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1=head1 NAME
2
3perlvar - Perl predefined variables
4
5=head1 DESCRIPTION
6
7=head2 Predefined Names
8
5a964f20 9The following names have special meaning to Perl. Most
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10punctuation names have reasonable mnemonics, or analogs in the
11shells. Nevertheless, if you wish to use long variable names,
12you need only say
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13
14 use English;
15
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16at the top of your program. This aliases all the short names to the long
17names in the current package. Some even have medium names, generally
18borrowed from B<awk>. In general, it's best to use the
a0d0e21e 19
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20 use English '-no_match_vars';
21
22invocation if you don't need $PREMATCH, $MATCH, or $POSTMATCH, as it avoids
23a certain performance hit with the use of regular expressions. See
24L<English>.
25
26Variables that depend on the currently selected filehandle may be set by
27calling an appropriate object method on the IO::Handle object, although
28this is less efficient than using the regular built-in variables. (Summary
29lines below for this contain the word HANDLE.) First you must say
a0d0e21e 30
19799a22 31 use IO::Handle;
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32
33after which you may use either
34
35 method HANDLE EXPR
36
5a964f20 37or more safely,
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38
39 HANDLE->method(EXPR)
40
14218588 41Each method returns the old value of the IO::Handle attribute.
a1ce9542 42The methods each take an optional EXPR, which, if supplied, specifies the
19799a22 43new value for the IO::Handle attribute in question. If not supplied,
14218588 44most methods do nothing to the current value--except for
a0d0e21e 45autoflush(), which will assume a 1 for you, just to be different.
a1ce9542 46
14218588 47Because loading in the IO::Handle class is an expensive operation, you should
19799a22 48learn how to use the regular built-in variables.
a0d0e21e 49
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50A few of these variables are considered "read-only". This means that if
51you try to assign to this variable, either directly or indirectly through
52a reference, you'll raise a run-time exception.
a0d0e21e 53
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54You should be very careful when modifying the default values of most
55special variables described in this document. In most cases you want
56to localize these variables before changing them, since if you don't,
57the change may affect other modules which rely on the default values
58of the special variables that you have changed. This is one of the
59correct ways to read the whole file at once:
60
61 open my $fh, "foo" or die $!;
62 local $/; # enable localized slurp mode
63 my $content = <$fh>;
64 close $fh;
65
66But the following code is quite bad:
67
68 open my $fh, "foo" or die $!;
69 undef $/; # enable slurp mode
70 my $content = <$fh>;
71 close $fh;
72
73since some other module, may want to read data from some file in the
74default "line mode", so if the code we have just presented has been
75executed, the global value of C<$/> is now changed for any other code
76running inside the same Perl interpreter.
77
78Usually when a variable is localized you want to make sure that this
79change affects the shortest scope possible. So unless you are already
80inside some short C<{}> block, you should create one yourself. For
81example:
82
83 my $content = '';
84 open my $fh, "foo" or die $!;
85 {
86 local $/;
87 $content = <$fh>;
88 }
89 close $fh;
90
91Here is an example of how your own code can go broken:
92
93 for (1..5){
94 nasty_break();
95 print "$_ ";
96 }
97 sub nasty_break {
98 $_ = 5;
99 # do something with $_
100 }
101
102You probably expect this code to print:
103
104 1 2 3 4 5
105
106but instead you get:
107
108 5 5 5 5 5
109
110Why? Because nasty_break() modifies C<$_> without localizing it
111first. The fix is to add local():
112
113 local $_ = 5;
114
115It's easy to notice the problem in such a short example, but in more
116complicated code you are looking for trouble if you don't localize
117changes to the special variables.
118
fb73857a 119The following list is ordered by scalar variables first, then the
87275199 120arrays, then the hashes.
fb73857a 121
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122=over 8
123
124=item $ARG
125
126=item $_
127
128The default input and pattern-searching space. The following pairs are
129equivalent:
130
19799a22 131 while (<>) {...} # equivalent only in while!
54310121 132 while (defined($_ = <>)) {...}
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133
134 /^Subject:/
135 $_ =~ /^Subject:/
136
137 tr/a-z/A-Z/
138 $_ =~ tr/a-z/A-Z/
139
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140 chomp
141 chomp($_)
a0d0e21e 142
54310121 143Here are the places where Perl will assume $_ even if you
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144don't use it:
145
146=over 3
147
148=item *
149
150Various unary functions, including functions like ord() and int(), as well
151as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
152STDIN.
153
154=item *
155
156Various list functions like print() and unlink().
157
158=item *
159
160The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
161without an C<=~> operator.
162
54310121 163=item *
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164
165The default iterator variable in a C<foreach> loop if no other
166variable is supplied.
167
54310121 168=item *
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169
170The implicit iterator variable in the grep() and map() functions.
171
54310121 172=item *
cb1a09d0 173
c47ff5f1 174The default place to put an input record when a C<< <FH> >>
cb1a09d0 175operation's result is tested by itself as the sole criterion of a C<while>
14218588 176test. Outside a C<while> test, this will not happen.
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177
178=back
179
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180As C<$_> is a global variable, this may lead in some cases to unwanted
181side-effects. As of perl 5.9.1, you can now use a lexical version of
182C<$_> by declaring it in a file or in a block with C<my>. Moreover,
183declaring C<our $> restores the global C<$_> in the current scope.
184
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185(Mnemonic: underline is understood in certain operations.)
186
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187=back
188
189=over 8
190
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191=item $a
192
193=item $b
194
195Special package variables when using sort(), see L<perlfunc/sort>.
196Because of this specialness $a and $b don't need to be declared
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197(using use vars, or our()) even when using the C<strict 'vars'> pragma.
198Don't lexicalize them with C<my $a> or C<my $b> if you want to be
199able to use them in the sort() comparison block or function.
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200
201=back
202
203=over 8
204
c47ff5f1 205=item $<I<digits>>
a0d0e21e 206
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207Contains the subpattern from the corresponding set of capturing
208parentheses from the last pattern match, not counting patterns
209matched in nested blocks that have been exited already. (Mnemonic:
210like \digits.) These variables are all read-only and dynamically
211scoped to the current BLOCK.
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212
213=item $MATCH
214
215=item $&
216
217The string matched by the last successful pattern match (not counting
218any matches hidden within a BLOCK or eval() enclosed by the current
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219BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
220and dynamically scoped to the current BLOCK.
a0d0e21e 221
19ddd453 222The use of this variable anywhere in a program imposes a considerable
667e1aea 223performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 224
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225=item $PREMATCH
226
227=item $`
228
229The string preceding whatever was matched by the last successful
230pattern match (not counting any matches hidden within a BLOCK or eval
a8f8344d 231enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
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232string.) This variable is read-only.
233
19ddd453 234The use of this variable anywhere in a program imposes a considerable
667e1aea 235performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 236
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237=item $POSTMATCH
238
239=item $'
240
241The string following whatever was matched by the last successful
242pattern match (not counting any matches hidden within a BLOCK or eval()
a8f8344d 243enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
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244string.) Example:
245
22d0716c 246 local $_ = 'abcdefghi';
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247 /def/;
248 print "$`:$&:$'\n"; # prints abc:def:ghi
249
19799a22 250This variable is read-only and dynamically scoped to the current BLOCK.
a0d0e21e 251
19ddd453 252The use of this variable anywhere in a program imposes a considerable
667e1aea 253performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 254
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255=item $LAST_PAREN_MATCH
256
257=item $+
258
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259The text matched by the last bracket of the last successful search pattern.
