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Move uid & gid variable docs next to each other
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1=head1 NAME
3perlvar - Perl predefined variables
b0c22438 7=head2 The Syntax of Variable Names
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>.
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<$^W> (dollar-sign caret
b0c22438 23C<W>) is the scalar variable whose name is the single character
0b9346e6 24control-C<W>. This is better than typing a literal control-C<W>
b0c22438 25into your program.
27Since Perl 5.6, Perl variable names may be alphanumeric
28strings that begin with control characters (or better yet, a caret).
29These variables must be written in the form C<${^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:
0b9346e6 44 ENV STDIN
b0c22438 48 SIG
69520822 50In particular, the special C<${^_XYZ}> variables are always taken
b0c22438 51to be in package C<main>, regardless of any C<package> declarations
52presently in scope.
a0d0e21e 55
0b9346e6 56The following names have special meaning to Perl. Most punctuation
57names have reasonable mnemonics, or analogs in the shells.
58Nevertheless, if you wish to use long variable names, you need only say:
a0d0e21e 59
0b9346e6 60 use English;
a0d0e21e 61
62at the top of your program. This aliases all the short names to the long
63names in the current package. Some even have medium names, generally
84dabc03 64borrowed from B<awk>. To avoid a performance hit, if you don't need the
65C<$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
81=item $ARG
83=item $_
a054c801 84X<$_> X<$ARG>
a0d0e21e 85
b0c22438 86The default input and pattern-searching space. The following pairs are
0b9346e6 89 while (<>) {...} # equivalent only in while!
90 while (defined($_ = <>)) {...}
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<$_> even if you don't use it:
103=over 3
105=item *
84dabc03 107The following functions use C<$_> as a default argument:
db1511c8 108
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.
115=item *
117All file tests (C<-f>, C<-d>) except for C<-t>, which defaults to STDIN.
118See L<perlfunc/-X>
120=item *
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 *
127The default iterator variable in a C<foreach> loop if no other
128variable is supplied.
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()>.
138=item *
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.
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
166=item $"
69520822 169When an array or an array slice is interpolated into a double-quoted
170string or a similar context such as C</.../>, its elements are
171separated by this value. Default is a space. For example, this:
0b9346e6 173 print "The array is: @array\n";
69520822 174
175is equivalent to this:
0b9346e6 177 print "The array is: " . join($", @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
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
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 $)
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
259=item $REAL_USER_ID
261=item $UID
263=item $<
264X<< $< >> X<$UID> X<$REAL_USER_ID>
266The real uid of this process. You can change both the real uid and the
267effective uid at the same time by using C<POSIX::setuid()>. Since
268changes to C<< $< >> require a system call, check C<$!> after a change
269attempt to detect any possible errors.
271Mnemonic: it's the uid you came I<from>, if you're running setuid.
275=item $EUID
277=item $>
280The effective uid of this process. For example:
282 $< = $>; # set real to effective uid
283 ($<,$>) = ($>,$<); # swap real and effective uids
285You can change both the effective uid and the real uid at the same
286time by using C<POSIX::setuid()>. Changes to C<< $> >> require a check
287to C<$!> to detect any possible errors after an attempted change.
289C<< $< >> and C<< $> >> can be swapped only on machines
290supporting C<setreuid()>.
292Mnemonic: it's the uid you went I<to>, if you're running setuid.
b0c22438 294=item $PROGRAM_NAME
a0d0e21e 295
b0c22438 296=item $0
a0d0e21e 298
b0c22438 299Contains the name of the program being executed.
a0d0e21e 300
69520822 301On some (but not all) operating systems assigning to C<$0> modifies
7333b1c4 302the argument area that the C<ps> program sees. On some platforms you
b0c22438 303may have to use special C<ps> options or a different C<ps> to see the
7333b1c4 304changes. Modifying the C<$0> is more useful as a way of indicating the
b0c22438 305current program state than it is for hiding the program you're
a0d0e21e 307
69520822 308Note that there are platform-specific limitations on the maximum
b0c22438 309length of C<$0>. In the most extreme case it may be limited to the
310space occupied by the original C<$0>.
fcc7d916 311
b0c22438 312In some platforms there may be arbitrary amount of padding, for
313example space characters, after the modified name as shown by C<ps>.
314In some platforms this padding may extend all the way to the original
315length of the argument area, no matter what you do (this is the case
316for example with Linux 2.2).
fcc7d916 317
b0c22438 318Note for BSD users: setting C<$0> does not completely remove "perl"
319from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
320result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
321and the " (perl)" suffix are shown depends on your exact BSD variant
322and version). This is an operating system feature, Perl cannot help it.
fcc7d916 323
b0c22438 324In multithreaded scripts Perl coordinates the threads so that any
325thread may modify its copy of the C<$0> and the change becomes visible
326to ps(1) (assuming the operating system plays along). Note that
327the view of C<$0> the other threads have will not change since they
328have their own copies of it.
fcc7d916 329
b0c22438 330If the program has been given to perl via the switches C<-e> or C<-E>,
331C<$0> will contain the string C<"-e">.
fcc7d916 332
b0c22438 333On Linux as of perl 5.14 the legacy process name will be set with
0b9346e6 334C<prctl(2)>, in addition to altering the POSIX name via C<argv[0]> as
b0c22438 335perl has done since version 4.000. Now system utilities that read the
336legacy process name such as ps, top and killall will recognize the
337name you set when assigning to C<$0>. The string you supply will be
338cut off at 16 bytes, this is a limitation imposed by Linux.
fcc7d916 339
b0c22438 340Mnemonic: same as B<sh> and B<ksh>.
0b9346e6 341
344=item $SUBSEP
346=item $;
349The subscript separator for multidimensional array emulation. If you
350refer to a hash element as
352 $foo{$a,$b,$c}
354it really means
356 $foo{join($;, $a, $b, $c)}
358But don't put
360 @foo{$a,$b,$c} # a slice--note the @
362which means
364 ($foo{$a},$foo{$b},$foo{$c})
7333b1c4 366Default is "\034", the same as SUBSEP in B<awk>. If your keys contain
0b9346e6 367binary data there might not be any safe value for C<$;>.
369Consider using "real" multidimensional arrays as described
370in L<perllol>.
372Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
0b9346e6 374=item $a
376=item $b
377X<$a> X<$b>
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.
b0c22438 385=item $COMPILING
a0d0e21e 386
b0c22438 387=item $^C
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
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.
417This variable was added in Perl 5.8.2.
419=item %ENV
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.
b0c22438 426=item $SYSTEM_FD_MAX
5b2b9c68 427
b0c22438 428=item $^F
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
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'>.
10c97e5d 448=item ${^GLOBAL_PHASE}
d30227f4 449X<${^GLOBAL_PHASE}>
10c97e5d 450
451The current phase of the perl interpreter.
bda934ba 453Possible values are:
10c97e5d 454
455=over 8
457=item CONSTRUCT
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
465=item START
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.
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}>.
478=item CHECK
480Execution of any C<CHECK> blocks.
482=item INIT
484Similar to "CHECK", but for C<INIT>-blocks, not C<CHECK> blocks.
486=item RUN
488The main run-time, i.e. the execution of C<PL_main_root>.
490=item END
492Execution of any C<END> blocks.
494=item DESTRUCT
496Global destruction.
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.
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.
508An example of all of the phases Perl code can see:
510 BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }
512 INIT { print "init-time: ${^GLOBAL_PHASE}\n" }
514 CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }
516 {
517 package Print::Phase;
519 sub new {
520 my ($class, $time) = @_;
521 return bless \$time, $class;
522 }
524 sub DESTROY {
525 my $self = shift;
526 print "$$self: ${^GLOBAL_PHASE}\n";
527 }
528 }
530 print "run-time: ${^GLOBAL_PHASE}\n";
532 my $runtime = Print::Phase->new(
533 "lexical variables are garbage collected before END"
534 );
536 END { print "end-time: ${^GLOBAL_PHASE}\n" }
538 our $destruct = Print::Phase->new(
539 "package variables are garbage collected after END"
540 );
542This will print out
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
606When putting items into C<%^H>, in order to avoid conflicting with other
607users of the hash there is a convention regarding which keys to use.
