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November 3, 2010 06:38Subject:

Regular expression substitution matching all possible matchesMessage ID:

loom.20101103T142012-957@post.gmane.orgThis is a question about perl regexp usage. I hope it's okay to post it here; it seemed too advanced for the beginners list. It may lead on to a possible feature request for the perl core. I would like to perform match-and-substitute finding all possible matches of a regular expression within a string. By 'all possible matches' I mean all productions from that regular expression, not just the production that perl's regexp engine happened to find first. So given the regexp /\Ax*/ and the string 'xxx', I would expect to see matches '', 'x', 'xx', and 'xxx'. At first sight it appears that the /g flag will provide this, but in fact it just tells perl to keep matching once it has consumed some of the input string, while I want it to keep backtracking and finding more possible matches even after a success. The perlre manual page suggests that using (*FAIL) together with ?{} can do it: 'xxx' =~ /\Ax*(?{say $&})(*FAIL)/; This gives the expected result. Great! But now how to do a s/// substitution? I would like to find all possible matches and get all resulting substitutions. So far I have hand-rolled code for that in perl: my $string = 'xxx'; my @m; $string =~ /\A(x*)(?{push @m, [ ${^PREMATCH}, ${^POSTMATCH}, [ @- ], [ @+ ] ]})(*FAIL)/p; my $replacement = '.$1.'; foreach (@m) { my ($pre, $post, $starts, $ends) = @$_; my @captured; foreach my $n (1 .. $#$starts) { my $start = $starts->[$n] // die; my $end = $ends->[$n] // die; push @captured, substr($string, $start, $end - $start); } (my $r = $replacement) =~ s{\$(\d+)}{$captured[$1 - 1] // die}ge; say $pre . $r . $post; } This appears to work but it is unsatisfactory. Can I coax perl's regexp engine into finding all possible matches and gathering them into an array? Would there be an appetite for adding a new regexp flag to find all possible matches? It would be a useful teaching tool, and would also make it possible to find the longest or shortest match. -- Ed Avis <eda@waniasset.com>Thread Next

**Regular expression substitution matching all possible matches**by Ed Avis- Re: Regular expression substitution matching all possible matches by Aristotle Pagaltzis
- Re: Regular expression substitution matching all possible matches by Abigail
- Re: Regular expression substitution matching all possible matches by Aristotle Pagaltzis
- Re: Regular expression substitution matching all possible matches by hv
- Re: Regular expression substitution matching all possible matches by Abigail
- Re: Regular expression substitution matching all possible matches by Dr.Ruud
- Re: Regular expression substitution matching all possible matches by Abigail
- Re: Regular expression substitution matching all possible matches by Ruud H.G. van Tol
- Re: Regular expression substitution matching all possible matches by Ronald J Kimball
- Re: Regular expression substitution matching all possible matches by Dr.Ruud
- Re: Regular expression substitution matching all possible matches by Ed Avis
- Re: Regular expression substitution matching all possible matches by Zefram

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