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1=head1 NAME
2
3perlvar - Perl predefined variables
4
5=head1 DESCRIPTION
6
b0c22438 7=head2 The Syntax of Variable Names
8
0b9346e6 9Variable names in Perl can have several formats. Usually, they
b0c22438 10must begin with a letter or underscore, in which case they can be
11arbitrarily long (up to an internal limit of 251 characters) and
12may contain letters, digits, underscores, or the special sequence
0b9346e6 13C<::> or C<'>. In this case, the part before the last C<::> or
b0c22438 14C<'> is taken to be a I<package qualifier>; see L<perlmod>.
15
16Perl variable names may also be a sequence of digits or a single
0b9346e6 17punctuation or control character. These names are all reserved for
b0c22438 18special uses by Perl; for example, the all-digits names are used
19to hold data captured by backreferences after a regular expression
0b9346e6 20match. Perl has a special syntax for the single-control-character
b0c22438 21names: It understands C<^X> (caret C<X>) to mean the control-C<X>
0b9346e6 22character. For example, the notation C<$^W> (dollar-sign caret
b0c22438 23C<W>) is the scalar variable whose name is the single character
0b9346e6 24control-C<W>. This is better than typing a literal control-C<W>
b0c22438 25into your program.
26
27Since Perl 5.6, Perl variable names may be alphanumeric
28strings that begin with control characters (or better yet, a caret).
29These variables must be written in the form C<${^Foo}>; the braces
0b9346e6 30are not optional. C<${^Foo}> denotes the scalar variable whose
31name is a control-C<F> followed by two C<o>'s. These variables are
b0c22438 32reserved for future special uses by Perl, except for the ones that
0b9346e6 33begin with C<^_> (control-underscore or caret-underscore). No
b0c22438 34control-character name that begins with C<^_> will acquire a special
35meaning in any future version of Perl; such names may therefore be
0b9346e6 36used safely in programs. C<$^_> itself, however, I<is> reserved.
b0c22438 37
38Perl identifiers that begin with digits, control characters, or
39punctuation characters are exempt from the effects of the C<package>
40declaration and are always forced to be in package C<main>; they are
0b9346e6 41also exempt from C<strict 'vars'> errors. A few other names are also
b0c22438 42exempt in these ways:
43
0b9346e6 44 ENV STDIN
45 INC STDOUT
46 ARGV STDERR
47 ARGVOUT
b0c22438 48 SIG
49
69520822 50In particular, the special C<${^_XYZ}> variables are always taken
b0c22438 51to be in package C<main>, regardless of any C<package> declarations
52presently in scope.
53
54=head1 SPECIAL VARIABLES
a0d0e21e 55
0b9346e6 56The following names have special meaning to Perl. Most punctuation
57names have reasonable mnemonics, or analogs in the shells.
58Nevertheless, if you wish to use long variable names, you need only say:
a0d0e21e 59
0b9346e6 60 use English;
a0d0e21e 61
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62at the top of your program. This aliases all the short names to the long
63names in the current package. Some even have medium names, generally
84dabc03 64borrowed from B<awk>. To avoid a performance hit, if you don't need the
65C<$PREMATCH>, C<$MATCH>, or C<$POSTMATCH> it's best to use the C<English>
66module without them:
a0d0e21e 67
0b9346e6 68 use English '-no_match_vars';
a1ce9542 69
0b9346e6 70Before you continue, note the sort order for variables. In general, we
71first list the variables in case-insensitive, almost-lexigraphical
72order (ignoring the C<{> or C<^> preceding words, as in C<${^UNICODE}>
73or C<$^T>), although C<$_> and C<@_> move up to the top of the pile.
74For variables with the same identifier, we list it in order of scalar,
75array, hash, and bareword.
a1ce9542 76
b0c22438 77=head2 General Variables
a0d0e21e 78
84dabc03 79=over 8
80
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81=item $ARG
82
83=item $_
a054c801 84X<$_> X<$ARG>
a0d0e21e 85
b0c22438 86The default input and pattern-searching space. The following pairs are
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87equivalent:
88
0b9346e6 89 while (<>) {...} # equivalent only in while!
90 while (defined($_ = <>)) {...}
a0d0e21e 91
0b9346e6 92 /^Subject:/
93 $_ =~ /^Subject:/
a0d0e21e 94
0b9346e6 95 tr/a-z/A-Z/
96 $_ =~ tr/a-z/A-Z/
a0d0e21e 97
0b9346e6 98 chomp
99 chomp($_)
a0d0e21e 100
0b9346e6 101Here are the places where Perl will assume C<$_> even if you don't use it:
cb1a09d0
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102
103=over 3
104
105=item *
106
84dabc03 107The following functions use C<$_> as a default argument:
db1511c8 108
b0169937
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109abs, alarm, chomp, chop, chr, chroot, cos, defined, eval, exp, glob,
110hex, int, lc, lcfirst, length, log, lstat, mkdir, oct, ord, pos, print,
111quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only),
b0c18621 112rmdir, sin, split (on its second argument), sqrt, stat, study, uc, ucfirst,
b0169937 113unlink, unpack.
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114
115=item *
116
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117All file tests (C<-f>, C<-d>) except for C<-t>, which defaults to STDIN.
118See L<perlfunc/-X>
119
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120=item *
121
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122The pattern matching operations C<m//>, C<s///> and C<tr///> (aka C<y///>)
123when used without an C<=~> operator.
cb1a09d0 124
54310121 125=item *
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126
127The default iterator variable in a C<foreach> loop if no other
128variable is supplied.
129
54310121 130=item *
cb1a09d0 131
b0c22438 132The implicit iterator variable in the C<grep()> and C<map()> functions.
cb1a09d0 133
54310121 134=item *
cb1a09d0 135
b0c22438 136The implicit variable of C<given()>.
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137
138=item *
139
c47ff5f1 140The default place to put an input record when a C<< <FH> >>
cb1a09d0 141operation's result is tested by itself as the sole criterion of a C<while>
b0c22438 142test. Outside a C<while> test, this will not happen.
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143
144=back
145
59f00321 146As C<$_> is a global variable, this may lead in some cases to unwanted
b0c22438 147side-effects. As of perl 5.9.1, you can now use a lexical version of
148C<$_> by declaring it in a file or in a block with C<my>. Moreover,
4fd88bf8 149declaring C<our $_> restores the global C<$_> in the current scope.
59f00321 150
b0c22438 151Mnemonic: underline is understood in certain operations.
a0d0e21e 152
0b9346e6 153=item @ARG
cde0cee5 154
0b9346e6 155=item @_
156X<@_> X<@ARG>
a0d0e21e 157
0b9346e6 158Within a subroutine the array C<@_> contains the parameters passed to
159that subroutine. Inside a subroutine, C<@_> is the default array for
160the array operators C<push>, C<pop>, C<shift>, and C<unshift>.
a0d0e21e 161
0b9346e6 162See L<perlsub>.
a0d0e21e 163
1311257d 164=item $LIST_SEPARATOR
165
166=item $"
167X<$"> X<$LIST_SEPARATOR>
168
69520822 169When an array or an array slice is interpolated into a double-quoted
170string or a similar context such as C</.../>, its elements are
171separated by this value. Default is a space. For example, this:
172
0b9346e6 173 print "The array is: @array\n";
69520822 174
175is equivalent to this:
176
0b9346e6 177 print "The array is: " . join($", @array) . "\n";
69520822 178
179Mnemonic: works in double-quoted context.
1311257d 180
b0c22438 181=item $PROCESS_ID
cde0cee5 182
b0c22438 183=item $PID
a0d0e21e 184
b0c22438 185=item $$
186X<$$> X<$PID> X<$PROCESS_ID>
a0d0e21e 187
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188The process number of the Perl running this script. Though you I<can> set
189this variable, doing so is generally discouraged, although it can be
190invaluable for some testing purposes. It will be reset automatically
b0c22438 191across C<fork()> calls.
a0d0e21e 192
b0c22438 193Note for Linux users: on Linux, the C functions C<getpid()> and
194C<getppid()> return different values from different threads. In order to
195be portable, this behavior is not reflected by C<$$>, whose value remains
196consistent across threads. If you want to call the underlying C<getpid()>,
197you may use the CPAN module C<Linux::Pid>.
a0d0e21e 198
b0c22438 199Mnemonic: same as shells.
ad83b128 200
b0c22438 201=item $REAL_GROUP_ID
a01268b5 202
b0c22438 203=item $GID
a01268b5 204
b0c22438 205=item $(
206X<$(> X<$GID> X<$REAL_GROUP_ID>
a01268b5 207
b0c22438 208The real gid of this process. If you are on a machine that supports
209membership in multiple groups simultaneously, gives a space separated
210list of groups you are in. The first number is the one returned by
211C<getgid()>, and the subsequent ones by C<getgroups()>, one of which may be
212the same as the first number.
a01268b5 213
b0c22438 214However, a value assigned to C<$(> must be a single number used to
215set the real gid. So the value given by C<$(> should I<not> be assigned
216back to C<$(> without being forced numeric, such as by adding zero. Note
217that this is different to the effective gid (C<$)>) which does take a
218list.
fe307981 219
b0c22438 220You can change both the real gid and the effective gid at the same
221time by using C<POSIX::setgid()>. Changes to C<$(> require a check to C<$!>
222to detect any possible errors after an attempted change.
6cef1e77 223
b0c22438 224Mnemonic: parentheses are used to I<group> things. The real gid is the
225group you I<left>, if you're running setgid.
6cef1e77 226
b0c22438 227=item $EFFECTIVE_GROUP_ID
8e08999f 228
b0c22438 229=item $EGID
81714fb9 230
b0c22438 231=item $)
232X<$)> X<$EGID> X<$EFFECTIVE_GROUP_ID>
81714fb9 233
b0c22438 234The effective gid of this process. If you are on a machine that
235supports membership in multiple groups simultaneously, gives a space
236separated list of groups you are in. The first number is the one
237returned by C<getegid()>, and the subsequent ones by C<getgroups()>,
238one of which may be the same as the first number.
81714fb9 239
b0c22438 240Similarly, a value assigned to C<$)> must also be a space-separated
241list of numbers. The first number sets the effective gid, and
242the rest (if any) are passed to C<setgroups()>. To get the effect of an
243empty list for C<setgroups()>, just repeat the new effective gid; that is,
244to force an effective gid of 5 and an effectively empty C<setgroups()>
245list, say C< $) = "5 5" >.
81714fb9 246
b0c22438 247You can change both the effective gid and the real gid at the same
248time by using C<POSIX::setgid()> (use only a single numeric argument).
249Changes to C<$)> require a check to C<$!> to detect any possible errors
250after an attempted change.
44a2ac75 251
b0c22438 252C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
253machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
254and C<$)> can be swapped only on machines supporting C<setregid()>.
3195cf34 255
b0c22438 256Mnemonic: parentheses are used to I<group> things. The effective gid
257is the group that's I<right> for you, if you're running setgid.
