Commit | Line | Data |
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a0d0e21e LW |
1 | =head1 NAME |
2 | ||
3 | perltrap - Perl traps for the unwary | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
9fda99eb DC |
7 | The biggest trap of all is forgetting to C<use warnings> or use the B<-w> |
8 | switch; see L<perllexwarn> and L<perlrun>. The second biggest trap is not | |
9 | making your entire program runnable under C<use strict>. The third biggest | |
10 | trap is not reading the list of changes in this version of Perl; see | |
11 | L<perldelta>. | |
a0d0e21e LW |
12 | |
13 | =head2 Awk Traps | |
14 | ||
15 | Accustomed B<awk> users should take special note of the following: | |
16 | ||
17 | =over 4 | |
18 | ||
19 | =item * | |
20 | ||
21 | The English module, loaded via | |
22 | ||
23 | use English; | |
24 | ||
54310121 | 25 | allows you to refer to special variables (like C<$/>) with names (like |
19799a22 | 26 | $RS), as though they were in B<awk>; see L<perlvar> for details. |
a0d0e21e LW |
27 | |
28 | =item * | |
29 | ||
30 | Semicolons are required after all simple statements in Perl (except | |
31 | at the end of a block). Newline is not a statement delimiter. | |
32 | ||
33 | =item * | |
34 | ||
35 | Curly brackets are required on C<if>s and C<while>s. | |
36 | ||
37 | =item * | |
38 | ||
5db417f7 | 39 | Variables begin with "$", "@" or "%" in Perl. |
a0d0e21e LW |
40 | |
41 | =item * | |
42 | ||
43 | Arrays index from 0. Likewise string positions in substr() and | |
44 | index(). | |
45 | ||
46 | =item * | |
47 | ||
48 | You have to decide whether your array has numeric or string indices. | |
49 | ||
50 | =item * | |
51 | ||
aa689395 | 52 | Hash values do not spring into existence upon mere reference. |
a0d0e21e LW |
53 | |
54 | =item * | |
55 | ||
56 | You have to decide whether you want to use string or numeric | |
57 | comparisons. | |
58 | ||
59 | =item * | |
60 | ||
61 | Reading an input line does not split it for you. You get to split it | |
54310121 | 62 | to an array yourself. And the split() operator has different |
63 | arguments than B<awk>'s. | |
a0d0e21e LW |
64 | |
65 | =item * | |
66 | ||
67 | The current input line is normally in $_, not $0. It generally does | |
68 | not have the newline stripped. ($0 is the name of the program | |
69 | executed.) See L<perlvar>. | |
70 | ||
71 | =item * | |
72 | ||
c47ff5f1 | 73 | $<I<digit>> does not refer to fields--it refers to substrings matched |
8b0a4b75 | 74 | by the last match pattern. |
a0d0e21e LW |
75 | |
76 | =item * | |
77 | ||
78 | The print() statement does not add field and record separators unless | |
8b0a4b75 | 79 | you set C<$,> and C<$\>. You can set $OFS and $ORS if you're using |
a0d0e21e LW |
80 | the English module. |
81 | ||
82 | =item * | |
83 | ||
84 | You must open your files before you print to them. | |
85 | ||
86 | =item * | |
87 | ||
88 | The range operator is "..", not comma. The comma operator works as in | |
89 | C. | |
90 | ||
91 | =item * | |
92 | ||
93 | The match operator is "=~", not "~". ("~" is the one's complement | |
94 | operator, as in C.) | |
95 | ||
96 | =item * | |
97 | ||
98 | The exponentiation operator is "**", not "^". "^" is the XOR | |
99 | operator, as in C. (You know, one could get the feeling that B<awk> is | |
100 | basically incompatible with C.) | |
101 | ||
102 | =item * | |
103 | ||
104 | The concatenation operator is ".", not the null string. (Using the | |
5f05dabc | 105 | null string would render C</pat/ /pat/> unparsable, because the third slash |
106 | would be interpreted as a division operator--the tokenizer is in fact | |
c47ff5f1 | 107 | slightly context sensitive for operators like "/", "?", and ">". |
a0d0e21e LW |
108 | And in fact, "." itself can be the beginning of a number.) |
109 | ||
110 | =item * | |
111 | ||
112 | The C<next>, C<exit>, and C<continue> keywords work differently. | |
113 | ||
114 | =item * | |
115 | ||
116 | ||
117 | The following variables work differently: | |
118 | ||
119 | Awk Perl | |
9fda99eb | 120 | ARGC scalar @ARGV (compare with $#ARGV) |
a0d0e21e LW |
121 | ARGV[0] $0 |
122 | FILENAME $ARGV | |
123 | FNR $. - something | |
124 | FS (whatever you like) | |
125 | NF $#Fld, or some such | |
126 | NR $. | |
127 | OFMT $# | |
128 | OFS $, | |
129 | ORS $\ | |
130 | RLENGTH length($&) | |
131 | RS $/ | |
132 | RSTART length($`) | |
133 | SUBSEP $; | |
134 | ||
135 | =item * | |
136 | ||
137 | You cannot set $RS to a pattern, only a string. | |
138 | ||
139 | =item * | |
140 | ||
141 | When in doubt, run the B<awk> construct through B<a2p> and see what it | |
142 | gives you. | |
143 | ||
144 | =back | |
145 | ||
6ec4bd10 | 146 | =head2 C/C++ Traps |
a0d0e21e | 147 | |
6ec4bd10 | 148 | Cerebral C and C++ programmers should take note of the following: |
a0d0e21e LW |
149 | |
150 | =over 4 | |
151 | ||
152 | =item * | |
153 | ||
154 | Curly brackets are required on C<if>'s and C<while>'s. | |
155 | ||
156 | =item * | |
157 | ||
158 | You must use C<elsif> rather than C<else if>. | |
159 | ||
160 | =item * | |
161 | ||
6ec4bd10 MS |
162 | The C<break> and C<continue> keywords from C become in Perl C<last> |
163 | and C<next>, respectively. Unlike in C, these do I<not> work within a | |
164 | C<do { } while> construct. See L<perlsyn/"Loop Control">. | |
a0d0e21e LW |
165 | |
166 | =item * | |
167 | ||
6ec4bd10 MS |
168 | There's no switch statement. (But it's easy to build one on the fly, |
169 | see L<perlsyn/"Basic BLOCKs and Switch Statements">) | |
a0d0e21e LW |
170 | |
171 | =item * | |
172 | ||
5db417f7 | 173 | Variables begin with "$", "@" or "%" in Perl. |
a0d0e21e LW |
174 | |
175 | =item * | |
176 | ||
a0d0e21e LW |
177 | Comments begin with "#", not "/*". |
178 | ||
179 | =item * | |
180 | ||
181 | You can't take the address of anything, although a similar operator | |
5f05dabc | 182 | in Perl is the backslash, which creates a reference. |
a0d0e21e LW |
183 | |
184 | =item * | |
185 | ||
4633a7c4 LW |
186 | C<ARGV> must be capitalized. C<$ARGV[0]> is C's C<argv[1]>, and C<argv[0]> |
187 | ends up in C<$0>. | |
a0d0e21e LW |
188 | |
189 | =item * | |
190 | ||
191 | System calls such as link(), unlink(), rename(), etc. return nonzero for | |
9fda99eb | 192 | success, not 0. (system(), however, returns zero for success.) |
a0d0e21e LW |
193 | |
194 | =item * | |
195 | ||
196 | Signal handlers deal with signal names, not numbers. Use C<kill -l> | |
197 | to find their names on your system. | |
198 | ||
199 | =back | |
200 | ||
201 | =head2 Sed Traps | |
202 | ||
203 | Seasoned B<sed> programmers should take note of the following: | |
204 | ||
205 | =over 4 | |
206 | ||
207 | =item * | |
208 | ||
209 | Backreferences in substitutions use "$" rather than "\". | |
210 | ||
211 | =item * | |
212 | ||
213 | The pattern matching metacharacters "(", ")", and "|" do not have backslashes | |
214 | in front. | |
215 | ||
216 | =item * | |
217 | ||
218 | The range operator is C<...>, rather than comma. | |
