you do (this is the case
223 for example with Linux 2.2).
225 Note for BSD users: setting C<$0> does not completely remove "perl"
226 from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
227 result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
228 and the " (perl)" suffix are shown depends on your exact BSD variant
229 and version). This is an operating system feature, Perl cannot help it.
231 In multithreaded scripts Perl coordinates the threads so that any
232 thread may modify its copy of the C<$0> and the change becomes visible
233 to ps(1) (assuming the operating system plays along). Note that
234 the view of C<$0> the other threads have will not change since they
235 have their own copies of it.
237 If the program has been given to perl via the switches C<-e> or C<-E>,
238 C<$0> will contain the string C<"-e">.
240 On Linux as of perl 5.14 the legacy process name will be set with
241 C<prctl(2)>, in addition to altering the POSIX name via C<argv[0]> as
242 perl has done since version 4.000. Now system utilities that read the
243 legacy process name such as ps, top and killall will recognize the
244 name you set when assigning to C<$0>. The string you supply will be
245 cut off at 16 bytes, this is a limitation imposed by Linux.
247 Mnemonic: same as B<sh> and B<ksh>.
254 X<$(> X<$GID> X<$REAL_GROUP_ID>
256 The real gid of this process. If you are on a machine that supports
257 membership in multiple groups simultaneously, gives a space separated
258 list of groups you are in. The first number is the one returned by
259 C<getgid()>, and the subsequent ones by C<getgroups()>, one of which may be
260 the same as the first number.
262 However, a value assigned to C<$(> must be a single number used to
263 set the real gid. So the value given by C<$(> should I<not> be assigned
264 back to C<$(> without being forced numeric, such as by adding zero. Note
265 that this is different to the effective gid (C<$)>) which does take a
268 You can change both the real gid and the effective gid at the same
269 time by using C<POSIX::setgid()>. Changes to C<$(> require a check to C<$!>
270 to detect any possible errors after an attempted change.
272 Mnemonic: parentheses are used to I<group> things. The real gid is the
273 group you I<left>, if you're running setgid.
275 =item $EFFECTIVE_GROUP_ID
280 X<$)> X<$EGID> X<$EFFECTIVE_GROUP_ID>
282 The effective gid of this process. If you are on a machine that
283 supports membership in multiple groups simultaneously, gives a space
284 separated list of groups you are in. The first number is the one
285 returned by C<getegid()>, and the subsequent ones by C<getgroups()>,
286 one of which may be the same as the first number.
288 Similarly, a value assigned to C<$)> must also be a space-separated
289 list of numbers. The first number sets the effective gid, and
290 the rest (if any) are passed to C<setgroups()>. To get the effect of an
291 empty list for C<setgroups()>, just repeat the new effective gid; that is,
292 to force an effective gid of 5 and an effectively empty C<setgroups()>
293 list, say C< $) = "5 5" >.
295 You can change both the effective gid and the real gid at the same
296 time by using C<POSIX::setgid()> (use only a single numeric argument).
297 Changes to C<$)> require a check to C<$!> to detect any possible errors
298 after an attempted change.
300 C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
301 machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
302 and C<$)> can be swapped only on machines supporting C<setregid()>.
304 Mnemonic: parentheses are used to I<group> things. The effective gid
305 is the group that's I<right> for you, if you're running setgid.
312 X<< $< >> X<$UID> X<$REAL_USER_ID>
314 The real uid of this process. You can change both the real uid and the
315 effective uid at the same time by using C<POSIX::setuid()>. Since
316 changes to C<< $< >> require a system call, check C<$!> after a change
317 attempt to detect any possible errors.
319 Mnemonic: it's the uid you came I<from>, if you're running setuid.
321 =item $EFFECTIVE_USER_ID
326 X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID>
328 The effective uid of this process. For example:
330 $< = $>; # set real to effective uid
331 ($<,$>) = ($>,$<); # swap real and effective uids
333 You can change both the effective uid and the real uid at the same
334 time by using C<POSIX::setuid()>. Changes to C<< $> >> require a check
335 to C<$!> to detect any possible errors after an attempted change.
337 C<< $< >> and C<< $> >> can be swapped only on machines
338 supporting C<setreuid()>.
340 Mnemonic: it's the uid you went I<to>, if you're running setuid.
342 =item $SUBSCRIPT_SEPARATOR
347 X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR>
349 The subscript separator for multidimensional array emulation. If you
350 refer to a hash element as
356 $foo{join($;, $a, $b, $c)}
360 @foo{$a,$b,$c} # a slice--note the @
364 ($foo{$a},$foo{$b},$foo{$c})
366 Default is "\034", the same as SUBSEP in B<awk>. If your keys contain
367 binary data there might not be any safe value for C<$;>.
369 Consider using "real" multidimensional arrays as described
372 Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
379 Special package variables when using C<sort()>, see L<perlfunc/sort>.
380 Because of this specialness C<$a> and C<$b> don't need to be declared
381 (using C<use vars>, or C<our()>) even when using the C<strict 'vars'>
382 pragma. Don't lexicalize them with C<my $a> or C<my $b> if you want to
383 be able to use them in the C<sort()> comparison block or function.
388 The hash C<%ENV> contains your current environment. Setting a
389 value in C<ENV> changes the environment for any child processes
390 you subsequently C<fork()> off.
395 X<$^F> X<$SYSTEM_FD_MAX>
397 The maximum system file descriptor, ordinarily 2. System file
398 descriptors are passed to C<exec()>ed processes, while higher file
399 descriptors are not. Also, during an C<open()>, system file descriptors are
400 preserved even if the C<open()> fails (ordinary file descriptors are
401 closed before the C<open()> is attempted). The close-on-exec
402 status of a file descriptor will be decided according to the value of
403 C<$^F> when the corresponding file, pipe, or socket was opened, not the
404 time of the C<exec()>.
409 The array C<@F> contains the fields of each line read in when autosplit
410 mode is turned on. See L<perlrun> for the B<-a> switch. This array
411 is package-specific, and must be declared or given a full package name
412 if not in package main when running under C<strict 'vars'>.
417 The array C<@INC> contains the list of places that the C<do EXPR>,
418 C<require>, or C<use> constructs look for their library files. It
419 initially consists of the arguments to any B<-I> command-line
420 switches, followed by the default Perl library, probably
421 F</usr/local/lib/perl>, followed by ".", to represent the current
422 directory. ("." will not be appended if taint checks are enabled,
423 either by C<-T> or by C<-t>.) If you need to modify this at runtime,
424 you should use the C<use lib> pragma to get the machine-dependent
425 library properly loaded also:
427 use lib '/mypath/libdir/';
430 You can also insert hooks into the file inclusion system by putting Perl
431 code directly into C<@INC>. Those hooks may be subroutine references, array
432 references or blessed objects. See L<perlfunc/require> for details.
437 The hash C<%INC> contains entries for each filename included via the
438 C<do>, C<require>, or C<use> operators. The key is the filename
439 you specified (with module names converted to pathnames), and the
440 value is the location of the file found. The C<require>
441 operator uses this hash to determine whether a particular file has
442 already been included.
