3 perltrap - Perl traps for the unwary
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
15 Accustomed B<awk> users should take special note of the following:
21 A Perl program executes only once, not once for each input line. You can
22 do an implicit loop with C<-n> or C<-p>.
26 The English module, loaded via
30 allows you to refer to special variables (like C<$/>) with names (like
31 $RS), as though they were in B<awk>; see L<perlvar> for details.
35 Semicolons are required after all simple statements in Perl (except
36 at the end of a block). Newline is not a statement delimiter.
40 Curly brackets are required on C<if>s and C<while>s.
44 Variables begin with "$", "@" or "%" in Perl.
48 Arrays index from 0. Likewise string positions in substr() and
53 You have to decide whether your array has numeric or string indices.
57 Hash values do not spring into existence upon mere reference.
61 You have to decide whether you want to use string or numeric
66 Reading an input line does not split it for you. You get to split it
67 to an array yourself. And the split() operator has different
68 arguments than B<awk>'s.
72 The current input line is normally in $_, not $0. It generally does
73 not have the newline stripped. ($0 is the name of the program
74 executed.) See L<perlvar>.
78 $<I<digit>> does not refer to fields--it refers to substrings matched
79 by the last match pattern.
83 The print() statement does not add field and record separators unless
84 you set C<$,> and C<$\>. You can set $OFS and $ORS if you're using
89 You must open your files before you print to them.
93 The range operator is "..", not comma. The comma operator works as in
98 The match operator is "=~", not "~". ("~" is the one's complement
103 The exponentiation operator is "**", not "^". "^" is the XOR
104 operator, as in C. (You know, one could get the feeling that B<awk> is
105 basically incompatible with C.)
109 The concatenation operator is ".", not the null string. (Using the
110 null string would render C</pat/ /pat/> unparsable, because the third slash
111 would be interpreted as a division operator--the tokenizer is in fact
112 slightly context sensitive for operators like "/", "?", and ">".
113 And in fact, "." itself can be the beginning of a number.)
117 The C<next>, C<exit>, and C<continue> keywords work differently.
122 The following variables work differently:
125 ARGC scalar @ARGV (compare with $#ARGV)
129 FS (whatever you like)
130 NF $#Fld, or some such
142 You cannot set $RS to a pattern, only a string.
146 When in doubt, run the B<awk> construct through B<a2p> and see what it
153 Cerebral C and C++ programmers should take note of the following:
159 Curly brackets are required on C<if>'s and C<while>'s.
163 You must use C<elsif> rather than C<else if>.
167 The C<break> and C<continue> keywords from C become in Perl C<last>
168 and C<next>, respectively. Unlike in C, these do I<not> work within a
169 C<do { } while> construct. See L<perlsyn/"Loop Control">.
173 There's no switch statement. (But it's easy to build one on the fly,
174 see L<perlsyn/"Basic BLOCKs and Switch Statements">)
178 Variables begin with "$", "@" or "%" in Perl.
182 Comments begin with "#", not "/*" or "//". Perl may interpret C/C++
183 comments as division operators, unterminated regular expressions or
184 the defined-or operator.
188 You can't take the address of anything, although a similar operator
189 in Perl is the backslash, which creates a reference.
193 C<ARGV> must be capitalized. C<$ARGV[0]> is C's C<argv[1]>, and C<argv[0]>
198 System calls such as link(), unlink(), rename(), etc. return nonzero for
199 success, not 0. (system(), however, returns zero for success.)
203 Signal handlers deal with signal names, not numbers. Use C<kill -l>
204 to find their names on your system.
210 Seasoned B<sed> programmers should take note of the following:
216 A Perl program executes only once, not once for each input line. You can
217 do an implicit loop with C<-n> or C<-p>.
221 Backreferences in substitutions use "$" rather than "\".
225 The pattern matching metacharacters "(", ")", and "|" do not have backslashes
230 The range operator is C<...>, rather than comma.
236 Sharp shell programmers should take note of the following:
242 The backtick operator does variable interpolation without regard to
243 the presence of single quotes in the command.
247 The backtick operator does no translation of the return value, unlike B<csh>.
251 Shells (especially B<csh>) do several levels of substitution on each
252 command line. Perl does substitution in only certain constructs
253 such as double quotes, backticks, angle brackets, and search patterns.