219 | ||
220 | =back | |
221 | ||
222 | =head2 Shell Traps | |
223 | ||
224 | Sharp shell programmers should take note of the following: | |
225 | ||
226 | =over 4 | |
227 | ||
228 | =item * | |
229 | ||
54310121 | 230 | The backtick operator does variable interpolation without regard to |
a0d0e21e LW |
231 | the presence of single quotes in the command. |
232 | ||
233 | =item * | |
234 | ||
54310121 | 235 | The backtick operator does no translation of the return value, unlike B<csh>. |
a0d0e21e LW |
236 | |
237 | =item * | |
238 | ||
239 | Shells (especially B<csh>) do several levels of substitution on each | |
5f05dabc | 240 | command line. Perl does substitution in only certain constructs |
54310121 | 241 | such as double quotes, backticks, angle brackets, and search patterns. |
a0d0e21e LW |
242 | |
243 | =item * | |
244 | ||
245 | Shells interpret scripts a little bit at a time. Perl compiles the | |
246 | entire program before executing it (except for C<BEGIN> blocks, which | |
247 | execute at compile time). | |
248 | ||
249 | =item * | |
250 | ||
251 | The arguments are available via @ARGV, not $1, $2, etc. | |
252 | ||
253 | =item * | |
254 | ||
255 | The environment is not automatically made available as separate scalar | |
256 | variables. | |
257 | ||
258 | =back | |
259 | ||
260 | =head2 Perl Traps | |
261 | ||
262 | Practicing Perl Programmers should take note of the following: | |
263 | ||
264 | =over 4 | |
265 | ||
266 | =item * | |
267 | ||
268 | Remember that many operations behave differently in a list | |
269 | context than they do in a scalar one. See L<perldata> for details. | |
270 | ||
271 | =item * | |
272 | ||
68dc0745 | 273 | Avoid barewords if you can, especially all lowercase ones. |
54310121 | 274 | You can't tell by just looking at it whether a bareword is |
275 | a function or a string. By using quotes on strings and | |
5f05dabc | 276 | parentheses on function calls, you won't ever get them confused. |
a0d0e21e LW |
277 | |
278 | =item * | |
279 | ||
54310121 | 280 | You cannot discern from mere inspection which builtins |
281 | are unary operators (like chop() and chdir()) | |
a0d0e21e | 282 | and which are list operators (like print() and unlink()). |
9fda99eb DC |
283 | (Unless prototyped, user-defined subroutines can B<only> be list |
284 | operators, never unary ones.) See L<perlop> and L<perlsub>. | |
a0d0e21e LW |
285 | |
286 | =item * | |
287 | ||
748a9306 | 288 | People have a hard time remembering that some functions |
a0d0e21e | 289 | default to $_, or @ARGV, or whatever, but that others which |
54310121 | 290 | you might expect to do not. |
a0d0e21e | 291 | |
6dbacca0 | 292 | =item * |
a0d0e21e | 293 | |
c47ff5f1 | 294 | The <FH> construct is not the name of the filehandle, it is a readline |
5f05dabc | 295 | operation on that handle. The data read is assigned to $_ only if the |
748a9306 LW |
296 | file read is the sole condition in a while loop: |
297 | ||
298 | while (<FH>) { } | |
54310121 | 299 | while (defined($_ = <FH>)) { }.. |
748a9306 LW |
300 | <FH>; # data discarded! |
301 | ||
6dbacca0 | 302 | =item * |
748a9306 | 303 | |
19799a22 | 304 | Remember not to use C<=> when you need C<=~>; |
a0d0e21e LW |
305 | these two constructs are quite different: |
306 | ||
307 | $x = /foo/; | |
308 | $x =~ /foo/; | |
309 | ||
310 | =item * | |
311 | ||
54310121 | 312 | The C<do {}> construct isn't a real loop that you can use |
a0d0e21e LW |
313 | loop control on. |
314 | ||
315 | =item * | |
316 | ||
54310121 | 317 | Use C<my()> for local variables whenever you can get away with |
318 | it (but see L<perlform> for where you can't). | |
319 | Using C<local()> actually gives a local value to a global | |
a0d0e21e LW |
320 | variable, which leaves you open to unforeseen side-effects |
321 | of dynamic scoping. | |
322 | ||
c07a80fd | 323 | =item * |
324 | ||
325 | If you localize an exported variable in a module, its exported value will | |
326 | not change. The local name becomes an alias to a new value but the | |
327 | external name is still an alias for the original. | |
328 | ||
a0d0e21e LW |
329 | =back |
330 | ||
5f05dabc | 331 | =head2 Perl4 to Perl5 Traps |
a0d0e21e | 332 | |
54310121 | 333 | Practicing Perl4 Programmers should take note of the following |
6dbacca0 | 334 | Perl4-to-Perl5 specific traps. |
335 | ||
336 | They're crudely ordered according to the following list: | |
a0d0e21e LW |
337 | |
338 | =over 4 | |
339 | ||
6dbacca0 | 340 | =item Discontinuance, Deprecation, and BugFix traps |
a0d0e21e | 341 | |
6dbacca0 | 342 | Anything that's been fixed as a perl4 bug, removed as a perl4 feature |
343 | or deprecated as a perl4 feature with the intent to encourage usage of | |
344 | some other perl5 feature. | |
a0d0e21e | 345 | |
6dbacca0 | 346 | =item Parsing Traps |
748a9306 | 347 | |
6dbacca0 | 348 | Traps that appear to stem from the new parser. |
a0d0e21e | 349 | |
6dbacca0 | 350 | =item Numerical Traps |
a0d0e21e | 351 | |
6dbacca0 | 352 | Traps having to do with numerical or mathematical operators. |
a0d0e21e | 353 | |
6dbacca0 | 354 | =item General data type traps |
a0d0e21e | 355 | |
6dbacca0 | 356 | Traps involving perl standard data types. |
a0d0e21e | 357 | |
6dbacca0 | 358 | =item Context Traps - scalar, list contexts |
359 | ||
360 | Traps related to context within lists, scalar statements/declarations. | |
361 | ||
362 | =item Precedence Traps | |
363 | ||
364 | Traps related to the precedence of parsing, evaluation, and execution of | |
365 | code. | |
366 | ||
367 | =item General Regular Expression Traps using s///, etc. | |
368 | ||
369 | Traps related to the use of pattern matching. | |
370 | ||
371 | =item Subroutine, Signal, Sorting Traps | |
372 | ||
373 | Traps related to the use of signals and signal handlers, general subroutines, | |
374 | and sorting, along with sorting subroutines. | |
375 | ||
376 | =item OS Traps | |
377 | ||
378 | OS-specific traps. | |
379 | ||
380 | =item DBM Traps | |
381 | ||
382 | Traps specific to the use of C<dbmopen()>, and specific dbm implementations. | |
383 | ||
384 | =item Unclassified Traps | |
385 | ||
386 | Everything else. | |
387 | ||
388 | =back | |
389 | ||
390 | If you find an example of a conversion trap that is not listed here, | |
4375e838 | 391 | please submit it to <F<perlbug@perl.org>> for inclusion. |
9f1b1f2d GS |
392 | Also note that at least some of these can be caught with the |
393 | C<use warnings> pragma or the B<-w> switch. | |
6dbacca0 | 394 | |
395 | =head2 Discontinuance, Deprecation, and BugFix traps | |
396 | ||
397 | Anything that has been discontinued, deprecated, or fixed as | |
54310121 | 398 | a bug from perl4. |
a0d0e21e | 399 | |
6dbacca0 | 400 | =over 4 |
401 | ||
54310121 | 402 | =item * Discontinuance |
6dbacca0 | 403 | |
404 | Symbols starting with "_" are no longer forced into package main, except | |
405 | for C<$_> itself (and C<@_>, etc.). | |
406 | ||
407 | package test; | |
408 | $_legacy = 1; | |
cb1a09d0 | 409 | |
