This is a live mirror of the Perl 5 development currently hosted at https://github.com/perl/perl5
POD spelling patches
[perl5.git] / pod / perltrap.pod
CommitLineData
a0d0e21e
LW
1=head1 NAME
2
3perltrap - Perl traps for the unwary
4
5=head1 DESCRIPTION
6
cb1a09d0
AD
7The biggest trap of all is forgetting to use the B<-w> switch; see
8L<perlrun>. The second biggest trap is not making your entire program
9runnable under C<use strict>.
a0d0e21e
LW
10
11=head2 Awk Traps
12
13Accustomed B<awk> users should take special note of the following:
14
15=over 4
16
17=item *
18
19The English module, loaded via
20
21 use English;
22
23allows you to refer to special variables (like $RS) as
24though they were in B<awk>; see L<perlvar> for details.
25
26=item *
27
28Semicolons are required after all simple statements in Perl (except
29at the end of a block). Newline is not a statement delimiter.
30
31=item *
32
33Curly brackets are required on C<if>s and C<while>s.
34
35=item *
36
37Variables begin with "$" or "@" in Perl.
38
39=item *
40
41Arrays index from 0. Likewise string positions in substr() and
42index().
43
44=item *
45
46You have to decide whether your array has numeric or string indices.
47
48=item *
49
50Associative array values do not spring into existence upon mere
51reference.
52
53=item *
54
55You have to decide whether you want to use string or numeric
56comparisons.
57
58=item *
59
60Reading an input line does not split it for you. You get to split it
6dbacca0 61yourself to an array. And the split() operator has different
a0d0e21e
LW
62arguments.
63
64=item *
65
66The current input line is normally in $_, not $0. It generally does
67not have the newline stripped. ($0 is the name of the program
68executed.) See L<perlvar>.
69
70=item *
71
72$<I<digit>> does not refer to fields--it refers to substrings matched by
73the last match pattern.
74
75=item *
76
77The print() statement does not add field and record separators unless
78you set C<$,> and C<$.>. You can set $OFS and $ORS if you're using
79the English module.
80
81=item *
82
83You must open your files before you print to them.
84
85=item *
86
87The range operator is "..", not comma. The comma operator works as in
88C.
89
90=item *
91
92The match operator is "=~", not "~". ("~" is the one's complement
93operator, as in C.)
94
95=item *
96
97The exponentiation operator is "**", not "^". "^" is the XOR
98operator, as in C. (You know, one could get the feeling that B<awk> is
99basically incompatible with C.)
100
101=item *
102
103The concatenation operator is ".", not the null string. (Using the
104null string would render C</pat/ /pat/> unparsable, since the third slash
105would be interpreted as a division operator--the tokener is in fact
106slightly context sensitive for operators like "/", "?", and ">".
107And in fact, "." itself can be the beginning of a number.)
108
109=item *
110
111The C<next>, C<exit>, and C<continue> keywords work differently.
112
113=item *
114
115
116The following variables work differently:
117
118 Awk Perl
119 ARGC $#ARGV or scalar @ARGV
120 ARGV[0] $0
121 FILENAME $ARGV
122 FNR $. - something
123 FS (whatever you like)
124 NF $#Fld, or some such
125 NR $.
126 OFMT $#
127 OFS $,
128 ORS $\
129 RLENGTH length($&)
130 RS $/
131 RSTART length($`)
132 SUBSEP $;
133
134=item *
135
136You cannot set $RS to a pattern, only a string.
137
138=item *
139
140When in doubt, run the B<awk> construct through B<a2p> and see what it
141gives you.
142
143=back
144
145=head2 C Traps
146
147Cerebral C programmers should take note of the following:
148
149=over 4
150
151=item *
152
153Curly brackets are required on C<if>'s and C<while>'s.
154
155=item *
156
157You must use C<elsif> rather than C<else if>.
158
159=item *
160
161The C<break> and C<continue> keywords from C become in
162Perl C<last> and C<next>, respectively.
163Unlike in C, these do I<NOT> work within a C<do { } while> construct.
164
165=item *
166
167There's no switch statement. (But it's easy to build one on the fly.)
168
169=item *
170
171Variables begin with "$" or "@" in Perl.
172
173=item *
174
6dbacca0 175C<printf()> does not implement the "*" format for interpolating
a0d0e21e
LW
176field widths, but it's trivial to use interpolation of double-quoted
177strings to achieve the same effect.
178
179=item *
180
181Comments begin with "#", not "/*".
182
183=item *
184
185You can't take the address of anything, although a similar operator
186in Perl 5 is the backslash, which creates a reference.
187
188=item *
189
4633a7c4
LW
190C<ARGV> must be capitalized. C<$ARGV[0]> is C's C<argv[1]>, and C<argv[0]>
191ends up in C<$0>.
a0d0e21e
LW
192
193=item *
194
195System calls such as link(), unlink(), rename(), etc. return nonzero for
196success, not 0.
197
198=item *
199
200Signal handlers deal with signal names, not numbers. Use C<kill -l>
201to find their names on your system.
