=head1 DESCRIPTION
-The biggest trap of all is forgetting to use the B<-w> switch; see
-L<perlrun>. The second biggest trap is not making your entire program
-runnable under C<use strict>. The third biggest trap is not reading
-the list of changes in this version of Perl; see L<perldelta>.
+The biggest trap of all is forgetting to C<use warnings> or use the B<-w>
+switch; see L<perllexwarn> and L<perlrun>. The second biggest trap is not
+making your entire program runnable under C<use strict>. The third biggest
+trap is not reading the list of changes in this version of Perl; see
+L<perldelta>.
=head2 Awk Traps
=item *
+A Perl program executes only once, not once for each input line. You can
+do an implicit loop with C<-n> or C<-p>.
+
+=item *
+
The English module, loaded via
use English;
=item *
-$E<lt>I<digit>E<gt> does not refer to fields--it refers to substrings matched
+$<I<digit>> does not refer to fields--it refers to substrings matched
by the last match pattern.
=item *
The concatenation operator is ".", not the null string. (Using the
null string would render C</pat/ /pat/> unparsable, because the third slash
would be interpreted as a division operator--the tokenizer is in fact
-slightly context sensitive for operators like "/", "?", and "E<gt>".
+slightly context sensitive for operators like "/", "?", and ">".
And in fact, "." itself can be the beginning of a number.)
=item *
The following variables work differently:
Awk Perl
- ARGC $#ARGV or scalar @ARGV
+ ARGC scalar @ARGV (compare with $#ARGV)
ARGV[0] $0
FILENAME $ARGV
FNR $. - something
=back
-=head2 C Traps
+=head2 C/C++ Traps
-Cerebral C programmers should take note of the following:
+Cerebral C and C++ programmers should take note of the following:
=over 4
=item *
-The C<break> and C<continue> keywords from C become in
-Perl C<last> and C<next>, respectively.
-Unlike in C, these do I<not> work within a C<do { } while> construct.
+The C<break> and C<continue> keywords from C become in Perl C<last>
+and C<next>, respectively. Unlike in C, these do I<not> work within a
+C<do { } while> construct. See L<perlsyn/"Loop Control">.
=item *
-There's no switch statement. (But it's easy to build one on the fly.)
+The switch statement is called C<given/when> and only available in
+perl 5.10 or newer. See L<perlsyn/"Switch Statements">.
=item *
=item *
-C<printf()> does not implement the "*" format for interpolating
-field widths, but it's trivial to use interpolation of double-quoted
-strings to achieve the same effect.
-
-=item *
-
-Comments begin with "#", not "/*".
+Comments begin with "#", not "/*" or "//". Perl may interpret C/C++
+comments as division operators, unterminated regular expressions or
+the defined-or operator.
=item *
=item *
System calls such as link(), unlink(), rename(), etc. return nonzero for
-success, not 0.
+success, not 0. (system(), however, returns zero for success.)
=item *
=back
+=head2 JavaScript Traps
+
+Judicious JavaScript programmers should take note of the following:
+
+=over 4
+
+=item *
+
+In Perl, binary C<+> is always addition. C<$string1 + $string2> converts
+both strings to numbers and then adds them. To concatenate two strings,
+use the C<.> operator.
+
+=item *
+
+The C<+> unary operator doesn't do anything in Perl. It exists to avoid
+syntactic ambiguities.
+
+=item *
+
+Unlike C<for...in>, Perl's C<for> (also spelled C<foreach>) does not allow
+the left-hand side to be an arbitrary expression. It must be a variable:
+
+ for my $variable (keys %hash) {
+ ...
+ }
+
+Furthermore, don't forget the C<keys> in there, as
+C<foreach my $kv (%hash) {}> iterates over the keys and values, and is
+generally not useful ($kv would be a key, then a value, and so on).
+
+=item *
+
+To iterate over the indices of an array, use C<foreach my $i (0 .. $#array)
+{}>. C<foreach my $v (@array) {}> iterates over the values.
+
+=item *
+
+Perl requires braces following C<if>, C<while>, C<foreach>, etc.
+
+=item *
+
+In Perl, C<else if> is spelled C<elsif>.
+
+=item *
+
+C<? :> has higher precedence than assignment. In JavaScript, one can
+write:
+
+ condition ? do_something() : variable = 3
+
+and the variable is only assigned if the condition is false. In Perl, you
+need parentheses:
+
+ $condition ? do_something() : ($variable = 3);
+
+Or just use C<if>.
+
+=item *
+
+Perl requires semicolons to separate statements.
+
+=item *
+
+Variables declared with C<my> only affect code I<after> the declaration.
+You cannot write C<$x = 1; my $x;> and expect the first assignment to
+affect the same variable. It will instead assign to an C<$x> declared
+previously in an outer scope, or to a global variable.
+
+Note also that the variable is not visible until the following
+I<statement>. This means that in C<my $x = 1 + $x> the second $x refers
+to one declared previously.
