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1=head1 NAME
2
3perlsyn - Perl syntax
4
5=head1 DESCRIPTION
6
7A Perl script consists of a sequence of declarations and statements.
8The only things that need to be declared in Perl are report formats
9and subroutines. See the sections below for more information on those
10declarations. All uninitialized user-created objects are assumed to
11start with a null or 0 value until they are defined by some explicit
12operation such as assignment. (Though you can get warnings about the
13use of undefined values if you like.) The sequence of statements is
14executed just once, unlike in B<sed> and B<awk> scripts, where the
15sequence of statements is executed for each input line. While this means
16that you must explicitly loop over the lines of your input file (or
17files), it also means you have much more control over which files and
18which lines you look at. (Actually, I'm lying--it is possible to do an
19implicit loop with either the B<-n> or B<-p> switch. It's just not the
20mandatory default like it is in B<sed> and B<awk>.)
21
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22=head2 Declarations
23
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24Perl is, for the most part, a free-form language. (The only
25exception to this is format declarations, for obvious reasons.) Comments
26are indicated by the "#" character, and extend to the end of the line. If
27you attempt to use C</* */> C-style comments, it will be interpreted
28either as division or pattern matching, depending on the context, and C++
4633a7c4 29C<//> comments just look like a null regular expression, so don't do
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30that.
31
32A declaration can be put anywhere a statement can, but has no effect on
33the execution of the primary sequence of statements--declarations all
34take effect at compile time. Typically all the declarations are put at
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35the beginning or the end of the script. However, if you're using
36lexically-scoped private variables created with my(), you'll have to make sure
37your format or subroutine definition is within the same block scope
38as the my if you expect to to be able to access those private variables.
a0d0e21e 39
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40Declaring a subroutine allows a subroutine name to be used as if it were a
41list operator from that point forward in the program. You can declare a
42subroutine without defining it by saying just
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43
44 sub myname;
45 $me = myname $0 or die "can't get myname";
46
4633a7c4 47Note that it functions as a list operator though, not as a unary
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48operator, so be careful to use C<or> instead of C<||> there.
49
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50Subroutines declarations can also be loaded up with the C<require> statement
51or both loaded and imported into your namespace with a C<use> statement.
52See L<perlmod> for details on this.
a0d0e21e 53
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54A statement sequence may contain declarations of lexically-scoped
55variables, but apart from declaring a variable name, the declaration acts
56like an ordinary statement, and is elaborated within the sequence of
57statements as if it were an ordinary statement. That means it actually
58has both compile-time and run-time effects.
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59
60=head2 Simple statements
61
62The only kind of simple statement is an expression evaluated for its
63side effects. Every simple statement must be terminated with a
64semicolon, unless it is the final statement in a block, in which case
65the semicolon is optional. (A semicolon is still encouraged there if the
748a9306 66block takes up more than one line, since you may eventually add another line.)
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67Note that there are some operators like C<eval {}> and C<do {}> that look
68like compound statements, but aren't (they're just TERMs in an expression),
4633a7c4 69and thus need an explicit termination if used as the last item in a statement.
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70
71Any simple statement may optionally be followed by a I<SINGLE> modifier,
72just before the terminating semicolon (or block ending). The possible
73modifiers are:
74
75 if EXPR
76 unless EXPR
77 while EXPR
78 until EXPR
79
80The C<if> and C<unless> modifiers have the expected semantics,
81presuming you're a speaker of English. The C<while> and C<until>
82modifiers also have the usual "while loop" semantics (conditional
83evaluated first), except when applied to a do-BLOCK (or to the
84now-deprecated do-SUBROUTINE statement), in which case the block
85executes once before the conditional is evaluated. This is so that you
86can write loops like:
87
88 do {
4633a7c4 89 $line = <STDIN>;
a0d0e21e 90 ...
4633a7c4 91 } until $line eq ".\n";
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92
93See L<perlfunc/do>. Note also that the loop control
94statements described later will I<NOT> work in this construct, since
95modifiers don't take loop labels. Sorry. You can always wrap
4633a7c4 96another block around it to do that sort of thing.