202
203=back
204
205=head2 Sed Traps
206
207Seasoned B<sed> programmers should take note of the following:
208
209=over 4
210
211=item *
212
213Backreferences in substitutions use "$" rather than "\".
214
215=item *
216
217The pattern matching metacharacters "(", ")", and "|" do not have backslashes
218in front.
219
220=item *
221
222The range operator is C<...>, rather than comma.
223
224=back
225
226=head2 Shell Traps
227
228Sharp shell programmers should take note of the following:
229
230=over 4
231
232=item *
233
6dbacca0 234The backtick operator does variable interpolation without regard to
a0d0e21e
LW
235the presence of single quotes in the command.
236
237=item *
238
239The backtick operator does no translation of the return value, unlike B<csh>.
240
241=item *
242
243Shells (especially B<csh>) do several levels of substitution on each
244command line. Perl does substitution only in certain constructs
245such as double quotes, backticks, angle brackets, and search patterns.
246
247=item *
248
249Shells interpret scripts a little bit at a time. Perl compiles the
250entire program before executing it (except for C<BEGIN> blocks, which
251execute at compile time).
252
253=item *
254
255The arguments are available via @ARGV, not $1, $2, etc.
256
257=item *
258
259The environment is not automatically made available as separate scalar
260variables.
261
262=back
263
264=head2 Perl Traps
265
266Practicing Perl Programmers should take note of the following:
267
268=over 4
269
270=item *
271
272Remember that many operations behave differently in a list
273context than they do in a scalar one. See L<perldata> for details.
274
275=item *
276
277Avoid barewords if you can, especially all lower-case ones.
278You can't tell just by looking at it whether a bareword is
279a function or a string. By using quotes on strings and
280parens on function calls, you won't ever get them confused.
281
282=item *
283
284You cannot discern from mere inspection which built-ins
285are unary operators (like chop() and chdir())
286and which are list operators (like print() and unlink()).
287(User-defined subroutines can B<only> be list operators, never
288unary ones.) See L<perlop>.
289
290=item *
291
748a9306 292People have a hard time remembering that some functions
a0d0e21e
LW
293default to $_, or @ARGV, or whatever, but that others which
294you might expect to do not.
295
6dbacca0 296=item *
a0d0e21e 297
748a9306
LW
298The <FH> construct is not the name of the filehandle, it is a readline
299operation on that handle. The data read is only assigned to $_ if the
300file read is the sole condition in a while loop:
301
302 while (<FH>) { }
303 while ($_ = <FH>) { }..
304 <FH>; # data discarded!
305
6dbacca0 306=item *
748a9306 307
a0d0e21e
LW
308Remember not to use "C<=>" when you need "C<=~>";
309these two constructs are quite different:
310
311 $x = /foo/;
312 $x =~ /foo/;
313
314=item *
315
316The C<do {}> construct isn't a real loop that you can use
317loop control on.
318
319=item *
320
6dbacca0 321Use C<my()> for local variables whenever you can get away with
a0d0e21e 322it (but see L<perlform> for where you can't).
6dbacca0 323Using C<local()> actually gives a local value to a global
a0d0e21e
LW
324variable, which leaves you open to unforeseen side-effects
325of dynamic scoping.
326
c07a80fd
PP
327=item *
328
329If you localize an exported variable in a module, its exported value will
330not change. The local name becomes an alias to a new value but the
331external name is still an alias for the original.
332
a0d0e21e
LW
333=back
334
6dbacca0 335=head2 Perl4 to Perl5 Traps
a0d0e21e 336
6dbacca0
PP
337Practicing Perl4 Programmers should take note of the following
338Perl4-to-Perl5 specific traps.
339
340They're crudely ordered according to the following list:
a0d0e21e
LW
341
342=over 4
343
6dbacca0 344=item Discontinuance, Deprecation, and BugFix traps
a0d0e21e 345
6dbacca0
PP
346Anything that's been fixed as a perl4 bug, removed as a perl4 feature
347or deprecated as a perl4 feature with the intent to encourage usage of
348some other perl5 feature.
a0d0e21e 349
6dbacca0 350=item Parsing Traps
748a9306 351
6dbacca0 352Traps that appear to stem from the new parser.
a0d0e21e 353
6dbacca0 354=item Numerical Traps
a0d0e21e 355
6dbacca0 356Traps having to do with numerical or mathematical operators.
a0d0e21e 357
6dbacca0 358=item General data type traps
a0d0e21e 359
6dbacca0 360Traps involving perl standard data types.
a0d0e21e 361
6dbacca0
PP
362=item Context Traps - scalar, list contexts
363
364Traps related to context within lists, scalar statements/declarations.
365
366=item Precedence Traps
367
368Traps related to the precedence of parsing, evaluation, and execution of
369code.
370
371=item General Regular Expression Traps using s///, etc.
372
373Traps related to the use of pattern matching.
374
375=item Subroutine, Signal, Sorting Traps
376
377Traps related to the use of signals and signal handlers, general subroutines,
378and sorting, along with sorting subroutines.
379
380=item OS Traps
381
382OS-specific traps.
383
384=item DBM Traps
385
386Traps specific to the use of C<dbmopen()>, and specific dbm implementations.
