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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<${^_XYZ}> variables are always taken
b0c22438 51to be in package C<main>, regardless of any C<package> declarations
52presently in scope.
53
54=head1 SPECIAL VARIABLES
a0d0e21e 55
0b9346e6 56The following names have special meaning to Perl. Most punctuation
57names have reasonable mnemonics, or analogs in the shells.
58Nevertheless, if you wish to use long variable names, you need only say:
a0d0e21e 59
0b9346e6 60 use English;
a0d0e21e 61
a1ce9542
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62at the top of your program. This aliases all the short names to the long
63names in the current package. Some even have medium names, generally
84dabc03 64borrowed from B<awk>. To avoid a performance hit, if you don't need the
65C<$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
b0169937
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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.
cb1a09d0
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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
b0c22438 188The process number of the Perl running this script. You should
189consider this variable read-only, although it will be altered
190across C<fork()> calls.
a0d0e21e 191
b0c22438 192Note for Linux users: on Linux, the C functions C<getpid()> and
193C<getppid()> return different values from different threads. In order to
194be portable, this behavior is not reflected by C<$$>, whose value remains
195consistent across threads. If you want to call the underlying C<getpid()>,
196you may use the CPAN module C<Linux::Pid>.
a0d0e21e 197
b0c22438 198Mnemonic: same as shells.
ad83b128 199
b0c22438 200=item $REAL_GROUP_ID
a01268b5 201
b0c22438 202=item $GID
a01268b5 203
b0c22438 204=item $(
205X<$(> X<$GID> X<$REAL_GROUP_ID>
a01268b5 206
b0c22438 207The real gid of this process. If you are on a machine that supports
208membership in multiple groups simultaneously, gives a space separated
209list of groups you are in. The first number is the one returned by
210C<getgid()>, and the subsequent ones by C<getgroups()>, one of which may be
211the same as the first number.
a01268b5 212
b0c22438 213However, a value assigned to C<$(> must be a single number used to
214set the real gid. So the value given by C<$(> should I<not> be assigned
215back to C<$(> without being forced numeric, such as by adding zero. Note
216that this is different to the effective gid (C<$)>) which does take a
217list.
fe307981 218
b0c22438 219You can change both the real gid and the effective gid at the same
220time by using C<POSIX::setgid()>. Changes to C<$(> require a check to C<$!>
221to detect any possible errors after an attempted change.
6cef1e77 222
b0c22438 223Mnemonic: parentheses are used to I<group> things. The real gid is the
224group you I<left>, if you're running setgid.
6cef1e77 225
b0c22438 226=item $EFFECTIVE_GROUP_ID
8e08999f 227
b0c22438 228=item $EGID
81714fb9 229
b0c22438 230=item $)
231X<$)> X<$EGID> X<$EFFECTIVE_GROUP_ID>
81714fb9 232
b0c22438 233The effective gid of this process. If you are on a machine that
234supports membership in multiple groups simultaneously, gives a space
235separated list of groups you are in. The first number is the one
236returned by C<getegid()>, and the subsequent ones by C<getgroups()>,
237one of which may be the same as the first number.
81714fb9 238
b0c22438 239Similarly, a value assigned to C<$)> must also be a space-separated
240list of numbers. The first number sets the effective gid, and
241the rest (if any) are passed to C<setgroups()>. To get the effect of an
242empty list for C<setgroups()>, just repeat the new effective gid; that is,
243to force an effective gid of 5 and an effectively empty C<setgroups()>
244list, say C< $) = "5 5" >.
81714fb9 245
b0c22438 246You can change both the effective gid and the real gid at the same
247time by using C<POSIX::setgid()> (use only a single numeric argument).
248Changes to C<$)> require a check to C<$!> to detect any possible errors
249after an attempted change.
44a2ac75 250
b0c22438 251C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
252machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
253and C<$)> can be swapped only on machines supporting C<setregid()>.
3195cf34 254
b0c22438 255Mnemonic: parentheses are used to I<group> things. The effective gid
256is the group that's I<right> for you, if you're running setgid.
44a2ac75 257
b0c22438 258=item $PROGRAM_NAME
a0d0e21e 259
b0c22438 260=item $0
261X<$0> X<$PROGRAM_NAME>
a0d0e21e 262
b0c22438 263Contains the name of the program being executed.
a0d0e21e 264
69520822 265On some (but not all) operating systems assigning to C<$0> modifies
b0c22438 266the argument area that the C<ps> program sees. On some platforms you
267may have to use special C<ps> options or a different C<ps> to see the
268changes. Modifying the C<$0> is more useful as a way of indicating the
269current program state than it is for hiding the program you're
270running.
a0d0e21e 271
69520822 272Note that there are platform-specific limitations on the maximum
b0c22438 273length of C<$0>. In the most extreme case it may be limited to the
274space occupied by the original C<$0>.
fcc7d916 275
b0c22438 276In some platforms there may be arbitrary amount of padding, for
277example space characters, after the modified name as shown by C<ps>.
278In some platforms this padding may extend all the way to the original
279length of the argument area, no matter what you do (this is the case
280for example with Linux 2.2).
fcc7d916 281
b0c22438 282Note for BSD users: setting C<$0> does not completely remove "perl"
283from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
284result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
285and the " (perl)" suffix are shown depends on your exact BSD variant
286and version). This is an operating system feature, Perl cannot help it.
fcc7d916 287
b0c22438 288In multithreaded scripts Perl coordinates the threads so that any
289thread may modify its copy of the C<$0> and the change becomes visible
290to ps(1) (assuming the operating system plays along). Note that
291the view of C<$0> the other threads have will not change since they
292have their own copies of it.
fcc7d916 293
b0c22438 294If the program has been given to perl via the switches C<-e> or C<-E>,
295C<$0> will contain the string C<"-e">.
fcc7d916 296
b0c22438 297On Linux as of perl 5.14 the legacy process name will be set with
0b9346e6 298C<prctl(2)>, in addition to altering the POSIX name via C<argv[0]> as
b0c22438 299perl has done since version 4.000. Now system utilities that read the
300legacy process name such as ps, top and killall will recognize the
301name you set when assigning to C<$0>. The string you supply will be
302cut off at 16 bytes, this is a limitation imposed by Linux.
fcc7d916 303
b0c22438 304Mnemonic: same as B<sh> and B<ksh>.
0b9346e6 305
306=item $SUBSCRIPT_SEPARATOR
307
308=item $SUBSEP
309
310=item $;
311X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR>
312
313The subscript separator for multidimensional array emulation. If you
314refer to a hash element as
315
316 $foo{$a,$b,$c}
317
318it really means
319
320 $foo{join($;, $a, $b, $c)}
321
322But don't put
323
324 @foo{$a,$b,$c} # a slice--note the @
325
326which means
327
328 ($foo{$a},$foo{$b},$foo{$c})
329
330Default is "\034", the same as SUBSEP in B<awk>. If your keys contain
331binary data there might not be any safe value for C<$;>.
332
333Consider using "real" multidimensional arrays as described
334in L<perllol>.
335
336Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
337
338=item $REAL_USER_ID
339
340=item $UID
341
342=item $<
343X<< $< >> X<$UID> X<$REAL_USER_ID>
344
345The real uid of this process. You can change both the real uid and the
346effective uid at the same time by using C<POSIX::setuid()>. Since
347changes to C<< $< >> require a system call, check C<$!> after a change
348attempt to detect any possible errors.
349
350Mnemonic: it's the uid you came I<from>, if you're running setuid.
351
352=item $EFFECTIVE_USER_ID
353
354=item $EUID
355
356=item $>
357X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID>
358
359The effective uid of this process. For example:
360
361 $< = $>; # set real to effective uid
362 ($<,$>) = ($>,$<); # swap real and effective uids
363
364You can change both the effective uid and the real uid at the same
365time by using C<POSIX::setuid()>. Changes to C<< $> >> require a check
366to C<$!> to detect any possible errors after an attempted change.
367
368C<< $< >> and C<< $> >> can be swapped only on machines
369supporting C<setreuid()>.
370
371Mnemonic: it's the uid you went I<to>, if you're running setuid.
372
373=item $a
374
375=item $b
376X<$a> X<$b>
377
378Special package variables when using C<sort()>, see L<perlfunc/sort>.
379Because of this specialness C<$a> and C<$b> don't need to be declared
380(using C<use vars>, or C<our()>) even when using the C<strict 'vars'>
381pragma. Don't lexicalize them with C<my $a> or C<my $b> if you want to
382be able to use them in the C<sort()> comparison block or function.
383
b0c22438 384=item $COMPILING
a0d0e21e 385
b0c22438 386=item $^C
387X<$^C> X<$COMPILING>
a0d0e21e 388
b0c22438 389The current value of the flag associated with the B<-c> switch.
390Mainly of use with B<-MO=...> to allow code to alter its behavior
391when being compiled, such as for example to C<AUTOLOAD> at compile
392time rather than normal, deferred loading. Setting
393C<$^C = 1> is similar to calling C<B::minus_c>.
a0d0e21e 394
b0c22438 395This variable was added in Perl 5.6.
a0d0e21e 396
b0c22438 397=item $DEBUGGING
a0d0e21e 398
b0c22438 399=item $^D
400X<$^D> X<$DEBUGGING>
a0d0e21e 401
b0c22438 402The current value of the debugging flags. May be read or set. Like its
403command-line equivalent, you can use numeric or symbolic values, eg
404C<$^D = 10> or C<$^D = "st">.