444 If the file was loaded via a hook (e.g. a subroutine reference, see
445 L<perlfunc/require> for a description of these hooks), this hook is
446 by default inserted into C<%INC> in place of a filename. Note, however,
447 that the hook may have set the C<%INC> entry by itself to provide some more
453 X<$^I> X<$INPLACE_EDIT>
455 The current value of the inplace-edit extension. Use C<undef> to disable
458 Mnemonic: value of B<-i> switch.
463 By default, running out of memory is an untrappable, fatal error.
464 However, if suitably built, Perl can use the contents of C<$^M>
465 as an emergency memory pool after C<die()>ing. Suppose that your Perl
466 were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
469 $^M = 'a' x (1 << 16);
471 would allocate a 64K buffer for use in an emergency. See the
472 F<INSTALL> file in the Perl distribution for information on how to
473 add custom C compilation flags when compiling perl. To discourage casual
474 use of this advanced feature, there is no L<English|English> long name for
477 This variable was added in Perl 5.004.
484 The name of the operating system under which this copy of Perl was
485 built, as determined during the configuration process. For examples
486 see L<perlport/PLATFORMS>.
488 The value is identical to C<$Config{'osname'}>. See also L<Config>
489 and the B<-V> command-line switch documented in L<perlrun>.
491 In Windows platforms, C<$^O> is not very helpful: since it is always
492 C<MSWin32>, it doesn't tell the difference between
493 95/98/ME/NT/2000/XP/CE/.NET. Use C<Win32::GetOSName()> or
494 Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
495 between the variants.
497 This variable was added in Perl 5.003.
502 The hash C<%SIG> contains signal handlers for signals. For example:
504 sub handler { # 1st argument is signal name
506 print "Caught a SIG$sig--shutting down\n";
511 $SIG{'INT'} = \&handler;
512 $SIG{'QUIT'} = \&handler;
514 $SIG{'INT'} = 'DEFAULT'; # restore default action
515 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
517 Using a value of C<'IGNORE'> usually has the effect of ignoring the
518 signal, except for the C<CHLD> signal. See L<perlipc> for more about
521 Here are some other examples:
523 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
524 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
525 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
526 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
528 Be sure not to use a bareword as the name of a signal handler,
529 lest you inadvertently call it.
531 If your system has the C<sigaction()> function then signal handlers
532 are installed using it. This means you get reliable signal handling.
534 The default delivery policy of signals changed in Perl 5.8.0 from
535 immediate (also known as "unsafe") to deferred, also known as "safe
536 signals". See L<perlipc> for more information.
538 Certain internal hooks can be also set using the C<%SIG> hash. The
539 routine indicated by C<$SIG{__WARN__}> is called when a warning
540 message is about to be printed. The warning message is passed as the
541 first argument. The presence of a C<__WARN__> hook causes the
542 ordinary printing of warnings to C<STDERR> to be suppressed. You can
543 use this to save warnings in a variable, or turn warnings into fatal
546 local $SIG{__WARN__} = sub { die $_[0] };
549 As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
550 disable warnings using the empty subroutine:
552 local $SIG{__WARN__} = sub {};
554 The routine indicated by C<$SIG{__DIE__}> is called when a fatal
555 exception is about to be thrown. The error message is passed as the
556 first argument. When a C<__DIE__> hook routine returns, the exception
557 processing continues as it would have in the absence of the hook,
558 unless the hook routine itself exits via a C<goto>, a loop exit, or a
559 C<die()>. The C<__DIE__> handler is explicitly disabled during the
560 call, so that you can die from a C<__DIE__> handler. Similarly for
563 Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
564 even inside an C<eval()>. Do not use this to rewrite a pending
565 exception in C<$@>, or as a bizarre substitute for overriding
566 C<CORE::GLOBAL::die()>. This strange action at a distance may be fixed
567 in a future release so that C<$SIG{__DIE__}> is only called if your
568 program is about to exit, as was the original intent. Any other use is
571 C<__DIE__>/C<__WARN__> handlers are very special in one respect: they
572 may be called to report (probable) errors found by the parser. In such
573 a case the parser may be in inconsistent state, so any attempt to
574 evaluate Perl code from such a handler will probably result in a
575 segfault. This means that warnings or errors that result from parsing
576 Perl should be used with extreme caution, like this:
578 require Carp if defined $^S;
579 Carp::confess("Something wrong") if defined &Carp::confess;
580 die "Something wrong, but could not load Carp to give backtrace...
581 To see backtrace try starting Perl with -MCarp switch";
583 Here the first line will load C<Carp> I<unless> it is the parser who
584 called the handler. The second line will print backtrace and die if
585 C<Carp> was available. The third line will be executed only if C<Carp> was
588 Having to even think about the C<$^S> variable in your exception
589 handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
590 invites grievous and difficult to track down errors. Avoid it
591 and use an C<END{}> or CORE::GLOBAL::die override instead.
593 See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
594 L<warnings> for additional information.
601 The time at which the program began running, in seconds since the
602 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
603 and B<-C> filetests are based on this value.
608 X<$^V> X<$PERL_VERSION>
610 The revision, version, and subversion of the Perl interpreter,
611 represented as a C<version> object.
613 This variable first appeared in perl 5.6.0; earlier versions of perl
614 will see an undefined value. Before perl 5.10.0 C<$^V> was represented
617 C<$^V> can be used to determine whether the Perl interpreter executing
618 a script is in the right range of versions. For example:
620 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
622 To convert C<$^V> into its string representation use C<sprintf()>'s
625 printf "version is v%vd\n", $^V; # Perl's version
627 See the documentation of C<use VERSION> and C<require VERSION>
628 for a convenient way to fail if the running Perl interpreter is too old.
630 See also C<$]> for an older representation of the Perl version.
632 This variable was added in Perl 5.6.
634 Mnemonic: use ^V for Version Control.
636 =item ${^WIN32_SLOPPY_STAT}
637 X<${^WIN32_SLOPPY_STAT}> X<sitecustomize> X<sitecustomize.pl>
639 If this variable is set to a true value, then C<stat()> on Windows will
640 not try to open the file. This means that the link count cannot be
641 determined and file attributes may be out of date if additional
642 hardlinks to the file exist. On the other hand, not opening the file
643 is considerably faster, especially for files on network drives.
645 This variable could be set in the F<sitecustomize.pl> file to
646 configure the local Perl installation to use "sloppy" C<stat()> by
647 default. See the documentation for B<-f> in
648 L<perlrun|perlrun/"Command Switches"> for more information about site
651 This variable was added in Perl 5.10.
653 =item $EXECUTABLE_NAME
656 X<$^X> X<$EXECUTABLE_NAME>
658 The name used to execute the current copy of Perl, from C's
659 C<argv[0]> or (where supported) F</proc/self/exe>.
661 Depending on the host operating system, the value of C<$^X> may be
662 a relative or absolute pathname of the perl program file, or may
663 be the string used to invoke perl but not the pathname of the
664 perl program file. Also, most operating systems permit invoking
665 programs that are not in the PATH environment variable, so there
666 is no guarantee that the value of C<$^X> is in PATH. For VMS, the
667 value may or may not include a version number.