257 Shells interpret scripts a little bit at a time. Perl compiles the
258 entire program before executing it (except for C<BEGIN> blocks, which
259 execute at compile time).
263 The arguments are available via @ARGV, not $1, $2, etc.
267 The environment is not automatically made available as separate scalar
274 Practicing Perl Programmers should take note of the following:
280 Remember that many operations behave differently in a list
281 context than they do in a scalar one. See L<perldata> for details.
285 Avoid barewords if you can, especially all lowercase ones.
286 You can't tell by just looking at it whether a bareword is
287 a function or a string. By using quotes on strings and
288 parentheses on function calls, you won't ever get them confused.
292 You cannot discern from mere inspection which builtins
293 are unary operators (like chop() and chdir())
294 and which are list operators (like print() and unlink()).
295 (Unless prototyped, user-defined subroutines can B<only> be list
296 operators, never unary ones.) See L<perlop> and L<perlsub>.
300 People have a hard time remembering that some functions
301 default to $_, or @ARGV, or whatever, but that others which
302 you might expect to do not.
306 The <FH> construct is not the name of the filehandle, it is a readline
307 operation on that handle. The data read is assigned to $_ only if the
308 file read is the sole condition in a while loop:
311 while (defined($_ = <FH>)) { }..
312 <FH>; # data discarded!
316 Remember not to use C<=> when you need C<=~>;
317 these two constructs are quite different:
324 The C<do {}> construct isn't a real loop that you can use
329 Use C<my()> for local variables whenever you can get away with
330 it (but see L<perlform> for where you can't).
331 Using C<local()> actually gives a local value to a global
332 variable, which leaves you open to unforeseen side-effects
337 If you localize an exported variable in a module, its exported value will
338 not change. The local name becomes an alias to a new value but the
339 external name is still an alias for the original.
343 =head2 Perl4 to Perl5 Traps
345 Practicing Perl4 Programmers should take note of the following
346 Perl4-to-Perl5 specific traps.
348 They're crudely ordered according to the following list:
352 =item Discontinuance, Deprecation, and BugFix traps
354 Anything that's been fixed as a perl4 bug, removed as a perl4 feature
355 or deprecated as a perl4 feature with the intent to encourage usage of
356 some other perl5 feature.
360 Traps that appear to stem from the new parser.
362 =item Numerical Traps
364 Traps having to do with numerical or mathematical operators.
366 =item General data type traps
368 Traps involving perl standard data types.
370 =item Context Traps - scalar, list contexts
372 Traps related to context within lists, scalar statements/declarations.
374 =item Precedence Traps
376 Traps related to the precedence of parsing, evaluation, and execution of
379 =item General Regular Expression Traps using s///, etc.
381 Traps related to the use of pattern matching.
383 =item Subroutine, Signal, Sorting Traps
385 Traps related to the use of signals and signal handlers, general subroutines,
386 and sorting, along with sorting subroutines.
394 Traps specific to the use of C<dbmopen()>, and specific dbm implementations.
396 =item Unclassified Traps
402 If you find an example of a conversion trap that is not listed here,
403 please submit it to <F<perlbug@perl.org>> for inclusion.
404 Also note that at least some of these can be caught with the
405 C<use warnings> pragma or the B<-w> switch.
407 =head2 Discontinuance, Deprecation, and BugFix traps
409 Anything that has been discontinued, deprecated, or fixed as
414 =item * Discontinuance
416 Symbols starting with "_" are no longer forced into package main, except
417 for C<$_> itself (and C<@_>, etc.).
423 print "\$_legacy is ",$_legacy,"\n";
425 # perl4 prints: $_legacy is 1
426 # perl5 prints: $_legacy is
430 Double-colon is now a valid package separator in a variable name. Thus these
431 behave differently in perl4 vs. perl5, because the packages don't exist.
433 $a=1;$b=2;$c=3;$var=4;
435 print "$var::abc::xyz\n";
437 # perl4 prints: 1::2::3 4::abc::xyz
440 Given that C<::> is now the preferred package delimiter, it is debatable
441 whether this should be classed as a bug or not.
