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I think I have perlvar ordered and proper now
[perl5.git] / pod / perlvar.pod
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
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
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 $)
81714fb9 232
b0c22438 233The effective gid of this process. If you are on a machine that
234supports membership in multiple groups simultaneously, gives a space
235separated list of groups you are in. The first number is the one
236returned by C<getegid()>, and the subsequent ones by C<getgroups()>,
237one of which may be the same as the first number.
81714fb9 238
b0c22438 239Similarly, a value assigned to C<$)> 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
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
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
308=item $SUBSEP
310=item $;
313The subscript separator for multidimensional array emulation. If you
314refer to a hash element as
316 $foo{$a,$b,$c}
318it really means
320 $foo{join($;, $a, $b, $c)}
322But don't put
324 @foo{$a,$b,$c} # a slice--note the @
326which means
328 ($foo{$a},$foo{$b},$foo{$c})
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<$;>.
333Consider using "real" multidimensional arrays as described
334in L<perllol>.
336Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
338=item $REAL_USER_ID
340=item $UID
342=item $<
343X<< $< >> X<$UID> X<$REAL_USER_ID>
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.
350Mnemonic: it's the uid you came I<from>, if you're running setuid.
354=item $EUID
356=item $>
359The effective uid of this process. For example:
361 $< = $>; # set real to effective uid
362 ($<,$>) = ($>,$<); # swap real and effective uids
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.
368C<< $< >> and C<< $> >> can be swapped only on machines
369supporting C<setreuid()>.
371Mnemonic: it's the uid you went I<to>, if you're running setuid.
373=item $a
375=item $b
376X<$a> X<$b>
378Special package variables when using C<sort()>, see L<perlfunc/sort>.
379Because of this specialness C<$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.
b0c22438 384=item $COMPILING
a0d0e21e 385
b0c22438 386=item $^C
a0d0e21e 388
b0c22438 389The current value of the flag associated with the B<-c> switch.
390Mainly of use with B<-MO=...> to allow code to alter its behavior
391when being compiled, such as for example to C<AUTOLOAD> at compile
392time rather than normal, deferred loading. Setting
393C<$^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 $^D
a0d0e21e 401
b0c22438 402The current value of the debugging flags. May be read or set. Like its
403command-line equivalent, you can use numeric or symbolic values, eg
404C<$^D = 10> or C<$^D = "st">.
68dc0745 405
b0c22438 406Mnemonic: value of B<-D> switch.
5b2b9c68 407
0b9346e6 408=item ${^ENCODING}
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.
416This variable was added in Perl 5.8.2.
418=item %ENV
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.
b0c22438 425=item $SYSTEM_FD_MAX
5b2b9c68 426
b0c22438 427=item $^F
5b2b9c68 429
b0c22438 430The maximum system file descriptor, ordinarily 2. System file
431descriptors are passed to C<exec()>ed processes, while higher file
432descriptors are not. Also, during an C<open()>, system file descriptors are
433preserved even if the C<open()> fails (ordinary file descriptors are
434closed before the C<open()> is attempted). The close-on-exec
435status of a file descriptor will be decided according to the value of
436C<$^F> when the corresponding file, pipe, or socket was opened, not the
437time of the C<exec()>.
5b2b9c68 438
0b9346e6 439=item @F
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'>.
b0c22438 447=item $^H
883faa13 448
b0c22438 449WARNING: This variable is strictly for internal use only. Its availability,
450behavior, and contents are subject to change without notice.
a0d0e21e 451
b0c22438 452This variable contains compile-time hints for the Perl interpreter. At the
453end of compilation of a BLOCK the value of this variable is restored to the
454value when the interpreter started to compile the BLOCK.
a0d0e21e 455
b0c22438 456When perl begins to parse any block construct that provides a lexical scope
457(e.g., eval body, required file, subroutine body, loop body, or conditional
458block), the existing value of C<$^H> is saved, but its value is left unchanged.
459When the compilation of the block is completed, it regains the saved value.
460Between the points where its value is saved and restored, code that
461executes within BEGIN blocks is free to change the value of C<$^H>.
a0d0e21e 462
b0c22438 463This behavior provides the semantic of lexical scoping, and is used in,
464for instance, the C<use strict> pragma.
a0d0e21e 465
b0c22438 466The contents should be an integer; different bits of it are used for
467different pragmatic flags. Here's an example:
a0d0e21e 468
0b9346e6 469 sub add_100 { $^H |= 0x100 }
a0d0e21e 470
0b9346e6 471 sub foo {
472 BEGIN { add_100() }
473 bar->baz($boon);
474 }
a0d0e21e 475
b0c22438 476Consider what happens during execution of the BEGIN block. At this point
477the BEGIN block has already been compiled, but the body of C<foo()> is still
478being compiled. The new value of C<$^H> will therefore be visible only while
479the body of C<foo()> is being compiled.
a0d0e21e 480
b0c22438 481Substitution of the above BEGIN block with:
a0d0e21e 482
0b9346e6 483 BEGIN { require strict; strict->import('vars') }
a0d0e21e 484
b0c22438 485demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
486version of the same lexical pragma:
a0d0e21e 487
0b9346e6 488 BEGIN { require strict; strict->import('vars') if $condition }
a0d0e21e 489
b0c22438 490This variable was added in Perl 5.003.
a0d0e21e 491
b0c22438 492=item %^H
a0d0e21e 493
b0c22438 494The C<%^H> hash provides the same scoping semantic as C<$^H>. This makes it
495useful for implementation of lexically scoped pragmas. See L<perlpragma>.
a0d0e21e 496
b0c22438 497This variable was added in Perl 5.6.
a0d0e21e 498
0b9346e6 499=item @INC
502The array C<@INC> contains the list of places that the C<do EXPR>,
503C<require>, or C<use> constructs look for their library files. It
504initially consists of the arguments to any B<-I> command-line
505switches, followed by the default Perl library, probably
506F</usr/local/lib/perl>, followed by ".", to represent the current
507directory. ("." will not be appended if taint checks are enabled,
508either by C<-T> or by C<-t>.) If you need to modify this at runtime,
509you should use the C<use lib> pragma to get the machine-dependent
510library properly loaded also:
512 use lib '/mypath/libdir/';
513 use SomeMod;
515You can also insert hooks into the file inclusion system by putting Perl
516code directly into C<@INC>. Those hooks may be subroutine references, array
517references or blessed objects. See L<perlfunc/require> for details.
519=item %INC
522The hash C<%INC> contains entries for each filename included via the
523C<do>, C<require>, or C<use> operators. The key is the filename
524you specified (with module names converted to pathnames), and the
525value is the location of the file found. The C<require>
526operator uses this hash to determine whether a particular file has
527already been included.
529If the file was loaded via a hook (e.g. a subroutine reference, see
530L<perlfunc/require> for a description of these hooks), this hook is
531by default inserted into C<%INC> in place of a filename. Note, however,
532that the hook may have set the C<%INC> entry by itself to provide some more
533specific info.
b0c22438 535=item $INPLACE_EDIT
a0d0e21e 536
b0c22438 537=item $^I
a0d0e21e 539
b0c22438 540The current value of the inplace-edit extension. Use C<undef> to disable
541inplace editing.
a0d0e21e 542
b0c22438 543Mnemonic: value of B<-i> switch.
a0d0e21e 544
b0c22438 545=item $^M
a0d0e21e 547
b0c22438 548By default, running out of memory is an untrappable, fatal error.