6dbacca0 | 410 | package main; |
411 | print "\$_legacy is ",$_legacy,"\n"; | |
54310121 | 412 | |
6dbacca0 | 413 | # perl4 prints: $_legacy is 1 |
414 | # perl5 prints: $_legacy is | |
415 | ||
54310121 | 416 | =item * Deprecation |
6dbacca0 | 417 | |
418 | Double-colon is now a valid package separator in a variable name. Thus these | |
5f05dabc | 419 | behave differently in perl4 vs. perl5, because the packages don't exist. |
6dbacca0 | 420 | |
421 | $a=1;$b=2;$c=3;$var=4; | |
422 | print "$a::$b::$c "; | |
cb1a09d0 | 423 | print "$var::abc::xyz\n"; |
c47ff5f1 | 424 | |
6dbacca0 | 425 | # perl4 prints: 1::2::3 4::abc::xyz |
426 | # perl5 prints: 3 | |
cb1a09d0 | 427 | |
6dbacca0 | 428 | Given that C<::> is now the preferred package delimiter, it is debatable |
429 | whether this should be classed as a bug or not. | |
430 | (The older package delimiter, ' ,is used here) | |
cb1a09d0 | 431 | |
6dbacca0 | 432 | $x = 10 ; |
433 | print "x=${'x}\n" ; | |
54310121 | 434 | |
6dbacca0 | 435 | # perl4 prints: x=10 |
436 | # perl5 prints: Can't find string terminator "'" anywhere before EOF | |
a0d0e21e | 437 | |
5e77893f MG |
438 | You can avoid this problem, and remain compatible with perl4, if you |
439 | always explicitly include the package name: | |
440 | ||
441 | $x = 10 ; | |
442 | print "x=${main'x}\n" ; | |
443 | ||
54310121 | 444 | Also see precedence traps, for parsing C<$:>. |
a0d0e21e | 445 | |
6dbacca0 | 446 | =item * BugFix |
a0d0e21e | 447 | |
6dbacca0 | 448 | The second and third arguments of C<splice()> are now evaluated in scalar |
449 | context (as the Camel says) rather than list context. | |
a0d0e21e | 450 | |
1d2dff63 GS |
451 | sub sub1{return(0,2) } # return a 2-element list |
452 | sub sub2{ return(1,2,3)} # return a 3-element list | |
54310121 | 453 | @a1 = ("a","b","c","d","e"); |
6dbacca0 | 454 | @a2 = splice(@a1,&sub1,&sub2); |
455 | print join(' ',@a2),"\n"; | |
54310121 | 456 | |
6dbacca0 | 457 | # perl4 prints: a b |
54310121 | 458 | # perl5 prints: c d e |
a0d0e21e | 459 | |
54310121 | 460 | =item * Discontinuance |
a0d0e21e | 461 | |
6dbacca0 | 462 | You can't do a C<goto> into a block that is optimized away. Darn. |
a0d0e21e | 463 | |
6dbacca0 | 464 | goto marker1; |
a0d0e21e | 465 | |
54310121 | 466 | for(1){ |
6dbacca0 | 467 | marker1: |
468 | print "Here I is!\n"; | |
54310121 | 469 | } |
470 | ||
6dbacca0 | 471 | # perl4 prints: Here I is! |
9fda99eb | 472 | # perl5 errors: Can't "goto" into the middle of a foreach loop |
6dbacca0 | 473 | |
54310121 | 474 | =item * Discontinuance |
6dbacca0 | 475 | |
476 | It is no longer syntactically legal to use whitespace as the name | |
477 | of a variable, or as a delimiter for any kind of quote construct. | |
54310121 | 478 | Double darn. |
6dbacca0 | 479 | |
480 | $a = ("foo bar"); | |
481 | $b = q baz ; | |
482 | print "a is $a, b is $b\n"; | |
54310121 | 483 | |
6dbacca0 | 484 | # perl4 prints: a is foo bar, b is baz |
54310121 | 485 | # perl5 errors: Bareword found where operator expected |
5e378fdf | 486 | |
6dbacca0 | 487 | =item * Discontinuance |
488 | ||
489 | The archaic while/if BLOCK BLOCK syntax is no longer supported. | |
490 | ||
491 | if { 1 } { | |
492 | print "True!"; | |
493 | } | |
494 | else { | |
495 | print "False!"; | |
496 | } | |
54310121 | 497 | |
6dbacca0 | 498 | # perl4 prints: True! |
499 | # perl5 errors: syntax error at test.pl line 1, near "if {" | |
500 | ||
501 | =item * BugFix | |
502 | ||
503 | The C<**> operator now binds more tightly than unary minus. | |
504 | It was documented to work this way before, but didn't. | |
505 | ||
506 | print -4**2,"\n"; | |
54310121 | 507 | |
6dbacca0 | 508 | # perl4 prints: 16 |
509 | # perl5 prints: -16 | |
510 | ||
54310121 | 511 | =item * Discontinuance |
6dbacca0 | 512 | |
513 | The meaning of C<foreach{}> has changed slightly when it is iterating over a | |
514 | list which is not an array. This used to assign the list to a | |
515 | temporary array, but no longer does so (for efficiency). This means | |
516 | that you'll now be iterating over the actual values, not over copies of | |
517 | the values. Modifications to the loop variable can change the original | |
518 | values. | |
519 | ||
520 | @list = ('ab','abc','bcd','def'); | |
521 | foreach $var (grep(/ab/,@list)){ | |
522 | $var = 1; | |
523 | } | |
524 | print (join(':',@list)); | |
54310121 | 525 | |
6dbacca0 | 526 | # perl4 prints: ab:abc:bcd:def |
527 | # perl5 prints: 1:1:bcd:def | |
528 | ||
529 | To retain Perl4 semantics you need to assign your list | |
54310121 | 530 | explicitly to a temporary array and then iterate over that. For |
6dbacca0 | 531 | example, you might need to change |
532 | ||
533 | foreach $var (grep(/ab/,@list)){ | |
534 | ||
535 | to | |
536 | ||
537 | foreach $var (@tmp = grep(/ab/,@list)){ | |
538 | ||
539 | Otherwise changing $var will clobber the values of @list. (This most often | |
540 | happens when you use C<$_> for the loop variable, and call subroutines in | |
541 | the loop that don't properly localize C<$_>.) | |
542 | ||
5e378fdf | 543 | =item * Discontinuance |
544 | ||
545 | C<split> with no arguments now behaves like C<split ' '> (which doesn't | |
546 | return an initial null field if $_ starts with whitespace), it used to | |
547 | behave like C<split /\s+/> (which does). | |
548 | ||
549 | $_ = ' hi mom'; | |
550 | print join(':', split); | |
551 | ||
552 | # perl4 prints: :hi:mom | |
553 | # perl5 prints: hi:mom | |
554 | ||
55497cff | 555 | =item * BugFix |
556 | ||
9607fc9c | 557 | Perl 4 would ignore any text which was attached to an B<-e> switch, |
55497cff | 558 | always taking the code snippet from the following arg. Additionally, it |
9607fc9c | 559 | would silently accept an B<-e> switch without a following arg. Both of |
55497cff | 560 | these behaviors have been fixed. |
561 | ||
562 | perl -e'print "attached to -e"' 'print "separate arg"' | |
54310121 | 563 | |
55497cff | 564 | # perl4 prints: separate arg |
565 | # perl5 prints: attached to -e | |
54310121 | 566 | |
55497cff | 567 | perl -e |
568 | ||
569 | # perl4 prints: | |
570 | # perl5 dies: No code specified for -e. | |
571 | ||
572 | =item * Discontinuance | |
573 | ||
574 | In Perl 4 the return value of C<push> was undocumented, but it was | |
575 | actually the last value being pushed onto the target list. In Perl 5 | |
576 | the return value of C<push> is documented, but has changed, it is the | |
577 | number of elements in the resulting list. | |
578 | ||
579 | @x = ('existing'); | |
580 | print push(@x, 'first new', 'second new'); | |
54310121 | 581 | |
55497cff | 582 | # perl4 prints: second new |
583 | # perl5 prints: 3 | |
584 | ||
6dbacca0 | 585 | =item * Deprecation |
586 | ||
587 | Some error messages will be different. | |
588 | ||
54310121 | 589 | =item * Discontinuance |
6dbacca0 | 590 | |
46836f5c GS |
591 | In Perl 4, if in list context the delimiters to the first argument of |
592 | C<split()> were C<??>, the result would be placed in C<@_> as well as | |