442 (The older package delimiter, ' ,is used here)
448 # perl5 prints: Can't find string terminator "'" anywhere before EOF
450 You can avoid this problem, and remain compatible with perl4, if you
451 always explicitly include the package name:
454 print "x=${main'x}\n" ;
456 Also see precedence traps, for parsing C<$:>.
460 The second and third arguments of C<splice()> are now evaluated in scalar
461 context (as the Camel says) rather than list context.
463 sub sub1{return(0,2) } # return a 2-element list
464 sub sub2{ return(1,2,3)} # return a 3-element list
465 @a1 = ("a","b","c","d","e");
466 @a2 = splice(@a1,&sub1,&sub2);
467 print join(' ',@a2),"\n";
470 # perl5 prints: c d e
472 =item * Discontinuance
474 You can't do a C<goto> into a block that is optimized away. Darn.
480 print "Here I is!\n";
483 # perl4 prints: Here I is!
484 # perl5 errors: Can't "goto" into the middle of a foreach loop
486 =item * Discontinuance
488 It is no longer syntactically legal to use whitespace as the name
489 of a variable, or as a delimiter for any kind of quote construct.
494 print "a is $a, b is $b\n";
496 # perl4 prints: a is foo bar, b is baz
497 # perl5 errors: Bareword found where operator expected
499 =item * Discontinuance
501 The archaic while/if BLOCK BLOCK syntax is no longer supported.
510 # perl4 prints: True!
511 # perl5 errors: syntax error at test.pl line 1, near "if {"
515 The C<**> operator now binds more tightly than unary minus.
516 It was documented to work this way before, but didn't.
523 =item * Discontinuance
525 The meaning of C<foreach{}> has changed slightly when it is iterating over a
526 list which is not an array. This used to assign the list to a
527 temporary array, but no longer does so (for efficiency). This means
528 that you'll now be iterating over the actual values, not over copies of
529 the values. Modifications to the loop variable can change the original
532 @list = ('ab','abc','bcd','def');
533 foreach $var (grep(/ab/,@list)){
536 print (join(':',@list));
538 # perl4 prints: ab:abc:bcd:def
539 # perl5 prints: 1:1:bcd:def
541 To retain Perl4 semantics you need to assign your list
542 explicitly to a temporary array and then iterate over that. For
543 example, you might need to change
545 foreach $var (grep(/ab/,@list)){
549 foreach $var (@tmp = grep(/ab/,@list)){
551 Otherwise changing $var will clobber the values of @list. (This most often
552 happens when you use C<$_> for the loop variable, and call subroutines in
553 the loop that don't properly localize C<$_>.)
555 =item * Discontinuance
557 C<split> with no arguments now behaves like C<split ' '> (which doesn't
558 return an initial null field if $_ starts with whitespace), it used to
559 behave like C<split /\s+/> (which does).
562 print join(':', split);
564 # perl4 prints: :hi:mom
565 # perl5 prints: hi:mom
569 Perl 4 would ignore any text which was attached to an B<-e> switch,
570 always taking the code snippet from the following arg. Additionally, it
571 would silently accept an B<-e> switch without a following arg. Both of
572 these behaviors have been fixed.
574 perl -e'print "attached to -e"' 'print "separate arg"'
576 # perl4 prints: separate arg
577 # perl5 prints: attached to -e
582 # perl5 dies: No code specified for -e.
584 =item * Discontinuance
586 In Perl 4 the return value of C<push> was undocumented, but it was
587 actually the last value being pushed onto the target list. In Perl 5
588 the return value of C<push> is documented, but has changed, it is the
589 number of elements in the resulting list.
592 print push(@x, 'first new', 'second new');
594 # perl4 prints: second new
599 Some error messages will be different.
601 =item * Discontinuance
603 In Perl 4, if in list context the delimiters to the first argument of
604 C<split()> were C<??>, the result would be placed in C<@_> as well as
605 being returned. Perl 5 has more respect for your subroutine arguments.