68dc0745 405
b0c22438 406Mnemonic: value of B<-D> switch.
5b2b9c68 407
0b9346e6 408=item ${^ENCODING}
5b442a2a 409X<${^ENCODING}>
0b9346e6 410
411The I<object reference> to the C<Encode> object that is used to convert
412the source code to Unicode. Thanks to this variable your Perl script
413does not have to be written in UTF-8. Default is I<undef>. The direct
414manipulation of this variable is highly discouraged.
415
416This variable was added in Perl 5.8.2.
417
418=item %ENV
419X<%ENV>
420
421The hash C<%ENV> contains your current environment. Setting a
422value in C<ENV> changes the environment for any child processes
423you subsequently C<fork()> off.
424
b0c22438 425=item $SYSTEM_FD_MAX
5b2b9c68 426
b0c22438 427=item $^F
428X<$^F> X<$SYSTEM_FD_MAX>
5b2b9c68 429
b0c22438 430The maximum system file descriptor, ordinarily 2. System file
431descriptors are passed to C<exec()>ed processes, while higher file
432descriptors are not. Also, during an C<open()>, system file descriptors are
433preserved even if the C<open()> fails (ordinary file descriptors are
434closed before the C<open()> is attempted). The close-on-exec
435status of a file descriptor will be decided according to the value of
436C<$^F> when the corresponding file, pipe, or socket was opened, not the
437time of the C<exec()>.
5b2b9c68 438
0b9346e6 439=item @F
440X<@F>
441
442The array C<@F> contains the fields of each line read in when autosplit
443mode is turned on. See L<perlrun> for the B<-a> switch. This array
444is package-specific, and must be declared or given a full package name
445if not in package main when running under C<strict 'vars'>.
446
b0c22438 447=item $^H
5b442a2a 448X<$^H>
883faa13 449
b0c22438 450WARNING: This variable is strictly for internal use only. Its availability,
451behavior, and contents are subject to change without notice.
a0d0e21e 452
b0c22438 453This variable contains compile-time hints for the Perl interpreter. At the
454end of compilation of a BLOCK the value of this variable is restored to the
455value when the interpreter started to compile the BLOCK.
a0d0e21e 456
b0c22438 457When perl begins to parse any block construct that provides a lexical scope
458(e.g., eval body, required file, subroutine body, loop body, or conditional
459block), the existing value of C<$^H> is saved, but its value is left unchanged.
460When the compilation of the block is completed, it regains the saved value.
461Between the points where its value is saved and restored, code that
462executes within BEGIN blocks is free to change the value of C<$^H>.
a0d0e21e 463
b0c22438 464This behavior provides the semantic of lexical scoping, and is used in,
465for instance, the C<use strict> pragma.
a0d0e21e 466
b0c22438 467The contents should be an integer; different bits of it are used for
468different pragmatic flags. Here's an example:
a0d0e21e 469
0b9346e6 470 sub add_100 { $^H |= 0x100 }
a0d0e21e 471
0b9346e6 472 sub foo {
473 BEGIN { add_100() }
474 bar->baz($boon);
475 }
a0d0e21e 476
b0c22438 477Consider what happens during execution of the BEGIN block. At this point
478the BEGIN block has already been compiled, but the body of C<foo()> is still
479being compiled. The new value of C<$^H> will therefore be visible only while
480the body of C<foo()> is being compiled.
a0d0e21e 481
b0c22438 482Substitution of the above BEGIN block with:
a0d0e21e 483
0b9346e6 484 BEGIN { require strict; strict->import('vars') }
a0d0e21e 485
b0c22438 486demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
487version of the same lexical pragma:
a0d0e21e 488
0b9346e6 489 BEGIN { require strict; strict->import('vars') if $condition }
a0d0e21e 490
b0c22438 491This variable was added in Perl 5.003.
a0d0e21e 492
b0c22438 493=item %^H
5b442a2a 494X<%^H>
a0d0e21e 495
b0c22438 496The C<%^H> hash provides the same scoping semantic as C<$^H>. This makes it
497useful for implementation of lexically scoped pragmas. See L<perlpragma>.
a0d0e21e 498
b0c22438 499This variable was added in Perl 5.6.
a0d0e21e 500
0b9346e6 501=item @INC
502X<@INC>
503
504The array C<@INC> contains the list of places that the C<do EXPR>,
505C<require>, or C<use> constructs look for their library files. It
506initially consists of the arguments to any B<-I> command-line
507switches, followed by the default Perl library, probably
508F</usr/local/lib/perl>, followed by ".", to represent the current
509directory. ("." will not be appended if taint checks are enabled,
510either by C<-T> or by C<-t>.) If you need to modify this at runtime,
511you should use the C<use lib> pragma to get the machine-dependent
512library properly loaded also:
513
514 use lib '/mypath/libdir/';
515 use SomeMod;
516
517You can also insert hooks into the file inclusion system by putting Perl
518code directly into C<@INC>. Those hooks may be subroutine references, array
519references or blessed objects. See L<perlfunc/require> for details.
520
521=item %INC
522X<%INC>
523
524The hash C<%INC> contains entries for each filename included via the
525C<do>, C<require>, or C<use> operators. The key is the filename
526you specified (with module names converted to pathnames), and the
527value is the location of the file found. The C<require>
528operator uses this hash to determine whether a particular file has
529already been included.
530
531If the file was loaded via a hook (e.g. a subroutine reference, see
532L<perlfunc/require> for a description of these hooks), this hook is
533by default inserted into C<%INC> in place of a filename. Note, however,
534that the hook may have set the C<%INC> entry by itself to provide some more
535specific info.
536
b0c22438 537=item $INPLACE_EDIT
a0d0e21e 538
b0c22438 539=item $^I
540X<$^I> X<$INPLACE_EDIT>
a0d0e21e 541
b0c22438 542The current value of the inplace-edit extension. Use C<undef> to disable
543inplace editing.
a0d0e21e 544
b0c22438 545Mnemonic: value of B<-i> switch.
a0d0e21e 546
b0c22438 547=item $^M
548X<$^M>
a0d0e21e 549
b0c22438 550By default, running out of memory is an untrappable, fatal error.
551However, if suitably built, Perl can use the contents of C<$^M>
552as an emergency memory pool after C<die()>ing. Suppose that your Perl
553were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
554Then
a0d0e21e 555
0b9346e6 556 $^M = 'a' x (1 << 16);
a0d0e21e 557
b0c22438 558would allocate a 64K buffer for use in an emergency. See the
559F<INSTALL> file in the Perl distribution for information on how to
560add custom C compilation flags when compiling perl. To discourage casual
561use of this advanced feature, there is no L<English|English> long name for
562this variable.
a0d0e21e 563
b0c22438 564This variable was added in Perl 5.004.
a0d0e21e 565
b0c22438 566=item $OSNAME
a0d0e21e 567
b0c22438 568=item $^O
569X<$^O> X<$OSNAME>
a0d0e21e 570
b0c22438 571The name of the operating system under which this copy of Perl was
572built, as determined during the configuration process. For examples
573see L<perlport/PLATFORMS>.
a0d0e21e 574
b0c22438 575The value is identical to C<$Config{'osname'}>. See also L<Config>
576and the B<-V> command-line switch documented in L<perlrun>.
a0d0e21e 577
b0c22438 578In Windows platforms, C<$^O> is not very helpful: since it is always
579C<MSWin32>, it doesn't tell the difference between
58095/98/ME/NT/2000/XP/CE/.NET. Use C<Win32::GetOSName()> or
581Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
582between the variants.
a0d0e21e 583
b0c22438 584This variable was added in Perl 5.003.
a0d0e21e 585
b0c22438 586=item ${^OPEN}
5b442a2a 587X<${^OPEN}>
a0d0e21e 588
b0c22438 589An internal variable used by PerlIO. A string in two parts, separated
590by a C<\0> byte, the first part describes the input layers, the second
591part describes the output layers.
a0d0e21e 592
b0c22438 593This variable was added in Perl 5.8.2.
a0d0e21e 594
b0c22438 595=item $PERLDB
a0d0e21e 596
b0c22438 597=item $^P
598X<$^P> X<$PERLDB>
a0d0e21e 599
b0c22438 600The internal variable for debugging support. The meanings of the
601various bits are subject to change, but currently indicate:
a0d0e21e 602
b0c22438 603=over 6
a0d0e21e 604
b0c22438 605=item 0x01
a0d0e21e 606
b0c22438 607Debug subroutine enter/exit.
a0d0e21e 608
b0c22438 609=item 0x02
a0d0e21e 610
b0c22438 611Line-by-line debugging. Causes C<DB::DB()> subroutine to be called for each
612statement executed. Also causes saving source code lines (like 0x400).
a0d0e21e 613
b0c22438 614=item 0x04
fe307981 615
b0c22438 616Switch off optimizations.
6cef1e77 617
b0c22438 618=item 0x08
6cef1e77 619
b0c22438 620Preserve more data for future interactive inspections.
6cef1e77 621
b0c22438 622=item 0x10
4ba05bdc 623
b0c22438 624Keep info about source lines on which a subroutine is defined.
4ba05bdc 625
b0c22438 626=item 0x20
4ba05bdc 627
b0c22438 628Start with single-step on.
4ba05bdc 629
b0c22438 630=item 0x40
4ba05bdc 631
b0c22438 632Use subroutine address instead of name when reporting.