608A module should use only keys that begin with the module's name (the
609name of its main package) and a "/" character. For example, a module
610C<Foo::Bar> should use keys such as C<Foo::Bar/baz>.
b0c22438 612This variable was added in Perl 5.6.
a0d0e21e 613
0b9346e6 614=item @INC
617The array C<@INC> contains the list of places that the C<do EXPR>,
7333b1c4 618C<require>, or C<use> constructs look for their library files. It
0b9346e6 619initially consists of the arguments to any B<-I> command-line
620switches, followed by the default Perl library, probably
621F</usr/local/lib/perl>, followed by ".", to represent the current
7333b1c4 622directory. ("." will not be appended if taint checks are enabled,
0b9346e6 623either by C<-T> or by C<-t>.) If you need to modify this at runtime,
624you should use the C<use lib> pragma to get the machine-dependent
625library properly loaded also:
627 use lib '/mypath/libdir/';
628 use SomeMod;
630You can also insert hooks into the file inclusion system by putting Perl
631code directly into C<@INC>. Those hooks may be subroutine references, array
632references or blessed objects. See L<perlfunc/require> for details.
634=item %INC
637The hash C<%INC> contains entries for each filename included via the
638C<do>, C<require>, or C<use> operators. The key is the filename
639you specified (with module names converted to pathnames), and the
640value is the location of the file found. The C<require>
641operator uses this hash to determine whether a particular file has
642already been included.
644If the file was loaded via a hook (e.g. a subroutine reference, see
645L<perlfunc/require> for a description of these hooks), this hook is
646by default inserted into C<%INC> in place of a filename. Note, however,
647that the hook may have set the C<%INC> entry by itself to provide some more
648specific info.
b0c22438 650=item $INPLACE_EDIT
a0d0e21e 651
b0c22438 652=item $^I
a0d0e21e 654
b0c22438 655The current value of the inplace-edit extension. Use C<undef> to disable
656inplace editing.
a0d0e21e 657
b0c22438 658Mnemonic: value of B<-i> switch.
a0d0e21e 659
b0c22438 660=item $^M
a0d0e21e 662
b0c22438 663By default, running out of memory is an untrappable, fatal error.
664However, if suitably built, Perl can use the contents of C<$^M>
665as an emergency memory pool after C<die()>ing. Suppose that your Perl
666were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
a0d0e21e 668
0b9346e6 669 $^M = 'a' x (1 << 16);
a0d0e21e 670
b0c22438 671would allocate a 64K buffer for use in an emergency. See the
672F<INSTALL> file in the Perl distribution for information on how to
673add custom C compilation flags when compiling perl. To discourage casual
674use of this advanced feature, there is no L<English|English> long name for
675this variable.
a0d0e21e 676
b0c22438 677This variable was added in Perl 5.004.
a0d0e21e 678
b0c22438 679=item $OSNAME
a0d0e21e 680
b0c22438 681=item $^O
682X<$^O> X<$OSNAME>
a0d0e21e 683
b0c22438 684The name of the operating system under which this copy of Perl was
685built, as determined during the configuration process. For examples
686see L<perlport/PLATFORMS>.
a0d0e21e 687
b0c22438 688The value is identical to C<$Config{'osname'}>. See also L<Config>
689and the B<-V> command-line switch documented in L<perlrun>.
a0d0e21e 690
b0c22438 691In Windows platforms, C<$^O> is not very helpful: since it is always
692C<MSWin32>, it doesn't tell the difference between
69395/98/ME/NT/2000/XP/CE/.NET. Use C<Win32::GetOSName()> or
694Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
695between the variants.
a0d0e21e 696
b0c22438 697This variable was added in Perl 5.003.
a0d0e21e 698
b0c22438 699=item ${^OPEN}
5b442a2a 700X<${^OPEN}>
a0d0e21e 701
b0c22438 702An internal variable used by PerlIO. A string in two parts, separated
703by a C<\0> byte, the first part describes the input layers, the second
704part describes the output layers.
a0d0e21e 705
28051109 706This variable was added in Perl 5.8.0.
a0d0e21e 707
b0c22438 708=item $PERLDB
a0d0e21e 709
b0c22438 710=item $^P
711X<$^P> X<$PERLDB>
a0d0e21e 712
b0c22438 713The internal variable for debugging support. The meanings of the
714various bits are subject to change, but currently indicate:
a0d0e21e 715
b0c22438 716=over 6
a0d0e21e 717
b0c22438 718=item 0x01
a0d0e21e 719
b0c22438 720Debug subroutine enter/exit.
a0d0e21e 721
b0c22438 722=item 0x02
a0d0e21e 723
b0c22438 724Line-by-line debugging. Causes C<DB::DB()> subroutine to be called for each
725statement executed. Also causes saving source code lines (like 0x400).
a0d0e21e 726
b0c22438 727=item 0x04
fe307981 728
b0c22438 729Switch off optimizations.
6cef1e77 730
b0c22438 731=item 0x08
6cef1e77 732
b0c22438 733Preserve more data for future interactive inspections.
6cef1e77 734
b0c22438 735=item 0x10
4ba05bdc 736
b0c22438 737Keep info about source lines on which a subroutine is defined.
4ba05bdc 738
b0c22438 739=item 0x20
4ba05bdc 740
b0c22438 741Start with single-step on.
4ba05bdc 742
b0c22438 743=item 0x40
4ba05bdc 744
b0c22438 745Use subroutine address instead of name when reporting.
4ba05bdc 746
b0c22438 747=item 0x80
4ba05bdc 748
b0c22438 749Report C<goto &subroutine> as well.
4ba05bdc 750
b0c22438 751=item 0x100
4ba05bdc 752
b0c22438 753Provide informative "file" names for evals based on the place they were compiled.
4ba05bdc 754
b0c22438 755=item 0x200
44a2ac75 756
b0c22438 757Provide informative names to anonymous subroutines based on the place they
758were compiled.
44a2ac75 759
b0c22438 760=item 0x400
44a2ac75 761
b0c22438 762Save source code lines into C<@{"_<$filename"}>.
44a2ac75 763
b0c22438 764=back
44a2ac75 765
b0c22438 766Some bits may be relevant at compile-time only, some at
7333b1c4 767run-time only. This is a new mechanism and the details may change.
b0c22438 768See also L<perldebguts>.
3195cf34 769
b0c22438 770=item %SIG
b0c22438 771X<%SIG>
a0d0e21e 772
b0c22438 773The hash C<%SIG> contains signal handlers for signals. For example:
a0d0e21e 774
0b9346e6 775 sub handler { # 1st argument is signal name
776 my($sig) = @_;
777 print "Caught a SIG$sig--shutting down\n";
778 close(LOG);
779 exit(0);
780 }
a0d0e21e 781
0b9346e6 782 $SIG{'INT'} = \&handler;
783 $SIG{'QUIT'} = \&handler;
784 ...
785 $SIG{'INT'} = 'DEFAULT'; # restore default action
786 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
a0d0e21e 787
b0c22438 788Using a value of C<'IGNORE'> usually has the effect of ignoring the
789signal, except for the C<CHLD> signal. See L<perlipc> for more about
790this special case.
a0d0e21e 791
b0c22438 792Here are some other examples:
a0d0e21e 793
0b9346e6 794 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
795 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
796 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
797 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
a0d0e21e 798
b0c22438 799Be sure not to use a bareword as the name of a signal handler,
800lest you inadvertently call it.
a0d0e21e 801
b0c22438 802If your system has the C<sigaction()> function then signal handlers
803are installed using it. This means you get reliable signal handling.
7b8d334a 804
b0c22438 805The default delivery policy of signals changed in Perl 5.8.0 from
806immediate (also known as "unsafe") to deferred, also known as "safe
7333b1c4 807signals". See L<perlipc> for more information.
aa689395 808
b0c22438 809Certain internal hooks can be also set using the C<%SIG> hash. The
810routine indicated by C<$SIG{__WARN__}> is called when a warning
7333b1c4 811message is about to be printed. The warning message is passed as the
812first argument. The presence of a C<__WARN__> hook causes the
b0c22438 813ordinary printing of warnings to C<STDERR> to be suppressed. You can
814use this to save warnings in a variable, or turn warnings into fatal
815errors, like this:
19799a22 816
0b9346e6 817 local $SIG{__WARN__} = sub { die $_[0] };
818 eval $proggie;
a8f8344d 819
b0c22438 820As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
821disable warnings using the empty subroutine:
f86702cc 822
0b9346e6 823 local $SIG{__WARN__} = sub {};
55602bd2 824
b0c22438 825The routine indicated by C<$SIG{__DIE__}> is called when a fatal
826exception is about to be thrown. The error message is passed as the
827first argument. When a C<__DIE__> hook routine returns, the exception
828processing continues as it would have in the absence of the hook,
829unless the hook routine itself exits via a C<goto>, a loop exit, or a
830C<die()>. The C<__DIE__> handler is explicitly disabled during the
831call, so that you can die from a C<__DIE__> handler. Similarly for
e5218da5 833
b0c22438 834Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
835even inside an C<eval()>. Do not use this to rewrite a pending
836exception in C<$@>, or as a bizarre substitute for overriding
837C<CORE::GLOBAL::die()>. This strange action at a distance may be fixed
838in a future release so that C<$SIG{__DIE__}> is only called if your
839program is about to exit, as was the original intent. Any other use is
842C<__DIE__>/C<__WARN__> handlers are very special in one respect: they
843may be called to report (probable) errors found by the parser. In such
844a case the parser may be in inconsistent state, so any attempt to
845evaluate Perl code from such a handler will probably result in a
846segfault. This means that warnings or errors that result from parsing
847Perl should be used with extreme caution, like this:
e5218da5 848
0b9346e6 849 require Carp if defined $^S;
850 Carp::confess("Something wrong") if defined &Carp::confess;
851 die "Something wrong, but could not load Carp to give backtrace...