44a2ac75 258
b0c22438 259=item $PROGRAM_NAME
a0d0e21e 260
b0c22438 261=item $0
262X<$0> X<$PROGRAM_NAME>
a0d0e21e 263
b0c22438 264Contains the name of the program being executed.
a0d0e21e 265
69520822 266On some (but not all) operating systems assigning to C<$0> modifies
7333b1c4 267the argument area that the C<ps> program sees. On some platforms you
b0c22438 268may have to use special C<ps> options or a different C<ps> to see the
7333b1c4 269changes. Modifying the C<$0> is more useful as a way of indicating the
b0c22438 270current program state than it is for hiding the program you're
271running.
a0d0e21e 272
69520822 273Note that there are platform-specific limitations on the maximum
b0c22438 274length of C<$0>. In the most extreme case it may be limited to the
275space occupied by the original C<$0>.
fcc7d916 276
b0c22438 277In some platforms there may be arbitrary amount of padding, for
278example space characters, after the modified name as shown by C<ps>.
279In some platforms this padding may extend all the way to the original
280length of the argument area, no matter what you do (this is the case
281for example with Linux 2.2).
fcc7d916 282
b0c22438 283Note for BSD users: setting C<$0> does not completely remove "perl"
284from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
285result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
286and the " (perl)" suffix are shown depends on your exact BSD variant
287and version). This is an operating system feature, Perl cannot help it.
fcc7d916 288
b0c22438 289In multithreaded scripts Perl coordinates the threads so that any
290thread may modify its copy of the C<$0> and the change becomes visible
291to ps(1) (assuming the operating system plays along). Note that
292the view of C<$0> the other threads have will not change since they
293have their own copies of it.
fcc7d916 294
b0c22438 295If the program has been given to perl via the switches C<-e> or C<-E>,
296C<$0> will contain the string C<"-e">.
fcc7d916 297
b0c22438 298On Linux as of perl 5.14 the legacy process name will be set with
0b9346e6 299C<prctl(2)>, in addition to altering the POSIX name via C<argv[0]> as
b0c22438 300perl has done since version 4.000. Now system utilities that read the
301legacy process name such as ps, top and killall will recognize the
302name you set when assigning to C<$0>. The string you supply will be
303cut off at 16 bytes, this is a limitation imposed by Linux.
fcc7d916 304
b0c22438 305Mnemonic: same as B<sh> and B<ksh>.
0b9346e6 306
307=item $SUBSCRIPT_SEPARATOR
308
309=item $SUBSEP
310
311=item $;
312X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR>
313
314The subscript separator for multidimensional array emulation. If you
315refer to a hash element as
316
317 $foo{$a,$b,$c}
318
319it really means
320
321 $foo{join($;, $a, $b, $c)}
322
323But don't put
324
325 @foo{$a,$b,$c} # a slice--note the @
326
327which means
328
329 ($foo{$a},$foo{$b},$foo{$c})
330
7333b1c4 331Default is "\034", the same as SUBSEP in B<awk>. If your keys contain
0b9346e6 332binary data there might not be any safe value for C<$;>.
333
334Consider using "real" multidimensional arrays as described
335in L<perllol>.
336
337Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
338
339=item $REAL_USER_ID
340
341=item $UID
342
343=item $<
344X<< $< >> X<$UID> X<$REAL_USER_ID>
345
346The real uid of this process. You can change both the real uid and the
347effective uid at the same time by using C<POSIX::setuid()>. Since
348changes to C<< $< >> require a system call, check C<$!> after a change
349attempt to detect any possible errors.
350
351Mnemonic: it's the uid you came I<from>, if you're running setuid.
352
353=item $EFFECTIVE_USER_ID
354
355=item $EUID
356
357=item $>
358X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID>
359
360The effective uid of this process. For example:
361
362 $< = $>; # set real to effective uid
363 ($<,$>) = ($>,$<); # swap real and effective uids
364
365You can change both the effective uid and the real uid at the same
366time by using C<POSIX::setuid()>. Changes to C<< $> >> require a check
367to C<$!> to detect any possible errors after an attempted change.
368
369C<< $< >> and C<< $> >> can be swapped only on machines
370supporting C<setreuid()>.
371
372Mnemonic: it's the uid you went I<to>, if you're running setuid.
373
374=item $a
375
376=item $b
377X<$a> X<$b>
378
379Special package variables when using C<sort()>, see L<perlfunc/sort>.
380Because 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'>
382pragma. Don't lexicalize them with C<my $a> or C<my $b> if you want to
383be able to use them in the C<sort()> comparison block or function.
384
b0c22438 385=item $COMPILING
a0d0e21e 386
b0c22438 387=item $^C
388X<$^C> X<$COMPILING>
a0d0e21e 389
b0c22438 390The current value of the flag associated with the B<-c> switch.
391Mainly of use with B<-MO=...> to allow code to alter its behavior
392when being compiled, such as for example to C<AUTOLOAD> at compile
7333b1c4 393time rather than normal, deferred loading. Setting
b0c22438 394C<$^C = 1> is similar to calling C<B::minus_c>.
a0d0e21e 395
b0c22438 396This variable was added in Perl 5.6.
a0d0e21e 397
b0c22438 398=item $DEBUGGING
a0d0e21e 399
b0c22438 400=item $^D
401X<$^D> X<$DEBUGGING>
a0d0e21e 402
b0c22438 403The current value of the debugging flags. May be read or set. Like its
404command-line equivalent, you can use numeric or symbolic values, eg
405C<$^D = 10> or C<$^D = "st">.
68dc0745 406
b0c22438 407Mnemonic: value of B<-D> switch.
5b2b9c68 408
0b9346e6 409=item ${^ENCODING}
5b442a2a 410X<${^ENCODING}>
0b9346e6 411
412The I<object reference> to the C<Encode> object that is used to convert
413the source code to Unicode. Thanks to this variable your Perl script
414does not have to be written in UTF-8. Default is I<undef>. The direct
415manipulation of this variable is highly discouraged.
416
417This variable was added in Perl 5.8.2.
418
419=item %ENV
420X<%ENV>
421
422The hash C<%ENV> contains your current environment. Setting a
423value in C<ENV> changes the environment for any child processes
424you subsequently C<fork()> off.
425
b0c22438 426=item $SYSTEM_FD_MAX
5b2b9c68 427
b0c22438 428=item $^F
429X<$^F> X<$SYSTEM_FD_MAX>
5b2b9c68 430
b0c22438 431The maximum system file descriptor, ordinarily 2. System file
432descriptors are passed to C<exec()>ed processes, while higher file
433descriptors are not. Also, during an C<open()>, system file descriptors are
434preserved even if the C<open()> fails (ordinary file descriptors are
435closed before the C<open()> is attempted). The close-on-exec
436status of a file descriptor will be decided according to the value of
437C<$^F> when the corresponding file, pipe, or socket was opened, not the
438time of the C<exec()>.
5b2b9c68 439
0b9346e6 440=item @F
441X<@F>
442
443The array C<@F> contains the fields of each line read in when autosplit
7333b1c4 444mode is turned on. See L<perlrun> for the B<-a> switch. This array
0b9346e6 445is package-specific, and must be declared or given a full package name
446if not in package main when running under C<strict 'vars'>.
447
10c97e5d 448=item ${^GLOBAL_PHASE}
d30227f4 449X<${^GLOBAL_PHASE}>
10c97e5d 450
451The current phase of the perl interpreter.
452
bda934ba 453Possible values are:
10c97e5d 454
455=over 8
456
457=item CONSTRUCT
458
459The C<PerlInterpreter*> is being constructed via C<perl_construct>. This
460value is mostly there for completeness and for use via the
461underlying C variable C<PL_phase>. It's not really possible for Perl
462code to be executed unless construction of the interpreter is
463finished.
464
465=item START
466
467This is the global compile-time. That includes, basically, every
468C<BEGIN> block executed directly or indirectly from during the
469compile-time of the top-level program.
470
471This phase is not called "BEGIN" to avoid confusion with
472C<BEGIN>-blocks, as those are executed during compile-time of any
473compilation unit, not just the top-level program. A new, localised
474compile-time entered at run-time, for example by constructs as
475C<eval "use SomeModule"> are not global interpreter phases, and
476therefore aren't reflected by C<${^GLOBAL_PHASE}>.
477
478=item CHECK
479
480Execution of any C<CHECK> blocks.
481
482=item INIT
483
484Similar to "CHECK", but for C<INIT>-blocks, not C<CHECK> blocks.
485
486=item RUN
487
488The main run-time, i.e. the execution of C<PL_main_root>.
489
490=item END
491
492Execution of any C<END> blocks.
493
494=item DESTRUCT
495
496Global destruction.
497
498=back
499
500Also note that there's no value for UNITCHECK-blocks. That's because
501those are run for each compilation unit individually, and therefore is
502not a global interpreter phase.
503
504Not every program has to go through each of the possible phases, but
505transition from one phase to another can only happen in the order
506described in the above list.
507
191f4b8c
COI
508An example of all of the phases Perl code can see:
509
510 BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }
511
512 INIT { print "init-time: ${^GLOBAL_PHASE}\n" }
513
514 CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }
515
516 {
517 package Print::Phase;
518
519 sub new {
520 my ($class, $time) = @_;
521 return bless \$time, $class;
522 }
523
524 sub DESTROY {
525 my $self = shift;
526 print "$$self: ${^GLOBAL_PHASE}\n";
527 }
528 }
529
530 print "run-time: ${^GLOBAL_PHASE}\n";
531
532 my $runtime = Print::Phase->new(
533 "lexical variables are garbage collected before END"
534 );
535
536 END { print "end-time: ${^GLOBAL_PHASE}\n" }
537
538 our $destruct = Print::Phase->new(
539 "package variables are garbage collected after END"
540 );
541
542This will print out
543
544 compile-time: START
545 check-time: CHECK
546 init-time: INIT
547 run-time: RUN
548 lexical variables are garbage collected before END: RUN
549 end-time: END
550 package variables are garbage collected after END: DESTRUCT
10c97e5d 551
6e896f9f 552This variable was added in Perl 5.14.0.
10c97e5d 553
b0c22438 554=item $^H
5b442a2a 555X<$^H>
883faa13 556
b0c22438 557WARNING: This variable is strictly for internal use only. Its availability,
558behavior, and contents are subject to change without notice.
a0d0e21e 559
b0c22438 560This variable contains compile-time hints for the Perl interpreter. At the
561end of compilation of a BLOCK the value of this variable is restored to the
562value when the interpreter started to compile the BLOCK.
a0d0e21e 563
b0c22438 564When perl begins to parse any block construct that provides a lexical scope
565(e.g., eval body, required file, subroutine body, loop body, or conditional
566block), the existing value of C<$^H> is saved, but its value is left unchanged.
567When the compilation of the block is completed, it regains the saved value.