593 | being returned. Perl 5 has more respect for your subroutine arguments. | |
594 | ||
595 | =item * Discontinuance | |
596 | ||
6dbacca0 | 597 | Some bugs may have been inadvertently removed. :-) |
598 | ||
599 | =back | |
600 | ||
601 | =head2 Parsing Traps | |
602 | ||
603 | Perl4-to-Perl5 traps from having to do with parsing. | |
604 | ||
605 | =over 4 | |
606 | ||
607 | =item * Parsing | |
608 | ||
609 | Note the space between . and = | |
610 | ||
611 | $string . = "more string"; | |
612 | print $string; | |
54310121 | 613 | |
6dbacca0 | 614 | # perl4 prints: more string |
615 | # perl5 prints: syntax error at - line 1, near ". =" | |
616 | ||
617 | =item * Parsing | |
618 | ||
619 | Better parsing in perl 5 | |
620 | ||
621 | sub foo {} | |
622 | &foo | |
623 | print("hello, world\n"); | |
54310121 | 624 | |
6dbacca0 | 625 | # perl4 prints: hello, world |
626 | # perl5 prints: syntax error | |
627 | ||
628 | =item * Parsing | |
629 | ||
630 | "if it looks like a function, it is a function" rule. | |
631 | ||
632 | ||
633 | ($foo == 1) ? "is one\n" : "is zero\n"; | |
54310121 | 634 | |
6dbacca0 | 635 | # perl4 prints: is zero |
636 | # perl5 warns: "Useless use of a constant in void context" if using -w | |
637 | ||
c12982c8 GS |
638 | =item * Parsing |
639 | ||
640 | String interpolation of the C<$#array> construct differs when braces | |
641 | are to used around the name. | |
642 | ||
9fda99eb | 643 | @a = (1..3); |
c12982c8 GS |
644 | print "${#a}"; |
645 | ||
646 | # perl4 prints: 2 | |
647 | # perl5 fails with syntax error | |
648 | ||
649 | @ = (1..3); | |
650 | print "$#{a}"; | |
651 | ||
652 | # perl4 prints: {a} | |
653 | # perl5 prints: 2 | |
654 | ||
bf1f8817 RB |
655 | =item * Parsing |
656 | ||
657 | When perl sees C<map {> (or C<grep {>), it has to guess whether the C<{> | |
658 | starts a BLOCK or a hash reference. If it guesses wrong, it will report | |
659 | a syntax error near the C<}> and the missing (or unexpected) comma. | |
660 | ||
661 | Use unary C<+> before C<{> on a hash reference, and unary C<+> applied | |
662 | to the first thing in a BLOCK (after C<{>), for perl to guess right all | |
663 | the time. (See L<perlfunc/map>.) | |
664 | ||
6dbacca0 | 665 | =back |
666 | ||
667 | =head2 Numerical Traps | |
668 | ||
669 | Perl4-to-Perl5 traps having to do with numerical operators, | |
670 | operands, or output from same. | |
671 | ||
672 | =over 5 | |
673 | ||
674 | =item * Numerical | |
675 | ||
a9709c40 AS |
676 | Formatted output and significant digits. In general, Perl 5 |
677 | tries to be more precise. For example, on a Solaris Sparc: | |
6dbacca0 | 678 | |
679 | print 7.373504 - 0, "\n"; | |
54310121 | 680 | printf "%20.18f\n", 7.373504 - 0; |
681 | ||
6dbacca0 | 682 | # Perl4 prints: |
a9709c40 AS |
683 | 7.3750399999999996141 |
684 | 7.375039999999999614 | |
54310121 | 685 | |
6dbacca0 | 686 | # Perl5 prints: |
687 | 7.373504 | |
a9709c40 AS |
688 | 7.375039999999999614 |
689 | ||
690 | Notice how the first result looks better in Perl 5. | |
691 | ||
692 | Your results may vary, since your floating point formatting routines | |
693 | and even floating point format may be slightly different. | |
6dbacca0 | 694 | |
695 | =item * Numerical | |
696 | ||
5f05dabc | 697 | This specific item has been deleted. It demonstrated how the auto-increment |
5e378fdf | 698 | operator would not catch when a number went over the signed int limit. Fixed |
a6006777 | 699 | in version 5.003_04. But always be wary when using large integers. |
700 | If in doubt: | |
6dbacca0 | 701 | |
5e378fdf | 702 | use Math::BigInt; |
6dbacca0 | 703 | |
54310121 | 704 | =item * Numerical |
6dbacca0 | 705 | |
706 | Assignment of return values from numeric equality tests | |
707 | does not work in perl5 when the test evaluates to false (0). | |
d1be9408 | 708 | Logical tests now return a null, instead of 0 |
a6006777 | 709 | |
6dbacca0 | 710 | $p = ($test == 1); |
711 | print $p,"\n"; | |
a6006777 | 712 | |
6dbacca0 | 713 | # perl4 prints: 0 |
714 | # perl5 prints: | |
715 | ||
dc848c6f | 716 | Also see L<"General Regular Expression Traps using s///, etc."> |
717 | for another example of this new feature... | |
6dbacca0 | 718 | |
651ad3b1 GS |
719 | =item * Bitwise string ops |
720 | ||
721 | When bitwise operators which can operate upon either numbers or | |
722 | strings (C<& | ^ ~>) are given only strings as arguments, perl4 would | |
723 | treat the operands as bitstrings so long as the program contained a call | |
724 | to the C<vec()> function. perl5 treats the string operands as bitstrings. | |
725 | (See L<perlop/Bitwise String Operators> for more details.) | |
726 | ||
727 | $fred = "10"; | |
728 | $barney = "12"; | |
729 | $betty = $fred & $barney; | |
730 | print "$betty\n"; | |
731 | # Uncomment the next line to change perl4's behavior | |
732 | # ($dummy) = vec("dummy", 0, 0); | |
733 | ||
734 | # Perl4 prints: | |
735 | 8 | |
736 | ||
737 | # Perl5 prints: | |
738 | 10 | |
739 | ||
740 | # If vec() is used anywhere in the program, both print: | |
741 | 10 | |
742 | ||
6dbacca0 | 743 | =back |
744 | ||
745 | =head2 General data type traps | |
746 | ||
747 | Perl4-to-Perl5 traps involving most data-types, and their usage | |
748 | within certain expressions and/or context. | |
749 | ||
750 | =over 5 | |
751 | ||
752 | =item * (Arrays) | |
753 | ||
754 | Negative array subscripts now count from the end of the array. | |
755 | ||
756 | @a = (1, 2, 3, 4, 5); | |
757 | print "The third element of the array is $a[3] also expressed as $a[-2] \n"; | |
54310121 | 758 | |
6dbacca0 | 759 | # perl4 prints: The third element of the array is 4 also expressed as |
760 | # perl5 prints: The third element of the array is 4 also expressed as 4 | |
761 | ||
762 | =item * (Arrays) | |
763 | ||
764 | Setting C<$#array> lower now discards array elements, and makes them | |
765 | impossible to recover. | |
766 | ||
54310121 | 767 | @a = (a,b,c,d,e); |
6dbacca0 | 768 | print "Before: ",join('',@a); |
54310121 | 769 | $#a =1; |
6dbacca0 | 770 | print ", After: ",join('',@a); |
771 | $#a =3; | |
772 | print ", Recovered: ",join('',@a),"\n"; | |
54310121 | 773 | |
6dbacca0 | 774 | # perl4 prints: Before: abcde, After: ab, Recovered: abcd |
775 | # perl5 prints: Before: abcde, After: ab, Recovered: ab | |
776 | ||
777 | =item * (Hashes) | |
778 | ||
779 | Hashes get defined before use | |
780 | ||
54310121 | 781 | local($s,@a,%h); |
6dbacca0 | 782 | die "scalar \$s defined" if defined($s); |
783 | die "array \@a defined" if defined(@a); | |
784 | die "hash \%h defined" if defined(%h); | |
54310121 | 785 | |
6dbacca0 | 786 | # perl4 prints: |
787 | # perl5 dies: hash %h defined | |
788 | ||
475342a6 GS |
789 | Perl will now generate a warning when it sees defined(@a) and |
790 | defined(%h). | |
791 | ||
6dbacca0 | 792 | =item * (Globs) |
793 | ||
794 | glob assignment from variable to variable will fail if the assigned | |
