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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
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377set, you'll get the record back in pieces. Trying to set the record
378size to zero or less will cause reading in the (rest of the) whole file.
5b2b9c68 379
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380On VMS, record reads are done with the equivalent of C<sysread>,
381so it's best not to mix record and non-record reads on the same
382file. (This is unlikely to be a problem, because any file you'd
83763826 383want to read in record mode is probably unusable in line mode.)
14218588 384Non-VMS systems do normal I/O, so it's safe to mix record and
19799a22 385non-record reads of a file.
5b2b9c68 386
14218588 387See also L<perlport/"Newlines">. Also see C<$.>.
883faa13 388
fcc7d916 389=item HANDLE->autoflush(EXPR)
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390
391=item $OUTPUT_AUTOFLUSH
392
393=item $|
394
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395If set to nonzero, forces a flush right away and after every write
396or print on the currently selected output channel. Default is 0
14218588 397(regardless of whether the channel is really buffered by the
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398system or not; C<$|> tells you only whether you've asked Perl
399explicitly to flush after each write). STDOUT will
400typically be line buffered if output is to the terminal and block
401buffered otherwise. Setting this variable is useful primarily when
402you are outputting to a pipe or socket, such as when you are running
403a Perl program under B<rsh> and want to see the output as it's
404happening. This has no effect on input buffering. See L<perlfunc/getc>
405for that. (Mnemonic: when you want your pipes to be piping hot.)
a0d0e21e 406
46550894 407=item IO::Handle->output_field_separator EXPR
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408
409=item $OUTPUT_FIELD_SEPARATOR
410
411=item $OFS
412
413=item $,
414
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415The output field separator for the print operator. If defined, this
416value is printed between each of print's arguments. Default is C<undef>.
417(Mnemonic: what is printed when there is a "," in your print statement.)
a0d0e21e 418
46550894 419=item IO::Handle->output_record_separator EXPR
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420
421=item $OUTPUT_RECORD_SEPARATOR
422
423=item $ORS
424
425=item $\
426
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427The output record separator for the print operator. If defined, this
428value is printed after the last of print's arguments. Default is C<undef>.
429(Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
430Also, it's just like C<$/>, but it's what you get "back" from Perl.)
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431
432=item $LIST_SEPARATOR
433
434=item $"
435
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436This is like C<$,> except that it applies to array and slice values
437interpolated into a double-quoted string (or similar interpreted
438string). Default is a space. (Mnemonic: obvious, I think.)
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439
440=item $SUBSCRIPT_SEPARATOR
441
442=item $SUBSEP
443
444=item $;
445
54310121 446The subscript separator for multidimensional array emulation. If you
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447refer to a hash element as
448
449 $foo{$a,$b,$c}
450
451it really means
452
453 $foo{join($;, $a, $b, $c)}
454
455But don't put
456
457 @foo{$a,$b,$c} # a slice--note the @
458
459which means
460
461 ($foo{$a},$foo{$b},$foo{$c})
462
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463Default is "\034", the same as SUBSEP in B<awk>. If your
464keys contain binary data there might not be any safe value for C<$;>.
a0d0e21e 465(Mnemonic: comma (the syntactic subscript separator) is a
19799a22 466semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
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467taken for something more important.)
468
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469Consider using "real" multidimensional arrays as described
470in L<perllol>.
a0d0e21e 471
fcc7d916 472=item HANDLE->format_page_number(EXPR)
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473
474=item $FORMAT_PAGE_NUMBER
475
476=item $%
477
478The current page number of the currently selected output channel.
19799a22 479Used with formats.
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480(Mnemonic: % is page number in B<nroff>.)
481
fcc7d916 482=item HANDLE->format_lines_per_page(EXPR)
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483
484=item $FORMAT_LINES_PER_PAGE
485
486=item $=
487
488The current page length (printable lines) of the currently selected
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489output channel. Default is 60.
490Used with formats.
491(Mnemonic: = has horizontal lines.)
a0d0e21e 492
fcc7d916 493=item HANDLE->format_lines_left(EXPR)
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494
495=item $FORMAT_LINES_LEFT
496
497=item $-
498
499The number of lines left on the page of the currently selected output
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500channel.
501Used with formats.
502(Mnemonic: lines_on_page - lines_printed.)
a0d0e21e 503
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504=item @LAST_MATCH_START
505
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506=item @-
507
19799a22 508$-[0] is the offset of the start of the last successful match.
6cef1e77 509C<$-[>I<n>C<]> is the offset of the start of the substring matched by
8f580fb8 510I<n>-th subpattern, or undef if the subpattern did not match.
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511
512Thus after a match against $_, $& coincides with C<substr $_, $-[0],
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513$+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
514$+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
515C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the last
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516matched subgroup in the last successful match. Contrast with
517C<$#+>, the number of subgroups in the regular expression. Compare
19799a22 518with C<@+>.
6cef1e77 519
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520This array holds the offsets of the beginnings of the last
521successful submatches in the currently active dynamic scope.
522C<$-[0]> is the offset into the string of the beginning of the
523entire match. The I<n>th element of this array holds the offset
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524of the I<n>th submatch, so C<$-[1]> is the offset where $1
525begins, C<$-[2]> the offset where $2 begins, and so on.
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526
527After a match against some variable $var:
528
529=over 5
530
4375e838 531=item C<$`> is the same as C<substr($var, 0, $-[0])>
4ba05bdc 532
4375e838 533=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
4ba05bdc 534
4375e838 535=item C<$'> is the same as C<substr($var, $+[0])>
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536
537=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
538
539=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
540
80dc6883 541=item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
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542
543=back
544
fcc7d916 545=item HANDLE->format_name(EXPR)
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546
547=item $FORMAT_NAME
548
549=item $~
550
551The name of the current report format for the currently selected output
14218588 552channel. Default is the name of the filehandle. (Mnemonic: brother to
19799a22 553C<$^>.)
a0d0e21e 554
fcc7d916 555=item HANDLE->format_top_name(EXPR)
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556
557=item $FORMAT_TOP_NAME
558
559=item $^
560
561The name of the current top-of-page format for the currently selected
14218588 562output channel. Default is the name of the filehandle with _TOP
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563appended. (Mnemonic: points to top of page.)