607 =item * Discontinuance
609 Some bugs may have been inadvertently removed. :-)
615 Perl4-to-Perl5 traps from having to do with parsing.
621 Note the space between . and =
623 $string . = "more string";
626 # perl4 prints: more string
627 # perl5 prints: syntax error at - line 1, near ". ="
631 Better parsing in perl 5
635 print("hello, world\n");
637 # perl4 prints: hello, world
638 # perl5 prints: syntax error
642 "if it looks like a function, it is a function" rule.
645 ($foo == 1) ? "is one\n" : "is zero\n";
647 # perl4 prints: is zero
648 # perl5 warns: "Useless use of a constant in void context" if using -w
652 String interpolation of the C<$#array> construct differs when braces
653 are to used around the name.
659 # perl5 fails with syntax error
669 When perl sees C<map {> (or C<grep {>), it has to guess whether the C<{>
670 starts a BLOCK or a hash reference. If it guesses wrong, it will report
671 a syntax error near the C<}> and the missing (or unexpected) comma.
673 Use unary C<+> before C<{> on a hash reference, and unary C<+> applied
674 to the first thing in a BLOCK (after C<{>), for perl to guess right all
675 the time. (See L<perlfunc/map>.)
679 =head2 Numerical Traps
681 Perl4-to-Perl5 traps having to do with numerical operators,
682 operands, or output from same.
688 Formatted output and significant digits. In general, Perl 5
689 tries to be more precise. For example, on a Solaris Sparc:
691 print 7.373504 - 0, "\n";
692 printf "%20.18f\n", 7.373504 - 0;
695 7.3750399999999996141
702 Notice how the first result looks better in Perl 5.
704 Your results may vary, since your floating point formatting routines
705 and even floating point format may be slightly different.
709 This specific item has been deleted. It demonstrated how the auto-increment
710 operator would not catch when a number went over the signed int limit. Fixed
711 in version 5.003_04. But always be wary when using large integers.
718 Assignment of return values from numeric equality tests
719 does not work in perl5 when the test evaluates to false (0).
720 Logical tests now return a null, instead of 0
728 Also see L<"General Regular Expression Traps using s///, etc.">
729 for another example of this new feature...
731 =item * Bitwise string ops
733 When bitwise operators which can operate upon either numbers or
734 strings (C<& | ^ ~>) are given only strings as arguments, perl4 would
735 treat the operands as bitstrings so long as the program contained a call
736 to the C<vec()> function. perl5 treats the string operands as bitstrings.
737 (See L<perlop/Bitwise String Operators> for more details.)
741 $betty = $fred & $barney;
743 # Uncomment the next line to change perl4's behavior
744 # ($dummy) = vec("dummy", 0, 0);
752 # If vec() is used anywhere in the program, both print:
757 =head2 General data type traps
759 Perl4-to-Perl5 traps involving most data-types, and their usage
760 within certain expressions and/or context.
766 Negative array subscripts now count from the end of the array.
768 @a = (1, 2, 3, 4, 5);
769 print "The third element of the array is $a[3] also expressed as $a[-2] \n";
771 # perl4 prints: The third element of the array is 4 also expressed as
772 # perl5 prints: The third element of the array is 4 also expressed as 4
776 Setting C<$#array> lower now discards array elements, and makes them
777 impossible to recover.
780 print "Before: ",join('',@a);
782 print ", After: ",join('',@a);
784 print ", Recovered: ",join('',@a),"\n";
786 # perl4 prints: Before: abcde, After: ab, Recovered: abcd
787 # perl5 prints: Before: abcde, After: ab, Recovered: ab
791 Hashes get defined before use
794 die "scalar \$s defined" if defined($s);
795 die "array \@a defined" if defined(@a);
796 die "hash \%h defined" if defined(%h);
799 # perl5 dies: hash %h defined
801 Perl will now generate a warning when it sees defined(@a) and
806 glob assignment from variable to variable will fail if the assigned
807 variable is localized subsequent to the assignment
809 @a = ("This is Perl 4");
814 # perl4 prints: This is Perl 4
819 Assigning C<undef> to a glob has no effect in Perl 5. In Perl 4
820 it undefines the associated scalar (but may have other side effects
821 including SEGVs). Perl 5 will also warn if C<undef> is assigned to a
822 typeglob. (Note that assigning C<undef> to a typeglob is different
823 than calling the C<undef> function on a typeglob (C<undef *foo>), which
824 has quite a few effects.