549However, if suitably built, Perl can use the contents of C<$^M>
550as an emergency memory pool after C<die()>ing. Suppose that your Perl
551were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
a0d0e21e 553
0b9346e6 554 $^M = 'a' x (1 << 16);
a0d0e21e 555
b0c22438 556would allocate a 64K buffer for use in an emergency. See the
557F<INSTALL> file in the Perl distribution for information on how to
558add custom C compilation flags when compiling perl. To discourage casual
559use of this advanced feature, there is no L<English|English> long name for
560this variable.
a0d0e21e 561
b0c22438 562This variable was added in Perl 5.004.
a0d0e21e 563
b0c22438 564=item $OSNAME
a0d0e21e 565
b0c22438 566=item $^O
567X<$^O> X<$OSNAME>
a0d0e21e 568
b0c22438 569The name of the operating system under which this copy of Perl was
570built, as determined during the configuration process. For examples
571see L<perlport/PLATFORMS>.
a0d0e21e 572
b0c22438 573The value is identical to C<$Config{'osname'}>. See also L<Config>
574and the B<-V> command-line switch documented in L<perlrun>.
a0d0e21e 575
b0c22438 576In Windows platforms, C<$^O> is not very helpful: since it is always
577C<MSWin32>, it doesn't tell the difference between
57895/98/ME/NT/2000/XP/CE/.NET. Use C<Win32::GetOSName()> or
579Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
580between the variants.
a0d0e21e 581
b0c22438 582This variable was added in Perl 5.003.
a0d0e21e 583
b0c22438 584=item ${^OPEN}
a0d0e21e 585
b0c22438 586An internal variable used by PerlIO. A string in two parts, separated
587by a C<\0> byte, the first part describes the input layers, the second
588part describes the output layers.
a0d0e21e 589
b0c22438 590This variable was added in Perl 5.8.2.
a0d0e21e 591
b0c22438 592=item $PERLDB
a0d0e21e 593
b0c22438 594=item $^P
595X<$^P> X<$PERLDB>
a0d0e21e 596
b0c22438 597The internal variable for debugging support. The meanings of the
598various bits are subject to change, but currently indicate:
a0d0e21e 599
b0c22438 600=over 6
a0d0e21e 601
b0c22438 602=item 0x01
a0d0e21e 603
b0c22438 604Debug subroutine enter/exit.
a0d0e21e 605
b0c22438 606=item 0x02
a0d0e21e 607
b0c22438 608Line-by-line debugging. Causes C<DB::DB()> subroutine to be called for each
609statement executed. Also causes saving source code lines (like 0x400).
a0d0e21e 610
b0c22438 611=item 0x04
fe307981 612
b0c22438 613Switch off optimizations.
6cef1e77 614
b0c22438 615=item 0x08
6cef1e77 616
b0c22438 617Preserve more data for future interactive inspections.
6cef1e77 618
b0c22438 619=item 0x10
4ba05bdc 620
b0c22438 621Keep info about source lines on which a subroutine is defined.
4ba05bdc 622
b0c22438 623=item 0x20
4ba05bdc 624
b0c22438 625Start with single-step on.
4ba05bdc 626
b0c22438 627=item 0x40
4ba05bdc 628
b0c22438 629Use subroutine address instead of name when reporting.
4ba05bdc 630
b0c22438 631=item 0x80
4ba05bdc 632
b0c22438 633Report C<goto &subroutine> as well.
4ba05bdc 634
b0c22438 635=item 0x100
4ba05bdc 636
b0c22438 637Provide informative "file" names for evals based on the place they were compiled.
4ba05bdc 638
b0c22438 639=item 0x200
44a2ac75 640
b0c22438 641Provide informative names to anonymous subroutines based on the place they
642were compiled.
44a2ac75 643
b0c22438 644=item 0x400
44a2ac75 645
b0c22438 646Save source code lines into C<@{"_<$filename"}>.
44a2ac75 647
b0c22438 648=back
44a2ac75 649
b0c22438 650Some bits may be relevant at compile-time only, some at
651run-time only. This is a new mechanism and the details may change.
652See also L<perldebguts>.
3195cf34 653
b0c22438 654=item %SIG
a0d0e21e 655
b0c22438 656=item $SIG{expr}
a0d0e21e 658
b0c22438 659The hash C<%SIG> contains signal handlers for signals. For example:
a0d0e21e 660
0b9346e6 661 sub handler { # 1st argument is signal name
662 my($sig) = @_;
663 print "Caught a SIG$sig--shutting down\n";
664 close(LOG);
665 exit(0);
666 }
a0d0e21e 667
0b9346e6 668 $SIG{'INT'} = \&handler;
669 $SIG{'QUIT'} = \&handler;
670 ...
671 $SIG{'INT'} = 'DEFAULT'; # restore default action
672 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
a0d0e21e 673
b0c22438 674Using a value of C<'IGNORE'> usually has the effect of ignoring the
675signal, except for the C<CHLD> signal. See L<perlipc> for more about
676this special case.
a0d0e21e 677
b0c22438 678Here are some other examples:
a0d0e21e 679
0b9346e6 680 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
681 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
682 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
683 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
a0d0e21e 684
b0c22438 685Be sure not to use a bareword as the name of a signal handler,
686lest you inadvertently call it.
a0d0e21e 687
b0c22438 688If your system has the C<sigaction()> function then signal handlers
689are installed using it. This means you get reliable signal handling.
7b8d334a 690
b0c22438 691The default delivery policy of signals changed in Perl 5.8.0 from
692immediate (also known as "unsafe") to deferred, also known as "safe
693signals". See L<perlipc> for more information.
aa689395 694
b0c22438 695Certain internal hooks can be also set using the C<%SIG> hash. The
696routine indicated by C<$SIG{__WARN__}> is called when a warning
697message is about to be printed. The warning message is passed as the
698first argument. The presence of a C<__WARN__> hook causes the
699ordinary printing of warnings to C<STDERR> to be suppressed. You can
700use this to save warnings in a variable, or turn warnings into fatal
701errors, like this:
19799a22 702
0b9346e6 703 local $SIG{__WARN__} = sub { die $_[0] };
704 eval $proggie;
a8f8344d 705
b0c22438 706As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
707disable warnings using the empty subroutine:
f86702cc 708
0b9346e6 709 local $SIG{__WARN__} = sub {};
55602bd2 710
b0c22438 711The routine indicated by C<$SIG{__DIE__}> is called when a fatal
712exception is about to be thrown. The error message is passed as the
713first argument. When a C<__DIE__> hook routine returns, the exception
714processing continues as it would have in the absence of the hook,
715unless the hook routine itself exits via a C<goto>, a loop exit, or a
716C<die()>. The C<__DIE__> handler is explicitly disabled during the
717call, so that you can die from a C<__DIE__> handler. Similarly for
e5218da5 719
b0c22438 720Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
721even inside an C<eval()>. Do not use this to rewrite a pending
722exception in C<$@>, or as a bizarre substitute for overriding
723C<CORE::GLOBAL::die()>. This strange action at a distance may be fixed
724in a future release so that C<$SIG{__DIE__}> is only called if your
725program is about to exit, as was the original intent. Any other use is
728C<__DIE__>/C<__WARN__> handlers are very special in one respect: they
729may be called to report (probable) errors found by the parser. In such
730a case the parser may be in inconsistent state, so any attempt to
731evaluate Perl code from such a handler will probably result in a
732segfault. This means that warnings or errors that result from parsing
733Perl should be used with extreme caution, like this:
e5218da5 734
0b9346e6 735 require Carp if defined $^S;
736 Carp::confess("Something wrong") if defined &Carp::confess;
737 die "Something wrong, but could not load Carp to give backtrace...
738 To see backtrace try starting Perl with -MCarp switch";
e5218da5 739
b0c22438 740Here the first line will load C<Carp> I<unless> it is the parser who
741called the handler. The second line will print backtrace and die if
742C<Carp> was available. The third line will be executed only if C<Carp> was
743not available.
0a378802 744
0b9346e6 745Having to even think about the C<$^S> variable in your exception
746handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
747invites grievous and difficult to track down errors. Avoid it
748and use an C<END{}> or CORE::GLOBAL::die override instead.
b0c22438 750See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
751L<warnings> for additional information.