564
46550894 565=item IO::Handle->format_line_break_characters EXPR
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566
567=item $FORMAT_LINE_BREAK_CHARACTERS
568
569=item $:
570
571The current set of characters after which a string may be broken to
54310121 572fill continuation fields (starting with ^) in a format. Default is
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573S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
574poetry is a part of a line.)
575
46550894 576=item IO::Handle->format_formfeed EXPR
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577
578=item $FORMAT_FORMFEED
579
580=item $^L
581
14218588 582What formats output as a form feed. Default is \f.
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583
584=item $ACCUMULATOR
585
586=item $^A
587
588The current value of the write() accumulator for format() lines. A format
19799a22 589contains formline() calls that put their result into C<$^A>. After
a0d0e21e 590calling its format, write() prints out the contents of C<$^A> and empties.
14218588 591So you never really see the contents of C<$^A> unless you call
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592formline() yourself and then look at it. See L<perlform> and
593L<perlfunc/formline()>.
594
595=item $CHILD_ERROR
596
597=item $?
598
54310121 599The status returned by the last pipe close, backtick (C<``>) command,
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600successful call to wait() or waitpid(), or from the system()
601operator. This is just the 16-bit status word returned by the
e5218da5 602traditional Unix wait() system call (or else is made up to look like it). Thus, the
c47ff5f1 603exit value of the subprocess is really (C<<< $? >> 8 >>>), and
19799a22
GS
604C<$? & 127> gives which signal, if any, the process died from, and
605C<$? & 128> reports whether there was a core dump. (Mnemonic:
606similar to B<sh> and B<ksh>.)
a0d0e21e 607
7b8d334a 608Additionally, if the C<h_errno> variable is supported in C, its value
14218588 609is returned via $? if any C<gethost*()> function fails.
7b8d334a 610
19799a22 611If you have installed a signal handler for C<SIGCHLD>, the
aa689395
PP
612value of C<$?> will usually be wrong outside that handler.
613
a8f8344d
PP
614Inside an C<END> subroutine C<$?> contains the value that is going to be
615given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
19799a22
GS
616change the exit status of your program. For example:
617
618 END {
619 $? = 1 if $? == 255; # die would make it 255
620 }
a8f8344d 621
aa689395 622Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
ff0cee69 623actual VMS exit status, instead of the default emulation of POSIX
9bc98430 624status; see L<perlvms/$?> for details.
f86702cc 625
55602bd2
IZ
626Also see L<Error Indicators>.
627
e5218da5
GA
628=item ${^CHILD_ERROR_NATIVE}
629
630The native status returned by the last pipe close, backtick (C<``>)
631command, successful call to wait() or waitpid(), or from the system()
632operator. On POSIX-like systems this value can be decoded with the
633WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED, WSTOPSIG
634and WIFCONTINUED functions provided by the L<POSIX> module.
635
636Under VMS this reflects the actual VMS exit status; i.e. it is the same
637as $? when the pragma C<use vmsish 'status'> is in effect.
638
0a378802
JH
639=item ${^ENCODING}
640
740bd165
PN
641The I<object reference> to the Encode object that is used to convert
642the source code to Unicode. Thanks to this variable your perl script
643does not have to be written in UTF-8. Default is I<undef>. The direct
644manipulation of this variable is highly discouraged. See L<encoding>
048c20cb 645for more details.
0a378802 646
a0d0e21e
LW
647=item $OS_ERROR
648
649=item $ERRNO
650
651=item $!
652
19799a22 653If used numerically, yields the current value of the C C<errno>
6ab308ee
JH
654variable, or in other words, if a system or library call fails, it
655sets this variable. This means that the value of C<$!> is meaningful
656only I<immediately> after a B<failure>:
657
658 if (open(FH, $filename)) {
659 # Here $! is meaningless.
660 ...
661 } else {
662 # ONLY here is $! meaningful.
663 ...
664 # Already here $! might be meaningless.
665 }
666 # Since here we might have either success or failure,
667 # here $! is meaningless.
668
669In the above I<meaningless> stands for anything: zero, non-zero,
670C<undef>. A successful system or library call does B<not> set
671the variable to zero.
672
271df126 673If used as a string, yields the corresponding system error string.
19799a22
GS
674You can assign a number to C<$!> to set I<errno> if, for instance,
675you want C<"$!"> to return the string for error I<n>, or you want
676to set the exit value for the die() operator. (Mnemonic: What just
677went bang?)
a0d0e21e 678
55602bd2
IZ
679Also see L<Error Indicators>.
680
4c5cef9b
MJD
681=item %!
682
683Each element of C<%!> has a true value only if C<$!> is set to that
684value. For example, C<$!{ENOENT}> is true if and only if the current
3be065a1
JH
685value of C<$!> is C<ENOENT>; that is, if the most recent error was
686"No such file or directory" (or its moral equivalent: not all operating
687systems give that exact error, and certainly not all languages).
688To check if a particular key is meaningful on your system, use
689C<exists $!{the_key}>; for a list of legal keys, use C<keys %!>.
690See L<Errno> for more information, and also see above for the
691validity of C<$!>.
4c5cef9b 692
5c055ba3
PP
693=item $EXTENDED_OS_ERROR
694
695=item $^E
696
22fae026
TM
697Error information specific to the current operating system. At
698the moment, this differs from C<$!> under only VMS, OS/2, and Win32
699(and for MacPerl). On all other platforms, C<$^E> is always just
700the same as C<$!>.
701
702Under VMS, C<$^E> provides the VMS status value from the last
703system error. This is more specific information about the last
704system error than that provided by C<$!>. This is particularly
d516a115 705important when C<$!> is set to B<EVMSERR>.
