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Make the description of $^M in perlvar a bit more clear
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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
6e2995f4 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
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414The output field separator for the print operator. If defined, this
415value is printed between each of print's arguments. Default is C<undef>.
416(Mnemonic: what is printed when there is a "," in your print statement.)
a0d0e21e 417
46550894 418=item IO::Handle->output_record_separator EXPR
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419
420=item $OUTPUT_RECORD_SEPARATOR
421
422=item $ORS
423
424=item $\
425
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426The output record separator for the print operator. If defined, this
427value is printed after the last of print's arguments. Default is C<undef>.
428(Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
429Also, it's just like C<$/>, but it's what you get "back" from Perl.)
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430
431=item $LIST_SEPARATOR
432
433=item $"
434
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435This is like C<$,> except that it applies to array and slice values
436interpolated into a double-quoted string (or similar interpreted
437string). Default is a space. (Mnemonic: obvious, I think.)
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438
439=item $SUBSCRIPT_SEPARATOR
440
441=item $SUBSEP
442
443=item $;
444
54310121 445The subscript separator for multidimensional array emulation. If you
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446refer to a hash element as
447
448 $foo{$a,$b,$c}
449
450it really means
451
452 $foo{join($;, $a, $b, $c)}
453
454But don't put
455
456 @foo{$a,$b,$c} # a slice--note the @
457
458which means
459
460 ($foo{$a},$foo{$b},$foo{$c})
461
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462Default is "\034", the same as SUBSEP in B<awk>. If your
463keys contain binary data there might not be any safe value for C<$;>.
a0d0e21e 464(Mnemonic: comma (the syntactic subscript separator) is a
19799a22 465semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
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466taken for something more important.)
467
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468Consider using "real" multidimensional arrays as described
469in L<perllol>.
a0d0e21e 470
fcc7d916 471=item HANDLE->format_page_number(EXPR)
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472
473=item $FORMAT_PAGE_NUMBER
474
475=item $%
476
477The current page number of the currently selected output channel.
19799a22 478Used with formats.
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479(Mnemonic: % is page number in B<nroff>.)
480
fcc7d916 481=item HANDLE->format_lines_per_page(EXPR)
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482
483=item $FORMAT_LINES_PER_PAGE
484
485=item $=
486
487The current page length (printable lines) of the currently selected
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488output channel. Default is 60.
489Used with formats.
490(Mnemonic: = has horizontal lines.)
a0d0e21e 491
fcc7d916 492=item HANDLE->format_lines_left(EXPR)
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493
494=item $FORMAT_LINES_LEFT
495
496=item $-
497
498The number of lines left on the page of the currently selected output
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499channel.
500Used with formats.
501(Mnemonic: lines_on_page - lines_printed.)
a0d0e21e 502
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503=item @LAST_MATCH_START
504
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505=item @-
506
19799a22 507$-[0] is the offset of the start of the last successful match.
6cef1e77 508C<$-[>I<n>C<]> is the offset of the start of the substring matched by
8f580fb8 509I<n>-th subpattern, or undef if the subpattern did not match.
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510
511Thus after a match against $_, $& coincides with C<substr $_, $-[0],
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512$+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
513$+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
514C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the last
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515matched subgroup in the last successful match. Contrast with
516C<$#+>, the number of subgroups in the regular expression. Compare
19799a22 517with C<@+>.
6cef1e77 518
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519This array holds the offsets of the beginnings of the last
520successful submatches in the currently active dynamic scope.
521C<$-[0]> is the offset into the string of the beginning of the
522entire match. The I<n>th element of this array holds the offset
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523of the I<n>th submatch, so C<$-[1]> is the offset where $1
524begins, C<$-[2]> the offset where $2 begins, and so on.
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525
526After a match against some variable $var:
527
528=over 5
529
4375e838 530=item C<$`> is the same as C<substr($var, 0, $-[0])>
4ba05bdc 531
4375e838 532=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
4ba05bdc 533
4375e838 534=item C<$'> is the same as C<substr($var, $+[0])>
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535
536=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
537
538=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
539
80dc6883 540=item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
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541
542=back
543
fcc7d916 544=item HANDLE->format_name(EXPR)
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545
546=item $FORMAT_NAME
547
548=item $~
549
550The name of the current report format for the currently selected output
14218588 551channel. Default is the name of the filehandle. (Mnemonic: brother to
19799a22 552C<$^>.)
a0d0e21e 553
fcc7d916 554=item HANDLE->format_top_name(EXPR)
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555
556=item $FORMAT_TOP_NAME
557
558=item $^
559
560The name of the current top-of-page format for the currently selected
14218588 561output channel. Default is the name of the filehandle with _TOP
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562appended. (Mnemonic: points to top of page.)
563
46550894 564=item IO::Handle->format_line_break_characters EXPR
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565
566=item $FORMAT_LINE_BREAK_CHARACTERS
567
568=item $:
569
570The current set of characters after which a string may be broken to
54310121 571fill continuation fields (starting with ^) in a format. Default is
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572S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
573poetry is a part of a line.)
574
46550894 575=item IO::Handle->format_formfeed EXPR
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576
577=item $FORMAT_FORMFEED
578
579=item $^L
580
14218588 581What formats output as a form feed. Default is \f.
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582
583=item $ACCUMULATOR
584
585=item $^A
586
587The current value of the write() accumulator for format() lines. A format
19799a22 588contains formline() calls that put their result into C<$^A>. After
a0d0e21e 589calling its format, write() prints out the contents of C<$^A> and empties.
14218588 590So you never really see the contents of C<$^A> unless you call
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591formline() yourself and then look at it. See L<perlform> and
592L<perlfunc/formline()>.
593
594=item $CHILD_ERROR
595
596=item $?