4ba05bdc 633
b0c22438 634=item 0x80
4ba05bdc 635
b0c22438 636Report C<goto &subroutine> as well.
4ba05bdc 637
b0c22438 638=item 0x100
4ba05bdc 639
b0c22438 640Provide informative "file" names for evals based on the place they were compiled.
4ba05bdc 641
b0c22438 642=item 0x200
44a2ac75 643
b0c22438 644Provide informative names to anonymous subroutines based on the place they
645were compiled.
44a2ac75 646
b0c22438 647=item 0x400
44a2ac75 648
b0c22438 649Save source code lines into C<@{"_<$filename"}>.
44a2ac75 650
b0c22438 651=back
44a2ac75 652
b0c22438 653Some bits may be relevant at compile-time only, some at
654run-time only. This is a new mechanism and the details may change.
655See also L<perldebguts>.
3195cf34 656
b0c22438 657=item %SIG
b0c22438 658X<%SIG>
a0d0e21e 659
b0c22438 660The hash C<%SIG> contains signal handlers for signals. For example:
a0d0e21e 661
0b9346e6 662 sub handler { # 1st argument is signal name
663 my($sig) = @_;
664 print "Caught a SIG$sig--shutting down\n";
665 close(LOG);
666 exit(0);
667 }
a0d0e21e 668
0b9346e6 669 $SIG{'INT'} = \&handler;
670 $SIG{'QUIT'} = \&handler;
671 ...
672 $SIG{'INT'} = 'DEFAULT'; # restore default action
673 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
a0d0e21e 674
b0c22438 675Using a value of C<'IGNORE'> usually has the effect of ignoring the
676signal, except for the C<CHLD> signal. See L<perlipc> for more about
677this special case.
a0d0e21e 678
b0c22438 679Here are some other examples:
a0d0e21e 680
0b9346e6 681 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
682 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
683 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
684 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
a0d0e21e 685
b0c22438 686Be sure not to use a bareword as the name of a signal handler,
687lest you inadvertently call it.
a0d0e21e 688
b0c22438 689If your system has the C<sigaction()> function then signal handlers
690are installed using it. This means you get reliable signal handling.
7b8d334a 691
b0c22438 692The default delivery policy of signals changed in Perl 5.8.0 from
693immediate (also known as "unsafe") to deferred, also known as "safe
694signals". See L<perlipc> for more information.
aa689395 695
b0c22438 696Certain internal hooks can be also set using the C<%SIG> hash. The
697routine indicated by C<$SIG{__WARN__}> is called when a warning
698message is about to be printed. The warning message is passed as the
699first argument. The presence of a C<__WARN__> hook causes the
700ordinary printing of warnings to C<STDERR> to be suppressed. You can
701use this to save warnings in a variable, or turn warnings into fatal
702errors, like this:
19799a22 703
0b9346e6 704 local $SIG{__WARN__} = sub { die $_[0] };
705 eval $proggie;
a8f8344d 706
b0c22438 707As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
708disable warnings using the empty subroutine:
f86702cc 709
0b9346e6 710 local $SIG{__WARN__} = sub {};
55602bd2 711
b0c22438 712The routine indicated by C<$SIG{__DIE__}> is called when a fatal
713exception is about to be thrown. The error message is passed as the
714first argument. When a C<__DIE__> hook routine returns, the exception
715processing continues as it would have in the absence of the hook,
716unless the hook routine itself exits via a C<goto>, a loop exit, or a
717C<die()>. The C<__DIE__> handler is explicitly disabled during the
718call, so that you can die from a C<__DIE__> handler. Similarly for
719C<__WARN__>.
e5218da5 720
b0c22438 721Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
722even inside an C<eval()>. Do not use this to rewrite a pending
723exception in C<$@>, or as a bizarre substitute for overriding
724C<CORE::GLOBAL::die()>. This strange action at a distance may be fixed
725in a future release so that C<$SIG{__DIE__}> is only called if your
726program is about to exit, as was the original intent. Any other use is
727deprecated.
728
729C<__DIE__>/C<__WARN__> handlers are very special in one respect: they
730may be called to report (probable) errors found by the parser. In such
731a case the parser may be in inconsistent state, so any attempt to
732evaluate Perl code from such a handler will probably result in a
733segfault. This means that warnings or errors that result from parsing
734Perl should be used with extreme caution, like this:
e5218da5 735
0b9346e6 736 require Carp if defined $^S;
737 Carp::confess("Something wrong") if defined &Carp::confess;
738 die "Something wrong, but could not load Carp to give backtrace...
739 To see backtrace try starting Perl with -MCarp switch";
e5218da5 740
b0c22438 741Here the first line will load C<Carp> I<unless> it is the parser who
742called the handler. The second line will print backtrace and die if
743C<Carp> was available. The third line will be executed only if C<Carp> was
744not available.
0a378802 745
0b9346e6 746Having to even think about the C<$^S> variable in your exception
747handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
748invites grievous and difficult to track down errors. Avoid it
749and use an C<END{}> or CORE::GLOBAL::die override instead.
750
b0c22438 751See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
752L<warnings> for additional information.
0a378802 753
b0c22438 754=item $BASETIME
6ab308ee 755
b0c22438 756=item $^T
757X<$^T> X<$BASETIME>
6ab308ee 758
b0c22438 759The time at which the program began running, in seconds since the
760epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
761and B<-C> filetests are based on this value.
a0d0e21e 762
b0c22438 763=item ${^TAINT}
5b442a2a 764X<${^TAINT}>
55602bd2 765
b0c22438 766Reflects if taint mode is on or off. 1 for on (the program was run with
767B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
0b9346e6 768B<-t> or B<-TU>).
daaddde1 769
b0c22438 770This variable is read-only.
daaddde1 771
b0c22438 772This variable was added in Perl 5.8.
4c5cef9b 773
b0c22438 774=item ${^UNICODE}
5b442a2a 775X<${^UNICODE}>
4c5cef9b 776
b0c22438 777Reflects certain Unicode settings of Perl. See L<perlrun>
778documentation for the C<-C> switch for more information about
0b9346e6 779the possible values.
5c055ba3 780
b0c22438 781This variable is set during Perl startup and is thereafter read-only.
5c055ba3 782
b0c22438 783This variable was added in Perl 5.8.2.
22fae026 784
b0c22438 785=item ${^UTF8CACHE}
5b442a2a 786X<${^UTF8CACHE}>
22fae026 787
b0c22438 788This variable controls the state of the internal UTF-8 offset caching code.
7891 for on (the default), 0 for off, -1 to debug the caching code by checking
790all its results against linear scans, and panicking on any discrepancy.
22fae026 791
b0c22438 792This variable was added in Perl 5.8.9.
22fae026 793
b0c22438 794=item ${^UTF8LOCALE}
5b442a2a 795X<${^UTF8LOCALE}>
5c055ba3 796
b0c22438 797This variable indicates whether a UTF-8 locale was detected by perl at
798startup. This information is used by perl when it's in
799adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
800switch); see L<perlrun> for more info on this.
55602bd2 801
b0c22438 802This variable was added in Perl 5.8.8.
a0d0e21e 803
b0c22438 804=item $PERL_VERSION
a0d0e21e 805
b0c22438 806=item $^V
807X<$^V> X<$PERL_VERSION>
a0d0e21e 808
b0c22438 809The revision, version, and subversion of the Perl interpreter,
810represented as a C<version> object.
748a9306 811
b0c22438 812This variable first appeared in perl 5.6.0; earlier versions of perl
813will see an undefined value. Before perl 5.10.0 C<$^V> was represented
814as a v-string.
55602bd2 815
b0c22438 816C<$^V> can be used to determine whether the Perl interpreter executing
817a script is in the right range of versions. For example:
a0d0e21e 818
0b9346e6 819 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
a0d0e21e 820
b0c22438 821To convert C<$^V> into its string representation use C<sprintf()>'s
822C<"%vd"> conversion:
a0d0e21e 823
0b9346e6 824 printf "version is v%vd\n", $^V; # Perl's version
a0d0e21e 825
b0c22438 826See the documentation of C<use VERSION> and C<require VERSION>
827for a convenient way to fail if the running Perl interpreter is too old.