22fae026 706
1c1c7f20
GS
707Under OS/2, C<$^E> is set to the error code of the last call to
708OS/2 API either via CRT, or directly from perl.
22fae026
TM
709
710Under Win32, C<$^E> always returns the last error information
711reported by the Win32 call C<GetLastError()> which describes
712the last error from within the Win32 API. Most Win32-specific
19799a22 713code will report errors via C<$^E>. ANSI C and Unix-like calls
22fae026
TM
714set C<errno> and so most portable Perl code will report errors
715via C<$!>.
716
717Caveats mentioned in the description of C<$!> generally apply to
718C<$^E>, also. (Mnemonic: Extra error explanation.)
5c055ba3 719
55602bd2
IZ
720Also see L<Error Indicators>.
721
a0d0e21e
LW
722=item $EVAL_ERROR
723
724=item $@
725
4a280ebe
JG
726The Perl syntax error message from the last eval() operator.
727If $@ is the null string, the last eval() parsed and executed
728correctly (although the operations you invoked may have failed in the
729normal fashion). (Mnemonic: Where was the syntax error "at"?)
a0d0e21e 730
19799a22 731Warning messages are not collected in this variable. You can,
a8f8344d 732however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
54310121 733as described below.
748a9306 734
55602bd2
IZ
735Also see L<Error Indicators>.
736
a0d0e21e
LW
737=item $PROCESS_ID
738
739=item $PID
740
741=item $$
742
19799a22
GS
743The process number of the Perl running this script. You should
744consider this variable read-only, although it will be altered
745across fork() calls. (Mnemonic: same as shells.)
a0d0e21e 746
4d76a344
RGS
747Note for Linux users: on Linux, the C functions C<getpid()> and
748C<getppid()> return different values from different threads. In order to
749be portable, this behavior is not reflected by C<$$>, whose value remains
750consistent across threads. If you want to call the underlying C<getpid()>,
e3256f86 751you may use the CPAN module C<Linux::Pid>.
4d76a344 752
a0d0e21e
LW
753=item $REAL_USER_ID
754
755=item $UID
756
757=item $<
758
19799a22 759The real uid of this process. (Mnemonic: it's the uid you came I<from>,
a043a685 760if you're running setuid.) You can change both the real uid and
a537debe
SP
761the effective uid at the same time by using POSIX::setuid(). Since
762changes to $< require a system call, check $! after a change attempt to
763detect any possible errors.
a0d0e21e
LW
764
765=item $EFFECTIVE_USER_ID
766
767=item $EUID
768
769=item $>
770
771The effective uid of this process. Example:
772
773 $< = $>; # set real to effective uid
774 ($<,$>) = ($>,$<); # swap real and effective uid
775
a043a685 776You can change both the effective uid and the real uid at the same
a537debe
SP
777time by using POSIX::setuid(). Changes to $> require a check to $!
778to detect any possible errors after an attempted change.
a043a685 779
19799a22 780(Mnemonic: it's the uid you went I<to>, if you're running setuid.)
c47ff5f1 781C<< $< >> and C<< $> >> can be swapped only on machines
8cc95fdb 782supporting setreuid().
a0d0e21e
LW
783
784=item $REAL_GROUP_ID
785
786=item $GID
787
788=item $(
789
790The real gid of this process. If you are on a machine that supports
791membership in multiple groups simultaneously, gives a space separated
792list of groups you are in. The first number is the one returned by
793getgid(), and the subsequent ones by getgroups(), one of which may be
8cc95fdb
PP
794the same as the first number.
795
19799a22
GS
796However, a value assigned to C<$(> must be a single number used to
797set the real gid. So the value given by C<$(> should I<not> be assigned
798back to C<$(> without being forced numeric, such as by adding zero.
8cc95fdb 799
a043a685 800You can change both the real gid and the effective gid at the same
a537debe
SP
801time by using POSIX::setgid(). Changes to $( require a check to $!
802to detect any possible errors after an attempted change.
a043a685 803
19799a22
GS
804(Mnemonic: parentheses are used to I<group> things. The real gid is the
805group you I<left>, if you're running setgid.)
a0d0e21e
LW
806
807=item $EFFECTIVE_GROUP_ID
808
809=item $EGID
810
811=item $)
812
813The effective gid of this process. If you are on a machine that
814supports membership in multiple groups simultaneously, gives a space
815separated list of groups you are in. The first number is the one
816returned by getegid(), and the subsequent ones by getgroups(), one of
8cc95fdb
PP
817which may be the same as the first number.
818
19799a22 819Similarly, a value assigned to C<$)> must also be a space-separated
14218588 820list of numbers. The first number sets the effective gid, and
8cc95fdb
PP
821the rest (if any) are passed to setgroups(). To get the effect of an
822empty list for setgroups(), just repeat the new effective gid; that is,
823to force an effective gid of 5 and an effectively empty setgroups()
824list, say C< $) = "5 5" >.
825
a043a685
GW
826You can change both the effective gid and the real gid at the same
827time by using POSIX::setgid() (use only a single numeric argument).
a537debe
SP
828Changes to $) require a check to $! to detect any possible errors
829after an attempted change.
a043a685 830
19799a22
GS
831(Mnemonic: parentheses are used to I<group> things. The effective gid
832is the group that's I<right> for you, if you're running setgid.)
a0d0e21e 833
c47ff5f1 834C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
19799a22
GS
835machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
836and C<$)> can be swapped only on machines supporting setregid().
a0d0e21e
LW
837
838=item $PROGRAM_NAME
839
840=item $0
841
80bca1b4
JH
842Contains the name of the program being executed.