597
54310121 598The status returned by the last pipe close, backtick (C<``>) command,
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599successful call to wait() or waitpid(), or from the system()
600operator. This is just the 16-bit status word returned by the
e5218da5 601traditional Unix wait() system call (or else is made up to look like it). Thus, the
c47ff5f1 602exit value of the subprocess is really (C<<< $? >> 8 >>>), and
19799a22
GS
603C<$? & 127> gives which signal, if any, the process died from, and
604C<$? & 128> reports whether there was a core dump. (Mnemonic:
605similar to B<sh> and B<ksh>.)
a0d0e21e 606
7b8d334a 607Additionally, if the C<h_errno> variable is supported in C, its value
14218588 608is returned via $? if any C<gethost*()> function fails.
7b8d334a 609
19799a22 610If you have installed a signal handler for C<SIGCHLD>, the
aa689395 611value of C<$?> will usually be wrong outside that handler.
612
a8f8344d 613Inside an C<END> subroutine C<$?> contains the value that is going to be
614given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
19799a22
GS
615change the exit status of your program. For example:
616
617 END {
618 $? = 1 if $? == 255; # die would make it 255
619 }
a8f8344d 620
aa689395 621Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
ff0cee69 622actual VMS exit status, instead of the default emulation of POSIX
9bc98430 623status; see L<perlvms/$?> for details.
f86702cc 624
55602bd2
IZ
625Also see L<Error Indicators>.
626
e5218da5
GA
627=item ${^CHILD_ERROR_NATIVE}
628
629The native status returned by the last pipe close, backtick (C<``>)
630command, successful call to wait() or waitpid(), or from the system()
631operator. On POSIX-like systems this value can be decoded with the
632WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED, WSTOPSIG
633and WIFCONTINUED functions provided by the L<POSIX> module.
634
635Under VMS this reflects the actual VMS exit status; i.e. it is the same
636as $? when the pragma C<use vmsish 'status'> is in effect.
637
0a378802
JH
638=item ${^ENCODING}
639
740bd165
PN
640The I<object reference> to the Encode object that is used to convert
641the source code to Unicode. Thanks to this variable your perl script
642does not have to be written in UTF-8. Default is I<undef>. The direct
643manipulation of this variable is highly discouraged. See L<encoding>
048c20cb 644for more details.
0a378802 645
a0d0e21e
LW
646=item $OS_ERROR
647
648=item $ERRNO
649
650=item $!
651
19799a22 652If used numerically, yields the current value of the C C<errno>
6ab308ee
JH
653variable, or in other words, if a system or library call fails, it
654sets this variable. This means that the value of C<$!> is meaningful
655only I<immediately> after a B<failure>:
656
657 if (open(FH, $filename)) {
658 # Here $! is meaningless.
659 ...
660 } else {
661 # ONLY here is $! meaningful.
662 ...
663 # Already here $! might be meaningless.
664 }
665 # Since here we might have either success or failure,
666 # here $! is meaningless.
667
668In the above I<meaningless> stands for anything: zero, non-zero,
669C<undef>. A successful system or library call does B<not> set
670the variable to zero.
671
271df126 672If used as a string, yields the corresponding system error string.
19799a22
GS
673You can assign a number to C<$!> to set I<errno> if, for instance,
674you want C<"$!"> to return the string for error I<n>, or you want
675to set the exit value for the die() operator. (Mnemonic: What just
676went bang?)
a0d0e21e 677
55602bd2
IZ
678Also see L<Error Indicators>.
679
4c5cef9b
MJD
680=item %!
681
682Each element of C<%!> has a true value only if C<$!> is set to that
683value. For example, C<$!{ENOENT}> is true if and only if the current
3be065a1
JH
684value of C<$!> is C<ENOENT>; that is, if the most recent error was
685"No such file or directory" (or its moral equivalent: not all operating
686systems give that exact error, and certainly not all languages).
687To check if a particular key is meaningful on your system, use
688C<exists $!{the_key}>; for a list of legal keys, use C<keys %!>.
689See L<Errno> for more information, and also see above for the
690validity of C<$!>.
4c5cef9b 691
5c055ba3 692=item $EXTENDED_OS_ERROR
693
694=item $^E
695
22fae026
TM
696Error information specific to the current operating system. At
697the moment, this differs from C<$!> under only VMS, OS/2, and Win32
698(and for MacPerl). On all other platforms, C<$^E> is always just
699the same as C<$!>.
700
701Under VMS, C<$^E> provides the VMS status value from the last
702system error. This is more specific information about the last
703system error than that provided by C<$!>. This is particularly
d516a115 704important when C<$!> is set to B<EVMSERR>.
22fae026 705
1c1c7f20
GS
706Under OS/2, C<$^E> is set to the error code of the last call to
707OS/2 API either via CRT, or directly from perl.
22fae026
TM
708
709Under Win32, C<$^E> always returns the last error information
710reported by the Win32 call C<GetLastError()> which describes
711the last error from within the Win32 API. Most Win32-specific
19799a22 712code will report errors via C<$^E>. ANSI C and Unix-like calls
22fae026
TM
713set C<errno> and so most portable Perl code will report errors
714via C<$!>.
715
716Caveats mentioned in the description of C<$!> generally apply to
717C<$^E>, also. (Mnemonic: Extra error explanation.)
5c055ba3 718
55602bd2
IZ
719Also see L<Error Indicators>.
720
a0d0e21e
LW
721=item $EVAL_ERROR
722
723=item $@
724
4a280ebe
JG
725The Perl syntax error message from the last eval() operator.