+
+=item *
+
+C<my> variables are scoped to the current block, not to the current
+function. If you write C<{my $x;} $x;>, the second C<$x> does not refer to
+the one declared inside the block.
+
+=item *
+
+An object's members cannot be made accessible as variables. The closest
+Perl equivalent to C<with(object) { method() }> is C<for>, which can alias
+C<$_> to the object:
+
+ for ($object) {
+ $_->method;
+ }
+
+=item *
+
+The object or class on which a method is called is passed as one of the
+method's arguments, not as a separate C<this> value.
+
+=back
+
=head2 Sed Traps
Seasoned B<sed> programmers should take note of the following:
=item *
+A Perl program executes only once, not once for each input line. You can
+do an implicit loop with C<-n> or C<-p>.
+
+=item *
+
Backreferences in substitutions use "$" rather than "\".
=item *
The environment is not automatically made available as separate scalar
variables.
+=item *
+
+The shell's C<test> uses "=", "!=", "<" etc for string comparisons and "-eq",
+"-ne", "-lt" etc for numeric comparisons. This is the reverse of Perl, which
+uses C<eq>, C<ne>, C<lt> for string comparisons, and C<==>, C<!=> C<< < >> etc
+for numeric comparisons.
+
=back
=head2 Perl Traps
You cannot discern from mere inspection which builtins
are unary operators (like chop() and chdir())
and which are list operators (like print() and unlink()).
-(User-defined subroutines can be B<only> list operators, never
-unary ones.) See L<perlop>.
+(Unless prototyped, user-defined subroutines can B<only> be list
+operators, never unary ones.) See L<perlop> and L<perlsub>.
=item *
=item *
-The E<lt>FHE<gt> construct is not the name of the filehandle, it is a readline
+The <FH> construct is not the name of the filehandle, it is a readline
operation on that handle. The data read is assigned to $_ only if the
file read is the sole condition in a while loop:
=back
-=head2 Perl4 to Perl5 Traps
-
-Practicing Perl4 Programmers should take note of the following
-Perl4-to-Perl5 specific traps.
-
-They're crudely ordered according to the following list:
-
-=over 4
-
-=item Discontinuance, Deprecation, and BugFix traps
-
-Anything that's been fixed as a perl4 bug, removed as a perl4 feature
-or deprecated as a perl4 feature with the intent to encourage usage of
-some other perl5 feature.
-
-=item Parsing Traps
-
-Traps that appear to stem from the new parser.
-
-=item Numerical Traps
-
-Traps having to do with numerical or mathematical operators.
-
-=item General data type traps
-
-Traps involving perl standard data types.
-
-=item Context Traps - scalar, list contexts
-
-Traps related to context within lists, scalar statements/declarations.
-
-=item Precedence Traps
-
-Traps related to the precedence of parsing, evaluation, and execution of
-code.
-
-=item General Regular Expression Traps using s///, etc.
-
-Traps related to the use of pattern matching.
-
-=item Subroutine, Signal, Sorting Traps
-
-Traps related to the use of signals and signal handlers, general subroutines,
-and sorting, along with sorting subroutines.
-
-=item OS Traps
-
-OS-specific traps.
-
-=item DBM Traps
-
-Traps specific to the use of C<dbmopen()>, and specific dbm implementations.
-
-=item Unclassified Traps
-
-Everything else.
-
-=back
-
-If you find an example of a conversion trap that is not listed here,
-please submit it to Bill Middleton <F<wjm@best.com>> for inclusion.
-Also note that at least some of these can be caught with B<-w>.
-
-=head2 Discontinuance, Deprecation, and BugFix traps
-
-Anything that has been discontinued, deprecated, or fixed as
-a bug from perl4.
-
-=over 4
-
-=item * Discontinuance
-
-Symbols starting with "_" are no longer forced into package main, except
-for C<$_> itself (and C<@_>, etc.).
-
- package test;
- $_legacy = 1;
-
- package main;
- print "\$_legacy is ",$_legacy,"\n";
-
- # perl4 prints: $_legacy is 1
- # perl5 prints: $_legacy is
-
-=item * Deprecation
-
-Double-colon is now a valid package separator in a variable name. Thus these
-behave differently in perl4 vs. perl5, because the packages don't exist.
-
- $a=1;$b=2;$c=3;$var=4;
- print "$a::$b::$c ";
- print "$var::abc::xyz\n";
-
- # perl4 prints: 1::2::3 4::abc::xyz
- # perl5 prints: 3
-
-Given that C<::> is now the preferred package delimiter, it is debatable
-whether this should be classed as a bug or not.
-(The older package delimiter, ' ,is used here)
-
- $x = 10 ;
- print "x=${'x}\n" ;
-
- # perl4 prints: x=10
- # perl5 prints: Can't find string terminator "'" anywhere before EOF
-
-You can avoid this problem, and remain compatible with perl4, if you
-always explicitly include the package name:
-
- $x = 10 ;
- print "x=${main'x}\n" ;
-
-Also see precedence traps, for parsing C<$:>.
