=head1 NAME
-perlfaq6 - Regular Expressions ($Revision: 1.20 $, $Date: 2003/01/03 20:05:28 $)
+perlfaq6 - Regular Expressions ($Revision: 1.27 $, $Date: 2004/11/03 22:52:16 $)
=head1 DESCRIPTION
but don't get your hopes up. Until then, you can use these examples
if you really need to do this.
-Use the four argument form of sysread to continually add to
+If you have File::Stream, this is easy.
+
+ use File::Stream;
+ my $stream = File::Stream->new(
+ $filehandle,
+ separator => qr/\s*,\s*/,
+ );
+
+ print "$_\n" while <$stream>;
+
+If you don't have File::Stream, you have to do a little more work.
+
+You can use the four argument form of sysread to continually add to
a buffer. After you add to the buffer, you check if you have a
complete line (using your regular expression).
also that any regex special characters will be acted on unless you
precede the substitution with \Q. Here's an example:
- $string = "to die?";
- $lhs = "die?";
- $rhs = "sleep, no more";
+ $string = "Placido P. Octopus";
+ $regex = "P.";
+
+ $string =~ s/$regex/Polyp/;
+ # $string is now "Polypacido P. Octopus"
- $string =~ s/\Q$lhs/$rhs/;
- # $string is now "to sleep no more"
+Because C<.> is special in regular expressions, and can match any
+single character, the regex C<P.> here has matched the <Pl> in the
+original string.
-Without the \Q, the regex would also spuriously match "di".
+To escape the special meaning of C<.>, we use C<\Q>:
+
+ $string = "Placido P. Octopus";
+ $regex = "P.";
+
+ $string =~ s/\Q$regex/Polyp/;
+ # $string is now "Placido Polyp Octopus"
+
+The use of C<\Q> causes the <.> in the regex to be treated as a
+regular character, so that C<P.> matches a C<P> followed by a dot.
=head2 What is C</o> really for?
$/ = undef;
$_ = <>;
- s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#$2#gs
+ s#/\*[^*]*\*+([^/*][^*]*\*+)*/|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse;
print;
This could, of course, be more legibly written with the C</x> modifier, adding
. ## Anything other char
[^/"'\\]* ## Chars which doesn't start a comment, string or escape
)
- }{$2}gxs;
+ }{defined $2 ? $2 : ""}gxse;
A slight modification also removes C++ comments:
- s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#$2#gs;
+ s#/\*[^*]*\*+([^/*][^*]*\*+)*/|//[^\n]*|("(\\.|[^"\\])*"|'(\\.|[^'\\])*'|.[^/"'\\]*)#defined $2 ? $2 : ""#gse;
=head2 Can I use Perl regular expressions to match balanced text?
hope to know on these matters (a full citation appears in
L<perlfaq2>).
-=head2 What's wrong with using grep or map in a void context?
+=head2 What's wrong with using grep in a void context?
-The problem is that both grep and map build a return list,
-regardless of the context. This means you're making Perl go
-to the trouble of building a list that you then just throw away.
-If the list is large, you waste both time and space. If your
-intent is to iterate over the list then use a for loop for this
+The problem is that grep builds a return list, regardless of the context.
+This means you're making Perl go to the trouble of building a list that
+you then just throw away. If the list is large, you waste both time and space.
+If your intent is to iterate over the list, then use a for loop for this
purpose.
+In perls older than 5.8.1, map suffers from this problem as well.
+But since 5.8.1, this has been fixed, and map is context aware - in void
+context, no lists are constructed.
+
=head2 How can I match strings with multibyte characters?
Starting from Perl 5.6 Perl has had some level of multibyte character
}
Here's another, slightly less painful, way to do it from Benjamin
-Goldberg:
+Goldberg, who uses a zero-width negative look-behind assertion.
- $martian =~ m/
- (?!<[A-Z])
- (?:[A-Z][A-Z])*?
- GX
- /x;
+ print "found GX!\n" if $martian =~ m/
+ (?<![A-Z])
+ (?:[A-Z][A-Z])*?
+ GX
+ /x;
This succeeds if the "martian" character GX is in the string, and fails
-otherwise. If you don't like using (?!<), you can replace (?!<[A-Z])
-with (?:^|[^A-Z]).
+otherwise. If you don't like using (?<!), a zero-width negative
+look-behind assertion, you can replace (?<![A-Z]) with (?:^|[^A-Z]).
It does have the drawback of putting the wrong thing in $-[0] and $+[0],
but this usually can be worked around.