726If $@ is the null string, the last eval() parsed and executed
727correctly (although the operations you invoked may have failed in the
728normal fashion). (Mnemonic: Where was the syntax error "at"?)
a0d0e21e 729
19799a22 730Warning messages are not collected in this variable. You can,
a8f8344d 731however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
54310121 732as described below.
748a9306 733
55602bd2
IZ
734Also see L<Error Indicators>.
735
a0d0e21e
LW
736=item $PROCESS_ID
737
738=item $PID
739
740=item $$
741
19799a22
GS
742The process number of the Perl running this script. You should
743consider this variable read-only, although it will be altered
744across fork() calls. (Mnemonic: same as shells.)
a0d0e21e 745
4d76a344
RGS
746Note for Linux users: on Linux, the C functions C<getpid()> and
747C<getppid()> return different values from different threads. In order to
748be portable, this behavior is not reflected by C<$$>, whose value remains
749consistent across threads. If you want to call the underlying C<getpid()>,
e3256f86 750you may use the CPAN module C<Linux::Pid>.
4d76a344 751
a0d0e21e
LW
752=item $REAL_USER_ID
753
754=item $UID
755
756=item $<
757
19799a22 758The real uid of this process. (Mnemonic: it's the uid you came I<from>,
a043a685 759if you're running setuid.) You can change both the real uid and
a537debe
SP
760the effective uid at the same time by using POSIX::setuid(). Since
761changes to $< require a system call, check $! after a change attempt to
762detect any possible errors.
a0d0e21e
LW
763
764=item $EFFECTIVE_USER_ID
765
766=item $EUID
767
768=item $>
769
770The effective uid of this process. Example:
771
772 $< = $>; # set real to effective uid
773 ($<,$>) = ($>,$<); # swap real and effective uid
774
a043a685 775You can change both the effective uid and the real uid at the same
a537debe
SP
776time by using POSIX::setuid(). Changes to $> require a check to $!
777to detect any possible errors after an attempted change.
a043a685 778
19799a22 779(Mnemonic: it's the uid you went I<to>, if you're running setuid.)
c47ff5f1 780C<< $< >> and C<< $> >> can be swapped only on machines
8cc95fdb 781supporting setreuid().
a0d0e21e
LW
782
783=item $REAL_GROUP_ID
784
785=item $GID
786
787=item $(
788
789The real gid of this process. If you are on a machine that supports
790membership in multiple groups simultaneously, gives a space separated
791list of groups you are in. The first number is the one returned by
792getgid(), and the subsequent ones by getgroups(), one of which may be
8cc95fdb 793the same as the first number.
794
19799a22
GS
795However, a value assigned to C<$(> must be a single number used to
796set the real gid. So the value given by C<$(> should I<not> be assigned
797back to C<$(> without being forced numeric, such as by adding zero.
8cc95fdb 798
a043a685 799You can change both the real gid and the effective gid at the same
a537debe
SP
800time by using POSIX::setgid(). Changes to $( require a check to $!
801to detect any possible errors after an attempted change.
a043a685 802
19799a22
GS
803(Mnemonic: parentheses are used to I<group> things. The real gid is the
804group you I<left>, if you're running setgid.)
a0d0e21e
LW
805
806=item $EFFECTIVE_GROUP_ID
807
808=item $EGID
809
810=item $)
811
812The effective gid of this process. If you are on a machine that
813supports membership in multiple groups simultaneously, gives a space
814separated list of groups you are in. The first number is the one
815returned by getegid(), and the subsequent ones by getgroups(), one of
8cc95fdb 816which may be the same as the first number.
817
19799a22 818Similarly, a value assigned to C<$)> must also be a space-separated
14218588 819list of numbers. The first number sets the effective gid, and
8cc95fdb 820the rest (if any) are passed to setgroups(). To get the effect of an
821empty list for setgroups(), just repeat the new effective gid; that is,
822to force an effective gid of 5 and an effectively empty setgroups()
823list, say C< $) = "5 5" >.
824
a043a685
GW
825You can change both the effective gid and the real gid at the same
826time by using POSIX::setgid() (use only a single numeric argument).
a537debe
SP
827Changes to $) require a check to $! to detect any possible errors
828after an attempted change.
a043a685 829
19799a22
GS
830(Mnemonic: parentheses are used to I<group> things. The effective gid
831is the group that's I<right> for you, if you're running setgid.)
a0d0e21e 832
c47ff5f1 833C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
19799a22
GS
834machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
835and C<$)> can be swapped only on machines supporting setregid().
a0d0e21e
LW
836
837=item $PROGRAM_NAME
838
839=item $0
840
80bca1b4
JH
841Contains the name of the program being executed.
842
843On some (read: not all) operating systems assigning to C<$0> modifies
844the argument area that the C<ps> program sees. On some platforms you
845may have to use special C<ps> options or a different C<ps> to see the
846changes. Modifying the $0 is more useful as a way of indicating the
847current program state than it is for hiding the program you're
848running. (Mnemonic: same as B<sh> and B<ksh>.)
f9cbb277 849
cf525c36 850Note that there are platform specific limitations on the maximum
f9cbb277
JH
851length of C<$0>. In the most extreme case it may be limited to the
852space occupied by the original C<$0>.
a0d0e21e 853
80bca1b4
JH
854In some platforms there may be arbitrary amount of padding, for
855example space characters, after the modified name as shown by C<ps>.
dda345b7 856In some platforms this padding may extend all the way to the original
c80e2480
JH
857length of the argument area, no matter what you do (this is the case
858for example with Linux 2.2).
80bca1b4 859
4bc88a62 860Note for BSD users: setting C<$0> does not completely remove "perl"
6a4647a3
JH
861from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
862result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
863and the " (perl)" suffix are shown depends on your exact BSD variant
864and version). This is an operating system feature, Perl cannot help it.