-
-=item * BugFix
-
-The second and third arguments of C<splice()> are now evaluated in scalar
-context (as the Camel says) rather than list context.
-
- sub sub1{return(0,2) } # return a 2-element list
- sub sub2{ return(1,2,3)} # return a 3-element list
- @a1 = ("a","b","c","d","e");
- @a2 = splice(@a1,&sub1,&sub2);
- print join(' ',@a2),"\n";
-
- # perl4 prints: a b
- # perl5 prints: c d e
-
-=item * Discontinuance
-
-You can't do a C<goto> into a block that is optimized away. Darn.
-
- goto marker1;
-
- for(1){
- marker1:
- print "Here I is!\n";
- }
-
- # perl4 prints: Here I is!
- # perl5 dumps core (SEGV)
-
-=item * Discontinuance
-
-It is no longer syntactically legal to use whitespace as the name
-of a variable, or as a delimiter for any kind of quote construct.
-Double darn.
-
- $a = ("foo bar");
- $b = q baz ;
- print "a is $a, b is $b\n";
-
- # perl4 prints: a is foo bar, b is baz
- # perl5 errors: Bareword found where operator expected
-
-=item * Discontinuance
-
-The archaic while/if BLOCK BLOCK syntax is no longer supported.
-
- if { 1 } {
- print "True!";
- }
- else {
- print "False!";
- }
-
- # perl4 prints: True!
- # perl5 errors: syntax error at test.pl line 1, near "if {"
-
-=item * BugFix
-
-The C<**> operator now binds more tightly than unary minus.
-It was documented to work this way before, but didn't.
-
- print -4**2,"\n";
-
- # perl4 prints: 16
- # perl5 prints: -16
-
-=item * Discontinuance
-
-The meaning of C<foreach{}> has changed slightly when it is iterating over a
-list which is not an array. This used to assign the list to a
-temporary array, but no longer does so (for efficiency). This means
-that you'll now be iterating over the actual values, not over copies of
-the values. Modifications to the loop variable can change the original
-values.
-
- @list = ('ab','abc','bcd','def');
- foreach $var (grep(/ab/,@list)){
- $var = 1;
- }
- print (join(':',@list));
-
- # perl4 prints: ab:abc:bcd:def
- # perl5 prints: 1:1:bcd:def
-
-To retain Perl4 semantics you need to assign your list
-explicitly to a temporary array and then iterate over that. For
-example, you might need to change
-
- foreach $var (grep(/ab/,@list)){
-
-to
-
- foreach $var (@tmp = grep(/ab/,@list)){
-
-Otherwise changing $var will clobber the values of @list. (This most often
-happens when you use C<$_> for the loop variable, and call subroutines in
-the loop that don't properly localize C<$_>.)
-
-=item * Discontinuance
-
-C<split> with no arguments now behaves like C<split ' '> (which doesn't
-return an initial null field if $_ starts with whitespace), it used to
-behave like C<split /\s+/> (which does).
-
- $_ = ' hi mom';
- print join(':', split);
-
- # perl4 prints: :hi:mom
- # perl5 prints: hi:mom
-
-=item * BugFix
-
-Perl 4 would ignore any text which was attached to an B<-e> switch,
-always taking the code snippet from the following arg. Additionally, it
-would silently accept an B<-e> switch without a following arg. Both of
-these behaviors have been fixed.
-
- perl -e'print "attached to -e"' 'print "separate arg"'
-
- # perl4 prints: separate arg
- # perl5 prints: attached to -e
-
- perl -e
-
- # perl4 prints:
- # perl5 dies: No code specified for -e.
-
-=item * Discontinuance
-
-In Perl 4 the return value of C<push> was undocumented, but it was
-actually the last value being pushed onto the target list. In Perl 5
-the return value of C<push> is documented, but has changed, it is the
-number of elements in the resulting list.
-
- @x = ('existing');
- print push(@x, 'first new', 'second new');
-
- # perl4 prints: second new
- # perl5 prints: 3
-
-=item * Discontinuance
-
-In Perl 4 (and versions of Perl 5 before 5.004), C<'\r'> characters in
-Perl code were silently allowed, although they could cause (mysterious!)
-failures in certain constructs, particularly here documents. Now,
-C<'\r'> characters cause an immediate fatal error. (Note: In this
-example, the notation B<\015> represents the incorrect line
-ending. Depending upon your text viewer, it will look different.)
-
- print "foo";\015
- print "bar";
-
- # perl4 prints: foobar
- # perl5.003 prints: foobar
- # perl5.004 dies: Illegal character \015 (carriage return)
-
-See L<perldiag> for full details.
-
-=item * Deprecation
-
-Some error messages will be different.