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97
98=head2 Compound statements
99
100In Perl, a sequence of statements that defines a scope is called a block.
101Sometimes a block is delimited by the file containing it (in the case
102of a required file, or the program as a whole), and sometimes a block
103is delimited by the extent of a string (in the case of an eval).
104
105But generally, a block is delimited by curly brackets, also known as braces.
106We will call this syntactic construct a BLOCK.
107
108The following compound statements may be used to control flow:
109
110 if (EXPR) BLOCK
111 if (EXPR) BLOCK else BLOCK
112 if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
113 LABEL while (EXPR) BLOCK
114 LABEL while (EXPR) BLOCK continue BLOCK
115 LABEL for (EXPR; EXPR; EXPR) BLOCK
748a9306 116 LABEL foreach VAR (LIST) BLOCK
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117 LABEL BLOCK continue BLOCK
118
119Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
120not statements. This means that the curly brackets are I<required>--no
121dangling statements allowed. If you want to write conditionals without
122curly brackets there are several other ways to do it. The following
123all do the same thing:
124
125 if (!open(FOO)) { die "Can't open $FOO: $!"; }
126 die "Can't open $FOO: $!" unless open(FOO);
127 open(FOO) or die "Can't open $FOO: $!"; # FOO or bust!
128 open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
129 # a bit exotic, that last one
130
131The C<if> statement is straightforward. Since BLOCKs are always
132bounded by curly brackets, there is never any ambiguity about which
133C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
134the sense of the test is reversed.
135
136The C<while> statement executes the block as long as the expression is
137true (does not evaluate to the null string or 0 or "0"). The LABEL is
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138optional, and if present, consists of an identifier followed by a colon.
139The LABEL identifies the loop for the loop control statements C<next>,
140C<last>, and C<redo>. If the LABEL is omitted, the loop control statement
141refers to the innermost enclosing loop. This may include dynamically
142looking back your call-stack at run time to find the LABEL. Such
143desperate behavior triggers a warning if you use the B<-w> flag.
144
145If there is a C<continue> BLOCK, it is always executed just before the
146conditional is about to be evaluated again, just like the third part of a
147C<for> loop in C. Thus it can be used to increment a loop variable, even
148when the loop has been continued via the C<next> statement (which is
149similar to the C C<continue> statement).
150
151=head2 Loop Control
152
153The C<next> command is like the C<continue> statement in C; it starts
154the next iteration of the loop:
155
156 LINE: while (<STDIN>) {
157 next LINE if /^#/; # discard comments
158 ...
159 }
160
161The C<last> command is like the C<break> statement in C (as used in
162loops); it immediately exits the loop in question. The
163C<continue> block, if any, is not executed:
164
165 LINE: while (<STDIN>) {
166 last LINE if /^$/; # exit when done with header
167 ...
168 }
169
170The C<redo> command restarts the loop block without evaluating the
171conditional again. The C<continue> block, if any, is I<not> executed.
172This command is normally used by programs that want to lie to themselves
173about what was just input.
174
175For example, when processing a file like F</etc/termcap>.
176If your input lines might end in backslashes to indicate continuation, you
177want to skip ahead and get the next record.
178
179 while (<>) {
180 chomp;
181 if (s/\\$//) {
182 $_ .= <>;
183 redo unless eof();
184 }
185 # now process $_
186 }
187
188which is Perl short-hand for the more explicitly written version:
189
190 LINE: while ($line = <ARGV>) {
191 chomp($line);
192 if ($line =~ s/\\$//) {
193 $line .= <ARGV>;
194 redo LINE unless eof(); # not eof(ARGV)!
195 }
196 # now process $line
197 }
198
199Or here's a a simpleminded Pascal comment stripper (warning: assumes no { or } in strings)
200
201 LINE: while (<STDIN>) {
202 while (s|({.*}.*){.*}|$1 |) {}
203 s|{.*}| |;
204 if (s|{.*| |) {
205 $front = $_;
206 while (<STDIN>) {
207 if (/}/) { # end of comment?