260This is useful if you don't know which one of a set of alternative patterns
261matched. For example:
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262
263 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
264
265(Mnemonic: be positive and forward looking.)
19799a22 266This variable is read-only and dynamically scoped to the current BLOCK.
a0d0e21e 267
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268=item $^N
269
270The text matched by the used group most-recently closed (i.e. the group
271with the rightmost closing parenthesis) of the last successful search
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272pattern. (Mnemonic: the (possibly) Nested parenthesis that most
273recently closed.)
274
210b36aa 275This is primarily used inside C<(?{...})> blocks for examining text
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276recently matched. For example, to effectively capture text to a variable
277(in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
278
279 (?:(...)(?{ $var = $^N }))
280
281By setting and then using C<$var> in this way relieves you from having to
282worry about exactly which numbered set of parentheses they are.
283
284This variable is dynamically scoped to the current BLOCK.
285
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286=item @LAST_MATCH_END
287
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288=item @+
289
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290This array holds the offsets of the ends of the last successful
291submatches in the currently active dynamic scope. C<$+[0]> is
292the offset into the string of the end of the entire match. This
293is the same value as what the C<pos> function returns when called
294on the variable that was matched against. The I<n>th element
295of this array holds the offset of the I<n>th submatch, so
296C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
297past where $2 ends, and so on. You can use C<$#+> to determine
298how many subgroups were in the last successful match. See the
299examples given for the C<@-> variable.
6cef1e77 300
fcc7d916 301=item HANDLE->input_line_number(EXPR)
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302
303=item $INPUT_LINE_NUMBER
304
305=item $NR
306
307=item $.
308
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309Current line number for the last filehandle accessed.
310
311Each filehandle in Perl counts the number of lines that have been read
312from it. (Depending on the value of C<$/>, Perl's idea of what
313constitutes a line may not match yours.) When a line is read from a
314filehandle (via readline() or C<< <> >>), or when tell() or seek() is
315called on it, C<$.> becomes an alias to the line counter for that
316filehandle.
317
318You can adjust the counter by assigning to C<$.>, but this will not
319actually move the seek pointer. I<Localizing C<$.> will not localize
320the filehandle's line count>. Instead, it will localize perl's notion
321of which filehandle C<$.> is currently aliased to.
322
323C<$.> is reset when the filehandle is closed, but B<not> when an open
324filehandle is reopened without an intervening close(). For more
e48df184 325details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
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326an explicit close, line numbers increase across ARGV files (but see
327examples in L<perlfunc/eof>).
328
329You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
330line counter for a given filehandle without having to worry about
331which handle you last accessed.
332
333(Mnemonic: many programs use "." to mean the current line number.)
334
335=item IO::Handle->input_record_separator(EXPR)
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336
337=item $INPUT_RECORD_SEPARATOR
338
339=item $RS
340
341=item $/
342
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343The input record separator, newline by default. This
344influences Perl's idea of what a "line" is. Works like B<awk>'s RS
19799a22 345variable, including treating empty lines as a terminator if set to
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346the null string. (An empty line cannot contain any spaces
347or tabs.) You may set it to a multi-character string to match a
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348multi-character terminator, or to C<undef> to read through the end
349of file. Setting it to C<"\n\n"> means something slightly
350different than setting to C<"">, if the file contains consecutive
351empty lines. Setting to C<""> will treat two or more consecutive
352empty lines as a single empty line. Setting to C<"\n\n"> will
353blindly assume that the next input character belongs to the next
14218588 354paragraph, even if it's a newline. (Mnemonic: / delimits
19799a22 355line boundaries when quoting poetry.)
a0d0e21e 356
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357 local $/; # enable "slurp" mode
358 local $_ = <FH>; # whole file now here
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359 s/\n[ \t]+/ /g;
360
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361Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
362better for something. :-)
68dc0745 363
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364Setting C<$/> to a reference to an integer, scalar containing an integer, or
365scalar that's convertible to an integer will attempt to read records
5b2b9c68 366instead of lines, with the maximum record size being the referenced
19799a22 367integer. So this:
5b2b9c68 368
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369 local $/ = \32768; # or \"32768", or \$var_containing_32768
370 open my $fh, $myfile or die $!;
371 local $_ = <$fh>;
5b2b9c68 372
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373will read a record of no more than 32768 bytes from FILE. If you're
374not reading from a record-oriented file (or your OS doesn't have
375record-oriented files), then you'll likely get a full chunk of data
376with every read. If a record is larger than the record size you've
377set, you'll get the record back in pieces.
5b2b9c68 378
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379On VMS, record reads are done with the equivalent of C<sysread>,
380so it's best not to mix record and non-record reads on the same
381file. (This is unlikely to be a problem, because any file you'd
83763826 382want to read in record mode is probably unusable in line mode.)
14218588 383Non-VMS systems do normal I/O, so it's safe to mix record and
19799a22 384non-record reads of a file.
5b2b9c68 385
14218588 386See also L<perlport/"Newlines">. Also see C<$.>.
883faa13 387
fcc7d916 388=item HANDLE->autoflush(EXPR)
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389
390=item $OUTPUT_AUTOFLUSH
391
392=item $|
393
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394If set to nonzero, forces a flush right away and after every write
395or print on the currently selected output channel. Default is 0
14218588 396(regardless of whether the channel is really buffered by the
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397system or not; C<$|> tells you only whether you've asked Perl
398explicitly to flush after each write). STDOUT will
399typically be line buffered if output is to the terminal and block
400buffered otherwise. Setting this variable is useful primarily when
401you are outputting to a pipe or socket, such as when you are running
402a Perl program under B<rsh> and want to see the output as it's
403happening. This has no effect on input buffering. See L<perlfunc/getc>
404for that. (Mnemonic: when you want your pipes to be piping hot.)
a0d0e21e 405
46550894 406=item IO::Handle->output_field_separator EXPR
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407
408=item $OUTPUT_FIELD_SEPARATOR
409
410=item $OFS
411
412=item $,
413
414The output field separator for the print operator. Ordinarily the
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415print operator simply prints out its arguments without further
416adornment. To get behavior more like B<awk>, set this variable as
417you would set B<awk>'s OFS variable to specify what is printed
418between fields. (Mnemonic: what is printed when there is a "," in
419your print statement.)
a0d0e21e 420
46550894 421=item IO::Handle->output_record_separator EXPR
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422
423=item $OUTPUT_RECORD_SEPARATOR
424
425=item $ORS
426
427=item $\
428
429The output record separator for the print operator. Ordinarily the
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430print operator simply prints out its arguments as is, with no
431trailing newline or other end-of-record string added. To get
432behavior more like B<awk>, set this variable as you would set
433B<awk>'s ORS variable to specify what is printed at the end of the
434print. (Mnemonic: you set C<$\> instead of adding "\n" at the
435end of the print. Also, it's just like C<$/>, but it's what you
436get "back" from Perl.)
