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Re: [perl #20933] \substr reuses lvalues (sometimes)
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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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180(Mnemonic: underline is understood in certain operations.)
181
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182=back
183
184=over 8
185
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186=item $a
187
188=item $b
189
190Special package variables when using sort(), see L<perlfunc/sort>.
191Because of this specialness $a and $b don't need to be declared
192(using local(), use vars, or our()) even when using the strict
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193vars pragma. Don't lexicalize them with C<my $a> or C<my $b>
194if you want to be able to use them in the sort() comparison block
195or function.
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196
197=back
198
199=over 8
200
c47ff5f1 201=item $<I<digits>>
a0d0e21e 202
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203Contains the subpattern from the corresponding set of capturing
204parentheses from the last pattern match, not counting patterns
205matched in nested blocks that have been exited already. (Mnemonic:
206like \digits.) These variables are all read-only and dynamically
207scoped to the current BLOCK.
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208
209=item $MATCH
210
211=item $&
212
213The string matched by the last successful pattern match (not counting
214any matches hidden within a BLOCK or eval() enclosed by the current
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215BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
216and dynamically scoped to the current BLOCK.
a0d0e21e 217
19ddd453 218The use of this variable anywhere in a program imposes a considerable
667e1aea 219performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 220
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221=item $PREMATCH
222
223=item $`
224
225The string preceding whatever was matched by the last successful
226pattern match (not counting any matches hidden within a BLOCK or eval
a8f8344d 227enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
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228string.) This variable is read-only.
229
19ddd453 230The use of this variable anywhere in a program imposes a considerable
667e1aea 231performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 232
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233=item $POSTMATCH
234
235=item $'
236
237The string following whatever was matched by the last successful
238pattern match (not counting any matches hidden within a BLOCK or eval()
a8f8344d 239enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
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240string.) Example:
241
22d0716c 242 local $_ = 'abcdefghi';
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243 /def/;
244 print "$`:$&:$'\n"; # prints abc:def:ghi
245
19799a22 246This variable is read-only and dynamically scoped to the current BLOCK.
a0d0e21e 247
19ddd453 248The use of this variable anywhere in a program imposes a considerable
667e1aea 249performance penalty on all regular expression matches. See L</BUGS>.
19ddd453 250
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251=item $LAST_PAREN_MATCH
252
253=item $+
254
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255The text matched by the last bracket of the last successful search pattern.
256This is useful if you don't know which one of a set of alternative patterns
257matched. For example:
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258
259 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
260
261(Mnemonic: be positive and forward looking.)
19799a22 262This variable is read-only and dynamically scoped to the current BLOCK.
a0d0e21e 263
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264=item $^N
265
266The text matched by the used group most-recently closed (i.e. the group
267with the rightmost closing parenthesis) of the last successful search
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268pattern. (Mnemonic: the (possibly) Nested parenthesis that most
269recently closed.)
270
210b36aa 271This is primarily used inside C<(?{...})> blocks for examining text
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272recently matched. For example, to effectively capture text to a variable
273(in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
274
275 (?:(...)(?{ $var = $^N }))
276
277By setting and then using C<$var> in this way relieves you from having to
278worry about exactly which numbered set of parentheses they are.
279
280This variable is dynamically scoped to the current BLOCK.
281
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282=item @LAST_MATCH_END
283
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284=item @+
285
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286This array holds the offsets of the ends of the last successful
287submatches in the currently active dynamic scope. C<$+[0]> is
288the offset into the string of the end of the entire match. This
289is the same value as what the C<pos> function returns when called
290on the variable that was matched against. The I<n>th element
291of this array holds the offset of the I<n>th submatch, so
292C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
293past where $2 ends, and so on. You can use C<$#+> to determine
294how many subgroups were in the last successful match. See the
295examples given for the C<@-> variable.
6cef1e77 296
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297=item $MULTILINE_MATCHING
298
299=item $*
300
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301Set to a non-zero integer value to do multi-line matching within a
302string, 0 (or undefined) to tell Perl that it can assume that strings
303contain a single line, for the purpose of optimizing pattern matches.
304Pattern matches on strings containing multiple newlines can produce
305confusing results when C<$*> is 0 or undefined. Default is undefined.
306(Mnemonic: * matches multiple things.) This variable influences the
307interpretation of only C<^> and C<$>. A literal newline can be searched
308for even when C<$* == 0>.
a0d0e21e 309
19799a22 310Use of C<$*> is deprecated in modern Perl, supplanted by
5a964f20 311the C</s> and C</m> modifiers on pattern matching.
a0d0e21e 312
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313Assigning a non-numerical value to C<$*> triggers a warning (and makes
314C<$*> act if C<$* == 0>), while assigning a numerical value to C<$*>
315makes that an implicit C<int> is applied on the value.
316
fcc7d916 317=item HANDLE->input_line_number(EXPR)
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318
319=item $INPUT_LINE_NUMBER
320
321=item $NR
322
323=item $.
324
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325Current line number for the last filehandle accessed.
326
327Each filehandle in Perl counts the number of lines that have been read
328from it. (Depending on the value of C<$/>, Perl's idea of what
329constitutes a line may not match yours.) When a line is read from a
330filehandle (via readline() or C<< <> >>), or when tell() or seek() is
331called on it, C<$.> becomes an alias to the line counter for that
332filehandle.
333
334You can adjust the counter by assigning to C<$.>, but this will not
335actually move the seek pointer. I<Localizing C<$.> will not localize
336the filehandle's line count>. Instead, it will localize perl's notion
337of which filehandle C<$.> is currently aliased to.
338
339C<$.> is reset when the filehandle is closed, but B<not> when an open
340filehandle is reopened without an intervening close(). For more
e48df184 341details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
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342an explicit close, line numbers increase across ARGV files (but see
343examples in L<perlfunc/eof>).
344
345You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
346line counter for a given filehandle without having to worry about
347which handle you last accessed.
348
349(Mnemonic: many programs use "." to mean the current line number.)