852 To see backtrace try starting Perl with -MCarp switch";
e5218da5 853
b0c22438 854Here the first line will load C<Carp> I<unless> it is the parser who
855called the handler. The second line will print backtrace and die if
856C<Carp> was available. The third line will be executed only if C<Carp> was
857not available.
0a378802 858
0b9346e6 859Having to even think about the C<$^S> variable in your exception
7333b1c4 860handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
0b9346e6 861invites grievous and difficult to track down errors. Avoid it
862and use an C<END{}> or CORE::GLOBAL::die override instead.
b0c22438 864See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
865L<warnings> for additional information.
0a378802 866
b0c22438 867=item $BASETIME
6ab308ee 868
b0c22438 869=item $^T
870X<$^T> X<$BASETIME>
6ab308ee 871
b0c22438 872The time at which the program began running, in seconds since the
873epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
874and B<-C> filetests are based on this value.
a0d0e21e 875
b0c22438 876=item ${^TAINT}
5b442a2a 877X<${^TAINT}>
55602bd2 878
b0c22438 879Reflects if taint mode is on or off. 1 for on (the program was run with
880B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
0b9346e6 881B<-t> or B<-TU>).
daaddde1 882
b0c22438 883This variable is read-only.
daaddde1 884
b0c22438 885This variable was added in Perl 5.8.
4c5cef9b 886
b0c22438 887=item ${^UNICODE}
5b442a2a 888X<${^UNICODE}>
4c5cef9b 889
7333b1c4 890Reflects certain Unicode settings of Perl. See L<perlrun>
b0c22438 891documentation for the C<-C> switch for more information about
0b9346e6 892the possible values.
5c055ba3 893
b0c22438 894This variable is set during Perl startup and is thereafter read-only.
5c055ba3 895
b0c22438 896This variable was added in Perl 5.8.2.
22fae026 897
b0c22438 898=item ${^UTF8CACHE}
5b442a2a 899X<${^UTF8CACHE}>
22fae026 900
b0c22438 901This variable controls the state of the internal UTF-8 offset caching code.
9021 for on (the default), 0 for off, -1 to debug the caching code by checking
903all its results against linear scans, and panicking on any discrepancy.
22fae026 904
b0c22438 905This variable was added in Perl 5.8.9.
22fae026 906
b0c22438 907=item ${^UTF8LOCALE}
5b442a2a 908X<${^UTF8LOCALE}>
5c055ba3 909
b0c22438 910This variable indicates whether a UTF-8 locale was detected by perl at
911startup. This information is used by perl when it's in
912adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
913switch); see L<perlrun> for more info on this.
55602bd2 914
b0c22438 915This variable was added in Perl 5.8.8.
a0d0e21e 916
b0c22438 917=item $PERL_VERSION
a0d0e21e 918
b0c22438 919=item $^V
a0d0e21e 921
b0c22438 922The revision, version, and subversion of the Perl interpreter,
923represented as a C<version> object.
748a9306 924
b0c22438 925This variable first appeared in perl 5.6.0; earlier versions of perl
926will see an undefined value. Before perl 5.10.0 C<$^V> was represented
927as a v-string.
55602bd2 928
b0c22438 929C<$^V> can be used to determine whether the Perl interpreter executing
930a script is in the right range of versions. For example:
a0d0e21e 931
0b9346e6 932 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
a0d0e21e 933
b0c22438 934To convert C<$^V> into its string representation use C<sprintf()>'s
935C<"%vd"> conversion:
a0d0e21e 936
0b9346e6 937 printf "version is v%vd\n", $^V; # Perl's version
a0d0e21e 938
b0c22438 939See the documentation of C<use VERSION> and C<require VERSION>
940for a convenient way to fail if the running Perl interpreter is too old.
4d76a344 941
b0c22438 942See also C<$]> for an older representation of the Perl version.
a0d0e21e 943
b0c22438 944This variable was added in Perl 5.6.
a0d0e21e 945
b0c22438 946Mnemonic: use ^V for Version Control.
a0d0e21e 947
b0c22438 948=item ${^WIN32_SLOPPY_STAT}
5b442a2a 949X<${^WIN32_SLOPPY_STAT}> X<sitecustomize> X<>
a0d0e21e 950
b0c22438 951If this variable is set to a true value, then C<stat()> on Windows will
952not try to open the file. This means that the link count cannot be
953determined and file attributes may be out of date if additional
954hardlinks to the file exist. On the other hand, not opening the file
955is considerably faster, especially for files on network drives.
a0d0e21e 956
b0c22438 957This variable could be set in the F<> file to
958configure the local Perl installation to use "sloppy" C<stat()> by
959default. See the documentation for B<-f> in
960L<perlrun|perlrun/"Command Switches"> for more information about site
a0d0e21e 962
b0c22438 963This variable was added in Perl 5.10.
a0d0e21e 964
b0c22438 965=item $EXECUTABLE_NAME
a0d0e21e 966
b0c22438 967=item $^X
a0d0e21e 969
b0c22438 970The name used to execute the current copy of Perl, from C's
971C<argv[0]> or (where supported) F</proc/self/exe>.
a043a685 972
b0c22438 973Depending on the host operating system, the value of C<$^X> may be
974a relative or absolute pathname of the perl program file, or may
975be the string used to invoke perl but not the pathname of the
976perl program file. Also, most operating systems permit invoking
977programs that are not in the PATH environment variable, so there
978is no guarantee that the value of C<$^X> is in PATH. For VMS, the
979value may or may not include a version number.
a0d0e21e 980
b0c22438 981You usually can use the value of C<$^X> to re-invoke an independent
982copy of the same perl that is currently running, e.g.,
a0d0e21e 983
0b9346e6 984 @first_run = `$^X -le "print int rand 100 for 1..100"`;
a0d0e21e 985
b0c22438 986But recall that not all operating systems support forking or
987capturing of the output of commands, so this complex statement
988may not be portable.
a0d0e21e 989
b0c22438 990It is not safe to use the value of C<$^X> as a path name of a file,
991as some operating systems that have a mandatory suffix on
992executable files do not require use of the suffix when invoking
993a command. To convert the value of C<$^X> to a path name, use the
994following statements:
8cc95fdb 995
0b9346e6 996 # Build up a set of file names (not command names).
997 use Config;
998 my $this_perl = $^X;
999 if ($^O ne 'VMS') {
1000 $this_perl .= $Config{_exe}
1001 unless $this_perl =~ m/$Config{_exe}$/i;
1002 }
8cc95fdb 1003
b0c22438 1004Because many operating systems permit anyone with read access to
1005the Perl program file to make a copy of it, patch the copy, and
1006then execute the copy, the security-conscious Perl programmer
1007should take care to invoke the installed copy of perl, not the
1008copy referenced by C<$^X>. The following statements accomplish
1009this goal, and produce a pathname that can be invoked as a
1010command or referenced as a file.
a043a685 1011
0b9346e6 1012 use Config;
1013 my $secure_perl_path = $Config{perlpath};
1014 if ($^O ne 'VMS') {
1015 $secure_perl_path .= $Config{_exe}
1016 unless $secure_perl_path =~ m/$Config{_exe}$/i;
1017 }
a0d0e21e 1018
b0c22438 1019=back
a0d0e21e 1020
b0c22438 1021=head2 Variables related to regular expressions
1023Most of the special variables related to regular expressions are side
1024effects. Perl sets these variables when it has a successful match, so
1025you should check the match result before using them. For instance:
1027 if( /P(A)TT(ER)N/ ) {
1028 print "I found $1 and $2\n";
1029 }
0b9346e6 1031These variables are read-only and dynamically-scoped, unless we note
b0c22438 1032otherwise.