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437
438=item $LIST_SEPARATOR
439
440=item $"
441
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442This is like C<$,> except that it applies to array and slice values
443interpolated into a double-quoted string (or similar interpreted
444string). Default is a space. (Mnemonic: obvious, I think.)
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445
446=item $SUBSCRIPT_SEPARATOR
447
448=item $SUBSEP
449
450=item $;
451
54310121 452The subscript separator for multidimensional array emulation. If you
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453refer to a hash element as
454
455 $foo{$a,$b,$c}
456
457it really means
458
459 $foo{join($;, $a, $b, $c)}
460
461But don't put
462
463 @foo{$a,$b,$c} # a slice--note the @
464
465which means
466
467 ($foo{$a},$foo{$b},$foo{$c})
468
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469Default is "\034", the same as SUBSEP in B<awk>. If your
470keys contain binary data there might not be any safe value for C<$;>.
a0d0e21e 471(Mnemonic: comma (the syntactic subscript separator) is a
19799a22 472semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
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473taken for something more important.)
474
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475Consider using "real" multidimensional arrays as described
476in L<perllol>.
a0d0e21e 477
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478=item $#
479
480The output format for printed numbers. This variable is a half-hearted
481attempt to emulate B<awk>'s OFMT variable. There are times, however,
14218588 482when B<awk> and Perl have differing notions of what counts as
19799a22 483numeric. The initial value is "%.I<n>g", where I<n> is the value
6e2995f4 484of the macro DBL_DIG from your system's F<float.h>. This is different from
19799a22 485B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#>
6e2995f4 486explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
a0d0e21e 487
19799a22 488Use of C<$#> is deprecated.
a0d0e21e 489
fcc7d916 490=item HANDLE->format_page_number(EXPR)
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491
492=item $FORMAT_PAGE_NUMBER
493
494=item $%
495
496The current page number of the currently selected output channel.
19799a22 497Used with formats.
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498(Mnemonic: % is page number in B<nroff>.)
499
fcc7d916 500=item HANDLE->format_lines_per_page(EXPR)
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501
502=item $FORMAT_LINES_PER_PAGE
503
504=item $=
505
506The current page length (printable lines) of the currently selected
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507output channel. Default is 60.
508Used with formats.
509(Mnemonic: = has horizontal lines.)
a0d0e21e 510
fcc7d916 511=item HANDLE->format_lines_left(EXPR)
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512
513=item $FORMAT_LINES_LEFT
514
515=item $-
516
517The number of lines left on the page of the currently selected output
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518channel.
519Used with formats.
520(Mnemonic: lines_on_page - lines_printed.)
a0d0e21e 521
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522=item @LAST_MATCH_START
523
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524=item @-
525
19799a22 526$-[0] is the offset of the start of the last successful match.
6cef1e77 527C<$-[>I<n>C<]> is the offset of the start of the substring matched by
8f580fb8 528I<n>-th subpattern, or undef if the subpattern did not match.
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529
530Thus after a match against $_, $& coincides with C<substr $_, $-[0],
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531$+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
532$+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
c47ff5f1 533C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last
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534matched subgroup in the last successful match. Contrast with
535C<$#+>, the number of subgroups in the regular expression. Compare
19799a22 536with C<@+>.
6cef1e77 537
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538This array holds the offsets of the beginnings of the last
539successful submatches in the currently active dynamic scope.
540C<$-[0]> is the offset into the string of the beginning of the
541entire match. The I<n>th element of this array holds the offset
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542of the I<n>th submatch, so C<$-[1]> is the offset where $1
543begins, C<$-[2]> the offset where $2 begins, and so on.
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544
545After a match against some variable $var:
546
547=over 5
548
4375e838 549=item C<$`> is the same as C<substr($var, 0, $-[0])>
4ba05bdc 550
4375e838 551=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
4ba05bdc 552
4375e838 553=item C<$'> is the same as C<substr($var, $+[0])>
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554
555=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
556
557=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
558
4375e838 559=item C<$3> is the same as C<substr $var, $-[3], $+[3] - $-[3])>
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560
561=back
562
fcc7d916 563=item HANDLE->format_name(EXPR)
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564
565=item $FORMAT_NAME
566
567=item $~
568
569The name of the current report format for the currently selected output
14218588 570channel. Default is the name of the filehandle. (Mnemonic: brother to
19799a22 571C<$^>.)
a0d0e21e 572
fcc7d916 573=item HANDLE->format_top_name(EXPR)
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574
575=item $FORMAT_TOP_NAME
576
577=item $^
578
579The name of the current top-of-page format for the currently selected
14218588 580output channel. Default is the name of the filehandle with _TOP
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581appended. (Mnemonic: points to top of page.)
582
46550894 583=item IO::Handle->format_line_break_characters EXPR
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584
585=item $FORMAT_LINE_BREAK_CHARACTERS
586
587=item $:
588
589The current set of characters after which a string may be broken to
54310121 590fill continuation fields (starting with ^) in a format. Default is
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591S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
592poetry is a part of a line.)
593
46550894 594=item IO::Handle->format_formfeed EXPR
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595
596=item $FORMAT_FORMFEED
597
598=item $^L
599
14218588 600What formats output as a form feed. Default is \f.
a0d0e21e
LW
601
602=item $ACCUMULATOR
603
604=item $^A
605
606The current value of the write() accumulator for format() lines. A format
19799a22 607contains formline() calls that put their result into C<$^A>. After
a0d0e21e 608calling its format, write() prints out the contents of C<$^A> and empties.
14218588 609So you never really see the contents of C<$^A> unless you call
a0d0e21e
LW
610formline() yourself and then look at it. See L<perlform> and
611L<perlfunc/formline()>.
612
613=item $CHILD_ERROR
614
615=item $?
616
54310121 617The status returned by the last pipe close, backtick (C<``>) command,
19799a22
GS
618successful call to wait() or waitpid(), or from the system()
619operator. This is just the 16-bit status word returned by the
620wait() system call (or else is made up to look like it). Thus, the
c47ff5f1 621exit value of the subprocess is really (C<<< $? >> 8 >>>), and
19799a22
GS
622C<$? & 127> gives which signal, if any, the process died from, and
623C<$? & 128> reports whether there was a core dump. (Mnemonic:
624similar to B<sh> and B<ksh>.)
a0d0e21e 625
7b8d334a 626Additionally, if the C<h_errno> variable is supported in C, its value
14218588 627is returned via $? if any C<gethost*()> function fails.
7b8d334a 628
19799a22 629If you have installed a signal handler for C<SIGCHLD>, the
aa689395
PP
630value of C<$?> will usually be wrong outside that handler.
631
a8f8344d
PP
632Inside an C<END> subroutine C<$?> contains the value that is going to be
633given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
19799a22
GS
634change the exit status of your program. For example:
635
636 END {
637 $? = 1 if $? == 255; # die would make it 255
638 }
a8f8344d 639
aa689395 640Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
ff0cee69 641actual VMS exit status, instead of the default emulation of POSIX
9bc98430 642status; see L<perlvms/$?> for details.
f86702cc 643
55602bd2
IZ
644Also see L<Error Indicators>.
