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