4bc88a62 865
e2975953
JH
866In multithreaded scripts Perl coordinates the threads so that any
867thread may modify its copy of the C<$0> and the change becomes visible
cf525c36 868to ps(1) (assuming the operating system plays along). Note that
80bca1b4
JH
869the view of C<$0> the other threads have will not change since they
870have their own copies of it.
e2975953 871
a0d0e21e
LW
872=item $[
873
874The index of the first element in an array, and of the first character
19799a22
GS
875in a substring. Default is 0, but you could theoretically set it
876to 1 to make Perl behave more like B<awk> (or Fortran) when
877subscripting and when evaluating the index() and substr() functions.
878(Mnemonic: [ begins subscripts.)
a0d0e21e 879
19799a22
GS
880As of release 5 of Perl, assignment to C<$[> is treated as a compiler
881directive, and cannot influence the behavior of any other file.
f83ed198 882(That's why you can only assign compile-time constants to it.)
19799a22 883Its use is highly discouraged.
a0d0e21e 884
f83ed198 885Note that, unlike other compile-time directives (such as L<strict>),
af7a0647
RGS
886assignment to C<$[> can be seen from outer lexical scopes in the same file.
887However, you can use local() on it to strictly bind its value to a
f83ed198
RGS
888lexical block.
889
a0d0e21e
LW
890=item $]
891
54310121 892The version + patchlevel / 1000 of the Perl interpreter. This variable
893can be used to determine whether the Perl interpreter executing a
894script is in the right range of versions. (Mnemonic: Is this version
895of perl in the right bracket?) Example:
a0d0e21e
LW
896
897 warn "No checksumming!\n" if $] < 3.019;
898
54310121 899See also the documentation of C<use VERSION> and C<require VERSION>
19799a22 900for a convenient way to fail if the running Perl interpreter is too old.
a0d0e21e 901
0c8d858b
MS
902The floating point representation can sometimes lead to inaccurate
903numeric comparisons. See C<$^V> for a more modern representation of
904the Perl version that allows accurate string comparisons.
16070b82 905
305aace0
NIS
906=item $COMPILING
907
908=item $^C
909
19799a22
GS
910The current value of the flag associated with the B<-c> switch.
911Mainly of use with B<-MO=...> to allow code to alter its behavior
912when being compiled, such as for example to AUTOLOAD at compile
913time rather than normal, deferred loading. See L<perlcc>. Setting
914C<$^C = 1> is similar to calling C<B::minus_c>.
305aace0 915
a0d0e21e
LW
916=item $DEBUGGING
917
918=item $^D
919
920The current value of the debugging flags. (Mnemonic: value of B<-D>
b4ab917c
DM
921switch.) May be read or set. Like its command-line equivalent, you can use
922numeric or symbolic values, eg C<$^D = 10> or C<$^D = "st">.
a0d0e21e 923
a3621e74
YO
924=item ${^RE_DEBUG_FLAGS}
925
926The current value of the regex debugging flags. Set to 0 for no debug output
927even when the re 'debug' module is loaded. See L<re> for details.
928
0111c4fd 929=item ${^RE_TRIE_MAXBUF}
a3621e74
YO
930
931Controls how certain regex optimisations are applied and how much memory they
932utilize. This value by default is 65536 which corresponds to a 512kB temporary
933cache. Set this to a higher value to trade memory for speed when matching
934large alternations. Set it to a lower value if you want the optimisations to
935be as conservative of memory as possible but still occur, and set it to a
936negative value to prevent the optimisation and conserve the most memory.
937Under normal situations this variable should be of no interest to you.
938
a0d0e21e
LW
939=item $SYSTEM_FD_MAX
940
941=item $^F
942
943The maximum system file descriptor, ordinarily 2. System file
944descriptors are passed to exec()ed processes, while higher file
945descriptors are not. Also, during an open(), system file descriptors are
946preserved even if the open() fails. (Ordinary file descriptors are
19799a22 947closed before the open() is attempted.) The close-on-exec
a0d0e21e 948status of a file descriptor will be decided according to the value of
8d2a6795
GS
949C<$^F> when the corresponding file, pipe, or socket was opened, not the
950time of the exec().
a0d0e21e 951
6e2995f4 952=item $^H
953
0462a1ab
GS
954WARNING: This variable is strictly for internal use only. Its availability,
955behavior, and contents are subject to change without notice.
956
957This variable contains compile-time hints for the Perl interpreter. At the
958end of compilation of a BLOCK the value of this variable is restored to the
959value when the interpreter started to compile the BLOCK.
960
961When perl begins to parse any block construct that provides a lexical scope
962(e.g., eval body, required file, subroutine body, loop body, or conditional
963block), the existing value of $^H is saved, but its value is left unchanged.
964When the compilation of the block is completed, it regains the saved value.
965Between the points where its value is saved and restored, code that
966executes within BEGIN blocks is free to change the value of $^H.
967
968This behavior provides the semantic of lexical scoping, and is used in,
969for instance, the C<use strict> pragma.
970
971The contents should be an integer; different bits of it are used for
972different pragmatic flags. Here's an example:
973
974 sub add_100 { $^H |= 0x100 }
975
976 sub foo {
977 BEGIN { add_100() }
978 bar->baz($boon);
979 }
980
981Consider what happens during execution of the BEGIN block. At this point
982the BEGIN block has already been compiled, but the body of foo() is still
983being compiled. The new value of $^H will therefore be visible only while
984the body of foo() is being compiled.
985
986Substitution of the above BEGIN block with:
987
988 BEGIN { require strict; strict->import('vars') }
989
990demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
991version of the same lexical pragma:
992
993 BEGIN { require strict; strict->import('vars') if $condition }
994
995=item %^H
996
997WARNING: This variable is strictly for internal use only. Its availability,
998behavior, and contents are subject to change without notice.
