The code for /^ab?(c|d)$/ is
perl -e '""=~/(?{a})(?{$^R.b})?((?{ $^R.c})|(?{ $^R.d}))(?{print "$^R\
+n"})(?=a)(?!a)/'
# output:
abc
abd
ac
ad
A more complex example follows, first some notes.
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.
#!/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__
Output
$ ./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!!
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