669 You usually can use the value of C<$^X> to re-invoke an independent
670 copy of the same perl that is currently running, e.g.,
672 @first_run = `$^X -le "print int rand 100 for 1..100"`;
674 But recall that not all operating systems support forking or
675 capturing of the output of commands, so this complex statement
678 It is not safe to use the value of C<$^X> as a path name of a file,
679 as some operating systems that have a mandatory suffix on
680 executable files do not require use of the suffix when invoking
681 a command. To convert the value of C<$^X> to a path name, use the
682 following statements:
684 # Build up a set of file names (not command names).
688 $this_perl .= $Config{_exe}
689 unless $this_perl =~ m/$Config{_exe}$/i;
692 Because many operating systems permit anyone with read access to
693 the Perl program file to make a copy of it, patch the copy, and
694 then execute the copy, the security-conscious Perl programmer
695 should take care to invoke the installed copy of perl, not the
696 copy referenced by C<$^X>. The following statements accomplish
697 this goal, and produce a pathname that can be invoked as a
698 command or referenced as a file.
701 my $secure_perl_path = $Config{perlpath};
703 $secure_perl_path .= $Config{_exe}
704 unless $secure_perl_path =~ m/$Config{_exe}$/i;
709 =head2 Variables related to regular expressions
711 Most of the special variables related to regular expressions are side
712 effects. Perl sets these variables when it has a successful match, so
713 you should check the match result before using them. For instance:
715 if( /P(A)TT(ER)N/ ) {
716 print "I found $1 and $2\n";
719 These variables are read-only and dynamically-scoped, unless we note
722 The dynamic nature of the regular expression variables means that
723 their value is limited to the block that they are in, as demonstrated
726 my $outer = 'Wallace and Grommit';
727 my $inner = 'Mutt and Jeff';
729 my $pattern = qr/(\S+) and (\S+)/;
731 sub show_n { print "\$1 is $1; \$2 is $2\n" }
735 show_n() if $outer =~ m/$pattern/;
738 show_n() if $inner =~ m/$pattern/;
744 The output shows that while in the C<OUTER> block, the values of C<$1>
745 and C<$2> are from the match against C<$outer>. Inside the C<INNER>
746 block, the values of C<$1> and C<$2> are from the match against
747 C<$inner>, but only until the end of the block (i.e. the dynamic
748 scope). After the C<INNER> block completes, the values of C<$1> and
749 C<$2> return to the values for the match against C<$outer> even though
750 we have not made another match:
752 $1 is Wallace; $2 is Grommit
753 $1 is Mutt; $2 is Jeff
754 $1 is Wallace; $2 is Grommit
756 Due to an unfortunate accident of Perl's implementation, C<use
757 English> imposes a considerable performance penalty on all regular
758 expression matches in a program because it uses the C<$`>, C<$&>, and
759 C<$'>, regardless of whether they occur in the scope of C<use
760 English>. For that reason, saying C<use English> in libraries is
761 strongly discouraged unless you import it without the match variables:
763 use English '-no_match_vars'
765 The C<Devel::NYTProf> and C<Devel::FindAmpersand>
766 modules can help you find uses of these
767 problematic match variables in your code.
769 Since Perl 5.10, you can use the C</p> match operator flag and the
770 C<${^PREMATCH}>, C<${^MATCH}>, and C<${^POSTMATCH}> variables instead
771 so you only suffer the performance penalties.
775 =item $<I<digits>> ($1, $2, ...)
778 Contains the subpattern from the corresponding set of capturing
779 parentheses from the last successful pattern match, not counting patterns
780 matched in nested blocks that have been exited already.
782 These variables are read-only and dynamically-scoped.
784 Mnemonic: like \digits.
791 The string matched by the last successful pattern match (not counting
792 any matches hidden within a BLOCK or C<eval()> enclosed by the current
795 The use of this variable anywhere in a program imposes a considerable
796 performance penalty on all regular expression matches. To avoid this
797 penalty, you can extract the same substring by using L</@->. Starting
798 with Perl 5.10, you can use the C</p> match flag and the C<${^MATCH}>
799 variable to do the same thing for particular match operations.
801 This variable is read-only and dynamically-scoped.
803 Mnemonic: like C<&> in some editors.
808 This is similar to C<$&> (C<$MATCH>) except that it does not incur the
809 performance penalty associated with that variable, and is only guaranteed
810 to return a defined value when the pattern was compiled or executed with
813 This variable was added in Perl 5.10.
815 This variable is read-only and dynamically-scoped.
820 X<$`> X<$PREMATCH> X<${^PREMATCH}>
822 The string preceding whatever was matched by the last successful
823 pattern match, not counting any matches hidden within a BLOCK or C<eval>
824 enclosed by the current BLOCK.
826 The use of this variable anywhere in a program imposes a considerable
827 performance penalty on all regular expression matches. To avoid this
828 penalty, you can extract the same substring by using L</@->. Starting
829 with Perl 5.10, you can use the C</p> match flag and the
830 C<${^PREMATCH}> variable to do the same thing for particular match
833 This variable is read-only and dynamically-scoped.
835 Mnemonic: C<`> often precedes a quoted string.
838 X<$`> X<${^PREMATCH}>
840 This is similar to C<$`> ($PREMATCH) except that it does not incur the
841 performance penalty associated with that variable, and is only guaranteed
842 to return a defined value when the pattern was compiled or executed with
845 This variable was added in Perl 5.10
847 This variable is read-only and dynamically-scoped.
852 X<$'> X<$POSTMATCH> X<${^POSTMATCH}> X<@->
854 The string following whatever was matched by the last successful
855 pattern match (not counting any matches hidden within a BLOCK or C<eval()>
856 enclosed by the current BLOCK). Example:
858 local $_ = 'abcdefghi';
860 print "$`:$&:$'\n"; # prints abc:def:ghi
862 The use of this variable anywhere in a program imposes a considerable
863 performance penalty on all regular expression matches.
864 To avoid this penalty, you can extract the same substring by
865 using L</@->. Starting with Perl 5.10, you can use the C</p> match flag
866 and the C<${^POSTMATCH}> variable to do the same thing for particular
869 This variable is read-only and dynamically-scoped.
871 Mnemonic: C<'> often follows a quoted string.
874 X<${^POSTMATCH}> X<$'> X<$POSTMATCH>
876 This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
877 performance penalty associated with that variable, and is only guaranteed
878 to return a defined value when the pattern was compiled or executed with
881 This variable was added in Perl 5.10.
883 This variable is read-only and dynamically-scoped.
885 =item $LAST_PAREN_MATCH
888 X<$+> X<$LAST_PAREN_MATCH>
890 The text matched by the last bracket of the last successful search pattern.
891 This is useful if you don't know which one of a set of alternative patterns
892 matched. For example:
894 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
896 This variable is read-only and dynamically-scoped.
898 Mnemonic: be positive and forward looking.
900 =item $LAST_SUBMATCH_RESULT
903 X<$^N> X<$LAST_SUBMATCH_RESULT>
905 The text matched by the used group most-recently closed (i.e. the group
906 with the rightmost closing parenthesis) of the last successful search
909 This is primarily used inside C<(?{...})> blocks for examining text
910 recently matched. For example, to effectively capture text to a variable
911 (in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
913 (?:(...)(?{ $var = $^N }))
915 By setting and then using C<$var> in this way relieves you from having to
916 worry about exactly which numbered set of parentheses they are.