350
351=item IO::Handle->input_record_separator(EXPR)
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352
353=item $INPUT_RECORD_SEPARATOR
354
355=item $RS
356
357=item $/
358
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359The input record separator, newline by default. This
360influences Perl's idea of what a "line" is. Works like B<awk>'s RS
19799a22 361variable, including treating empty lines as a terminator if set to
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362the null string. (An empty line cannot contain any spaces
363or tabs.) You may set it to a multi-character string to match a
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364multi-character terminator, or to C<undef> to read through the end
365of file. Setting it to C<"\n\n"> means something slightly
366different than setting to C<"">, if the file contains consecutive
367empty lines. Setting to C<""> will treat two or more consecutive
368empty lines as a single empty line. Setting to C<"\n\n"> will
369blindly assume that the next input character belongs to the next
14218588 370paragraph, even if it's a newline. (Mnemonic: / delimits
19799a22 371line boundaries when quoting poetry.)
a0d0e21e 372
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373 local $/; # enable "slurp" mode
374 local $_ = <FH>; # whole file now here
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375 s/\n[ \t]+/ /g;
376
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377Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
378better for something. :-)
68dc0745 379
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380Setting C<$/> to a reference to an integer, scalar containing an integer, or
381scalar that's convertible to an integer will attempt to read records
5b2b9c68 382instead of lines, with the maximum record size being the referenced
19799a22 383integer. So this:
5b2b9c68 384
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385 local $/ = \32768; # or \"32768", or \$var_containing_32768
386 open my $fh, $myfile or die $!;
387 local $_ = <$fh>;
5b2b9c68 388
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389will read a record of no more than 32768 bytes from FILE. If you're
390not reading from a record-oriented file (or your OS doesn't have
391record-oriented files), then you'll likely get a full chunk of data
392with every read. If a record is larger than the record size you've
393set, you'll get the record back in pieces.
5b2b9c68 394
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395On VMS, record reads are done with the equivalent of C<sysread>,
396so it's best not to mix record and non-record reads on the same
397file. (This is unlikely to be a problem, because any file you'd
83763826 398want to read in record mode is probably unusable in line mode.)
14218588 399Non-VMS systems do normal I/O, so it's safe to mix record and
19799a22 400non-record reads of a file.
5b2b9c68 401
14218588 402See also L<perlport/"Newlines">. Also see C<$.>.
883faa13 403
fcc7d916 404=item HANDLE->autoflush(EXPR)
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405
406=item $OUTPUT_AUTOFLUSH
407
408=item $|
409
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410If set to nonzero, forces a flush right away and after every write
411or print on the currently selected output channel. Default is 0
14218588 412(regardless of whether the channel is really buffered by the
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413system or not; C<$|> tells you only whether you've asked Perl
414explicitly to flush after each write). STDOUT will
415typically be line buffered if output is to the terminal and block
416buffered otherwise. Setting this variable is useful primarily when
417you are outputting to a pipe or socket, such as when you are running
418a Perl program under B<rsh> and want to see the output as it's
419happening. This has no effect on input buffering. See L<perlfunc/getc>
420for that. (Mnemonic: when you want your pipes to be piping hot.)
a0d0e21e 421
46550894 422=item IO::Handle->output_field_separator EXPR
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423
424=item $OUTPUT_FIELD_SEPARATOR
425
426=item $OFS
427
428=item $,
429
430The output field separator for the print operator. Ordinarily the
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431print operator simply prints out its arguments without further
432adornment. To get behavior more like B<awk>, set this variable as
433you would set B<awk>'s OFS variable to specify what is printed
434between fields. (Mnemonic: what is printed when there is a "," in
435your print statement.)
a0d0e21e 436
46550894 437=item IO::Handle->output_record_separator EXPR
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438
439=item $OUTPUT_RECORD_SEPARATOR
440
441=item $ORS
442
443=item $\
444
445The output record separator for the print operator. Ordinarily the
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446print operator simply prints out its arguments as is, with no
447trailing newline or other end-of-record string added. To get
448behavior more like B<awk>, set this variable as you would set
449B<awk>'s ORS variable to specify what is printed at the end of the
450print. (Mnemonic: you set C<$\> instead of adding "\n" at the
451end of the print. Also, it's just like C<$/>, but it's what you
452get "back" from Perl.)
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453
454=item $LIST_SEPARATOR
455
456=item $"
457
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458This is like C<$,> except that it applies to array and slice values
459interpolated into a double-quoted string (or similar interpreted
460string). Default is a space. (Mnemonic: obvious, I think.)
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461
462=item $SUBSCRIPT_SEPARATOR
463
464=item $SUBSEP
465
466=item $;
467
54310121 468The subscript separator for multidimensional array emulation. If you
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469refer to a hash element as
470
471 $foo{$a,$b,$c}
472
473it really means
474
475 $foo{join($;, $a, $b, $c)}
476
477But don't put
478
479 @foo{$a,$b,$c} # a slice--note the @
480
481which means
482
483 ($foo{$a},$foo{$b},$foo{$c})
484
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485Default is "\034", the same as SUBSEP in B<awk>. If your
486keys contain binary data there might not be any safe value for C<$;>.
a0d0e21e 487(Mnemonic: comma (the syntactic subscript separator) is a
19799a22 488semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
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489taken for something more important.)
490
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491Consider using "real" multidimensional arrays as described
492in L<perllol>.
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493
494=item $OFMT
495
496=item $#
497
498The output format for printed numbers. This variable is a half-hearted
499attempt to emulate B<awk>'s OFMT variable. There are times, however,
14218588 500when B<awk> and Perl have differing notions of what counts as
19799a22 501numeric. The initial value is "%.I<n>g", where I<n> is the value
6e2995f4 502of the macro DBL_DIG from your system's F<float.h>. This is different from
19799a22 503B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#>
6e2995f4 504explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
a0d0e21e 505
19799a22 506Use of C<$#> is deprecated.
a0d0e21e 507
fcc7d916 508=item HANDLE->format_page_number(EXPR)
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509
510=item $FORMAT_PAGE_NUMBER
511
512=item $%
513
514The current page number of the currently selected output channel.
19799a22 515Used with formats.
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516(Mnemonic: % is page number in B<nroff>.)
517
fcc7d916 518=item HANDLE->format_lines_per_page(EXPR)
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519
520=item $FORMAT_LINES_PER_PAGE
521
522=item $=
523
524The current page length (printable lines) of the currently selected
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525output channel. Default is 60.
526Used with formats.
527(Mnemonic: = has horizontal lines.)
a0d0e21e 528
fcc7d916 529=item HANDLE->format_lines_left(EXPR)
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530
531=item $FORMAT_LINES_LEFT
532
533=item $-
534
535The number of lines left on the page of the currently selected output
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536channel.
537Used with formats.
538(Mnemonic: lines_on_page - lines_printed.)
a0d0e21e 539
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540=item @LAST_MATCH_START
541
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542=item @-
543
19799a22 544$-[0] is the offset of the start of the last successful match.
6cef1e77 545C<$-[>I<n>C<]> is the offset of the start of the substring matched by
8f580fb8 546I<n>-th subpattern, or undef if the subpattern did not match.