999
1000The %^H hash provides the same scoping semantic as $^H. This makes it
1001useful for implementation of lexically scoped pragmas.
6e2995f4 1002
a0d0e21e
LW
1003=item $INPLACE_EDIT
1004
1005=item $^I
1006
1007The current value of the inplace-edit extension. Use C<undef> to disable
1008inplace editing. (Mnemonic: value of B<-i> switch.)
1009
fb73857a 1010=item $^M
1011
19799a22
GS
1012By default, running out of memory is an untrappable, fatal error.
1013However, if suitably built, Perl can use the contents of C<$^M>
1014as an emergency memory pool after die()ing. Suppose that your Perl
0acca065 1015were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
19799a22 1016Then
fb73857a 1017
19799a22 1018 $^M = 'a' x (1 << 16);
fb73857a 1019
51ee6500 1020would allocate a 64K buffer for use in an emergency. See the
19799a22 1021F<INSTALL> file in the Perl distribution for information on how to
0acca065
RGS
1022add custom C compilation flags when compiling perl. To discourage casual
1023use of this advanced feature, there is no L<English|English> long name for
1024this variable.
fb73857a 1025
5c055ba3 1026=item $OSNAME
6e2995f4 1027
5c055ba3 1028=item $^O
1029
1030The name of the operating system under which this copy of Perl was
1031built, as determined during the configuration process. The value
19799a22
GS
1032is identical to C<$Config{'osname'}>. See also L<Config> and the
1033B<-V> command-line switch documented in L<perlrun>.
5c055ba3 1034
443f6d01 1035In Windows platforms, $^O is not very helpful: since it is always
7f510801
GS
1036C<MSWin32>, it doesn't tell the difference between
103795/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() or
1038Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
1039between the variants.
916d64a3 1040
e2e27056
JH
1041=item ${^OPEN}
1042
1043An internal variable used by PerlIO. A string in two parts, separated
fae2c0fb
RGS
1044by a C<\0> byte, the first part describes the input layers, the second
1045part describes the output layers.
e2e27056 1046
a0d0e21e
LW
1047=item $PERLDB
1048
1049=item $^P
1050
19799a22
GS
1051The internal variable for debugging support. The meanings of the
1052various bits are subject to change, but currently indicate:
84902520
TB
1053
1054=over 6
1055
1056=item 0x01
1057
1058Debug subroutine enter/exit.
1059
1060=item 0x02
1061
1062Line-by-line debugging.
1063
1064=item 0x04
1065
1066Switch off optimizations.
1067
1068=item 0x08
1069
1070Preserve more data for future interactive inspections.
1071
1072=item 0x10
1073
1074Keep info about source lines on which a subroutine is defined.
1075
1076=item 0x20
1077
1078Start with single-step on.
1079
83ee9e09
GS
1080=item 0x40
1081
1082Use subroutine address instead of name when reporting.
1083
1084=item 0x80
1085
1086Report C<goto &subroutine> as well.
1087
1088=item 0x100
1089
1090Provide informative "file" names for evals based on the place they were compiled.
1091
1092=item 0x200
1093
1094Provide informative names to anonymous subroutines based on the place they
1095were compiled.
1096
7619c85e
RG
1097=item 0x400
1098
1099Debug assertion subroutines enter/exit.
1100
84902520
TB
1101=back
1102
19799a22
GS
1103Some bits may be relevant at compile-time only, some at
1104run-time only. This is a new mechanism and the details may change.
a0d0e21e 1105
66558a10
GS
1106=item $LAST_REGEXP_CODE_RESULT
1107
b9ac3b5b
GS
1108=item $^R
1109
19799a22
GS
1110The result of evaluation of the last successful C<(?{ code })>
1111regular expression assertion (see L<perlre>). May be written to.
b9ac3b5b 1112
66558a10
GS
1113=item $EXCEPTIONS_BEING_CAUGHT
1114
fb73857a 1115=item $^S
1116
fa05a9fd
IT
1117Current state of the interpreter.
1118
1119 $^S State
1120 --------- -------------------
1121 undef Parsing module/eval
1122 true (1) Executing an eval
1123 false (0) Otherwise
1124
1125The first state may happen in $SIG{__DIE__} and $SIG{__WARN__} handlers.
fb73857a 1126
a0d0e21e
LW
1127=item $BASETIME
1128
1129=item $^T
1130
19799a22 1131The time at which the program began running, in seconds since the
5f05dabc 1132epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
19799a22 1133and B<-C> filetests are based on this value.
a0d0e21e 1134
7c36658b
MS
1135=item ${^TAINT}
1136
9aa05f58
RGS
1137Reflects if taint mode is on or off. 1 for on (the program was run with
1138B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
18e8c5b0 1139B<-t> or B<-TU>). This variable is read-only.
7c36658b 1140
a05d7ebb
JH
1141=item ${^UNICODE}
1142
ab9e1bb7
JH
1143Reflects certain Unicode settings of Perl. See L<perlrun>
1144documentation for the C<-C> switch for more information about
1145the possible values. This variable is set during Perl startup
1146and is thereafter read-only.
fde18df1 1147
ea8eae40
RGS
1148=item ${^UTF8LOCALE}
1149
1150This variable indicates whether an UTF-8 locale was detected by perl at
1151startup. This information is used by perl when it's in
1152adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
1153switch); see L<perlrun> for more info on this.
1154
44dcb63b 1155=item $PERL_VERSION
b459063d 1156
16070b82
GS
1157=item $^V
1158
1159The revision, version, and subversion of the Perl interpreter, represented
da2094fd 1160as a string composed of characters with those ordinals. Thus in Perl v5.6.0
44dcb63b
GS
1161it equals C<chr(5) . chr(6) . chr(0)> and will return true for
1162C<$^V eq v5.6.0>. Note that the characters in this string value can
1163potentially be in Unicode range.