843
844On some (read: not all) operating systems assigning to C<$0> modifies
845the argument area that the C<ps> program sees. On some platforms you
846may have to use special C<ps> options or a different C<ps> to see the
847changes. Modifying the $0 is more useful as a way of indicating the
848current program state than it is for hiding the program you're
849running. (Mnemonic: same as B<sh> and B<ksh>.)
f9cbb277 850
cf525c36 851Note that there are platform specific limitations on the maximum
f9cbb277
JH
852length of C<$0>. In the most extreme case it may be limited to the
853space occupied by the original C<$0>.
a0d0e21e 854
80bca1b4
JH
855In some platforms there may be arbitrary amount of padding, for
856example space characters, after the modified name as shown by C<ps>.
dda345b7 857In some platforms this padding may extend all the way to the original
c80e2480
JH
858length of the argument area, no matter what you do (this is the case
859for example with Linux 2.2).
80bca1b4 860
4bc88a62 861Note for BSD users: setting C<$0> does not completely remove "perl"
6a4647a3
JH
862from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
863result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
864and the " (perl)" suffix are shown depends on your exact BSD variant
865and version). This is an operating system feature, Perl cannot help it.
4bc88a62 866
e2975953
JH
867In multithreaded scripts Perl coordinates the threads so that any
868thread may modify its copy of the C<$0> and the change becomes visible
cf525c36 869to ps(1) (assuming the operating system plays along). Note that
80bca1b4
JH
870the view of C<$0> the other threads have will not change since they
871have their own copies of it.
e2975953 872
a0d0e21e
LW
873=item $[
874
875The index of the first element in an array, and of the first character
19799a22
GS
876in a substring. Default is 0, but you could theoretically set it
877to 1 to make Perl behave more like B<awk> (or Fortran) when
878subscripting and when evaluating the index() and substr() functions.
879(Mnemonic: [ begins subscripts.)
a0d0e21e 880
19799a22
GS
881As of release 5 of Perl, assignment to C<$[> is treated as a compiler
882directive, and cannot influence the behavior of any other file.
f83ed198 883(That's why you can only assign compile-time constants to it.)
19799a22 884Its use is highly discouraged.
a0d0e21e 885
f83ed198 886Note that, unlike other compile-time directives (such as L<strict>),
af7a0647
RGS
887assignment to C<$[> can be seen from outer lexical scopes in the same file.
888However, you can use local() on it to strictly bind its value to a
f83ed198
RGS
889lexical block.
890
a0d0e21e
LW
891=item $]
892
54310121
PP
893The version + patchlevel / 1000 of the Perl interpreter. This variable
894can be used to determine whether the Perl interpreter executing a
895script is in the right range of versions. (Mnemonic: Is this version
896of perl in the right bracket?) Example:
a0d0e21e
LW
897
898 warn "No checksumming!\n" if $] < 3.019;
899
54310121 900See also the documentation of C<use VERSION> and C<require VERSION>
19799a22 901for a convenient way to fail if the running Perl interpreter is too old.
a0d0e21e 902
0c8d858b
MS
903The floating point representation can sometimes lead to inaccurate
904numeric comparisons. See C<$^V> for a more modern representation of
905the Perl version that allows accurate string comparisons.
16070b82 906
305aace0
NIS
907=item $COMPILING
908
909=item $^C
910
19799a22
GS
911The current value of the flag associated with the B<-c> switch.
912Mainly of use with B<-MO=...> to allow code to alter its behavior
913when being compiled, such as for example to AUTOLOAD at compile
914time rather than normal, deferred loading. See L<perlcc>. Setting
915C<$^C = 1> is similar to calling C<B::minus_c>.
305aace0 916
a0d0e21e
LW
917=item $DEBUGGING
918
919=item $^D
920
921The current value of the debugging flags. (Mnemonic: value of B<-D>
b4ab917c
DM
922switch.) May be read or set. Like its command-line equivalent, you can use
923numeric or symbolic values, eg C<$^D = 10> or C<$^D = "st">.
a0d0e21e 924
a3621e74
YO
925=item ${^RE_DEBUG_FLAGS}
926
927The current value of the regex debugging flags. Set to 0 for no debug output
928even when the re 'debug' module is loaded. See L<re> for details.
929
0111c4fd 930=item ${^RE_TRIE_MAXBUF}
a3621e74
YO
931
932Controls how certain regex optimisations are applied and how much memory they
933utilize. This value by default is 65536 which corresponds to a 512kB temporary
934cache. Set this to a higher value to trade memory for speed when matching
935large alternations. Set it to a lower value if you want the optimisations to
936be as conservative of memory as possible but still occur, and set it to a
937negative value to prevent the optimisation and conserve the most memory.
938Under normal situations this variable should be of no interest to you.
939
a0d0e21e
LW
940=item $SYSTEM_FD_MAX
941
942=item $^F
943
944The maximum system file descriptor, ordinarily 2. System file
945descriptors are passed to exec()ed processes, while higher file
946descriptors are not. Also, during an open(), system file descriptors are
947preserved even if the open() fails. (Ordinary file descriptors are
19799a22 948closed before the open() is attempted.) The close-on-exec
a0d0e21e 949status of a file descriptor will be decided according to the value of
8d2a6795
GS
950C<$^F> when the corresponding file, pipe, or socket was opened, not the
951time of the exec().
a0d0e21e 952
6e2995f4
PP
953=item $^H
954
0462a1ab
GS
955WARNING: This variable is strictly for internal use only. Its availability,
956behavior, and contents are subject to change without notice.
957
958This variable contains compile-time hints for the Perl interpreter. At the
959end of compilation of a BLOCK the value of this variable is restored to the
960value when the interpreter started to compile the BLOCK.
961
962When perl begins to parse any block construct that provides a lexical scope
963(e.g., eval body, required file, subroutine body, loop body, or conditional
964block), the existing value of $^H is saved, but its value is left unchanged.
965When the compilation of the block is completed, it regains the saved value.
966Between the points where its value is saved and restored, code that
967executes within BEGIN blocks is free to change the value of $^H.
968
969This behavior provides the semantic of lexical scoping, and is used in,
970for instance, the C<use strict> pragma.
