This isn't reusable code, just a trick I found interesting and worth exploring. See the discussion (and code!) at Regexp generating strings? for some alternatives./^ab?(c|d)$/ => qw( abc abd ac ad )
The idea is to (manually) replace the atoms with code assertions, building up a hyotheical match, printing it and then failing in order to backtrack and try again.
Brad
Update: Regexp::Genex
A more complex example follows, first some notes.perl -e '""=~/(?{a})(?{$^R.b})?((?{ $^R.c})|(?{ $^R.d}))(?{print "$^R\ +n"})(?=a)(?!a)/' # output: abc abd ac ad
The above will produce a warning "Quantifier unexpected on zero-length expression..." under -w because of the (?{$^R.b})?. Suppress it with "no warnings 'regexp';"
The $^R variable contains the result of the last (?{}) assert and is localized for backtracking. (My first version had (?{ local $_ = $_ . "b" }) instead)
I tried the more complex /reg(ular\s+)?exp?(ression)?/ but ran into a few interesting, but so far surmountable problems.
Firstly, I haven't tried to tackle classes such as \s yet. I changed it to '_'. (YAPE::Regex has expanded classes)
I couldn't convince perl to repeatedly run zero-width assertions at the same position in the string. To see why search for "infinite" in perlre :). (If anyone knows how, let me know). To work around I made a string of 'a's and tacked an 'a' onto each quantified assertion. A happy side-effect is that the length of the input string bounds the quantifiers (somehow, but I ain't doing the math).
Also, the '*' quantifier is apparently optimized to not try to repeatedly match in the same position. I use {0,3} instead, as it isn't optimized. perlre again.
Greediness controls the search/display order.
I'm looking for a good impossible assertion to put
at the end. Currently, (?!a)(?=a) since the obvious
'x' is also optimized. perl -Mre=debug tells why:
floating `x' at 0..2147483647 (checking floating)
I might be able to avoid the 'a's with m/.../g and resetting pos. I haven't investigated, but I doubt it.
Output#!/usr/bin/perl -w #orig: reg(ular\s+)?exp?(ression)?!*? #mod: ^reg(ular_+)?exp?(ression)?!{0,3}? no warnings 'regexp'; # zero-length + quantifier 'aa' =~ /^ (?{'reg'}) ( (?{ $^R.'ular'}) (?: (?{ $^R."_" }) a )+ )? (?{ $^R.'ex'}) (?{ $^R.'p'})? ((?{ $^R.'ression'}))? (?: (?{ $^R."!" }) a ){0,3}? (?{print "$^R\n"; }) (?=a)(?!a) # fail /x; __END__
</readmore$ ./genex.pl regular__expression regular__exp regular__exression regular__ex regular_expression regular_expression! regular_exp regular_exp! regular_exression regular_exression! regular_ex regular_ex! regexpression regexpression! regexpression!! regexp regexp! regexp!! regexression regexression! regexression!! regex regex! regex!!
In reply to Generating regex strings with a regex by bsb
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