568Between the points where its value is saved and restored, code that
569executes within BEGIN blocks is free to change the value of C<$^H>.
a0d0e21e 570
b0c22438 571This behavior provides the semantic of lexical scoping, and is used in,
572for instance, the C<use strict> pragma.
a0d0e21e 573
b0c22438 574The contents should be an integer; different bits of it are used for
575different pragmatic flags. Here's an example:
a0d0e21e 576
0b9346e6 577 sub add_100 { $^H |= 0x100 }
a0d0e21e 578
0b9346e6 579 sub foo {
580 BEGIN { add_100() }
581 bar->baz($boon);
582 }
a0d0e21e 583
b0c22438 584Consider what happens during execution of the BEGIN block. At this point
585the BEGIN block has already been compiled, but the body of C<foo()> is still
586being compiled. The new value of C<$^H> will therefore be visible only while
587the body of C<foo()> is being compiled.
a0d0e21e 588
7333b1c4 589Substitution of C<BEGIN { add_100() }> block with:
a0d0e21e 590
0b9346e6 591 BEGIN { require strict; strict->import('vars') }
a0d0e21e 592
7333b1c4 593demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
b0c22438 594version of the same lexical pragma:
a0d0e21e 595
0b9346e6 596 BEGIN { require strict; strict->import('vars') if $condition }
a0d0e21e 597
b0c22438 598This variable was added in Perl 5.003.
a0d0e21e 599
b0c22438 600=item %^H
5b442a2a 601X<%^H>
a0d0e21e 602
b0c22438 603The C<%^H> hash provides the same scoping semantic as C<$^H>. This makes it
604useful for implementation of lexically scoped pragmas. See L<perlpragma>.
a0d0e21e 605
b0c22438 606This variable was added in Perl 5.6.
a0d0e21e 607
0b9346e6 608=item @INC
609X<@INC>
610
611The array C<@INC> contains the list of places that the C<do EXPR>,
7333b1c4 612C<require>, or C<use> constructs look for their library files. It
0b9346e6 613initially consists of the arguments to any B<-I> command-line
614switches, followed by the default Perl library, probably
615F</usr/local/lib/perl>, followed by ".", to represent the current
7333b1c4 616directory. ("." will not be appended if taint checks are enabled,
0b9346e6 617either by C<-T> or by C<-t>.) If you need to modify this at runtime,
618you should use the C<use lib> pragma to get the machine-dependent
619library properly loaded also:
620
621 use lib '/mypath/libdir/';
622 use SomeMod;
623
624You can also insert hooks into the file inclusion system by putting Perl
625code directly into C<@INC>. Those hooks may be subroutine references, array
626references or blessed objects. See L<perlfunc/require> for details.
627
628=item %INC
629X<%INC>
630
631The hash C<%INC> contains entries for each filename included via the
632C<do>, C<require>, or C<use> operators. The key is the filename
633you specified (with module names converted to pathnames), and the
634value is the location of the file found. The C<require>
635operator uses this hash to determine whether a particular file has
636already been included.
637
638If the file was loaded via a hook (e.g. a subroutine reference, see
639L<perlfunc/require> for a description of these hooks), this hook is
640by default inserted into C<%INC> in place of a filename. Note, however,
641that the hook may have set the C<%INC> entry by itself to provide some more
642specific info.
643
b0c22438 644=item $INPLACE_EDIT
a0d0e21e 645
b0c22438 646=item $^I
647X<$^I> X<$INPLACE_EDIT>
a0d0e21e 648
b0c22438 649The current value of the inplace-edit extension. Use C<undef> to disable
650inplace editing.
a0d0e21e 651
b0c22438 652Mnemonic: value of B<-i> switch.
a0d0e21e 653
b0c22438 654=item $^M
655X<$^M>
a0d0e21e 656
b0c22438 657By default, running out of memory is an untrappable, fatal error.
658However, if suitably built, Perl can use the contents of C<$^M>
659as an emergency memory pool after C<die()>ing. Suppose that your Perl
660were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
661Then
a0d0e21e 662
0b9346e6 663 $^M = 'a' x (1 << 16);
a0d0e21e 664
b0c22438 665would allocate a 64K buffer for use in an emergency. See the
666F<INSTALL> file in the Perl distribution for information on how to
667add custom C compilation flags when compiling perl. To discourage casual
668use of this advanced feature, there is no L<English|English> long name for
669this variable.
a0d0e21e 670
b0c22438 671This variable was added in Perl 5.004.
a0d0e21e 672
b0c22438 673=item $OSNAME
a0d0e21e 674
b0c22438 675=item $^O
676X<$^O> X<$OSNAME>
a0d0e21e 677
b0c22438 678The name of the operating system under which this copy of Perl was
679built, as determined during the configuration process. For examples
680see L<perlport/PLATFORMS>.
a0d0e21e 681
b0c22438 682The value is identical to C<$Config{'osname'}>. See also L<Config>
683and the B<-V> command-line switch documented in L<perlrun>.
a0d0e21e 684
b0c22438 685In Windows platforms, C<$^O> is not very helpful: since it is always
686C<MSWin32>, it doesn't tell the difference between
68795/98/ME/NT/2000/XP/CE/.NET. Use C<Win32::GetOSName()> or
688Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
689between the variants.
a0d0e21e 690
b0c22438 691This variable was added in Perl 5.003.
a0d0e21e 692
b0c22438 693=item ${^OPEN}
5b442a2a 694X<${^OPEN}>
a0d0e21e 695
b0c22438 696An internal variable used by PerlIO. A string in two parts, separated
697by a C<\0> byte, the first part describes the input layers, the second
698part describes the output layers.
a0d0e21e 699
b0c22438 700This variable was added in Perl 5.8.2.
a0d0e21e 701
b0c22438 702=item $PERLDB
a0d0e21e 703
b0c22438 704=item $^P
705X<$^P> X<$PERLDB>
a0d0e21e 706
b0c22438 707The internal variable for debugging support. The meanings of the
708various bits are subject to change, but currently indicate:
a0d0e21e 709
b0c22438 710=over 6
a0d0e21e 711
b0c22438 712=item 0x01
a0d0e21e 713
b0c22438 714Debug subroutine enter/exit.
a0d0e21e 715
b0c22438 716=item 0x02
a0d0e21e 717
b0c22438 718Line-by-line debugging. Causes C<DB::DB()> subroutine to be called for each
719statement executed. Also causes saving source code lines (like 0x400).
a0d0e21e 720
b0c22438 721=item 0x04
fe307981 722
b0c22438 723Switch off optimizations.
6cef1e77 724
b0c22438 725=item 0x08
6cef1e77 726
b0c22438 727Preserve more data for future interactive inspections.
6cef1e77 728
b0c22438 729=item 0x10
4ba05bdc 730
b0c22438 731Keep info about source lines on which a subroutine is defined.
4ba05bdc 732
b0c22438 733=item 0x20
4ba05bdc 734
b0c22438 735Start with single-step on.
4ba05bdc 736
b0c22438 737=item 0x40
4ba05bdc 738
b0c22438 739Use subroutine address instead of name when reporting.
4ba05bdc 740
b0c22438 741=item 0x80
4ba05bdc 742
b0c22438 743Report C<goto &subroutine> as well.
4ba05bdc 744
b0c22438 745=item 0x100
4ba05bdc 746
b0c22438 747Provide informative "file" names for evals based on the place they were compiled.
4ba05bdc 748
b0c22438 749=item 0x200
44a2ac75 750
b0c22438 751Provide informative names to anonymous subroutines based on the place they
752were compiled.
44a2ac75 753
b0c22438 754=item 0x400
44a2ac75 755
b0c22438 756Save source code lines into C<@{"_<$filename"}>.
44a2ac75 757
b0c22438 758=back
44a2ac75 759
b0c22438 760Some bits may be relevant at compile-time only, some at
7333b1c4 761run-time only. This is a new mechanism and the details may change.
b0c22438 762See also L<perldebguts>.
3195cf34 763
b0c22438 764=item %SIG
b0c22438 765X<%SIG>
a0d0e21e 766
b0c22438 767The hash C<%SIG> contains signal handlers for signals. For example:
a0d0e21e 768
0b9346e6 769 sub handler { # 1st argument is signal name
770 my($sig) = @_;
771 print "Caught a SIG$sig--shutting down\n";
772 close(LOG);
773 exit(0);
774 }
a0d0e21e 775
0b9346e6 776 $SIG{'INT'} = \&handler;
777 $SIG{'QUIT'} = \&handler;
778 ...
779 $SIG{'INT'} = 'DEFAULT'; # restore default action
780 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
a0d0e21e 781
b0c22438 782Using a value of C<'IGNORE'> usually has the effect of ignoring the
783signal, except for the C<CHLD> signal. See L<perlipc> for more about
784this special case.
a0d0e21e 785
b0c22438 786Here are some other examples:
a0d0e21e 787
0b9346e6 788 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
789 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
790 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
791 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
a0d0e21e 792
b0c22438 793Be sure not to use a bareword as the name of a signal handler,
794lest you inadvertently call it.
a0d0e21e 795
b0c22438 796If your system has the C<sigaction()> function then signal handlers
797are installed using it. This means you get reliable signal handling.
7b8d334a 798
b0c22438 799The default delivery policy of signals changed in Perl 5.8.0 from
800immediate (also known as "unsafe") to deferred, also known as "safe
7333b1c4 801signals". See L<perlipc> for more information.
aa689395 802
b0c22438 803Certain internal hooks can be also set using the C<%SIG> hash. The
804routine indicated by C<$SIG{__WARN__}> is called when a warning
7333b1c4 805message is about to be printed. The warning message is passed as the
806first argument. The presence of a C<__WARN__> hook causes the
b0c22438 807ordinary printing of warnings to C<STDERR> to be suppressed. You can
808use this to save warnings in a variable, or turn warnings into fatal
809errors, like this:
19799a22 810
0b9346e6 811 local $SIG{__WARN__} = sub { die $_[0] };
812 eval $proggie;
a8f8344d 813
b0c22438 814As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
815disable warnings using the empty subroutine:
f86702cc 816
0b9346e6 817 local $SIG{__WARN__} = sub {};
55602bd2 818
b0c22438 819The routine indicated by C<$SIG{__DIE__}> is called when a fatal
820exception is about to be thrown. The error message is passed as the
821first argument. When a C<__DIE__> hook routine returns, the exception
822processing continues as it would have in the absence of the hook,
823unless the hook routine itself exits via a C<goto>, a loop exit, or a
824C<die()>. The C<__DIE__> handler is explicitly disabled during the
825call, so that you can die from a C<__DIE__> handler. Similarly for
826C<__WARN__>.
e5218da5 827
b0c22438 828Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
829even inside an C<eval()>. Do not use this to rewrite a pending
830exception in C<$@>, or as a bizarre substitute for overriding
831C<CORE::GLOBAL::die()>. This strange action at a distance may be fixed
832in a future release so that C<$SIG{__DIE__}> is only called if your
833program is about to exit, as was the original intent. Any other use is
834deprecated.