0b9346e6 1034The dynamic nature of the regular expression variables means that
1035their value is limited to the block that they are in, as demonstrated
1036by this bit of code:
b0c22438 1037
1038 my $outer = 'Wallace and Grommit';
1039 my $inner = 'Mutt and Jeff';
0b9346e6 1040
b0c22438 1041 my $pattern = qr/(\S+) and (\S+)/;
0b9346e6 1042
b0c22438 1043 sub show_n { print "\$1 is $1; \$2 is $2\n" }
0b9346e6 1044
b0c22438 1045 {
1046 OUTER:
1047 show_n() if $outer =~ m/$pattern/;
0b9346e6 1048
b0c22438 1049 INNER: {
1050 show_n() if $inner =~ m/$pattern/;
1051 }
0b9346e6 1052
b0c22438 1053 show_n();
1054 }
0b9346e6 1056The output shows that while in the C<OUTER> block, the values of C<$1>
1057and C<$2> are from the match against C<$outer>. Inside the C<INNER>
1058block, the values of C<$1> and C<$2> are from the match against
1059C<$inner>, but only until the end of the block (i.e. the dynamic
1060scope). After the C<INNER> block completes, the values of C<$1> and
1061C<$2> return to the values for the match against C<$outer> even though
b0c22438 1062we have not made another match:
1064 $1 is Wallace; $2 is Grommit
1065 $1 is Mutt; $2 is Jeff
1066 $1 is Wallace; $2 is Grommit
a0d0e21e 1067
0b9346e6 1068Due to an unfortunate accident of Perl's implementation, C<use
1069English> imposes a considerable performance penalty on all regular
1070expression matches in a program because it uses the C<$`>, C<$&>, and
1071C<$'>, regardless of whether they occur in the scope of C<use
1072English>. For that reason, saying C<use English> in libraries is
1073strongly discouraged unless you import it without the match variables:
1075 use English '-no_match_vars'
1077The C<Devel::NYTProf> and C<Devel::FindAmpersand>
1078modules can help you find uses of these
0b9346e6 1079problematic match variables in your code.
1081Since Perl 5.10, you can use the C</p> match operator flag and the
1082C<${^PREMATCH}>, C<${^MATCH}>, and C<${^POSTMATCH}> variables instead
1083so you only suffer the performance penalties.
b0c22438 1085=over 8
a0d0e21e 1086
b0c22438 1087=item $<I<digits>> ($1, $2, ...)
1088X<$1> X<$2> X<$3>
8cc95fdb 1089
b0c22438 1090Contains the subpattern from the corresponding set of capturing
1091parentheses from the last successful pattern match, not counting patterns
1092matched in nested blocks that have been exited already.
8cc95fdb 1093
b0c22438 1094These variables are read-only and dynamically-scoped.
a043a685 1095
b0c22438 1096Mnemonic: like \digits.
a0d0e21e 1097
b0c22438 1098=item $MATCH
a0d0e21e 1099
b0c22438 1100=item $&
1101X<$&> X<$MATCH>
a0d0e21e 1102
b0c22438 1103The string matched by the last successful pattern match (not counting
1104any matches hidden within a BLOCK or C<eval()> enclosed by the current
a0d0e21e 1106
b0c22438 1107The use of this variable anywhere in a program imposes a considerable
0b9346e6 1108performance penalty on all regular expression matches. To avoid this
1109penalty, you can extract the same substring by using L</@->. Starting
1110with Perl 5.10, you can use the </p> match flag and the C<${^MATCH}>
1111variable to do the same thing for particular match operations.
80bca1b4 1112
b0c22438 1113This variable is read-only and dynamically-scoped.
f9cbb277 1114
b0c22438 1115Mnemonic: like C<&> in some editors.
0b9346e6 1116
b0c22438 1117=item ${^MATCH}
a0d0e21e 1119
b0c22438 1120This is similar to C<$&> (C<$MATCH>) except that it does not incur the
1121performance penalty associated with that variable, and is only guaranteed
1122to return a defined value when the pattern was compiled or executed with
1123the C</p> modifier.
80bca1b4 1124
b0c22438 1125This variable was added in Perl 5.10.
4bc88a62 1126
b0c22438 1127This variable is read-only and dynamically-scoped.
e2975953 1128
b0c22438 1129=item $PREMATCH
52c447a8 1130
b0c22438 1131=item $`
5b442a2a 1132X<$`> X<$PREMATCH> X<${^PREMATCH}>
7636ea95 1133
b0c22438 1134The string preceding whatever was matched by the last successful
1135pattern match, not counting any matches hidden within a BLOCK or C<eval>
0b9346e6 1136enclosed by the current BLOCK.
a0d0e21e 1137
b0c22438 1138The use of this variable anywhere in a program imposes a considerable
0b9346e6 1139performance penalty on all regular expression matches. To avoid this
1140penalty, you can extract the same substring by using L</@->. Starting
1141with Perl 5.10, you can use the </p> match flag and the
1142C<${^PREMATCH}> variable to do the same thing for particular match
a0d0e21e 1144
b0c22438 1145This variable is read-only and dynamically-scoped.
a0d0e21e 1146
b0c22438 1147Mnemonic: C<`> often precedes a quoted string.
f83ed198 1148
b0c22438 1149=item ${^PREMATCH}
5b442a2a 1150X<$`> X<${^PREMATCH}>
a0d0e21e 1151
b0c22438 1152This is similar to C<$`> ($PREMATCH) except that it does not incur the
1153performance penalty associated with that variable, and is only guaranteed
1154to return a defined value when the pattern was compiled or executed with
1155the C</p> modifier.
a0d0e21e 1156
b0c22438 1157This variable was added in Perl 5.10
a0d0e21e 1158
b0c22438 1159This variable is read-only and dynamically-scoped.
a0d0e21e 1160
b0c22438 1161=item $POSTMATCH
16070b82 1162
b0c22438 1163=item $'
5b442a2a 1164X<$'> X<$POSTMATCH> X<${^POSTMATCH}> X<@->
305aace0 1165
b0c22438 1166The string following whatever was matched by the last successful
1167pattern match (not counting any matches hidden within a BLOCK or C<eval()>
1168enclosed by the current BLOCK). Example:
305aace0 1169
0b9346e6 1170 local $_ = 'abcdefghi';
1171 /def/;
1172 print "$`:$&:$'\n"; # prints abc:def:ghi
305aace0 1173
b0c22438 1174The use of this variable anywhere in a program imposes a considerable
0b9346e6 1175performance penalty on all regular expression matches.
1176To avoid this penalty, you can extract the same substring by
b0c22438 1177using L</@->. Starting with Perl 5.10, you can use the </p> match flag
0b9346e6 1178and the C<${^POSTMATCH}> variable to do the same thing for particular
b0c22438 1179match operations.
a0d0e21e 1180
b0c22438 1181This variable is read-only and dynamically-scoped.
1183Mnemonic: C<'> often follows a quoted string.
1185=item ${^POSTMATCH}
5b442a2a 1186X<${^POSTMATCH}> X<$'> X<$POSTMATCH>
b0c22438 1187
1188This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
1189performance penalty associated with that variable, and is only guaranteed
1190to return a defined value when the pattern was compiled or executed with
1191the C</p> modifier.
1193This variable was added in Perl 5.10.
1195This variable is read-only and dynamically-scoped.
1199=item $+
1202The text matched by the last bracket of the last successful search pattern.
1203This is useful if you don't know which one of a set of alternative patterns
1204matched. For example:
0b9346e6 1206 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
b0c22438 1207
1208This variable is read-only and dynamically-scoped.
1210Mnemonic: be positive and forward looking.
1214=item $^N
5b442a2a 1215X<$^N> X<$LAST_SUBMATCH_RESULT>
b0c22438 1216
1217The text matched by the used group most-recently closed (i.e. the group
1218with the rightmost closing parenthesis) of the last successful search
1221This is primarily used inside C<(?{...})> blocks for examining text
1222recently matched. For example, to effectively capture text to a variable
1223(in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
0b9346e6 1225 (?:(...)(?{ $var = $^N }))
b0c22438 1226
1227By setting and then using C<$var> in this way relieves you from having to
1228worry about exactly which numbered set of parentheses they are.
1230This variable was added in Perl 5.8.
1232Mnemonic: the (possibly) Nested parenthesis that most recently closed.