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547
548Thus after a match against $_, $& coincides with C<substr $_, $-[0],
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549$+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
550$+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
c47ff5f1 551C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last
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552matched subgroup in the last successful match. Contrast with
553C<$#+>, the number of subgroups in the regular expression. Compare
19799a22 554with C<@+>.
6cef1e77 555
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556This array holds the offsets of the beginnings of the last
557successful submatches in the currently active dynamic scope.
558C<$-[0]> is the offset into the string of the beginning of the
559entire match. The I<n>th element of this array holds the offset
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560of the I<n>th submatch, so C<$-[1]> is the offset where $1
561begins, C<$-[2]> the offset where $2 begins, and so on.
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562
563After a match against some variable $var:
564
565=over 5
566
4375e838 567=item C<$`> is the same as C<substr($var, 0, $-[0])>
4ba05bdc 568
4375e838 569=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
4ba05bdc 570
4375e838 571=item C<$'> is the same as C<substr($var, $+[0])>
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572
573=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
574
575=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
576
4375e838 577=item C<$3> is the same as C<substr $var, $-[3], $+[3] - $-[3])>
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578
579=back
580
fcc7d916 581=item HANDLE->format_name(EXPR)
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582
583=item $FORMAT_NAME
584
585=item $~
586
587The name of the current report format for the currently selected output
14218588 588channel. Default is the name of the filehandle. (Mnemonic: brother to
19799a22 589C<$^>.)
a0d0e21e 590
fcc7d916 591=item HANDLE->format_top_name(EXPR)
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592
593=item $FORMAT_TOP_NAME
594
595=item $^
596
597The name of the current top-of-page format for the currently selected
14218588 598output channel. Default is the name of the filehandle with _TOP
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599appended. (Mnemonic: points to top of page.)
600
46550894 601=item IO::Handle->format_line_break_characters EXPR
a0d0e21e
LW
602
603=item $FORMAT_LINE_BREAK_CHARACTERS
604
605=item $:
606
607The current set of characters after which a string may be broken to
54310121 608fill continuation fields (starting with ^) in a format. Default is
a0d0e21e
LW
609S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
610poetry is a part of a line.)
611
46550894 612=item IO::Handle->format_formfeed EXPR
a0d0e21e
LW
613
614=item $FORMAT_FORMFEED
615
616=item $^L
617
14218588 618What formats output as a form feed. Default is \f.
a0d0e21e
LW
619
620=item $ACCUMULATOR
621
622=item $^A
623
624The current value of the write() accumulator for format() lines. A format
19799a22 625contains formline() calls that put their result into C<$^A>. After
a0d0e21e 626calling its format, write() prints out the contents of C<$^A> and empties.
14218588 627So you never really see the contents of C<$^A> unless you call
a0d0e21e
LW
628formline() yourself and then look at it. See L<perlform> and
629L<perlfunc/formline()>.
630
631=item $CHILD_ERROR
632
633=item $?
634
54310121 635The status returned by the last pipe close, backtick (C<``>) command,
19799a22
GS
636successful call to wait() or waitpid(), or from the system()
637operator. This is just the 16-bit status word returned by the
638wait() system call (or else is made up to look like it). Thus, the
c47ff5f1 639exit value of the subprocess is really (C<<< $? >> 8 >>>), and
19799a22
GS
640C<$? & 127> gives which signal, if any, the process died from, and
641C<$? & 128> reports whether there was a core dump. (Mnemonic:
642similar to B<sh> and B<ksh>.)
a0d0e21e 643
7b8d334a 644Additionally, if the C<h_errno> variable is supported in C, its value
14218588 645is returned via $? if any C<gethost*()> function fails.
7b8d334a 646
19799a22 647If you have installed a signal handler for C<SIGCHLD>, the
aa689395
PP
648value of C<$?> will usually be wrong outside that handler.
649
a8f8344d
PP
650Inside an C<END> subroutine C<$?> contains the value that is going to be
651given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
19799a22
GS
652change the exit status of your program. For example:
653
654 END {
655 $? = 1 if $? == 255; # die would make it 255
656 }
a8f8344d 657
aa689395 658Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
ff0cee69 659actual VMS exit status, instead of the default emulation of POSIX
9bc98430 660status; see L<perlvms/$?> for details.
f86702cc 661
55602bd2
IZ
662Also see L<Error Indicators>.
663
0a378802
JH
664=item ${^ENCODING}
665
740bd165
PN
666The I<object reference> to the Encode object that is used to convert
667the source code to Unicode. Thanks to this variable your perl script
668does not have to be written in UTF-8. Default is I<undef>. The direct
669manipulation of this variable is highly discouraged. See L<encoding>
048c20cb 670for more details.
0a378802 671
a0d0e21e
LW
672=item $OS_ERROR
673
674=item $ERRNO
675
676=item $!
677
19799a22 678If used numerically, yields the current value of the C C<errno>
6ab308ee
JH
679variable, or in other words, if a system or library call fails, it
680sets this variable. This means that the value of C<$!> is meaningful
681only I<immediately> after a B<failure>:
682
683 if (open(FH, $filename)) {
684 # Here $! is meaningless.
685 ...
686 } else {
687 # ONLY here is $! meaningful.
688 ...
689 # Already here $! might be meaningless.
690 }
691 # Since here we might have either success or failure,
692 # here $! is meaningless.
693
694In the above I<meaningless> stands for anything: zero, non-zero,
695C<undef>. A successful system or library call does B<not> set
696the variable to zero.
697
19799a22
GS
698If used an a string, yields the corresponding system error string.
699You can assign a number to C<$!> to set I<errno> if, for instance,
700you want C<"$!"> to return the string for error I<n>, or you want
701to set the exit value for the die() operator. (Mnemonic: What just
702went bang?)
a0d0e21e 703
55602bd2
IZ
704Also see L<Error Indicators>.
705
4c5cef9b
MJD
706=item %!
707
708Each element of C<%!> has a true value only if C<$!> is set to that
709value. For example, C<$!{ENOENT}> is true if and only if the current
3be065a1
JH
710value of C<$!> is C<ENOENT>; that is, if the most recent error was
711"No such file or directory" (or its moral equivalent: not all operating
712systems give that exact error, and certainly not all languages).
713To check if a particular key is meaningful on your system, use
714C<exists $!{the_key}>; for a list of legal keys, use C<keys %!>.
715See L<Errno> for more information, and also see above for the
716validity of C<$!>.
4c5cef9b 717
5c055ba3
PP
718=item $EXTENDED_OS_ERROR
719
720=item $^E
721
22fae026
TM
722Error information specific to the current operating system. At
723the moment, this differs from C<$!> under only VMS, OS/2, and Win32
724(and for MacPerl). On all other platforms, C<$^E> is always just
725the same as C<$!>.