918 This variable was added in Perl 5.8.
920 Mnemonic: the (possibly) Nested parenthesis that most recently closed.
922 =item @LAST_MATCH_END
925 X<@+> X<@LAST_MATCH_END>
927 This array holds the offsets of the ends of the last successful
928 submatches in the currently active dynamic scope. C<$+[0]> is
929 the offset into the string of the end of the entire match. This
930 is the same value as what the C<pos> function returns when called
931 on the variable that was matched against. The I<n>th element
932 of this array holds the offset of the I<n>th submatch, so
933 C<$+[1]> is the offset past where C<$1> ends, C<$+[2]> the offset
934 past where C<$2> ends, and so on. You can use C<$#+> to determine
935 how many subgroups were in the last successful match. See the
936 examples given for the C<@-> variable.
938 This variable was added in Perl 5.6.
940 =item %LAST_PAREN_MATCH
943 X<%+> X<%LAST_PAREN_MATCH>
945 Similar to C<@+>, the C<%+> hash allows access to the named capture
946 buffers, should they exist, in the last successful match in the
947 currently active dynamic scope.
949 For example, C<$+{foo}> is equivalent to C<$1> after the following match:
951 'foo' =~ /(?<foo>foo)/;
953 The keys of the C<%+> hash list only the names of buffers that have
954 captured (and that are thus associated to defined values).
956 The underlying behaviour of C<%+> is provided by the
957 L<Tie::Hash::NamedCapture> module.
959 B<Note:> C<%-> and C<%+> are tied views into a common internal hash
960 associated with the last successful regular expression. Therefore mixing
961 iterative access to them via C<each> may have unpredictable results.
962 Likewise, if the last successful match changes, then the results may be
965 This variable was added in Perl 5.10.
967 This variable is read-only and dynamically-scoped.
969 =item @LAST_MATCH_START
972 X<@-> X<@LAST_MATCH_START>
974 C<$-[0]> is the offset of the start of the last successful match.
975 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
976 I<n>-th subpattern, or undef if the subpattern did not match.
978 Thus, after a match against C<$_>, C<$&> coincides with C<substr $_, $-[0],
979 $+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
980 $+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
981 C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the last
982 matched subgroup in the last successful match. Contrast with
983 C<$#+>, the number of subgroups in the regular expression. Compare
986 This array holds the offsets of the beginnings of the last
987 successful submatches in the currently active dynamic scope.
988 C<$-[0]> is the offset into the string of the beginning of the
989 entire match. The I<n>th element of this array holds the offset
990 of the I<n>th submatch, so C<$-[1]> is the offset where C<$1>
991 begins, C<$-[2]> the offset where C<$2> begins, and so on.
993 After a match against some variable C<$var>:
997 =item C<$`> is the same as C<substr($var, 0, $-[0])>
999 =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
1001 =item C<$'> is the same as C<substr($var, $+[0])>
1003 =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
1005 =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
1007 =item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
1011 This variable was added in Perl 5.6.
1013 =item %LAST_MATCH_START
1016 X<%-> X<%LAST_MATCH_START>
1018 Similar to C<%+>, this variable allows access to the named capture groups
1019 in the last successful match in the currently active dynamic scope. To
1020 each capture group name found in the regular expression, it associates a
1021 reference to an array containing the list of values captured by all
1022 buffers with that name (should there be several of them), in the order
1027 if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
1028 foreach my $bufname (sort keys %-) {
1029 my $ary = $-{$bufname};
1030 foreach my $idx (0..$#$ary) {
1031 print "\$-{$bufname}[$idx] : ",
1032 (defined($ary->[$idx]) ? "'$ary->[$idx]'" : "undef"),
1045 The keys of the C<%-> hash correspond to all buffer names found in
1046 the regular expression.
1048 The behaviour of C<%-> is implemented via the
1049 L<Tie::Hash::NamedCapture> module.
1051 B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1052 associated with the last successful regular expression. Therefore mixing
1053 iterative access to them via C<each> may have unpredictable results.
1054 Likewise, if the last successful match changes, then the results may be
1057 This variable was added in Perl 5.10
1059 This variable is read-only and dynamically-scoped.
1061 =item $LAST_REGEXP_CODE_RESULT
1064 X<$^R> X<$LAST_REGEXP_CODE_RESULT>
1066 The result of evaluation of the last successful C<(?{ code })>
1067 regular expression assertion (see L<perlre>). May be written to.
1069 This variable was added in Perl 5.005.
1071 =item ${^RE_DEBUG_FLAGS}
1072 X<${^RE_DEBUG_FLAGS}>
1074 The current value of the regex debugging flags. Set to 0 for no debug output
1075 even when the C<re 'debug'> module is loaded. See L<re> for details.
1077 This variable was added in Perl 5.10.
1079 =item ${^RE_TRIE_MAXBUF}
1080 X<${^RE_TRIE_MAXBUF}>
1082 Controls how certain regex optimisations are applied and how much memory they
1083 utilize. This value by default is 65536 which corresponds to a 512kB temporary
1084 cache. Set this to a higher value to trade memory for speed when matching
1085 large alternations. Set it to a lower value if you want the optimisations to
1086 be as conservative of memory as possible but still occur, and set it to a
1087 negative value to prevent the optimisation and conserve the most memory.
1088 Under normal situations this variable should be of no interest to you.
1090 This variable was added in Perl 5.10.
1094 =head2 Variables related to filehandles
1096 Variables that depend on the currently selected filehandle may be set
1097 by calling an appropriate object method on the C<IO::Handle> object,
1098 although this is less efficient than using the regular built-in
1099 variables. (Summary lines below for this contain the word HANDLE.)
1104 after which you may use either
1110 HANDLE->method(EXPR)
1112 Each method returns the old value of the C<IO::Handle> attribute. The
1113 methods each take an optional EXPR, which, if supplied, specifies the
1114 new value for the C<IO::Handle> attribute in question. If not
1115 supplied, most methods do nothing to the current value--except for
1116 C<autoflush()>, which will assume a 1 for you, just to be different.
1118 Because loading in the C<IO::Handle> class is an expensive operation,
1119 you should learn how to use the regular built-in variables.
1121 A few of these variables are considered "read-only". This means that
1122 if you try to assign to this variable, either directly or indirectly
1123 through a reference, you'll raise a run-time exception.
1125 You should be very careful when modifying the default values of most
1126 special variables described in this document. In most cases you want
1127 to localize these variables before changing them, since if you don't,
1128 the change may affect other modules which rely on the default values
1129 of the special variables that you have changed. This is one of the
1130 correct ways to read the whole file at once:
1132 open my $fh, "<", "foo" or die $!;
1133 local $/; # enable localized slurp mode
1134 my $content = <$fh>;
1137 But the following code is quite bad:
1139 open my $fh, "<", "foo" or die $!;
1140 undef $/; # enable slurp mode
1141 my $content = <$fh>;
1144 since some other module, may want to read data from some file in the
1145 default "line mode", so if the code we have just presented has been
1146 executed, the global value of C<$/> is now changed for any other code
1147 running inside the same Perl interpreter.