1234=item @LAST_MATCH_END
1236=item @+
1237X<@+> X<@LAST_MATCH_END>
1239This array holds the offsets of the ends of the last successful
1240submatches in the currently active dynamic scope. C<$+[0]> is
1241the offset into the string of the end of the entire match. This
1242is the same value as what the C<pos> function returns when called
1243on the variable that was matched against. The I<n>th element
1244of this array holds the offset of the I<n>th submatch, so
1245C<$+[1]> is the offset past where C<$1> ends, C<$+[2]> the offset
7333b1c4 1246past where C<$2> ends, and so on. You can use C<$#+> to determine
b0c22438 1247how many subgroups were in the last successful match. See the
1248examples given for the C<@-> variable.
1250This variable was added in Perl 5.6.
1254=item %+
5b442a2a 1255X<%+> X<%LAST_PAREN_MATCH>
b0c22438 1256
1257Similar to C<@+>, the C<%+> hash allows access to the named capture
1258buffers, should they exist, in the last successful match in the
1259currently active dynamic scope.
1261For example, C<$+{foo}> is equivalent to C<$1> after the following match:
0b9346e6 1263 'foo' =~ /(?<foo>foo)/;
b0c22438 1264
1265The keys of the C<%+> hash list only the names of buffers that have
1266captured (and that are thus associated to defined values).
1268The underlying behaviour of C<%+> is provided by the
1269L<Tie::Hash::NamedCapture> module.
1271B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1272associated with the last successful regular expression. Therefore mixing
1273iterative access to them via C<each> may have unpredictable results.
1274Likewise, if the last successful match changes, then the results may be
1277This variable was added in Perl 5.10.
a0d0e21e 1278
b0c22438 1279This variable is read-only and dynamically-scoped.
1283=item @-
1286C<$-[0]> is the offset of the start of the last successful match.
1287C<$-[>I<n>C<]> is the offset of the start of the substring matched by
1288I<n>-th subpattern, or undef if the subpattern did not match.
1290Thus, after a match against C<$_>, C<$&> coincides with C<substr $_, $-[0],
1291$+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
1292$+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
1293C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the last
1294matched subgroup in the last successful match. Contrast with
1295C<$#+>, the number of subgroups in the regular expression. Compare
1296with C<@+>.
1298This array holds the offsets of the beginnings of the last
1299successful submatches in the currently active dynamic scope.
1300C<$-[0]> is the offset into the string of the beginning of the
7333b1c4 1301entire match. The I<n>th element of this array holds the offset
b0c22438 1302of the I<n>th submatch, so C<$-[1]> is the offset where C<$1>
1303begins, C<$-[2]> the offset where C<$2> begins, and so on.
1305After a match against some variable C<$var>:
1307=over 5
1309=item C<$`> is the same as C<substr($var, 0, $-[0])>
1311=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
1313=item C<$'> is the same as C<substr($var, $+[0])>
1315=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
1317=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
1319=item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
1323This variable was added in Perl 5.6.
5b442a2a 1325=item %LAST_MATCH_START
b0c22438 1327=item %-
5b442a2a 1328X<%-> X<%LAST_MATCH_START>
b0c22438 1329
1330Similar to C<%+>, this variable allows access to the named capture groups
1331in the last successful match in the currently active dynamic scope. To
1332each capture group name found in the regular expression, it associates a
1333reference to an array containing the list of values captured by all
1334buffers with that name (should there be several of them), in the order
1335where they appear.
1337Here's an example:
1339 if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
1340 foreach my $bufname (sort keys %-) {
1341 my $ary = $-{$bufname};
1342 foreach my $idx (0..$#$ary) {
1343 print "\$-{$bufname}[$idx] : ",
1344 (defined($ary->[$idx]) ? "'$ary->[$idx]'" : "undef"),
1345 "\n";
1346 }
1347 }
1348 }
1350would print out:
0b9346e6 1352 $-{A}[0] : '1'
1353 $-{A}[1] : '3'
1354 $-{B}[0] : '2'
1355 $-{B}[1] : '4'
b0c22438 1356
1357The keys of the C<%-> hash correspond to all buffer names found in
1358the regular expression.
1360The behaviour of C<%-> is implemented via the
1361L<Tie::Hash::NamedCapture> module.
1363B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1364associated with the last successful regular expression. Therefore mixing
1365iterative access to them via C<each> may have unpredictable results.
1366Likewise, if the last successful match changes, then the results may be
1369This variable was added in Perl 5.10
1371This variable is read-only and dynamically-scoped.
1375=item $^R
1378The result of evaluation of the last successful C<(?{ code })>
1379regular expression assertion (see L<perlre>). May be written to.
1381This variable was added in Perl 5.005.
a0d0e21e 1382
a3621e74 1383=item ${^RE_DEBUG_FLAGS}
ca1b95ae 1384X<${^RE_DEBUG_FLAGS}>
1386The current value of the regex debugging flags. Set to 0 for no debug output
b0c22438 1387even when the C<re 'debug'> module is loaded. See L<re> for details.
1389This variable was added in Perl 5.10.
a3621e74 1390
0111c4fd 1391=item ${^RE_TRIE_MAXBUF}
ca1b95ae 1392X<${^RE_TRIE_MAXBUF}>
1394Controls how certain regex optimisations are applied and how much memory they
1395utilize. This value by default is 65536 which corresponds to a 512kB temporary
1396cache. Set this to a higher value to trade memory for speed when matching
1397large alternations. Set it to a lower value if you want the optimisations to
1398be as conservative of memory as possible but still occur, and set it to a
1399negative value to prevent the optimisation and conserve the most memory.
1400Under normal situations this variable should be of no interest to you.
b0c22438 1402This variable was added in Perl 5.10.
a0d0e21e 1403
b0c22438 1404=back
a0d0e21e 1405
b0c22438 1406=head2 Variables related to filehandles
a0d0e21e 1407
b0c22438 1408Variables that depend on the currently selected filehandle may be set
1409by calling an appropriate object method on the C<IO::Handle> object,
1410although this is less efficient than using the regular built-in
1411variables. (Summary lines below for this contain the word HANDLE.)
1412First you must say
6e2995f4 1413
0b9346e6 1414 use IO::Handle;
0462a1ab 1415
b0c22438 1416after which you may use either
0462a1ab 1417
0b9346e6 1418 method HANDLE EXPR
0462a1ab 1419
b0c22438 1420or more safely,
0462a1ab 1421
0b9346e6 1422 HANDLE->method(EXPR)
0462a1ab 1423
b0c22438 1424Each method returns the old value of the C<IO::Handle> attribute. The
1425methods each take an optional EXPR, which, if supplied, specifies the
1426new value for the C<IO::Handle> attribute in question. If not
1427supplied, most methods do nothing to the current value--except for
1428C<autoflush()>, which will assume a 1 for you, just to be different.
0462a1ab 1429
b0c22438 1430Because loading in the C<IO::Handle> class is an expensive operation,
1431you should learn how to use the regular built-in variables.
1433A few of these variables are considered "read-only". This means that
1434if you try to assign to this variable, either directly or indirectly
1435through a reference, you'll raise a run-time exception.
1437You should be very careful when modifying the default values of most
1438special variables described in this document. In most cases you want
1439to localize these variables before changing them, since if you don't,
1440the change may affect other modules which rely on the default values
1441of the special variables that you have changed. This is one of the
1442correct ways to read the whole file at once:
0b9346e6 1444 open my $fh, "<", "foo" or die $!;
1445 local $/; # enable localized slurp mode
1446 my $content = <$fh>;
1447 close $fh;
b0c22438 1448
1449But the following code is quite bad:
0b9346e6 1451 open my $fh, "<", "foo" or die $!;
1452 undef $/; # enable slurp mode
1453 my $content = <$fh>;
1454 close $fh;
b0c22438 1455
1456since some other module, may want to read data from some file in the
1457default "line mode", so if the code we have just presented has been
1458executed, the global value of C<$/> is now changed for any other code
1459running inside the same Perl interpreter.