4d76a344 828
b0c22438 829See also C<$]> for an older representation of the Perl version.
a0d0e21e 830
b0c22438 831This variable was added in Perl 5.6.
a0d0e21e 832
b0c22438 833Mnemonic: use ^V for Version Control.
a0d0e21e 834
b0c22438 835=item ${^WIN32_SLOPPY_STAT}
5b442a2a 836X<${^WIN32_SLOPPY_STAT}> X<sitecustomize> X<sitecustomize.pl>
a0d0e21e 837
b0c22438 838If this variable is set to a true value, then C<stat()> on Windows will
839not try to open the file. This means that the link count cannot be
840determined and file attributes may be out of date if additional
841hardlinks to the file exist. On the other hand, not opening the file
842is considerably faster, especially for files on network drives.
a0d0e21e 843
b0c22438 844This variable could be set in the F<sitecustomize.pl> file to
845configure the local Perl installation to use "sloppy" C<stat()> by
846default. See the documentation for B<-f> in
847L<perlrun|perlrun/"Command Switches"> for more information about site
848customization.
a0d0e21e 849
b0c22438 850This variable was added in Perl 5.10.
a0d0e21e 851
b0c22438 852=item $EXECUTABLE_NAME
a0d0e21e 853
b0c22438 854=item $^X
855X<$^X> X<$EXECUTABLE_NAME>
a0d0e21e 856
b0c22438 857The name used to execute the current copy of Perl, from C's
858C<argv[0]> or (where supported) F</proc/self/exe>.
a043a685 859
b0c22438 860Depending on the host operating system, the value of C<$^X> may be
861a relative or absolute pathname of the perl program file, or may
862be the string used to invoke perl but not the pathname of the
863perl program file. Also, most operating systems permit invoking
864programs that are not in the PATH environment variable, so there
865is no guarantee that the value of C<$^X> is in PATH. For VMS, the
866value may or may not include a version number.
a0d0e21e 867
b0c22438 868You usually can use the value of C<$^X> to re-invoke an independent
869copy of the same perl that is currently running, e.g.,
a0d0e21e 870
0b9346e6 871 @first_run = `$^X -le "print int rand 100 for 1..100"`;
a0d0e21e 872
b0c22438 873But recall that not all operating systems support forking or
874capturing of the output of commands, so this complex statement
875may not be portable.
a0d0e21e 876
b0c22438 877It is not safe to use the value of C<$^X> as a path name of a file,
878as some operating systems that have a mandatory suffix on
879executable files do not require use of the suffix when invoking
880a command. To convert the value of C<$^X> to a path name, use the
881following statements:
8cc95fdb 882
0b9346e6 883 # Build up a set of file names (not command names).
884 use Config;
885 my $this_perl = $^X;
886 if ($^O ne 'VMS') {
887 $this_perl .= $Config{_exe}
888 unless $this_perl =~ m/$Config{_exe}$/i;
889 }
8cc95fdb 890
b0c22438 891Because many operating systems permit anyone with read access to
892the Perl program file to make a copy of it, patch the copy, and
893then execute the copy, the security-conscious Perl programmer
894should take care to invoke the installed copy of perl, not the
895copy referenced by C<$^X>. The following statements accomplish
896this goal, and produce a pathname that can be invoked as a
897command or referenced as a file.
a043a685 898
0b9346e6 899 use Config;
900 my $secure_perl_path = $Config{perlpath};
901 if ($^O ne 'VMS') {
902 $secure_perl_path .= $Config{_exe}
903 unless $secure_perl_path =~ m/$Config{_exe}$/i;
904 }
a0d0e21e 905
b0c22438 906=back
a0d0e21e 907
b0c22438 908=head2 Variables related to regular expressions
909
910Most of the special variables related to regular expressions are side
911effects. Perl sets these variables when it has a successful match, so
912you should check the match result before using them. For instance:
913
914 if( /P(A)TT(ER)N/ ) {
915 print "I found $1 and $2\n";
916 }
917
0b9346e6 918These variables are read-only and dynamically-scoped, unless we note
b0c22438 919otherwise.
920
0b9346e6 921The dynamic nature of the regular expression variables means that
922their value is limited to the block that they are in, as demonstrated
923by this bit of code:
b0c22438 924
925 my $outer = 'Wallace and Grommit';
926 my $inner = 'Mutt and Jeff';
0b9346e6 927
b0c22438 928 my $pattern = qr/(\S+) and (\S+)/;
0b9346e6 929
b0c22438 930 sub show_n { print "\$1 is $1; \$2 is $2\n" }
0b9346e6 931
b0c22438 932 {
933 OUTER:
934 show_n() if $outer =~ m/$pattern/;
0b9346e6 935
b0c22438 936 INNER: {
937 show_n() if $inner =~ m/$pattern/;
938 }
0b9346e6 939
b0c22438 940 show_n();
941 }
942
0b9346e6 943The output shows that while in the C<OUTER> block, the values of C<$1>
944and C<$2> are from the match against C<$outer>. Inside the C<INNER>
945block, the values of C<$1> and C<$2> are from the match against
946C<$inner>, but only until the end of the block (i.e. the dynamic
947scope). After the C<INNER> block completes, the values of C<$1> and
948C<$2> return to the values for the match against C<$outer> even though
b0c22438 949we have not made another match:
950
951 $1 is Wallace; $2 is Grommit
952 $1 is Mutt; $2 is Jeff
953 $1 is Wallace; $2 is Grommit
a0d0e21e 954
0b9346e6 955Due to an unfortunate accident of Perl's implementation, C<use
956English> imposes a considerable performance penalty on all regular
957expression matches in a program because it uses the C<$`>, C<$&>, and
958C<$'>, regardless of whether they occur in the scope of C<use
959English>. For that reason, saying C<use English> in libraries is
960strongly discouraged unless you import it without the match variables:
961
962 use English '-no_match_vars'
963
964The C<Devel::NYTProf> module can help you find uses of these
965problematic match variables in your code.
966
967Since Perl 5.10, you can use the C</p> match operator flag and the
968C<${^PREMATCH}>, C<${^MATCH}>, and C<${^POSTMATCH}> variables instead
969so you only suffer the performance penalties.
970
b0c22438 971=over 8
a0d0e21e 972
b0c22438 973=item $<I<digits>> ($1, $2, ...)
974X<$1> X<$2> X<$3>
8cc95fdb 975
b0c22438 976Contains the subpattern from the corresponding set of capturing
977parentheses from the last successful pattern match, not counting patterns
978matched in nested blocks that have been exited already.
8cc95fdb 979
b0c22438 980These variables are read-only and dynamically-scoped.
a043a685 981
b0c22438 982Mnemonic: like \digits.
a0d0e21e 983
b0c22438 984=item $MATCH
a0d0e21e 985
b0c22438 986=item $&
987X<$&> X<$MATCH>
a0d0e21e 988
b0c22438 989The string matched by the last successful pattern match (not counting
990any matches hidden within a BLOCK or C<eval()> enclosed by the current
991BLOCK).
a0d0e21e 992
b0c22438 993The use of this variable anywhere in a program imposes a considerable
0b9346e6 994performance penalty on all regular expression matches. To avoid this
995penalty, you can extract the same substring by using L</@->. Starting
996with Perl 5.10, you can use the </p> match flag and the C<${^MATCH}>
997variable to do the same thing for particular match operations.
80bca1b4 998
b0c22438 999This variable is read-only and dynamically-scoped.
f9cbb277 1000
b0c22438 1001Mnemonic: like C<&> in some editors.
0b9346e6 1002
b0c22438 1003=item ${^MATCH}
1004X<${^MATCH}>
a0d0e21e 1005
b0c22438 1006This is similar to C<$&> (C<$MATCH>) except that it does not incur the
1007performance penalty associated with that variable, and is only guaranteed
1008to return a defined value when the pattern was compiled or executed with
1009the C</p> modifier.
80bca1b4 1010
b0c22438 1011This variable was added in Perl 5.10.
4bc88a62 1012
b0c22438 1013This variable is read-only and dynamically-scoped.
e2975953 1014
b0c22438 1015=item $PREMATCH
52c447a8 1016
b0c22438 1017=item $`
5b442a2a 1018X<$`> X<$PREMATCH> X<${^PREMATCH}>
7636ea95 1019
b0c22438 1020The string preceding whatever was matched by the last successful
1021pattern match, not counting any matches hidden within a BLOCK or C<eval>
0b9346e6 1022enclosed by the current BLOCK.
a0d0e21e 1023
b0c22438 1024The use of this variable anywhere in a program imposes a considerable
0b9346e6 1025performance penalty on all regular expression matches. To avoid this
1026penalty, you can extract the same substring by using L</@->. Starting
1027with Perl 5.10, you can use the </p> match flag and the
1028C<${^PREMATCH}> variable to do the same thing for particular match
1029operations.
a0d0e21e 1030
b0c22438 1031This variable is read-only and dynamically-scoped.
a0d0e21e 1032
b0c22438 1033Mnemonic: C<`> often precedes a quoted string.
f83ed198 1034
b0c22438 1035=item ${^PREMATCH}
5b442a2a 1036X<$`> X<${^PREMATCH}>
a0d0e21e 1037
b0c22438 1038This is similar to C<$`> ($PREMATCH) except that it does not incur the
1039performance penalty associated with that variable, and is only guaranteed
1040to return a defined value when the pattern was compiled or executed with
1041the C</p> modifier.
a0d0e21e 1042
b0c22438 1043This variable was added in Perl 5.10
a0d0e21e 1044
b0c22438 1045This variable is read-only and dynamically-scoped.
a0d0e21e 1046
b0c22438 1047=item $POSTMATCH
16070b82 1048
b0c22438 1049=item $'
5b442a2a 1050X<$'> X<$POSTMATCH> X<${^POSTMATCH}> X<@->
305aace0 1051
b0c22438 1052The string following whatever was matched by the last successful
1053pattern match (not counting any matches hidden within a BLOCK or C<eval()>
1054enclosed by the current BLOCK). Example:
305aace0 1055
0b9346e6 1056 local $_ = 'abcdefghi';
1057 /def/;
1058 print "$`:$&:$'\n"; # prints abc:def:ghi
305aace0 1059
b0c22438 1060The use of this variable anywhere in a program imposes a considerable
0b9346e6 1061performance penalty on all regular expression matches.
1062To avoid this penalty, you can extract the same substring by
b0c22438 1063using L</@->. Starting with Perl 5.10, you can use the </p> match flag
0b9346e6 1064and the C<${^POSTMATCH}> variable to do the same thing for particular
b0c22438 1065match operations.
a0d0e21e 1066
b0c22438 1067This variable is read-only and dynamically-scoped.