0a378802 752
b0c22438 753=item $BASETIME
6ab308ee 754
b0c22438 755=item $^T
756X<$^T> X<$BASETIME>
6ab308ee 757
b0c22438 758The time at which the program began running, in seconds since the
759epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
760and B<-C> filetests are based on this value.
a0d0e21e 761
b0c22438 762=item ${^TAINT}
55602bd2 763
b0c22438 764Reflects if taint mode is on or off. 1 for on (the program was run with
765B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
0b9346e6 766B<-t> or B<-TU>).
daaddde1 767
b0c22438 768This variable is read-only.
daaddde1 769
b0c22438 770This variable was added in Perl 5.8.
4c5cef9b 771
b0c22438 772=item ${^UNICODE}
4c5cef9b 773
b0c22438 774Reflects certain Unicode settings of Perl. See L<perlrun>
775documentation for the C<-C> switch for more information about
0b9346e6 776the possible values.
5c055ba3 777
b0c22438 778This variable is set during Perl startup and is thereafter read-only.
5c055ba3 779
b0c22438 780This variable was added in Perl 5.8.2.
22fae026 781
b0c22438 782=item ${^UTF8CACHE}
22fae026 783
b0c22438 784This variable controls the state of the internal UTF-8 offset caching code.
7851 for on (the default), 0 for off, -1 to debug the caching code by checking
786all its results against linear scans, and panicking on any discrepancy.
22fae026 787
b0c22438 788This variable was added in Perl 5.8.9.
22fae026 789
b0c22438 790=item ${^UTF8LOCALE}
5c055ba3 791
b0c22438 792This variable indicates whether a UTF-8 locale was detected by perl at
793startup. This information is used by perl when it's in
794adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
795switch); see L<perlrun> for more info on this.
55602bd2 796
b0c22438 797This variable was added in Perl 5.8.8.
a0d0e21e 798
b0c22438 799=item $PERL_VERSION
a0d0e21e 800
b0c22438 801=item $^V
a0d0e21e 803
b0c22438 804The revision, version, and subversion of the Perl interpreter,
805represented as a C<version> object.
748a9306 806
b0c22438 807This variable first appeared in perl 5.6.0; earlier versions of perl
808will see an undefined value. Before perl 5.10.0 C<$^V> was represented
809as a v-string.
55602bd2 810
b0c22438 811C<$^V> can be used to determine whether the Perl interpreter executing
812a script is in the right range of versions. For example:
a0d0e21e 813
0b9346e6 814 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
a0d0e21e 815
b0c22438 816To convert C<$^V> into its string representation use C<sprintf()>'s
817C<"%vd"> conversion:
a0d0e21e 818
0b9346e6 819 printf "version is v%vd\n", $^V; # Perl's version
a0d0e21e 820
b0c22438 821See the documentation of C<use VERSION> and C<require VERSION>
822for a convenient way to fail if the running Perl interpreter is too old.
4d76a344 823
b0c22438 824See also C<$]> for an older representation of the Perl version.
a0d0e21e 825
b0c22438 826This variable was added in Perl 5.6.
a0d0e21e 827
b0c22438 828Mnemonic: use ^V for Version Control.
a0d0e21e 829
b0c22438 830=item ${^WIN32_SLOPPY_STAT}
831X<sitecustomize> X<>
a0d0e21e 832
b0c22438 833If this variable is set to a true value, then C<stat()> on Windows will
834not try to open the file. This means that the link count cannot be
835determined and file attributes may be out of date if additional
836hardlinks to the file exist. On the other hand, not opening the file
837is considerably faster, especially for files on network drives.
a0d0e21e 838
b0c22438 839This variable could be set in the F<> file to
840configure the local Perl installation to use "sloppy" C<stat()> by
841default. See the documentation for B<-f> in
842L<perlrun|perlrun/"Command Switches"> for more information about site
a0d0e21e 844
b0c22438 845This variable was added in Perl 5.10.
a0d0e21e 846
b0c22438 847=item $EXECUTABLE_NAME
a0d0e21e 848
b0c22438 849=item $^X
a0d0e21e 851
b0c22438 852The name used to execute the current copy of Perl, from C's
853C<argv[0]> or (where supported) F</proc/self/exe>.
a043a685 854
b0c22438 855Depending on the host operating system, the value of C<$^X> may be
856a relative or absolute pathname of the perl program file, or may
857be the string used to invoke perl but not the pathname of the
858perl program file. Also, most operating systems permit invoking
859programs that are not in the PATH environment variable, so there
860is no guarantee that the value of C<$^X> is in PATH. For VMS, the
861value may or may not include a version number.
a0d0e21e 862
b0c22438 863You usually can use the value of C<$^X> to re-invoke an independent
864copy of the same perl that is currently running, e.g.,
a0d0e21e 865
0b9346e6 866 @first_run = `$^X -le "print int rand 100 for 1..100"`;
a0d0e21e 867
b0c22438 868But recall that not all operating systems support forking or
869capturing of the output of commands, so this complex statement
870may not be portable.
a0d0e21e 871
b0c22438 872It is not safe to use the value of C<$^X> as a path name of a file,
873as some operating systems that have a mandatory suffix on
874executable files do not require use of the suffix when invoking
875a command. To convert the value of C<$^X> to a path name, use the
876following statements:
8cc95fdb 877
0b9346e6 878 # Build up a set of file names (not command names).
879 use Config;
880 my $this_perl = $^X;
881 if ($^O ne 'VMS') {
882 $this_perl .= $Config{_exe}
883 unless $this_perl =~ m/$Config{_exe}$/i;
884 }
8cc95fdb 885
b0c22438 886Because many operating systems permit anyone with read access to
887the Perl program file to make a copy of it, patch the copy, and
888then execute the copy, the security-conscious Perl programmer
889should take care to invoke the installed copy of perl, not the
890copy referenced by C<$^X>. The following statements accomplish
891this goal, and produce a pathname that can be invoked as a
892command or referenced as a file.
a043a685 893
0b9346e6 894 use Config;
895 my $secure_perl_path = $Config{perlpath};
896 if ($^O ne 'VMS') {
897 $secure_perl_path .= $Config{_exe}
898 unless $secure_perl_path =~ m/$Config{_exe}$/i;
899 }
a0d0e21e 900
b0c22438 901=back
a0d0e21e 902
b0c22438 903=head2 Variables related to regular expressions
905Most of the special variables related to regular expressions are side
906effects. Perl sets these variables when it has a successful match, so
907you should check the match result before using them. For instance:
909 if( /P(A)TT(ER)N/ ) {
910 print "I found $1 and $2\n";
911 }
0b9346e6 913These variables are read-only and dynamically-scoped, unless we note
b0c22438 914otherwise.
0b9346e6 916The dynamic nature of the regular expression variables means that
917their value is limited to the block that they are in, as demonstrated
918by this bit of code:
b0c22438 919
920 my $outer = 'Wallace and Grommit';
921 my $inner = 'Mutt and Jeff';
0b9346e6 922
b0c22438 923 my $pattern = qr/(\S+) and (\S+)/;
0b9346e6 924
b0c22438 925 sub show_n { print "\$1 is $1; \$2 is $2\n" }
0b9346e6 926
b0c22438 927 {
928 OUTER:
929 show_n() if $outer =~ m/$pattern/;
0b9346e6 930
b0c22438 931 INNER: {
932 show_n() if $inner =~ m/$pattern/;
933 }
0b9346e6 934
b0c22438 935 show_n();
936 }
0b9346e6 938The output shows that while in the C<OUTER> block, the values of C<$1>
939and C<$2> are from the match against C<$outer>. Inside the C<INNER>
940block, the values of C<$1> and C<$2> are from the match against
941C<$inner>, but only until the end of the block (i.e. the dynamic
942scope). After the C<INNER> block completes, the values of C<$1> and
943C<$2> return to the values for the match against C<$outer> even though
b0c22438 944we have not made another match:
946 $1 is Wallace; $2 is Grommit
947 $1 is Mutt; $2 is Jeff
948 $1 is Wallace; $2 is Grommit
a0d0e21e 949
0b9346e6 950Due to an unfortunate accident of Perl's implementation, C<use
951English> imposes a considerable performance penalty on all regular
952expression matches in a program because it uses the C<$`>, C<$&>, and
953C<$'>, regardless of whether they occur in the scope of C<use
954English>. For that reason, saying C<use English> in libraries is
955strongly discouraged unless you import it without the match variables:
957 use English '-no_match_vars'
959The C<Devel::NYTProf> module can help you find uses of these
960problematic match variables in your code.