726
727Under VMS, C<$^E> provides the VMS status value from the last
728system error. This is more specific information about the last
729system error than that provided by C<$!>. This is particularly
d516a115 730important when C<$!> is set to B<EVMSERR>.
22fae026 731
1c1c7f20
GS
732Under OS/2, C<$^E> is set to the error code of the last call to
733OS/2 API either via CRT, or directly from perl.
22fae026
TM
734
735Under Win32, C<$^E> always returns the last error information
736reported by the Win32 call C<GetLastError()> which describes
737the last error from within the Win32 API. Most Win32-specific
19799a22 738code will report errors via C<$^E>. ANSI C and Unix-like calls
22fae026
TM
739set C<errno> and so most portable Perl code will report errors
740via C<$!>.
741
742Caveats mentioned in the description of C<$!> generally apply to
743C<$^E>, also. (Mnemonic: Extra error explanation.)
5c055ba3 744
55602bd2
IZ
745Also see L<Error Indicators>.
746
a0d0e21e
LW
747=item $EVAL_ERROR
748
749=item $@
750
4a280ebe
JG
751The Perl syntax error message from the last eval() operator.
752If $@ is the null string, the last eval() parsed and executed
753correctly (although the operations you invoked may have failed in the
754normal fashion). (Mnemonic: Where was the syntax error "at"?)
a0d0e21e 755
19799a22 756Warning messages are not collected in this variable. You can,
a8f8344d 757however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
54310121 758as described below.
748a9306 759
55602bd2
IZ
760Also see L<Error Indicators>.
761
a0d0e21e
LW
762=item $PROCESS_ID
763
764=item $PID
765
766=item $$
767
19799a22
GS
768The process number of the Perl running this script. You should
769consider this variable read-only, although it will be altered
770across fork() calls. (Mnemonic: same as shells.)
a0d0e21e 771
4d76a344
RGS
772Note for Linux users: on Linux, the C functions C<getpid()> and
773C<getppid()> return different values from different threads. In order to
774be portable, this behavior is not reflected by C<$$>, whose value remains
775consistent across threads. If you want to call the underlying C<getpid()>,
e3256f86 776you may use the CPAN module C<Linux::Pid>.
4d76a344 777
a0d0e21e
LW
778=item $REAL_USER_ID
779
780=item $UID
781
782=item $<
783
19799a22 784The real uid of this process. (Mnemonic: it's the uid you came I<from>,
a043a685
GW
785if you're running setuid.) You can change both the real uid and
786the effective uid at the same time by using POSIX::setuid().
a0d0e21e
LW
787
788=item $EFFECTIVE_USER_ID
789
790=item $EUID
791
792=item $>
793
794The effective uid of this process. Example:
795
796 $< = $>; # set real to effective uid
797 ($<,$>) = ($>,$<); # swap real and effective uid
798
a043a685
GW
799You can change both the effective uid and the real uid at the same
800time by using POSIX::setuid().
801
19799a22 802(Mnemonic: it's the uid you went I<to>, if you're running setuid.)
c47ff5f1 803C<< $< >> and C<< $> >> can be swapped only on machines
8cc95fdb 804supporting setreuid().
a0d0e21e
LW
805
806=item $REAL_GROUP_ID
807
808=item $GID
809
810=item $(
811
812The real gid of this process. If you are on a machine that supports
813membership in multiple groups simultaneously, gives a space separated
814list of groups you are in. The first number is the one returned by
815getgid(), and the subsequent ones by getgroups(), one of which may be
8cc95fdb
PP
816the same as the first number.
817
19799a22
GS
818However, a value assigned to C<$(> must be a single number used to
819set the real gid. So the value given by C<$(> should I<not> be assigned
820back to C<$(> without being forced numeric, such as by adding zero.
8cc95fdb 821
a043a685
GW
822You can change both the real gid and the effective gid at the same
823time by using POSIX::setgid().
824
19799a22
GS
825(Mnemonic: parentheses are used to I<group> things. The real gid is the
826group you I<left>, if you're running setgid.)
a0d0e21e
LW
827
828=item $EFFECTIVE_GROUP_ID
829
830=item $EGID
831
832=item $)
833
834The effective gid of this process. If you are on a machine that
835supports membership in multiple groups simultaneously, gives a space
836separated list of groups you are in. The first number is the one
837returned by getegid(), and the subsequent ones by getgroups(), one of
8cc95fdb
PP
838which may be the same as the first number.
839
19799a22 840Similarly, a value assigned to C<$)> must also be a space-separated
14218588 841list of numbers. The first number sets the effective gid, and
8cc95fdb
PP
842the rest (if any) are passed to setgroups(). To get the effect of an
843empty list for setgroups(), just repeat the new effective gid; that is,
844to force an effective gid of 5 and an effectively empty setgroups()
845list, say C< $) = "5 5" >.
846
a043a685
GW
847You can change both the effective gid and the real gid at the same
848time by using POSIX::setgid() (use only a single numeric argument).
849
19799a22
GS
850(Mnemonic: parentheses are used to I<group> things. The effective gid
851is the group that's I<right> for you, if you're running setgid.)
a0d0e21e 852
c47ff5f1 853C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
19799a22
GS
854machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
855and C<$)> can be swapped only on machines supporting setregid().
a0d0e21e
LW
856
857=item $PROGRAM_NAME
858
859=item $0
860
19799a22
GS
861Contains the name of the program being executed. On some operating
862systems assigning to C<$0> modifies the argument area that the B<ps>
863program sees. This is more useful as a way of indicating the current
864program state than it is for hiding the program you're running.
a0d0e21e
LW
865(Mnemonic: same as B<sh> and B<ksh>.)
866
4bc88a62
PS
867Note for BSD users: setting C<$0> does not completely remove "perl"
868from the ps(1) output. For example, setting C<$0> to C<"foobar"> will
869result in C<"perl: foobar (perl)">. This is an operating system
870feature.
871
e2975953
JH
872In multithreaded scripts Perl coordinates the threads so that any
873thread may modify its copy of the C<$0> and the change becomes visible
874to ps(1) (assuming the operating system plays along).
875
a0d0e21e
LW
876=item $[
877
878The index of the first element in an array, and of the first character
19799a22
GS
879in a substring. Default is 0, but you could theoretically set it
880to 1 to make Perl behave more like B<awk> (or Fortran) when
881subscripting and when evaluating the index() and substr() functions.