971
972The contents should be an integer; different bits of it are used for
973different pragmatic flags. Here's an example:
974
975 sub add_100 { $^H |= 0x100 }
976
977 sub foo {
978 BEGIN { add_100() }
979 bar->baz($boon);
980 }
981
982Consider what happens during execution of the BEGIN block. At this point
983the BEGIN block has already been compiled, but the body of foo() is still
984being compiled. The new value of $^H will therefore be visible only while
985the body of foo() is being compiled.
986
987Substitution of the above BEGIN block with:
988
989 BEGIN { require strict; strict->import('vars') }
990
991demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
992version of the same lexical pragma:
993
994 BEGIN { require strict; strict->import('vars') if $condition }
995
996=item %^H
997
998WARNING: This variable is strictly for internal use only. Its availability,
999behavior, and contents are subject to change without notice.
1000
1001The %^H hash provides the same scoping semantic as $^H. This makes it
1002useful for implementation of lexically scoped pragmas.
6e2995f4 1003
a0d0e21e
LW
1004=item $INPLACE_EDIT
1005
1006=item $^I
1007
1008The current value of the inplace-edit extension. Use C<undef> to disable
1009inplace editing. (Mnemonic: value of B<-i> switch.)
1010
fb73857a
PP
1011=item $^M
1012
19799a22
GS
1013By default, running out of memory is an untrappable, fatal error.
1014However, if suitably built, Perl can use the contents of C<$^M>
1015as an emergency memory pool after die()ing. Suppose that your Perl
0acca065 1016were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
19799a22 1017Then
fb73857a 1018
19799a22 1019 $^M = 'a' x (1 << 16);
fb73857a 1020
51ee6500 1021would allocate a 64K buffer for use in an emergency. See the
19799a22 1022F<INSTALL> file in the Perl distribution for information on how to
0acca065
RGS
1023add custom C compilation flags when compiling perl. To discourage casual
1024use of this advanced feature, there is no L<English|English> long name for
1025this variable.
fb73857a 1026
5c055ba3 1027=item $OSNAME
6e2995f4 1028
5c055ba3
PP
1029=item $^O
1030
1031The name of the operating system under which this copy of Perl was
1032built, as determined during the configuration process. The value
19799a22
GS
1033is identical to C<$Config{'osname'}>. See also L<Config> and the
1034B<-V> command-line switch documented in L<perlrun>.
5c055ba3 1035
443f6d01 1036In Windows platforms, $^O is not very helpful: since it is always
7f510801
GS
1037C<MSWin32>, it doesn't tell the difference between
103895/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() or
1039Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
1040between the variants.
916d64a3 1041
e2e27056
JH
1042=item ${^OPEN}
1043
1044An internal variable used by PerlIO. A string in two parts, separated
fae2c0fb
RGS
1045by a C<\0> byte, the first part describes the input layers, the second
1046part describes the output layers.
e2e27056 1047
a0d0e21e
LW
1048=item $PERLDB
1049
1050=item $^P
1051
19799a22
GS
1052The internal variable for debugging support. The meanings of the
1053various bits are subject to change, but currently indicate:
84902520
TB
1054
1055=over 6
1056
1057=item 0x01
1058
1059Debug subroutine enter/exit.
1060
1061=item 0x02
1062
1063Line-by-line debugging.
1064
1065=item 0x04
1066
1067Switch off optimizations.
1068
1069=item 0x08
1070
1071Preserve more data for future interactive inspections.
1072
1073=item 0x10
1074
1075Keep info about source lines on which a subroutine is defined.
1076
1077=item 0x20
1078
1079Start with single-step on.
1080
83ee9e09
GS
1081=item 0x40
1082
1083Use subroutine address instead of name when reporting.
1084
1085=item 0x80
1086
1087Report C<goto &subroutine> as well.
1088
1089=item 0x100
1090
1091Provide informative "file" names for evals based on the place they were compiled.
1092
1093=item 0x200
1094
1095Provide informative names to anonymous subroutines based on the place they
1096were compiled.
1097
7619c85e
RG
1098=item 0x400
1099
1100Debug assertion subroutines enter/exit.
1101
84902520
TB
1102=back
1103
19799a22
GS
1104Some bits may be relevant at compile-time only, some at
1105run-time only. This is a new mechanism and the details may change.
a0d0e21e 1106
66558a10
GS
1107=item $LAST_REGEXP_CODE_RESULT
1108
b9ac3b5b
GS
1109=item $^R
1110
19799a22
GS
1111The result of evaluation of the last successful C<(?{ code })>
1112regular expression assertion (see L<perlre>). May be written to.
b9ac3b5b 1113
66558a10
GS
1114=item $EXCEPTIONS_BEING_CAUGHT
1115
fb73857a
PP
1116=item $^S
1117
fa05a9fd
IST
1118Current state of the interpreter.
1119
1120 $^S State
1121 --------- -------------------
1122 undef Parsing module/eval
1123 true (1) Executing an eval
1124 false (0) Otherwise
1125
1126The first state may happen in $SIG{__DIE__} and $SIG{__WARN__} handlers.
fb73857a 1127
a0d0e21e
LW
1128=item $BASETIME
1129
1130=item $^T
1131
19799a22 1132The time at which the program began running, in seconds since the
5f05dabc 1133epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
19799a22 1134and B<-C> filetests are based on this value.
a0d0e21e 1135
7c36658b
MS
1136=item ${^TAINT}
1137
9aa05f58
RGS
1138Reflects if taint mode is on or off. 1 for on (the program was run with
1139B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
18e8c5b0 1140B<-t> or B<-TU>). This variable is read-only.
7c36658b 1141
a05d7ebb
JH
1142=item ${^UNICODE}
1143
ab9e1bb7
JH
1144Reflects certain Unicode settings of Perl. See L<perlrun>
1145documentation for the C<-C> switch for more information about
1146the possible values. This variable is set during Perl startup
1147and is thereafter read-only.
fde18df1 1148
e07ea26a
NC
1149=item ${^UTF8CACHE}
1150
1151This variable controls the state of the internal UTF-8 offset caching code.