1068
1069Mnemonic: C<'> often follows a quoted string.
1070
1071=item ${^POSTMATCH}
5b442a2a 1072X<${^POSTMATCH}> X<$'> X<$POSTMATCH>
b0c22438 1073
1074This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
1075performance penalty associated with that variable, and is only guaranteed
1076to return a defined value when the pattern was compiled or executed with
1077the C</p> modifier.
1078
1079This variable was added in Perl 5.10.
1080
1081This variable is read-only and dynamically-scoped.
1082
1083=item $LAST_PAREN_MATCH
1084
1085=item $+
1086X<$+> X<$LAST_PAREN_MATCH>
1087
1088The text matched by the last bracket of the last successful search pattern.
1089This is useful if you don't know which one of a set of alternative patterns
1090matched. For example:
1091
0b9346e6 1092 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
b0c22438 1093
1094This variable is read-only and dynamically-scoped.
1095
1096Mnemonic: be positive and forward looking.
1097
1098=item $LAST_SUBMATCH_RESULT
1099
1100=item $^N
5b442a2a 1101X<$^N> X<$LAST_SUBMATCH_RESULT>
b0c22438 1102
1103The text matched by the used group most-recently closed (i.e. the group
1104with the rightmost closing parenthesis) of the last successful search
1105pattern.
1106
1107This is primarily used inside C<(?{...})> blocks for examining text
1108recently matched. For example, to effectively capture text to a variable
1109(in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
1110
0b9346e6 1111 (?:(...)(?{ $var = $^N }))
b0c22438 1112
1113By setting and then using C<$var> in this way relieves you from having to
1114worry about exactly which numbered set of parentheses they are.
1115
1116This variable was added in Perl 5.8.
1117
1118Mnemonic: the (possibly) Nested parenthesis that most recently closed.
1119
1120=item @LAST_MATCH_END
1121
1122=item @+
1123X<@+> X<@LAST_MATCH_END>
1124
1125This array holds the offsets of the ends of the last successful
1126submatches in the currently active dynamic scope. C<$+[0]> is
1127the offset into the string of the end of the entire match. This
1128is the same value as what the C<pos> function returns when called
1129on the variable that was matched against. The I<n>th element
1130of this array holds the offset of the I<n>th submatch, so
1131C<$+[1]> is the offset past where C<$1> ends, C<$+[2]> the offset
1132past where C<$2> ends, and so on. You can use C<$#+> to determine
1133how many subgroups were in the last successful match. See the
1134examples given for the C<@-> variable.
1135
1136This variable was added in Perl 5.6.
1137
1138=item %LAST_PAREN_MATCH
1139
1140=item %+
5b442a2a 1141X<%+> X<%LAST_PAREN_MATCH>
b0c22438 1142
1143Similar to C<@+>, the C<%+> hash allows access to the named capture
1144buffers, should they exist, in the last successful match in the
1145currently active dynamic scope.
1146
1147For example, C<$+{foo}> is equivalent to C<$1> after the following match:
1148
0b9346e6 1149 'foo' =~ /(?<foo>foo)/;
b0c22438 1150
1151The keys of the C<%+> hash list only the names of buffers that have
1152captured (and that are thus associated to defined values).
1153
1154The underlying behaviour of C<%+> is provided by the
1155L<Tie::Hash::NamedCapture> module.
1156
1157B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1158associated with the last successful regular expression. Therefore mixing
1159iterative access to them via C<each> may have unpredictable results.
1160Likewise, if the last successful match changes, then the results may be
1161surprising.
1162
1163This variable was added in Perl 5.10.
a0d0e21e 1164
b0c22438 1165This variable is read-only and dynamically-scoped.
1166
1167=item @LAST_MATCH_START
1168
1169=item @-
1170X<@-> X<@LAST_MATCH_START>
1171
1172C<$-[0]> is the offset of the start of the last successful match.
1173C<$-[>I<n>C<]> is the offset of the start of the substring matched by
1174I<n>-th subpattern, or undef if the subpattern did not match.
1175
1176Thus, after a match against C<$_>, C<$&> coincides with C<substr $_, $-[0],
1177$+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
1178$+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
1179C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the last
1180matched subgroup in the last successful match. Contrast with
1181C<$#+>, the number of subgroups in the regular expression. Compare
1182with C<@+>.
1183
1184This array holds the offsets of the beginnings of the last
1185successful submatches in the currently active dynamic scope.
1186C<$-[0]> is the offset into the string of the beginning of the
1187entire match. The I<n>th element of this array holds the offset
1188of the I<n>th submatch, so C<$-[1]> is the offset where C<$1>
1189begins, C<$-[2]> the offset where C<$2> begins, and so on.
1190
1191After a match against some variable C<$var>:
1192
1193=over 5
1194
1195=item C<$`> is the same as C<substr($var, 0, $-[0])>
1196
1197=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
1198
1199=item C<$'> is the same as C<substr($var, $+[0])>
1200
1201=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
1202
1203=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
1204
1205=item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
1206
1207=back
1208
1209This variable was added in Perl 5.6.
1210
5b442a2a 1211=item %LAST_MATCH_START
1212
b0c22438 1213=item %-
5b442a2a 1214X<%-> X<%LAST_MATCH_START>
b0c22438 1215
1216Similar to C<%+>, this variable allows access to the named capture groups
1217in the last successful match in the currently active dynamic scope. To
1218each capture group name found in the regular expression, it associates a
1219reference to an array containing the list of values captured by all
1220buffers with that name (should there be several of them), in the order
1221where they appear.
1222
1223Here's an example:
1224
1225 if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
1226 foreach my $bufname (sort keys %-) {
1227 my $ary = $-{$bufname};
1228 foreach my $idx (0..$#$ary) {
1229 print "\$-{$bufname}[$idx] : ",
1230 (defined($ary->[$idx]) ? "'$ary->[$idx]'" : "undef"),
1231 "\n";
1232 }
1233 }
1234 }
1235
1236would print out:
1237
0b9346e6 1238 $-{A}[0] : '1'
1239 $-{A}[1] : '3'
1240 $-{B}[0] : '2'
1241 $-{B}[1] : '4'
b0c22438 1242
1243The keys of the C<%-> hash correspond to all buffer names found in
1244the regular expression.
1245
1246The behaviour of C<%-> is implemented via the
1247L<Tie::Hash::NamedCapture> module.
1248
1249B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1250associated with the last successful regular expression. Therefore mixing
1251iterative access to them via C<each> may have unpredictable results.
1252Likewise, if the last successful match changes, then the results may be
1253surprising.
1254
1255This variable was added in Perl 5.10
1256
1257This variable is read-only and dynamically-scoped.
1258
1259=item $LAST_REGEXP_CODE_RESULT
1260
1261=item $^R
1262X<$^R> X<$LAST_REGEXP_CODE_RESULT>
1263
1264The result of evaluation of the last successful C<(?{ code })>
1265regular expression assertion (see L<perlre>). May be written to.
1266
1267This variable was added in Perl 5.005.
a0d0e21e 1268
a3621e74
YO
1269=item ${^RE_DEBUG_FLAGS}
1270
1271The current value of the regex debugging flags. Set to 0 for no debug output
b0c22438 1272even when the C<re 'debug'> module is loaded. See L<re> for details.
1273
1274This variable was added in Perl 5.10.
a3621e74 1275
0111c4fd 1276=item ${^RE_TRIE_MAXBUF}
a3621e74
YO
1277
1278Controls how certain regex optimisations are applied and how much memory they
1279utilize. This value by default is 65536 which corresponds to a 512kB temporary
1280cache. Set this to a higher value to trade memory for speed when matching
1281large alternations. Set it to a lower value if you want the optimisations to
1282be as conservative of memory as possible but still occur, and set it to a
1283negative value to prevent the optimisation and conserve the most memory.
1284Under normal situations this variable should be of no interest to you.
1285
b0c22438 1286This variable was added in Perl 5.10.
a0d0e21e 1287
b0c22438 1288=back
a0d0e21e 1289
b0c22438 1290=head2 Variables related to filehandles
a0d0e21e 1291
b0c22438 1292Variables that depend on the currently selected filehandle may be set
1293by calling an appropriate object method on the C<IO::Handle> object,
1294although this is less efficient than using the regular built-in
1295variables. (Summary lines below for this contain the word HANDLE.)
1296First you must say
6e2995f4 1297
0b9346e6 1298 use IO::Handle;
0462a1ab 1299
b0c22438 1300after which you may use either
0462a1ab 1301
0b9346e6 1302 method HANDLE EXPR
0462a1ab 1303
b0c22438 1304or more safely,
0462a1ab 1305
0b9346e6 1306 HANDLE->method(EXPR)
0462a1ab 1307
b0c22438 1308Each method returns the old value of the C<IO::Handle> attribute. The
1309methods each take an optional EXPR, which, if supplied, specifies the
1310new value for the C<IO::Handle> attribute in question. If not
1311supplied, most methods do nothing to the current value--except for
1312C<autoflush()>, which will assume a 1 for you, just to be different.
0462a1ab 1313
b0c22438 1314Because loading in the C<IO::Handle> class is an expensive operation,
1315you should learn how to use the regular built-in variables.
1316
1317A few of these variables are considered "read-only". This means that
1318if you try to assign to this variable, either directly or indirectly
1319through a reference, you'll raise a run-time exception.