-
-=item * Discontinuance
-
-Some bugs may have been inadvertently removed. :-)
-
-=back
-
-=head2 Parsing Traps
-
-Perl4-to-Perl5 traps from having to do with parsing.
-
-=over 4
-
-=item * Parsing
-
-Note the space between . and =
-
- $string . = "more string";
- print $string;
-
- # perl4 prints: more string
- # perl5 prints: syntax error at - line 1, near ". ="
-
-=item * Parsing
-
-Better parsing in perl 5
-
- sub foo {}
- &foo
- print("hello, world\n");
-
- # perl4 prints: hello, world
- # perl5 prints: syntax error
-
-=item * Parsing
-
-"if it looks like a function, it is a function" rule.
-
- print
- ($foo == 1) ? "is one\n" : "is zero\n";
-
- # perl4 prints: is zero
- # perl5 warns: "Useless use of a constant in void context" if using -w
-
-=item * Parsing
-
-String interpolation of the C<$#array> construct differs when braces
-are to used around the name.
-
- @ = (1..3);
- print "${#a}";
-
- # perl4 prints: 2
- # perl5 fails with syntax error
-
- @ = (1..3);
- print "$#{a}";
-
- # perl4 prints: {a}
- # perl5 prints: 2
-
-=back
-
-=head2 Numerical Traps
-
-Perl4-to-Perl5 traps having to do with numerical operators,
-operands, or output from same.
-
-=over 5
-
-=item * Numerical
-
-Formatted output and significant digits
-
- print 7.373504 - 0, "\n";
- printf "%20.18f\n", 7.373504 - 0;
-
- # Perl4 prints:
- 7.375039999999996141
- 7.37503999999999614
-
- # Perl5 prints:
- 7.373504
- 7.37503999999999614
-
-=item * Numerical
-
-This specific item has been deleted. It demonstrated how the auto-increment
-operator would not catch when a number went over the signed int limit. Fixed
-in version 5.003_04. But always be wary when using large integers.
-If in doubt:
-
- use Math::BigInt;
-
-=item * Numerical
-
-Assignment of return values from numeric equality tests
-does not work in perl5 when the test evaluates to false (0).
-Logical tests now return an null, instead of 0
-
- $p = ($test == 1);
- print $p,"\n";
-
- # perl4 prints: 0
- # perl5 prints:
-
-Also see L<"General Regular Expression Traps using s///, etc.">
-for another example of this new feature...
-
-=item * Bitwise string ops
-
-When bitwise operators which can operate upon either numbers or
-strings (C<& | ^ ~>) are given only strings as arguments, perl4 would
-treat the operands as bitstrings so long as the program contained a call
-to the C<vec()> function. perl5 treats the string operands as bitstrings.
-(See L<perlop/Bitwise String Operators> for more details.)
-
- $fred = "10";
- $barney = "12";
- $betty = $fred & $barney;
- print "$betty\n";
- # Uncomment the next line to change perl4's behavior
- # ($dummy) = vec("dummy", 0, 0);
-
- # Perl4 prints:
- 8
-
- # Perl5 prints:
- 10
-
- # If vec() is used anywhere in the program, both print:
- 10
-
-=back
-
-=head2 General data type traps
-
-Perl4-to-Perl5 traps involving most data-types, and their usage
-within certain expressions and/or context.
-
-=over 5
-
-=item * (Arrays)
-
-Negative array subscripts now count from the end of the array.
-
- @a = (1, 2, 3, 4, 5);
- print "The third element of the array is $a[3] also expressed as $a[-2] \n";
-
- # perl4 prints: The third element of the array is 4 also expressed as
- # perl5 prints: The third element of the array is 4 also expressed as 4
-
-=item * (Arrays)
-
-Setting C<$#array> lower now discards array elements, and makes them
-impossible to recover.
-
- @a = (a,b,c,d,e);
- print "Before: ",join('',@a);
- $#a =1;
- print ", After: ",join('',@a);
- $#a =3;
- print ", Recovered: ",join('',@a),"\n";
-
- # perl4 prints: Before: abcde, After: ab, Recovered: abcd
- # perl5 prints: Before: abcde, After: ab, Recovered: ab
-
-=item * (Hashes)
-
-Hashes get defined before use
-
- local($s,@a,%h);
- die "scalar \$s defined" if defined($s);
- die "array \@a defined" if defined(@a);
- die "hash \%h defined" if defined(%h);
-
- # perl4 prints:
- # perl5 dies: hash %h defined
-
-Perl will now generate a warning when it sees defined(@a) and
-defined(%h).
-
-=item * (Globs)
-
-glob assignment from variable to variable will fail if the assigned
-variable is localized subsequent to the assignment
-
- @a = ("This is Perl 4");
- *b = *a;
- local(@a);
- print @b,"\n";
-
- # perl4 prints: This is Perl 4
- # perl5 prints:
-
-=item * (Globs)
-
-Assigning C<undef> to a glob has no effect in Perl 5. In Perl 4
-it undefines the associated scalar (but may have other side effects
-including SEGVs).