645
0a378802
JH
646=item ${^ENCODING}
647
740bd165
PN
648The I<object reference> to the Encode object that is used to convert
649the source code to Unicode. Thanks to this variable your perl script
650does not have to be written in UTF-8. Default is I<undef>. The direct
651manipulation of this variable is highly discouraged. See L<encoding>
048c20cb 652for more details.
0a378802 653
a0d0e21e
LW
654=item $OS_ERROR
655
656=item $ERRNO
657
658=item $!
659
19799a22 660If used numerically, yields the current value of the C C<errno>
6ab308ee
JH
661variable, or in other words, if a system or library call fails, it
662sets this variable. This means that the value of C<$!> is meaningful
663only I<immediately> after a B<failure>:
664
665 if (open(FH, $filename)) {
666 # Here $! is meaningless.
667 ...
668 } else {
669 # ONLY here is $! meaningful.
670 ...
671 # Already here $! might be meaningless.
672 }
673 # Since here we might have either success or failure,
674 # here $! is meaningless.
675
676In the above I<meaningless> stands for anything: zero, non-zero,
677C<undef>. A successful system or library call does B<not> set
678the variable to zero.
679
271df126 680If used as a string, yields the corresponding system error string.
19799a22
GS
681You can assign a number to C<$!> to set I<errno> if, for instance,
682you want C<"$!"> to return the string for error I<n>, or you want
683to set the exit value for the die() operator. (Mnemonic: What just
684went bang?)
a0d0e21e 685
55602bd2
IZ
686Also see L<Error Indicators>.
687
4c5cef9b
MJD
688=item %!
689
690Each element of C<%!> has a true value only if C<$!> is set to that
691value. For example, C<$!{ENOENT}> is true if and only if the current
3be065a1
JH
692value of C<$!> is C<ENOENT>; that is, if the most recent error was
693"No such file or directory" (or its moral equivalent: not all operating
694systems give that exact error, and certainly not all languages).
695To check if a particular key is meaningful on your system, use
696C<exists $!{the_key}>; for a list of legal keys, use C<keys %!>.
697See L<Errno> for more information, and also see above for the
698validity of C<$!>.
4c5cef9b 699
5c055ba3
PP
700=item $EXTENDED_OS_ERROR
701
702=item $^E
703
22fae026
TM
704Error information specific to the current operating system. At
705the moment, this differs from C<$!> under only VMS, OS/2, and Win32
706(and for MacPerl). On all other platforms, C<$^E> is always just
707the same as C<$!>.
708
709Under VMS, C<$^E> provides the VMS status value from the last
710system error. This is more specific information about the last
711system error than that provided by C<$!>. This is particularly
d516a115 712important when C<$!> is set to B<EVMSERR>.
22fae026 713
1c1c7f20
GS
714Under OS/2, C<$^E> is set to the error code of the last call to
715OS/2 API either via CRT, or directly from perl.
22fae026
TM
716
717Under Win32, C<$^E> always returns the last error information
718reported by the Win32 call C<GetLastError()> which describes
719the last error from within the Win32 API. Most Win32-specific
19799a22 720code will report errors via C<$^E>. ANSI C and Unix-like calls
22fae026
TM
721set C<errno> and so most portable Perl code will report errors
722via C<$!>.
723
724Caveats mentioned in the description of C<$!> generally apply to
725C<$^E>, also. (Mnemonic: Extra error explanation.)
5c055ba3 726
55602bd2
IZ
727Also see L<Error Indicators>.
728
a0d0e21e
LW
729=item $EVAL_ERROR
730
731=item $@
732
4a280ebe
JG
733The Perl syntax error message from the last eval() operator.
734If $@ is the null string, the last eval() parsed and executed
735correctly (although the operations you invoked may have failed in the
736normal fashion). (Mnemonic: Where was the syntax error "at"?)
a0d0e21e 737
19799a22 738Warning messages are not collected in this variable. You can,
a8f8344d 739however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
54310121 740as described below.
748a9306 741
55602bd2
IZ
742Also see L<Error Indicators>.
743
a0d0e21e
LW
744=item $PROCESS_ID
745
746=item $PID
747
748=item $$
749
19799a22
GS
750The process number of the Perl running this script. You should
751consider this variable read-only, although it will be altered
752across fork() calls. (Mnemonic: same as shells.)
a0d0e21e 753
4d76a344
RGS
754Note for Linux users: on Linux, the C functions C<getpid()> and
755C<getppid()> return different values from different threads. In order to
756be portable, this behavior is not reflected by C<$$>, whose value remains
757consistent across threads. If you want to call the underlying C<getpid()>,
e3256f86 758you may use the CPAN module C<Linux::Pid>.
4d76a344 759
a0d0e21e
LW
760=item $REAL_USER_ID
761
762=item $UID
763
764=item $<
765
19799a22 766The real uid of this process. (Mnemonic: it's the uid you came I<from>,
a043a685
GW
767if you're running setuid.) You can change both the real uid and
768the effective uid at the same time by using POSIX::setuid().
a0d0e21e
LW
769
770=item $EFFECTIVE_USER_ID
771
772=item $EUID
773
774=item $>
775
776The effective uid of this process. Example:
777
778 $< = $>; # set real to effective uid
779 ($<,$>) = ($>,$<); # swap real and effective uid
780
a043a685
GW
781You can change both the effective uid and the real uid at the same
782time by using POSIX::setuid().
783
19799a22 784(Mnemonic: it's the uid you went I<to>, if you're running setuid.)
c47ff5f1 785C<< $< >> and C<< $> >> can be swapped only on machines
8cc95fdb 786supporting setreuid().
a0d0e21e
LW
787
788=item $REAL_GROUP_ID
789
790=item $GID
791
792=item $(
793
794The real gid of this process. If you are on a machine that supports
795membership in multiple groups simultaneously, gives a space separated
796list of groups you are in. The first number is the one returned by
797getgid(), and the subsequent ones by getgroups(), one of which may be
8cc95fdb
PP
798the same as the first number.
799
19799a22
GS
800However, a value assigned to C<$(> must be a single number used to
801set the real gid. So the value given by C<$(> should I<not> be assigned
802back to C<$(> without being forced numeric, such as by adding zero.
8cc95fdb 803
a043a685
GW
804You can change both the real gid and the effective gid at the same
805time by using POSIX::setgid().
806
19799a22
GS
807(Mnemonic: parentheses are used to I<group> things. The real gid is the
808group you I<left>, if you're running setgid.)
a0d0e21e
LW
809
810=item $EFFECTIVE_GROUP_ID
811
812=item $EGID
813
814=item $)
815
816The effective gid of this process. If you are on a machine that
817supports membership in multiple groups simultaneously, gives a space
818separated list of groups you are in. The first number is the one
819returned by getegid(), and the subsequent ones by getgroups(), one of
8cc95fdb
PP
820which may be the same as the first number.
821
19799a22 822Similarly, a value assigned to C<$)> must also be a space-separated
14218588 823list of numbers. The first number sets the effective gid, and
8cc95fdb
PP
824the rest (if any) are passed to setgroups(). To get the effect of an
825empty list for setgroups(), just repeat the new effective gid; that is,
826to force an effective gid of 5 and an effectively empty setgroups()
827list, say C< $) = "5 5" >.
828
a043a685
GW
829You can change both the effective gid and the real gid at the same
830time by using POSIX::setgid() (use only a single numeric argument).