795 | variable is localized subsequent to the assignment | |
796 | ||
797 | @a = ("This is Perl 4"); | |
798 | *b = *a; | |
799 | local(@a); | |
800 | print @b,"\n"; | |
54310121 | 801 | |
6dbacca0 | 802 | # perl4 prints: This is Perl 4 |
803 | # perl5 prints: | |
54310121 | 804 | |
a3cb178b | 805 | =item * (Globs) |
54310121 | 806 | |
a3cb178b GS |
807 | Assigning C<undef> to a glob has no effect in Perl 5. In Perl 4 |
808 | it undefines the associated scalar (but may have other side effects | |
9fda99eb DC |
809 | including SEGVs). Perl 5 will also warn if C<undef> is assigned to a |
810 | typeglob. (Note that assigning C<undef> to a typeglob is different | |
811 | than calling the C<undef> function on a typeglob (C<undef *foo>), which | |
812 | has quite a few effects. | |
813 | ||
814 | $foo = "bar"; | |
815 | *foo = undef; | |
816 | print $foo; | |
817 | ||
818 | # perl4 prints: | |
819 | # perl4 warns: "Use of uninitialized variable" if using -w | |
820 | # perl5 prints: bar | |
821 | # perl5 warns: "Undefined value assigned to typeglob" if using -w | |
5e378fdf | 822 | |
6dbacca0 | 823 | =item * (Scalar String) |
824 | ||
825 | Changes in unary negation (of strings) | |
826 | This change effects both the return value and what it | |
827 | does to auto(magic)increment. | |
828 | ||
829 | $x = "aaa"; | |
830 | print ++$x," : "; | |
831 | print -$x," : "; | |
832 | print ++$x,"\n"; | |
54310121 | 833 | |
6dbacca0 | 834 | # perl4 prints: aab : -0 : 1 |
835 | # perl5 prints: aab : -aab : aac | |
836 | ||
837 | =item * (Constants) | |
838 | ||
839 | perl 4 lets you modify constants: | |
840 | ||
841 | $foo = "x"; | |
842 | &mod($foo); | |
843 | for ($x = 0; $x < 3; $x++) { | |
844 | &mod("a"); | |
845 | } | |
846 | sub mod { | |
847 | print "before: $_[0]"; | |
848 | $_[0] = "m"; | |
849 | print " after: $_[0]\n"; | |
850 | } | |
54310121 | 851 | |
6dbacca0 | 852 | # perl4: |
853 | # before: x after: m | |
854 | # before: a after: m | |
855 | # before: m after: m | |
856 | # before: m after: m | |
54310121 | 857 | |
6dbacca0 | 858 | # Perl5: |
859 | # before: x after: m | |
860 | # Modification of a read-only value attempted at foo.pl line 12. | |
861 | # before: a | |
862 | ||
863 | =item * (Scalars) | |
864 | ||
865 | The behavior is slightly different for: | |
866 | ||
867 | print "$x", defined $x | |
54310121 | 868 | |
6dbacca0 | 869 | # perl 4: 1 |
870 | # perl 5: <no output, $x is not called into existence> | |
871 | ||
872 | =item * (Variable Suicide) | |
873 | ||
874 | Variable suicide behavior is more consistent under Perl 5. | |
aa689395 | 875 | Perl5 exhibits the same behavior for hashes and scalars, |
5f05dabc | 876 | that perl4 exhibits for only scalars. |
6dbacca0 | 877 | |
878 | $aGlobal{ "aKey" } = "global value"; | |
879 | print "MAIN:", $aGlobal{"aKey"}, "\n"; | |
880 | $GlobalLevel = 0; | |
881 | &test( *aGlobal ); | |
882 | ||
883 | sub test { | |
884 | local( *theArgument ) = @_; | |
885 | local( %aNewLocal ); # perl 4 != 5.001l,m | |
54310121 | 886 | $aNewLocal{"aKey"} = "this should never appear"; |
6dbacca0 | 887 | print "SUB: ", $theArgument{"aKey"}, "\n"; |
888 | $aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print | |
889 | $GlobalLevel++; | |
890 | if( $GlobalLevel<4 ) { | |
891 | &test( *aNewLocal ); | |
892 | } | |
893 | } | |
54310121 | 894 | |
6dbacca0 | 895 | # Perl4: |
896 | # MAIN:global value | |
897 | # SUB: global value | |
898 | # SUB: level 0 | |
899 | # SUB: level 1 | |
900 | # SUB: level 2 | |
54310121 | 901 | |
6dbacca0 | 902 | # Perl5: |
903 | # MAIN:global value | |
904 | # SUB: global value | |
905 | # SUB: this should never appear | |
906 | # SUB: this should never appear | |
907 | # SUB: this should never appear | |
908 | ||
84dc3c4d | 909 | =back |
6dbacca0 | 910 | |
911 | =head2 Context Traps - scalar, list contexts | |
912 | ||
913 | =over 5 | |
914 | ||
915 | =item * (list context) | |
916 | ||
917 | The elements of argument lists for formats are now evaluated in list | |
918 | context. This means you can interpolate list values now. | |
919 | ||
920 | @fmt = ("foo","bar","baz"); | |
921 | format STDOUT= | |
922 | @<<<<< @||||| @>>>>> | |
923 | @fmt; | |
924 | . | |
54310121 | 925 | write; |
926 | ||
6dbacca0 | 927 | # perl4 errors: Please use commas to separate fields in file |
928 | # perl5 prints: foo bar baz | |
929 | ||
930 | =item * (scalar context) | |
931 | ||
54310121 | 932 | The C<caller()> function now returns a false value in a scalar context |
933 | if there is no caller. This lets library files determine if they're | |
6dbacca0 | 934 | being required. |
935 | ||
936 | caller() ? (print "You rang?\n") : (print "Got a 0\n"); | |
54310121 | 937 | |
6dbacca0 | 938 | # perl4 errors: There is no caller |
939 | # perl5 prints: Got a 0 | |
5e378fdf | 940 | |
6dbacca0 | 941 | =item * (scalar context) |
942 | ||
943 | The comma operator in a scalar context is now guaranteed to give a | |
944 | scalar context to its arguments. | |
945 | ||
946 | @y= ('a','b','c'); | |
947 | $x = (1, 2, @y); | |
948 | print "x = $x\n"; | |
54310121 | 949 | |
6dbacca0 | 950 | # Perl4 prints: x = c # Thinks list context interpolates list |
951 | # Perl5 prints: x = 3 # Knows scalar uses length of list | |
952 | ||
953 | =item * (list, builtin) | |
954 | ||
9fda99eb DC |
955 | C<sprintf()> is prototyped as ($;@), so its first argument is given scalar |
956 | context. Thus, if passed an array, it will probably not do what you want, | |
957 | unlike Perl 4: | |
6dbacca0 | 958 | |
959 | @z = ('%s%s', 'foo', 'bar'); | |
960 | $x = sprintf(@z); | |
9fda99eb | 961 | print $x; |
54310121 | 962 | |
9fda99eb DC |
963 | # perl4 prints: foobar |
964 | # perl5 prints: 3 | |
6dbacca0 | 965 | |
9fda99eb | 966 | C<printf()> works the same as it did in Perl 4, though: |
6dbacca0 | 967 | |
9fda99eb | 968 | @z = ('%s%s', 'foo', 'bar'); |
6dbacca0 | 969 | printf STDOUT (@z); |
54310121 | 970 | |
6dbacca0 | 971 | # perl4 prints: foobar |
972 | # perl5 prints: foobar | |
973 | ||
6dbacca0 | 974 | =back |
975 | ||
976 | =head2 Precedence Traps | |
977 | ||
978 | Perl4-to-Perl5 traps involving precedence order. | |
979 | ||
f4b17341 GS |
980 | Perl 4 has almost the same precedence rules as Perl 5 for the operators |
981 | that they both have. Perl 4 however, seems to have had some | |
982 | inconsistencies that made the behavior differ from what was documented. | |
983 | ||
84dc3c4d | 984 | =over 5 |
985 | ||
5e378fdf | 986 | =item * Precedence |
987 | ||
8dbef698 JM |
988 | LHS vs. RHS of any assignment operator. LHS is evaluated first |
989 | in perl4, second in perl5; this can affect the relationship | |
990 | between side-effects in sub-expressions. | |
5e378fdf | 991 | |
992 | @arr = ( 'left', 'right' ); | |
993 | $a{shift @arr} = shift @arr; | |
994 | print join( ' ', keys %a ); | |
995 | ||
996 | # perl4 prints: left | |
997 | # perl5 prints: right | |
998 | ||
999 | =item * Precedence | |
6dbacca0 | 1000 | |
1001 | These are now semantic errors because of precedence: | |