1320
1321You should be very careful when modifying the default values of most
1322special variables described in this document. In most cases you want
1323to localize these variables before changing them, since if you don't,
1324the change may affect other modules which rely on the default values
1325of the special variables that you have changed. This is one of the
1326correct ways to read the whole file at once:
1327
0b9346e6 1328 open my $fh, "<", "foo" or die $!;
1329 local $/; # enable localized slurp mode
1330 my $content = <$fh>;
1331 close $fh;
b0c22438 1332
1333But the following code is quite bad:
1334
0b9346e6 1335 open my $fh, "<", "foo" or die $!;
1336 undef $/; # enable slurp mode
1337 my $content = <$fh>;
1338 close $fh;
b0c22438 1339
1340since some other module, may want to read data from some file in the
1341default "line mode", so if the code we have just presented has been
1342executed, the global value of C<$/> is now changed for any other code
1343running inside the same Perl interpreter.
1344
1345Usually when a variable is localized you want to make sure that this
1346change affects the shortest scope possible. So unless you are already
1347inside some short C<{}> block, you should create one yourself. For
1348example:
1349
0b9346e6 1350 my $content = '';
1351 open my $fh, "<", "foo" or die $!;
1352 {
1353 local $/;
1354 $content = <$fh>;
1355 }
1356 close $fh;
0462a1ab 1357
b0c22438 1358Here is an example of how your own code can go broken:
0462a1ab 1359
0b9346e6 1360 for ( 1..3 ){
1361 $\ = "\r\n";
1362 nasty_break();
1363 print "$_";
1364 }
1365
1366 sub nasty_break {
1367 $\ = "\f";
1368 # do something with $_
1369 }
0462a1ab 1370
0b9346e6 1371You probably expect this code to print the equivalent of
0462a1ab 1372
0b9346e6 1373 "1\r\n2\r\n3\r\n"
0462a1ab 1374
b0c22438 1375but instead you get:
0462a1ab 1376
0b9346e6 1377 "1\f2\f3\f"
0462a1ab 1378
0b9346e6 1379Why? Because C<nasty_break()> modifies C<$\> without localizing it
1380first. The value you set in C<nasty_break()> is still there when you
1381return. The fix is to add C<local()> so the value doesn't leak out of
1382C<nasty_break()>:
6e2995f4 1383
0b9346e6 1384 local $\ = "\f";
a0d0e21e 1385
b0c22438 1386It's easy to notice the problem in such a short example, but in more
1387complicated code you are looking for trouble if you don't localize
1388changes to the special variables.
a0d0e21e 1389
b0c22438 1390=over 8
a0d0e21e 1391
b0c22438 1392=item $ARGV
1393X<$ARGV>
fb73857a 1394
b0c22438 1395contains the name of the current file when reading from <>.
1396
1397=item @ARGV
1398X<@ARGV>
1399
1400The array @ARGV contains the command-line arguments intended for
1401the script. C<$#ARGV> is generally the number of arguments minus
1402one, because C<$ARGV[0]> is the first argument, I<not> the program's
1403command name itself. See C<$0> for the command name.
1404
84dabc03 1405=item ARGV
1406X<ARGV>
1407
1408The special filehandle that iterates over command-line filenames in
1409C<@ARGV>. Usually written as the null filehandle in the angle operator
1410C<< <> >>. Note that currently C<ARGV> only has its magical effect
1411within the C<< <> >> operator; elsewhere it is just a plain filehandle
1412corresponding to the last file opened by C<< <> >>. In particular,
1413passing C<\*ARGV> as a parameter to a function that expects a filehandle
1414may not cause your function to automatically read the contents of all the
1415files in C<@ARGV>.
1416
b0c22438 1417=item ARGVOUT
1418X<ARGVOUT>
1419
1420The special filehandle that points to the currently open output file
1421when doing edit-in-place processing with B<-i>. Useful when you have
1422to do a lot of inserting and don't want to keep modifying C<$_>. See
1423L<perlrun> for the B<-i> switch.
1424
5b442a2a 1425=item Handle->output_field_separator( EXPR )
84dabc03 1426
1427=item $OUTPUT_FIELD_SEPARATOR
1428
1429=item $OFS
1430
1431=item $,
1432X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>
1433
1434The output field separator for the print operator. If defined, this
1435value is printed between each of print's arguments. Default is C<undef>.
1436
1437Mnemonic: what is printed when there is a "," in your print statement.
1438
5b442a2a 1439=item HANDLE->input_line_number( EXPR )
b0c22438 1440
1441=item $INPUT_LINE_NUMBER
1442
1443=item $NR
1444
1445=item $.
1446X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
1447
1448Current line number for the last filehandle accessed.
1449
1450Each filehandle in Perl counts the number of lines that have been read
1451from it. (Depending on the value of C<$/>, Perl's idea of what
1452constitutes a line may not match yours.) When a line is read from a
1453filehandle (via C<readline()> or C<< <> >>), or when C<tell()> or
1454C<seek()> is called on it, C<$.> becomes an alias to the line counter
1455for that filehandle.
1456
1457You can adjust the counter by assigning to C<$.>, but this will not
1458actually move the seek pointer. I<Localizing C<$.> will not localize
1459the filehandle's line count>. Instead, it will localize perl's notion
1460of which filehandle C<$.> is currently aliased to.
1461
1462C<$.> is reset when the filehandle is closed, but B<not> when an open
1463filehandle is reopened without an intervening C<close()>. For more
1464details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
1465an explicit close, line numbers increase across C<ARGV> files (but see
1466examples in L<perlfunc/eof>).
1467
1468You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
1469line counter for a given filehandle without having to worry about
1470which handle you last accessed.
1471
1472Mnemonic: many programs use "." to mean the current line number.
1473
5b442a2a 1474=item HANDLE->input_record_separator( EXPR )
b0c22438 1475
1476=item $INPUT_RECORD_SEPARATOR
1477
1478=item $RS
1479
1480=item $/
1481X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR>
1482
84dabc03 1483The input record separator, newline by default. This influences Perl's
1484idea of what a "line" is. Works like B<awk>'s RS variable, including
1485treating empty lines as a terminator if set to the null string (an
1486empty line cannot contain any spaces or tabs). You may set it to a
1487multi-character string to match a multi-character terminator, or to
1488C<undef> to read through the end of file. Setting it to C<"\n\n">
1489means something slightly different than setting to C<"">, if the file
1490contains consecutive empty lines. Setting to C<""> will treat two or
1491more consecutive empty lines as a single empty line. Setting to
1492C<"\n\n"> will blindly assume that the next input character belongs to
1493the next paragraph, even if it's a newline.
b0c22438 1494
1495 local $/; # enable "slurp" mode
1496 local $_ = <FH>; # whole file now here
1497 s/\n[ \t]+/ /g;
1498
1499Remember: the value of C<$/> is a string, not a regex. B<awk> has to
1500be better for something. :-)
1501
1502Setting C<$/> to a reference to an integer, scalar containing an
1503integer, or scalar that's convertible to an integer will attempt to
1504read records instead of lines, with the maximum record size being the
1505referenced integer. So this:
1506
1507 local $/ = \32768; # or \"32768", or \$var_containing_32768
1508 open my $fh, "<", $myfile or die $!;
1509 local $_ = <$fh>;
fb73857a 1510
b0c22438 1511will read a record of no more than 32768 bytes from FILE. If you're
1512not reading from a record-oriented file (or your OS doesn't have
1513record-oriented files), then you'll likely get a full chunk of data
1514with every read. If a record is larger than the record size you've
1515set, you'll get the record back in pieces. Trying to set the record
1516size to zero or less will cause reading in the (rest of the) whole file.
6e2995f4 1517
b0c22438 1518On VMS, record reads are done with the equivalent of C<sysread>,
1519so it's best not to mix record and non-record reads on the same
5b442a2a 1520file. (This is unlikely to be a problem, because any file you'd
b0c22438 1521want to read in record mode is probably unusable in line mode.)
1522Non-VMS systems do normal I/O, so it's safe to mix record and
1523non-record reads of a file.
5c055ba3 1524
b0c22438 1525See also L<perlport/"Newlines">. Also see C<$.>.
9bf22702 1526
b0c22438 1527Mnemonic: / delimits line boundaries when quoting poetry.
5c055ba3 1528
5b442a2a 1529=item Handle->output_record_separator( EXPR )
84902520 1530
b0c22438 1531=item $OUTPUT_RECORD_SEPARATOR
84902520 1532
b0c22438 1533=item $ORS
84902520 1534
b0c22438 1535=item $\
1536X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR>
84902520 1537
b0c22438 1538The output record separator for the print operator. If defined, this
1539value is printed after the last of print's arguments. Default is C<undef>.
84902520 1540
b0c22438 1541Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
1542Also, it's just like C<$/>, but it's what you get "back" from Perl.
84902520 1543
5b442a2a 1544=item HANDLE->autoflush( EXPR )
1545
1546=item $OUTPUT_AUTOFLUSH
1547
84dabc03 1548=item $|
1549X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
84902520 1550
84dabc03 1551If set to nonzero, forces a flush right away and after every write or
1552print on the currently selected output channel. Default is 0
1553(regardless of whether the channel is really buffered by the system or
1554not; C<$|> tells you only whether you've asked Perl explicitly to
1555flush after each write). STDOUT will typically be line buffered if
5b442a2a 1556output is to the terminal and block buffered otherwise. Setting this
84dabc03 1557variable is useful primarily when you are outputting to a pipe or
1558socket, such as when you are running a Perl program under B<rsh> and
5b442a2a 1559want to see the output as it's happening. This has no effect on input
1560buffering. See L<perlfunc/getc> for that. See L<perldoc/select> on
84dabc03 1561how to select the output channel. See also L<IO::Handle>.