831 # perl4 warns: "Use of uninitialized variable" if using -w
833 # perl5 warns: "Undefined value assigned to typeglob" if using -w
835 =item * (Scalar String)
837 Changes in unary negation (of strings)
838 This change effects both the return value and what it
839 does to auto(magic)increment.
846 # perl4 prints: aab : -0 : 1
847 # perl5 prints: aab : -aab : aac
851 perl 4 lets you modify constants:
855 for ($x = 0; $x < 3; $x++) {
859 print "before: $_[0]";
861 print " after: $_[0]\n";
872 # Modification of a read-only value attempted at foo.pl line 12.
877 The behavior is slightly different for:
879 print "$x", defined $x
882 # perl 5: <no output, $x is not called into existence>
884 =item * (Variable Suicide)
886 Variable suicide behavior is more consistent under Perl 5.
887 Perl5 exhibits the same behavior for hashes and scalars,
888 that perl4 exhibits for only scalars.
890 $aGlobal{ "aKey" } = "global value";
891 print "MAIN:", $aGlobal{"aKey"}, "\n";
896 local( *theArgument ) = @_;
897 local( %aNewLocal ); # perl 4 != 5.001l,m
898 $aNewLocal{"aKey"} = "this should never appear";
899 print "SUB: ", $theArgument{"aKey"}, "\n";
900 $aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print
902 if( $GlobalLevel<4 ) {
917 # SUB: this should never appear
918 # SUB: this should never appear
919 # SUB: this should never appear
923 =head2 Context Traps - scalar, list contexts
927 =item * (list context)
929 The elements of argument lists for formats are now evaluated in list
930 context. This means you can interpolate list values now.
932 @fmt = ("foo","bar","baz");
939 # perl4 errors: Please use commas to separate fields in file
940 # perl5 prints: foo bar baz
942 =item * (scalar context)
944 The C<caller()> function now returns a false value in a scalar context
945 if there is no caller. This lets library files determine if they're
948 caller() ? (print "You rang?\n") : (print "Got a 0\n");
950 # perl4 errors: There is no caller
951 # perl5 prints: Got a 0
953 =item * (scalar context)
955 The comma operator in a scalar context is now guaranteed to give a
956 scalar context to its arguments.
962 # Perl4 prints: x = c # Thinks list context interpolates list
963 # Perl5 prints: x = 3 # Knows scalar uses length of list
965 =item * (list, builtin)
967 C<sprintf()> is prototyped as ($;@), so its first argument is given scalar
968 context. Thus, if passed an array, it will probably not do what you want,
971 @z = ('%s%s', 'foo', 'bar');
975 # perl4 prints: foobar
978 C<printf()> works the same as it did in Perl 4, though:
980 @z = ('%s%s', 'foo', 'bar');
983 # perl4 prints: foobar
984 # perl5 prints: foobar
988 =head2 Precedence Traps
990 Perl4-to-Perl5 traps involving precedence order.
992 Perl 4 has almost the same precedence rules as Perl 5 for the operators
993 that they both have. Perl 4 however, seems to have had some
994 inconsistencies that made the behavior differ from what was documented.
1000 LHS vs. RHS of any assignment operator. LHS is evaluated first
1001 in perl4, second in perl5; this can affect the relationship
1002 between side-effects in sub-expressions.
1004 @arr = ( 'left', 'right' );
1005 $a{shift @arr} = shift @arr;
1006 print join( ' ', keys %a );
1008 # perl4 prints: left
1009 # perl5 prints: right
1013 These are now semantic errors because of precedence:
1015 @list = (1,2,3,4,5);
1016 %map = ("a",1,"b",2,"c",3,"d",4);
1017 $n = shift @list + 2; # first item in list plus 2
1019 $m = keys %map + 2; # number of items in hash plus 2
1022 # perl4 prints: n is 3, m is 6
1023 # perl5 errors and fails to compile
1027 The precedence of assignment operators is now the same as the precedence
1028 of assignment. Perl 4 mistakenly gave them the precedence of the associated
1029 operator. So you now must parenthesize them in expressions like
1031 /foo/ ? ($a += 2) : ($a -= 2);
1035 /foo/ ? $a += 2 : $a -= 2
1037 would be erroneously parsed as
1039 (/foo/ ? $a += 2 : $a) -= 2;
1043 $a += /foo/ ? 1 : 2;
1045 now works as a C programmer would expect.