1149 Usually when a variable is localized you want to make sure that this
1150 change affects the shortest scope possible. So unless you are already
1151 inside some short C<{}> block, you should create one yourself. For
1155 open my $fh, "<", "foo" or die $!;
1162 Here is an example of how your own code can go broken:
1172 # do something with $_
1175 You probably expect this code to print the equivalent of
1179 but instead you get:
1183 Why? Because C<nasty_break()> modifies C<$\> without localizing it
1184 first. The value you set in C<nasty_break()> is still there when you
1185 return. The fix is to add C<local()> so the value doesn't leak out of
1190 It's easy to notice the problem in such a short example, but in more
1191 complicated code you are looking for trouble if you don't localize
1192 changes to the special variables.
1199 Contains the name of the current file when reading from C<< <> >>.
1204 The array C<@ARGV> contains the command-line arguments intended for
1205 the script. C<$#ARGV> is generally the number of arguments minus
1206 one, because C<$ARGV[0]> is the first argument, I<not> the program's
1207 command name itself. See L</$0> for the command name.
1212 The special filehandle that iterates over command-line filenames in
1213 C<@ARGV>. Usually written as the null filehandle in the angle operator
1214 C<< <> >>. Note that currently C<ARGV> only has its magical effect
1215 within the C<< <> >> operator; elsewhere it is just a plain filehandle
1216 corresponding to the last file opened by C<< <> >>. In particular,
1217 passing C<\*ARGV> as a parameter to a function that expects a filehandle
1218 may not cause your function to automatically read the contents of all the
1224 The special filehandle that points to the currently open output file
1225 when doing edit-in-place processing with B<-i>. Useful when you have
1226 to do a lot of inserting and don't want to keep modifying C<$_>. See
1227 L<perlrun> for the B<-i> switch.
1229 =item Handle->output_field_separator( EXPR )
1231 =item $OUTPUT_FIELD_SEPARATOR
1236 X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>
1238 The output field separator for the print operator. If defined, this
1239 value is printed between each of print's arguments. Default is C<undef>.
1241 Mnemonic: what is printed when there is a "," in your print statement.
1243 =item HANDLE->input_line_number( EXPR )
1245 =item $INPUT_LINE_NUMBER
1250 X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
1252 Current line number for the last filehandle accessed.
1254 Each filehandle in Perl counts the number of lines that have been read
1255 from it. (Depending on the value of C<$/>, Perl's idea of what
1256 constitutes a line may not match yours.) When a line is read from a
1257 filehandle (via C<readline()> or C<< <> >>), or when C<tell()> or
1258 C<seek()> is called on it, C<$.> becomes an alias to the line counter
1259 for that filehandle.
1261 You can adjust the counter by assigning to C<$.>, but this will not
1262 actually move the seek pointer. I<Localizing C<$.> will not localize
1263 the filehandle's line count>. Instead, it will localize perl's notion
1264 of which filehandle C<$.> is currently aliased to.
1266 C<$.> is reset when the filehandle is closed, but B<not> when an open
1267 filehandle is reopened without an intervening C<close()>. For more
1268 details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
1269 an explicit close, line numbers increase across C<ARGV> files (but see
1270 examples in L<perlfunc/eof>).
1272 You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
1273 line counter for a given filehandle without having to worry about
1274 which handle you last accessed.
1276 Mnemonic: many programs use "." to mean the current line number.
1278 =item HANDLE->input_record_separator( EXPR )
1280 =item $INPUT_RECORD_SEPARATOR
1285 X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR>
1287 The input record separator, newline by default. This influences Perl's
1288 idea of what a "line" is. Works like B<awk>'s RS variable, including
1289 treating empty lines as a terminator if set to the null string (an
1290 empty line cannot contain any spaces or tabs). You may set it to a
1291 multi-character string to match a multi-character terminator, or to
1292 C<undef> to read through the end of file. Setting it to C<"\n\n">
1293 means something slightly different than setting to C<"">, if the file
1294 contains consecutive empty lines. Setting to C<""> will treat two or
1295 more consecutive empty lines as a single empty line. Setting to
1296 C<"\n\n"> will blindly assume that the next input character belongs to
1297 the next paragraph, even if it's a newline.
1299 local $/; # enable "slurp" mode
1300 local $_ = <FH>; # whole file now here
1303 Remember: the value of C<$/> is a string, not a regex. B<awk> has to
1304 be better for something. :-)
1306 Setting C<$/> to a reference to an integer, scalar containing an
1307 integer, or scalar that's convertible to an integer will attempt to
1308 read records instead of lines, with the maximum record size being the
1309 referenced integer. So this:
1311 local $/ = \32768; # or \"32768", or \$var_containing_32768
1312 open my $fh, "<", $myfile or die $!;
1315 will read a record of no more than 32768 bytes from FILE. If you're
1316 not reading from a record-oriented file (or your OS doesn't have
1317 record-oriented files), then you'll likely get a full chunk of data
1318 with every read. If a record is larger than the record size you've
1319 set, you'll get the record back in pieces. Trying to set the record
1320 size to zero or less will cause reading in the (rest of the) whole file.
1322 On VMS, record reads are done with the equivalent of C<sysread>,
1323 so it's best not to mix record and non-record reads on the same
1324 file. (This is unlikely to be a problem, because any file you'd
1325 want to read in record mode is probably unusable in line mode.)
1326 Non-VMS systems do normal I/O, so it's safe to mix record and
1327 non-record reads of a file.
1329 See also L<perlport/"Newlines">. Also see L</$.>.
1331 Mnemonic: / delimits line boundaries when quoting poetry.
1333 =item Handle->output_record_separator( EXPR )
1335 =item $OUTPUT_RECORD_SEPARATOR
1340 X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR>
1342 The output record separator for the print operator. If defined, this
1343 value is printed after the last of print's arguments. Default is C<undef>.
1345 Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
1346 Also, it's just like C<$/>, but it's what you get "back" from Perl.
1348 =item HANDLE->autoflush( EXPR )
1350 =item $OUTPUT_AUTOFLUSH
1353 X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
1355 If set to nonzero, forces a flush right away and after every write or
1356 print on the currently selected output channel. Default is 0
1357 (regardless of whether the channel is really buffered by the system or
1358 not; C<$|> tells you only whether you've asked Perl explicitly to
1359 flush after each write). STDOUT will typically be line buffered if
1360 output is to the terminal and block buffered otherwise. Setting this
1361 variable is useful primarily when you are outputting to a pipe or
1362 socket, such as when you are running a Perl program under B<rsh> and
1363 want to see the output as it's happening. This has no effect on input
1364 buffering. See L<perlfunc/getc> for that. See L<perlfunc/select> on
1365 how to select the output channel. See also L<IO::Handle>.