208 s|^|$front{|;
209 redo LINE;
210 }
211 }
212 }
213 print;
214 }
215
216Note that if there were a C<continue> block on the above code, it would get
217executed even on discarded lines.
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218
219If the word C<while> is replaced by the word C<until>, the sense of the
220test is reversed, but the conditional is still tested before the first
221iteration.
222
223In either the C<if> or the C<while> statement, you may replace "(EXPR)"
224with a BLOCK, and the conditional is true if the value of the last
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225statement in that block is true. While this "feature" continues to work in
226version 5, it has been deprecated, so please change any occurrences of "if BLOCK" to
227"if (do BLOCK)".
228
229=head2 For and Foreach
a0d0e21e 230
4633a7c4 231Perl's C-style C<for> loop works exactly like the corresponding C<while> loop:
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232
233 for ($i = 1; $i < 10; $i++) {
234 ...
235 }
236
237is the same as
238
239 $i = 1;
240 while ($i < 10) {
241 ...
242 } continue {
243 $i++;
244 }
245
4633a7c4 246The C<foreach> loop iterates over a normal list value and sets the
a0d0e21e 247variable VAR to be each element of the list in turn. The variable is
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248implicitly local to the loop and regains its former value upon exiting the
249loop. If the variable was previously declared with C<my>, it uses that
250variable instead of the global one, but it's still localized to the loop.
251This can cause problems if you have subroutine or format declarations
252within that block's scope.
253
254The C<foreach> keyword is actually a synonym for the C<for> keyword, so
255you can use C<foreach> for readability or C<for> for brevity. If VAR is
256omitted, $_ is set to each value. If LIST is an actual array (as opposed
257to an expression returning a list value), you can modify each element of
258the array by modifying VAR inside the loop. That's because the C<foreach>
259loop index variable is an implicit alias for each item in the list that
260you're looping over.
261
748a9306 262Examples:
a0d0e21e 263
4633a7c4 264 for (@ary) { s/foo/bar/ }
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265
266 foreach $elem (@elements) {
267 $elem *= 2;
268 }
269
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270 for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
271 print $count, "\n"; sleep(1);
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272 }
273
274 for (1..15) { print "Merry Christmas\n"; }
275
4633a7c4 276 foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
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277 print "Item: $item\n";
278 }
279
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280Here's how a C programmer might code up a particular algorithm in Perl:
281
282 for ($i = 0; $i < @ary1; $i++) {
283 for ($j = 0; $j < @ary2; $j++) {
284 if ($ary1[$i] > $ary2[$j]) {
285 last; # can't go to outer :-(
286 }
287 $ary1[$i] += $ary2[$j];
288 }
289 }
290
291Whereas here's how a Perl programmer more confortable with the idiom might
292do it this way:
293
294 OUTER: foreach $i (@ary1) {
295 INNER: foreach $j (@ary2) {
296 next OUTER if $i > $j;
297 $i += $j;
298 }
299 }
300
301See how much easier this is? It's cleaner, safer, and faster.
302It's cleaner because it's less noisy.
303It's safer because if code gets added
304between the inner and outer loops later, you won't accidentally excecute
305it because you've explicitly asked to iterate the other loop rather than
306merely terminating the inner one.
307And it's faster because Perl exececute C<foreach> statement more
308rapidly than it would the equivalent C<for> loop.
309
310=head2 Basic BLOCKs and Switch Statements
311
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312A BLOCK by itself (labeled or not) is semantically equivalent to a loop
313that executes once. Thus you can use any of the loop control
314statements in it to leave or restart the block. The C<continue> block
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315is optional.
316
317The BLOCK construct is particularly nice for doing case
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318structures.