-
-=item * (Scalar String)
-
-Changes in unary negation (of strings)
-This change effects both the return value and what it
-does to auto(magic)increment.
-
- $x = "aaa";
- print ++$x," : ";
- print -$x," : ";
- print ++$x,"\n";
-
- # perl4 prints: aab : -0 : 1
- # perl5 prints: aab : -aab : aac
-
-=item * (Constants)
-
-perl 4 lets you modify constants:
-
- $foo = "x";
- &mod($foo);
- for ($x = 0; $x < 3; $x++) {
- &mod("a");
- }
- sub mod {
- print "before: $_[0]";
- $_[0] = "m";
- print " after: $_[0]\n";
- }
-
- # perl4:
- # before: x after: m
- # before: a after: m
- # before: m after: m
- # before: m after: m
-
- # Perl5:
- # before: x after: m
- # Modification of a read-only value attempted at foo.pl line 12.
- # before: a
-
-=item * (Scalars)
-
-The behavior is slightly different for:
-
- print "$x", defined $x
-
- # perl 4: 1
- # perl 5: <no output, $x is not called into existence>
-
-=item * (Variable Suicide)
-
-Variable suicide behavior is more consistent under Perl 5.
-Perl5 exhibits the same behavior for hashes and scalars,
-that perl4 exhibits for only scalars.
-
- $aGlobal{ "aKey" } = "global value";
- print "MAIN:", $aGlobal{"aKey"}, "\n";
- $GlobalLevel = 0;
- &test( *aGlobal );
-
- sub test {
- local( *theArgument ) = @_;
- local( %aNewLocal ); # perl 4 != 5.001l,m
- $aNewLocal{"aKey"} = "this should never appear";
- print "SUB: ", $theArgument{"aKey"}, "\n";
- $aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print
- $GlobalLevel++;
- if( $GlobalLevel<4 ) {
- &test( *aNewLocal );
- }
- }
-
- # Perl4:
- # MAIN:global value
- # SUB: global value
- # SUB: level 0
- # SUB: level 1
- # SUB: level 2
-
- # Perl5:
- # MAIN:global value
- # SUB: global value
- # SUB: this should never appear
- # SUB: this should never appear
- # SUB: this should never appear
-
-=back
-
-=head2 Context Traps - scalar, list contexts
-
-=over 5
-
-=item * (list context)
-
-The elements of argument lists for formats are now evaluated in list
-context. This means you can interpolate list values now.
-
- @fmt = ("foo","bar","baz");
- format STDOUT=
- @<<<<< @||||| @>>>>>
- @fmt;
- .
- write;
-
- # perl4 errors: Please use commas to separate fields in file
- # perl5 prints: foo bar baz
-
-=item * (scalar context)
-
-The C<caller()> function now returns a false value in a scalar context
-if there is no caller. This lets library files determine if they're
-being required.
-
- caller() ? (print "You rang?\n") : (print "Got a 0\n");
-
- # perl4 errors: There is no caller
- # perl5 prints: Got a 0
-
-=item * (scalar context)
-
-The comma operator in a scalar context is now guaranteed to give a
-scalar context to its arguments.
-
- @y= ('a','b','c');
- $x = (1, 2, @y);
- print "x = $x\n";
-
- # Perl4 prints: x = c # Thinks list context interpolates list
- # Perl5 prints: x = 3 # Knows scalar uses length of list
-
-=item * (list, builtin)
-
-C<sprintf()> funkiness (array argument converted to scalar array count)
-This test could be added to t/op/sprintf.t
-
- @z = ('%s%s', 'foo', 'bar');
- $x = sprintf(@z);
- if ($x eq 'foobar') {print "ok 2\n";} else {print "not ok 2 '$x'\n";}
-
- # perl4 prints: ok 2
- # perl5 prints: not ok 2
-
-C<printf()> works fine, though:
-
- printf STDOUT (@z);
- print "\n";
-
- # perl4 prints: foobar
- # perl5 prints: foobar
-
-Probably a bug.
-
-=back
-
-=head2 Precedence Traps
-
-Perl4-to-Perl5 traps involving precedence order.
-
-Perl 4 has almost the same precedence rules as Perl 5 for the operators
-that they both have. Perl 4 however, seems to have had some
-inconsistencies that made the behavior differ from what was documented.
-
-=over 5
-
-=item * Precedence
-
-LHS vs. RHS of any assignment operator. LHS is evaluated first
-in perl4, second in perl5; this can affect the relationship
-between side-effects in sub-expressions.