962Since Perl 5.10, you can use the C</p> match operator flag and the
963C<${^PREMATCH}>, C<${^MATCH}>, and C<${^POSTMATCH}> variables instead
964so you only suffer the performance penalties.
b0c22438 966=over 8
a0d0e21e 967
b0c22438 968=item $<I<digits>> ($1, $2, ...)
969X<$1> X<$2> X<$3>
8cc95fdb 970
b0c22438 971Contains the subpattern from the corresponding set of capturing
972parentheses from the last successful pattern match, not counting patterns
973matched in nested blocks that have been exited already.
8cc95fdb 974
b0c22438 975These variables are read-only and dynamically-scoped.
a043a685 976
b0c22438 977Mnemonic: like \digits.
a0d0e21e 978
b0c22438 979=item $MATCH
a0d0e21e 980
b0c22438 981=item $&
982X<$&> X<$MATCH>
a0d0e21e 983
b0c22438 984The string matched by the last successful pattern match (not counting
985any matches hidden within a BLOCK or C<eval()> enclosed by the current
a0d0e21e 987
b0c22438 988The use of this variable anywhere in a program imposes a considerable
0b9346e6 989performance penalty on all regular expression matches. To avoid this
990penalty, you can extract the same substring by using L</@->. Starting
991with Perl 5.10, you can use the </p> match flag and the C<${^MATCH}>
992variable to do the same thing for particular match operations.
80bca1b4 993
b0c22438 994This variable is read-only and dynamically-scoped.
f9cbb277 995
b0c22438 996Mnemonic: like C<&> in some editors.
0b9346e6 997
b0c22438 998=item ${^MATCH}
a0d0e21e 1000
b0c22438 1001This is similar to C<$&> (C<$MATCH>) except that it does not incur the
1002performance penalty associated with that variable, and is only guaranteed
1003to return a defined value when the pattern was compiled or executed with
1004the C</p> modifier.
80bca1b4 1005
b0c22438 1006This variable was added in Perl 5.10.
4bc88a62 1007
b0c22438 1008This variable is read-only and dynamically-scoped.
e2975953 1009
b0c22438 1010=item $PREMATCH
52c447a8 1011
b0c22438 1012=item $`
1013X<$`> X<$PREMATCH>
7636ea95 1014
b0c22438 1015The string preceding whatever was matched by the last successful
1016pattern match, not counting any matches hidden within a BLOCK or C<eval>
0b9346e6 1017enclosed by the current BLOCK.
a0d0e21e 1018
b0c22438 1019The use of this variable anywhere in a program imposes a considerable
0b9346e6 1020performance penalty on all regular expression matches. To avoid this
1021penalty, you can extract the same substring by using L</@->. Starting
1022with Perl 5.10, you can use the </p> match flag and the
1023C<${^PREMATCH}> variable to do the same thing for particular match
a0d0e21e 1025
b0c22438 1026This variable is read-only and dynamically-scoped.
a0d0e21e 1027
b0c22438 1028Mnemonic: C<`> often precedes a quoted string.
f83ed198 1029
b0c22438 1030=item ${^PREMATCH}
a0d0e21e 1032
b0c22438 1033This is similar to C<$`> ($PREMATCH) except that it does not incur the
1034performance penalty associated with that variable, and is only guaranteed
1035to return a defined value when the pattern was compiled or executed with
1036the C</p> modifier.
a0d0e21e 1037
b0c22438 1038This variable was added in Perl 5.10
a0d0e21e 1039
b0c22438 1040This variable is read-only and dynamically-scoped.
a0d0e21e 1041
b0c22438 1042=item $POSTMATCH
16070b82 1043
b0c22438 1044=item $'
1045X<$'> X<$POSTMATCH>
305aace0 1046
b0c22438 1047The string following whatever was matched by the last successful
1048pattern match (not counting any matches hidden within a BLOCK or C<eval()>
1049enclosed by the current BLOCK). Example:
305aace0 1050
0b9346e6 1051 local $_ = 'abcdefghi';
1052 /def/;
1053 print "$`:$&:$'\n"; # prints abc:def:ghi
305aace0 1054
b0c22438 1055The use of this variable anywhere in a program imposes a considerable
0b9346e6 1056performance penalty on all regular expression matches.
1057To avoid this penalty, you can extract the same substring by
b0c22438 1058using L</@->. Starting with Perl 5.10, you can use the </p> match flag
0b9346e6 1059and the C<${^POSTMATCH}> variable to do the same thing for particular
b0c22438 1060match operations.
a0d0e21e 1061
b0c22438 1062This variable is read-only and dynamically-scoped.
1064Mnemonic: C<'> often follows a quoted string.
1066=item ${^POSTMATCH}
1069This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
1070performance penalty associated with that variable, and is only guaranteed
1071to return a defined value when the pattern was compiled or executed with
1072the C</p> modifier.
1074This variable was added in Perl 5.10.
1076This variable is read-only and dynamically-scoped.
1080=item $+
1083The text matched by the last bracket of the last successful search pattern.
1084This is useful if you don't know which one of a set of alternative patterns
1085matched. For example:
0b9346e6 1087 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
b0c22438 1088
1089This variable is read-only and dynamically-scoped.
1091Mnemonic: be positive and forward looking.
1095=item $^N
1098The text matched by the used group most-recently closed (i.e. the group
1099with the rightmost closing parenthesis) of the last successful search
1102This is primarily used inside C<(?{...})> blocks for examining text
1103recently matched. For example, to effectively capture text to a variable
1104(in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
0b9346e6 1106 (?:(...)(?{ $var = $^N }))
b0c22438 1107
1108By setting and then using C<$var> in this way relieves you from having to
1109worry about exactly which numbered set of parentheses they are.
1111This variable was added in Perl 5.8.
1113Mnemonic: the (possibly) Nested parenthesis that most recently closed.
1115=item @LAST_MATCH_END
1117=item @+
1118X<@+> X<@LAST_MATCH_END>
1120This array holds the offsets of the ends of the last successful
1121submatches in the currently active dynamic scope. C<$+[0]> is
1122the offset into the string of the end of the entire match. This
1123is the same value as what the C<pos> function returns when called
1124on the variable that was matched against. The I<n>th element
1125of this array holds the offset of the I<n>th submatch, so
1126C<$+[1]> is the offset past where C<$1> ends, C<$+[2]> the offset
1127past where C<$2> ends, and so on. You can use C<$#+> to determine
1128how many subgroups were in the last successful match. See the
1129examples given for the C<@-> variable.
1131This variable was added in Perl 5.6.
1135=item %+
1138Similar to C<@+>, the C<%+> hash allows access to the named capture
1139buffers, should they exist, in the last successful match in the
1140currently active dynamic scope.
1142For example, C<$+{foo}> is equivalent to C<$1> after the following match:
0b9346e6 1144 'foo' =~ /(?<foo>foo)/;
b0c22438 1145
1146The keys of the C<%+> hash list only the names of buffers that have
1147captured (and that are thus associated to defined values).
1149The underlying behaviour of C<%+> is provided by the
1150L<Tie::Hash::NamedCapture> module.
