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

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

#!/usr/bin/perl -w
#orig:  reg(ular\s+)?exp?(ression)?!*?
...
(?=a)(?!a)  # fail
/x;
__END__

$ ./genex.pl 
regular__expression
...
regex
regex!
regex!!