882(Mnemonic: [ begins subscripts.)
a0d0e21e 883
19799a22
GS
884As of release 5 of Perl, assignment to C<$[> is treated as a compiler
885directive, and cannot influence the behavior of any other file.
886Its use is highly discouraged.
a0d0e21e 887
a0d0e21e
LW
888=item $]
889
54310121
PP
890The version + patchlevel / 1000 of the Perl interpreter. This variable
891can be used to determine whether the Perl interpreter executing a
892script is in the right range of versions. (Mnemonic: Is this version
893of perl in the right bracket?) Example:
a0d0e21e
LW
894
895 warn "No checksumming!\n" if $] < 3.019;
896
54310121 897See also the documentation of C<use VERSION> and C<require VERSION>
19799a22 898for a convenient way to fail if the running Perl interpreter is too old.
a0d0e21e 899
0c8d858b
MS
900The floating point representation can sometimes lead to inaccurate
901numeric comparisons. See C<$^V> for a more modern representation of
902the Perl version that allows accurate string comparisons.
16070b82 903
305aace0
NIS
904=item $COMPILING
905
906=item $^C
907
19799a22
GS
908The current value of the flag associated with the B<-c> switch.
909Mainly of use with B<-MO=...> to allow code to alter its behavior
910when being compiled, such as for example to AUTOLOAD at compile
911time rather than normal, deferred loading. See L<perlcc>. Setting
912C<$^C = 1> is similar to calling C<B::minus_c>.
305aace0 913
a0d0e21e
LW
914=item $DEBUGGING
915
916=item $^D
917
918The current value of the debugging flags. (Mnemonic: value of B<-D>
919switch.)
920
921=item $SYSTEM_FD_MAX
922
923=item $^F
924
925The maximum system file descriptor, ordinarily 2. System file
926descriptors are passed to exec()ed processes, while higher file
927descriptors are not. Also, during an open(), system file descriptors are
928preserved even if the open() fails. (Ordinary file descriptors are
19799a22 929closed before the open() is attempted.) The close-on-exec
a0d0e21e 930status of a file descriptor will be decided according to the value of
8d2a6795
GS
931C<$^F> when the corresponding file, pipe, or socket was opened, not the
932time of the exec().
a0d0e21e 933
6e2995f4
PP
934=item $^H
935
0462a1ab
GS
936WARNING: This variable is strictly for internal use only. Its availability,
937behavior, and contents are subject to change without notice.
938
939This variable contains compile-time hints for the Perl interpreter. At the
940end of compilation of a BLOCK the value of this variable is restored to the
941value when the interpreter started to compile the BLOCK.
942
943When perl begins to parse any block construct that provides a lexical scope
944(e.g., eval body, required file, subroutine body, loop body, or conditional
945block), the existing value of $^H is saved, but its value is left unchanged.
946When the compilation of the block is completed, it regains the saved value.
947Between the points where its value is saved and restored, code that
948executes within BEGIN blocks is free to change the value of $^H.
949
950This behavior provides the semantic of lexical scoping, and is used in,
951for instance, the C<use strict> pragma.
952
953The contents should be an integer; different bits of it are used for
954different pragmatic flags. Here's an example:
955
956 sub add_100 { $^H |= 0x100 }
957
958 sub foo {
959 BEGIN { add_100() }
960 bar->baz($boon);
961 }
962
963Consider what happens during execution of the BEGIN block. At this point
964the BEGIN block has already been compiled, but the body of foo() is still
965being compiled. The new value of $^H will therefore be visible only while
966the body of foo() is being compiled.
967
968Substitution of the above BEGIN block with:
969
970 BEGIN { require strict; strict->import('vars') }
971
972demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
973version of the same lexical pragma:
974
975 BEGIN { require strict; strict->import('vars') if $condition }
976
977=item %^H
978
979WARNING: This variable is strictly for internal use only. Its availability,
980behavior, and contents are subject to change without notice.
981
982The %^H hash provides the same scoping semantic as $^H. This makes it
983useful for implementation of lexically scoped pragmas.
6e2995f4 984
a0d0e21e
LW
985=item $INPLACE_EDIT
986
987=item $^I
988
989The current value of the inplace-edit extension. Use C<undef> to disable
990inplace editing. (Mnemonic: value of B<-i> switch.)
991
fb73857a
PP
992=item $^M
993
19799a22
GS
994By default, running out of memory is an untrappable, fatal error.
995However, if suitably built, Perl can use the contents of C<$^M>
996as an emergency memory pool after die()ing. Suppose that your Perl
997were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.
998Then
fb73857a 999
19799a22 1000 $^M = 'a' x (1 << 16);
fb73857a 1001
51ee6500 1002would allocate a 64K buffer for use in an emergency. See the
19799a22
GS
1003F<INSTALL> file in the Perl distribution for information on how to
1004enable this option. To discourage casual use of this advanced
4ec0190b 1005feature, there is no L<English|English> long name for this variable.
fb73857a 1006
5c055ba3 1007=item $OSNAME
6e2995f4 1008
5c055ba3
PP
1009=item $^O
1010
1011The name of the operating system under which this copy of Perl was
1012built, as determined during the configuration process. The value
19799a22
GS
1013is identical to C<$Config{'osname'}>. See also L<Config> and the
1014B<-V> command-line switch documented in L<perlrun>.
5c055ba3 1015
443f6d01 1016In Windows platforms, $^O is not very helpful: since it is always
7f510801
GS
1017C<MSWin32>, it doesn't tell the difference between
101895/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() or
1019Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
1020between the variants.
916d64a3 1021
e2e27056
JH
1022=item ${^OPEN}
1023
1024An internal variable used by PerlIO. A string in two parts, separated
fae2c0fb
RGS
1025by a C<\0> byte, the first part describes the input layers, the second
1026part describes the output layers.
e2e27056 1027
a0d0e21e
LW
1028=item $PERLDB
1029
1030=item $^P
1031
19799a22
GS
1032The internal variable for debugging support. The meanings of the
1033various bits are subject to change, but currently indicate:
84902520
TB
1034
1035=over 6
1036
1037=item 0x01
1038
1039Debug subroutine enter/exit.
1040
1041=item 0x02
1042
1043Line-by-line debugging.
1044
1045=item 0x04
1046
1047Switch off optimizations.
1048
1049=item 0x08
1050
1051Preserve more data for future interactive inspections.
1052
1053=item 0x10
1054
1055Keep info about source lines on which a subroutine is defined.
1056
1057=item 0x20
1058
1059Start with single-step on.
1060
83ee9e09
GS
1061=item 0x40
1062
1063Use subroutine address instead of name when reporting.