1152B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1153associated with the last successful regular expression. Therefore mixing
1154iterative access to them via C<each> may have unpredictable results.
1155Likewise, if the last successful match changes, then the results may be
1158This variable was added in Perl 5.10.
a0d0e21e 1159
b0c22438 1160This variable is read-only and dynamically-scoped.
1164=item @-
1167C<$-[0]> is the offset of the start of the last successful match.
1168C<$-[>I<n>C<]> is the offset of the start of the substring matched by
1169I<n>-th subpattern, or undef if the subpattern did not match.
1171Thus, after a match against C<$_>, C<$&> coincides with C<substr $_, $-[0],
1172$+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
1173$+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
1174C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the last
1175matched subgroup in the last successful match. Contrast with
1176C<$#+>, the number of subgroups in the regular expression. Compare
1177with C<@+>.
1179This array holds the offsets of the beginnings of the last
1180successful submatches in the currently active dynamic scope.
1181C<$-[0]> is the offset into the string of the beginning of the
1182entire match. The I<n>th element of this array holds the offset
1183of the I<n>th submatch, so C<$-[1]> is the offset where C<$1>
1184begins, C<$-[2]> the offset where C<$2> begins, and so on.
1186After a match against some variable C<$var>:
1188=over 5
1190=item C<$`> is the same as C<substr($var, 0, $-[0])>
1192=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
1194=item C<$'> is the same as C<substr($var, $+[0])>
1196=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
1198=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
1200=item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
1204This variable was added in Perl 5.6.
1206=item %-
1209Similar to C<%+>, this variable allows access to the named capture groups
1210in the last successful match in the currently active dynamic scope. To
1211each capture group name found in the regular expression, it associates a
1212reference to an array containing the list of values captured by all
1213buffers with that name (should there be several of them), in the order
1214where they appear.
1216Here's an example:
1218 if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
1219 foreach my $bufname (sort keys %-) {
1220 my $ary = $-{$bufname};
1221 foreach my $idx (0..$#$ary) {
1222 print "\$-{$bufname}[$idx] : ",
1223 (defined($ary->[$idx]) ? "'$ary->[$idx]'" : "undef"),
1224 "\n";
1225 }
1226 }
1227 }
1229would print out:
0b9346e6 1231 $-{A}[0] : '1'
1232 $-{A}[1] : '3'
1233 $-{B}[0] : '2'
1234 $-{B}[1] : '4'
b0c22438 1235
1236The keys of the C<%-> hash correspond to all buffer names found in
1237the regular expression.
1239The behaviour of C<%-> is implemented via the
1240L<Tie::Hash::NamedCapture> module.
1242B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1243associated with the last successful regular expression. Therefore mixing
1244iterative access to them via C<each> may have unpredictable results.
1245Likewise, if the last successful match changes, then the results may be
1248This variable was added in Perl 5.10
1250This variable is read-only and dynamically-scoped.
1254=item $^R
1257The result of evaluation of the last successful C<(?{ code })>
1258regular expression assertion (see L<perlre>). May be written to.
1260This variable was added in Perl 5.005.
a0d0e21e 1261
1262=item ${^RE_DEBUG_FLAGS}
1264The current value of the regex debugging flags. Set to 0 for no debug output
b0c22438 1265even when the C<re 'debug'> module is loaded. See L<re> for details.
1267This variable was added in Perl 5.10.
a3621e74 1268
0111c4fd 1269=item ${^RE_TRIE_MAXBUF}
1271Controls how certain regex optimisations are applied and how much memory they
1272utilize. This value by default is 65536 which corresponds to a 512kB temporary
1273cache. Set this to a higher value to trade memory for speed when matching
1274large alternations. Set it to a lower value if you want the optimisations to
1275be as conservative of memory as possible but still occur, and set it to a
1276negative value to prevent the optimisation and conserve the most memory.
1277Under normal situations this variable should be of no interest to you.
b0c22438 1279This variable was added in Perl 5.10.
a0d0e21e 1280
b0c22438 1281=back
a0d0e21e 1282
b0c22438 1283=head2 Variables related to filehandles
a0d0e21e 1284
b0c22438 1285Variables that depend on the currently selected filehandle may be set
1286by calling an appropriate object method on the C<IO::Handle> object,
1287although this is less efficient than using the regular built-in
1288variables. (Summary lines below for this contain the word HANDLE.)
1289First you must say
6e2995f4 1290
0b9346e6 1291 use IO::Handle;
0462a1ab 1292
b0c22438 1293after which you may use either
0462a1ab 1294
0b9346e6 1295 method HANDLE EXPR
0462a1ab 1296
b0c22438 1297or more safely,
0462a1ab 1298
0b9346e6 1299 HANDLE->method(EXPR)
0462a1ab 1300
b0c22438 1301Each method returns the old value of the C<IO::Handle> attribute. The
1302methods each take an optional EXPR, which, if supplied, specifies the
1303new value for the C<IO::Handle> attribute in question. If not
1304supplied, most methods do nothing to the current value--except for
1305C<autoflush()>, which will assume a 1 for you, just to be different.
0462a1ab 1306
b0c22438 1307Because loading in the C<IO::Handle> class is an expensive operation,
1308you should learn how to use the regular built-in variables.
1310A few of these variables are considered "read-only". This means that
1311if you try to assign to this variable, either directly or indirectly
1312through a reference, you'll raise a run-time exception.
1314You should be very careful when modifying the default values of most
1315special variables described in this document. In most cases you want
1316to localize these variables before changing them, since if you don't,
1317the change may affect other modules which rely on the default values
1318of the special variables that you have changed. This is one of the
1319correct ways to read the whole file at once:
0b9346e6 1321 open my $fh, "<", "foo" or die $!;
1322 local $/; # enable localized slurp mode
1323 my $content = <$fh>;
1324 close $fh;
b0c22438 1325
1326But the following code is quite bad:
0b9346e6 1328 open my $fh, "<", "foo" or die $!;
1329 undef $/; # enable slurp mode
1330 my $content = <$fh>;
1331 close $fh;
b0c22438 1332
1333since some other module, may want to read data from some file in the
1334default "line mode", so if the code we have just presented has been
1335executed, the global value of C<$/> is now changed for any other code
1336running inside the same Perl interpreter.
1338Usually when a variable is localized you want to make sure that this
1339change affects the shortest scope possible. So unless you are already
1340inside some short C<{}> block, you should create one yourself. For
0b9346e6 1343 my $content = '';
1344 open my $fh, "<", "foo" or die $!;
1345 {
1346 local $/;
1347 $content = <$fh>;
1348 }
1349 close $fh;
0462a1ab 1350
b0c22438 1351Here is an example of how your own code can go broken:
0462a1ab 1352
0b9346e6 1353 for ( 1..3 ){
1354 $\ = "\r\n";
1355 nasty_break();
1356 print "$_";
1357 }
1359 sub nasty_break {
1360 $\ = "\f";
1361 # do something with $_
1362 }
0462a1ab 1363
0b9346e6 1364You probably expect this code to print the equivalent of
0462a1ab 1365
0b9346e6 1366 "1\r\n2\r\n3\r\n"
0462a1ab 1367
b0c22438 1368but instead you get:
0462a1ab 1369
0b9346e6 1370 "1\f2\f3\f"
0462a1ab 1371
0b9346e6 1372Why? Because C<nasty_break()> modifies C<$\> without localizing it
1373first. The value you set in C<nasty_break()> is still there when you
1374return. The fix is to add C<local()> so the value doesn't leak out of
6e2995f4 1376
0b9346e6 1377 local $\ = "\f";
a0d0e21e 1378
b0c22438 1379It's easy to notice the problem in such a short example, but in more
1380complicated code you are looking for trouble if you don't localize
1381changes to the special variables.
a0d0e21e 1382
b0c22438 1383=over 8
a0d0e21e 1384
b0c22438 1385=item $ARGV
fb73857a 1387
b0c22438 1388contains the name of the current file when reading from <>.