387
388=item Unclassified Traps
389
390Everything else.
391
392=back
393
394If you find an example of a conversion trap that is not listed here,
395please submit it to Bill Middleton F<wjm@best.com> for inclusion.
396Also note that at least some of these can be caught with C<-w>.
397
398=head2 Discontinuance, Deprecation, and BugFix traps
399
400Anything that has been discontinued, deprecated, or fixed as
401a bug from perl4.
a0d0e21e 402
6dbacca0
PP
403=over 4
404
405=item * Discontinuance
406
407Symbols starting with "_" are no longer forced into package main, except
408for C<$_> itself (and C<@_>, etc.).
409
410 package test;
411 $_legacy = 1;
cb1a09d0 412
6dbacca0
PP
413 package main;
414 print "\$_legacy is ",$_legacy,"\n";
415
416 # perl4 prints: $_legacy is 1
417 # perl5 prints: $_legacy is
418
419=item * Deprecation
420
421Double-colon is now a valid package separator in a variable name. Thus these
422behave differently in perl4 vs. perl5, since the packages don't exist.
423
424 $a=1;$b=2;$c=3;$var=4;
425 print "$a::$b::$c ";
cb1a09d0 426 print "$var::abc::xyz\n";
6dbacca0
PP
427
428 # perl4 prints: 1::2::3 4::abc::xyz
429 # perl5 prints: 3
cb1a09d0 430
6dbacca0
PP
431Given that C<::> is now the preferred package delimiter, it is debatable
432whether this should be classed as a bug or not.
433(The older package delimiter, ' ,is used here)
cb1a09d0 434
6dbacca0
PP
435 $x = 10 ;
436 print "x=${'x}\n" ;
437
438 # perl4 prints: x=10
439 # perl5 prints: Can't find string terminator "'" anywhere before EOF
a0d0e21e 440
6dbacca0 441Also see precedence traps, for parsing C<$:>.
a0d0e21e 442
6dbacca0 443=item * BugFix
a0d0e21e 444
6dbacca0
PP
445The second and third arguments of C<splice()> are now evaluated in scalar
446context (as the Camel says) rather than list context.
a0d0e21e 447
6dbacca0
PP
448 sub sub1{return(0,2) } # return a 2-elem array
449 sub sub2{ return(1,2,3)} # return a 3-elem array
450 @a1 = ("a","b","c","d","e");
451 @a2 = splice(@a1,&sub1,&sub2);
452 print join(' ',@a2),"\n";
453
454 # perl4 prints: a b
455 # perl5 prints: c d e
a0d0e21e 456
6dbacca0 457=item * Discontinuance
a0d0e21e 458
6dbacca0 459You can't do a C<goto> into a block that is optimized away. Darn.
a0d0e21e 460
6dbacca0 461 goto marker1;
a0d0e21e 462
6dbacca0
PP
463 for(1){
464 marker1:
465 print "Here I is!\n";
466 }
467
468 # perl4 prints: Here I is!
469 # perl5 dumps core (SEGV)
470
471=item * Discontinuance
472
473It is no longer syntactically legal to use whitespace as the name
474of a variable, or as a delimiter for any kind of quote construct.
475Double darn.
476
477 $a = ("foo bar");
478 $b = q baz ;
479 print "a is $a, b is $b\n";
480
481 # perl4 prints: a is foo bar, b is baz
482 # perl5 errors: Bare word found where operator expected
5e378fdf 483
6dbacca0
PP
484=item * Discontinuance
485
486The archaic while/if BLOCK BLOCK syntax is no longer supported.
487
488 if { 1 } {
489 print "True!";
490 }
491 else {
492 print "False!";
493 }
494
495 # perl4 prints: True!
496 # perl5 errors: syntax error at test.pl line 1, near "if {"
497
498=item * BugFix
499
500The C<**> operator now binds more tightly than unary minus.
501It was documented to work this way before, but didn't.
502
503 print -4**2,"\n";
504
505 # perl4 prints: 16
506 # perl5 prints: -16
507
508=item * Discontinuance
509
510The meaning of C<foreach{}> has changed slightly when it is iterating over a
511list which is not an array. This used to assign the list to a
512temporary array, but no longer does so (for efficiency). This means
513that you'll now be iterating over the actual values, not over copies of
514the values. Modifications to the loop variable can change the original
515values.
516
517 @list = ('ab','abc','bcd','def');
518 foreach $var (grep(/ab/,@list)){
519 $var = 1;
520 }
521 print (join(':',@list));
522
523 # perl4 prints: ab:abc:bcd:def
524 # perl5 prints: 1:1:bcd:def
525
526To retain Perl4 semantics you need to assign your list
527explicitly to a temporary array and then iterate over that. For
528example, you might need to change
529
530 foreach $var (grep(/ab/,@list)){
531
532to
533
534 foreach $var (@tmp = grep(/ab/,@list)){
535
536Otherwise changing $var will clobber the values of @list. (This most often
537happens when you use C<$_> for the loop variable, and call subroutines in
538the loop that don't properly localize C<$_>.)