831
19799a22
GS
832(Mnemonic: parentheses are used to I<group> things. The effective gid
833is the group that's I<right> for you, if you're running setgid.)
a0d0e21e 834
c47ff5f1 835C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
19799a22
GS
836machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
837and C<$)> can be swapped only on machines supporting setregid().
a0d0e21e
LW
838
839=item $PROGRAM_NAME
840
841=item $0
842
80bca1b4
JH
843Contains the name of the program being executed.
844
845On some (read: not all) operating systems assigning to C<$0> modifies
846the argument area that the C<ps> program sees. On some platforms you
847may have to use special C<ps> options or a different C<ps> to see the
848changes. Modifying the $0 is more useful as a way of indicating the
849current program state than it is for hiding the program you're
850running. (Mnemonic: same as B<sh> and B<ksh>.)
f9cbb277
JH
851
852Note that there are platform specific limitations on the the maximum
853length of C<$0>. In the most extreme case it may be limited to the
854space occupied by the original C<$0>.
a0d0e21e 855
80bca1b4
JH
856In some platforms there may be arbitrary amount of padding, for
857example space characters, after the modified name as shown by C<ps>.
dda345b7 858In some platforms this padding may extend all the way to the original
c80e2480
JH
859length of the argument area, no matter what you do (this is the case
860for example with Linux 2.2).
80bca1b4 861
4bc88a62 862Note for BSD users: setting C<$0> does not completely remove "perl"
6a4647a3
JH
863from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
864result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
865and the " (perl)" suffix are shown depends on your exact BSD variant
866and version). This is an operating system feature, Perl cannot help it.
4bc88a62 867
e2975953
JH
868In multithreaded scripts Perl coordinates the threads so that any
869thread may modify its copy of the C<$0> and the change becomes visible
80bca1b4
JH
870to ps(1) (assuming the operating system plays along). Note that the
871the view of C<$0> the other threads have will not change since they
872have their own copies of it.
e2975953 873
a0d0e21e
LW
874=item $[
875
876The index of the first element in an array, and of the first character
19799a22
GS
877in a substring. Default is 0, but you could theoretically set it
878to 1 to make Perl behave more like B<awk> (or Fortran) when
879subscripting and when evaluating the index() and substr() functions.
880(Mnemonic: [ begins subscripts.)
a0d0e21e 881
19799a22
GS
882As of release 5 of Perl, assignment to C<$[> is treated as a compiler
883directive, and cannot influence the behavior of any other file.
f83ed198 884(That's why you can only assign compile-time constants to it.)
19799a22 885Its use is highly discouraged.
a0d0e21e 886
f83ed198
RGS
887Note that, unlike other compile-time directives (such as L<strict>),
888assignment to $[ can be seen from outer lexical scopes in the same file.
889However, you can use local() on it to strictly bound its value to a
890lexical block.
891
a0d0e21e
LW
892=item $]
893
54310121
PP
894The version + patchlevel / 1000 of the Perl interpreter. This variable
895can be used to determine whether the Perl interpreter executing a
896script is in the right range of versions. (Mnemonic: Is this version
897of perl in the right bracket?) Example:
a0d0e21e
LW
898
899 warn "No checksumming!\n" if $] < 3.019;
900
54310121 901See also the documentation of C<use VERSION> and C<require VERSION>
19799a22 902for a convenient way to fail if the running Perl interpreter is too old.
a0d0e21e 903
0c8d858b
MS
904The floating point representation can sometimes lead to inaccurate
905numeric comparisons. See C<$^V> for a more modern representation of
906the Perl version that allows accurate string comparisons.
16070b82 907
305aace0
NIS
908=item $COMPILING
909
910=item $^C
911
19799a22
GS
912The current value of the flag associated with the B<-c> switch.
913Mainly of use with B<-MO=...> to allow code to alter its behavior
914when being compiled, such as for example to AUTOLOAD at compile
915time rather than normal, deferred loading. See L<perlcc>. Setting
916C<$^C = 1> is similar to calling C<B::minus_c>.
305aace0 917
a0d0e21e
LW
918=item $DEBUGGING
919
920=item $^D
921
922The current value of the debugging flags. (Mnemonic: value of B<-D>
b4ab917c
DM
923switch.) May be read or set. Like its command-line equivalent, you can use
924numeric or symbolic values, eg C<$^D = 10> or C<$^D = "st">.
a0d0e21e
LW
925
926=item $SYSTEM_FD_MAX
927
928=item $^F
929
930The maximum system file descriptor, ordinarily 2. System file
931descriptors are passed to exec()ed processes, while higher file
932descriptors are not. Also, during an open(), system file descriptors are
933preserved even if the open() fails. (Ordinary file descriptors are
19799a22 934closed before the open() is attempted.) The close-on-exec
a0d0e21e 935status of a file descriptor will be decided according to the value of
8d2a6795
GS
936C<$^F> when the corresponding file, pipe, or socket was opened, not the
937time of the exec().
a0d0e21e 938
6e2995f4
PP
939=item $^H
940
0462a1ab
GS
941WARNING: This variable is strictly for internal use only. Its availability,
942behavior, and contents are subject to change without notice.
943
944This variable contains compile-time hints for the Perl interpreter. At the
945end of compilation of a BLOCK the value of this variable is restored to the
946value when the interpreter started to compile the BLOCK.
947
948When perl begins to parse any block construct that provides a lexical scope
949(e.g., eval body, required file, subroutine body, loop body, or conditional
950block), the existing value of $^H is saved, but its value is left unchanged.
951When the compilation of the block is completed, it regains the saved value.
952Between the points where its value is saved and restored, code that
953executes within BEGIN blocks is free to change the value of $^H.
954
955This behavior provides the semantic of lexical scoping, and is used in,
956for instance, the C<use strict> pragma.
957
958The contents should be an integer; different bits of it are used for
959different pragmatic flags. Here's an example:
960
961 sub add_100 { $^H |= 0x100 }
962
963 sub foo {
964 BEGIN { add_100() }
965 bar->baz($boon);
966 }
967
968Consider what happens during execution of the BEGIN block. At this point
969the BEGIN block has already been compiled, but the body of foo() is still
970being compiled. The new value of $^H will therefore be visible only while
971the body of foo() is being compiled.
972
973Substitution of the above BEGIN block with:
974
975 BEGIN { require strict; strict->import('vars') }
976
977demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
978version of the same lexical pragma:
979
980 BEGIN { require strict; strict->import('vars') if $condition }
981
982=item %^H
983
984WARNING: This variable is strictly for internal use only. Its availability,
985behavior, and contents are subject to change without notice.
986
987The %^H hash provides the same scoping semantic as $^H. This makes it
988useful for implementation of lexically scoped pragmas.
6e2995f4 989
a0d0e21e
LW
990=item $INPLACE_EDIT
991
992=item $^I
993
994The current value of the inplace-edit extension. Use C<undef> to disable
995inplace editing. (Mnemonic: value of B<-i> switch.)
996
fb73857a
PP
997=item $^M
998
19799a22
GS
999By default, running out of memory is an untrappable, fatal error.
1000However, if suitably built, Perl can use the contents of C<$^M>
1001as an emergency memory pool after die()ing. Suppose that your Perl
1002were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.