835
836C<__DIE__>/C<__WARN__> handlers are very special in one respect: they
837may be called to report (probable) errors found by the parser. In such
838a case the parser may be in inconsistent state, so any attempt to
839evaluate Perl code from such a handler will probably result in a
840segfault. This means that warnings or errors that result from parsing
841Perl should be used with extreme caution, like this:
e5218da5 842
0b9346e6 843 require Carp if defined $^S;
844 Carp::confess("Something wrong") if defined &Carp::confess;
845 die "Something wrong, but could not load Carp to give backtrace...
846 To see backtrace try starting Perl with -MCarp switch";
e5218da5 847
b0c22438 848Here the first line will load C<Carp> I<unless> it is the parser who
849called the handler. The second line will print backtrace and die if
850C<Carp> was available. The third line will be executed only if C<Carp> was
851not available.
0a378802 852
0b9346e6 853Having to even think about the C<$^S> variable in your exception
7333b1c4 854handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
0b9346e6 855invites grievous and difficult to track down errors. Avoid it
856and use an C<END{}> or CORE::GLOBAL::die override instead.
857
b0c22438 858See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
859L<warnings> for additional information.
0a378802 860
b0c22438 861=item $BASETIME
6ab308ee 862
b0c22438 863=item $^T
864X<$^T> X<$BASETIME>
6ab308ee 865
b0c22438 866The time at which the program began running, in seconds since the
867epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
868and B<-C> filetests are based on this value.
a0d0e21e 869
b0c22438 870=item ${^TAINT}
5b442a2a 871X<${^TAINT}>
55602bd2 872
b0c22438 873Reflects if taint mode is on or off. 1 for on (the program was run with
874B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
0b9346e6 875B<-t> or B<-TU>).
daaddde1 876
b0c22438 877This variable is read-only.
daaddde1 878
b0c22438 879This variable was added in Perl 5.8.
4c5cef9b 880
b0c22438 881=item ${^UNICODE}
5b442a2a 882X<${^UNICODE}>
4c5cef9b 883
7333b1c4 884Reflects certain Unicode settings of Perl. See L<perlrun>
b0c22438 885documentation for the C<-C> switch for more information about
0b9346e6 886the possible values.
5c055ba3 887
b0c22438 888This variable is set during Perl startup and is thereafter read-only.
5c055ba3 889
b0c22438 890This variable was added in Perl 5.8.2.
22fae026 891
b0c22438 892=item ${^UTF8CACHE}
5b442a2a 893X<${^UTF8CACHE}>
22fae026 894
b0c22438 895This variable controls the state of the internal UTF-8 offset caching code.
8961 for on (the default), 0 for off, -1 to debug the caching code by checking
897all its results against linear scans, and panicking on any discrepancy.
22fae026 898
b0c22438 899This variable was added in Perl 5.8.9.
22fae026 900
b0c22438 901=item ${^UTF8LOCALE}
5b442a2a 902X<${^UTF8LOCALE}>
5c055ba3 903
b0c22438 904This variable indicates whether a UTF-8 locale was detected by perl at
905startup. This information is used by perl when it's in
906adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
907switch); see L<perlrun> for more info on this.
55602bd2 908
b0c22438 909This variable was added in Perl 5.8.8.
a0d0e21e 910
b0c22438 911=item $PERL_VERSION
a0d0e21e 912
b0c22438 913=item $^V
914X<$^V> X<$PERL_VERSION>
a0d0e21e 915
b0c22438 916The revision, version, and subversion of the Perl interpreter,
917represented as a C<version> object.
748a9306 918
b0c22438 919This variable first appeared in perl 5.6.0; earlier versions of perl
920will see an undefined value. Before perl 5.10.0 C<$^V> was represented
921as a v-string.
55602bd2 922
b0c22438 923C<$^V> can be used to determine whether the Perl interpreter executing
924a script is in the right range of versions. For example:
a0d0e21e 925
0b9346e6 926 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
a0d0e21e 927
b0c22438 928To convert C<$^V> into its string representation use C<sprintf()>'s
929C<"%vd"> conversion:
a0d0e21e 930
0b9346e6 931 printf "version is v%vd\n", $^V; # Perl's version
a0d0e21e 932
b0c22438 933See the documentation of C<use VERSION> and C<require VERSION>
934for a convenient way to fail if the running Perl interpreter is too old.
4d76a344 935
b0c22438 936See also C<$]> for an older representation of the Perl version.
a0d0e21e 937
b0c22438 938This variable was added in Perl 5.6.
a0d0e21e 939
b0c22438 940Mnemonic: use ^V for Version Control.
a0d0e21e 941
b0c22438 942=item ${^WIN32_SLOPPY_STAT}
5b442a2a 943X<${^WIN32_SLOPPY_STAT}> X<sitecustomize> X<sitecustomize.pl>
a0d0e21e 944
b0c22438 945If this variable is set to a true value, then C<stat()> on Windows will
946not try to open the file. This means that the link count cannot be
947determined and file attributes may be out of date if additional
948hardlinks to the file exist. On the other hand, not opening the file
949is considerably faster, especially for files on network drives.
a0d0e21e 950
b0c22438 951This variable could be set in the F<sitecustomize.pl> file to
952configure the local Perl installation to use "sloppy" C<stat()> by
953default. See the documentation for B<-f> in
954L<perlrun|perlrun/"Command Switches"> for more information about site
955customization.
a0d0e21e 956
b0c22438 957This variable was added in Perl 5.10.
a0d0e21e 958
b0c22438 959=item $EXECUTABLE_NAME
a0d0e21e 960
b0c22438 961=item $^X
962X<$^X> X<$EXECUTABLE_NAME>
a0d0e21e 963
b0c22438 964The name used to execute the current copy of Perl, from C's
965C<argv[0]> or (where supported) F</proc/self/exe>.
a043a685 966
b0c22438 967Depending on the host operating system, the value of C<$^X> may be
968a relative or absolute pathname of the perl program file, or may
969be the string used to invoke perl but not the pathname of the
970perl program file. Also, most operating systems permit invoking
971programs that are not in the PATH environment variable, so there
972is no guarantee that the value of C<$^X> is in PATH. For VMS, the
973value may or may not include a version number.
a0d0e21e 974
b0c22438 975You usually can use the value of C<$^X> to re-invoke an independent
976copy of the same perl that is currently running, e.g.,
a0d0e21e 977
0b9346e6 978 @first_run = `$^X -le "print int rand 100 for 1..100"`;
a0d0e21e 979
b0c22438 980But recall that not all operating systems support forking or
981capturing of the output of commands, so this complex statement
982may not be portable.
a0d0e21e 983
b0c22438 984It is not safe to use the value of C<$^X> as a path name of a file,
985as some operating systems that have a mandatory suffix on
986executable files do not require use of the suffix when invoking
987a command. To convert the value of C<$^X> to a path name, use the
988following statements:
8cc95fdb 989
0b9346e6 990 # Build up a set of file names (not command names).
991 use Config;
992 my $this_perl = $^X;
993 if ($^O ne 'VMS') {
994 $this_perl .= $Config{_exe}
995 unless $this_perl =~ m/$Config{_exe}$/i;
996 }
8cc95fdb 997
b0c22438 998Because many operating systems permit anyone with read access to
999the Perl program file to make a copy of it, patch the copy, and
1000then execute the copy, the security-conscious Perl programmer
1001should take care to invoke the installed copy of perl, not the
1002copy referenced by C<$^X>. The following statements accomplish
1003this goal, and produce a pathname that can be invoked as a
1004command or referenced as a file.
a043a685 1005
0b9346e6 1006 use Config;
1007 my $secure_perl_path = $Config{perlpath};
1008 if ($^O ne 'VMS') {
1009 $secure_perl_path .= $Config{_exe}
1010 unless $secure_perl_path =~ m/$Config{_exe}$/i;
1011 }
a0d0e21e 1012
b0c22438 1013=back
a0d0e21e 1014
b0c22438 1015=head2 Variables related to regular expressions
1016
1017Most of the special variables related to regular expressions are side
1018effects. Perl sets these variables when it has a successful match, so
1019you should check the match result before using them. For instance:
1020
1021 if( /P(A)TT(ER)N/ ) {
1022 print "I found $1 and $2\n";
1023 }
1024
0b9346e6 1025These variables are read-only and dynamically-scoped, unless we note
b0c22438 1026otherwise.
1027
0b9346e6 1028The dynamic nature of the regular expression variables means that
1029their value is limited to the block that they are in, as demonstrated
1030by this bit of code:
b0c22438 1031
1032 my $outer = 'Wallace and Grommit';
1033 my $inner = 'Mutt and Jeff';
0b9346e6 1034
b0c22438 1035 my $pattern = qr/(\S+) and (\S+)/;
0b9346e6 1036
b0c22438 1037 sub show_n { print "\$1 is $1; \$2 is $2\n" }
0b9346e6 1038
b0c22438 1039 {
1040 OUTER:
1041 show_n() if $outer =~ m/$pattern/;
0b9346e6 1042
b0c22438 1043 INNER: {
1044 show_n() if $inner =~ m/$pattern/;
1045 }
0b9346e6 1046
b0c22438 1047 show_n();
1048 }
1049
0b9346e6 1050The output shows that while in the C<OUTER> block, the values of C<$1>
1051and C<$2> are from the match against C<$outer>. Inside the C<INNER>
1052block, the values of C<$1> and C<$2> are from the match against
1053C<$inner>, but only until the end of the block (i.e. the dynamic
1054scope). After the C<INNER> block completes, the values of C<$1> and
1055C<$2> return to the values for the match against C<$outer> even though
b0c22438 1056we have not made another match:
1057
1058 $1 is Wallace; $2 is Grommit
1059 $1 is Mutt; $2 is Jeff
1060 $1 is Wallace; $2 is Grommit
a0d0e21e 1061
0b9346e6 1062Due to an unfortunate accident of Perl's implementation, C<use
1063English> imposes a considerable performance penalty on all regular
1064expression matches in a program because it uses the C<$`>, C<$&>, and
1065C<$'>, regardless of whether they occur in the scope of C<use
1066English>. For that reason, saying C<use English> in libraries is
1067strongly discouraged unless you import it without the match variables:
1068
1069 use English '-no_match_vars'
1070
d8a75b5a
FC
1071The C<Devel::NYTProf> and C<Devel::FindAmpersand>
1072modules can help you find uses of these
0b9346e6 1073problematic match variables in your code.
1074
1075Since Perl 5.10, you can use the C</p> match operator flag and the
1076C<${^PREMATCH}>, C<${^MATCH}>, and C<${^POSTMATCH}> variables instead
1077so you only suffer the performance penalties.
1078
b0c22438 1079=over 8
a0d0e21e 1080
b0c22438 1081=item $<I<digits>> ($1, $2, ...)
1082X<$1> X<$2> X<$3>
8cc95fdb 1083
b0c22438 1084Contains the subpattern from the corresponding set of capturing
1085parentheses from the last successful pattern match, not counting patterns
1086matched in nested blocks that have been exited already.