539
5e378fdf
PP
540=item * Discontinuance
541
542C<split> with no arguments now behaves like C<split ' '> (which doesn't
543return an initial null field if $_ starts with whitespace), it used to
544behave like C<split /\s+/> (which does).
545
546 $_ = ' hi mom';
547 print join(':', split);
548
549 # perl4 prints: :hi:mom
550 # perl5 prints: hi:mom
551
6dbacca0
PP
552=item * Deprecation
553
554Some error messages will be different.
555
556=item * Discontinuance
557
558Some bugs may have been inadvertently removed. :-)
559
560=back
561
562=head2 Parsing Traps
563
564Perl4-to-Perl5 traps from having to do with parsing.
565
566=over 4
567
568=item * Parsing
569
570Note the space between . and =
571
572 $string . = "more string";
573 print $string;
574
575 # perl4 prints: more string
576 # perl5 prints: syntax error at - line 1, near ". ="
577
578=item * Parsing
579
580Better parsing in perl 5
581
582 sub foo {}
583 &foo
584 print("hello, world\n");
585
586 # perl4 prints: hello, world
587 # perl5 prints: syntax error
588
589=item * Parsing
590
591"if it looks like a function, it is a function" rule.
592
593 print
594 ($foo == 1) ? "is one\n" : "is zero\n";
595
596 # perl4 prints: is zero
597 # perl5 warns: "Useless use of a constant in void context" if using -w
598
599=back
600
601=head2 Numerical Traps
602
603Perl4-to-Perl5 traps having to do with numerical operators,
604operands, or output from same.
605
606=over 5
607
608=item * Numerical
609
610Formatted output and significant digits
611
612 print 7.373504 - 0, "\n";
613 printf "%20.18f\n", 7.373504 - 0;
614
615 # Perl4 prints:
616 7.375039999999996141
617 7.37503999999999614
618
619 # Perl5 prints:
620 7.373504
621 7.37503999999999614
622
623=item * Numerical
624
5e378fdf
PP
625This specific item has been deleted. It demonstrated how the autoincrement
626operator would not catch when a number went over the signed int limit. Fixed
627in 5.003_04. But always be wary when using large ints. If in doubt:
6dbacca0 628
5e378fdf 629 use Math::BigInt;
6dbacca0
PP
630
631=item * Numerical
632
633Assignment of return values from numeric equality tests
634does not work in perl5 when the test evaluates to false (0).
635Logical tests now return an null, instead of 0
636
637 $p = ($test == 1);
638 print $p,"\n";
639
640 # perl4 prints: 0
641 # perl5 prints:
642
643Also see the L<General Regular Expression Traps> tests for another example
644of this new feature...
645
646=back
647
648=head2 General data type traps
649
650Perl4-to-Perl5 traps involving most data-types, and their usage
651within certain expressions and/or context.
652
653=over 5
654
655=item * (Arrays)
656
657Negative array subscripts now count from the end of the array.
658
659 @a = (1, 2, 3, 4, 5);
660 print "The third element of the array is $a[3] also expressed as $a[-2] \n";
661
662 # perl4 prints: The third element of the array is 4 also expressed as
663 # perl5 prints: The third element of the array is 4 also expressed as 4
664
665=item * (Arrays)
666
667Setting C<$#array> lower now discards array elements, and makes them
668impossible to recover.
669
670 @a = (a,b,c,d,e);
671 print "Before: ",join('',@a);
672 $#a =1;
673 print ", After: ",join('',@a);
674 $#a =3;
675 print ", Recovered: ",join('',@a),"\n";
676
677 # perl4 prints: Before: abcde, After: ab, Recovered: abcd
678 # perl5 prints: Before: abcde, After: ab, Recovered: ab
679
680=item * (Hashes)
681
682Hashes get defined before use
683
684 local($s,@a,%h);
685 die "scalar \$s defined" if defined($s);
686 die "array \@a defined" if defined(@a);
687 die "hash \%h defined" if defined(%h);
688
689 # perl4 prints:
690 # perl5 dies: hash %h defined
691
692=item * (Globs)
693
694glob assignment from variable to variable will fail if the assigned
695variable is localized subsequent to the assignment
696
697 @a = ("This is Perl 4");
698 *b = *a;
699 local(@a);
700 print @b,"\n";
701
702 # perl4 prints: This is Perl 4
703 # perl5 prints:
704
705 # Another example
706
707 *fred = *barney; # fred is aliased to barney
708 @barney = (1, 2, 4);
709 # @fred;
710 print "@fred"; # should print "1, 2, 4"
711
712 # perl4 prints: 1 2 4
713 # perl5 prints: Literal @fred now requires backslash
5e378fdf 714
6dbacca0
PP
715=item * (Scalar String)
716
717Changes in unary negation (of strings)
718This change effects both the return value and what it
719does to auto(magic)increment.