1367 Mnemonic: when you want your pipes to be piping hot.
1371 =head3 Variables related to formats
1373 The special variables for formats are a subset of those for
1374 filehandles. See L<perlform> for more information about Perl's
1382 X<$^A> X<$ACCUMULATOR>
1384 The current value of the C<write()> accumulator for C<format()> lines.
1385 A format contains C<formline()> calls that put their result into
1386 C<$^A>. After calling its format, C<write()> prints out the contents
1387 of C<$^A> and empties. So you never really see the contents of C<$^A>
1388 unless you call C<formline()> yourself and then look at it. See
1389 L<perlform> and L<perlfunc/"formline PICTURE,LIST">.
1391 =item HANDLE->format_formfeed(EXPR)
1393 =item $FORMAT_FORMFEED
1396 X<$^L> X<$FORMAT_FORMFEED>
1398 What formats output as a form feed. The default is C<\f>.
1400 =item HANDLE->format_page_number(EXPR)
1402 =item $FORMAT_PAGE_NUMBER
1405 X<$%> X<$FORMAT_PAGE_NUMBER>
1407 The current page number of the currently selected output channel.
1409 Mnemonic: C<%> is page number in B<nroff>.
1411 =item HANDLE->format_lines_left(EXPR)
1413 =item $FORMAT_LINES_LEFT
1416 X<$-> X<$FORMAT_LINES_LEFT>
1418 The number of lines left on the page of the currently selected output
1421 Mnemonic: lines_on_page - lines_printed.
1423 =item Handle->format_line_break_characters EXPR
1425 =item $FORMAT_LINE_BREAK_CHARACTERS
1428 X<$:> X<FORMAT_LINE_BREAK_CHARACTERS>
1430 The current set of characters after which a string may be broken to
1431 fill continuation fields (starting with C<^>) in a format. The default is
1432 S<" \n-">, to break on a space, newline, or a hyphen.
1434 Mnemonic: a "colon" in poetry is a part of a line.
1436 =item HANDLE->format_lines_per_page(EXPR)
1438 =item $FORMAT_LINES_PER_PAGE
1441 X<$=> X<$FORMAT_LINES_PER_PAGE>
1443 The current page length (printable lines) of the currently selected
1444 output channel. The default is 60.
1446 Mnemonic: = has horizontal lines.
1448 =item HANDLE->format_top_name(EXPR)
1450 =item $FORMAT_TOP_NAME
1453 X<$^> X<$FORMAT_TOP_NAME>
1455 The name of the current top-of-page format for the currently selected
1456 output channel. The default is the name of the filehandle with C<_TOP>
1457 appended. For example, the default format top name for the C<STDOUT>
1458 filehanlde is C<STDOUT_TOP>.
1460 Mnemonic: points to top of page.
1462 =item HANDLE->format_name(EXPR)
1467 X<$~> X<$FORMAT_NAME>
1469 The name of the current report format for the currently selected
1470 output channel. The default format name is the same as the filehandle
1471 name. For example, the default format name for the C<STDOUT>
1472 filehandle is just C<STDOUT>.
1474 Mnemonic: brother to C<$^>.
1478 =head2 Error Variables
1479 X<error> X<exception>
1481 The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1482 about different types of error conditions that may appear during
1483 execution of a Perl program. The variables are shown ordered by
1484 the "distance" between the subsystem which reported the error and
1485 the Perl process. They correspond to errors detected by the Perl
1486 interpreter, C library, operating system, or an external program,
1489 To illustrate the differences between these variables, consider the
1490 following Perl expression, which uses a single-quoted string. After
1491 execution of this statement, perl may have set all four special error
1495 open my $pipe, "/cdrom/install |" or die $!;
1497 close $pipe or die "bad pipe: $?, $!";
1500 When perl executes the C<eval()> expression, it translates the
1501 C<open()>, C<< <PIPE> >>, and C<close> calls in the C run-time library
1502 and thence to the operating system kernel. perl sets C<$!> to
1503 the C library's C<errno> if one of these calls fails.
1505 C<$@> is set if the string to be C<eval>-ed did not compile (this may
1506 happen if C<open> or C<close> were imported with bad prototypes), or
1507 if Perl code executed during evaluation C<die()>d. In these cases the
1508 value of C<$@> is the compile error, or the argument to C<die> (which
1509 will interpolate C<$!> and C<$?>). (See also L<Fatal>, though.)
1511 Under a few operating systems, C<$^E> may contain a more verbose error
1512 indicator, such as in this case, "CDROM tray not closed." Systems that
1513 do not support extended error messages leave C<$^E> the same as C<$!>.
1515 Finally, C<$?> may be set to non-0 value if the external program
1516 F</cdrom/install> fails. The upper eight bits reflect specific error
1517 conditions encountered by the program (the program's C<exit()> value).
1518 The lower eight bits reflect mode of failure, like signal death and
1519 core dump information. See L<wait(2)> for details. In contrast to
1520 C<$!> and C<$^E>, which are set only if error condition is detected,
1521 the variable C<$?> is set on each C<wait> or pipe C<close>,
1522 overwriting the old value. This is more like C<$@>, which on every
1523 C<eval()> is always set on failure and cleared on success.
1525 For more details, see the individual descriptions at C<$@>, C<$!>,
1530 =item ${^CHILD_ERROR_NATIVE}
1531 X<$^CHILD_ERROR_NATIVE>
1533 The native status returned by the last pipe close, backtick (C<``>)
1534 command, successful call to C<wait()> or C<waitpid()>, or from the
1535 C<system()> operator. On POSIX-like systems this value can be decoded
1536 with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED,
1537 WSTOPSIG and WIFCONTINUED functions provided by the L<POSIX> module.
1539 Under VMS this reflects the actual VMS exit status; i.e. it is the
1540 same as C<$?> when the pragma C<use vmsish 'status'> is in effect.
1542 This variable was added in Perl 5.8.9.
1544 =item $EXTENDED_OS_ERROR
1547 X<$^E> X<$EXTENDED_OS_ERROR>
1549 Error information specific to the current operating system. At the
1550 moment, this differs from C<$!> under only VMS, OS/2, and Win32 (and
1551 for MacPerl). On all other platforms, C<$^E> is always just the same
1554 Under VMS, C<$^E> provides the VMS status value from the last system
1555 error. This is more specific information about the last system error
1556 than that provided by C<$!>. This is particularly important when C<$!>
1557 is set to B<EVMSERR>.
1559 Under OS/2, C<$^E> is set to the error code of the last call to OS/2
1560 API either via CRT, or directly from perl.
1562 Under Win32, C<$^E> always returns the last error information reported
1563 by the Win32 call C<GetLastError()> which describes the last error
1564 from within the Win32 API. Most Win32-specific code will report errors
1565 via C<$^E>. ANSI C and Unix-like calls set C<errno> and so most
1566 portable Perl code will report errors via C<$!>.