16d9fe92
NC
11521 for on (the default), 0 for off, -1 to debug the caching code by checking
1153all its results against linear scans, and panicking on any discrepancy.
e07ea26a 1154
ea8eae40
RGS
1155=item ${^UTF8LOCALE}
1156
1157This variable indicates whether an UTF-8 locale was detected by perl at
1158startup. This information is used by perl when it's in
1159adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
1160switch); see L<perlrun> for more info on this.
1161
44dcb63b 1162=item $PERL_VERSION
b459063d 1163
16070b82
GS
1164=item $^V
1165
1166The revision, version, and subversion of the Perl interpreter, represented
da2094fd 1167as a string composed of characters with those ordinals. Thus in Perl v5.6.0
44dcb63b
GS
1168it equals C<chr(5) . chr(6) . chr(0)> and will return true for
1169C<$^V eq v5.6.0>. Note that the characters in this string value can
1170potentially be in Unicode range.
16070b82
GS
1171
1172This can be used to determine whether the Perl interpreter executing a
1173script is in the right range of versions. (Mnemonic: use ^V for Version
44dcb63b 1174Control.) Example:
16070b82 1175
3fd4402b 1176 warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0;
16070b82 1177
aa2f2a36
AMS
1178To convert C<$^V> into its string representation use sprintf()'s
1179C<"%vd"> conversion:
1180
1181 printf "version is v%vd\n", $^V; # Perl's version
1182
44dcb63b 1183See the documentation of C<use VERSION> and C<require VERSION>
16070b82
GS
1184for a convenient way to fail if the running Perl interpreter is too old.
1185
1186See also C<$]> for an older representation of the Perl version.
1187
a0d0e21e
LW
1188=item $WARNING
1189
1190=item $^W
1191
19799a22
GS
1192The current value of the warning switch, initially true if B<-w>
1193was used, false otherwise, but directly modifiable. (Mnemonic:
4438c4b7
JH
1194related to the B<-w> switch.) See also L<warnings>.
1195
6a818117 1196=item ${^WARNING_BITS}
4438c4b7
JH
1197
1198The current set of warning checks enabled by the C<use warnings> pragma.
1199See the documentation of C<warnings> for more details.
a0d0e21e
LW
1200
1201=item $EXECUTABLE_NAME
1202
1203=item $^X
1204
e71940de 1205The name used to execute the current copy of Perl, from C's
21c1191d 1206C<argv[0]> or (where supported) F</proc/self/exe>.
38e4f4ae 1207
e71940de
PG
1208Depending on the host operating system, the value of $^X may be
1209a relative or absolute pathname of the perl program file, or may
1210be the string used to invoke perl but not the pathname of the
1211perl program file. Also, most operating systems permit invoking
1212programs that are not in the PATH environment variable, so there
a10d74f3
PG
1213is no guarantee that the value of $^X is in PATH. For VMS, the
1214value may or may not include a version number.
38e4f4ae 1215
e71940de
PG
1216You usually can use the value of $^X to re-invoke an independent
1217copy of the same perl that is currently running, e.g.,
1218
1219 @first_run = `$^X -le "print int rand 100 for 1..100"`;
1220
1221But recall that not all operating systems support forking or
1222capturing of the output of commands, so this complex statement
1223may not be portable.
38e4f4ae 1224
e71940de
PG
1225It is not safe to use the value of $^X as a path name of a file,
1226as some operating systems that have a mandatory suffix on
1227executable files do not require use of the suffix when invoking
1228a command. To convert the value of $^X to a path name, use the
1229following statements:
1230
304dea91 1231 # Build up a set of file names (not command names).
e71940de 1232 use Config;
68fb0eb7
PG
1233 $this_perl = $^X;
1234 if ($^O ne 'VMS')
1235 {$this_perl .= $Config{_exe}
1236 unless $this_perl =~ m/$Config{_exe}$/i;}
e71940de
PG
1237
1238Because many operating systems permit anyone with read access to
1239the Perl program file to make a copy of it, patch the copy, and
1240then execute the copy, the security-conscious Perl programmer
1241should take care to invoke the installed copy of perl, not the
1242copy referenced by $^X. The following statements accomplish
1243this goal, and produce a pathname that can be invoked as a
1244command or referenced as a file.
38e4f4ae
SB
1245
1246 use Config;
68fb0eb7
PG
1247 $secure_perl_path = $Config{perlpath};
1248 if ($^O ne 'VMS')
1249 {$secure_perl_path .= $Config{_exe}
1250 unless $secure_perl_path =~ m/$Config{_exe}$/i;}
a0d0e21e 1251
2d84a16a
DM
1252=item ARGV
1253
1254The special filehandle that iterates over command-line filenames in
1255C<@ARGV>. Usually written as the null filehandle in the angle operator
1256C<< <> >>. Note that currently C<ARGV> only has its magical effect
1257within the C<< <> >> operator; elsewhere it is just a plain filehandle
1258corresponding to the last file opened by C<< <> >>. In particular,
1259passing C<\*ARGV> as a parameter to a function that expects a filehandle
1260may not cause your function to automatically read the contents of all the
1261files in C<@ARGV>.
1262
a0d0e21e
LW
1263=item $ARGV
1264
c47ff5f1 1265contains the name of the current file when reading from <>.
a0d0e21e
LW
1266
1267=item @ARGV
1268
19799a22 1269The array @ARGV contains the command-line arguments intended for
14218588 1270the script. C<$#ARGV> is generally the number of arguments minus
19799a22
GS
1271one, because C<$ARGV[0]> is the first argument, I<not> the program's
1272command name itself. See C<$0> for the command name.
a0d0e21e 1273
5ccee41e
JA
1274=item ARGVOUT
1275
1276The special filehandle that points to the currently open output file
1277when doing edit-in-place processing with B<-i>. Useful when you have
1278to do a lot of inserting and don't want to keep modifying $_. See
1279L<perlrun> for the B<-i> switch.