-
- @arr = ( 'left', 'right' );
- $a{shift @arr} = shift @arr;
- print join( ' ', keys %a );
-
- # perl4 prints: left
- # perl5 prints: right
-
-=item * Precedence
-
-These are now semantic errors because of precedence:
-
- @list = (1,2,3,4,5);
- %map = ("a",1,"b",2,"c",3,"d",4);
- $n = shift @list + 2; # first item in list plus 2
- print "n is $n, ";
- $m = keys %map + 2; # number of items in hash plus 2
- print "m is $m\n";
-
- # perl4 prints: n is 3, m is 6
- # perl5 errors and fails to compile
-
-=item * Precedence
-
-The precedence of assignment operators is now the same as the precedence
-of assignment. Perl 4 mistakenly gave them the precedence of the associated
-operator. So you now must parenthesize them in expressions like
-
- /foo/ ? ($a += 2) : ($a -= 2);
-
-Otherwise
-
- /foo/ ? $a += 2 : $a -= 2
-
-would be erroneously parsed as
-
- (/foo/ ? $a += 2 : $a) -= 2;
-
-On the other hand,
-
- $a += /foo/ ? 1 : 2;
-
-now works as a C programmer would expect.
-
-=item * Precedence
-
- open FOO || die;
-
-is now incorrect. You need parentheses around the filehandle.
-Otherwise, perl5 leaves the statement as its default precedence:
-
- open(FOO || die);
-
- # perl4 opens or dies
- # perl5 errors: Precedence problem: open FOO should be open(FOO)
-
-=item * Precedence
-
-perl4 gives the special variable, C<$:> precedence, where perl5
-treats C<$::> as main C<package>
-
- $a = "x"; print "$::a";
-
- # perl 4 prints: -:a
- # perl 5 prints: x
-
-=item * Precedence
-
-perl4 had buggy precedence for the file test operators vis-a-vis
-the assignment operators. Thus, although the precedence table
-for perl4 leads one to believe C<-e $foo .= "q"> should parse as
-C<((-e $foo) .= "q")>, it actually parses as C<(-e ($foo .= "q"))>.
-In perl5, the precedence is as documented.
-
- -e $foo .= "q"
-
- # perl4 prints: no output
- # perl5 prints: Can't modify -e in concatenation
-
-=item * Precedence
-
-In perl4, keys(), each() and values() were special high-precedence operators
-that operated on a single hash, but in perl5, they are regular named unary
-operators. As documented, named unary operators have lower precedence
-than the arithmetic and concatenation operators C<+ - .>, but the perl4
-variants of these operators actually bind tighter than C<+ - .>.
-Thus, for:
-
- %foo = 1..10;
- print keys %foo - 1
-
- # perl4 prints: 4
- # perl5 prints: Type of arg 1 to keys must be hash (not subtraction)
-
-The perl4 behavior was probably more useful, if less consistent.
-
-=back
-
-=head2 General Regular Expression Traps using s///, etc.
-
-All types of RE traps.
-
-=over 5
-
-=item * Regular Expression
-
-C<s'$lhs'$rhs'> now does no interpolation on either side. It used to
-interpolate $lhs but not $rhs. (And still does not match a literal
-'$' in string)
-
- $a=1;$b=2;
- $string = '1 2 $a $b';
- $string =~ s'$a'$b';
- print $string,"\n";
-
- # perl4 prints: $b 2 $a $b
- # perl5 prints: 1 2 $a $b
-
-=item * Regular Expression
-
-C<m//g> now attaches its state to the searched string rather than the
-regular expression. (Once the scope of a block is left for the sub, the
-state of the searched string is lost)
-
- $_ = "ababab";
- while(m/ab/g){
- &doit("blah");
- }
- sub doit{local($_) = shift; print "Got $_ "}
-
- # perl4 prints: blah blah blah
- # perl5 prints: infinite loop blah...
-
-=item * Regular Expression
-
-Currently, if you use the C<m//o> qualifier on a regular expression
-within an anonymous sub, I<all> closures generated from that anonymous
-sub will use the regular expression as it was compiled when it was used
-the very first time in any such closure. For instance, if you say
-
- sub build_match {
- my($left,$right) = @_;
- return sub { $_[0] =~ /$left stuff $right/o; };
- }
-
-build_match() will always return a sub which matches the contents of
-$left and $right as they were the I<first> time that build_match()
-was called, not as they are in the current call.
-
-This is probably a bug, and may change in future versions of Perl.
-
-=item * Regular Expression
-
-If no parentheses are used in a match, Perl4 sets C<$+> to
-the whole match, just like C<$&>. Perl5 does not.
-
- "abcdef" =~ /b.*e/;
- print "\$+ = $+\n";
-
- # perl4 prints: bcde
- # perl5 prints:
-
-=item * Regular Expression
-
-substitution now returns the null string if it fails
-
- $string = "test";
- $value = ($string =~ s/foo//);
- print $value, "\n";
-
- # perl4 prints: 0
- # perl5 prints:
-
-Also see L<Numerical Traps> for another example of this new feature.