1051 is now incorrect. You need parentheses around the filehandle.
1052 Otherwise, perl5 leaves the statement as its default precedence:
1056 # perl4 opens or dies
1057 # perl5 opens FOO, dying only if 'FOO' is false, i.e. never
1061 perl4 gives the special variable, C<$:> precedence, where perl5
1062 treats C<$::> as main C<package>
1064 $a = "x"; print "$::a";
1066 # perl 4 prints: -:a
1071 perl4 had buggy precedence for the file test operators vis-a-vis
1072 the assignment operators. Thus, although the precedence table
1073 for perl4 leads one to believe C<-e $foo .= "q"> should parse as
1074 C<((-e $foo) .= "q")>, it actually parses as C<(-e ($foo .= "q"))>.
1075 In perl5, the precedence is as documented.
1079 # perl4 prints: no output
1080 # perl5 prints: Can't modify -e in concatenation
1084 In perl4, keys(), each() and values() were special high-precedence operators
1085 that operated on a single hash, but in perl5, they are regular named unary
1086 operators. As documented, named unary operators have lower precedence
1087 than the arithmetic and concatenation operators C<+ - .>, but the perl4
1088 variants of these operators actually bind tighter than C<+ - .>.
1095 # perl5 prints: Type of arg 1 to keys must be hash (not subtraction)
1097 The perl4 behavior was probably more useful, if less consistent.
1101 =head2 General Regular Expression Traps using s///, etc.
1103 All types of RE traps.
1107 =item * Regular Expression
1109 C<s'$lhs'$rhs'> now does no interpolation on either side. It used to
1110 interpolate $lhs but not $rhs. (And still does not match a literal
1114 $string = '1 2 $a $b';
1115 $string =~ s'$a'$b';
1118 # perl4 prints: $b 2 $a $b
1119 # perl5 prints: 1 2 $a $b
1121 =item * Regular Expression
1123 C<m//g> now attaches its state to the searched string rather than the
1124 regular expression. (Once the scope of a block is left for the sub, the
1125 state of the searched string is lost)
1131 sub doit{local($_) = shift; print "Got $_ "}
1133 # perl4 prints: Got blah Got blah Got blah Got blah
1134 # perl5 prints: infinite loop blah...
1136 =item * Regular Expression
1138 Currently, if you use the C<m//o> qualifier on a regular expression
1139 within an anonymous sub, I<all> closures generated from that anonymous
1140 sub will use the regular expression as it was compiled when it was used
1141 the very first time in any such closure. For instance, if you say
1144 my($left,$right) = @_;
1145 return sub { $_[0] =~ /$left stuff $right/o; };
1147 $good = build_match('foo','bar');
1148 $bad = build_match('baz','blarch');
1149 print $good->('foo stuff bar') ? "ok\n" : "not ok\n";
1150 print $bad->('baz stuff blarch') ? "ok\n" : "not ok\n";
1151 print $bad->('foo stuff bar') ? "not ok\n" : "ok\n";
1153 For most builds of Perl5, this will print:
1158 build_match() will always return a sub which matches the contents of
1159 $left and $right as they were the I<first> time that build_match()
1160 was called, not as they are in the current call.
1162 =item * Regular Expression
1164 If no parentheses are used in a match, Perl4 sets C<$+> to
1165 the whole match, just like C<$&>. Perl5 does not.
1170 # perl4 prints: bcde
1173 =item * Regular Expression
1175 substitution now returns the null string if it fails
1178 $value = ($string =~ s/foo//);
1184 Also see L<Numerical Traps> for another example of this new feature.
1186 =item * Regular Expression
1188 C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no
1192 $string =~ s`^`hostname`;
1193 print $string, "\n";
1195 # perl4 prints: <the local hostname>
1196 # perl5 prints: hostname
1198 =item * Regular Expression
1200 Stricter parsing of variables used in regular expressions
1202 s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
1204 # perl4: compiles w/o error
1205 # perl5: with Scalar found where operator expected ..., near "$opt$plus"
1207 an added component of this example, apparently from the same script, is
1208 the actual value of the s'd string after the substitution.