1562
1563Mnemonic: when you want your pipes to be piping hot.
1564
1565=back
84902520 1566
b0c22438 1567=head3 Variables related to formats
83ee9e09 1568
b0c22438 1569The special variables for formats are a subset of those for
1570filehandles so they have
83ee9e09 1571
b0c22438 1572See L<perlform> for more information about Perl's formats.
83ee9e09 1573
b0c22438 1574=over 8
83ee9e09 1575
84dabc03 1576=item $ACCUMULATOR
1577
1578=item $^A
1579X<$^A> X<$ACCUMULATOR>
1580
1581The current value of the C<write()> accumulator for C<format()> lines.
1582A format contains C<formline()> calls that put their result into
1583C<$^A>. After calling its format, C<write()> prints out the contents
1584of C<$^A> and empties. So you never really see the contents of C<$^A>
1585unless you call C<formline()> yourself and then look at it. See
1586L<perlform> and L<perlfunc/formline()>.
1587
5b442a2a 1588=item HANDLE->format_formfeed(EXPR)
1589
1590=item $FORMAT_FORMFEED
1591
84dabc03 1592=item $^L
1593X<$^L> X<$FORMAT_FORMFEED>
1594
1595What formats output as a form feed. The default is C<\f>.
1596
b0c22438 1597=item HANDLE->format_page_number(EXPR)
83ee9e09 1598
b0c22438 1599=item $FORMAT_PAGE_NUMBER
83ee9e09 1600
b0c22438 1601=item $%
1602X<$%> X<$FORMAT_PAGE_NUMBER>
83ee9e09 1603
b0c22438 1604The current page number of the currently selected output channel.
83ee9e09 1605
b0c22438 1606Mnemonic: C<%> is page number in B<nroff>.
7619c85e 1607
b0c22438 1608=item HANDLE->format_lines_left(EXPR)
b9ac3b5b 1609
b0c22438 1610=item $FORMAT_LINES_LEFT
66558a10 1611
b0c22438 1612=item $-
1613X<$-> X<$FORMAT_LINES_LEFT>
fb73857a 1614
b0c22438 1615The number of lines left on the page of the currently selected output
1616channel.
fa05a9fd 1617
b0c22438 1618Mnemonic: lines_on_page - lines_printed.
fa05a9fd 1619
84dabc03 1620=item Handle->format_line_break_characters EXPR
fb73857a 1621
84dabc03 1622=item $FORMAT_LINE_BREAK_CHARACTERS
a0d0e21e 1623
84dabc03 1624=item $:
1625X<$:> X<FORMAT_LINE_BREAK_CHARACTERS>
a0d0e21e 1626
84dabc03 1627The current set of characters after which a string may be broken to
1628fill continuation fields (starting with C<^>) in a format. The default is
1629S<" \n-">, to break on a space, newline, or a hyphen.
a0d0e21e 1630
84dabc03 1631Mnemonic: a "colon" in poetry is a part of a line.
1632
1633=item HANDLE->format_lines_per_page(EXPR)
1634
1635=item $FORMAT_LINES_PER_PAGE
1636
1637=item $=
1638X<$=> X<$FORMAT_LINES_PER_PAGE>
1639
1640The current page length (printable lines) of the currently selected
1641output channel. The default is 60.
1642
1643Mnemonic: = has horizontal lines.
7c36658b 1644
b0c22438 1645=item HANDLE->format_top_name(EXPR)
7c36658b 1646
b0c22438 1647=item $FORMAT_TOP_NAME
a05d7ebb 1648
b0c22438 1649=item $^
1650X<$^> X<$FORMAT_TOP_NAME>
fde18df1 1651
b0c22438 1652The name of the current top-of-page format for the currently selected
1653output channel. The default is the name of the filehandle with C<_TOP>
1654appended. For example, the default format top name for the C<STDOUT>
1655filehanlde is C<STDOUT_TOP>.
e07ea26a 1656
b0c22438 1657Mnemonic: points to top of page.
e07ea26a 1658
84dabc03 1659=item HANDLE->format_name(EXPR)
16070b82 1660
84dabc03 1661=item $FORMAT_NAME
aa2f2a36 1662
84dabc03 1663=item $~
1664X<$~> X<$FORMAT_NAME>
aa2f2a36 1665
84dabc03 1666The name of the current report format for the currently selected
1667output channel. The default format name is the same as the filehandle
1668name. For example, the default format name for the C<STDOUT>
1669filehandle is just C<STDOUT>.
16070b82 1670
84dabc03 1671Mnemonic: brother to C<$^>.
16070b82 1672
b0c22438 1673=back
a0d0e21e 1674
84dabc03 1675=head2 Error Variables
b0c22438 1676X<error> X<exception>
a0d0e21e 1677
b0c22438 1678The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1679about different types of error conditions that may appear during
1680execution of a Perl program. The variables are shown ordered by
1681the "distance" between the subsystem which reported the error and
1682the Perl process. They correspond to errors detected by the Perl
1683interpreter, C library, operating system, or an external program,
1684respectively.
4438c4b7 1685
b0c22438 1686To illustrate the differences between these variables, consider the
1687following Perl expression, which uses a single-quoted string:
4438c4b7 1688
b0c22438 1689 eval q{
1690 open my $pipe, "/cdrom/install |" or die $!;
1691 my @res = <$pipe>;
1692 close $pipe or die "bad pipe: $?, $!";
1693 };
a0d0e21e 1694
2f373948 1695After execution of this statement all four variables may have been set.
2a8c8378 1696
84dabc03 1697C<$@> is set if the string to be C<eval>-ed did not compile (this may
1698happen if C<open> or C<close> were imported with bad prototypes), or
0b9346e6 1699if Perl code executed during evaluation C<die()>d . In these cases the
1700value of C<$@> is the compile error, or the argument to C<die> (which
84dabc03 1701will interpolate C<$!> and C<$?>). (See also L<Fatal>, though.)
2a8c8378 1702
0b9346e6 1703When the C<eval()> expression above is executed, C<open()>, C<< <PIPE> >>,
1704and C<close> are translated to calls in the C run-time library and
b0c22438 1705thence to the operating system kernel. C<$!> is set to the C library's
1706C<errno> if one of these calls fails.
2a8c8378 1707
84dabc03 1708Under a few operating systems, C<$^E> may contain a more verbose error
1709indicator, such as in this case, "CDROM tray not closed." Systems that
1710do not support extended error messages leave C<$^E> the same as C<$!>.
a0d0e21e 1711
b0c22438 1712Finally, C<$?> may be set to non-0 value if the external program
84dabc03 1713F</cdrom/install> fails. The upper eight bits reflect specific error
1714conditions encountered by the program (the program's C<exit()> value).
1715The lower eight bits reflect mode of failure, like signal death and
1716core dump information. See C<wait(2)> for details. In contrast to
1717C<$!> and C<$^E>, which are set only if error condition is detected,
1718the variable C<$?> is set on each C<wait> or pipe C<close>,
1719overwriting the old value. This is more like C<$@>, which on every
1720C<eval()> is always set on failure and cleared on success.
a0d0e21e 1721
b0c22438 1722For more details, see the individual descriptions at C<$@>, C<$!>,
1723C<$^E>, and C<$?>.
38e4f4ae 1724
0b9346e6 1725=over 8
1726
b0c22438 1727=item ${^CHILD_ERROR_NATIVE}
1728X<$^CHILD_ERROR_NATIVE>
a0d0e21e 1729
b0c22438 1730The native status returned by the last pipe close, backtick (C<``>)
1731command, successful call to C<wait()> or C<waitpid()>, or from the
1732C<system()> operator. On POSIX-like systems this value can be decoded
1733with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED,
1734WSTOPSIG and WIFCONTINUED functions provided by the L<POSIX> module.
a0d0e21e 1735
b0c22438 1736Under VMS this reflects the actual VMS exit status; i.e. it is the
1737same as C<$?> when the pragma C<use vmsish 'status'> is in effect.
a0d0e21e 1738
b0c22438 1739This variable was added in Perl 5.8.9.
a0d0e21e 1740
5b442a2a 1741=item $EXTENDED_OS_ERROR
1742
84dabc03 1743=item $^E
1744X<$^E> X<$EXTENDED_OS_ERROR>
1745
1746Error information specific to the current operating system. At the
1747moment, this differs from C<$!> under only VMS, OS/2, and Win32 (and
1748for MacPerl). On all other platforms, C<$^E> is always just the same
1749as C<$!>.
1750
1751Under VMS, C<$^E> provides the VMS status value from the last system
1752error. This is more specific information about the last system error
1753than that provided by C<$!>. This is particularly important when C<$!>
1754is set to B<EVMSERR>.
1755
1756Under OS/2, C<$^E> is set to the error code of the last call to OS/2
1757API either via CRT, or directly from perl.
1758
1759Under Win32, C<$^E> always returns the last error information reported
1760by the Win32 call C<GetLastError()> which describes the last error
1761from within the Win32 API. Most Win32-specific code will report errors
1762via C<$^E>. ANSI C and Unix-like calls set C<errno> and so most
1763portable Perl code will report errors via C<$!>.
1764
1765Caveats mentioned in the description of C<$!> generally apply to
1766C<$^E>, also.