1002 | ||
1003 | @list = (1,2,3,4,5); | |
1004 | %map = ("a",1,"b",2,"c",3,"d",4); | |
1005 | $n = shift @list + 2; # first item in list plus 2 | |
1006 | print "n is $n, "; | |
1007 | $m = keys %map + 2; # number of items in hash plus 2 | |
1008 | print "m is $m\n"; | |
54310121 | 1009 | |
6dbacca0 | 1010 | # perl4 prints: n is 3, m is 6 |
1011 | # perl5 errors and fails to compile | |
1012 | ||
1013 | =item * Precedence | |
a0d0e21e | 1014 | |
4633a7c4 LW |
1015 | The precedence of assignment operators is now the same as the precedence |
1016 | of assignment. Perl 4 mistakenly gave them the precedence of the associated | |
1017 | operator. So you now must parenthesize them in expressions like | |
1018 | ||
1019 | /foo/ ? ($a += 2) : ($a -= 2); | |
a6006777 | 1020 | |
4633a7c4 LW |
1021 | Otherwise |
1022 | ||
6dbacca0 | 1023 | /foo/ ? $a += 2 : $a -= 2 |
4633a7c4 LW |
1024 | |
1025 | would be erroneously parsed as | |
1026 | ||
1027 | (/foo/ ? $a += 2 : $a) -= 2; | |
1028 | ||
1029 | On the other hand, | |
1030 | ||
54310121 | 1031 | $a += /foo/ ? 1 : 2; |
4633a7c4 LW |
1032 | |
1033 | now works as a C programmer would expect. | |
1034 | ||
6dbacca0 | 1035 | =item * Precedence |
4633a7c4 | 1036 | |
6dbacca0 | 1037 | open FOO || die; |
a0d0e21e | 1038 | |
5f05dabc | 1039 | is now incorrect. You need parentheses around the filehandle. |
1040 | Otherwise, perl5 leaves the statement as its default precedence: | |
a0d0e21e | 1041 | |
6dbacca0 | 1042 | open(FOO || die); |
54310121 | 1043 | |
6dbacca0 | 1044 | # perl4 opens or dies |
9fda99eb | 1045 | # perl5 opens FOO, dying only if 'FOO' is false, i.e. never |
a0d0e21e | 1046 | |
6dbacca0 | 1047 | =item * Precedence |
a0d0e21e | 1048 | |
6dbacca0 | 1049 | perl4 gives the special variable, C<$:> precedence, where perl5 |
1050 | treats C<$::> as main C<package> | |
a0d0e21e | 1051 | |
6dbacca0 | 1052 | $a = "x"; print "$::a"; |
54310121 | 1053 | |
6dbacca0 | 1054 | # perl 4 prints: -:a |
1055 | # perl 5 prints: x | |
5e378fdf | 1056 | |
6dbacca0 | 1057 | =item * Precedence |
a0d0e21e | 1058 | |
f4b17341 GS |
1059 | perl4 had buggy precedence for the file test operators vis-a-vis |
1060 | the assignment operators. Thus, although the precedence table | |
1061 | for perl4 leads one to believe C<-e $foo .= "q"> should parse as | |
1062 | C<((-e $foo) .= "q")>, it actually parses as C<(-e ($foo .= "q"))>. | |
1063 | In perl5, the precedence is as documented. | |
54310121 | 1064 | |
1065 | -e $foo .= "q" | |
a0d0e21e | 1066 | |
6dbacca0 | 1067 | # perl4 prints: no output |
1068 | # perl5 prints: Can't modify -e in concatenation | |
a0d0e21e | 1069 | |
f4b17341 GS |
1070 | =item * Precedence |
1071 | ||
1072 | In perl4, keys(), each() and values() were special high-precedence operators | |
1073 | that operated on a single hash, but in perl5, they are regular named unary | |
1074 | operators. As documented, named unary operators have lower precedence | |
1075 | than the arithmetic and concatenation operators C<+ - .>, but the perl4 | |
1076 | variants of these operators actually bind tighter than C<+ - .>. | |
1077 | Thus, for: | |
1078 | ||
1079 | %foo = 1..10; | |
1080 | print keys %foo - 1 | |
1081 | ||
1082 | # perl4 prints: 4 | |
1083 | # perl5 prints: Type of arg 1 to keys must be hash (not subtraction) | |
1084 | ||
1085 | The perl4 behavior was probably more useful, if less consistent. | |
1086 | ||
6dbacca0 | 1087 | =back |
1088 | ||
1089 | =head2 General Regular Expression Traps using s///, etc. | |
1090 | ||
1091 | All types of RE traps. | |
1092 | ||
1093 | =over 5 | |
1094 | ||
1095 | =item * Regular Expression | |
1096 | ||
1097 | C<s'$lhs'$rhs'> now does no interpolation on either side. It used to | |
19799a22 | 1098 | interpolate $lhs but not $rhs. (And still does not match a literal |
6dbacca0 | 1099 | '$' in string) |
1100 | ||
1101 | $a=1;$b=2; | |
1102 | $string = '1 2 $a $b'; | |
1103 | $string =~ s'$a'$b'; | |
1104 | print $string,"\n"; | |
54310121 | 1105 | |
6dbacca0 | 1106 | # perl4 prints: $b 2 $a $b |
1107 | # perl5 prints: 1 2 $a $b | |
1108 | ||
1109 | =item * Regular Expression | |
a0d0e21e LW |
1110 | |
1111 | C<m//g> now attaches its state to the searched string rather than the | |
6dbacca0 | 1112 | regular expression. (Once the scope of a block is left for the sub, the |
1113 | state of the searched string is lost) | |
1114 | ||
1115 | $_ = "ababab"; | |
1116 | while(m/ab/g){ | |
1117 | &doit("blah"); | |
1118 | } | |
1119 | sub doit{local($_) = shift; print "Got $_ "} | |
54310121 | 1120 | |
9fda99eb | 1121 | # perl4 prints: Got blah Got blah Got blah Got blah |
6dbacca0 | 1122 | # perl5 prints: infinite loop blah... |
1123 | ||
1124 | =item * Regular Expression | |
1125 | ||
68dc0745 | 1126 | Currently, if you use the C<m//o> qualifier on a regular expression |
1127 | within an anonymous sub, I<all> closures generated from that anonymous | |
1128 | sub will use the regular expression as it was compiled when it was used | |
1129 | the very first time in any such closure. For instance, if you say | |
1130 | ||
1131 | sub build_match { | |
1132 | my($left,$right) = @_; | |
1133 | return sub { $_[0] =~ /$left stuff $right/o; }; | |
1134 | } | |
9fda99eb DC |
1135 | $good = build_match('foo','bar'); |
1136 | $bad = build_match('baz','blarch'); | |
1137 | print $good->('foo stuff bar') ? "ok\n" : "not ok\n"; | |
1138 | print $bad->('baz stuff blarch') ? "ok\n" : "not ok\n"; | |
1139 | print $bad->('foo stuff bar') ? "not ok\n" : "ok\n"; | |
1140 | ||
1141 | For most builds of Perl5, this will print: | |
1142 | ok | |
1143 | not ok | |
1144 | not ok | |
68dc0745 | 1145 | |
1146 | build_match() will always return a sub which matches the contents of | |
19799a22 | 1147 | $left and $right as they were the I<first> time that build_match() |
68dc0745 | 1148 | was called, not as they are in the current call. |
1149 | ||
68dc0745 | 1150 | =item * Regular Expression |
1151 | ||
6dbacca0 | 1152 | If no parentheses are used in a match, Perl4 sets C<$+> to |
1153 | the whole match, just like C<$&>. Perl5 does not. | |
1154 | ||
1155 | "abcdef" =~ /b.*e/; | |
1156 | print "\$+ = $+\n"; | |
54310121 | 1157 | |
6dbacca0 | 1158 | # perl4 prints: bcde |
1159 | # perl5 prints: | |
1160 | ||
1161 | =item * Regular Expression | |
1162 | ||
1163 | substitution now returns the null string if it fails | |
1164 | ||
1165 | $string = "test"; | |
1166 | $value = ($string =~ s/foo//); | |
1167 | print $value, "\n"; | |
54310121 | 1168 | |
6dbacca0 | 1169 | # perl4 prints: 0 |
1170 | # perl5 prints: | |
1171 | ||
1172 | Also see L<Numerical Traps> for another example of this new feature. | |
1173 | ||
1174 | =item * Regular Expression | |
1175 | ||
54310121 | 1176 | C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no |
1177 | backtick expansion | |
6dbacca0 | 1178 | |
1179 | $string = ""; | |
1180 | $string =~ s`^`hostname`; | |
1181 | print $string, "\n"; | |
54310121 | 1182 | |
6dbacca0 | 1183 | # perl4 prints: <the local hostname> |