1064
1065=item 0x80
1066
1067Report C<goto &subroutine> as well.
1068
1069=item 0x100
1070
1071Provide informative "file" names for evals based on the place they were compiled.
1072
1073=item 0x200
1074
1075Provide informative names to anonymous subroutines based on the place they
1076were compiled.
1077
84902520
TB
1078=back
1079
19799a22
GS
1080Some bits may be relevant at compile-time only, some at
1081run-time only. This is a new mechanism and the details may change.
a0d0e21e 1082
66558a10
GS
1083=item $LAST_REGEXP_CODE_RESULT
1084
b9ac3b5b
GS
1085=item $^R
1086
19799a22
GS
1087The result of evaluation of the last successful C<(?{ code })>
1088regular expression assertion (see L<perlre>). May be written to.
b9ac3b5b 1089
66558a10
GS
1090=item $EXCEPTIONS_BEING_CAUGHT
1091
fb73857a
PP
1092=item $^S
1093
1094Current state of the interpreter. Undefined if parsing of the current
1095module/eval is not finished (may happen in $SIG{__DIE__} and
19799a22 1096$SIG{__WARN__} handlers). True if inside an eval(), otherwise false.
fb73857a 1097
a0d0e21e
LW
1098=item $BASETIME
1099
1100=item $^T
1101
19799a22 1102The time at which the program began running, in seconds since the
5f05dabc 1103epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
19799a22 1104and B<-C> filetests are based on this value.
a0d0e21e 1105
7c36658b
MS
1106=item ${^TAINT}
1107
9aa05f58
RGS
1108Reflects if taint mode is on or off. 1 for on (the program was run with
1109B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
18e8c5b0 1110B<-t> or B<-TU>). This variable is read-only.
7c36658b 1111
fde18df1
JH
1112=item ${^UTF8_LOCALE}
1113
1114Reflects whether the locale settings indicated the use of UTF-8 and that
1115the use of UTF-8 was enabled either by the C<-C> command line switch or
1116by setting the PERL_UTF8_LOCALE environment variable to a true value.
1117This variable is read-only. If true, the STDIN is expected to be in
1118UTF-8, the STDOUT and STDERR are in UTF-8, and C<:utf8> is the default
1119file open layer. See L<perluniintro>, L<perlfunc/open>, and L<open>
1120for more information.
1121
44dcb63b 1122=item $PERL_VERSION
b459063d 1123
16070b82
GS
1124=item $^V
1125
1126The revision, version, and subversion of the Perl interpreter, represented
da2094fd 1127as a string composed of characters with those ordinals. Thus in Perl v5.6.0
44dcb63b
GS
1128it equals C<chr(5) . chr(6) . chr(0)> and will return true for
1129C<$^V eq v5.6.0>. Note that the characters in this string value can
1130potentially be in Unicode range.
16070b82
GS
1131
1132This can be used to determine whether the Perl interpreter executing a
1133script is in the right range of versions. (Mnemonic: use ^V for Version
44dcb63b 1134Control.) Example:
16070b82 1135
3fd4402b 1136 warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0;
16070b82 1137
aa2f2a36
AMS
1138To convert C<$^V> into its string representation use sprintf()'s
1139C<"%vd"> conversion:
1140
1141 printf "version is v%vd\n", $^V; # Perl's version
1142
44dcb63b 1143See the documentation of C<use VERSION> and C<require VERSION>
16070b82
GS
1144for a convenient way to fail if the running Perl interpreter is too old.
1145
1146See also C<$]> for an older representation of the Perl version.
1147
a0d0e21e
LW
1148=item $WARNING
1149
1150=item $^W
1151
19799a22
GS
1152The current value of the warning switch, initially true if B<-w>
1153was used, false otherwise, but directly modifiable. (Mnemonic:
4438c4b7
JH
1154related to the B<-w> switch.) See also L<warnings>.
1155
6a818117 1156=item ${^WARNING_BITS}
4438c4b7
JH
1157
1158The current set of warning checks enabled by the C<use warnings> pragma.
1159See the documentation of C<warnings> for more details.
a0d0e21e
LW
1160
1161=item $EXECUTABLE_NAME
1162
1163=item $^X
1164
e71940de 1165The name used to execute the current copy of Perl, from C's
38e4f4ae
SB
1166C<argv[0]>.
1167
e71940de
PG
1168Depending on the host operating system, the value of $^X may be
1169a relative or absolute pathname of the perl program file, or may
1170be the string used to invoke perl but not the pathname of the
1171perl program file. Also, most operating systems permit invoking
1172programs that are not in the PATH environment variable, so there
a10d74f3
PG
1173is no guarantee that the value of $^X is in PATH. For VMS, the
1174value may or may not include a version number.
38e4f4ae 1175
e71940de
PG
1176You usually can use the value of $^X to re-invoke an independent
1177copy of the same perl that is currently running, e.g.,
1178
1179 @first_run = `$^X -le "print int rand 100 for 1..100"`;
1180
1181But recall that not all operating systems support forking or
1182capturing of the output of commands, so this complex statement
1183may not be portable.
38e4f4ae 1184
e71940de
PG
1185It is not safe to use the value of $^X as a path name of a file,
1186as some operating systems that have a mandatory suffix on
1187executable files do not require use of the suffix when invoking
1188a command. To convert the value of $^X to a path name, use the
1189following statements:
1190
1191# Build up a set of file names (not command names).
1192 use Config;
68fb0eb7
PG
1193 $this_perl = $^X;
1194 if ($^O ne 'VMS')
1195 {$this_perl .= $Config{_exe}
1196 unless $this_perl =~ m/$Config{_exe}$/i;}
e71940de
PG
1197
1198Because many operating systems permit anyone with read access to
1199the Perl program file to make a copy of it, patch the copy, and
1200then execute the copy, the security-conscious Perl programmer
1201should take care to invoke the installed copy of perl, not the
1202copy referenced by $^X. The following statements accomplish
1203this goal, and produce a pathname that can be invoked as a
1204command or referenced as a file.
38e4f4ae
SB
1205
1206 use Config;
68fb0eb7
PG
1207 $secure_perl_path = $Config{perlpath};
1208 if ($^O ne 'VMS')
1209 {$secure_perl_path .= $Config{_exe}
1210 unless $secure_perl_path =~ m/$Config{_exe}$/i;}
a0d0e21e 1211
2d84a16a
DM
1212=item ARGV
1213
1214The special filehandle that iterates over command-line filenames in
1215C<@ARGV>. Usually written as the null filehandle in the angle operator
1216C<< <> >>. Note that currently C<ARGV> only has its magical effect
1217within the C<< <> >> operator; elsewhere it is just a plain filehandle
1218corresponding to the last file opened by C<< <> >>. In particular,
1219passing C<\*ARGV> as a parameter to a function that expects a filehandle
1220may not cause your function to automatically read the contents of all the
1221files in C<@ARGV>.