720
721 $x = "aaa";
722 print ++$x," : ";
723 print -$x," : ";
724 print ++$x,"\n";
725
726 # perl4 prints: aab : -0 : 1
727 # perl5 prints: aab : -aab : aac
728
729=item * (Constants)
730
731perl 4 lets you modify constants:
732
733 $foo = "x";
734 &mod($foo);
735 for ($x = 0; $x < 3; $x++) {
736 &mod("a");
737 }
738 sub mod {
739 print "before: $_[0]";
740 $_[0] = "m";
741 print " after: $_[0]\n";
742 }
743
744 # perl4:
745 # before: x after: m
746 # before: a after: m
747 # before: m after: m
748 # before: m after: m
749
750 # Perl5:
751 # before: x after: m
752 # Modification of a read-only value attempted at foo.pl line 12.
753 # before: a
754
755=item * (Scalars)
756
757The behavior is slightly different for:
758
759 print "$x", defined $x
760
761 # perl 4: 1
762 # perl 5: <no output, $x is not called into existence>
763
764=item * (Variable Suicide)
765
766Variable suicide behavior is more consistent under Perl 5.
767Perl5 exhibits the same behavior for associative arrays and scalars,
768that perl4 exhibits only for scalars.
769
770 $aGlobal{ "aKey" } = "global value";
771 print "MAIN:", $aGlobal{"aKey"}, "\n";
772 $GlobalLevel = 0;
773 &test( *aGlobal );
774
775 sub test {
776 local( *theArgument ) = @_;
777 local( %aNewLocal ); # perl 4 != 5.001l,m
778 $aNewLocal{"aKey"} = "this should never appear";
779 print "SUB: ", $theArgument{"aKey"}, "\n";
780 $aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print
781 $GlobalLevel++;
782 if( $GlobalLevel<4 ) {
783 &test( *aNewLocal );
784 }
785 }
786
787 # Perl4:
788 # MAIN:global value
789 # SUB: global value
790 # SUB: level 0
791 # SUB: level 1
792 # SUB: level 2
793
794 # Perl5:
795 # MAIN:global value
796 # SUB: global value
797 # SUB: this should never appear
798 # SUB: this should never appear
799 # SUB: this should never appear
800
84dc3c4d 801=back
6dbacca0
PP
802
803=head2 Context Traps - scalar, list contexts
804
805=over 5
806
807=item * (list context)
808
809The elements of argument lists for formats are now evaluated in list
810context. This means you can interpolate list values now.
811
812 @fmt = ("foo","bar","baz");
813 format STDOUT=
814 @<<<<< @||||| @>>>>>
815 @fmt;
816 .
817 write;
818
819 # perl4 errors: Please use commas to separate fields in file
820 # perl5 prints: foo bar baz
821
822=item * (scalar context)
823
824The C<caller()> function now returns a false value in a scalar context
825if there is no caller. This lets library files determine if they're
826being required.
827
828 caller() ? (print "You rang?\n") : (print "Got a 0\n");
829
830 # perl4 errors: There is no caller
831 # perl5 prints: Got a 0
5e378fdf 832
6dbacca0
PP
833=item * (scalar context)
834
835The comma operator in a scalar context is now guaranteed to give a
836scalar context to its arguments.
837
838 @y= ('a','b','c');
839 $x = (1, 2, @y);
840 print "x = $x\n";
841
842 # Perl4 prints: x = c # Thinks list context interpolates list
843 # Perl5 prints: x = 3 # Knows scalar uses length of list
844
845=item * (list, builtin)
846
847C<sprintf()> funkiness (array argument converted to scalar array count)
848This test could be added to t/op/sprintf.t
849
850 @z = ('%s%s', 'foo', 'bar');
851 $x = sprintf(@z);
852 if ($x eq 'foobar') {print "ok 2\n";} else {print "not ok 2 '$x'\n";}
853
854 # perl4 prints: ok 2
855 # perl5 prints: not ok 2
856
857C<printf()> works fine, though:
858
859 printf STDOUT (@z);
860 print "\n";
861
862 # perl4 prints: foobar
863 # perl5 prints: foobar
864
865Probably a bug.
866
867=back
868
869=head2 Precedence Traps
870
871Perl4-to-Perl5 traps involving precedence order.
872
84dc3c4d
PP
873=over 5
874
5e378fdf
PP
875=item * Precedence
876
877LHS vs. RHS when both sides are getting an op.
878
879 @arr = ( 'left', 'right' );
880 $a{shift @arr} = shift @arr;
881 print join( ' ', keys %a );
882
883 # perl4 prints: left
884 # perl5 prints: right
885
886=item * Precedence
6dbacca0
PP
887
888These are now semantic errors because of precedence:
889
890 @list = (1,2,3,4,5);
891 %map = ("a",1,"b",2,"c",3,"d",4);
892 $n = shift @list + 2; # first item in list plus 2
893 print "n is $n, ";
894 $m = keys %map + 2; # number of items in hash plus 2
895 print "m is $m\n";
896
897 # perl4 prints: n is 3, m is 6
898 # perl5 errors and fails to compile
899
900=item * Precedence
a0d0e21e 901
4633a7c4
LW
902The precedence of assignment operators is now the same as the precedence
903of assignment. Perl 4 mistakenly gave them the precedence of the associated
904operator. So you now must parenthesize them in expressions like
905
906 /foo/ ? ($a += 2) : ($a -= 2);
6dbacca0 907
4633a7c4
LW
908Otherwise
909
6dbacca0 910 /foo/ ? $a += 2 : $a -= 2
4633a7c4
LW
911
912would be erroneously parsed as
913
914 (/foo/ ? $a += 2 : $a) -= 2;
915
916On the other hand,
917
6dbacca0 918 $a += /foo/ ? 1 : 2;
4633a7c4
LW
919
920now works as a C programmer would expect.