8cc95fdb 1087
b0c22438 1088These variables are read-only and dynamically-scoped.
a043a685 1089
b0c22438 1090Mnemonic: like \digits.
a0d0e21e 1091
b0c22438 1092=item $MATCH
a0d0e21e 1093
b0c22438 1094=item $&
1095X<$&> X<$MATCH>
a0d0e21e 1096
b0c22438 1097The string matched by the last successful pattern match (not counting
1098any matches hidden within a BLOCK or C<eval()> enclosed by the current
1099BLOCK).
a0d0e21e 1100
b0c22438 1101The use of this variable anywhere in a program imposes a considerable
0b9346e6 1102performance penalty on all regular expression matches. To avoid this
1103penalty, you can extract the same substring by using L</@->. Starting
1104with Perl 5.10, you can use the </p> match flag and the C<${^MATCH}>
1105variable to do the same thing for particular match operations.
80bca1b4 1106
b0c22438 1107This variable is read-only and dynamically-scoped.
f9cbb277 1108
b0c22438 1109Mnemonic: like C<&> in some editors.
0b9346e6 1110
b0c22438 1111=item ${^MATCH}
1112X<${^MATCH}>
a0d0e21e 1113
b0c22438 1114This is similar to C<$&> (C<$MATCH>) except that it does not incur the
1115performance penalty associated with that variable, and is only guaranteed
1116to return a defined value when the pattern was compiled or executed with
1117the C</p> modifier.
80bca1b4 1118
b0c22438 1119This variable was added in Perl 5.10.
4bc88a62 1120
b0c22438 1121This variable is read-only and dynamically-scoped.
e2975953 1122
b0c22438 1123=item $PREMATCH
52c447a8 1124
b0c22438 1125=item $`
5b442a2a 1126X<$`> X<$PREMATCH> X<${^PREMATCH}>
7636ea95 1127
b0c22438 1128The string preceding whatever was matched by the last successful
1129pattern match, not counting any matches hidden within a BLOCK or C<eval>
0b9346e6 1130enclosed by the current BLOCK.
a0d0e21e 1131
b0c22438 1132The use of this variable anywhere in a program imposes a considerable
0b9346e6 1133performance penalty on all regular expression matches. To avoid this
1134penalty, you can extract the same substring by using L</@->. Starting
1135with Perl 5.10, you can use the </p> match flag and the
1136C<${^PREMATCH}> variable to do the same thing for particular match
1137operations.
a0d0e21e 1138
b0c22438 1139This variable is read-only and dynamically-scoped.
a0d0e21e 1140
b0c22438 1141Mnemonic: C<`> often precedes a quoted string.
f83ed198 1142
b0c22438 1143=item ${^PREMATCH}
5b442a2a 1144X<$`> X<${^PREMATCH}>
a0d0e21e 1145
b0c22438 1146This is similar to C<$`> ($PREMATCH) except that it does not incur the
1147performance penalty associated with that variable, and is only guaranteed
1148to return a defined value when the pattern was compiled or executed with
1149the C</p> modifier.
a0d0e21e 1150
b0c22438 1151This variable was added in Perl 5.10
a0d0e21e 1152
b0c22438 1153This variable is read-only and dynamically-scoped.
a0d0e21e 1154
b0c22438 1155=item $POSTMATCH
16070b82 1156
b0c22438 1157=item $'
5b442a2a 1158X<$'> X<$POSTMATCH> X<${^POSTMATCH}> X<@->
305aace0 1159
b0c22438 1160The string following whatever was matched by the last successful
1161pattern match (not counting any matches hidden within a BLOCK or C<eval()>
1162enclosed by the current BLOCK). Example:
305aace0 1163
0b9346e6 1164 local $_ = 'abcdefghi';
1165 /def/;
1166 print "$`:$&:$'\n"; # prints abc:def:ghi
305aace0 1167
b0c22438 1168The use of this variable anywhere in a program imposes a considerable
0b9346e6 1169performance penalty on all regular expression matches.
1170To avoid this penalty, you can extract the same substring by
b0c22438 1171using L</@->. Starting with Perl 5.10, you can use the </p> match flag
0b9346e6 1172and the C<${^POSTMATCH}> variable to do the same thing for particular
b0c22438 1173match operations.
a0d0e21e 1174
b0c22438 1175This variable is read-only and dynamically-scoped.
1176
1177Mnemonic: C<'> often follows a quoted string.
1178
1179=item ${^POSTMATCH}
5b442a2a 1180X<${^POSTMATCH}> X<$'> X<$POSTMATCH>
b0c22438 1181
1182This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
1183performance penalty associated with that variable, and is only guaranteed
1184to return a defined value when the pattern was compiled or executed with
1185the C</p> modifier.
1186
1187This variable was added in Perl 5.10.
1188
1189This variable is read-only and dynamically-scoped.
1190
1191=item $LAST_PAREN_MATCH
1192
1193=item $+
1194X<$+> X<$LAST_PAREN_MATCH>
1195
1196The text matched by the last bracket of the last successful search pattern.
1197This is useful if you don't know which one of a set of alternative patterns
1198matched. For example:
1199
0b9346e6 1200 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
b0c22438 1201
1202This variable is read-only and dynamically-scoped.
1203
1204Mnemonic: be positive and forward looking.
1205
1206=item $LAST_SUBMATCH_RESULT
1207
1208=item $^N
5b442a2a 1209X<$^N> X<$LAST_SUBMATCH_RESULT>
b0c22438 1210
1211The text matched by the used group most-recently closed (i.e. the group
1212with the rightmost closing parenthesis) of the last successful search
1213pattern.
1214
1215This is primarily used inside C<(?{...})> blocks for examining text
1216recently matched. For example, to effectively capture text to a variable
1217(in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
1218
0b9346e6 1219 (?:(...)(?{ $var = $^N }))
b0c22438 1220
1221By setting and then using C<$var> in this way relieves you from having to
1222worry about exactly which numbered set of parentheses they are.
1223
1224This variable was added in Perl 5.8.
1225
1226Mnemonic: the (possibly) Nested parenthesis that most recently closed.
1227
1228=item @LAST_MATCH_END
1229
1230=item @+
1231X<@+> X<@LAST_MATCH_END>
1232
1233This array holds the offsets of the ends of the last successful
1234submatches in the currently active dynamic scope. C<$+[0]> is
1235the offset into the string of the end of the entire match. This
1236is the same value as what the C<pos> function returns when called
1237on the variable that was matched against. The I<n>th element
1238of this array holds the offset of the I<n>th submatch, so
1239C<$+[1]> is the offset past where C<$1> ends, C<$+[2]> the offset
7333b1c4 1240past where C<$2> ends, and so on. You can use C<$#+> to determine
b0c22438 1241how many subgroups were in the last successful match. See the
1242examples given for the C<@-> variable.
1243
1244This variable was added in Perl 5.6.
1245
1246=item %LAST_PAREN_MATCH
1247
1248=item %+
5b442a2a 1249X<%+> X<%LAST_PAREN_MATCH>
b0c22438 1250
1251Similar to C<@+>, the C<%+> hash allows access to the named capture
1252buffers, should they exist, in the last successful match in the
1253currently active dynamic scope.
1254
1255For example, C<$+{foo}> is equivalent to C<$1> after the following match:
1256
0b9346e6 1257 'foo' =~ /(?<foo>foo)/;
b0c22438 1258
1259The keys of the C<%+> hash list only the names of buffers that have
1260captured (and that are thus associated to defined values).
1261
1262The underlying behaviour of C<%+> is provided by the
1263L<Tie::Hash::NamedCapture> module.
1264
1265B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1266associated with the last successful regular expression. Therefore mixing
1267iterative access to them via C<each> may have unpredictable results.
1268Likewise, if the last successful match changes, then the results may be
1269surprising.
1270
1271This variable was added in Perl 5.10.
a0d0e21e 1272
b0c22438 1273This variable is read-only and dynamically-scoped.
1274
1275=item @LAST_MATCH_START
1276
1277=item @-
1278X<@-> X<@LAST_MATCH_START>
1279
1280C<$-[0]> is the offset of the start of the last successful match.
1281C<$-[>I<n>C<]> is the offset of the start of the substring matched by
1282I<n>-th subpattern, or undef if the subpattern did not match.
1283
1284Thus, after a match against C<$_>, C<$&> coincides with C<substr $_, $-[0],
1285$+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
1286$+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
1287C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the last
1288matched subgroup in the last successful match. Contrast with
1289C<$#+>, the number of subgroups in the regular expression. Compare
1290with C<@+>.
1291
1292This array holds the offsets of the beginnings of the last
1293successful submatches in the currently active dynamic scope.
1294C<$-[0]> is the offset into the string of the beginning of the
7333b1c4 1295entire match. The I<n>th element of this array holds the offset
b0c22438 1296of the I<n>th submatch, so C<$-[1]> is the offset where C<$1>
1297begins, C<$-[2]> the offset where C<$2> begins, and so on.
1298
1299After a match against some variable C<$var>:
1300
1301=over 5
1302
1303=item C<$`> is the same as C<substr($var, 0, $-[0])>
1304
1305=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
1306
1307=item C<$'> is the same as C<substr($var, $+[0])>
1308
1309=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
1310
1311=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
1312
1313=item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
1314
1315=back
1316
1317This variable was added in Perl 5.6.
1318
5b442a2a 1319=item %LAST_MATCH_START
1320
b0c22438 1321=item %-
5b442a2a 1322X<%-> X<%LAST_MATCH_START>
b0c22438 1323
1324Similar to C<%+>, this variable allows access to the named capture groups
1325in the last successful match in the currently active dynamic scope. To
1326each capture group name found in the regular expression, it associates a
1327reference to an array containing the list of values captured by all
1328buffers with that name (should there be several of them), in the order
1329where they appear.
1330
1331Here's an example:
1332
1333 if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
1334 foreach my $bufname (sort keys %-) {
1335 my $ary = $-{$bufname};
1336 foreach my $idx (0..$#$ary) {
1337 print "\$-{$bufname}[$idx] : ",
1338 (defined($ary->[$idx]) ? "'$ary->[$idx]'" : "undef"),
1339 "\n";
1340 }
1341 }
1342 }
1343
1344would print out:
1345
0b9346e6 1346 $-{A}[0] : '1'
1347 $-{A}[1] : '3'
1348 $-{B}[0] : '2'
1349 $-{B}[1] : '4'
b0c22438 1350
1351The keys of the C<%-> hash correspond to all buffer names found in
1352the regular expression.
1353
1354The behaviour of C<%-> is implemented via the
1355L<Tie::Hash::NamedCapture> module.
1356
1357B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1358associated with the last successful regular expression. Therefore mixing
1359iterative access to them via C<each> may have unpredictable results.
1360Likewise, if the last successful match changes, then the results may be
1361surprising.