1461Usually when a variable is localized you want to make sure that this
1462change affects the shortest scope possible. So unless you are already
1463inside some short C<{}> block, you should create one yourself. For
0b9346e6 1466 my $content = '';
1467 open my $fh, "<", "foo" or die $!;
1468 {
1469 local $/;
1470 $content = <$fh>;
1471 }
1472 close $fh;
0462a1ab 1473
b0c22438 1474Here is an example of how your own code can go broken:
0462a1ab 1475
0b9346e6 1476 for ( 1..3 ){
1477 $\ = "\r\n";
1478 nasty_break();
1479 print "$_";
1480 }
1482 sub nasty_break {
1483 $\ = "\f";
1484 # do something with $_
1485 }
0462a1ab 1486
0b9346e6 1487You probably expect this code to print the equivalent of
0462a1ab 1488
0b9346e6 1489 "1\r\n2\r\n3\r\n"
0462a1ab 1490
b0c22438 1491but instead you get:
0462a1ab 1492
0b9346e6 1493 "1\f2\f3\f"
0462a1ab 1494
0b9346e6 1495Why? Because C<nasty_break()> modifies C<$\> without localizing it
1496first. The value you set in C<nasty_break()> is still there when you
1497return. The fix is to add C<local()> so the value doesn't leak out of
6e2995f4 1499
0b9346e6 1500 local $\ = "\f";
a0d0e21e 1501
b0c22438 1502It's easy to notice the problem in such a short example, but in more
1503complicated code you are looking for trouble if you don't localize
1504changes to the special variables.
a0d0e21e 1505
b0c22438 1506=over 8
a0d0e21e 1507
b0c22438 1508=item $ARGV
fb73857a 1510
ca1b95ae 1511Contains the name of the current file when reading from C<< <> >>.
b0c22438 1512
1513=item @ARGV
ca1b95ae 1516The array C<@ARGV> contains the command-line arguments intended for
b0c22438 1517the script. C<$#ARGV> is generally the number of arguments minus
1518one, because C<$ARGV[0]> is the first argument, I<not> the program's
57f6eff5 1519command name itself. See L</$0> for the command name.
b0c22438 1520
84dabc03 1521=item ARGV
1524The special filehandle that iterates over command-line filenames in
1525C<@ARGV>. Usually written as the null filehandle in the angle operator
1526C<< <> >>. Note that currently C<ARGV> only has its magical effect
1527within the C<< <> >> operator; elsewhere it is just a plain filehandle
1528corresponding to the last file opened by C<< <> >>. In particular,
1529passing C<\*ARGV> as a parameter to a function that expects a filehandle
1530may not cause your function to automatically read the contents of all the
1531files in C<@ARGV>.
b0c22438 1533=item ARGVOUT
1536The special filehandle that points to the currently open output file
1537when doing edit-in-place processing with B<-i>. Useful when you have
1538to do a lot of inserting and don't want to keep modifying C<$_>. See
1539L<perlrun> for the B<-i> switch.
5b442a2a 1541=item Handle->output_field_separator( EXPR )
84dabc03 1542
1545=item $OFS
1547=item $,
1550The output field separator for the print operator. If defined, this
1551value is printed between each of print's arguments. Default is C<undef>.
1553Mnemonic: what is printed when there is a "," in your print statement.
5b442a2a 1555=item HANDLE->input_line_number( EXPR )
b0c22438 1556
1559=item $NR
1561=item $.
1562X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
1564Current line number for the last filehandle accessed.
1566Each filehandle in Perl counts the number of lines that have been read
7333b1c4 1567from it. (Depending on the value of C<$/>, Perl's idea of what
b0c22438 1568constitutes a line may not match yours.) When a line is read from a
1569filehandle (via C<readline()> or C<< <> >>), or when C<tell()> or
1570C<seek()> is called on it, C<$.> becomes an alias to the line counter
1571for that filehandle.
1573You can adjust the counter by assigning to C<$.>, but this will not
1574actually move the seek pointer. I<Localizing C<$.> will not localize
1575the filehandle's line count>. Instead, it will localize perl's notion
1576of which filehandle C<$.> is currently aliased to.
1578C<$.> is reset when the filehandle is closed, but B<not> when an open
1579filehandle is reopened without an intervening C<close()>. For more
1580details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
1581an explicit close, line numbers increase across C<ARGV> files (but see
1582examples in L<perlfunc/eof>).
1584You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
1585line counter for a given filehandle without having to worry about
1586which handle you last accessed.
1588Mnemonic: many programs use "." to mean the current line number.
5b442a2a 1590=item HANDLE->input_record_separator( EXPR )
b0c22438 1591
1594=item $RS
1596=item $/
84dabc03 1599The input record separator, newline by default. This influences Perl's
7333b1c4 1600idea of what a "line" is. Works like B<awk>'s RS variable, including
84dabc03 1601treating empty lines as a terminator if set to the null string (an
1602empty line cannot contain any spaces or tabs). You may set it to a
1603multi-character string to match a multi-character terminator, or to
1604C<undef> to read through the end of file. Setting it to C<"\n\n">
1605means something slightly different than setting to C<"">, if the file
1606contains consecutive empty lines. Setting to C<""> will treat two or
1607more consecutive empty lines as a single empty line. Setting to
1608C<"\n\n"> will blindly assume that the next input character belongs to
1609the next paragraph, even if it's a newline.
b0c22438 1610
1611 local $/; # enable "slurp" mode
1612 local $_ = <FH>; # whole file now here
1613 s/\n[ \t]+/ /g;
7333b1c4 1615Remember: the value of C<$/> is a string, not a regex. B<awk> has to
b0c22438 1616be better for something. :-)
1618Setting C<$/> to a reference to an integer, scalar containing an
1619integer, or scalar that's convertible to an integer will attempt to
1620read records instead of lines, with the maximum record size being the
1621referenced integer. So this:
1623 local $/ = \32768; # or \"32768", or \$var_containing_32768
1624 open my $fh, "<", $myfile or die $!;
1625 local $_ = <$fh>;
fb73857a 1626
7333b1c4 1627will read a record of no more than 32768 bytes from FILE. If you're
b0c22438 1628not reading from a record-oriented file (or your OS doesn't have
1629record-oriented files), then you'll likely get a full chunk of data
7333b1c4 1630with every read. If a record is larger than the record size you've
1631set, you'll get the record back in pieces. Trying to set the record
b0c22438 1632size to zero or less will cause reading in the (rest of the) whole file.
6e2995f4 1633
b0c22438 1634On VMS, record reads are done with the equivalent of C<sysread>,
1635so it's best not to mix record and non-record reads on the same
5b442a2a 1636file. (This is unlikely to be a problem, because any file you'd
b0c22438 1637want to read in record mode is probably unusable in line mode.)
1638Non-VMS systems do normal I/O, so it's safe to mix record and
1639non-record reads of a file.
5c055ba3 1640
57f6eff5 1641See also L<perlport/"Newlines">. Also see L</$.>.
9bf22702 1642
b0c22438 1643Mnemonic: / delimits line boundaries when quoting poetry.
5c055ba3 1644
5b442a2a 1645=item Handle->output_record_separator( EXPR )
84902520 1646
b0c22438 1647=item $OUTPUT_RECORD_SEPARATOR
84902520 1648
b0c22438 1649=item $ORS
84902520 1650
b0c22438 1651=item $\
84902520 1653
b0c22438 1654The output record separator for the print operator. If defined, this
1655value is printed after the last of print's arguments. Default is C<undef>.
84902520 1656
b0c22438 1657Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
1658Also, it's just like C<$/>, but it's what you get "back" from Perl.
84902520 1659
5b442a2a 1660=item HANDLE->autoflush( EXPR )
84dabc03 1664=item $|
1665X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
84902520 1666
84dabc03 1667If set to nonzero, forces a flush right away and after every write or
7333b1c4 1668print on the currently selected output channel. Default is 0
84dabc03 1669(regardless of whether the channel is really buffered by the system or
1670not; C<$|> tells you only whether you've asked Perl explicitly to
1671flush after each write). STDOUT will typically be line buffered if
5b442a2a 1672output is to the terminal and block buffered otherwise. Setting this
84dabc03 1673variable is useful primarily when you are outputting to a pipe or
1674socket, such as when you are running a Perl program under B<rsh> and
5b442a2a 1675want to see the output as it's happening. This has no effect on input
c003e62a 1676buffering. See L<perlfunc/getc> for that. See L<perlfunc/select> on
84dabc03 1677how to select the output channel. See also L<IO::Handle>.
1679Mnemonic: when you want your pipes to be piping hot.
84902520 1682
b0c22438 1683=head3 Variables related to formats
83ee9e09 1684
b0c22438 1685The special variables for formats are a subset of those for
69b55ccc 1686filehandles. See L<perlform> for more information about Perl's
83ee9e09 1688
b0c22438 1689=over 8
83ee9e09 1690
84dabc03 1691=item $ACCUMULATOR
1693=item $^A
1696The current value of the C<write()> accumulator for C<format()> lines.
1697A format contains C<formline()> calls that put their result into
7333b1c4 1698C<$^A>. After calling its format, C<write()> prints out the contents
84dabc03 1699of C<$^A> and empties. So you never really see the contents of C<$^A>
1700unless you call C<formline()> yourself and then look at it. See
96090e4f 1701L<perlform> and L<perlfunc/"formline PICTURE,LIST">.