921
6dbacca0 922=item * Precedence
4633a7c4 923
6dbacca0 924 open FOO || die;
a0d0e21e 925
6dbacca0
PP
926is now incorrect. You need parens around the filehandle.
927Otherwise, perl5 leaves the statement as it's default precedence:
a0d0e21e 928
6dbacca0
PP
929 open(FOO || die);
930
931 # perl4 opens or dies
932 # perl5 errors: Precedence problem: open FOO should be open(FOO)
a0d0e21e 933
6dbacca0 934=item * Precedence
a0d0e21e 935
6dbacca0
PP
936perl4 gives the special variable, C<$:> precedence, where perl5
937treats C<$::> as main C<package>
a0d0e21e 938
6dbacca0
PP
939 $a = "x"; print "$::a";
940
941 # perl 4 prints: -:a
942 # perl 5 prints: x
5e378fdf 943
6dbacca0 944=item * Precedence
a0d0e21e 945
6dbacca0 946concatenation precedence over filetest operator?
a0d0e21e 947
6dbacca0
PP
948 -e $foo .= "q"
949
950 # perl4 prints: no output
951 # perl5 prints: Can't modify -e in concatenation
a0d0e21e 952
6dbacca0 953=item * Precedence
a0d0e21e 954
6dbacca0
PP
955Assignment to value takes precedence over assignment to key in
956perl5 when using the shift operator on both sides.
957
958 @arr = ( 'left', 'right' );
959 $a{shift @arr} = shift @arr;
960 print join( ' ', keys %a );
961
962 # perl4 prints: left
963 # perl5 prints: right
964
965=back
966
967=head2 General Regular Expression Traps using s///, etc.
968
969All types of RE traps.
970
971=over 5
972
973=item * Regular Expression
974
975C<s'$lhs'$rhs'> now does no interpolation on either side. It used to
976interpolate C<$lhs> but not C<$rhs>. (And still does not match a literal
977'$' in string)
978
979 $a=1;$b=2;
980 $string = '1 2 $a $b';
981 $string =~ s'$a'$b';
982 print $string,"\n";
983
984 # perl4 prints: $b 2 $a $b
985 # perl5 prints: 1 2 $a $b
986
987=item * Regular Expression
a0d0e21e
LW
988
989C<m//g> now attaches its state to the searched string rather than the
6dbacca0
PP
990regular expression. (Once the scope of a block is left for the sub, the
991state of the searched string is lost)
992
993 $_ = "ababab";
994 while(m/ab/g){
995 &doit("blah");
996 }
997 sub doit{local($_) = shift; print "Got $_ "}
998
999 # perl4 prints: blah blah blah
1000 # perl5 prints: infinite loop blah...
1001
1002=item * Regular Expression
1003
1004If no parentheses are used in a match, Perl4 sets C<$+> to
1005the whole match, just like C<$&>. Perl5 does not.
1006
1007 "abcdef" =~ /b.*e/;
1008 print "\$+ = $+\n";
1009
1010 # perl4 prints: bcde
1011 # perl5 prints:
1012
1013=item * Regular Expression
1014
1015substitution now returns the null string if it fails
1016
1017 $string = "test";
1018 $value = ($string =~ s/foo//);
1019 print $value, "\n";
1020
1021 # perl4 prints: 0
1022 # perl5 prints:
1023
1024Also see L<Numerical Traps> for another example of this new feature.
1025
1026=item * Regular Expression
1027
1028C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no
1029backtick expansion
1030
1031 $string = "";
1032 $string =~ s`^`hostname`;
1033 print $string, "\n";
1034
1035 # perl4 prints: <the local hostname>
1036 # perl5 prints: hostname
1037
1038=item * Regular Expression
1039
1040Stricter parsing of variables used in regular expressions
1041
1042 s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
1043
1044 # perl4: compiles w/o error
1045 # perl5: with Scalar found where operator expected ..., near "$opt$plus"
1046
1047an added component of this example, apparently from the same script, is
1048the actual value of the s'd string after the substitution.
1049C<[$opt]> is a character class in perl4 and an array subscript in perl5
1050
1051 $grpc = 'a';
1052 $opt = 'r';
1053 $_ = 'bar';
1054 s/^([^$grpc]*$grpc[$opt]?)/foo/;
1055 print ;
1056
1057 # perl4 prints: foo
1058 # perl5 prints: foobar
1059
1060=item * Regular Expression
1061
1062Under perl5, C<m?x?> matches only once, like C<?x?>. Under perl4, it matched
1063repeatedly, like C</x/> or C<m!x!>.