1568 Caveats mentioned in the description of C<$!> generally apply to
1571 This variable was added in Perl 5.003.
1573 Mnemonic: Extra error explanation.
1575 =item $EXCEPTIONS_BEING_CAUGHT
1578 X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>
1580 Current state of the interpreter.
1583 --------- -------------------
1584 undef Parsing module/eval
1585 true (1) Executing an eval
1588 The first state may happen in C<$SIG{__DIE__}> and C<$SIG{__WARN__}>
1591 This variable was added in Perl 5.004.
1598 The current value of the warning switch, initially true if B<-w> was
1599 used, false otherwise, but directly modifiable.
1601 See also L<warnings>.
1603 Mnemonic: related to the B<-w> switch.
1605 =item ${^WARNING_BITS}
1608 The current set of warning checks enabled by the C<use warnings> pragma.
1609 See the documentation of C<warnings> for more details.
1611 This variable was added in Perl 5.10.
1618 X<$!> X<$ERRNO> X<$OS_ERROR>
1620 When referenced, C<$!> retrieves the current value
1621 of the C C<errno> integer variable.
1622 If C<$!> is assigned a numerical value, that value is stored in C<errno>.
1623 When referenced as a string, C<$!> yields the system error string
1624 corresponding to C<errno>.
1626 Many system or library calls set C<errno> if they fail,
1627 to indicate the cause of failure. They usually do B<not>
1628 set C<errno> to zero if they succeed. This means C<errno>,
1629 hence C<$!>, is meaningful only I<immediately> after a B<failure>:
1631 if (open my $fh, "<", $filename) {
1632 # Here $! is meaningless.
1636 # ONLY here is $! meaningful.
1638 # Already here $! might be meaningless.
1640 # Since here we might have either success or failure,
1641 # $! is meaningless.
1643 Here, I<meaningless> means that C<$!> may be unrelated to the outcome
1644 of the C<open()> operator. Assignment to C<$!> is similarly ephemeral.
1645 It can be used immediately before invoking the C<die()> operator,
1646 to set the exit value, or to inspect the system error string
1647 corresponding to error I<n>, or to restore C<$!> to a meaningful state.
1649 Mnemonic: What just went bang?
1656 X<%!> X<%OS_ERROR> X<%ERRNO>
1658 Each element of C<%!> has a true value only if C<$!> is set to that
1659 value. For example, C<$!{ENOENT}> is true if and only if the current
1660 value of C<$!> is C<ENOENT>; that is, if the most recent error was "No
1661 such file or directory" (or its moral equivalent: not all operating
1662 systems give that exact error, and certainly not all languages). To
1663 check if a particular key is meaningful on your system, use C<exists
1664 $!{the_key}>; for a list of legal keys, use C<keys %!>. See L<Errno>
1665 for more information, and also see L</$!>.
1667 This variable was added in Perl 5.005.
1672 X<$?> X<$CHILD_ERROR>
1674 The status returned by the last pipe close, backtick (C<``>) command,
1675 successful call to C<wait()> or C<waitpid()>, or from the C<system()>
1676 operator. This is just the 16-bit status word returned by the
1677 traditional Unix C<wait()> system call (or else is made up to look
1678 like it). Thus, the exit value of the subprocess is really (C<<< $? >>
1679 8 >>>), and C<$? & 127> gives which signal, if any, the process died
1680 from, and C<$? & 128> reports whether there was a core dump.
1682 Additionally, if the C<h_errno> variable is supported in C, its value
1683 is returned via C<$?> if any C<gethost*()> function fails.
1685 If you have installed a signal handler for C<SIGCHLD>, the
1686 value of C<$?> will usually be wrong outside that handler.
1688 Inside an C<END> subroutine C<$?> contains the value that is going to be
1689 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
1690 change the exit status of your program. For example:
1693 $? = 1 if $? == 255; # die would make it 255
1696 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
1697 actual VMS exit status, instead of the default emulation of POSIX
1698 status; see L<perlvms/$?> for details.
1700 Mnemonic: similar to B<sh> and B<ksh>.
1705 X<$@> X<$EVAL_ERROR>
1707 The Perl syntax error message from the last C<eval()> operator. If C<$@> is
1708 the null string, the last C<eval()> parsed and executed correctly
1709 (although the operations you invoked may have failed in the normal
1712 Warning messages are not collected in this variable. You can, however,
1713 set up a routine to process warnings by setting C<$SIG{__WARN__}> as
1714 described in L</%SIG>.
1716 Mnemonic: Where was the syntax error "at"?
1720 =head2 Variables related to the interpreter state
1722 These variables provide information about the current interpreter state.
1729 X<$^C> X<$COMPILING>
1731 The current value of the flag associated with the B<-c> switch.
1732 Mainly of use with B<-MO=...> to allow code to alter its behavior
1733 when being compiled, such as for example to C<AUTOLOAD> at compile
1734 time rather than normal, deferred loading. Setting
1735 C<$^C = 1> is similar to calling C<B::minus_c>.
1737 This variable was added in Perl 5.6.
1742 X<$^D> X<$DEBUGGING>
1744 The current value of the debugging flags. May be read or set. Like its
1745 command-line equivalent, you can use numeric or symbolic values, eg
1746 C<$^D = 10> or C<$^D = "st">.
1748 Mnemonic: value of B<-D> switch.
1753 The I<object reference> to the C<Encode> object that is used to convert
1754 the source code to Unicode. Thanks to this variable your Perl script
1755 does not have to be written in UTF-8. Default is I<undef>. The direct
1756 manipulation of this variable is highly discouraged.
1758 This variable was added in Perl 5.8.2.
1760 =item ${^GLOBAL_PHASE}
1763 The current phase of the perl interpreter.
1765 Possible values are:
1771 The C<PerlInterpreter*> is being constructed via C<perl_construct>. This
1772 value is mostly there for completeness and for use via the
1773 underlying C variable C<PL_phase>. It's not really possible for Perl
1774 code to be executed unless construction of the interpreter is
1779 This is the global compile-time. That includes, basically, every
1780 C<BEGIN> block executed directly or indirectly from during the
1781 compile-time of the top-level program.
1783 This phase is not called "BEGIN" to avoid confusion with
1784 C<BEGIN>-blocks, as those are executed during compile-time of any
1785 compilation unit, not just the top-level program. A new, localised
1786 compile-time entered at run-time, for example by constructs as
1787 C<eval "use SomeModule"> are not global interpreter phases, and
1788 therefore aren't reflected by C<${^GLOBAL_PHASE}>.
1792 Execution of any C<CHECK> blocks.
1796 Similar to "CHECK", but for C<INIT>-blocks, not C<CHECK> blocks.
1800 The main run-time, i.e. the execution of C<PL_main_root>.
1804 Execution of any C<END> blocks.
1812 Also note that there's no value for UNITCHECK-blocks. That's because
1813 those are run for each compilation unit individually, and therefore is
1814 not a global interpreter phase.
1816 Not every program has to go through each of the possible phases, but
1817 transition from one phase to another can only happen in the order
1818 described in the above list.
