/^ab?(c|d)$/ => qw( abc abd ac ad )

##</code><code>##

perl -e '""=~/(?{a})(?{$^R.b})?((?{ $^R.c})|(?{ $^R.d}))(?{print "$^R\n"})(?=a)(?!a)/'
# output:
abc
abd
ac
ad

##</code><code>##

#!/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__

##</code><code>##

$ ./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!!