1003Then
fb73857a 1004
19799a22 1005 $^M = 'a' x (1 << 16);
fb73857a 1006
51ee6500 1007would allocate a 64K buffer for use in an emergency. See the
19799a22
GS
1008F<INSTALL> file in the Perl distribution for information on how to
1009enable this option. To discourage casual use of this advanced
4ec0190b 1010feature, there is no L<English|English> long name for this variable.
fb73857a 1011
5c055ba3 1012=item $OSNAME
6e2995f4 1013
5c055ba3
PP
1014=item $^O
1015
1016The name of the operating system under which this copy of Perl was
1017built, as determined during the configuration process. The value
19799a22
GS
1018is identical to C<$Config{'osname'}>. See also L<Config> and the
1019B<-V> command-line switch documented in L<perlrun>.
5c055ba3 1020
443f6d01 1021In Windows platforms, $^O is not very helpful: since it is always
7f510801
GS
1022C<MSWin32>, it doesn't tell the difference between
102395/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() or
1024Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
1025between the variants.
916d64a3 1026
e2e27056
JH
1027=item ${^OPEN}
1028
1029An internal variable used by PerlIO. A string in two parts, separated
fae2c0fb
RGS
1030by a C<\0> byte, the first part describes the input layers, the second
1031part describes the output layers.
e2e27056 1032
a0d0e21e
LW
1033=item $PERLDB
1034
1035=item $^P
1036
19799a22
GS
1037The internal variable for debugging support. The meanings of the
1038various bits are subject to change, but currently indicate:
84902520
TB
1039
1040=over 6
1041
1042=item 0x01
1043
1044Debug subroutine enter/exit.
1045
1046=item 0x02
1047
1048Line-by-line debugging.
1049
1050=item 0x04
1051
1052Switch off optimizations.
1053
1054=item 0x08
1055
1056Preserve more data for future interactive inspections.
1057
1058=item 0x10
1059
1060Keep info about source lines on which a subroutine is defined.
1061
1062=item 0x20
1063
1064Start with single-step on.
1065
83ee9e09
GS
1066=item 0x40
1067
1068Use subroutine address instead of name when reporting.
1069
1070=item 0x80
1071
1072Report C<goto &subroutine> as well.
1073
1074=item 0x100
1075
1076Provide informative "file" names for evals based on the place they were compiled.
1077
1078=item 0x200
1079
1080Provide informative names to anonymous subroutines based on the place they
1081were compiled.
1082
7619c85e
RG
1083=item 0x400
1084
1085Debug assertion subroutines enter/exit.
1086
84902520
TB
1087=back
1088
19799a22
GS
1089Some bits may be relevant at compile-time only, some at
1090run-time only. This is a new mechanism and the details may change.
a0d0e21e 1091
66558a10
GS
1092=item $LAST_REGEXP_CODE_RESULT
1093
b9ac3b5b
GS
1094=item $^R
1095
19799a22
GS
1096The result of evaluation of the last successful C<(?{ code })>
1097regular expression assertion (see L<perlre>). May be written to.
b9ac3b5b 1098
66558a10
GS
1099=item $EXCEPTIONS_BEING_CAUGHT
1100
fb73857a
PP
1101=item $^S
1102
fa05a9fd
IST
1103Current state of the interpreter.
1104
1105 $^S State
1106 --------- -------------------
1107 undef Parsing module/eval
1108 true (1) Executing an eval
1109 false (0) Otherwise
1110
1111The first state may happen in $SIG{__DIE__} and $SIG{__WARN__} handlers.
fb73857a 1112
a0d0e21e
LW
1113=item $BASETIME
1114
1115=item $^T
1116
19799a22 1117The time at which the program began running, in seconds since the
5f05dabc 1118epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
19799a22 1119and B<-C> filetests are based on this value.
a0d0e21e 1120
7c36658b
MS
1121=item ${^TAINT}
1122
9aa05f58
RGS
1123Reflects if taint mode is on or off. 1 for on (the program was run with
1124B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
18e8c5b0 1125B<-t> or B<-TU>). This variable is read-only.
7c36658b 1126
a05d7ebb
JH
1127=item ${^UNICODE}
1128
ab9e1bb7
JH
1129Reflects certain Unicode settings of Perl. See L<perlrun>
1130documentation for the C<-C> switch for more information about
1131the possible values. This variable is set during Perl startup
1132and is thereafter read-only.
fde18df1 1133
44dcb63b 1134=item $PERL_VERSION
b459063d 1135
16070b82
GS
1136=item $^V
1137
1138The revision, version, and subversion of the Perl interpreter, represented
da2094fd 1139as a string composed of characters with those ordinals. Thus in Perl v5.6.0
44dcb63b
GS
1140it equals C<chr(5) . chr(6) . chr(0)> and will return true for
1141C<$^V eq v5.6.0>. Note that the characters in this string value can
1142potentially be in Unicode range.
16070b82
GS
1143
1144This can be used to determine whether the Perl interpreter executing a
1145script is in the right range of versions. (Mnemonic: use ^V for Version
44dcb63b 1146Control.) Example:
16070b82 1147
3fd4402b 1148 warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0;
16070b82 1149
aa2f2a36
AMS
1150To convert C<$^V> into its string representation use sprintf()'s
1151C<"%vd"> conversion:
1152
1153 printf "version is v%vd\n", $^V; # Perl's version
1154
44dcb63b 1155See the documentation of C<use VERSION> and C<require VERSION>
16070b82
GS
1156for a convenient way to fail if the running Perl interpreter is too old.
1157
1158See also C<$]> for an older representation of the Perl version.
1159
a0d0e21e
LW
1160=item $WARNING
1161
1162=item $^W
1163
19799a22
GS
1164The current value of the warning switch, initially true if B<-w>
1165was used, false otherwise, but directly modifiable. (Mnemonic:
4438c4b7
JH
1166related to the B<-w> switch.) See also L<warnings>.
1167
6a818117 1168=item ${^WARNING_BITS}
4438c4b7
JH
1169
1170The current set of warning checks enabled by the C<use warnings> pragma.
1171See the documentation of C<warnings> for more details.
a0d0e21e
LW
1172
1173=item $EXECUTABLE_NAME
1174
1175=item $^X
1176
e71940de 1177The name used to execute the current copy of Perl, from C's
38e4f4ae
SB
1178C<argv[0]>.
1179
e71940de
PG
1180Depending on the host operating system, the value of $^X may be
1181a relative or absolute pathname of the perl program file, or may
1182be the string used to invoke perl but not the pathname of the
1183perl program file. Also, most operating systems permit invoking
1184programs that are not in the PATH environment variable, so there
a10d74f3
PG
1185is no guarantee that the value of $^X is in PATH. For VMS, the
1186value may or may not include a version number.
38e4f4ae 1187
e71940de
PG
1188You usually can use the value of $^X to re-invoke an independent
1189copy of the same perl that is currently running, e.g.,
1190
1191 @first_run = `$^X -le "print int rand 100 for 1..100"`;
1192
1193But recall that not all operating systems support forking or
1194capturing of the output of commands, so this complex statement
1195may not be portable.