16070b82
GS
1164
1165This can be used to determine whether the Perl interpreter executing a
1166script is in the right range of versions. (Mnemonic: use ^V for Version
44dcb63b 1167Control.) Example:
16070b82 1168
3fd4402b 1169 warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0;
16070b82 1170
aa2f2a36
AMS
1171To convert C<$^V> into its string representation use sprintf()'s
1172C<"%vd"> conversion:
1173
1174 printf "version is v%vd\n", $^V; # Perl's version
1175
44dcb63b 1176See the documentation of C<use VERSION> and C<require VERSION>
16070b82
GS
1177for a convenient way to fail if the running Perl interpreter is too old.
1178
1179See also C<$]> for an older representation of the Perl version.
1180
a0d0e21e
LW
1181=item $WARNING
1182
1183=item $^W
1184
19799a22
GS
1185The current value of the warning switch, initially true if B<-w>
1186was used, false otherwise, but directly modifiable. (Mnemonic:
4438c4b7
JH
1187related to the B<-w> switch.) See also L<warnings>.
1188
6a818117 1189=item ${^WARNING_BITS}
4438c4b7
JH
1190
1191The current set of warning checks enabled by the C<use warnings> pragma.
1192See the documentation of C<warnings> for more details.
a0d0e21e
LW
1193
1194=item $EXECUTABLE_NAME
1195
1196=item $^X
1197
e71940de 1198The name used to execute the current copy of Perl, from C's
38e4f4ae
SB
1199C<argv[0]>.
1200
e71940de
PG
1201Depending on the host operating system, the value of $^X may be
1202a relative or absolute pathname of the perl program file, or may
1203be the string used to invoke perl but not the pathname of the
1204perl program file. Also, most operating systems permit invoking
1205programs that are not in the PATH environment variable, so there
a10d74f3
PG
1206is no guarantee that the value of $^X is in PATH. For VMS, the
1207value may or may not include a version number.
38e4f4ae 1208
e71940de
PG
1209You usually can use the value of $^X to re-invoke an independent
1210copy of the same perl that is currently running, e.g.,
1211
1212 @first_run = `$^X -le "print int rand 100 for 1..100"`;
1213
1214But recall that not all operating systems support forking or
1215capturing of the output of commands, so this complex statement
1216may not be portable.
38e4f4ae 1217
e71940de
PG
1218It is not safe to use the value of $^X as a path name of a file,
1219as some operating systems that have a mandatory suffix on
1220executable files do not require use of the suffix when invoking
1221a command. To convert the value of $^X to a path name, use the
1222following statements:
1223
304dea91 1224 # Build up a set of file names (not command names).
e71940de 1225 use Config;
68fb0eb7
PG
1226 $this_perl = $^X;
1227 if ($^O ne 'VMS')
1228 {$this_perl .= $Config{_exe}
1229 unless $this_perl =~ m/$Config{_exe}$/i;}
e71940de
PG
1230
1231Because many operating systems permit anyone with read access to
1232the Perl program file to make a copy of it, patch the copy, and
1233then execute the copy, the security-conscious Perl programmer
1234should take care to invoke the installed copy of perl, not the
1235copy referenced by $^X. The following statements accomplish
1236this goal, and produce a pathname that can be invoked as a
1237command or referenced as a file.
38e4f4ae
SB
1238
1239 use Config;
68fb0eb7
PG
1240 $secure_perl_path = $Config{perlpath};
1241 if ($^O ne 'VMS')
1242 {$secure_perl_path .= $Config{_exe}
1243 unless $secure_perl_path =~ m/$Config{_exe}$/i;}
a0d0e21e 1244
2d84a16a
DM
1245=item ARGV
1246
1247The special filehandle that iterates over command-line filenames in
1248C<@ARGV>. Usually written as the null filehandle in the angle operator
1249C<< <> >>. Note that currently C<ARGV> only has its magical effect
1250within the C<< <> >> operator; elsewhere it is just a plain filehandle
1251corresponding to the last file opened by C<< <> >>. In particular,
1252passing C<\*ARGV> as a parameter to a function that expects a filehandle
1253may not cause your function to automatically read the contents of all the
1254files in C<@ARGV>.
1255
a0d0e21e
LW
1256=item $ARGV
1257
c47ff5f1 1258contains the name of the current file when reading from <>.
a0d0e21e
LW
1259
1260=item @ARGV
1261
19799a22 1262The array @ARGV contains the command-line arguments intended for
14218588 1263the script. C<$#ARGV> is generally the number of arguments minus
19799a22
GS
1264one, because C<$ARGV[0]> is the first argument, I<not> the program's
1265command name itself. See C<$0> for the command name.
a0d0e21e 1266
5ccee41e
JA
1267=item ARGVOUT
1268
1269The special filehandle that points to the currently open output file
1270when doing edit-in-place processing with B<-i>. Useful when you have
1271to do a lot of inserting and don't want to keep modifying $_. See
1272L<perlrun> for the B<-i> switch.
1273
9b0e6e7a
JP
1274=item @F
1275
1276The array @F contains the fields of each line read in when autosplit
1277mode is turned on. See L<perlrun> for the B<-a> switch. This array
1278is package-specific, and must be declared or given a full package name
1279if not in package main when running under C<strict 'vars'>.