319
320 SWITCH: {
321 if (/^abc/) { $abc = 1; last SWITCH; }
322 if (/^def/) { $def = 1; last SWITCH; }
323 if (/^xyz/) { $xyz = 1; last SWITCH; }
324 $nothing = 1;
325 }
326
327There is no official switch statement in Perl, because there are
328already several ways to write the equivalent. In addition to the
329above, you could write
330
331 SWITCH: {
332 $abc = 1, last SWITCH if /^abc/;
333 $def = 1, last SWITCH if /^def/;
334 $xyz = 1, last SWITCH if /^xyz/;
335 $nothing = 1;
336 }
337
338(That's actually not as strange as it looks one you realize that you can
339use loop control "operators" within an expression, That's just the normal
340C comma operator.)
341
342or
343
344 SWITCH: {
345 /^abc/ && do { $abc = 1; last SWITCH; };
346 /^def/ && do { $def = 1; last SWITCH; };
347 /^xyz/ && do { $xyz = 1; last SWITCH; };
348 $nothing = 1;
349 }
350
351or formatted so it stands out more as a "proper" switch statement:
352
353 SWITCH: {
354 /^abc/ && do {
355 $abc = 1;
356 last SWITCH;
357 };
358
359 /^def/ && do {
360 $def = 1;
361 last SWITCH;
362 };
363
364 /^xyz/ && do {
365 $xyz = 1;
366 last SWITCH;
367 };
368 $nothing = 1;
369 }
370
371or
372
373 SWITCH: {
374 /^abc/ and $abc = 1, last SWITCH;
375 /^def/ and $def = 1, last SWITCH;
376 /^xyz/ and $xyz = 1, last SWITCH;
377 $nothing = 1;
378 }
379
380or even, horrors,
381
382 if (/^abc/)
383 { $abc = 1 }
384 elsif (/^def/)
385 { $def = 1 }
386 elsif (/^xyz/)
387 { $xyz = 1 }
388 else
389 { $nothing = 1 }
390
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391
392A common idiom for a switch statement is to use C<foreach>'s aliasing to make
393a temporary assignment to $_ for convenient matching:
394
395 SWITCH: for ($where) {
396 /In Card Names/ && do { push @flags, '-e'; last; };
397 /Anywhere/ && do { push @flags, '-h'; last; };
398 /In Rulings/ && do { last; };
399 die "unknown value for form variable where: `$where'";
400 }
401
402=head2 Goto
403
404Although not for the faint of heart, Perl does support a C<goto> statement.
405A loop's LABEL is not actually a valid target for a C<goto>;
406it's just the name of the loop. There are three forms: goto-LABEL,
407goto-EXPR, and goto-&NAME.
408
409The goto-LABEL form finds the statement labeled with LABEL and resumes
410execution there. It may not be used to go into any construct that
411requires initialization, such as a subroutine or a foreach loop. It
412also can't be used to go into a construct that is optimized away. It
413can be used to go almost anywhere else within the dynamic scope,
414including out of subroutines, but it's usually better to use some other
415construct such as last or die. The author of Perl has never felt the
416need to use this form of goto (in Perl, that is--C is another matter).
417
418The goto-EXPR form expects a label name, whose scope will be resolved
419dynamically. This allows for computed gotos per FORTRAN, but isn't
420necessarily recommended if you're optimizing for maintainability:
421
422 goto ("FOO", "BAR", "GLARCH")[$i];
423
424The goto-&NAME form is highly magical, and substitutes a call to the
425named subroutine for the currently running subroutine. This is used by
426AUTOLOAD() subroutines that wish to load another subroutine and then
427pretend that the other subroutine had been called in the first place
428(except that any modifications to @_ in the current subroutine are
429propagated to the other subroutine.) After the C<goto>, not even caller()
430will be able to tell that this routine was called first.
431
432In almost cases like this, it's usually a far, far better idea to use the
433structured control flow mechanisms of C<next>, C<last>, or C<redo> insetad
434resorting to a C<goto>. For certain applications, the catch and throw pair of
435C<eval{}> and die() for exception processing can also be a prudent approach.