38e4f4ae 1196
e71940de
PG
1197It is not safe to use the value of $^X as a path name of a file,
1198as some operating systems that have a mandatory suffix on
1199executable files do not require use of the suffix when invoking
1200a command. To convert the value of $^X to a path name, use the
1201following statements:
1202
1203# Build up a set of file names (not command names).
1204 use Config;
68fb0eb7
PG
1205 $this_perl = $^X;
1206 if ($^O ne 'VMS')
1207 {$this_perl .= $Config{_exe}
1208 unless $this_perl =~ m/$Config{_exe}$/i;}
e71940de
PG
1209
1210Because many operating systems permit anyone with read access to
1211the Perl program file to make a copy of it, patch the copy, and
1212then execute the copy, the security-conscious Perl programmer
1213should take care to invoke the installed copy of perl, not the
1214copy referenced by $^X. The following statements accomplish
1215this goal, and produce a pathname that can be invoked as a
1216command or referenced as a file.
38e4f4ae
SB
1217
1218 use Config;
68fb0eb7
PG
1219 $secure_perl_path = $Config{perlpath};
1220 if ($^O ne 'VMS')
1221 {$secure_perl_path .= $Config{_exe}
1222 unless $secure_perl_path =~ m/$Config{_exe}$/i;}
a0d0e21e 1223
2d84a16a
DM
1224=item ARGV
1225
1226The special filehandle that iterates over command-line filenames in
1227C<@ARGV>. Usually written as the null filehandle in the angle operator
1228C<< <> >>. Note that currently C<ARGV> only has its magical effect
1229within the C<< <> >> operator; elsewhere it is just a plain filehandle
1230corresponding to the last file opened by C<< <> >>. In particular,
1231passing C<\*ARGV> as a parameter to a function that expects a filehandle
1232may not cause your function to automatically read the contents of all the
1233files in C<@ARGV>.
1234
a0d0e21e
LW
1235=item $ARGV
1236
c47ff5f1 1237contains the name of the current file when reading from <>.
a0d0e21e
LW
1238
1239=item @ARGV
1240
19799a22 1241The array @ARGV contains the command-line arguments intended for
14218588 1242the script. C<$#ARGV> is generally the number of arguments minus
19799a22
GS
1243one, because C<$ARGV[0]> is the first argument, I<not> the program's
1244command name itself. See C<$0> for the command name.
a0d0e21e 1245
5ccee41e
JA
1246=item ARGVOUT
1247
1248The special filehandle that points to the currently open output file
1249when doing edit-in-place processing with B<-i>. Useful when you have
1250to do a lot of inserting and don't want to keep modifying $_. See
1251L<perlrun> for the B<-i> switch.
1252
9b0e6e7a
JP
1253=item @F
1254
1255The array @F contains the fields of each line read in when autosplit
1256mode is turned on. See L<perlrun> for the B<-a> switch. This array
1257is package-specific, and must be declared or given a full package name
1258if not in package main when running under C<strict 'vars'>.
1259
a0d0e21e
LW
1260=item @INC
1261
19799a22
GS
1262The array @INC contains the list of places that the C<do EXPR>,
1263C<require>, or C<use> constructs look for their library files. It
1264initially consists of the arguments to any B<-I> command-line
1265switches, followed by the default Perl library, probably
1266F</usr/local/lib/perl>, followed by ".", to represent the current
e48df184
RGS
1267directory. ("." will not be appended if taint checks are enabled, either by
1268C<-T> or by C<-t>.) If you need to modify this at runtime, you should use
19799a22
GS
1269the C<use lib> pragma to get the machine-dependent library properly
1270loaded also:
a0d0e21e 1271
cb1a09d0
AD
1272 use lib '/mypath/libdir/';
1273 use SomeMod;
303f2f76 1274
d54b56d5
RGS
1275You can also insert hooks into the file inclusion system by putting Perl
1276code directly into @INC. Those hooks may be subroutine references, array
1277references or blessed objects. See L<perlfunc/require> for details.
1278
fb73857a
PP
1279=item @_
1280
1281Within a subroutine the array @_ contains the parameters passed to that
19799a22 1282subroutine. See L<perlsub>.
fb73857a 1283
a0d0e21e
LW
1284=item %INC
1285
19799a22
GS
1286The hash %INC contains entries for each filename included via the
1287C<do>, C<require>, or C<use> operators. The key is the filename
1288you specified (with module names converted to pathnames), and the
14218588 1289value is the location of the file found. The C<require>
87275199 1290operator uses this hash to determine whether a particular file has
19799a22 1291already been included.
a0d0e21e 1292
89ccab8c
RGS
1293If the file was loaded via a hook (e.g. a subroutine reference, see
1294L<perlfunc/require> for a description of these hooks), this hook is
9ae8cd5b
RGS
1295by default inserted into %INC in place of a filename. Note, however,
1296that the hook may have set the %INC entry by itself to provide some more
1297specific info.
44f0be63 1298
b687b08b
TC
1299=item %ENV
1300
1301=item $ENV{expr}
a0d0e21e
LW
1302
1303The hash %ENV contains your current environment. Setting a
19799a22
GS
1304value in C<ENV> changes the environment for any child processes
1305you subsequently fork() off.
a0d0e21e 1306
b687b08b
TC
1307=item %SIG
1308
1309=item $SIG{expr}
a0d0e21e 1310
14218588 1311The hash %SIG contains signal handlers for signals. For example:
a0d0e21e
LW
1312
1313 sub handler { # 1st argument is signal name
fb73857a 1314 my($sig) = @_;
a0d0e21e
LW
1315 print "Caught a SIG$sig--shutting down\n";
1316 close(LOG);
1317 exit(0);
1318 }
1319
fb73857a
PP
1320 $SIG{'INT'} = \&handler;
1321 $SIG{'QUIT'} = \&handler;
a0d0e21e 1322 ...
19799a22 1323 $SIG{'INT'} = 'DEFAULT'; # restore default action
a0d0e21e
LW
1324 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1325
f648820c
GS
1326Using a value of C<'IGNORE'> usually has the effect of ignoring the
1327signal, except for the C<CHLD> signal. See L<perlipc> for more about
1328this special case.
1329
19799a22 1330Here are some other examples:
a0d0e21e 1331
fb73857a 1332 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
a0d0e21e 1333 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
19799a22 1334 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
a0d0e21e
LW
1335 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1336
19799a22
GS
1337Be sure not to use a bareword as the name of a signal handler,
1338lest you inadvertently call it.
748a9306 1339
44a8e56a 1340If your system has the sigaction() function then signal handlers are
9ce5b4ad 1341installed using it. This means you get reliable signal handling.
44a8e56a 1342
9ce5b4ad
SG
1343The default delivery policy of signals changed in Perl 5.8.0 from
1344immediate (also known as "unsafe") to deferred, also known as
1345"safe signals". See L<perlipc> for more information.