1280
a0d0e21e
LW
1281=item @INC
1282
19799a22
GS
1283The array @INC contains the list of places that the C<do EXPR>,
1284C<require>, or C<use> constructs look for their library files. It
1285initially consists of the arguments to any B<-I> command-line
1286switches, followed by the default Perl library, probably
1287F</usr/local/lib/perl>, followed by ".", to represent the current
e48df184
RGS
1288directory. ("." will not be appended if taint checks are enabled, either by
1289C<-T> or by C<-t>.) If you need to modify this at runtime, you should use
19799a22
GS
1290the C<use lib> pragma to get the machine-dependent library properly
1291loaded also:
a0d0e21e 1292
cb1a09d0
AD
1293 use lib '/mypath/libdir/';
1294 use SomeMod;
303f2f76 1295
d54b56d5
RGS
1296You can also insert hooks into the file inclusion system by putting Perl
1297code directly into @INC. Those hooks may be subroutine references, array
1298references or blessed objects. See L<perlfunc/require> for details.
1299
fb73857a 1300=item @_
1301
1302Within a subroutine the array @_ contains the parameters passed to that
19799a22 1303subroutine. See L<perlsub>.
fb73857a 1304
a0d0e21e
LW
1305=item %INC
1306
19799a22
GS
1307The hash %INC contains entries for each filename included via the
1308C<do>, C<require>, or C<use> operators. The key is the filename
1309you specified (with module names converted to pathnames), and the
14218588 1310value is the location of the file found. The C<require>
87275199 1311operator uses this hash to determine whether a particular file has
19799a22 1312already been included.
a0d0e21e 1313
89ccab8c
RGS
1314If the file was loaded via a hook (e.g. a subroutine reference, see
1315L<perlfunc/require> for a description of these hooks), this hook is
9ae8cd5b
RGS
1316by default inserted into %INC in place of a filename. Note, however,
1317that the hook may have set the %INC entry by itself to provide some more
1318specific info.
44f0be63 1319
b687b08b
TC
1320=item %ENV
1321
1322=item $ENV{expr}
a0d0e21e
LW
1323
1324The hash %ENV contains your current environment. Setting a
19799a22
GS
1325value in C<ENV> changes the environment for any child processes
1326you subsequently fork() off.
a0d0e21e 1327
b687b08b
TC
1328=item %SIG
1329
1330=item $SIG{expr}
a0d0e21e 1331
14218588 1332The hash %SIG contains signal handlers for signals. For example:
a0d0e21e
LW
1333
1334 sub handler { # 1st argument is signal name
fb73857a 1335 my($sig) = @_;
a0d0e21e
LW
1336 print "Caught a SIG$sig--shutting down\n";
1337 close(LOG);
1338 exit(0);
1339 }
1340
fb73857a 1341 $SIG{'INT'} = \&handler;
1342 $SIG{'QUIT'} = \&handler;
a0d0e21e 1343 ...
19799a22 1344 $SIG{'INT'} = 'DEFAULT'; # restore default action
a0d0e21e
LW
1345 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1346
f648820c
GS
1347Using a value of C<'IGNORE'> usually has the effect of ignoring the
1348signal, except for the C<CHLD> signal. See L<perlipc> for more about
1349this special case.
1350
19799a22 1351Here are some other examples:
a0d0e21e 1352
fb73857a 1353 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
a0d0e21e 1354 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
19799a22 1355 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
a0d0e21e
LW
1356 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1357
19799a22
GS
1358Be sure not to use a bareword as the name of a signal handler,
1359lest you inadvertently call it.
748a9306 1360
44a8e56a 1361If your system has the sigaction() function then signal handlers are
9ce5b4ad 1362installed using it. This means you get reliable signal handling.
44a8e56a 1363
9ce5b4ad
SG
1364The default delivery policy of signals changed in Perl 5.8.0 from
1365immediate (also known as "unsafe") to deferred, also known as
1366"safe signals". See L<perlipc> for more information.
45c0772f 1367
748a9306 1368Certain internal hooks can be also set using the %SIG hash. The
a8f8344d 1369routine indicated by C<$SIG{__WARN__}> is called when a warning message is
748a9306
LW
1370about to be printed. The warning message is passed as the first
1371argument. The presence of a __WARN__ hook causes the ordinary printing
1372of warnings to STDERR to be suppressed. You can use this to save warnings
1373in a variable, or turn warnings into fatal errors, like this:
1374
1375 local $SIG{__WARN__} = sub { die $_[0] };
1376 eval $proggie;
1377
a8f8344d 1378The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
748a9306
LW
1379is about to be thrown. The error message is passed as the first
1380argument. When a __DIE__ hook routine returns, the exception
1381processing continues as it would have in the absence of the hook,
cb1a09d0 1382unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
774d564b 1383The C<__DIE__> handler is explicitly disabled during the call, so that you
fb73857a 1384can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1385
19799a22
GS
1386Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1387even inside an eval(). Do not use this to rewrite a pending exception
1388in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
1389This strange action at a distance may be fixed in a future release
1390so that C<$SIG{__DIE__}> is only called if your program is about
1391to exit, as was the original intent. Any other use is deprecated.
1392
1393C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1394they may be called to report (probable) errors found by the parser.
1395In such a case the parser may be in inconsistent state, so any
1396attempt to evaluate Perl code from such a handler will probably
1397result in a segfault. This means that warnings or errors that
1398result from parsing Perl should be used with extreme caution, like
1399this:
fb73857a 1400
1401 require Carp if defined $^S;
1402 Carp::confess("Something wrong") if defined &Carp::confess;
1403 die "Something wrong, but could not load Carp to give backtrace...
1404 To see backtrace try starting Perl with -MCarp switch";
1405
1406Here the first line will load Carp I<unless> it is the parser who
1407called the handler. The second line will print backtrace and die if
1408Carp was available. The third line will be executed only if Carp was
1409not available.
1410
19799a22 1411See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
4438c4b7 1412L<warnings> for additional information.