1820 An example of all of the phases Perl code can see:
1822 BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }
1824 INIT { print "init-time: ${^GLOBAL_PHASE}\n" }
1826 CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }
1829 package Print::Phase;
1832 my ($class, $time) = @_;
1833 return bless \$time, $class;
1838 print "$$self: ${^GLOBAL_PHASE}\n";
1842 print "run-time: ${^GLOBAL_PHASE}\n";
1844 my $runtime = Print::Phase->new(
1845 "lexical variables are garbage collected before END"
1848 END { print "end-time: ${^GLOBAL_PHASE}\n" }
1850 our $destruct = Print::Phase->new(
1851 "package variables are garbage collected after END"
1860 lexical variables are garbage collected before END: RUN
1862 package variables are garbage collected after END: DESTRUCT
1864 This variable was added in Perl 5.14.0.
1869 WARNING: This variable is strictly for internal use only. Its availability,
1870 behavior, and contents are subject to change without notice.
1872 This variable contains compile-time hints for the Perl interpreter. At the
1873 end of compilation of a BLOCK the value of this variable is restored to the
1874 value when the interpreter started to compile the BLOCK.
1876 When perl begins to parse any block construct that provides a lexical scope
1877 (e.g., eval body, required file, subroutine body, loop body, or conditional
1878 block), the existing value of C<$^H> is saved, but its value is left unchanged.
1879 When the compilation of the block is completed, it regains the saved value.
1880 Between the points where its value is saved and restored, code that
1881 executes within BEGIN blocks is free to change the value of C<$^H>.
1883 This behavior provides the semantic of lexical scoping, and is used in,
1884 for instance, the C<use strict> pragma.
1886 The contents should be an integer; different bits of it are used for
1887 different pragmatic flags. Here's an example:
1889 sub add_100 { $^H |= 0x100 }
1896 Consider what happens during execution of the BEGIN block. At this point
1897 the BEGIN block has already been compiled, but the body of C<foo()> is still
1898 being compiled. The new value of C<$^H> will therefore be visible only while
1899 the body of C<foo()> is being compiled.
1901 Substitution of C<BEGIN { add_100() }> block with:
1903 BEGIN { require strict; strict->import('vars') }
1905 demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
1906 version of the same lexical pragma:
1908 BEGIN { require strict; strict->import('vars') if $condition }
1910 This variable was added in Perl 5.003.
1915 The C<%^H> hash provides the same scoping semantic as C<$^H>. This makes it
1916 useful for implementation of lexically scoped pragmas. See L<perlpragma>.
1918 When putting items into C<%^H>, in order to avoid conflicting with other
1919 users of the hash there is a convention regarding which keys to use.
1920 A module should use only keys that begin with the module's name (the
1921 name of its main package) and a "/" character. For example, a module
1922 C<Foo::Bar> should use keys such as C<Foo::Bar/baz>.
1924 This variable was added in Perl 5.6.
1929 An internal variable used by PerlIO. A string in two parts, separated
1930 by a C<\0> byte, the first part describes the input layers, the second
1931 part describes the output layers.
1933 This variable was added in Perl 5.8.0.
1940 The internal variable for debugging support. The meanings of the
1941 various bits are subject to change, but currently indicate:
1947 Debug subroutine enter/exit.
1951 Line-by-line debugging. Causes C<DB::DB()> subroutine to be called for each
1952 statement executed. Also causes saving source code lines (like 0x400).
1956 Switch off optimizations.
1960 Preserve more data for future interactive inspections.
1964 Keep info about source lines on which a subroutine is defined.
1968 Start with single-step on.
1972 Use subroutine address instead of name when reporting.
1976 Report C<goto &subroutine> as well.
1980 Provide informative "file" names for evals based on the place they were compiled.
1984 Provide informative names to anonymous subroutines based on the place they
1989 Save source code lines into C<@{"_<$filename"}>.
1993 Some bits may be relevant at compile-time only, some at
1994 run-time only. This is a new mechanism and the details may change.
1995 See also L<perldebguts>.
2000 Reflects if taint mode is on or off. 1 for on (the program was run with
2001 B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
2004 This variable is read-only.
2006 This variable was added in Perl 5.8.
2011 Reflects certain Unicode settings of Perl. See L<perlrun>
2012 documentation for the C<-C> switch for more information about
2013 the possible values.
2015 This variable is set during Perl startup and is thereafter read-only.
2017 This variable was added in Perl 5.8.2.
2022 This variable controls the state of the internal UTF-8 offset caching code.
2023 1 for on (the default), 0 for off, -1 to debug the caching code by checking
2024 all its results against linear scans, and panicking on any discrepancy.
2026 This variable was added in Perl 5.8.9.
2028 =item ${^UTF8LOCALE}
2031 This variable indicates whether a UTF-8 locale was detected by perl at
2032 startup. This information is used by perl when it's in
2033 adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
2034 switch); see L<perlrun> for more info on this.
2036 This variable was added in Perl 5.8.8.
2040 =head2 Deprecated and removed variables
2042 Deprecating a variable announces the intent of the perl maintainers to
2043 eventually remove the variable from the language. It may still be
2044 available despite its status. Using a deprecated variable triggers
2047 Once a variable is removed, its use triggers an error telling you
2048 the variable is unsupported.
2050 See L<perldiag> for details about error messages.
2059 C<$#> was a variable that could be used to format printed numbers.
2060 After a deprecation cycle, its magic was removed in Perl 5.10 and
2061 using it now triggers a warning: C<$# is no longer supported>.
2063 This is not the sigil you use in front of an array name to get the
2064 last index, like C<$#array>. That's still how you get the last index
2065 of an array in Perl. The two have nothing to do with each other.
2067 Deprecated in Perl 5.
2069 Removed in Perl 5.10.
2074 C<$*> was a variable that you could use to enable multiline matching.
2075 After a deprecation cycle, its magic was removed in Perl 5.10.
2076 Using it now triggers a warning: C<$* is no longer supported>.
2077 You should use the C</s> and C</m> regexp modifiers instead.
2079 Deprecated in Perl 5.
2081 Removed in Perl 5.10.
2086 X<$[> X<$ARRAY_BASE>
2088 C<$[> was a variable that you could use to offset the indexing of arrays
2089 and strings. After a deprecation cycle, the feature was removed in
2090 Perl 5.16. Two old ways of coping with the variability of the index
2091 offset, which were rendered obsolete in Perl 5.000 when C<$[> became
2092 effectively lexically scoped, are still supported: you can read it
2093 (always yielding zero) and you can assign zero to it.
2095 Deprecated in Perl 5.12.
2097 Removed in Perl 5.16.
2099 =item $OLD_PERL_VERSION
2102 X<$]> X<$OLD_PERL_VERSION>
2104 See L</$^V> for a more modern representation of the Perl version that allows
2105 accurate string comparisons.
2107 The version + patchlevel / 1000 of the Perl interpreter. This variable
2108 can be used to determine whether the Perl interpreter executing a
2109 script is in the right range of versions:
2111 warn "No checksumming!\n" if $] < 3.019;
2113 The floating point representation can sometimes lead to inaccurate
2114 numeric comparisons.
2116 See also the documentation of C<use VERSION> and C<require VERSION>
2117 for a convenient way to fail if the running Perl interpreter is too old.
2119 Mnemonic: Is this version of perl in the right bracket?
2121 Deprecated in Perl 5.6.