-
-=item * Regular Expression
-
-C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no
-backtick expansion
-
- $string = "";
- $string =~ s`^`hostname`;
- print $string, "\n";
-
- # perl4 prints: <the local hostname>
- # perl5 prints: hostname
-
-=item * Regular Expression
-
-Stricter parsing of variables used in regular expressions
-
- s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;
-
- # perl4: compiles w/o error
- # perl5: with Scalar found where operator expected ..., near "$opt$plus"
-
-an added component of this example, apparently from the same script, is
-the actual value of the s'd string after the substitution.
-C<[$opt]> is a character class in perl4 and an array subscript in perl5
-
- $grpc = 'a';
- $opt = 'r';
- $_ = 'bar';
- s/^([^$grpc]*$grpc[$opt]?)/foo/;
- print ;
-
- # perl4 prints: foo
- # perl5 prints: foobar
-
-=item * Regular Expression
-
-Under perl5, C<m?x?> matches only once, like C<?x?>. Under perl4, it matched
-repeatedly, like C</x/> or C<m!x!>.
-
- $test = "once";
- sub match { $test =~ m?once?; }
- &match();
- if( &match() ) {
- # m?x? matches more then once
- print "perl4\n";
- } else {
- # m?x? matches only once
- print "perl5\n";
- }
-
- # perl4 prints: perl4
- # perl5 prints: perl5
-
-
-=back
-
-=head2 Subroutine, Signal, Sorting Traps
-
-The general group of Perl4-to-Perl5 traps having to do with
-Signals, Sorting, and their related subroutines, as well as
-general subroutine traps. Includes some OS-Specific traps.
-
-=over 5
-
-=item * (Signals)
-
-Barewords that used to look like strings to Perl will now look like subroutine
-calls if a subroutine by that name is defined before the compiler sees them.
-
- sub SeeYa { warn"Hasta la vista, baby!" }
- $SIG{'TERM'} = SeeYa;
- print "SIGTERM is now $SIG{'TERM'}\n";
-
- # perl4 prints: SIGTERM is main'SeeYa
- # perl5 prints: SIGTERM is now main::1
-
-Use B<-w> to catch this one
-
-=item * (Sort Subroutine)
-
-reverse is no longer allowed as the name of a sort subroutine.
-
- sub reverse{ print "yup "; $a <=> $b }
- print sort reverse a,b,c;
-
- # perl4 prints: yup yup yup yup abc
- # perl5 prints: abc
-
-=item * warn() won't let you specify a filehandle.
-
-Although it _always_ printed to STDERR, warn() would let you specify a
-filehandle in perl4. With perl5 it does not.
-
- warn STDERR "Foo!";
-
- # perl4 prints: Foo!
- # perl5 prints: String found where operator expected
-
-=back
-
-=head2 OS Traps
-
-=over 5
-
-=item * (SysV)
-
-Under HPUX, and some other SysV OSes, one had to reset any signal handler,
-within the signal handler function, each time a signal was handled with
-perl4. With perl5, the reset is now done correctly. Any code relying
-on the handler _not_ being reset will have to be reworked.
-
-Since version 5.002, Perl uses sigaction() under SysV.
-
- sub gotit {
- print "Got @_... ";
- }
- $SIG{'INT'} = 'gotit';
-
- $| = 1;
- $pid = fork;
- if ($pid) {
- kill('INT', $pid);
- sleep(1);
- kill('INT', $pid);
- } else {
- while (1) {sleep(10);}
- }
-
- # perl4 (HPUX) prints: Got INT...
- # perl5 (HPUX) prints: Got INT... Got INT...
-
-=item * (SysV)
-
-Under SysV OSes, C<seek()> on a file opened to append C<E<gt>E<gt>> now does
-the right thing w.r.t. the fopen() manpage. e.g., - When a file is opened
-for append, it is impossible to overwrite information already in
-the file.
-
- open(TEST,">>seek.test");
- $start = tell TEST ;
- foreach(1 .. 9){
- print TEST "$_ ";
- }
- $end = tell TEST ;
- seek(TEST,$start,0);
- print TEST "18 characters here";
-
- # perl4 (solaris) seek.test has: 18 characters here
- # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here
-
-
-
-=back
-
-=head2 Interpolation Traps
-
-Perl4-to-Perl5 traps having to do with how things get interpolated
-within certain expressions, statements, contexts, or whatever.
-
-=over 5
-
-=item * Interpolation
-
-@ now always interpolates an array in double-quotish strings.
-
- print "To: someone@somewhere.com\n";
-
- # perl4 prints: To:someone@somewhere.com
- # perl5 errors : In string, @somewhere now must be written as \@somewhere
-
-=item * Interpolation
-
-Double-quoted strings may no longer end with an unescaped $ or @.
-
- $foo = "foo$";
- $bar = "bar@";
- print "foo is $foo, bar is $bar\n";
-
- # perl4 prints: foo is foo$, bar is bar@
- # perl5 errors: Final $ should be \$ or $name
-
-Note: perl5 DOES NOT error on the terminating @ in $bar
-
-=item * Interpolation
-
-Perl now sometimes evaluates arbitrary expressions inside braces that occur
-within double quotes (usually when the opening brace is preceded by C<$>
-or C<@>).