1222
a0d0e21e
LW
1223=item $ARGV
1224
c47ff5f1 1225contains the name of the current file when reading from <>.
a0d0e21e
LW
1226
1227=item @ARGV
1228
19799a22 1229The array @ARGV contains the command-line arguments intended for
14218588 1230the script. C<$#ARGV> is generally the number of arguments minus
19799a22
GS
1231one, because C<$ARGV[0]> is the first argument, I<not> the program's
1232command name itself. See C<$0> for the command name.
a0d0e21e 1233
5ccee41e
JA
1234=item ARGVOUT
1235
1236The special filehandle that points to the currently open output file
1237when doing edit-in-place processing with B<-i>. Useful when you have
1238to do a lot of inserting and don't want to keep modifying $_. See
1239L<perlrun> for the B<-i> switch.
1240
9b0e6e7a
JP
1241=item @F
1242
1243The array @F contains the fields of each line read in when autosplit
1244mode is turned on. See L<perlrun> for the B<-a> switch. This array
1245is package-specific, and must be declared or given a full package name
1246if not in package main when running under C<strict 'vars'>.
1247
a0d0e21e
LW
1248=item @INC
1249
19799a22
GS
1250The array @INC contains the list of places that the C<do EXPR>,
1251C<require>, or C<use> constructs look for their library files. It
1252initially consists of the arguments to any B<-I> command-line
1253switches, followed by the default Perl library, probably
1254F</usr/local/lib/perl>, followed by ".", to represent the current
e48df184
RGS
1255directory. ("." will not be appended if taint checks are enabled, either by
1256C<-T> or by C<-t>.) If you need to modify this at runtime, you should use
19799a22
GS
1257the C<use lib> pragma to get the machine-dependent library properly
1258loaded also:
a0d0e21e 1259
cb1a09d0
AD
1260 use lib '/mypath/libdir/';
1261 use SomeMod;
303f2f76 1262
d54b56d5
RGS
1263You can also insert hooks into the file inclusion system by putting Perl
1264code directly into @INC. Those hooks may be subroutine references, array
1265references or blessed objects. See L<perlfunc/require> for details.
1266
fb73857a
PP
1267=item @_
1268
1269Within a subroutine the array @_ contains the parameters passed to that
19799a22 1270subroutine. See L<perlsub>.
fb73857a 1271
a0d0e21e
LW
1272=item %INC
1273
19799a22
GS
1274The hash %INC contains entries for each filename included via the
1275C<do>, C<require>, or C<use> operators. The key is the filename
1276you specified (with module names converted to pathnames), and the
14218588 1277value is the location of the file found. The C<require>
87275199 1278operator uses this hash to determine whether a particular file has
19799a22 1279already been included.
a0d0e21e 1280
89ccab8c
RGS
1281If the file was loaded via a hook (e.g. a subroutine reference, see
1282L<perlfunc/require> for a description of these hooks), this hook is
9ae8cd5b
RGS
1283by default inserted into %INC in place of a filename. Note, however,
1284that the hook may have set the %INC entry by itself to provide some more
1285specific info.
44f0be63 1286
b687b08b
TC
1287=item %ENV
1288
1289=item $ENV{expr}
a0d0e21e
LW
1290
1291The hash %ENV contains your current environment. Setting a
19799a22
GS
1292value in C<ENV> changes the environment for any child processes
1293you subsequently fork() off.
a0d0e21e 1294
b687b08b
TC
1295=item %SIG
1296
1297=item $SIG{expr}
a0d0e21e 1298
14218588 1299The hash %SIG contains signal handlers for signals. For example:
a0d0e21e
LW
1300
1301 sub handler { # 1st argument is signal name
fb73857a 1302 my($sig) = @_;
a0d0e21e
LW
1303 print "Caught a SIG$sig--shutting down\n";
1304 close(LOG);
1305 exit(0);
1306 }
1307
fb73857a
PP
1308 $SIG{'INT'} = \&handler;
1309 $SIG{'QUIT'} = \&handler;
a0d0e21e 1310 ...
19799a22 1311 $SIG{'INT'} = 'DEFAULT'; # restore default action
a0d0e21e
LW
1312 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1313
f648820c
GS
1314Using a value of C<'IGNORE'> usually has the effect of ignoring the
1315signal, except for the C<CHLD> signal. See L<perlipc> for more about
1316this special case.
1317
19799a22 1318Here are some other examples:
a0d0e21e 1319
fb73857a 1320 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
a0d0e21e 1321 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
19799a22 1322 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
a0d0e21e
LW
1323 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1324
19799a22
GS
1325Be sure not to use a bareword as the name of a signal handler,
1326lest you inadvertently call it.
748a9306 1327
44a8e56a
PP
1328If your system has the sigaction() function then signal handlers are
1329installed using it. This means you get reliable signal handling. If
1330your system has the SA_RESTART flag it is used when signals handlers are
19799a22 1331installed. This means that system calls for which restarting is supported
44a8e56a
PP
1332continue rather than returning when a signal arrives. If you want your
1333system calls to be interrupted by signal delivery then do something like
1334this:
1335
1336 use POSIX ':signal_h';
1337
1338 my $alarm = 0;
1339 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
1340 or die "Error setting SIGALRM handler: $!\n";
1341
1342See L<POSIX>.
1343
748a9306 1344Certain internal hooks can be also set using the %SIG hash. The
a8f8344d 1345routine indicated by C<$SIG{__WARN__}> is called when a warning message is
748a9306
LW
1346about to be printed. The warning message is passed as the first
1347argument. The presence of a __WARN__ hook causes the ordinary printing
1348of warnings to STDERR to be suppressed. You can use this to save warnings
1349in a variable, or turn warnings into fatal errors, like this:
1350
1351 local $SIG{__WARN__} = sub { die $_[0] };
1352 eval $proggie;
1353
a8f8344d 1354The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
748a9306
LW
1355is about to be thrown. The error message is passed as the first
1356argument. When a __DIE__ hook routine returns, the exception
1357processing continues as it would have in the absence of the hook,
cb1a09d0 1358unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
774d564b 1359The C<__DIE__> handler is explicitly disabled during the call, so that you
fb73857a
PP
1360can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1361
19799a22
GS
1362Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1363even inside an eval(). Do not use this to rewrite a pending exception
1364in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
1365This strange action at a distance may be fixed in a future release
1366so that C<$SIG{__DIE__}> is only called if your program is about
1367to exit, as was the original intent. Any other use is deprecated.