1064
1065 $test = "once";
1066 sub match { $test =~ m?once?; }
1067 &match();
1068 if( &match() ) {
1069 # m?x? matches more then once
1070 print "perl4\n";
1071 } else {
1072 # m?x? matches only once
1073 print "perl5\n";
1074 }
1075
1076 # perl4 prints: perl4
1077 # perl5 prints: perl5
a0d0e21e 1078
a0d0e21e 1079
6dbacca0
PP
1080=back
1081
1082=head2 Subroutine, Signal, Sorting Traps
a0d0e21e 1083
6dbacca0
PP
1084The general group of Perl4-to-Perl5 traps having to do with
1085Signals, Sorting, and their related subroutines, as well as
1086general subroutine traps. Includes some OS-Specific traps.
a0d0e21e 1087
6dbacca0 1088=over 5
a0d0e21e 1089
6dbacca0 1090=item * (Signals)
a0d0e21e 1091
6dbacca0
PP
1092Barewords that used to look like strings to Perl will now look like subroutine
1093calls if a subroutine by that name is defined before the compiler sees them.
a0d0e21e 1094
6dbacca0
PP
1095 sub SeeYa { warn"Hasta la vista, baby!" }
1096 $SIG{'TERM'} = SeeYa;
1097 print "SIGTERM is now $SIG{'TERM'}\n";
1098
1099 # perl4 prints: SIGTERM is main'SeeYa
1100 # perl5 prints: SIGTERM is now main::1
a0d0e21e 1101
6dbacca0 1102Use B<-w> to catch this one
a0d0e21e 1103
6dbacca0 1104=item * (Sort Subroutine)
a0d0e21e 1105
6dbacca0 1106reverse is no longer allowed as the name of a sort subroutine.
a0d0e21e 1107
6dbacca0
PP
1108 sub reverse{ print "yup "; $a <=> $b }
1109 print sort reverse a,b,c;
1110
1111 # perl4 prints: yup yup yup yup abc
1112 # perl5 prints: abc
a0d0e21e 1113
5e378fdf
PP
1114=item * warn() specifically implies STDERR
1115
1116 warn STDERR "Foo!";
1117
1118 # perl4 prints: Foo!
1119 # perl5 prints: String found where operator expected
1120
6dbacca0 1121=back
a0d0e21e 1122
6dbacca0
PP
1123=head2 OS Traps
1124
1125=over 5
1126
1127=item * (SysV)
1128
1129Under HPUX, and some other SysV OS's, one had to reset any signal handler,
1130within the signal handler function, each time a signal was handled with
1131perl4. With perl5, the reset is now done correctly. Any code relying
1132on the handler _not_ being reset will have to be reworked.
1133
11345.002 and beyond uses sigaction() under SysV
1135
1136 sub gotit {
1137 print "Got @_... ";
1138 }
1139 $SIG{'INT'} = 'gotit';
1140
1141 $| = 1;
1142 $pid = fork;
1143 if ($pid) {
1144 kill('INT', $pid);
1145 sleep(1);
1146 kill('INT', $pid);
1147 } else {
1148 while (1) {sleep(10);}
1149 }
1150
1151 # perl4 (HPUX) prints: Got INT...
1152 # perl5 (HPUX) prints: Got INT... Got INT...
1153
1154=item * (SysV)
1155
1156Under SysV OS's, C<seek()> on a file opened to append C<E<gt>E<gt>> now does
1157the right thing w.r.t. the fopen() man page. e.g. - When a file is opened
1158for append, it is impossible to overwrite information already in
1159the file.
1160
1161 open(TEST,">>seek.test");
1162 $start = tell TEST ;
1163 foreach(1 .. 9){
1164 print TEST "$_ ";
1165 }
1166 $end = tell TEST ;
1167 seek(TEST,$start,0);
1168 print TEST "18 characters here";
1169
1170 # perl4 (solaris) seek.test has: 18 characters here
1171 # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here
a0d0e21e 1172
a0d0e21e 1173
a0d0e21e 1174
6dbacca0 1175=back
a0d0e21e 1176
6dbacca0 1177=head2 Interpolation Traps
a0d0e21e 1178
6dbacca0 1179=over 5
a0d0e21e 1180
6dbacca0 1181=item * Interpolation
a0d0e21e 1182
6dbacca0
PP
1183@ now always interpolates an array in double-quotish strings.
1184
1185 print "To: someone@somewhere.com\n";
1186
1187 # perl4 prints: To:someone@somewhere.com
1188 # perl5 errors : Literal @somewhere now requires backslash
1189
1190=item * Interpolation
1191
1192Perl4-to-Perl5 traps having to do with how things get interpolated
1193within certain expressions, statements, contexts, or whatever.
1194
1195Double-quoted strings may no longer end with an unescaped $ or @.
1196
1197 $foo = "foo$";
1198 $bar = "bar@";
1199 print "foo is $foo, bar is $bar\n";
1200
1201 # perl4 prints: foo is foo$, bar is bar@
1202 # perl5 errors: Final $ should be \$ or $name
1203
1204Note: perl5 DOES NOT error on the terminating @ in $bar
1205
1206=item * Interpolation
a0d0e21e 1207
748a9306 1208The construct "this is $$x" used to interpolate the pid at that
6dbacca0 1209point, but now apparently tries to dereference C<$x>. C<$$> by itself still
748a9306
LW
1210works fine, however.