1767
1768This variable was added in Perl 5.003.
1769
1770Mnemonic: Extra error explanation.
0b9346e6 1771
84dabc03 1772=item $EXCEPTIONS_BEING_CAUGHT
1773
1774=item $^S
1775X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>
1776
1777Current state of the interpreter.
1778
1779 $^S State
1780 --------- -------------------
1781 undef Parsing module/eval
1782 true (1) Executing an eval
1783 false (0) Otherwise
1784
1785The first state may happen in C<$SIG{__DIE__}> and C<$SIG{__WARN__}>
1786handlers.
1787
1788This variable was added in Perl 5.004.
1789
1790=item $WARNING
1791
1792=item $^W
1793X<$^W> X<$WARNING>
1794
1795The current value of the warning switch, initially true if B<-w> was
1796used, false otherwise, but directly modifiable.
1797
1798See also L<warnings>.
1799
0b9346e6 1800Mnemonic: related to the B<-w> switch.
84dabc03 1801
1802=item ${^WARNING_BITS}
1803
1804The current set of warning checks enabled by the C<use warnings> pragma.
1805See the documentation of C<warnings> for more details.
1806
1807This variable was added in Perl 5.10.
1808
b0c22438 1809=item $OS_ERROR
5ccee41e 1810
b0c22438 1811=item $ERRNO
5ccee41e 1812
b0c22438 1813=item $!
1814X<$!> X<$ERRNO> X<$OS_ERROR>
9b0e6e7a 1815
b0c22438 1816If used numerically, yields the current value of the C C<errno>
1817variable, or in other words, if a system or library call fails, it
1818sets this variable. This means that the value of C<$!> is meaningful
1819only I<immediately> after a B<failure>:
9b0e6e7a 1820
b0c22438 1821 if (open my $fh, "<", $filename) {
1822 # Here $! is meaningless.
1823 ...
1824 } else {
1825 # ONLY here is $! meaningful.
1826 ...
1827 # Already here $! might be meaningless.
1828 }
1829 # Since here we might have either success or failure,
1830 # here $! is meaningless.
a0d0e21e 1831
b0c22438 1832In the above I<meaningless> stands for anything: zero, non-zero,
84dabc03 1833C<undef>. A successful system or library call does B<not> set the
1834variable to zero.
a0d0e21e 1835
84dabc03 1836If used as a string, yields the corresponding system error string. You
1837can assign a number to C<$!> to set I<errno> if, for instance, you
1838want C<"$!"> to return the string for error I<n>, or you want to set
1839the exit value for the C<die()> operator.
d54b56d5 1840
b0c22438 1841Mnemonic: What just went bang?
314d39ce 1842
b0c22438 1843=item %OS_ERROR
fb73857a 1844
b0c22438 1845=item %ERRNO
fb73857a 1846
b0c22438 1847=item %!
5b442a2a 1848X<%!> X<%OS_ERROR> X<%ERRNO>
a0d0e21e 1849
b0c22438 1850Each element of C<%!> has a true value only if C<$!> is set to that
1851value. For example, C<$!{ENOENT}> is true if and only if the current
84dabc03 1852value of C<$!> is C<ENOENT>; that is, if the most recent error was "No
1853such file or directory" (or its moral equivalent: not all operating
1854systems give that exact error, and certainly not all languages). To
1855check if a particular key is meaningful on your system, use C<exists
1856$!{the_key}>; for a list of legal keys, use C<keys %!>. See L<Errno>
1857for more information, and also see above for the validity of C<$!>.
a0d0e21e 1858
b0c22438 1859This variable was added in Perl 5.005.
44f0be63 1860
84dabc03 1861=item $CHILD_ERROR
b687b08b 1862
84dabc03 1863=item $?
1864X<$?> X<$CHILD_ERROR>
a0d0e21e 1865
84dabc03 1866The status returned by the last pipe close, backtick (C<``>) command,
1867successful call to C<wait()> or C<waitpid()>, or from the C<system()>
1868operator. This is just the 16-bit status word returned by the
1869traditional Unix C<wait()> system call (or else is made up to look
1870like it). Thus, the exit value of the subprocess is really (C<<< $? >>
18718 >>>), and C<$? & 127> gives which signal, if any, the process died
1872from, and C<$? & 128> reports whether there was a core dump.
a0d0e21e 1873
84dabc03 1874Additionally, if the C<h_errno> variable is supported in C, its value
1875is returned via C<$?> if any C<gethost*()> function fails.
b687b08b 1876
84dabc03 1877If you have installed a signal handler for C<SIGCHLD>, the
1878value of C<$?> will usually be wrong outside that handler.
a0d0e21e 1879
84dabc03 1880Inside an C<END> subroutine C<$?> contains the value that is going to be
1881given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
1882change the exit status of your program. For example:
a0d0e21e 1883
84dabc03 1884 END {
1885 $? = 1 if $? == 255; # die would make it 255
1886 }
a0d0e21e 1887
84dabc03 1888Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
1889actual VMS exit status, instead of the default emulation of POSIX
1890status; see L<perlvms/$?> for details.
1891
1892Mnemonic: similar to B<sh> and B<ksh>.
a0d0e21e 1893
b0c22438 1894=item $EVAL_ERROR
f648820c 1895
b0c22438 1896=item $@
1897X<$@> X<$EVAL_ERROR>
a0d0e21e 1898
0b9346e6 1899The Perl syntax error message from the last C<eval()> operator. If C<$@> is
1900the null string, the last C<eval()> parsed and executed correctly
b0c22438 1901(although the operations you invoked may have failed in the normal
1902fashion).
a0d0e21e 1903
b0c22438 1904Warning messages are not collected in this variable. You can, however,
1905set up a routine to process warnings by setting C<$SIG{__WARN__}> as
1906described below.
748a9306 1907
b0c22438 1908Mnemonic: Where was the syntax error "at"?
7f315d2e 1909
b0c22438 1910=back
7f315d2e 1911
b0c22438 1912=head2 Deprecated and removed variables
7f315d2e 1913
0b9346e6 1914Deprecating a variable announces the intent of the perl maintainers to
84dabc03 1915eventually remove the variable from the langauge. It may still be
b0c22438 1916available despite its status. Using a deprecated variable triggers
1917a warning.
7f315d2e 1918
84dabc03 1919Once a variable is removed, its use triggers an error telling you
b0c22438 1920the variable is unsupported.
7f315d2e 1921
84dabc03 1922See L<perldiag> for details about error messages.
7f315d2e 1923
b0c22438 1924=over 8
7f315d2e 1925
5b442a2a 1926=item $OFMT
1927
84dabc03 1928=item $#
5b442a2a 1929X<$#> X<$OFMT>
84dabc03 1930
1931C<$#> was a variable that you could be use to format printed numbers.
1932After a deprecation cycle, its magic was removed in Perl 5.10 and
1933using it now triggers a warning: C<$# is no longer supported>.
1934
1935This is not the sigil you use in front of an array name to get the
1936last index, like C<$#array>. That's still how you get the last index
1937of an array in Perl. The two have nothing to do with each other.
1938
1939Deprecated in Perl 5.
1940
1941Removed in Perl 5.10.
1942
7f315d2e
CO
1943=item $*
1944X<$*>
1945
84dabc03 1946C<$*> was a variable that you could use to enable multiline matching.
7f315d2e
CO
1947After a deprecation cycle, its magic was removed in Perl 5.10.
1948Using it now triggers a warning: C<$* is no longer supported>.
84dabc03 1949You should use the C</s> and C</m> regexp modifiers instead.
7f315d2e 1950
b0c22438 1951Deprecated in Perl 5.
7f315d2e 1952
b0c22438 1953Removed in Perl 5.10.
7f315d2e 1954
5b442a2a 1955=item $ARRAY_BASE
1956
84dabc03 1957=item $[
5b442a2a 1958X<$[> X<$ARRAY_BASE>
84dabc03 1959
1960This variable stores the index of the first element in an array, and
1961of the first character in a substring. You use to be able to assign to
1962this variable, but you can't do that anymore. It's now always 0, like
0b9346e6 1963it should be.
84dabc03 1964
1965Mnemonic: [ begins subscripts.
1966
1967This variable is read-only.
1968
0b9346e6 1969Deprecated in Perl 5.12.
84dabc03 1970
5b442a2a 1971=item $OLD_PERL_VERSION
1972
b0c22438 1973=item $]
5b442a2a 1974X<$]> X<$OLD_PERL_VERSION>
55602bd2 1975
d4ba9bf2 1976See C<$^V> for a more modern representation of the Perl version that allows
1977accurate string comparisons.
1978
b0c22438 1979The version + patchlevel / 1000 of the Perl interpreter. This variable
1980can be used to determine whether the Perl interpreter executing a
1981script is in the right range of versions:
55602bd2 1982
b0c22438 1983 warn "No checksumming!\n" if $] < 3.019;
55602bd2 1984
d4ba9bf2 1985The floating point representation can sometimes lead to inaccurate
1986numeric comparisons.
1987
b0c22438 1988See also the documentation of C<use VERSION> and C<require VERSION>
1989for a convenient way to fail if the running Perl interpreter is too old.
55602bd2 1990
b0c22438 1991Mnemonic: Is this version of perl in the right bracket?
19799a22 1992
b0c22438 1993Deprecated in Perl 5.6.
19799a22 1994
b0c22438 1995=back
2b92dfce 1996
0b9346e6 1997=cut