=item *
The concatenation operator is ".", not the null string. (Using the
-null string would render C</pat/ /pat/> unparsable, since the third slash
-would be interpreted as a division operator--the tokener is in fact
+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>".
And in fact, "." itself can be the beginning of a number.)
=item *
You can't take the address of anything, although a similar operator
-in Perl 5 is the backslash, which creates a reference.
+in Perl is the backslash, which creates a reference.
=item *
=item *
-The backtick operator does variable interpolation without regard to
+The back-tick operator does variable interpolation without regard to
the presence of single quotes in the command.
=item *
-The backtick operator does no translation of the return value, unlike B<csh>.
+The back-tick operator does no translation of the return value, unlike B<csh>.
=item *
Shells (especially B<csh>) do several levels of substitution on each
-command line. Perl does substitution only in certain constructs
-such as double quotes, backticks, angle brackets, and search patterns.
+command line. Perl does substitution in only certain constructs
+such as double quotes, back-ticks, angle brackets, and search patterns.
=item *
=item *
Avoid barewords if you can, especially all lower-case ones.
-You can't tell just by looking at it whether a bareword is
+You can't tell by just looking at it whether a bareword is
a function or a string. By using quotes on strings and
-parens on function calls, you won't ever get them confused.
+parentheses on function calls, you won't ever get them confused.
=item *
You cannot discern from mere inspection which built-ins
are unary operators (like chop() and chdir())
and which are list operators (like print() and unlink()).
-(User-defined subroutines can B<only> be list operators, never
+(User-defined subroutines can be B<only> list operators, never
unary ones.) See L<perlop>.
=item *
=item *
The E<lt>FHE<gt> construct is not the name of the filehandle, it is a readline
-operation on that handle. The data read is only assigned to $_ if the
+operation on that handle. The data read is assigned to $_ only if the
file read is the sole condition in a while loop:
while (<FH>) { }
=back
-=head2 Perl4 to Perl5 Traps
+=head2 Perl4 to Perl5 Traps
Practicing Perl4 Programmers should take note of the following
Perl4-to-Perl5 specific traps.
=item * Deprecation
Double-colon is now a valid package separator in a variable name. Thus these
-behave differently in perl4 vs. perl5, since the packages don't exist.
+behave differently in perl4 vs. perl5, because the packages don't exist.
$a=1;$b=2;$c=3;$var=4;
print "$a::$b::$c ";
# perl4 prints: :hi:mom
# perl5 prints: hi:mom
+=item * BugFix
+
+Perl 4 would ignore any text which was attached to an C<-e> switch,
+always taking the code snippet from the following arg. Additionally, it
+would silently accept an C<-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 * Deprecation
Some error messages will be different.
=item * Numerical
-This specific item has been deleted. It demonstrated how the autoincrement
+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 5.003_04. But always be wary when using large ints. If in doubt:
+in version 5.003_04. But always be wary when using large integers.
+If in doubt:
use Math::BigInt;
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 the L<General Regular Expression Traps> tests for another example
-of this new feature...
+Also see the L<General Regular Expression Traps using s///, etc.>
+tests for another example of this new feature...
=back
Variable suicide behavior is more consistent under Perl 5.
Perl5 exhibits the same behavior for associative arrays and scalars,
-that perl4 exhibits only for scalars.
+that perl4 exhibits for only scalars.
$aGlobal{ "aKey" } = "global value";
print "MAIN:", $aGlobal{"aKey"}, "\n";
operator. So you now must parenthesize them in expressions like
/foo/ ? ($a += 2) : ($a -= 2);
-
+
Otherwise
/foo/ ? $a += 2 : $a -= 2
open FOO || die;
-is now incorrect. You need parens around the filehandle.
-Otherwise, perl5 leaves the statement as it's default precedence:
+is now incorrect. You need parentheses around the filehandle.
+Otherwise, perl5 leaves the statement as its default precedence:
open(FOO || die);
=item * Regular Expression
-C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no
-backtick expansion
+C<s`lhs`rhs`> (using back-ticks) is now a normal substitution, with no
+back-tick expansion
$string = "";
$string =~ s`^`hostname`;
# 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
perl4. With perl5, the reset is now done correctly. Any code relying
on the handler _not_ being reset will have to be reworked.
-5.002 and beyond uses sigaction() under SysV
+Since version 5.002, Perl uses sigaction() under SysV.
sub gotit {
print "Got @_... ";
=item * (SysV)
Under SysV OS's, C<seek()> on a file opened to append C<E<gt>E<gt>> now does
-the right thing w.r.t. the fopen() man page. e.g. - When a file is opened
+the right thing w.r.t. the fopen() man page. e.g., - When a file is opened
for append, it is impossible to overwrite information already in
the file.
projects / p5sagit/p5-mst-13.2.git / blobdiff