1390=item @ARGV
1393The array @ARGV contains the command-line arguments intended for
1394the script. C<$#ARGV> is generally the number of arguments minus
1395one, because C<$ARGV[0]> is the first argument, I<not> the program's
1396command name itself. See C<$0> for the command name.
84dabc03 1398=item ARGV
1401The special filehandle that iterates over command-line filenames in
1402C<@ARGV>. Usually written as the null filehandle in the angle operator
1403C<< <> >>. Note that currently C<ARGV> only has its magical effect
1404within the C<< <> >> operator; elsewhere it is just a plain filehandle
1405corresponding to the last file opened by C<< <> >>. In particular,
1406passing C<\*ARGV> as a parameter to a function that expects a filehandle
1407may not cause your function to automatically read the contents of all the
1408files in C<@ARGV>.
b0c22438 1410=item ARGVOUT
1413The special filehandle that points to the currently open output file
1414when doing edit-in-place processing with B<-i>. Useful when you have
1415to do a lot of inserting and don't want to keep modifying C<$_>. See
1416L<perlrun> for the B<-i> switch.
84dabc03 1418=item Handle->output_field_separator EXPR
1422=item $OFS
1424=item $,
1427The output field separator for the print operator. If defined, this
1428value is printed between each of print's arguments. Default is C<undef>.
1430Mnemonic: what is printed when there is a "," in your print statement.
b0c22438 1432=item HANDLE->input_line_number(EXPR)
1436=item $NR
1438=item $.
1439X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
1441Current line number for the last filehandle accessed.
1443Each filehandle in Perl counts the number of lines that have been read
1444from it. (Depending on the value of C<$/>, Perl's idea of what
1445constitutes a line may not match yours.) When a line is read from a
1446filehandle (via C<readline()> or C<< <> >>), or when C<tell()> or
1447C<seek()> is called on it, C<$.> becomes an alias to the line counter
1448for that filehandle.
1450You can adjust the counter by assigning to C<$.>, but this will not
1451actually move the seek pointer. I<Localizing C<$.> will not localize
1452the filehandle's line count>. Instead, it will localize perl's notion
1453of which filehandle C<$.> is currently aliased to.
1455C<$.> is reset when the filehandle is closed, but B<not> when an open
1456filehandle is reopened without an intervening C<close()>. For more
1457details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
1458an explicit close, line numbers increase across C<ARGV> files (but see
1459examples in L<perlfunc/eof>).
1461You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
1462line counter for a given filehandle without having to worry about
1463which handle you last accessed.
1465Mnemonic: many programs use "." to mean the current line number.
84dabc03 1467=item HANDLE->input_record_separator(EXPR)
b0c22438 1468
1471=item $RS
1473=item $/
84dabc03 1476The input record separator, newline by default. This influences Perl's
1477idea of what a "line" is. Works like B<awk>'s RS variable, including
1478treating empty lines as a terminator if set to the null string (an
1479empty line cannot contain any spaces or tabs). You may set it to a
1480multi-character string to match a multi-character terminator, or to
1481C<undef> to read through the end of file. Setting it to C<"\n\n">
1482means something slightly different than setting to C<"">, if the file
1483contains consecutive empty lines. Setting to C<""> will treat two or
1484more consecutive empty lines as a single empty line. Setting to
1485C<"\n\n"> will blindly assume that the next input character belongs to
1486the next paragraph, even if it's a newline.
b0c22438 1487
1488 local $/; # enable "slurp" mode
1489 local $_ = <FH>; # whole file now here
1490 s/\n[ \t]+/ /g;
1492Remember: the value of C<$/> is a string, not a regex. B<awk> has to
1493be better for something. :-)
1495Setting C<$/> to a reference to an integer, scalar containing an
1496integer, or scalar that's convertible to an integer will attempt to
1497read records instead of lines, with the maximum record size being the
1498referenced integer. So this:
1500 local $/ = \32768; # or \"32768", or \$var_containing_32768
1501 open my $fh, "<", $myfile or die $!;
1502 local $_ = <$fh>;
fb73857a 1503
b0c22438 1504will read a record of no more than 32768 bytes from FILE. If you're
1505not reading from a record-oriented file (or your OS doesn't have
1506record-oriented files), then you'll likely get a full chunk of data
1507with every read. If a record is larger than the record size you've
1508set, you'll get the record back in pieces. Trying to set the record
1509size to zero or less will cause reading in the (rest of the) whole file.
6e2995f4 1510
b0c22438 1511On VMS, record reads are done with the equivalent of C<sysread>,
1512so it's best not to mix record and non-record reads on the same
1513file. (This is unlikely to be a problem, because any file you'd
1514want to read in record mode is probably unusable in line mode.)
1515Non-VMS systems do normal I/O, so it's safe to mix record and
1516non-record reads of a file.
5c055ba3 1517
b0c22438 1518See also L<perlport/"Newlines">. Also see C<$.>.
9bf22702 1519
b0c22438 1520Mnemonic: / delimits line boundaries when quoting poetry.
5c055ba3 1521
b0c22438 1522=item HANDLE->autoflush(EXPR)
916d64a3 1523
b0c22438 1524=item $OUTPUT_AUTOFLUSH
e2e27056 1525
84dabc03 1526=item Handle->output_record_separator EXPR
84902520 1527
b0c22438 1528=item $OUTPUT_RECORD_SEPARATOR
84902520 1529
b0c22438 1530=item $ORS
84902520 1531
b0c22438 1532=item $\
84902520 1534
b0c22438 1535The output record separator for the print operator. If defined, this
1536value is printed after the last of print's arguments. Default is C<undef>.
84902520 1537
b0c22438 1538Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
1539Also, it's just like C<$/>, but it's what you get "back" from Perl.
84902520 1540
84dabc03 1541=item $|
1542X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
84902520 1543
84dabc03 1544If set to nonzero, forces a flush right away and after every write or
1545print on the currently selected output channel. Default is 0
1546(regardless of whether the channel is really buffered by the system or
1547not; C<$|> tells you only whether you've asked Perl explicitly to
1548flush after each write). STDOUT will typically be line buffered if
1549output is to the terminal and block buffered otherwise. Setting this
1550variable is useful primarily when you are outputting to a pipe or
1551socket, such as when you are running a Perl program under B<rsh> and
1552want to see the output as it's happening. This has no effect on input
1553buffering. See L<perlfunc/getc> for that. See L<perldoc/select> on
1554how to select the output channel. See also L<IO::Handle>.
1556Mnemonic: when you want your pipes to be piping hot.
84902520 1559
b0c22438 1560=head3 Variables related to formats
83ee9e09 1561
b0c22438 1562The special variables for formats are a subset of those for
1563filehandles so they have
83ee9e09 1564
b0c22438 1565See L<perlform> for more information about Perl's formats.
83ee9e09 1566
b0c22438 1567=over 8
83ee9e09 1568
84dabc03 1569=item HANDLE->format_formfeed(EXPR)
1573=item $ACCUMULATOR
1575=item $^A
1578The current value of the C<write()> accumulator for C<format()> lines.
1579A format contains C<formline()> calls that put their result into
1580C<$^A>. After calling its format, C<write()> prints out the contents
1581of C<$^A> and empties. So you never really see the contents of C<$^A>
1582unless you call C<formline()> yourself and then look at it. See
1583L<perlform> and L<perlfunc/formline()>.
1585=item $^L
1588What formats output as a form feed. The default is C<\f>.
b0c22438 1590=item HANDLE->format_page_number(EXPR)
83ee9e09 1591
b0c22438 1592=item $FORMAT_PAGE_NUMBER
83ee9e09 1593
b0c22438 1594=item $%
83ee9e09 1596
b0c22438 1597The current page number of the currently selected output channel.