1362
1363This variable was added in Perl 5.10
1364
1365This variable is read-only and dynamically-scoped.
1366
1367=item $LAST_REGEXP_CODE_RESULT
1368
1369=item $^R
1370X<$^R> X<$LAST_REGEXP_CODE_RESULT>
1371
1372The result of evaluation of the last successful C<(?{ code })>
1373regular expression assertion (see L<perlre>). May be written to.
1374
1375This variable was added in Perl 5.005.
a0d0e21e 1376
a3621e74 1377=item ${^RE_DEBUG_FLAGS}
ca1b95ae 1378X<${^RE_DEBUG_FLAGS}>
a3621e74
YO
1379
1380The current value of the regex debugging flags. Set to 0 for no debug output
b0c22438 1381even when the C<re 'debug'> module is loaded. See L<re> for details.
1382
1383This variable was added in Perl 5.10.
a3621e74 1384
0111c4fd 1385=item ${^RE_TRIE_MAXBUF}
ca1b95ae 1386X<${^RE_TRIE_MAXBUF}>
a3621e74
YO
1387
1388Controls how certain regex optimisations are applied and how much memory they
1389utilize. This value by default is 65536 which corresponds to a 512kB temporary
1390cache. Set this to a higher value to trade memory for speed when matching
1391large alternations. Set it to a lower value if you want the optimisations to
1392be as conservative of memory as possible but still occur, and set it to a
1393negative value to prevent the optimisation and conserve the most memory.
1394Under normal situations this variable should be of no interest to you.
1395
b0c22438 1396This variable was added in Perl 5.10.
a0d0e21e 1397
b0c22438 1398=back
a0d0e21e 1399
b0c22438 1400=head2 Variables related to filehandles
a0d0e21e 1401
b0c22438 1402Variables that depend on the currently selected filehandle may be set
1403by calling an appropriate object method on the C<IO::Handle> object,
1404although this is less efficient than using the regular built-in
1405variables. (Summary lines below for this contain the word HANDLE.)
1406First you must say
6e2995f4 1407
0b9346e6 1408 use IO::Handle;
0462a1ab 1409
b0c22438 1410after which you may use either
0462a1ab 1411
0b9346e6 1412 method HANDLE EXPR
0462a1ab 1413
b0c22438 1414or more safely,
0462a1ab 1415
0b9346e6 1416 HANDLE->method(EXPR)
0462a1ab 1417
b0c22438 1418Each method returns the old value of the C<IO::Handle> attribute. The
1419methods each take an optional EXPR, which, if supplied, specifies the
1420new value for the C<IO::Handle> attribute in question. If not
1421supplied, most methods do nothing to the current value--except for
1422C<autoflush()>, which will assume a 1 for you, just to be different.
0462a1ab 1423
b0c22438 1424Because loading in the C<IO::Handle> class is an expensive operation,
1425you should learn how to use the regular built-in variables.
1426
1427A few of these variables are considered "read-only". This means that
1428if you try to assign to this variable, either directly or indirectly
1429through a reference, you'll raise a run-time exception.
1430
1431You should be very careful when modifying the default values of most
1432special variables described in this document. In most cases you want
1433to localize these variables before changing them, since if you don't,
1434the change may affect other modules which rely on the default values
1435of the special variables that you have changed. This is one of the
1436correct ways to read the whole file at once:
1437
0b9346e6 1438 open my $fh, "<", "foo" or die $!;
1439 local $/; # enable localized slurp mode
1440 my $content = <$fh>;
1441 close $fh;
b0c22438 1442
1443But the following code is quite bad:
1444
0b9346e6 1445 open my $fh, "<", "foo" or die $!;
1446 undef $/; # enable slurp mode
1447 my $content = <$fh>;
1448 close $fh;
b0c22438 1449
1450since some other module, may want to read data from some file in the
1451default "line mode", so if the code we have just presented has been
1452executed, the global value of C<$/> is now changed for any other code
1453running inside the same Perl interpreter.
1454
1455Usually when a variable is localized you want to make sure that this
1456change affects the shortest scope possible. So unless you are already
1457inside some short C<{}> block, you should create one yourself. For
1458example:
1459
0b9346e6 1460 my $content = '';
1461 open my $fh, "<", "foo" or die $!;
1462 {
1463 local $/;
1464 $content = <$fh>;
1465 }
1466 close $fh;
0462a1ab 1467
b0c22438 1468Here is an example of how your own code can go broken:
0462a1ab 1469
0b9346e6 1470 for ( 1..3 ){
1471 $\ = "\r\n";
1472 nasty_break();
1473 print "$_";
1474 }
1475
1476 sub nasty_break {
1477 $\ = "\f";
1478 # do something with $_
1479 }
0462a1ab 1480
0b9346e6 1481You probably expect this code to print the equivalent of
0462a1ab 1482
0b9346e6 1483 "1\r\n2\r\n3\r\n"
0462a1ab 1484
b0c22438 1485but instead you get:
0462a1ab 1486
0b9346e6 1487 "1\f2\f3\f"
0462a1ab 1488
0b9346e6 1489Why? Because C<nasty_break()> modifies C<$\> without localizing it
1490first. The value you set in C<nasty_break()> is still there when you
1491return. The fix is to add C<local()> so the value doesn't leak out of
1492C<nasty_break()>:
6e2995f4 1493
0b9346e6 1494 local $\ = "\f";
a0d0e21e 1495
b0c22438 1496It's easy to notice the problem in such a short example, but in more
1497complicated code you are looking for trouble if you don't localize
1498changes to the special variables.
a0d0e21e 1499
b0c22438 1500=over 8
a0d0e21e 1501
b0c22438 1502=item $ARGV
1503X<$ARGV>
fb73857a 1504
ca1b95ae 1505Contains the name of the current file when reading from C<< <> >>.
b0c22438 1506
1507=item @ARGV
1508X<@ARGV>
1509
ca1b95ae 1510The array C<@ARGV> contains the command-line arguments intended for
b0c22438 1511the script. C<$#ARGV> is generally the number of arguments minus
1512one, because C<$ARGV[0]> is the first argument, I<not> the program's
57f6eff5 1513command name itself. See L</$0> for the command name.
b0c22438 1514
84dabc03 1515=item ARGV
1516X<ARGV>
1517
1518The special filehandle that iterates over command-line filenames in
1519C<@ARGV>. Usually written as the null filehandle in the angle operator
1520C<< <> >>. Note that currently C<ARGV> only has its magical effect
1521within the C<< <> >> operator; elsewhere it is just a plain filehandle
1522corresponding to the last file opened by C<< <> >>. In particular,
1523passing C<\*ARGV> as a parameter to a function that expects a filehandle
1524may not cause your function to automatically read the contents of all the
1525files in C<@ARGV>.
1526
b0c22438 1527=item ARGVOUT
1528X<ARGVOUT>
1529
1530The special filehandle that points to the currently open output file
1531when doing edit-in-place processing with B<-i>. Useful when you have
1532to do a lot of inserting and don't want to keep modifying C<$_>. See
1533L<perlrun> for the B<-i> switch.
1534
5b442a2a 1535=item Handle->output_field_separator( EXPR )
84dabc03 1536
1537=item $OUTPUT_FIELD_SEPARATOR
1538
1539=item $OFS
1540
1541=item $,
1542X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>
1543
1544The output field separator for the print operator. If defined, this
1545value is printed between each of print's arguments. Default is C<undef>.
1546
1547Mnemonic: what is printed when there is a "," in your print statement.
1548
5b442a2a 1549=item HANDLE->input_line_number( EXPR )
b0c22438 1550
1551=item $INPUT_LINE_NUMBER
1552
1553=item $NR
1554
1555=item $.
1556X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
1557
1558Current line number for the last filehandle accessed.
1559
1560Each filehandle in Perl counts the number of lines that have been read
7333b1c4 1561from it. (Depending on the value of C<$/>, Perl's idea of what
b0c22438 1562constitutes a line may not match yours.) When a line is read from a
1563filehandle (via C<readline()> or C<< <> >>), or when C<tell()> or
1564C<seek()> is called on it, C<$.> becomes an alias to the line counter
1565for that filehandle.
1566
1567You can adjust the counter by assigning to C<$.>, but this will not
1568actually move the seek pointer. I<Localizing C<$.> will not localize
1569the filehandle's line count>. Instead, it will localize perl's notion
1570of which filehandle C<$.> is currently aliased to.
1571
1572C<$.> is reset when the filehandle is closed, but B<not> when an open
1573filehandle is reopened without an intervening C<close()>. For more
1574details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
1575an explicit close, line numbers increase across C<ARGV> files (but see
1576examples in L<perlfunc/eof>).
1577
1578You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
1579line counter for a given filehandle without having to worry about
1580which handle you last accessed.
1581
1582Mnemonic: many programs use "." to mean the current line number.
1583
5b442a2a 1584=item HANDLE->input_record_separator( EXPR )
b0c22438 1585
1586=item $INPUT_RECORD_SEPARATOR
1587
1588=item $RS
1589
1590=item $/
1591X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR>
1592
84dabc03 1593The input record separator, newline by default. This influences Perl's
7333b1c4 1594idea of what a "line" is. Works like B<awk>'s RS variable, including
84dabc03 1595treating empty lines as a terminator if set to the null string (an
1596empty line cannot contain any spaces or tabs). You may set it to a
1597multi-character string to match a multi-character terminator, or to
1598C<undef> to read through the end of file. Setting it to C<"\n\n">
1599means something slightly different than setting to C<"">, if the file
1600contains consecutive empty lines. Setting to C<""> will treat two or
1601more consecutive empty lines as a single empty line. Setting to
1602C<"\n\n"> will blindly assume that the next input character belongs to
1603the next paragraph, even if it's a newline.
b0c22438 1604
1605 local $/; # enable "slurp" mode
1606 local $_ = <FH>; # whole file now here
1607 s/\n[ \t]+/ /g;
1608
7333b1c4 1609Remember: the value of C<$/> is a string, not a regex. B<awk> has to
b0c22438 1610be better for something. :-)
1611
1612Setting C<$/> to a reference to an integer, scalar containing an
1613integer, or scalar that's convertible to an integer will attempt to
1614read records instead of lines, with the maximum record size being the
1615referenced integer. So this:
1616
1617 local $/ = \32768; # or \"32768", or \$var_containing_32768
1618 open my $fh, "<", $myfile or die $!;
1619 local $_ = <$fh>;
fb73857a 1620
7333b1c4 1621will read a record of no more than 32768 bytes from FILE. If you're
b0c22438 1622not reading from a record-oriented file (or your OS doesn't have
1623record-oriented files), then you'll likely get a full chunk of data
7333b1c4 1624with every read. If a record is larger than the record size you've
1625set, you'll get the record back in pieces. Trying to set the record
b0c22438 1626size to zero or less will cause reading in the (rest of the) whole file.
