#!/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__