=head1 NAME
-perlfaq6 - Regular Expressions ($Revision: 1.18 $, $Date: 2002/10/30 18:44:21 $)
+perlfaq6 - Regular Expressions ($Revision: 1.20 $, $Date: 2003/01/03 20:05:28 $)
=head1 DESCRIPTION
littered with answers involving regular expressions. For example,
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
+this document (in L<perlfaq9>: ``How do I decode or create those %-encodings
on the web'' and L<perlfaq4>: ``How do I determine whether a scalar is
a number/whole/integer/float'', to be precise).
# now choose between them
} continue {
reset if eof(); # fix $.
- }
+ }
=head2 I put a regular expression into $/ but it didn't work. What's wrong?
-As of Perl 5.8.0, $/ has to be a string. This may change in 5.10,
+Up to Perl 5.8.0, $/ has to be a string. This may change in 5.10,
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
-a buffer. After you add to the buffer, you check if you have a
+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).
local $_ = "";
# do stuff here.
}
}
-
+
You can do the same thing with foreach and a match using the
c flag and the \G anchor, if you do not mind your entire file
being in memory at the end.
-
+
local $_ = "";
while( sysread FH, $_, 8192, length ) {
foreach my $record ( m/\G((?s).*?)your_pattern/gc ) {
my $mask = uc $old ^ $old;
uc $new | $mask .
- substr($mask, -1) x (length($new) - length($old))
+ substr($mask, -1) x (length($new) - length($old))
}
$a = "this is a TEsT case";
No matter which locale you are in, the alphabetic characters are
the characters in \w without the digits and the underscore.
As a regex, that looks like C</[^\W\d_]/>. Its complement,
-the non-alphabetics, is then everything in \W along with
-the digits and the underscore, or C</[\W\d_]/>.
+the non-alphabetics, is then everything in \W along with
+the digits and the underscore, or C</[\W\d_]/>.
=head2 How can I quote a variable to use in a regex?
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"
+
+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.
+
+To escape the special meaning of C<.>, we use C<\Q>:
+
+ $string = "Placido P. Octopus";
+ $regex = "P.";
- $string =~ s/\Q$lhs/$rhs/;
- # $string is now "to sleep no more"
+ $string =~ s/\Q$regex/Polyp/;
+ # $string is now "Placido Polyp Octopus"
-Without the \Q, the regex would also spuriously match "di".
+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?
Use the split function:
while (<>) {
- foreach $word ( split ) {
+ foreach $word ( split ) {
# do something with $word here
- }
+ }
}
Note that this isn't really a word in the English sense; it's just
If you wanted to do the same thing for lines, you wouldn't need a
regular expression:
- while (<>) {
+ while (<>) {
$seen{$_}++;
}
while ( ($line, $count) = each %seen ) {
@popstates = qw(CO ON MI WI MN);
while (defined($line = <>)) {
for $state (@popstates) {
- if ($line =~ /\b$state\b/i) {
+ if ($line =~ /\b$state\b/i) {
print $line;
last;
}
}
- }
+ }
That's because Perl has to recompile all those patterns for each of
the lines of the file. As of the 5.005 release, there's a much better
{
print "Found $1\n";
}
-
+
After the match fails at the letter C<a>, perl resets pos()
and the next match on the same string starts at the beginning.
For each line, the PARSER loop first tries to match a series
of digits followed by a word boundary. This match has to
start at the place the last match left off (or the beginning
-of the string on the first match). Since C<m/ \G( \d+\b
+of the string on the first match). Since C<m/ \G( \d+\b
)/gcx> uses the C<c> flag, if the string does not match that
regular expression, perl does not reset pos() and the next
match starts at the same position to try a different
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
(?:[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]).
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