1280
9b0e6e7a
JP
1281=item @F
1282
1283The array @F contains the fields of each line read in when autosplit
1284mode is turned on. See L<perlrun> for the B<-a> switch. This array
1285is package-specific, and must be declared or given a full package name
1286if not in package main when running under C<strict 'vars'>.
1287
a0d0e21e
LW
1288=item @INC
1289
19799a22
GS
1290The array @INC contains the list of places that the C<do EXPR>,
1291C<require>, or C<use> constructs look for their library files. It
1292initially consists of the arguments to any B<-I> command-line
1293switches, followed by the default Perl library, probably
1294F</usr/local/lib/perl>, followed by ".", to represent the current
e48df184
RGS
1295directory. ("." will not be appended if taint checks are enabled, either by
1296C<-T> or by C<-t>.) If you need to modify this at runtime, you should use
19799a22
GS
1297the C<use lib> pragma to get the machine-dependent library properly
1298loaded also:
a0d0e21e 1299
cb1a09d0
AD
1300 use lib '/mypath/libdir/';
1301 use SomeMod;
303f2f76 1302
d54b56d5
RGS
1303You can also insert hooks into the file inclusion system by putting Perl
1304code directly into @INC. Those hooks may be subroutine references, array
1305references or blessed objects. See L<perlfunc/require> for details.
1306
fb73857a
PP
1307=item @_
1308
1309Within a subroutine the array @_ contains the parameters passed to that
19799a22 1310subroutine. See L<perlsub>.
fb73857a 1311
a0d0e21e
LW
1312=item %INC
1313
19799a22
GS
1314The hash %INC contains entries for each filename included via the
1315C<do>, C<require>, or C<use> operators. The key is the filename
1316you specified (with module names converted to pathnames), and the
14218588 1317value is the location of the file found. The C<require>
87275199 1318operator uses this hash to determine whether a particular file has
19799a22 1319already been included.
a0d0e21e 1320
89ccab8c
RGS
1321If the file was loaded via a hook (e.g. a subroutine reference, see
1322L<perlfunc/require> for a description of these hooks), this hook is
9ae8cd5b
RGS
1323by default inserted into %INC in place of a filename. Note, however,
1324that the hook may have set the %INC entry by itself to provide some more
1325specific info.
44f0be63 1326
b687b08b
TC
1327=item %ENV
1328
1329=item $ENV{expr}
a0d0e21e
LW
1330
1331The hash %ENV contains your current environment. Setting a
19799a22
GS
1332value in C<ENV> changes the environment for any child processes
1333you subsequently fork() off.
a0d0e21e 1334
b687b08b
TC
1335=item %SIG
1336
1337=item $SIG{expr}
a0d0e21e 1338
14218588 1339The hash %SIG contains signal handlers for signals. For example:
a0d0e21e
LW
1340
1341 sub handler { # 1st argument is signal name
fb73857a 1342 my($sig) = @_;
a0d0e21e
LW
1343 print "Caught a SIG$sig--shutting down\n";
1344 close(LOG);
1345 exit(0);
1346 }
1347
fb73857a
PP
1348 $SIG{'INT'} = \&handler;
1349 $SIG{'QUIT'} = \&handler;
a0d0e21e 1350 ...
19799a22 1351 $SIG{'INT'} = 'DEFAULT'; # restore default action
a0d0e21e
LW
1352 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1353
f648820c
GS
1354Using a value of C<'IGNORE'> usually has the effect of ignoring the
1355signal, except for the C<CHLD> signal. See L<perlipc> for more about
1356this special case.
1357
19799a22 1358Here are some other examples:
a0d0e21e 1359
fb73857a 1360 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
a0d0e21e 1361 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
19799a22 1362 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
a0d0e21e
LW
1363 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1364
19799a22
GS
1365Be sure not to use a bareword as the name of a signal handler,
1366lest you inadvertently call it.
748a9306 1367
44a8e56a 1368If your system has the sigaction() function then signal handlers are
9ce5b4ad 1369installed using it. This means you get reliable signal handling.
44a8e56a 1370
9ce5b4ad
SG
1371The default delivery policy of signals changed in Perl 5.8.0 from
1372immediate (also known as "unsafe") to deferred, also known as
1373"safe signals". See L<perlipc> for more information.
45c0772f 1374
748a9306 1375Certain internal hooks can be also set using the %SIG hash. The
a8f8344d 1376routine indicated by C<$SIG{__WARN__}> is called when a warning message is
748a9306
LW
1377about to be printed. The warning message is passed as the first
1378argument. The presence of a __WARN__ hook causes the ordinary printing
1379of warnings to STDERR to be suppressed. You can use this to save warnings
1380in a variable, or turn warnings into fatal errors, like this:
1381
1382 local $SIG{__WARN__} = sub { die $_[0] };
1383 eval $proggie;
1384
a8f8344d 1385The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
748a9306
LW
1386is about to be thrown. The error message is passed as the first
1387argument. When a __DIE__ hook routine returns, the exception
1388processing continues as it would have in the absence of the hook,
cb1a09d0 1389unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
774d564b 1390The C<__DIE__> handler is explicitly disabled during the call, so that you
fb73857a
PP
1391can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1392
19799a22
GS
1393Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1394even inside an eval(). Do not use this to rewrite a pending exception
1395in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
1396This strange action at a distance may be fixed in a future release
1397so that C<$SIG{__DIE__}> is only called if your program is about
1398to exit, as was the original intent. Any other use is deprecated.
1399
1400C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1401they may be called to report (probable) errors found by the parser.
1402In such a case the parser may be in inconsistent state, so any
1403attempt to evaluate Perl code from such a handler will probably
1404result in a segfault. This means that warnings or errors that
1405result from parsing Perl should be used with extreme caution, like
1406this:
fb73857a
PP
1407
1408 require Carp if defined $^S;
1409 Carp::confess("Something wrong") if defined &Carp::confess;
1410 die "Something wrong, but could not load Carp to give backtrace...