84dabc03 1702
5b442a2a 1703=item HANDLE->format_formfeed(EXPR)
84dabc03 1707=item $^L
1710What formats output as a form feed. The default is C<\f>.
b0c22438 1712=item HANDLE->format_page_number(EXPR)
83ee9e09 1713
b0c22438 1714=item $FORMAT_PAGE_NUMBER
83ee9e09 1715
b0c22438 1716=item $%
83ee9e09 1718
b0c22438 1719The current page number of the currently selected output channel.
83ee9e09 1720
b0c22438 1721Mnemonic: C<%> is page number in B<nroff>.
7619c85e 1722
b0c22438 1723=item HANDLE->format_lines_left(EXPR)
b9ac3b5b 1724
b0c22438 1725=item $FORMAT_LINES_LEFT
66558a10 1726
b0c22438 1727=item $-
fb73857a 1729
b0c22438 1730The number of lines left on the page of the currently selected output
fa05a9fd 1732
b0c22438 1733Mnemonic: lines_on_page - lines_printed.
fa05a9fd 1734
84dabc03 1735=item Handle->format_line_break_characters EXPR
fb73857a 1736
a0d0e21e 1738
84dabc03 1739=item $:
a0d0e21e 1741
84dabc03 1742The current set of characters after which a string may be broken to
1743fill continuation fields (starting with C<^>) in a format. The default is
1744S<" \n-">, to break on a space, newline, or a hyphen.
a0d0e21e 1745
84dabc03 1746Mnemonic: a "colon" in poetry is a part of a line.
1748=item HANDLE->format_lines_per_page(EXPR)
1752=item $=
1755The current page length (printable lines) of the currently selected
1756output channel. The default is 60.
1758Mnemonic: = has horizontal lines.
7c36658b 1759
b0c22438 1760=item HANDLE->format_top_name(EXPR)
7c36658b 1761
b0c22438 1762=item $FORMAT_TOP_NAME
a05d7ebb 1763
b0c22438 1764=item $^
fde18df1 1766
b0c22438 1767The name of the current top-of-page format for the currently selected
1768output channel. The default is the name of the filehandle with C<_TOP>
1769appended. For example, the default format top name for the C<STDOUT>
1770filehanlde is C<STDOUT_TOP>.
e07ea26a 1771
b0c22438 1772Mnemonic: points to top of page.
e07ea26a 1773
84dabc03 1774=item HANDLE->format_name(EXPR)
16070b82 1775
84dabc03 1776=item $FORMAT_NAME
aa2f2a36 1777
84dabc03 1778=item $~
1779X<$~> X<$FORMAT_NAME>
aa2f2a36 1780
84dabc03 1781The name of the current report format for the currently selected
1782output channel. The default format name is the same as the filehandle
1783name. For example, the default format name for the C<STDOUT>
1784filehandle is just C<STDOUT>.
16070b82 1785
84dabc03 1786Mnemonic: brother to C<$^>.
16070b82 1787
b0c22438 1788=back
a0d0e21e 1789
84dabc03 1790=head2 Error Variables
b0c22438 1791X<error> X<exception>
a0d0e21e 1792
b0c22438 1793The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1794about different types of error conditions that may appear during
1795execution of a Perl program. The variables are shown ordered by
1796the "distance" between the subsystem which reported the error and
1797the Perl process. They correspond to errors detected by the Perl
1798interpreter, C library, operating system, or an external program,
4438c4b7 1800
b0c22438 1801To illustrate the differences between these variables, consider the
7fd683ff 1802following Perl expression, which uses a single-quoted string. After
1803execution of this statement, perl may have set all four special error
7333b1c4 1804variables:
4438c4b7 1805
ca1b95ae 1806 eval q{
7333b1c4 1807 open my $pipe, "/cdrom/install |" or die $!;
1808 my @res = <$pipe>;
1809 close $pipe or die "bad pipe: $?, $!";
1810 };
a0d0e21e 1811
7333b1c4 1812When perl executes the C<eval()> expression, it translates the
1813C<open()>, C<< <PIPE> >>, and C<close> calls in the C run-time library
69b55ccc 1814and thence to the operating system kernel. perl sets C<$!> to
7333b1c4 1815the C library's C<errno> if one of these calls fails.
2a8c8378 1816
84dabc03 1817C<$@> is set if the string to be C<eval>-ed did not compile (this may
1818happen if C<open> or C<close> were imported with bad prototypes), or
7333b1c4 1819if Perl code executed during evaluation C<die()>d. In these cases the
0b9346e6 1820value of C<$@> is the compile error, or the argument to C<die> (which
84dabc03 1821will interpolate C<$!> and C<$?>). (See also L<Fatal>, though.)
2a8c8378 1822
84dabc03 1823Under a few operating systems, C<$^E> may contain a more verbose error
1824indicator, such as in this case, "CDROM tray not closed." Systems that
1825do not support extended error messages leave C<$^E> the same as C<$!>.
a0d0e21e 1826
b0c22438 1827Finally, C<$?> may be set to non-0 value if the external program
84dabc03 1828F</cdrom/install> fails. The upper eight bits reflect specific error
1829conditions encountered by the program (the program's C<exit()> value).
1830The lower eight bits reflect mode of failure, like signal death and
57f6eff5 1831core dump information. See L<wait(2)> for details. In contrast to
84dabc03 1832C<$!> and C<$^E>, which are set only if error condition is detected,
1833the variable C<$?> is set on each C<wait> or pipe C<close>,
1834overwriting the old value. This is more like C<$@>, which on every
1835C<eval()> is always set on failure and cleared on success.
a0d0e21e 1836
b0c22438 1837For more details, see the individual descriptions at C<$@>, C<$!>,
1838C<$^E>, and C<$?>.
38e4f4ae 1839
0b9346e6 1840=over 8
b0c22438 1842=item ${^CHILD_ERROR_NATIVE}
a0d0e21e 1844
b0c22438 1845The native status returned by the last pipe close, backtick (C<``>)
1846command, successful call to C<wait()> or C<waitpid()>, or from the
1847C<system()> operator. On POSIX-like systems this value can be decoded
1849WSTOPSIG and WIFCONTINUED functions provided by the L<POSIX> module.
a0d0e21e 1850
b0c22438 1851Under VMS this reflects the actual VMS exit status; i.e. it is the
1852same as C<$?> when the pragma C<use vmsish 'status'> is in effect.
a0d0e21e 1853
b0c22438 1854This variable was added in Perl 5.8.9.
a0d0e21e 1855
5b442a2a 1856=item $EXTENDED_OS_ERROR
84dabc03 1858=item $^E
1861Error information specific to the current operating system. At the
1862moment, this differs from C<$!> under only VMS, OS/2, and Win32 (and
1863for MacPerl). On all other platforms, C<$^E> is always just the same
1864as C<$!>.
1866Under VMS, C<$^E> provides the VMS status value from the last system
1867error. This is more specific information about the last system error
1868than that provided by C<$!>. This is particularly important when C<$!>
1869is set to B<EVMSERR>.
1871Under OS/2, C<$^E> is set to the error code of the last call to OS/2
1872API either via CRT, or directly from perl.
1874Under Win32, C<$^E> always returns the last error information reported
1875by the Win32 call C<GetLastError()> which describes the last error
1876from within the Win32 API. Most Win32-specific code will report errors
1877via C<$^E>. ANSI C and Unix-like calls set C<errno> and so most
1878portable Perl code will report errors via C<$!>.
1880Caveats mentioned in the description of C<$!> generally apply to
1881C<$^E>, also.
1883This variable was added in Perl 5.003.
1885Mnemonic: Extra error explanation.
0b9346e6 1886
84dabc03 1887=item $EXCEPTIONS_BEING_CAUGHT
1889=item $^S
1892Current state of the interpreter.
ca1b95ae 1894 $^S State
1895 --------- -------------------
1896 undef Parsing module/eval
1897 true (1) Executing an eval
1898 false (0) Otherwise
84dabc03 1899
1900The first state may happen in C<$SIG{__DIE__}> and C<$SIG{__WARN__}>
1903This variable was added in Perl 5.004.
1905=item $WARNING
1907=item $^W
1908X<$^W> X<$WARNING>
1910The current value of the warning switch, initially true if B<-w> was
1911used, false otherwise, but directly modifiable.
1913See also L<warnings>.
0b9346e6 1915Mnemonic: related to the B<-w> switch.
84dabc03 1916
1917=item ${^WARNING_BITS}
ca1b95ae 1918X<${^WARNING_BITS}>
84dabc03 1919
1920The current set of warning checks enabled by the C<use warnings> pragma.