1184 | # perl5 prints: hostname | |
1185 | ||
1186 | =item * Regular Expression | |
1187 | ||
1188 | Stricter parsing of variables used in regular expressions | |
1189 | ||
1190 | s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o; | |
54310121 | 1191 | |
6dbacca0 | 1192 | # perl4: compiles w/o error |
1193 | # perl5: with Scalar found where operator expected ..., near "$opt$plus" | |
1194 | ||
1195 | an added component of this example, apparently from the same script, is | |
1196 | the actual value of the s'd string after the substitution. | |
1197 | C<[$opt]> is a character class in perl4 and an array subscript in perl5 | |
1198 | ||
54310121 | 1199 | $grpc = 'a'; |
6dbacca0 | 1200 | $opt = 'r'; |
1201 | $_ = 'bar'; | |
1202 | s/^([^$grpc]*$grpc[$opt]?)/foo/; | |
1203 | print ; | |
54310121 | 1204 | |
6dbacca0 | 1205 | # perl4 prints: foo |
1206 | # perl5 prints: foobar | |
1207 | ||
1208 | =item * Regular Expression | |
1209 | ||
1210 | Under perl5, C<m?x?> matches only once, like C<?x?>. Under perl4, it matched | |
1211 | repeatedly, like C</x/> or C<m!x!>. | |
1212 | ||
1213 | $test = "once"; | |
1214 | sub match { $test =~ m?once?; } | |
1215 | &match(); | |
1216 | if( &match() ) { | |
1217 | # m?x? matches more then once | |
1218 | print "perl4\n"; | |
54310121 | 1219 | } else { |
6dbacca0 | 1220 | # m?x? matches only once |
54310121 | 1221 | print "perl5\n"; |
6dbacca0 | 1222 | } |
54310121 | 1223 | |
6dbacca0 | 1224 | # perl4 prints: perl4 |
1225 | # perl5 prints: perl5 | |
a0d0e21e | 1226 | |
a0d0e21e | 1227 | |
6dbacca0 | 1228 | =back |
1229 | ||
1230 | =head2 Subroutine, Signal, Sorting Traps | |
a0d0e21e | 1231 | |
6dbacca0 | 1232 | The general group of Perl4-to-Perl5 traps having to do with |
1233 | Signals, Sorting, and their related subroutines, as well as | |
1234 | general subroutine traps. Includes some OS-Specific traps. | |
a0d0e21e | 1235 | |
6dbacca0 | 1236 | =over 5 |
a0d0e21e | 1237 | |
6dbacca0 | 1238 | =item * (Signals) |
a0d0e21e | 1239 | |
6dbacca0 | 1240 | Barewords that used to look like strings to Perl will now look like subroutine |
1241 | calls if a subroutine by that name is defined before the compiler sees them. | |
a0d0e21e | 1242 | |
6dbacca0 | 1243 | sub SeeYa { warn"Hasta la vista, baby!" } |
1244 | $SIG{'TERM'} = SeeYa; | |
1245 | print "SIGTERM is now $SIG{'TERM'}\n"; | |
54310121 | 1246 | |
9fda99eb DC |
1247 | # perl4 prints: SIGTERM is now main'SeeYa |
1248 | # perl5 prints: SIGTERM is now main::1 (and warns "Hasta la vista, baby!") | |
a0d0e21e | 1249 | |
6dbacca0 | 1250 | Use B<-w> to catch this one |
a0d0e21e | 1251 | |
6dbacca0 | 1252 | =item * (Sort Subroutine) |
a0d0e21e | 1253 | |
6dbacca0 | 1254 | reverse is no longer allowed as the name of a sort subroutine. |
a0d0e21e | 1255 | |
6dbacca0 | 1256 | sub reverse{ print "yup "; $a <=> $b } |
9fda99eb | 1257 | print sort reverse (2,1,3); |
54310121 | 1258 | |
9fda99eb DC |
1259 | # perl4 prints: yup yup 123 |
1260 | # perl5 prints: 123 | |
1261 | # perl5 warns (if using -w): Ambiguous call resolved as CORE::reverse() | |
a0d0e21e | 1262 | |
b996531f | 1263 | =item * warn() won't let you specify a filehandle. |
1264 | ||
1265 | Although it _always_ printed to STDERR, warn() would let you specify a | |
1266 | filehandle in perl4. With perl5 it does not. | |
5e378fdf | 1267 | |
1268 | warn STDERR "Foo!"; | |
1269 | ||
1270 | # perl4 prints: Foo! | |
54310121 | 1271 | # perl5 prints: String found where operator expected |
5e378fdf | 1272 | |
6dbacca0 | 1273 | =back |
a0d0e21e | 1274 | |
6dbacca0 | 1275 | =head2 OS Traps |
1276 | ||
1277 | =over 5 | |
1278 | ||
1279 | =item * (SysV) | |
1280 | ||
54310121 | 1281 | Under HPUX, and some other SysV OSes, one had to reset any signal handler, |
1282 | within the signal handler function, each time a signal was handled with | |
1283 | perl4. With perl5, the reset is now done correctly. Any code relying | |
6dbacca0 | 1284 | on the handler _not_ being reset will have to be reworked. |
1285 | ||
a6006777 | 1286 | Since version 5.002, Perl uses sigaction() under SysV. |
6dbacca0 | 1287 | |
1288 | sub gotit { | |
54310121 | 1289 | print "Got @_... "; |
1290 | } | |
6dbacca0 | 1291 | $SIG{'INT'} = 'gotit'; |
54310121 | 1292 | |
6dbacca0 | 1293 | $| = 1; |
1294 | $pid = fork; | |
1295 | if ($pid) { | |
1296 | kill('INT', $pid); | |
1297 | sleep(1); | |
1298 | kill('INT', $pid); | |
54310121 | 1299 | } else { |
6dbacca0 | 1300 | while (1) {sleep(10);} |
54310121 | 1301 | } |
1302 | ||
6dbacca0 | 1303 | # perl4 (HPUX) prints: Got INT... |
1304 | # perl5 (HPUX) prints: Got INT... Got INT... | |
1305 | ||
1306 | =item * (SysV) | |
1307 | ||
c47ff5f1 | 1308 | Under SysV OSes, C<seek()> on a file opened to append C<<< >> >>> now does |
54310121 | 1309 | the right thing w.r.t. the fopen() manpage. e.g., - When a file is opened |
6dbacca0 | 1310 | for append, it is impossible to overwrite information already in |
1311 | the file. | |
1312 | ||
1313 | open(TEST,">>seek.test"); | |
54310121 | 1314 | $start = tell TEST ; |
6dbacca0 | 1315 | foreach(1 .. 9){ |
1316 | print TEST "$_ "; | |
1317 | } | |
1318 | $end = tell TEST ; | |
1319 | seek(TEST,$start,0); | |
1320 | print TEST "18 characters here"; | |
54310121 | 1321 | |
6dbacca0 | 1322 | # perl4 (solaris) seek.test has: 18 characters here |
1323 | # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here | |
a0d0e21e | 1324 | |
a0d0e21e | 1325 | |
a0d0e21e | 1326 | |
6dbacca0 | 1327 | =back |
a0d0e21e | 1328 | |
6dbacca0 | 1329 | =head2 Interpolation Traps |
a0d0e21e | 1330 | |
8b0a4b75 | 1331 | Perl4-to-Perl5 traps having to do with how things get interpolated |
1332 | within certain expressions, statements, contexts, or whatever. | |
1333 | ||
6dbacca0 | 1334 | =over 5 |
a0d0e21e | 1335 | |
6dbacca0 | 1336 | =item * Interpolation |
a0d0e21e | 1337 | |
6dbacca0 | 1338 | @ now always interpolates an array in double-quotish strings. |
1339 | ||
54310121 | 1340 | print "To: someone@somewhere.com\n"; |
1341 | ||
6dbacca0 | 1342 | # perl4 prints: To:someone@somewhere.com |
8593bda5 GS |
1343 | # perl < 5.6.1, error : In string, @somewhere now must be written as \@somewhere |
1344 | # perl >= 5.6.1, warning : Possible unintended interpolation of @somewhere in string | |
6dbacca0 | 1345 | |
1346 | =item * Interpolation | |
1347 | ||
1fa58bec | 1348 | Double-quoted strings may no longer end with an unescaped $. |
6dbacca0 | 1349 | |
1350 | $foo = "foo$"; | |
1fa58bec | 1351 | print "foo is $foo\n"; |
54310121 | 1352 | |
1fa58bec | 1353 | # perl4 prints: foo is foo$ |
6dbacca0 | 1354 | # perl5 errors: Final $ should be \$ or $name |
1355 | ||
1356 | Note: perl5 DOES NOT error on the terminating @ in $bar | |
1357 | ||
1358 | =item * Interpolation | |
a0d0e21e | 1359 | |
8b0a4b75 | 1360 | Perl now sometimes evaluates arbitrary expressions inside braces that occur |
1361 | within double quotes (usually when the opening brace is preceded by C<$> | |
1362 | or C<@>). | |
1363 | ||