1411 To see backtrace try starting Perl with -MCarp switch";
1412
1413Here the first line will load Carp I<unless> it is the parser who
1414called the handler. The second line will print backtrace and die if
1415Carp was available. The third line will be executed only if Carp was
1416not available.
1417
19799a22 1418See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
4438c4b7 1419L<warnings> for additional information.
68dc0745 1420
a0d0e21e 1421=back
55602bd2
IZ
1422
1423=head2 Error Indicators
1424
19799a22
GS
1425The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1426about different types of error conditions that may appear during
1427execution of a Perl program. The variables are shown ordered by
1428the "distance" between the subsystem which reported the error and
1429the Perl process. They correspond to errors detected by the Perl
1430interpreter, C library, operating system, or an external program,
1431respectively.
55602bd2
IZ
1432
1433To illustrate the differences between these variables, consider the
19799a22 1434following Perl expression, which uses a single-quoted string:
55602bd2 1435
19799a22 1436 eval q{
22d0716c
SB
1437 open my $pipe, "/cdrom/install |" or die $!;
1438 my @res = <$pipe>;
1439 close $pipe or die "bad pipe: $?, $!";
19799a22 1440 };
55602bd2
IZ
1441
1442After execution of this statement all 4 variables may have been set.
1443
19799a22
GS
1444C<$@> is set if the string to be C<eval>-ed did not compile (this
1445may happen if C<open> or C<close> were imported with bad prototypes),
1446or if Perl code executed during evaluation die()d . In these cases
1447the value of $@ is the compile error, or the argument to C<die>
4cb1c523 1448(which will interpolate C<$!> and C<$?>). (See also L<Fatal>,
19799a22
GS
1449though.)
1450
c47ff5f1 1451When the eval() expression above is executed, open(), C<< <PIPE> >>,
19799a22
GS
1452and C<close> are translated to calls in the C run-time library and
1453thence to the operating system kernel. C<$!> is set to the C library's
1454C<errno> if one of these calls fails.
1455
1456Under a few operating systems, C<$^E> may contain a more verbose
1457error indicator, such as in this case, "CDROM tray not closed."
14218588 1458Systems that do not support extended error messages leave C<$^E>
19799a22
GS
1459the same as C<$!>.
1460
1461Finally, C<$?> may be set to non-0 value if the external program
1462F</cdrom/install> fails. The upper eight bits reflect specific
1463error conditions encountered by the program (the program's exit()
1464value). The lower eight bits reflect mode of failure, like signal
1465death and core dump information See wait(2) for details. In
1466contrast to C<$!> and C<$^E>, which are set only if error condition
1467is detected, the variable C<$?> is set on each C<wait> or pipe
1468C<close>, overwriting the old value. This is more like C<$@>, which
1469on every eval() is always set on failure and cleared on success.
2b92dfce 1470
19799a22
GS
1471For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1472and C<$?>.
2b92dfce
GS
1473
1474=head2 Technical Note on the Syntax of Variable Names
1475
19799a22
GS
1476Variable names in Perl can have several formats. Usually, they
1477must begin with a letter or underscore, in which case they can be
1478arbitrarily long (up to an internal limit of 251 characters) and
1479may contain letters, digits, underscores, or the special sequence
1480C<::> or C<'>. In this case, the part before the last C<::> or
1481C<'> is taken to be a I<package qualifier>; see L<perlmod>.
2b92dfce
GS
1482
1483Perl variable names may also be a sequence of digits or a single
1484punctuation or control character. These names are all reserved for
19799a22
GS
1485special uses by Perl; for example, the all-digits names are used
1486to hold data captured by backreferences after a regular expression
1487match. Perl has a special syntax for the single-control-character
1488names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1489character. For example, the notation C<$^W> (dollar-sign caret
1490C<W>) is the scalar variable whose name is the single character
1491control-C<W>. This is better than typing a literal control-C<W>
1492into your program.
2b92dfce 1493
87275199 1494Finally, new in Perl 5.6, Perl variable names may be alphanumeric
19799a22
GS
1495strings that begin with control characters (or better yet, a caret).
1496These variables must be written in the form C<${^Foo}>; the braces
1497are not optional. C<${^Foo}> denotes the scalar variable whose
1498name is a control-C<F> followed by two C<o>'s. These variables are
1499reserved for future special uses by Perl, except for the ones that
1500begin with C<^_> (control-underscore or caret-underscore). No
1501control-character name that begins with C<^_> will acquire a special
1502meaning in any future version of Perl; such names may therefore be
1503used safely in programs. C<$^_> itself, however, I<is> reserved.
1504
1fcb18de
RGS
1505Perl identifiers that begin with digits, control characters, or
1506punctuation characters are exempt from the effects of the C<package>
1507declaration and are always forced to be in package C<main>; they are
1508also exempt from C<strict 'vars'> errors. A few other names are also
1509exempt in these ways:
2b92dfce
GS
1510
1511 ENV STDIN
1512 INC STDOUT
1513 ARGV STDERR
5b88253b 1514 ARGVOUT _
2b92dfce
GS
1515 SIG
1516
1517In particular, the new special C<${^_XYZ}> variables are always taken
19799a22 1518to be in package C<main>, regardless of any C<package> declarations
747fafda 1519presently in scope.
2b92dfce 1520
19799a22
GS
1521=head1 BUGS
1522
1523Due to an unfortunate accident of Perl's implementation, C<use
1524English> imposes a considerable performance penalty on all regular
1525expression matches in a program, regardless of whether they occur
1526in the scope of C<use English>. For that reason, saying C<use
1527English> in libraries is strongly discouraged. See the
1528Devel::SawAmpersand module documentation from CPAN
1577cd80 1529( http://www.cpan.org/modules/by-module/Devel/ )
19799a22 1530for more information.
2b92dfce 1531
19799a22
GS
1532Having to even think about the C<$^S> variable in your exception
1533handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1534invites grievous and difficult to track down errors. Avoid it
1535and use an C<END{}> or CORE::GLOBAL::die override instead.