1921See the documentation of C<warnings> for more details.
1923This variable was added in Perl 5.10.
b0c22438 1925=item $OS_ERROR
5ccee41e 1926
b0c22438 1927=item $ERRNO
5ccee41e 1928
b0c22438 1929=item $!
1930X<$!> X<$ERRNO> X<$OS_ERROR>
9b0e6e7a 1931
1932When referenced, C<$!> retrieves the current value
1933of the C C<errno> integer variable.
1934If C<$!> is assigned a numerical value, that value is stored in C<errno>.
1935When referenced as a string, C<$!> yields the system error string
1936corresponding to C<errno>.
1938Many system or library calls set C<errno> if they fail,
1939to indicate the cause of failure. They usually do B<not>
1940set C<errno> to zero if they succeed. This means C<errno>,
1941hence C<$!>, is meaningful only I<immediately> after a B<failure>:
1943 if (open my $fh, "<", $filename) {
ca1b95ae 1944 # Here $! is meaningless.
1945 ...
7fd683ff 1946 }
ca1b95ae 1947 else {
1948 # ONLY here is $! meaningful.
1949 ...
1950 # Already here $! might be meaningless.
b0c22438 1951 }
1952 # Since here we might have either success or failure,
a73bef78 1953 # $! is meaningless.
a0d0e21e 1954
1955Here, I<meaningless> means that C<$!> may be unrelated to the outcome
1956of the C<open()> operator. Assignment to C<$!> is similarly ephemeral.
1957It can be used immediately before invoking the C<die()> operator,
1958to set the exit value, or to inspect the system error string
1959corresponding to error I<n>, or to restore C<$!> to a meaningful state.
d54b56d5 1960
b0c22438 1961Mnemonic: What just went bang?
314d39ce 1962
b0c22438 1963=item %OS_ERROR
fb73857a 1964
b0c22438 1965=item %ERRNO
fb73857a 1966
b0c22438 1967=item %!
5b442a2a 1968X<%!> X<%OS_ERROR> X<%ERRNO>
a0d0e21e 1969
b0c22438 1970Each element of C<%!> has a true value only if C<$!> is set to that
1971value. For example, C<$!{ENOENT}> is true if and only if the current
84dabc03 1972value of C<$!> is C<ENOENT>; that is, if the most recent error was "No
1973such file or directory" (or its moral equivalent: not all operating
1974systems give that exact error, and certainly not all languages). To
1975check if a particular key is meaningful on your system, use C<exists
1976$!{the_key}>; for a list of legal keys, use C<keys %!>. See L<Errno>
7333b1c4 1977for more information, and also see L</$!>.
a0d0e21e 1978
b0c22438 1979This variable was added in Perl 5.005.
44f0be63 1980
84dabc03 1981=item $CHILD_ERROR
b687b08b 1982
84dabc03 1983=item $?
1984X<$?> X<$CHILD_ERROR>
a0d0e21e 1985
84dabc03 1986The status returned by the last pipe close, backtick (C<``>) command,
1987successful call to C<wait()> or C<waitpid()>, or from the C<system()>
1988operator. This is just the 16-bit status word returned by the
1989traditional Unix C<wait()> system call (or else is made up to look
1990like it). Thus, the exit value of the subprocess is really (C<<< $? >>
19918 >>>), and C<$? & 127> gives which signal, if any, the process died
1992from, and C<$? & 128> reports whether there was a core dump.
a0d0e21e 1993
84dabc03 1994Additionally, if the C<h_errno> variable is supported in C, its value
1995is returned via C<$?> if any C<gethost*()> function fails.
b687b08b 1996
84dabc03 1997If you have installed a signal handler for C<SIGCHLD>, the
1998value of C<$?> will usually be wrong outside that handler.
a0d0e21e 1999
84dabc03 2000Inside an C<END> subroutine C<$?> contains the value that is going to be
2001given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
2002change the exit status of your program. For example:
a0d0e21e 2003
84dabc03 2004 END {
2005 $? = 1 if $? == 255; # die would make it 255
2006 }
a0d0e21e 2007
84dabc03 2008Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
2009actual VMS exit status, instead of the default emulation of POSIX
2010status; see L<perlvms/$?> for details.
2012Mnemonic: similar to B<sh> and B<ksh>.
a0d0e21e 2013
b0c22438 2014=item $EVAL_ERROR
f648820c 2015
b0c22438 2016=item $@
2017X<$@> X<$EVAL_ERROR>
a0d0e21e 2018
0b9346e6 2019The Perl syntax error message from the last C<eval()> operator. If C<$@> is
2020the null string, the last C<eval()> parsed and executed correctly
b0c22438 2021(although the operations you invoked may have failed in the normal
a0d0e21e 2023
b0c22438 2024Warning messages are not collected in this variable. You can, however,
2025set up a routine to process warnings by setting C<$SIG{__WARN__}> as
7333b1c4 2026described in L</%SIG>.
748a9306 2027
b0c22438 2028Mnemonic: Where was the syntax error "at"?
7f315d2e 2029
b0c22438 2030=back
7f315d2e 2031
b0c22438 2032=head2 Deprecated and removed variables
7f315d2e 2033
0b9346e6 2034Deprecating a variable announces the intent of the perl maintainers to
1c2e8cca 2035eventually remove the variable from the language. It may still be
b0c22438 2036available despite its status. Using a deprecated variable triggers
2037a warning.
7f315d2e 2038
84dabc03 2039Once a variable is removed, its use triggers an error telling you
b0c22438 2040the variable is unsupported.
7f315d2e 2041
84dabc03 2042See L<perldiag> for details about error messages.
7f315d2e 2043
b0c22438 2044=over 8
7f315d2e 2045
5b442a2a 2046=item $OFMT
84dabc03 2048=item $#
5b442a2a 2049X<$#> X<$OFMT>
84dabc03 2050
38e5787b 2051C<$#> was a variable that could be used to format printed numbers.
84dabc03 2052After a deprecation cycle, its magic was removed in Perl 5.10 and
2053using it now triggers a warning: C<$# is no longer supported>.
2055This is not the sigil you use in front of an array name to get the
2056last index, like C<$#array>. That's still how you get the last index
2057of an array in Perl. The two have nothing to do with each other.
2059Deprecated in Perl 5.
2061Removed in Perl 5.10.
2063=item $*
84dabc03 2066C<$*> was a variable that you could use to enable multiline matching.
2067After a deprecation cycle, its magic was removed in Perl 5.10.
2068Using it now triggers a warning: C<$* is no longer supported>.
84dabc03 2069You should use the C</s> and C</m> regexp modifiers instead.
7f315d2e 2070
b0c22438 2071Deprecated in Perl 5.
7f315d2e 2072
b0c22438 2073Removed in Perl 5.10.
7f315d2e 2074
5b442a2a 2075=item $ARRAY_BASE
84dabc03 2077=item $[
5b442a2a 2078X<$[> X<$ARRAY_BASE>
84dabc03 2079
2080C<$[> was a variable that you could use to offset the indexing of arrays
2081and strings. After a deprecation cycle, the feature was removed in
2082Perl 5.16. Two old ways of coping with the variability of the index
2083offset, which were rendered obsolete in Perl 5.000 when C<$[> became
2084effectively lexically scoped, are still supported: you can read it
2085(always yielding zero) and you can assign zero to it.
84dabc03 2086
0b9346e6 2087Deprecated in Perl 5.12.
84dabc03 2088
2089Removed in Perl 5.16.
5b442a2a 2091=item $OLD_PERL_VERSION
b0c22438 2093=item $]
5b442a2a 2094X<$]> X<$OLD_PERL_VERSION>
55602bd2 2095
57f6eff5 2096See L</$^V> for a more modern representation of the Perl version that allows
d4ba9bf2 2097accurate string comparisons.
b0c22438 2099The version + patchlevel / 1000 of the Perl interpreter. This variable
2100can be used to determine whether the Perl interpreter executing a
2101script is in the right range of versions:
55602bd2 2102
b0c22438 2103 warn "No checksumming!\n" if $] < 3.019;
55602bd2 2104
d4ba9bf2 2105The floating point representation can sometimes lead to inaccurate
2106numeric comparisons.
b0c22438 2108See also the documentation of C<use VERSION> and C<require VERSION>
2109for a convenient way to fail if the running Perl interpreter is too old.
55602bd2 2110
b0c22438 2111Mnemonic: Is this version of perl in the right bracket?
19799a22 2112
b0c22438 2113Deprecated in Perl 5.6.
19799a22 2114
b0c22438 2115=back
2b92dfce 2116
0b9346e6 2117=cut