=head1 NAME
-perlfaq6 - Regexes ($Revision: 1.27 $, $Date: 1999/05/23 16:08:30 $)
+perlfaq6 - Regular Expressions ($Revision: 1.12 $, $Date: 2002/06/01 22:31:09 $)
=head1 DESCRIPTION
decoding a URL and checking whether something is a number are handled
with regular expressions, but those answers are found elsewhere in
this document (in L<perlfaq9>: ``How do I decode or create those %-encodings
-on the web'' and L<perfaq4>: ``How do I determine whether a scalar is
+on the web'' and L<perlfaq4>: ``How do I determine whether a scalar is
a number/whole/integer/float'', to be precise).
=head2 How can I hope to use regular expressions without creating illegible and unmaintainable code?
=head2 I'm having trouble matching over more than one line. What's wrong?
-Either you don't have more than one line in the string you're looking at
-(probably), or else you aren't using the correct modifier(s) on your
-pattern (possibly).
+Either you don't have more than one line in the string you're looking
+at (probably), or else you aren't using the correct modifier(s) on
+your pattern (possibly).
There are many ways to get multiline data into a string. If you want
it to happen automatically while reading input, you'll want to set $/
close FH;
## Get a read/write filehandle to it.
- $fh = new FileHandle "+<file";
+ $fh = new IO::File "+<file";
## Attach it to a "stream" object.
use Net::Telnet;
print;
-And here it is as a subroutine, modelled after the above:
+And here it is as a subroutine, modeled after the above:
sub preserve_case($$) {
my ($old, $new) = @_;
sub preserve_case {
my ($from, $to) = @_;
my ($lf, $lt) = map length, @_;
-
+
if ($lt < $lf) { $from = substr $from, 0, $lt }
else { $from .= substr $to, $lf }
-
+
return uc $to | ($from ^ uc $from);
}
=head2 Can I use Perl regular expressions to match balanced text?
-Although Perl regular expressions are more powerful than "mathematical"
-regular expressions because they feature conveniences like backreferences
-(C<\1> and its ilk), they still aren't powerful enough--with
-the possible exception of bizarre and experimental features in the
-development-track releases of Perl. You still need to use non-regex
-techniques to parse balanced text, such as the text enclosed between
-matching parentheses or braces, for example.
+Historically, Perl regular expressions were not capable of matching
+balanced text. As of more recent versions of perl including 5.6.1
+experimental features have been added that make it possible to do this.
+Look at the documentation for the (??{ }) construct in recent perlre manual
+pages to see an example of matching balanced parentheses. Be sure to take
+special notice of the warnings present in the manual before making use
+of this feature.
+
+CPAN contains many modules that can be useful for matching text
+depending on the context. Damian Conway provides some useful
+patterns in Regexp::Common. The module Text::Balanced provides a
+general solution to this problem.
+
+One of the common applications of balanced text matching is working
+with XML and HTML. There are many modules available that support
+these needs. Two examples are HTML::Parser and XML::Parser. There
+are many others.
An elaborate subroutine (for 7-bit ASCII only) to pull out balanced
and possibly nested single chars, like C<`> and C<'>, C<{> and C<}>,
or C<(> and C<)> can be found in
-http://www.perl.com/CPAN/authors/id/TOMC/scripts/pull_quotes.gz .
+http://www.cpan.org/authors/id/TOMC/scripts/pull_quotes.gz .
-The C::Scan module from CPAN contains such subs for internal use,
+The C::Scan module from CPAN also contains such subs for internal use,
but they are undocumented.
=head2 What does it mean that regexes are greedy? How can I get around it?
=head2 What's wrong with using grep or map in a void context?
-Both grep and map build a return list, regardless of their context.
-This means you're making Perl go to the trouble of building up a
-return list that you then just ignore. That's no way to treat a
-programming language, you insensitive scoundrel!
+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
+purpose.
=head2 How can I match strings with multibyte characters?
-This is hard, and there's no good way. Perl does not directly support
-wide characters. It pretends that a byte and a character are
-synonymous. The following set of approaches was offered by Jeffrey
-Friedl, whose article in issue #5 of The Perl Journal talks about this
-very matter.
+Starting from Perl 5.6 Perl has had some level of multibyte character
+support. Perl 5.8 or later is recommended. Supported multibyte
+character repertoires include Unicode, and legacy encodings
+through the Encode module. See L<perluniintro>, L<perlunicode>,
+and L<Encode>.
+
+If you are stuck with older Perls, you can do Unicode with the
+C<Unicode::String> module, and character conversions using the
+C<Unicode::Map8> and C<Unicode::Map> modules. If you are using
+Japanese encodings, you might try using the jperl 5.005_03.
+
+Finally, the following set of approaches was offered by Jeffrey
+Friedl, whose article in issue #5 of The Perl Journal talks about
+this very matter.
Let's suppose you have some weird Martian encoding where pairs of
ASCII uppercase letters encode single Martian letters (i.e. the two
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
+Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
All rights reserved.
This documentation is free; you can redistribute it and/or modify it