6e2995f4 1627
b0c22438 1628On VMS, record reads are done with the equivalent of C<sysread>,
1629so it's best not to mix record and non-record reads on the same
5b442a2a 1630file. (This is unlikely to be a problem, because any file you'd
b0c22438 1631want to read in record mode is probably unusable in line mode.)
1632Non-VMS systems do normal I/O, so it's safe to mix record and
1633non-record reads of a file.
5c055ba3 1634
57f6eff5 1635See also L<perlport/"Newlines">. Also see L</$.>.
9bf22702 1636
b0c22438 1637Mnemonic: / delimits line boundaries when quoting poetry.
5c055ba3 1638
5b442a2a 1639=item Handle->output_record_separator( EXPR )
84902520 1640
b0c22438 1641=item $OUTPUT_RECORD_SEPARATOR
84902520 1642
b0c22438 1643=item $ORS
84902520 1644
b0c22438 1645=item $\
1646X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR>
84902520 1647
b0c22438 1648The output record separator for the print operator. If defined, this
1649value is printed after the last of print's arguments. Default is C<undef>.
84902520 1650
b0c22438 1651Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
1652Also, it's just like C<$/>, but it's what you get "back" from Perl.
84902520 1653
5b442a2a 1654=item HANDLE->autoflush( EXPR )
1655
1656=item $OUTPUT_AUTOFLUSH
1657
84dabc03 1658=item $|
1659X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
84902520 1660
84dabc03 1661If set to nonzero, forces a flush right away and after every write or
7333b1c4 1662print on the currently selected output channel. Default is 0
84dabc03 1663(regardless of whether the channel is really buffered by the system or
1664not; C<$|> tells you only whether you've asked Perl explicitly to
1665flush after each write). STDOUT will typically be line buffered if
5b442a2a 1666output is to the terminal and block buffered otherwise. Setting this
84dabc03 1667variable is useful primarily when you are outputting to a pipe or
1668socket, such as when you are running a Perl program under B<rsh> and
5b442a2a 1669want to see the output as it's happening. This has no effect on input
c003e62a 1670buffering. See L<perlfunc/getc> for that. See L<perlfunc/select> on
84dabc03 1671how to select the output channel. See also L<IO::Handle>.
1672
1673Mnemonic: when you want your pipes to be piping hot.
1674
1675=back
84902520 1676
b0c22438 1677=head3 Variables related to formats
83ee9e09 1678
b0c22438 1679The special variables for formats are a subset of those for
69b55ccc 1680filehandles. See L<perlform> for more information about Perl's
1681formats.
83ee9e09 1682
b0c22438 1683=over 8
83ee9e09 1684
84dabc03 1685=item $ACCUMULATOR
1686
1687=item $^A
1688X<$^A> X<$ACCUMULATOR>
1689
1690The current value of the C<write()> accumulator for C<format()> lines.
1691A format contains C<formline()> calls that put their result into
7333b1c4 1692C<$^A>. After calling its format, C<write()> prints out the contents
84dabc03 1693of C<$^A> and empties. So you never really see the contents of C<$^A>
1694unless you call C<formline()> yourself and then look at it. See
96090e4f 1695L<perlform> and L<perlfunc/"formline PICTURE,LIST">.
84dabc03 1696
5b442a2a 1697=item HANDLE->format_formfeed(EXPR)
1698
1699=item $FORMAT_FORMFEED
1700
84dabc03 1701=item $^L
1702X<$^L> X<$FORMAT_FORMFEED>
1703
1704What formats output as a form feed. The default is C<\f>.
1705
b0c22438 1706=item HANDLE->format_page_number(EXPR)
83ee9e09 1707
b0c22438 1708=item $FORMAT_PAGE_NUMBER
83ee9e09 1709
b0c22438 1710=item $%
1711X<$%> X<$FORMAT_PAGE_NUMBER>
83ee9e09 1712
b0c22438 1713The current page number of the currently selected output channel.
83ee9e09 1714
b0c22438 1715Mnemonic: C<%> is page number in B<nroff>.
7619c85e 1716
b0c22438 1717=item HANDLE->format_lines_left(EXPR)
b9ac3b5b 1718
b0c22438 1719=item $FORMAT_LINES_LEFT
66558a10 1720
b0c22438 1721=item $-
1722X<$-> X<$FORMAT_LINES_LEFT>
fb73857a 1723
b0c22438 1724The number of lines left on the page of the currently selected output
1725channel.
fa05a9fd 1726
b0c22438 1727Mnemonic: lines_on_page - lines_printed.
fa05a9fd 1728
84dabc03 1729=item Handle->format_line_break_characters EXPR
fb73857a 1730
84dabc03 1731=item $FORMAT_LINE_BREAK_CHARACTERS
a0d0e21e 1732
84dabc03 1733=item $:
1734X<$:> X<FORMAT_LINE_BREAK_CHARACTERS>
a0d0e21e 1735
84dabc03 1736The current set of characters after which a string may be broken to
1737fill continuation fields (starting with C<^>) in a format. The default is
1738S<" \n-">, to break on a space, newline, or a hyphen.
a0d0e21e 1739
84dabc03 1740Mnemonic: a "colon" in poetry is a part of a line.
1741
1742=item HANDLE->format_lines_per_page(EXPR)
1743
1744=item $FORMAT_LINES_PER_PAGE
1745
1746=item $=
1747X<$=> X<$FORMAT_LINES_PER_PAGE>
1748
1749The current page length (printable lines) of the currently selected
1750output channel. The default is 60.
1751
1752Mnemonic: = has horizontal lines.
7c36658b 1753
b0c22438 1754=item HANDLE->format_top_name(EXPR)
7c36658b 1755
b0c22438 1756=item $FORMAT_TOP_NAME
a05d7ebb 1757
b0c22438 1758=item $^
1759X<$^> X<$FORMAT_TOP_NAME>
fde18df1 1760
b0c22438 1761The name of the current top-of-page format for the currently selected
1762output channel. The default is the name of the filehandle with C<_TOP>
1763appended. For example, the default format top name for the C<STDOUT>
1764filehanlde is C<STDOUT_TOP>.
e07ea26a 1765
b0c22438 1766Mnemonic: points to top of page.
e07ea26a 1767
84dabc03 1768=item HANDLE->format_name(EXPR)
16070b82 1769
84dabc03 1770=item $FORMAT_NAME
aa2f2a36 1771
84dabc03 1772=item $~
1773X<$~> X<$FORMAT_NAME>
aa2f2a36 1774
84dabc03 1775The name of the current report format for the currently selected
1776output channel. The default format name is the same as the filehandle
1777name. For example, the default format name for the C<STDOUT>
1778filehandle is just C<STDOUT>.
16070b82 1779
84dabc03 1780Mnemonic: brother to C<$^>.
16070b82 1781
b0c22438 1782=back
a0d0e21e 1783
84dabc03 1784=head2 Error Variables
b0c22438 1785X<error> X<exception>
a0d0e21e 1786
b0c22438 1787The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1788about different types of error conditions that may appear during
1789execution of a Perl program. The variables are shown ordered by
1790the "distance" between the subsystem which reported the error and
1791the Perl process. They correspond to errors detected by the Perl
1792interpreter, C library, operating system, or an external program,
1793respectively.
4438c4b7 1794
b0c22438 1795To illustrate the differences between these variables, consider the
7fd683ff 1796following Perl expression, which uses a single-quoted string. After
1797execution of this statement, perl may have set all four special error
7333b1c4 1798variables:
4438c4b7 1799
ca1b95ae 1800 eval q{
7333b1c4 1801 open my $pipe, "/cdrom/install |" or die $!;
1802 my @res = <$pipe>;
1803 close $pipe or die "bad pipe: $?, $!";
1804 };
a0d0e21e 1805
7333b1c4 1806When perl executes the C<eval()> expression, it translates the
1807C<open()>, C<< <PIPE> >>, and C<close> calls in the C run-time library
69b55ccc 1808and thence to the operating system kernel. perl sets C<$!> to
7333b1c4 1809the C library's C<errno> if one of these calls fails.
2a8c8378 1810
84dabc03 1811C<$@> is set if the string to be C<eval>-ed did not compile (this may
1812happen if C<open> or C<close> were imported with bad prototypes), or
7333b1c4 1813if Perl code executed during evaluation C<die()>d. In these cases the
0b9346e6 1814value of C<$@> is the compile error, or the argument to C<die> (which
84dabc03 1815will interpolate C<$!> and C<$?>). (See also L<Fatal>, though.)
2a8c8378 1816
84dabc03 1817Under a few operating systems, C<$^E> may contain a more verbose error
1818indicator, such as in this case, "CDROM tray not closed." Systems that
1819do not support extended error messages leave C<$^E> the same as C<$!>.
a0d0e21e 1820
b0c22438 1821Finally, C<$?> may be set to non-0 value if the external program
84dabc03 1822F</cdrom/install> fails. The upper eight bits reflect specific error
1823conditions encountered by the program (the program's C<exit()> value).
1824The lower eight bits reflect mode of failure, like signal death and
57f6eff5 1825core dump information. See L<wait(2)> for details. In contrast to
84dabc03 1826C<$!> and C<$^E>, which are set only if error condition is detected,
1827the variable C<$?> is set on each C<wait> or pipe C<close>,
1828overwriting the old value. This is more like C<$@>, which on every
1829C<eval()> is always set on failure and cleared on success.
a0d0e21e 1830
b0c22438 1831For more details, see the individual descriptions at C<$@>, C<$!>,
1832C<$^E>, and C<$?>.
38e4f4ae 1833
0b9346e6 1834=over 8
1835
b0c22438 1836=item ${^CHILD_ERROR_NATIVE}
1837X<$^CHILD_ERROR_NATIVE>
a0d0e21e 1838
b0c22438 1839The native status returned by the last pipe close, backtick (C<``>)
1840command, successful call to C<wait()> or C<waitpid()>, or from the
1841C<system()> operator. On POSIX-like systems this value can be decoded
1842with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED,
1843WSTOPSIG and WIFCONTINUED functions provided by the L<POSIX> module.
a0d0e21e 1844
b0c22438 1845Under VMS this reflects the actual VMS exit status; i.e. it is the
1846same as C<$?> when the pragma C<use vmsish 'status'> is in effect.
a0d0e21e 1847
b0c22438 1848This variable was added in Perl 5.8.9.
a0d0e21e 1849
5b442a2a 1850=item $EXTENDED_OS_ERROR
1851
84dabc03 1852=item $^E
1853X<$^E> X<$EXTENDED_OS_ERROR>
1854
1855Error information specific to the current operating system. At the
1856moment, this differs from C<$!> under only VMS, OS/2, and Win32 (and
1857for MacPerl). On all other platforms, C<$^E> is always just the same
1858as C<$!>.
1859
1860Under VMS, C<$^E> provides the VMS status value from the last system
1861error. This is more specific information about the last system error
1862than that provided by C<$!>. This is particularly important when C<$!>
1863is set to B<EVMSERR>.