1364 | @www = "buz"; | |
1365 | $foo = "foo"; | |
1366 | $bar = "bar"; | |
1367 | sub foo { return "bar" }; | |
1368 | print "|@{w.w.w}|${main'foo}|"; | |
1369 | ||
1370 | # perl4 prints: |@{w.w.w}|foo| | |
1371 | # perl5 prints: |buz|bar| | |
1372 | ||
1373 | Note that you can C<use strict;> to ward off such trappiness under perl5. | |
1374 | ||
1375 | =item * Interpolation | |
1376 | ||
9fda99eb DC |
1377 | The construct "this is $$x" used to interpolate the pid at that point, but |
1378 | now tries to dereference $x. C<$$> by itself still works fine, however. | |
748a9306 | 1379 | |
9fda99eb DC |
1380 | $s = "a reference"; |
1381 | $x = *s; | |
6dbacca0 | 1382 | print "this is $$x\n"; |
748a9306 | 1383 | |
6dbacca0 | 1384 | # perl4 prints: this is XXXx (XXX is the current pid) |
9fda99eb | 1385 | # perl5 prints: this is a reference |
6dbacca0 | 1386 | |
1387 | =item * Interpolation | |
1388 | ||
54310121 | 1389 | Creation of hashes on the fly with C<eval "EXPR"> now requires either both |
1390 | C<$>'s to be protected in the specification of the hash name, or both curlies | |
6dbacca0 | 1391 | to be protected. If both curlies are protected, the result will be compatible |
1392 | with perl4 and perl5. This is a very common practice, and should be changed | |
1393 | to use the block form of C<eval{}> if possible. | |
c07a80fd | 1394 | |
6dbacca0 | 1395 | $hashname = "foobar"; |
1396 | $key = "baz"; | |
1397 | $value = 1234; | |
1398 | eval "\$$hashname{'$key'} = q|$value|"; | |
1399 | (defined($foobar{'baz'})) ? (print "Yup") : (print "Nope"); | |
1400 | ||
1401 | # perl4 prints: Yup | |
1402 | # perl5 prints: Nope | |
1403 | ||
1404 | Changing | |
1405 | ||
1406 | eval "\$$hashname{'$key'} = q|$value|"; | |
c07a80fd | 1407 | |
1408 | to | |
1409 | ||
6dbacca0 | 1410 | eval "\$\$hashname{'$key'} = q|$value|"; |
c07a80fd | 1411 | |
6dbacca0 | 1412 | causes the following result: |
c07a80fd | 1413 | |
6dbacca0 | 1414 | # perl4 prints: Nope |
1415 | # perl5 prints: Yup | |
c07a80fd | 1416 | |
6dbacca0 | 1417 | or, changing to |
a0d0e21e | 1418 | |
6dbacca0 | 1419 | eval "\$$hashname\{'$key'\} = q|$value|"; |
1420 | ||
1421 | causes the following result: | |
1422 | ||
1423 | # perl4 prints: Yup | |
1424 | # perl5 prints: Yup | |
1425 | # and is compatible for both versions | |
1426 | ||
1427 | ||
1428 | =item * Interpolation | |
1429 | ||
1430 | perl4 programs which unconsciously rely on the bugs in earlier perl versions. | |
1431 | ||
1432 | perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"' | |
54310121 | 1433 | |
6dbacca0 | 1434 | # perl4 prints: This is not perl5 |
1435 | # perl5 prints: This is perl5 | |
1436 | ||
1437 | =item * Interpolation | |
1438 | ||
418272e4 CW |
1439 | You also have to be careful about array and hash brackets during |
1440 | interpolation. | |
1441 | ||
1442 | print "$foo[" | |
1443 | ||
1444 | perl 4 prints: [ | |
1445 | perl 5 prints: syntax error | |
6dbacca0 | 1446 | |
1447 | print "$foo{" | |
1448 | ||
1449 | perl 4 prints: { | |
1450 | perl 5 prints: syntax error | |
1451 | ||
418272e4 CW |
1452 | Perl 5 is expecting to find an index or key name following the respective |
1453 | brackets, as well as an ending bracket of the appropriate type. In order | |
1454 | to mimic the behavior of Perl 4, you must escape the bracket like so. | |
1455 | ||
1456 | print "$foo\["; | |
1457 | print "$foo\{"; | |
1458 | ||
6dbacca0 | 1459 | =item * Interpolation |
1460 | ||
1461 | Similarly, watch out for: | |
1462 | ||
9fda99eb | 1463 | $foo = "baz"; |
6dbacca0 | 1464 | print "\$$foo{bar}\n"; |
54310121 | 1465 | |
9fda99eb | 1466 | # perl4 prints: $baz{bar} |
6dbacca0 | 1467 | # perl5 prints: $ |
1468 | ||
9fda99eb DC |
1469 | Perl 5 is looking for C<$foo{bar}> which doesn't exist, but perl 4 is |
1470 | happy just to expand $foo to "baz" by itself. Watch out for this | |
6dbacca0 | 1471 | especially in C<eval>'s. |
1472 | ||
1473 | =item * Interpolation | |
1474 | ||
1475 | C<qq()> string passed to C<eval> | |
1476 | ||
1477 | eval qq( | |
1478 | foreach \$y (keys %\$x\) { | |
1479 | \$count++; | |
1480 | } | |
1481 | ); | |
54310121 | 1482 | |
6dbacca0 | 1483 | # perl4 runs this ok |
54310121 | 1484 | # perl5 prints: Can't find string terminator ")" |
a0d0e21e | 1485 | |
6dbacca0 | 1486 | =back |
1487 | ||
1488 | =head2 DBM Traps | |
1489 | ||
1490 | General DBM traps. | |
1491 | ||
1492 | =over 5 | |
1493 | ||
1494 | =item * DBM | |
1495 | ||
1496 | Existing dbm databases created under perl4 (or any other dbm/ndbm tool) | |
1497 | may cause the same script, run under perl5, to fail. The build of perl5 | |
1498 | must have been linked with the same dbm/ndbm as the default for C<dbmopen()> | |
1499 | to function properly without C<tie>'ing to an extension dbm implementation. | |
1500 | ||
1501 | dbmopen (%dbm, "file", undef); | |
1502 | print "ok\n"; | |
1503 | ||
1504 | # perl4 prints: ok | |
1505 | # perl5 prints: ok (IFF linked with -ldbm or -lndbm) | |
1506 | ||
1507 | ||
1508 | =item * DBM | |
1509 | ||
1510 | Existing dbm databases created under perl4 (or any other dbm/ndbm tool) | |
1511 | may cause the same script, run under perl5, to fail. The error generated | |
1512 | when exceeding the limit on the key/value size will cause perl5 to exit | |
1513 | immediately. | |
1514 | ||
1515 | dbmopen(DB, "testdb",0600) || die "couldn't open db! $!"; | |
1516 | $DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm | |
1517 | print "YUP\n"; | |
1518 | ||
1519 | # perl4 prints: | |
1520 | dbm store returned -1, errno 28, key "trap" at - line 3. | |
1521 | YUP | |
1522 | ||
1523 | # perl5 prints: | |
1524 | dbm store returned -1, errno 28, key "trap" at - line 3. | |
a0d0e21e LW |
1525 | |
1526 | =back | |
6dbacca0 | 1527 | |
1528 | =head2 Unclassified Traps | |
1529 | ||
1530 | Everything else. | |
1531 | ||
84dc3c4d | 1532 | =over 5 |
1533 | ||
5db417f7 | 1534 | =item * C<require>/C<do> trap using returned value |
6dbacca0 | 1535 | |
1536 | If the file doit.pl has: | |
1537 | ||
1538 | sub foo { | |
1539 | $rc = do "./do.pl"; | |
1540 | return 8; | |
54310121 | 1541 | } |
6dbacca0 | 1542 | print &foo, "\n"; |
1543 | ||
1544 | And the do.pl file has the following single line: | |
1545 | ||
1546 | return 3; | |
1547 | ||
1548 | Running doit.pl gives the following: | |
1549 | ||
1550 | # perl 4 prints: 3 (aborts the subroutine early) | |
54310121 | 1551 | # perl 5 prints: 8 |
6dbacca0 | 1552 | |
1553 | Same behavior if you replace C<do> with C<require>. | |
1554 | ||
5db417f7 TB |
1555 | =item * C<split> on empty string with LIMIT specified |
1556 | ||
9fda99eb | 1557 | $string = ''; |
5db417f7 TB |
1558 | @list = split(/foo/, $string, 2) |
1559 | ||
1560 | Perl4 returns a one element list containing the empty string but Perl5 | |
1561 | returns an empty list. | |
1562 | ||
6dbacca0 | 1563 | =back |
1564 | ||
54310121 | 1565 | As always, if any of these are ever officially declared as bugs, |
6dbacca0 | 1566 | they'll be fixed and removed. |
1567 |