1211
6dbacca0 1212 print "this is $$x\n";
748a9306 1213
6dbacca0
PP
1214 # perl4 prints: this is XXXx (XXX is the current pid)
1215 # perl5 prints: this is
1216
1217=item * Interpolation
1218
1219Creation of hashes on the fly with C<eval "EXPR"> now requires either both
1220C<$>'s to be protected in the specification of the hash name, or both curlies
1221to be protected. If both curlies are protected, the result will be compatible
1222with perl4 and perl5. This is a very common practice, and should be changed
1223to use the block form of C<eval{}> if possible.
c07a80fd 1224
6dbacca0
PP
1225 $hashname = "foobar";
1226 $key = "baz";
1227 $value = 1234;
1228 eval "\$$hashname{'$key'} = q|$value|";
1229 (defined($foobar{'baz'})) ? (print "Yup") : (print "Nope");
1230
1231 # perl4 prints: Yup
1232 # perl5 prints: Nope
1233
1234Changing
1235
1236 eval "\$$hashname{'$key'} = q|$value|";
c07a80fd
PP
1237
1238to
1239
6dbacca0 1240 eval "\$\$hashname{'$key'} = q|$value|";
c07a80fd 1241
6dbacca0 1242causes the following result:
c07a80fd 1243
6dbacca0
PP
1244 # perl4 prints: Nope
1245 # perl5 prints: Yup
c07a80fd 1246
6dbacca0 1247or, changing to
a0d0e21e 1248
6dbacca0
PP
1249 eval "\$$hashname\{'$key'\} = q|$value|";
1250
1251causes the following result:
1252
1253 # perl4 prints: Yup
1254 # perl5 prints: Yup
1255 # and is compatible for both versions
1256
1257
1258=item * Interpolation
1259
1260perl4 programs which unconsciously rely on the bugs in earlier perl versions.
1261
1262 perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'
1263
1264 # perl4 prints: This is not perl5
1265 # perl5 prints: This is perl5
1266
1267=item * Interpolation
1268
1269You also have to be careful about array references.
1270
1271 print "$foo{"
1272
1273 perl 4 prints: {
1274 perl 5 prints: syntax error
1275
1276=item * Interpolation
1277
1278Similarly, watch out for:
1279
1280 $foo = "array";
1281 print "\$$foo{bar}\n";
1282
1283 # perl4 prints: $array{bar}
1284 # perl5 prints: $
1285
1286Perl 5 is looking for C<$array{bar}> which doesn't exist, but perl 4 is
1287happy just to expand $foo to "array" by itself. Watch out for this
1288especially in C<eval>'s.
1289
1290=item * Interpolation
1291
1292C<qq()> string passed to C<eval>
1293
1294 eval qq(
1295 foreach \$y (keys %\$x\) {
1296 \$count++;
1297 }
1298 );
1299
1300 # perl4 runs this ok
1301 # perl5 prints: Can't find string terminator ")"
a0d0e21e 1302
6dbacca0
PP
1303=back
1304
1305=head2 DBM Traps
1306
1307General DBM traps.
1308
1309=over 5
1310
1311=item * DBM
1312
1313Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1314may cause the same script, run under perl5, to fail. The build of perl5
1315must have been linked with the same dbm/ndbm as the default for C<dbmopen()>
1316to function properly without C<tie>'ing to an extension dbm implementation.
1317
1318 dbmopen (%dbm, "file", undef);
1319 print "ok\n";
1320
1321 # perl4 prints: ok
1322 # perl5 prints: ok (IFF linked with -ldbm or -lndbm)
1323
1324
1325=item * DBM
1326
1327Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1328may cause the same script, run under perl5, to fail. The error generated
1329when exceeding the limit on the key/value size will cause perl5 to exit
1330immediately.
1331
1332 dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";
1333 $DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm
1334 print "YUP\n";
1335
1336 # perl4 prints:
1337 dbm store returned -1, errno 28, key "trap" at - line 3.
1338 YUP
1339
1340 # perl5 prints:
1341 dbm store returned -1, errno 28, key "trap" at - line 3.
a0d0e21e
LW
1342
1343=back
6dbacca0
PP
1344
1345=head2 Unclassified Traps
1346
1347Everything else.
1348
84dc3c4d
PP
1349=over 5
1350
6dbacca0
PP
1351=item * Unclassified
1352
1353C<require>/C<do> trap using returned value
1354
1355If the file doit.pl has:
1356
1357 sub foo {
1358 $rc = do "./do.pl";
1359 return 8;
1360 }
1361 print &foo, "\n";
1362
1363And the do.pl file has the following single line:
1364
1365 return 3;
1366
1367Running doit.pl gives the following:
1368
1369 # perl 4 prints: 3 (aborts the subroutine early)
1370 # perl 5 prints: 8
1371
1372Same behavior if you replace C<do> with C<require>.
1373
1374=back
1375
1376As always, if any of these are ever officially declared as bugs,
1377they'll be fixed and removed.
1378