1864
1865Under OS/2, C<$^E> is set to the error code of the last call to OS/2
1866API either via CRT, or directly from perl.
1867
1868Under Win32, C<$^E> always returns the last error information reported
1869by the Win32 call C<GetLastError()> which describes the last error
1870from within the Win32 API. Most Win32-specific code will report errors
1871via C<$^E>. ANSI C and Unix-like calls set C<errno> and so most
1872portable Perl code will report errors via C<$!>.
1873
1874Caveats mentioned in the description of C<$!> generally apply to
1875C<$^E>, also.
1876
1877This variable was added in Perl 5.003.
1878
1879Mnemonic: Extra error explanation.
0b9346e6 1880
84dabc03 1881=item $EXCEPTIONS_BEING_CAUGHT
1882
1883=item $^S
1884X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>
1885
1886Current state of the interpreter.
1887
ca1b95ae 1888 $^S State
1889 --------- -------------------
1890 undef Parsing module/eval
1891 true (1) Executing an eval
1892 false (0) Otherwise
84dabc03 1893
1894The first state may happen in C<$SIG{__DIE__}> and C<$SIG{__WARN__}>
1895handlers.
1896
1897This variable was added in Perl 5.004.
1898
1899=item $WARNING
1900
1901=item $^W
1902X<$^W> X<$WARNING>
1903
1904The current value of the warning switch, initially true if B<-w> was
1905used, false otherwise, but directly modifiable.
1906
1907See also L<warnings>.
1908
0b9346e6 1909Mnemonic: related to the B<-w> switch.
84dabc03 1910
1911=item ${^WARNING_BITS}
ca1b95ae 1912X<${^WARNING_BITS}>
84dabc03 1913
1914The current set of warning checks enabled by the C<use warnings> pragma.
1915See the documentation of C<warnings> for more details.
1916
1917This variable was added in Perl 5.10.
1918
b0c22438 1919=item $OS_ERROR
5ccee41e 1920
b0c22438 1921=item $ERRNO
5ccee41e 1922
b0c22438 1923=item $!
1924X<$!> X<$ERRNO> X<$OS_ERROR>
9b0e6e7a 1925
a73bef78
JL
1926When referenced, C<$!> retrieves the current value
1927of the C C<errno> integer variable.
1928If C<$!> is assigned a numerical value, that value is stored in C<errno>.
1929When referenced as a string, C<$!> yields the system error string
1930corresponding to C<errno>.
1931
1932Many system or library calls set C<errno> if they fail,
1933to indicate the cause of failure. They usually do B<not>
1934set C<errno> to zero if they succeed. This means C<errno>,
1935hence C<$!>, is meaningful only I<immediately> after a B<failure>:
1936
1937 if (open my $fh, "<", $filename) {
ca1b95ae 1938 # Here $! is meaningless.
1939 ...
7fd683ff 1940 }
ca1b95ae 1941 else {
1942 # ONLY here is $! meaningful.
1943 ...
1944 # Already here $! might be meaningless.
b0c22438 1945 }
1946 # Since here we might have either success or failure,
a73bef78 1947 # $! is meaningless.
a0d0e21e 1948
a73bef78
JL
1949Here, I<meaningless> means that C<$!> may be unrelated to the outcome
1950of the C<open()> operator. Assignment to C<$!> is similarly ephemeral.
1951It can be used immediately before invoking the C<die()> operator,
1952to set the exit value, or to inspect the system error string
1953corresponding to error I<n>, or to restore C<$!> to a meaningful state.
d54b56d5 1954
b0c22438 1955Mnemonic: What just went bang?
314d39ce 1956
b0c22438 1957=item %OS_ERROR
fb73857a 1958
b0c22438 1959=item %ERRNO
fb73857a 1960
b0c22438 1961=item %!
5b442a2a 1962X<%!> X<%OS_ERROR> X<%ERRNO>
a0d0e21e 1963
b0c22438 1964Each element of C<%!> has a true value only if C<$!> is set to that
1965value. For example, C<$!{ENOENT}> is true if and only if the current
84dabc03 1966value of C<$!> is C<ENOENT>; that is, if the most recent error was "No
1967such file or directory" (or its moral equivalent: not all operating
1968systems give that exact error, and certainly not all languages). To
1969check if a particular key is meaningful on your system, use C<exists
1970$!{the_key}>; for a list of legal keys, use C<keys %!>. See L<Errno>
7333b1c4 1971for more information, and also see L</$!>.
a0d0e21e 1972
b0c22438 1973This variable was added in Perl 5.005.
44f0be63 1974
84dabc03 1975=item $CHILD_ERROR
b687b08b 1976
84dabc03 1977=item $?
1978X<$?> X<$CHILD_ERROR>
a0d0e21e 1979
84dabc03 1980The status returned by the last pipe close, backtick (C<``>) command,
1981successful call to C<wait()> or C<waitpid()>, or from the C<system()>
1982operator. This is just the 16-bit status word returned by the
1983traditional Unix C<wait()> system call (or else is made up to look
1984like it). Thus, the exit value of the subprocess is really (C<<< $? >>
19858 >>>), and C<$? & 127> gives which signal, if any, the process died
1986from, and C<$? & 128> reports whether there was a core dump.
a0d0e21e 1987
84dabc03 1988Additionally, if the C<h_errno> variable is supported in C, its value
1989is returned via C<$?> if any C<gethost*()> function fails.
b687b08b 1990
84dabc03 1991If you have installed a signal handler for C<SIGCHLD>, the
1992value of C<$?> will usually be wrong outside that handler.
a0d0e21e 1993
84dabc03 1994Inside an C<END> subroutine C<$?> contains the value that is going to be
1995given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
1996change the exit status of your program. For example:
a0d0e21e 1997
84dabc03 1998 END {
1999 $? = 1 if $? == 255; # die would make it 255
2000 }
a0d0e21e 2001
84dabc03 2002Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
2003actual VMS exit status, instead of the default emulation of POSIX
2004status; see L<perlvms/$?> for details.
2005
2006Mnemonic: similar to B<sh> and B<ksh>.
a0d0e21e 2007
b0c22438 2008=item $EVAL_ERROR
f648820c 2009
b0c22438 2010=item $@
2011X<$@> X<$EVAL_ERROR>
a0d0e21e 2012
0b9346e6 2013The Perl syntax error message from the last C<eval()> operator. If C<$@> is
2014the null string, the last C<eval()> parsed and executed correctly
b0c22438 2015(although the operations you invoked may have failed in the normal
2016fashion).
a0d0e21e 2017
b0c22438 2018Warning messages are not collected in this variable. You can, however,
2019set up a routine to process warnings by setting C<$SIG{__WARN__}> as
7333b1c4 2020described in L</%SIG>.
748a9306 2021
b0c22438 2022Mnemonic: Where was the syntax error "at"?
7f315d2e 2023
b0c22438 2024=back
7f315d2e 2025
b0c22438 2026=head2 Deprecated and removed variables
7f315d2e 2027
0b9346e6 2028Deprecating a variable announces the intent of the perl maintainers to
1c2e8cca 2029eventually remove the variable from the language. It may still be
b0c22438 2030available despite its status. Using a deprecated variable triggers
2031a warning.
7f315d2e 2032
84dabc03 2033Once a variable is removed, its use triggers an error telling you
b0c22438 2034the variable is unsupported.
7f315d2e 2035
84dabc03 2036See L<perldiag> for details about error messages.
7f315d2e 2037
b0c22438 2038=over 8
7f315d2e 2039
5b442a2a 2040=item $OFMT
2041
84dabc03 2042=item $#
5b442a2a 2043X<$#> X<$OFMT>
84dabc03 2044
38e5787b 2045C<$#> was a variable that could be used to format printed numbers.
84dabc03 2046After a deprecation cycle, its magic was removed in Perl 5.10 and
2047using it now triggers a warning: C<$# is no longer supported>.
2048
2049This is not the sigil you use in front of an array name to get the
2050last index, like C<$#array>. That's still how you get the last index
2051of an array in Perl. The two have nothing to do with each other.
2052
2053Deprecated in Perl 5.
2054
2055Removed in Perl 5.10.
2056
7f315d2e
CO
2057=item $*
2058X<$*>
2059
84dabc03 2060C<$*> was a variable that you could use to enable multiline matching.
7f315d2e
CO
2061After a deprecation cycle, its magic was removed in Perl 5.10.
2062Using it now triggers a warning: C<$* is no longer supported>.
84dabc03 2063You should use the C</s> and C</m> regexp modifiers instead.
7f315d2e 2064
b0c22438 2065Deprecated in Perl 5.
7f315d2e 2066
b0c22438 2067Removed in Perl 5.10.
7f315d2e 2068
5b442a2a 2069=item $ARRAY_BASE
2070
84dabc03 2071=item $[
5b442a2a 2072X<$[> X<$ARRAY_BASE>
84dabc03 2073
2074This variable stores the index of the first element in an array, and
ac0650a4
FC
2075of the first character in a substring. The default is 0, but you could
2076theoretically set it to 1 to make Perl behave more like B<awk> (or Fortran)
2077when subscripting and when evaluating the index() and substr() functions.
84dabc03 2078
ac0650a4
FC
2079As of release 5 of Perl, assignment to C<$[> is treated as a compiler
2080directive, and cannot influence the behavior of any other file.
2081(That's why you can only assign compile-time constants to it.)
2082Its use is highly discouraged.
84dabc03 2083
ac0650a4
FC
2084Prior to Perl 5.10, assignment to C<$[> could be seen from outer lexical
2085scopes in the same file, unlike other compile-time directives (such as
2086L<strict>). Using local() on it would bind its value strictly to a lexical
2087block. Now it is always lexically scoped.
2088
2089Mnemonic: [ begins subscripts.
84dabc03 2090
0b9346e6 2091Deprecated in Perl 5.12.
84dabc03 2092
5b442a2a 2093=item $OLD_PERL_VERSION
2094
b0c22438 2095=item $]
5b442a2a 2096X<$]> X<$OLD_PERL_VERSION>
55602bd2 2097
57f6eff5 2098See L</$^V> for a more modern representation of the Perl version that allows
d4ba9bf2 2099accurate string comparisons.
2100
b0c22438 2101The version + patchlevel / 1000 of the Perl interpreter. This variable
2102can be used to determine whether the Perl interpreter executing a
2103script is in the right range of versions:
55602bd2 2104
b0c22438 2105 warn "No checksumming!\n" if $] < 3.019;
55602bd2 2106
d4ba9bf2 2107The floating point representation can sometimes lead to inaccurate
2108numeric comparisons.
2109
b0c22438 2110See also the documentation of C<use VERSION> and C<require VERSION>
2111for a convenient way to fail if the running Perl interpreter is too old.
55602bd2 2112
b0c22438 2113Mnemonic: Is this version of perl in the right bracket?
19799a22 2114
b0c22438 2115Deprecated in Perl 5.6.
19799a22 2116
b0c22438 2117=back
2b92dfce 2118
0b9346e6 2119=cut