1209 C<[$opt]> is a character class in perl4 and an array subscript in perl5
1214 s/^([^$grpc]*$grpc[$opt]?)/foo/;
1218 # perl5 prints: foobar
1220 =item * Regular Expression
1222 Under perl5, C<m?x?> matches only once, like C<?x?>. Under perl4, it matched
1223 repeatedly, like C</x/> or C<m!x!>.
1226 sub match { $test =~ m?once?; }
1229 # m?x? matches more then once
1232 # m?x? matches only once
1236 # perl4 prints: perl4
1237 # perl5 prints: perl5
1239 =item * Regular Expression
1241 Unlike in Ruby, failed matches in Perl do not reset the match variables
1242 ($1, $2, ..., C<$`>, ...).
1246 =head2 Subroutine, Signal, Sorting Traps
1248 The general group of Perl4-to-Perl5 traps having to do with
1249 Signals, Sorting, and their related subroutines, as well as
1250 general subroutine traps. Includes some OS-Specific traps.
1256 Barewords that used to look like strings to Perl will now look like subroutine
1257 calls if a subroutine by that name is defined before the compiler sees them.
1259 sub SeeYa { warn"Hasta la vista, baby!" }
1260 $SIG{'TERM'} = SeeYa;
1261 print "SIGTERM is now $SIG{'TERM'}\n";
1263 # perl4 prints: SIGTERM is now main'SeeYa
1264 # perl5 prints: SIGTERM is now main::1 (and warns "Hasta la vista, baby!")
1266 Use B<-w> to catch this one
1268 =item * (Sort Subroutine)
1270 reverse is no longer allowed as the name of a sort subroutine.
1272 sub reverse{ print "yup "; $a <=> $b }
1273 print sort reverse (2,1,3);
1275 # perl4 prints: yup yup 123
1277 # perl5 warns (if using -w): Ambiguous call resolved as CORE::reverse()
1279 =item * warn() won't let you specify a filehandle.
1281 Although it _always_ printed to STDERR, warn() would let you specify a
1282 filehandle in perl4. With perl5 it does not.
1286 # perl4 prints: Foo!
1287 # perl5 prints: String found where operator expected
1297 Under HPUX, and some other SysV OSes, one had to reset any signal handler,
1298 within the signal handler function, each time a signal was handled with
1299 perl4. With perl5, the reset is now done correctly. Any code relying
1300 on the handler _not_ being reset will have to be reworked.
1302 Since version 5.002, Perl uses sigaction() under SysV.
1307 $SIG{'INT'} = 'gotit';
1316 while (1) {sleep(10);}
1319 # perl4 (HPUX) prints: Got INT...
1320 # perl5 (HPUX) prints: Got INT... Got INT...
1324 Under SysV OSes, C<seek()> on a file opened to append C<<< >> >>> now does
1325 the right thing w.r.t. the fopen() manpage. e.g., - When a file is opened
1326 for append, it is impossible to overwrite information already in
1329 open(TEST,">>seek.test");
1330 $start = tell TEST ;
1335 seek(TEST,$start,0);
1336 print TEST "18 characters here";
1338 # perl4 (solaris) seek.test has: 18 characters here
1339 # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here
1345 =head2 Interpolation Traps
1347 Perl4-to-Perl5 traps having to do with how things get interpolated
1348 within certain expressions, statements, contexts, or whatever.
1352 =item * Interpolation
1354 @ now always interpolates an array in double-quotish strings.
1356 print "To: someone@somewhere.com\n";
1358 # perl4 prints: To:someone@somewhere.com
1359 # perl < 5.6.1, error : In string, @somewhere now must be written as \@somewhere
1360 # perl >= 5.6.1, warning : Possible unintended interpolation of @somewhere in string
1362 =item * Interpolation
1364 Double-quoted strings may no longer end with an unescaped $.
1367 print "foo is $foo\n";
1369 # perl4 prints: foo is foo$
1370 # perl5 errors: Final $ should be \$ or $name
1372 Note: perl5 DOES NOT error on the terminating @ in $bar
1374 =item * Interpolation
1376 Perl now sometimes evaluates arbitrary expressions inside braces that occur
1377 within double quotes (usually when the opening brace is preceded by C<$>
1383 sub foo { return "bar" };
1384 print "|@{w.w.w}|${main'foo}|";
1386 # perl4 prints: |@{w.w.w}|foo|
1387 # perl5 prints: |buz|bar|
1389 Note that you can C<use strict;> to ward off such trappiness under perl5.
1391 =item * Interpolation
1393 The construct "this is $$x" used to interpolate the pid at that point, but
1394 now tries to dereference $x. C<$$> by itself still works fine, however.