68dc0745 1413
a0d0e21e 1414=back
55602bd2
IZ
1415
1416=head2 Error Indicators
1417
19799a22
GS
1418The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1419about different types of error conditions that may appear during
1420execution of a Perl program. The variables are shown ordered by
1421the "distance" between the subsystem which reported the error and
1422the Perl process. They correspond to errors detected by the Perl
1423interpreter, C library, operating system, or an external program,
1424respectively.
55602bd2
IZ
1425
1426To illustrate the differences between these variables, consider the
19799a22 1427following Perl expression, which uses a single-quoted string:
55602bd2 1428
19799a22 1429 eval q{
22d0716c
SB
1430 open my $pipe, "/cdrom/install |" or die $!;
1431 my @res = <$pipe>;
1432 close $pipe or die "bad pipe: $?, $!";
19799a22 1433 };
55602bd2
IZ
1434
1435After execution of this statement all 4 variables may have been set.
1436
19799a22
GS
1437C<$@> is set if the string to be C<eval>-ed did not compile (this
1438may happen if C<open> or C<close> were imported with bad prototypes),
1439or if Perl code executed during evaluation die()d . In these cases
1440the value of $@ is the compile error, or the argument to C<die>
4cb1c523 1441(which will interpolate C<$!> and C<$?>). (See also L<Fatal>,
19799a22
GS
1442though.)
1443
c47ff5f1 1444When the eval() expression above is executed, open(), C<< <PIPE> >>,
19799a22
GS
1445and C<close> are translated to calls in the C run-time library and
1446thence to the operating system kernel. C<$!> is set to the C library's
1447C<errno> if one of these calls fails.
1448
1449Under a few operating systems, C<$^E> may contain a more verbose
1450error indicator, such as in this case, "CDROM tray not closed."
14218588 1451Systems that do not support extended error messages leave C<$^E>
19799a22
GS
1452the same as C<$!>.
1453
1454Finally, C<$?> may be set to non-0 value if the external program
1455F</cdrom/install> fails. The upper eight bits reflect specific
1456error conditions encountered by the program (the program's exit()
1457value). The lower eight bits reflect mode of failure, like signal
1458death and core dump information See wait(2) for details. In
1459contrast to C<$!> and C<$^E>, which are set only if error condition
1460is detected, the variable C<$?> is set on each C<wait> or pipe
1461C<close>, overwriting the old value. This is more like C<$@>, which
1462on every eval() is always set on failure and cleared on success.
2b92dfce 1463
19799a22
GS
1464For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1465and C<$?>.
2b92dfce
GS
1466
1467=head2 Technical Note on the Syntax of Variable Names
1468
19799a22
GS
1469Variable names in Perl can have several formats. Usually, they
1470must begin with a letter or underscore, in which case they can be
1471arbitrarily long (up to an internal limit of 251 characters) and
1472may contain letters, digits, underscores, or the special sequence
1473C<::> or C<'>. In this case, the part before the last C<::> or
1474C<'> is taken to be a I<package qualifier>; see L<perlmod>.
2b92dfce
GS
1475
1476Perl variable names may also be a sequence of digits or a single
1477punctuation or control character. These names are all reserved for
19799a22
GS
1478special uses by Perl; for example, the all-digits names are used
1479to hold data captured by backreferences after a regular expression
1480match. Perl has a special syntax for the single-control-character
1481names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1482character. For example, the notation C<$^W> (dollar-sign caret
1483C<W>) is the scalar variable whose name is the single character
1484control-C<W>. This is better than typing a literal control-C<W>
1485into your program.
2b92dfce 1486
87275199 1487Finally, new in Perl 5.6, Perl variable names may be alphanumeric
19799a22
GS
1488strings that begin with control characters (or better yet, a caret).
1489These variables must be written in the form C<${^Foo}>; the braces
1490are not optional. C<${^Foo}> denotes the scalar variable whose
1491name is a control-C<F> followed by two C<o>'s. These variables are
1492reserved for future special uses by Perl, except for the ones that
1493begin with C<^_> (control-underscore or caret-underscore). No
1494control-character name that begins with C<^_> will acquire a special
1495meaning in any future version of Perl; such names may therefore be
1496used safely in programs. C<$^_> itself, however, I<is> reserved.
1497
1fcb18de
RGS
1498Perl identifiers that begin with digits, control characters, or
1499punctuation characters are exempt from the effects of the C<package>
1500declaration and are always forced to be in package C<main>; they are
1501also exempt from C<strict 'vars'> errors. A few other names are also
1502exempt in these ways:
2b92dfce
GS
1503
1504 ENV STDIN
1505 INC STDOUT
1506 ARGV STDERR
5b88253b 1507 ARGVOUT _
2b92dfce
GS
1508 SIG
1509
1510In particular, the new special C<${^_XYZ}> variables are always taken
19799a22 1511to be in package C<main>, regardless of any C<package> declarations
747fafda 1512presently in scope.
2b92dfce 1513
19799a22
GS
1514=head1 BUGS
1515
1516Due to an unfortunate accident of Perl's implementation, C<use
1517English> imposes a considerable performance penalty on all regular
1518expression matches in a program, regardless of whether they occur
1519in the scope of C<use English>. For that reason, saying C<use
1520English> in libraries is strongly discouraged. See the
1521Devel::SawAmpersand module documentation from CPAN
1577cd80 1522( http://www.cpan.org/modules/by-module/Devel/ )
19799a22 1523for more information.
2b92dfce 1524
19799a22
GS
1525Having to even think about the C<$^S> variable in your exception
1526handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1527invites grievous and difficult to track down errors. Avoid it
1528and use an C<END{}> or CORE::GLOBAL::die override instead.