use English;
allows you to refer to special variables (like C<$/>) with names (like
-C<$RS>), as though they were in B<awk>; see L<perlvar> for details.
+$RS), as though they were in B<awk>; see L<perlvar> for details.
=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.
+Unlike in C, these do I<not> work within a C<do { } while> construct.
=item *
=item *
-Remember not to use "C<=>" when you need "C<=~>";
+Remember not to use C<=> when you need C<=~>;
these two constructs are quite different:
$x = /foo/;
# perl4 prints: is zero
# perl5 warns: "Useless use of a constant in void context" if using -w
+=item * Parsing
+
+String interpolation of the C<$#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 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
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
=item * Precedence
-concatenation precedence over filetest operator?
+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 $foo .= "q"
# perl4 prints: no output
# perl5 prints: Can't modify -e in concatenation
+=item * Precedence
+
+In perl4, keys(), each() and values() were special high-precedence operators
+that operated on a single hash, but in perl5, they are regular named unary
+operators. As documented, named unary operators have lower precedence
+than the arithmetic and concatenation operators C<+ - .>, but the perl4
+variants of these operators actually bind tighter than C<+ - .>.
+Thus, for:
+
+ %foo = 1..10;
+ print keys %foo - 1
+
+ # perl4 prints: 4
+ # perl5 prints: Type of arg 1 to keys must be hash (not subtraction)
+
+The perl4 behavior was probably more useful, if less consistent.
+
=back
=head2 General Regular Expression Traps using s///, etc.
=item * Regular Expression
C<s'$lhs'$rhs'> now does no interpolation on either side. It used to
-interpolate C<$lhs> but not C<$rhs>. (And still does not match a literal
+interpolate $lhs but not $rhs. (And still does not match a literal
'$' in string)
$a=1;$b=2;
}
build_match() will always return a sub which matches the contents of
-C<$left> and C<$right> as they were the I<first> time that build_match()
+$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.
# perl5 prints: perl5
-=item * Regular Expression
-
-Under perl4 and upto version 5.003, a failed C<m//g> match used to
-reset the internal iterator, so that subsequent C<m//g> match attempts
-began from the beginning of the string. In perl version 5.004 and later,
-failed C<m//g> matches do not reset the iterator position (which can be
-found using the C<pos()> function--see L<perlfunc/pos>).
-
- $test = "foop";
- for (1..3) {
- print $1 while ($test =~ /(o)/g);
- # pos $test = 0; # to get old behavior
- }
-
- # perl4 prints: oooooo
- # perl5.004 prints: oo
-
-You may always reset the iterator yourself as shown in the commented line
-to get the old behavior.
-
=back
=head2 Subroutine, Signal, Sorting Traps
=item * Interpolation
The construct "this is $$x" used to interpolate the pid at that
-point, but now apparently tries to dereference C<$x>. C<$$> by itself still
+point, but now apparently tries to dereference $x. C<$$> by itself still
works fine, however.
print "this is $$x\n";
projects / p5sagit/p5-mst-13.2.git / blobdiff