1398 print "this is $$x\n";
1400 # perl4 prints: this is XXXx (XXX is the current pid)
1401 # perl5 prints: this is a reference
1403 =item * Interpolation
1405 Creation of hashes on the fly with C<eval "EXPR"> now requires either both
1406 C<$>'s to be protected in the specification of the hash name, or both curlies
1407 to be protected. If both curlies are protected, the result will be compatible
1408 with perl4 and perl5. This is a very common practice, and should be changed
1409 to use the block form of C<eval{}> if possible.
1411 $hashname = "foobar";
1414 eval "\$$hashname{'$key'} = q|$value|";
1415 (defined($foobar{'baz'})) ? (print "Yup") : (print "Nope");
1418 # perl5 prints: Nope
1422 eval "\$$hashname{'$key'} = q|$value|";
1426 eval "\$\$hashname{'$key'} = q|$value|";
1428 causes the following result:
1430 # perl4 prints: Nope
1435 eval "\$$hashname\{'$key'\} = q|$value|";
1437 causes the following result:
1441 # and is compatible for both versions
1444 =item * Interpolation
1446 perl4 programs which unconsciously rely on the bugs in earlier perl versions.
1448 perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'
1450 # perl4 prints: This is not perl5
1451 # perl5 prints: This is perl5
1453 =item * Interpolation
1455 You also have to be careful about array and hash brackets during
1461 perl 5 prints: syntax error
1466 perl 5 prints: syntax error
1468 Perl 5 is expecting to find an index or key name following the respective
1469 brackets, as well as an ending bracket of the appropriate type. In order
1470 to mimic the behavior of Perl 4, you must escape the bracket like so.
1475 =item * Interpolation
1477 Similarly, watch out for:
1480 print "\$$foo{bar}\n";
1482 # perl4 prints: $baz{bar}
1485 Perl 5 is looking for C<$foo{bar}> which doesn't exist, but perl 4 is
1486 happy just to expand $foo to "baz" by itself. Watch out for this
1487 especially in C<eval>'s.
1489 =item * Interpolation
1491 C<qq()> string passed to C<eval>
1494 foreach \$y (keys %\$x\) {
1499 # perl4 runs this ok
1500 # perl5 prints: Can't find string terminator ")"
1512 Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1513 may cause the same script, run under perl5, to fail. The build of perl5
1514 must have been linked with the same dbm/ndbm as the default for C<dbmopen()>
1515 to function properly without C<tie>'ing to an extension dbm implementation.
1517 dbmopen (%dbm, "file", undef);
1521 # perl5 prints: ok (IFF linked with -ldbm or -lndbm)
1526 Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
1527 may cause the same script, run under perl5, to fail. The error generated
1528 when exceeding the limit on the key/value size will cause perl5 to exit
1531 dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";
1532 $DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm
1536 dbm store returned -1, errno 28, key "trap" at - line 3.
1540 dbm store returned -1, errno 28, key "trap" at - line 3.
1544 =head2 Unclassified Traps
1550 =item * C<require>/C<do> trap using returned value
1552 If the file doit.pl has:
1560 And the do.pl file has the following single line:
1564 Running doit.pl gives the following:
1566 # perl 4 prints: 3 (aborts the subroutine early)
1569 Same behavior if you replace C<do> with C<require>.
1571 =item * C<split> on empty string with LIMIT specified
1574 @list = split(/foo/, $string, 2)
1576 Perl4 returns a one element list containing the empty string but Perl5
1577 returns an empty list.
1581 As always, if any of these are ever officially declared as bugs,
1582 they'll be fixed and removed.