45c0772f 1346
748a9306 1347Certain internal hooks can be also set using the %SIG hash. The
a8f8344d 1348routine indicated by C<$SIG{__WARN__}> is called when a warning message is
748a9306
LW
1349about to be printed. The warning message is passed as the first
1350argument. The presence of a __WARN__ hook causes the ordinary printing
1351of warnings to STDERR to be suppressed. You can use this to save warnings
1352in a variable, or turn warnings into fatal errors, like this:
1353
1354 local $SIG{__WARN__} = sub { die $_[0] };
1355 eval $proggie;
1356
a8f8344d 1357The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
748a9306
LW
1358is about to be thrown. The error message is passed as the first
1359argument. When a __DIE__ hook routine returns, the exception
1360processing continues as it would have in the absence of the hook,
cb1a09d0 1361unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
774d564b 1362The C<__DIE__> handler is explicitly disabled during the call, so that you
fb73857a
PP
1363can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1364
19799a22
GS
1365Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1366even inside an eval(). Do not use this to rewrite a pending exception
1367in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
1368This strange action at a distance may be fixed in a future release
1369so that C<$SIG{__DIE__}> is only called if your program is about
1370to exit, as was the original intent. Any other use is deprecated.
1371
1372C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1373they may be called to report (probable) errors found by the parser.
1374In such a case the parser may be in inconsistent state, so any
1375attempt to evaluate Perl code from such a handler will probably
1376result in a segfault. This means that warnings or errors that
1377result from parsing Perl should be used with extreme caution, like
1378this:
fb73857a
PP
1379
1380 require Carp if defined $^S;
1381 Carp::confess("Something wrong") if defined &Carp::confess;
1382 die "Something wrong, but could not load Carp to give backtrace...
1383 To see backtrace try starting Perl with -MCarp switch";
1384
1385Here the first line will load Carp I<unless> it is the parser who
1386called the handler. The second line will print backtrace and die if
1387Carp was available. The third line will be executed only if Carp was
1388not available.
1389
19799a22 1390See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
4438c4b7 1391L<warnings> for additional information.
68dc0745 1392
a0d0e21e 1393=back
55602bd2
IZ
1394
1395=head2 Error Indicators
1396
19799a22
GS
1397The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1398about different types of error conditions that may appear during
1399execution of a Perl program. The variables are shown ordered by
1400the "distance" between the subsystem which reported the error and
1401the Perl process. They correspond to errors detected by the Perl
1402interpreter, C library, operating system, or an external program,
1403respectively.
55602bd2
IZ
1404
1405To illustrate the differences between these variables, consider the
19799a22 1406following Perl expression, which uses a single-quoted string:
55602bd2 1407
19799a22 1408 eval q{
22d0716c
SB
1409 open my $pipe, "/cdrom/install |" or die $!;
1410 my @res = <$pipe>;
1411 close $pipe or die "bad pipe: $?, $!";
19799a22 1412 };
55602bd2
IZ
1413
1414After execution of this statement all 4 variables may have been set.
1415
19799a22
GS
1416C<$@> is set if the string to be C<eval>-ed did not compile (this
1417may happen if C<open> or C<close> were imported with bad prototypes),
1418or if Perl code executed during evaluation die()d . In these cases
1419the value of $@ is the compile error, or the argument to C<die>
1420(which will interpolate C<$!> and C<$?>!). (See also L<Fatal>,
1421though.)
1422
c47ff5f1 1423When the eval() expression above is executed, open(), C<< <PIPE> >>,
19799a22
GS
1424and C<close> are translated to calls in the C run-time library and
1425thence to the operating system kernel. C<$!> is set to the C library's
1426C<errno> if one of these calls fails.
1427
1428Under a few operating systems, C<$^E> may contain a more verbose
1429error indicator, such as in this case, "CDROM tray not closed."
14218588 1430Systems that do not support extended error messages leave C<$^E>
19799a22
GS
1431the same as C<$!>.
1432
1433Finally, C<$?> may be set to non-0 value if the external program
1434F</cdrom/install> fails. The upper eight bits reflect specific
1435error conditions encountered by the program (the program's exit()
1436value). The lower eight bits reflect mode of failure, like signal
1437death and core dump information See wait(2) for details. In
1438contrast to C<$!> and C<$^E>, which are set only if error condition
1439is detected, the variable C<$?> is set on each C<wait> or pipe
1440C<close>, overwriting the old value. This is more like C<$@>, which
1441on every eval() is always set on failure and cleared on success.
2b92dfce 1442
19799a22
GS
1443For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1444and C<$?>.
2b92dfce
GS
1445
1446=head2 Technical Note on the Syntax of Variable Names
1447
19799a22
GS
1448Variable names in Perl can have several formats. Usually, they
1449must begin with a letter or underscore, in which case they can be
1450arbitrarily long (up to an internal limit of 251 characters) and
1451may contain letters, digits, underscores, or the special sequence
1452C<::> or C<'>. In this case, the part before the last C<::> or
1453C<'> is taken to be a I<package qualifier>; see L<perlmod>.
2b92dfce
GS
1454
1455Perl variable names may also be a sequence of digits or a single
1456punctuation or control character. These names are all reserved for
19799a22
GS
1457special uses by Perl; for example, the all-digits names are used
1458to hold data captured by backreferences after a regular expression
1459match. Perl has a special syntax for the single-control-character
1460names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1461character. For example, the notation C<$^W> (dollar-sign caret
1462C<W>) is the scalar variable whose name is the single character
1463control-C<W>. This is better than typing a literal control-C<W>
1464into your program.
2b92dfce 1465
87275199 1466Finally, new in Perl 5.6, Perl variable names may be alphanumeric
19799a22
GS
1467strings that begin with control characters (or better yet, a caret).
1468These variables must be written in the form C<${^Foo}>; the braces
1469are not optional. C<${^Foo}> denotes the scalar variable whose
1470name is a control-C<F> followed by two C<o>'s. These variables are
1471reserved for future special uses by Perl, except for the ones that
1472begin with C<^_> (control-underscore or caret-underscore). No
1473control-character name that begins with C<^_> will acquire a special
1474meaning in any future version of Perl; such names may therefore be
1475used safely in programs. C<$^_> itself, however, I<is> reserved.
1476
1477Perl identifiers that begin with digits, control characters, or
2b92dfce 1478punctuation characters are exempt from the effects of the C<package>
747fafda
MJD
1479declaration and are always forced to be in package C<main>; they are
1480also exempt from C<strict 'vars'> errors. A few other names are also
1481exempt in these ways:
2b92dfce
GS
1482
1483 ENV STDIN
1484 INC STDOUT
1485 ARGV STDERR
5b88253b 1486 ARGVOUT _
2b92dfce
GS
1487 SIG
1488
1489In particular, the new special C<${^_XYZ}> variables are always taken
19799a22 1490to be in package C<main>, regardless of any C<package> declarations
747fafda 1491presently in scope.
2b92dfce 1492
19799a22
GS
1493=head1 BUGS
1494
1495Due to an unfortunate accident of Perl's implementation, C<use
1496English> imposes a considerable performance penalty on all regular
1497expression matches in a program, regardless of whether they occur
1498in the scope of C<use English>. For that reason, saying C<use
1499English> in libraries is strongly discouraged. See the
1500Devel::SawAmpersand module documentation from CPAN
1577cd80 1501( http://www.cpan.org/modules/by-module/Devel/ )
19799a22 1502for more information.
2b92dfce 1503
19799a22
GS
1504Having to even think about the C<$^S> variable in your exception
1505handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1506invites grievous and difficult to track down errors. Avoid it
1507and use an C<END{}> or CORE::GLOBAL::die override instead.