83ee9e09 1598
b0c22438 1599Mnemonic: C<%> is page number in B<nroff>.
7619c85e 1600
b0c22438 1601=item HANDLE->format_lines_left(EXPR)
b9ac3b5b 1602
b0c22438 1603=item $FORMAT_LINES_LEFT
66558a10 1604
b0c22438 1605=item $-
fb73857a 1607
b0c22438 1608The number of lines left on the page of the currently selected output
fa05a9fd 1610
b0c22438 1611Mnemonic: lines_on_page - lines_printed.
fa05a9fd 1612
84dabc03 1613=item Handle->format_line_break_characters EXPR
fb73857a 1614
a0d0e21e 1616
84dabc03 1617=item $:
a0d0e21e 1619
84dabc03 1620The current set of characters after which a string may be broken to
1621fill continuation fields (starting with C<^>) in a format. The default is
1622S<" \n-">, to break on a space, newline, or a hyphen.
a0d0e21e 1623
84dabc03 1624Mnemonic: a "colon" in poetry is a part of a line.
1626=item HANDLE->format_lines_per_page(EXPR)
1630=item $=
1633The current page length (printable lines) of the currently selected
1634output channel. The default is 60.
1636Mnemonic: = has horizontal lines.
7c36658b 1637
b0c22438 1638=item HANDLE->format_top_name(EXPR)
7c36658b 1639
b0c22438 1640=item $FORMAT_TOP_NAME
a05d7ebb 1641
b0c22438 1642=item $^
fde18df1 1644
b0c22438 1645The name of the current top-of-page format for the currently selected
1646output channel. The default is the name of the filehandle with C<_TOP>
1647appended. For example, the default format top name for the C<STDOUT>
1648filehanlde is C<STDOUT_TOP>.
e07ea26a 1649
b0c22438 1650Mnemonic: points to top of page.
e07ea26a 1651
84dabc03 1652=item HANDLE->format_name(EXPR)
16070b82 1653
84dabc03 1654=item $FORMAT_NAME
aa2f2a36 1655
84dabc03 1656=item $~
1657X<$~> X<$FORMAT_NAME>
aa2f2a36 1658
84dabc03 1659The name of the current report format for the currently selected
1660output channel. The default format name is the same as the filehandle
1661name. For example, the default format name for the C<STDOUT>
1662filehandle is just C<STDOUT>.
16070b82 1663
84dabc03 1664Mnemonic: brother to C<$^>.
16070b82 1665
b0c22438 1666=back
a0d0e21e 1667
84dabc03 1668=head2 Error Variables
b0c22438 1669X<error> X<exception>
a0d0e21e 1670
b0c22438 1671The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1672about different types of error conditions that may appear during
1673execution of a Perl program. The variables are shown ordered by
1674the "distance" between the subsystem which reported the error and
1675the Perl process. They correspond to errors detected by the Perl
1676interpreter, C library, operating system, or an external program,
4438c4b7 1678
b0c22438 1679To illustrate the differences between these variables, consider the
1680following Perl expression, which uses a single-quoted string:
4438c4b7 1681
b0c22438 1682 eval q{
1683 open my $pipe, "/cdrom/install |" or die $!;
1684 my @res = <$pipe>;
1685 close $pipe or die "bad pipe: $?, $!";
1686 };
a0d0e21e 1687
b0c22438 1688After execution of this statement all 4 variables may have been set.
2a8c8378 1689
84dabc03 1690C<$@> is set if the string to be C<eval>-ed did not compile (this may
1691happen if C<open> or C<close> were imported with bad prototypes), or
0b9346e6 1692if Perl code executed during evaluation C<die()>d . In these cases the
1693value of C<$@> is the compile error, or the argument to C<die> (which
84dabc03 1694will interpolate C<$!> and C<$?>). (See also L<Fatal>, though.)
2a8c8378 1695
0b9346e6 1696When the C<eval()> expression above is executed, C<open()>, C<< <PIPE> >>,
1697and C<close> are translated to calls in the C run-time library and
b0c22438 1698thence to the operating system kernel. C<$!> is set to the C library's
1699C<errno> if one of these calls fails.
2a8c8378 1700
84dabc03 1701Under a few operating systems, C<$^E> may contain a more verbose error
1702indicator, such as in this case, "CDROM tray not closed." Systems that
1703do not support extended error messages leave C<$^E> the same as C<$!>.
a0d0e21e 1704
b0c22438 1705Finally, C<$?> may be set to non-0 value if the external program
84dabc03 1706F</cdrom/install> fails. The upper eight bits reflect specific error
1707conditions encountered by the program (the program's C<exit()> value).
1708The lower eight bits reflect mode of failure, like signal death and
1709core dump information. See C<wait(2)> for details. In contrast to
1710C<$!> and C<$^E>, which are set only if error condition is detected,
1711the variable C<$?> is set on each C<wait> or pipe C<close>,
1712overwriting the old value. This is more like C<$@>, which on every
1713C<eval()> is always set on failure and cleared on success.
a0d0e21e 1714
b0c22438 1715For more details, see the individual descriptions at C<$@>, C<$!>,
1716C<$^E>, and C<$?>.
38e4f4ae 1717
0b9346e6 1718=over 8
84dabc03 1720=item $EXTENDED_OS_ERROR
2d84a16a 1721
b0c22438 1722=item ${^CHILD_ERROR_NATIVE}
a0d0e21e 1724
b0c22438 1725The native status returned by the last pipe close, backtick (C<``>)
1726command, successful call to C<wait()> or C<waitpid()>, or from the
1727C<system()> operator. On POSIX-like systems this value can be decoded
1729WSTOPSIG and WIFCONTINUED functions provided by the L<POSIX> module.
a0d0e21e 1730
b0c22438 1731Under VMS this reflects the actual VMS exit status; i.e. it is the
1732same as C<$?> when the pragma C<use vmsish 'status'> is in effect.
a0d0e21e 1733
b0c22438 1734This variable was added in Perl 5.8.9.
a0d0e21e 1735
84dabc03 1736=item $^E
1739Error information specific to the current operating system. At the
1740moment, this differs from C<$!> under only VMS, OS/2, and Win32 (and
1741for MacPerl). On all other platforms, C<$^E> is always just the same
1742as C<$!>.
1744Under VMS, C<$^E> provides the VMS status value from the last system
1745error. This is more specific information about the last system error
1746than that provided by C<$!>. This is particularly important when C<$!>
1747is set to B<EVMSERR>.
1749Under OS/2, C<$^E> is set to the error code of the last call to OS/2
1750API either via CRT, or directly from perl.
1752Under Win32, C<$^E> always returns the last error information reported
1753by the Win32 call C<GetLastError()> which describes the last error
1754from within the Win32 API. Most Win32-specific code will report errors
1755via C<$^E>. ANSI C and Unix-like calls set C<errno> and so most
1756portable Perl code will report errors via C<$!>.
1758Caveats mentioned in the description of C<$!> generally apply to
1759C<$^E>, also.
1761This variable was added in Perl 5.003.
1763Mnemonic: Extra error explanation.
0b9346e6 1764
84dabc03 1765=item $EXCEPTIONS_BEING_CAUGHT
1767=item $^S
1770Current state of the interpreter.
1772 $^S State
1773 --------- -------------------
1774 undef Parsing module/eval
1775 true (1) Executing an eval
1776 false (0) Otherwise
1778The first state may happen in C<$SIG{__DIE__}> and C<$SIG{__WARN__}>
1781This variable was added in Perl 5.004.
1783=item $WARNING
1785=item $^W
1786X<$^W> X<$WARNING>
1788The current value of the warning switch, initially true if B<-w> was
1789used, false otherwise, but directly modifiable.
1791See also L<warnings>.