1368
1369C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1370they may be called to report (probable) errors found by the parser.
1371In such a case the parser may be in inconsistent state, so any
1372attempt to evaluate Perl code from such a handler will probably
1373result in a segfault. This means that warnings or errors that
1374result from parsing Perl should be used with extreme caution, like
1375this:
fb73857a
PP
1376
1377 require Carp if defined $^S;
1378 Carp::confess("Something wrong") if defined &Carp::confess;
1379 die "Something wrong, but could not load Carp to give backtrace...
1380 To see backtrace try starting Perl with -MCarp switch";
1381
1382Here the first line will load Carp I<unless> it is the parser who
1383called the handler. The second line will print backtrace and die if
1384Carp was available. The third line will be executed only if Carp was
1385not available.
1386
19799a22 1387See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
4438c4b7 1388L<warnings> for additional information.
68dc0745 1389
a0d0e21e 1390=back
55602bd2
IZ
1391
1392=head2 Error Indicators
1393
19799a22
GS
1394The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1395about different types of error conditions that may appear during
1396execution of a Perl program. The variables are shown ordered by
1397the "distance" between the subsystem which reported the error and
1398the Perl process. They correspond to errors detected by the Perl
1399interpreter, C library, operating system, or an external program,
1400respectively.
55602bd2
IZ
1401
1402To illustrate the differences between these variables, consider the
19799a22 1403following Perl expression, which uses a single-quoted string:
55602bd2 1404
19799a22 1405 eval q{
22d0716c
SB
1406 open my $pipe, "/cdrom/install |" or die $!;
1407 my @res = <$pipe>;
1408 close $pipe or die "bad pipe: $?, $!";
19799a22 1409 };
55602bd2
IZ
1410
1411After execution of this statement all 4 variables may have been set.
1412
19799a22
GS
1413C<$@> is set if the string to be C<eval>-ed did not compile (this
1414may happen if C<open> or C<close> were imported with bad prototypes),
1415or if Perl code executed during evaluation die()d . In these cases
1416the value of $@ is the compile error, or the argument to C<die>
1417(which will interpolate C<$!> and C<$?>!). (See also L<Fatal>,
1418though.)
1419
c47ff5f1 1420When the eval() expression above is executed, open(), C<< <PIPE> >>,
19799a22
GS
1421and C<close> are translated to calls in the C run-time library and
1422thence to the operating system kernel. C<$!> is set to the C library's
1423C<errno> if one of these calls fails.
1424
1425Under a few operating systems, C<$^E> may contain a more verbose
1426error indicator, such as in this case, "CDROM tray not closed."
14218588 1427Systems that do not support extended error messages leave C<$^E>
19799a22
GS
1428the same as C<$!>.
1429
1430Finally, C<$?> may be set to non-0 value if the external program
1431F</cdrom/install> fails. The upper eight bits reflect specific
1432error conditions encountered by the program (the program's exit()
1433value). The lower eight bits reflect mode of failure, like signal
1434death and core dump information See wait(2) for details. In
1435contrast to C<$!> and C<$^E>, which are set only if error condition
1436is detected, the variable C<$?> is set on each C<wait> or pipe
1437C<close>, overwriting the old value. This is more like C<$@>, which
1438on every eval() is always set on failure and cleared on success.
2b92dfce 1439
19799a22
GS
1440For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1441and C<$?>.
2b92dfce
GS
1442
1443=head2 Technical Note on the Syntax of Variable Names
1444
19799a22
GS
1445Variable names in Perl can have several formats. Usually, they
1446must begin with a letter or underscore, in which case they can be
1447arbitrarily long (up to an internal limit of 251 characters) and
1448may contain letters, digits, underscores, or the special sequence
1449C<::> or C<'>. In this case, the part before the last C<::> or
1450C<'> is taken to be a I<package qualifier>; see L<perlmod>.
2b92dfce
GS
1451
1452Perl variable names may also be a sequence of digits or a single
1453punctuation or control character. These names are all reserved for
19799a22
GS
1454special uses by Perl; for example, the all-digits names are used
1455to hold data captured by backreferences after a regular expression
1456match. Perl has a special syntax for the single-control-character
1457names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1458character. For example, the notation C<$^W> (dollar-sign caret
1459C<W>) is the scalar variable whose name is the single character
1460control-C<W>. This is better than typing a literal control-C<W>
1461into your program.
2b92dfce 1462
87275199 1463Finally, new in Perl 5.6, Perl variable names may be alphanumeric
19799a22
GS
1464strings that begin with control characters (or better yet, a caret).
1465These variables must be written in the form C<${^Foo}>; the braces
1466are not optional. C<${^Foo}> denotes the scalar variable whose
1467name is a control-C<F> followed by two C<o>'s. These variables are
1468reserved for future special uses by Perl, except for the ones that
1469begin with C<^_> (control-underscore or caret-underscore). No
1470control-character name that begins with C<^_> will acquire a special
1471meaning in any future version of Perl; such names may therefore be
1472used safely in programs. C<$^_> itself, however, I<is> reserved.
1473
1474Perl identifiers that begin with digits, control characters, or
2b92dfce 1475punctuation characters are exempt from the effects of the C<package>
747fafda
MJD
1476declaration and are always forced to be in package C<main>; they are
1477also exempt from C<strict 'vars'> errors. A few other names are also
1478exempt in these ways:
2b92dfce
GS
1479
1480 ENV STDIN
1481 INC STDOUT
1482 ARGV STDERR
1483 ARGVOUT
1484 SIG
1485
1486In particular, the new special C<${^_XYZ}> variables are always taken
19799a22 1487to be in package C<main>, regardless of any C<package> declarations
747fafda 1488presently in scope.
2b92dfce 1489
19799a22
GS
1490=head1 BUGS
1491
1492Due to an unfortunate accident of Perl's implementation, C<use
1493English> imposes a considerable performance penalty on all regular
1494expression matches in a program, regardless of whether they occur
1495in the scope of C<use English>. For that reason, saying C<use
1496English> in libraries is strongly discouraged. See the
1497Devel::SawAmpersand module documentation from CPAN
1577cd80 1498( http://www.cpan.org/modules/by-module/Devel/ )
19799a22 1499for more information.
2b92dfce 1500
19799a22
GS
1501Having to even think about the C<$^S> variable in your exception
1502handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1503invites grievous and difficult to track down errors. Avoid it
1504and use an C<END{}> or CORE::GLOBAL::die override instead.