-
- @www = "buz";
- $foo = "foo";
- $bar = "bar";
- sub foo { return "bar" };
- print "|@{w.w.w}|${main'foo}|";
-
- # perl4 prints: |@{w.w.w}|foo|
- # perl5 prints: |buz|bar|
-
-Note that you can C<use strict;> to ward off such trappiness under perl5.
-
-=item * Interpolation
-
-The construct "this is $$x" used to interpolate the pid at that
-point, but now apparently tries to dereference $x. C<$$> by itself still
-works fine, however.
-
- print "this is $$x\n";
-
- # perl4 prints: this is XXXx (XXX is the current pid)
- # perl5 prints: this is
-
-=item * Interpolation
-
-Creation of hashes on the fly with C<eval "EXPR"> now requires either both
-C<$>'s to be protected in the specification of the hash name, or both curlies
-to be protected. If both curlies are protected, the result will be compatible
-with perl4 and perl5. This is a very common practice, and should be changed
-to use the block form of C<eval{}> if possible.
-
- $hashname = "foobar";
- $key = "baz";
- $value = 1234;
- eval "\$$hashname{'$key'} = q|$value|";
- (defined($foobar{'baz'})) ? (print "Yup") : (print "Nope");
-
- # perl4 prints: Yup
- # perl5 prints: Nope
-
-Changing
-
- eval "\$$hashname{'$key'} = q|$value|";
-
-to
-
- eval "\$\$hashname{'$key'} = q|$value|";
-
-causes the following result:
-
- # perl4 prints: Nope
- # perl5 prints: Yup
-
-or, changing to
-
- eval "\$$hashname\{'$key'\} = q|$value|";
-
-causes the following result:
-
- # perl4 prints: Yup
- # perl5 prints: Yup
- # and is compatible for both versions
-
-
-=item * Interpolation
-
-perl4 programs which unconsciously rely on the bugs in earlier perl versions.
-
- perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'
-
- # perl4 prints: This is not perl5
- # perl5 prints: This is perl5
-
-=item * Interpolation
-
-You also have to be careful about array references.
-
- print "$foo{"
-
- perl 4 prints: {
- perl 5 prints: syntax error
-
-=item * Interpolation
-
-Similarly, watch out for:
-
- $foo = "array";
- print "\$$foo{bar}\n";
-
- # perl4 prints: $array{bar}
- # perl5 prints: $
-
-Perl 5 is looking for C<$array{bar}> which doesn't exist, but perl 4 is
-happy just to expand $foo to "array" by itself. Watch out for this
-especially in C<eval>'s.
-
-=item * Interpolation
-
-C<qq()> string passed to C<eval>
-
- eval qq(
- foreach \$y (keys %\$x\) {
- \$count++;
- }
- );
-
- # perl4 runs this ok
- # perl5 prints: Can't find string terminator ")"
-
-=back
-
-=head2 DBM Traps
-
-General DBM traps.
-
-=over 5
-
-=item * DBM
-
-Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
-may cause the same script, run under perl5, to fail. The build of perl5
-must have been linked with the same dbm/ndbm as the default for C<dbmopen()>
-to function properly without C<tie>'ing to an extension dbm implementation.
-
- dbmopen (%dbm, "file", undef);
- print "ok\n";
-
- # perl4 prints: ok
- # perl5 prints: ok (IFF linked with -ldbm or -lndbm)
-
-
-=item * DBM
-
-Existing dbm databases created under perl4 (or any other dbm/ndbm tool)
-may cause the same script, run under perl5, to fail. The error generated
-when exceeding the limit on the key/value size will cause perl5 to exit
-immediately.
-
- dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";
- $DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm
- print "YUP\n";
-
- # perl4 prints:
- dbm store returned -1, errno 28, key "trap" at - line 3.
- YUP
-
- # perl5 prints:
- dbm store returned -1, errno 28, key "trap" at - line 3.
-
-=back
-
-=head2 Unclassified Traps
-
-Everything else.
-
-=over 5
-
-=item * C<require>/C<do> trap using returned value
-
-If the file doit.pl has:
-
- sub foo {
- $rc = do "./do.pl";
- return 8;
- }
- print &foo, "\n";
-
-And the do.pl file has the following single line:
-
- return 3;
-
-Running doit.pl gives the following:
-
- # perl 4 prints: 3 (aborts the subroutine early)
- # perl 5 prints: 8
-
-Same behavior if you replace C<do> with C<require>.
-
-=item * C<split> on empty string with LIMIT specified
-
- $string = '';
- @list = split(/foo/, $string, 2)
-
-Perl4 returns a one element list containing the empty string but Perl5
-returns an empty list.
-
-=back
-
As always, if any of these are ever officially declared as bugs,
they'll be fixed and removed.
https://perl5.git.perl.org / perl5.git / blobdiff