=head1 NAME
-perlfaq6 - Regexps ($Revision: 1.16 $, $Date: 1997/03/25 18:16:56 $)
+perlfaq6 - Regexps ($Revision: 1.17 $, $Date: 1997/04/24 22:44:10 $)
=head1 DESCRIPTION
this document (in the section on Data and the Networking one on
networking, to be precise).
-=head2 How can I hope to use regular expressions without creating illegible and unmaintainable code?
+=head2 How can I hope to use regular expressions without creating illegible and unmaintainable code?
Three techniques can make regular expressions maintainable and
understandable.
while ( <> ) {
while ( /\b(\w\S+)(\s+\1)+\b/gi ) {
print "Duplicate $1 at paragraph $.\n";
- }
- }
+ }
+ }
Here's code that finds sentences that begin with "From " (which would
be mangled by many mailers):
$/ must be a string, not a regular expression. Awk has to be better
for something. :-)
-Actually, you could do this if you don't mind reading the whole file into
+Actually, you could do this if you don't mind reading the whole file
+into memory:
undef $/;
@records = split /your_pattern/, <FH>;
=head2 How can I match a locale-smart version of C</[a-zA-Z]/>?
One alphabetic character would be C</[^\W\d_]/>, no matter what locale
-you're in. Non-alphabetics would be C</[\W\d_]/> (assuming you don't
+you're in. Non-alphabetics would be C</[\W\d_]/> (assuming you don't
consider an underscore a letter).
-=head2 How can I quote a variable to use in a regexp?
+=head2 How can I quote a variable to use in a regexp?
The Perl parser will expand $variable and @variable references in
regular expressions unless the delimiter is a single quote. Remember,
foreach $word ( split ) {
# do something with $word here
}
- }
+ }
-Note that this isn't really a word in the English sense; it's just
-chunks of consecutive non-whitespace characters.
+Note that this isn't really a word in the English sense; it's just
+chunks of consecutive non-whitespace characters.
To work with only alphanumeric sequences, you might consider
=head2 How can I print out a word-frequency or line-frequency summary?
To do this, you have to parse out each word in the input stream. We'll
-pretend that by word you mean chunk of alphabetics, hyphens, or
-apostrophes, rather than the non-whitespace chunk idea of a word given
+pretend that by word you mean chunk of alphabetics, hyphens, or
+apostrophes, rather than the non-whitespace chunk idea of a word given
in the previous question:
while (<>) {
while ( /(\b[^\W_\d][\w'-]+\b)/g ) { # misses "`sheep'"
$seen{$1}++;
- }
- }
+ }
+ }
while ( ($word, $count) = each %seen ) {
print "$count $word\n";
- }
+ }
If you wanted to do the same thing for lines, you wouldn't need a
regular expression:
while (<>) {
$seen{$_}++;
- }
+ }
while ( ($line, $count) = each %seen ) {
print "$count $line";
}
A more sophisticated use might involve a tokenizer. The following
lex-like example is courtesy of Jeffrey Friedl. It did not work in
-5.003 due to bugs in that release, but does work in 5.004 or better:
+5.003 due to bugs in that release, but does work in 5.004 or better.
+(Note the use of C</c>, which prevents a failed match with C</g> from
+resetting the search position back to the beginning of the string.)
while (<>) {
chomp;
PARSER: {
- m/ \G( \d+\b )/gx && do { print "number: $1\n"; redo; };
- m/ \G( \w+ )/gx && do { print "word: $1\n"; redo; };
- m/ \G( \s+ )/gx && do { print "space: $1\n"; redo; };
- m/ \G( [^\w\d]+ )/gx && do { print "other: $1\n"; redo; };
+ m/ \G( \d+\b )/gcx && do { print "number: $1\n"; redo; };
+ m/ \G( \w+ )/gcx && do { print "word: $1\n"; redo; };
+ m/ \G( \s+ )/gcx && do { print "space: $1\n"; redo; };
+ m/ \G( [^\w\d]+ )/gcx && do { print "other: $1\n"; redo; };
}
}
while (<>) {
chomp;
PARSER: {
- if ( /\G( \d+\b )/gx {
+ if ( /\G( \d+\b )/gcx {
print "number: $1\n";
redo PARSER;
}
- if ( /\G( \w+ )/gx {
+ if ( /\G( \w+ )/gcx {
print "word: $1\n";
redo PARSER;
}
- if ( /\G( \s+ )/gx {
+ if ( /\G( \s+ )/gcx {
print "space: $1\n";
redo PARSER;
}
- if ( /\G( [^\w\d]+ )/gx {
+ if ( /\G( [^\w\d]+ )/gcx {
print "other: $1\n";
redo PARSER;
}
grep() that's not better written as a C<for> (well, C<foreach>,
technically) loop.
-=head2 How can I match strings with multi-byte characters?
+=head2 How can I match strings with multibyte characters?
This is hard, and there's no good way. Perl does not directly support
wide characters. It pretends that a byte and a character are
Friedl, whose article in issue #5 of The Perl Journal talks about this
very matter.
-Let's suppose you have some weird Martian encoding where pairs of ASCII
-uppercase letters encode single Martian letters (i.e. the two bytes
-"CV" make a single Martian letter, as do the two bytes "SG", "VS",
-"XX", etc.). Other bytes represent single characters, just like ASCII.
+Let's suppose you have some weird Martian encoding where pairs of
+ASCII uppercase letters encode single Martian letters (i.e. the two
+bytes "CV" make a single Martian letter, as do the two bytes "SG",
+"VS", "XX", etc.). Other bytes represent single characters, just like
+ASCII.
-So, the string of Martian "I am CVSGXX!" uses 12 bytes to encode the nine
-characters 'I', ' ', 'a', 'm', ' ', 'CV', 'SG', 'XX', '!'.
+So, the string of Martian "I am CVSGXX!" uses 12 bytes to encode the
+nine characters 'I', ' ', 'a', 'm', ' ', 'CV', 'SG', 'XX', '!'.
Now, say you want to search for the single character C</GX/>. Perl
-doesn't know about Martian, so it'll find the two bytes "GX" in the
-"I am CVSGXX!" string, even though that character isn't there: it just
-looks like it is because "SG" is next to "XX", but there's no real "GX".
-This is a big problem.
+doesn't know about Martian, so it'll find the two bytes "GX" in the "I
+am CVSGXX!" string, even though that character isn't there: it just
+looks like it is because "SG" is next to "XX", but there's no real
+"GX". This is a big problem.
Here are a few ways, all painful, to deal with it:
Or like this:
while ($martian =~ m/\G([A-Z][A-Z]|.)/gs) { # \G probably unneeded
- print "found GX!\n", last if $1 eq 'GX';
+ print "found GX!\n", last if $1 eq 'GX';
}
Or like this:
die "sorry, Perl doesn't (yet) have Martian support )-:\n";
In addition, a sample program which converts half-width to full-width
-katakana (in Shift-JIS or EUC encoding) is available from CPAN as
+katakana (in Shift-JIS or EUC encoding) is available from CPAN as
=for Tom make it so
Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
All rights reserved. See L<perlfaq> for distribution information.
+
projects / p5sagit/p5-mst-13.2.git / blobdiff