0b9346e6 1793Mnemonic: related to the B<-w> switch.
84dabc03 1794
1795=item ${^WARNING_BITS}
1797The current set of warning checks enabled by the C<use warnings> pragma.
1798See the documentation of C<warnings> for more details.
1800This variable was added in Perl 5.10.
b0c22438 1802=item $OS_ERROR
5ccee41e 1803
b0c22438 1804=item $ERRNO
5ccee41e 1805
b0c22438 1806=item $!
1807X<$!> X<$ERRNO> X<$OS_ERROR>
9b0e6e7a 1808
b0c22438 1809If used numerically, yields the current value of the C C<errno>
1810variable, or in other words, if a system or library call fails, it
1811sets this variable. This means that the value of C<$!> is meaningful
1812only I<immediately> after a B<failure>:
9b0e6e7a 1813
b0c22438 1814 if (open my $fh, "<", $filename) {
1815 # Here $! is meaningless.
1816 ...
1817 } else {
1818 # ONLY here is $! meaningful.
1819 ...
1820 # Already here $! might be meaningless.
1821 }
1822 # Since here we might have either success or failure,
1823 # here $! is meaningless.
a0d0e21e 1824
b0c22438 1825In the above I<meaningless> stands for anything: zero, non-zero,
84dabc03 1826C<undef>. A successful system or library call does B<not> set the
1827variable to zero.
a0d0e21e 1828
84dabc03 1829If used as a string, yields the corresponding system error string. You
1830can assign a number to C<$!> to set I<errno> if, for instance, you
1831want C<"$!"> to return the string for error I<n>, or you want to set
1832the exit value for the C<die()> operator.
d54b56d5 1833
b0c22438 1834Mnemonic: What just went bang?
314d39ce 1835
b0c22438 1836=item %OS_ERROR
fb73857a 1837
b0c22438 1838=item %ERRNO
fb73857a 1839
b0c22438 1840=item %!
a0d0e21e 1842
b0c22438 1843Each element of C<%!> has a true value only if C<$!> is set to that
1844value. For example, C<$!{ENOENT}> is true if and only if the current
84dabc03 1845value of C<$!> is C<ENOENT>; that is, if the most recent error was "No
1846such file or directory" (or its moral equivalent: not all operating
1847systems give that exact error, and certainly not all languages). To
1848check if a particular key is meaningful on your system, use C<exists
1849$!{the_key}>; for a list of legal keys, use C<keys %!>. See L<Errno>
1850for more information, and also see above for the validity of C<$!>.
a0d0e21e 1851
b0c22438 1852This variable was added in Perl 5.005.
44f0be63 1853
84dabc03 1854=item $CHILD_ERROR
b687b08b 1855
84dabc03 1856=item $?
1857X<$?> X<$CHILD_ERROR>
a0d0e21e 1858
84dabc03 1859The status returned by the last pipe close, backtick (C<``>) command,
1860successful call to C<wait()> or C<waitpid()>, or from the C<system()>
1861operator. This is just the 16-bit status word returned by the
1862traditional Unix C<wait()> system call (or else is made up to look
1863like it). Thus, the exit value of the subprocess is really (C<<< $? >>
18648 >>>), and C<$? & 127> gives which signal, if any, the process died
1865from, and C<$? & 128> reports whether there was a core dump.
a0d0e21e 1866
84dabc03 1867Additionally, if the C<h_errno> variable is supported in C, its value
1868is returned via C<$?> if any C<gethost*()> function fails.
b687b08b 1869
84dabc03 1870If you have installed a signal handler for C<SIGCHLD>, the
1871value of C<$?> will usually be wrong outside that handler.
a0d0e21e 1872
84dabc03 1873Inside an C<END> subroutine C<$?> contains the value that is going to be
1874given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
1875change the exit status of your program. For example:
a0d0e21e 1876
84dabc03 1877 END {
1878 $? = 1 if $? == 255; # die would make it 255
1879 }
a0d0e21e 1880
84dabc03 1881Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
1882actual VMS exit status, instead of the default emulation of POSIX
1883status; see L<perlvms/$?> for details.
1885Mnemonic: similar to B<sh> and B<ksh>.
a0d0e21e 1886
b0c22438 1887=item $EVAL_ERROR
f648820c 1888
b0c22438 1889=item $@
1890X<$@> X<$EVAL_ERROR>
a0d0e21e 1891
0b9346e6 1892The Perl syntax error message from the last C<eval()> operator. If C<$@> is
1893the null string, the last C<eval()> parsed and executed correctly
b0c22438 1894(although the operations you invoked may have failed in the normal
a0d0e21e 1896
b0c22438 1897Warning messages are not collected in this variable. You can, however,
1898set up a routine to process warnings by setting C<$SIG{__WARN__}> as
1899described below.
748a9306 1900
b0c22438 1901Mnemonic: Where was the syntax error "at"?
7f315d2e 1902
b0c22438 1903=back
7f315d2e 1904
b0c22438 1905=head2 Deprecated and removed variables
7f315d2e 1906
0b9346e6 1907Deprecating a variable announces the intent of the perl maintainers to
84dabc03 1908eventually remove the variable from the langauge. It may still be
b0c22438 1909available despite its status. Using a deprecated variable triggers
1910a warning.
7f315d2e 1911
84dabc03 1912Once a variable is removed, its use triggers an error telling you
b0c22438 1913the variable is unsupported.
7f315d2e 1914
84dabc03 1915See L<perldiag> for details about error messages.
7f315d2e 1916
b0c22438 1917=over 8
7f315d2e 1918
84dabc03 1919=item $#
1922C<$#> was a variable that you could be use to format printed numbers.
1923After a deprecation cycle, its magic was removed in Perl 5.10 and
1924using it now triggers a warning: C<$# is no longer supported>.
1926This is not the sigil you use in front of an array name to get the
1927last index, like C<$#array>. That's still how you get the last index
1928of an array in Perl. The two have nothing to do with each other.
1930Deprecated in Perl 5.
1932Removed in Perl 5.10.
1934=item $*
84dabc03 1937C<$*> was a variable that you could use to enable multiline matching.
1938After a deprecation cycle, its magic was removed in Perl 5.10.
1939Using it now triggers a warning: C<$* is no longer supported>.
84dabc03 1940You should use the C</s> and C</m> regexp modifiers instead.
7f315d2e 1941
b0c22438 1942Deprecated in Perl 5.
7f315d2e 1943
b0c22438 1944Removed in Perl 5.10.
7f315d2e 1945
84dabc03 1946=item $[
1949This variable stores the index of the first element in an array, and
1950of the first character in a substring. You use to be able to assign to
1951this variable, but you can't do that anymore. It's now always 0, like
0b9346e6 1952it should be.
84dabc03 1953
1954Mnemonic: [ begins subscripts.
1956This variable is read-only.
0b9346e6 1958Deprecated in Perl 5.12.
84dabc03 1959
b0c22438 1960=item $]
55602bd2 1962
b0c22438 1963The version + patchlevel / 1000 of the Perl interpreter. This variable
1964can be used to determine whether the Perl interpreter executing a
1965script is in the right range of versions:
55602bd2 1966
b0c22438 1967 warn "No checksumming!\n" if $] < 3.019;
55602bd2 1968
b0c22438 1969See also the documentation of C<use VERSION> and C<require VERSION>
1970for a convenient way to fail if the running Perl interpreter is too old.
55602bd2 1971
b0c22438 1972The floating point representation can sometimes lead to inaccurate
1973numeric comparisons. See C<$^V> for a more modern representation of
1974the Perl version that allows accurate string comparisons.
55602bd2 1975
b0c22438 1976Mnemonic: Is this version of perl in the right bracket?
19799a22 1977
b0c22438 1978Deprecated in Perl 5.6.
19799a22 1979
b0c22438 1980=back
2b92dfce 1981
0b9346e6 1982=cut