#! /usr/bin/perl

my $g3 = "( d(?1)f(?1) | b(?1) | c )";      # (defe|be|c)
my $g2 = "( (?:au)* (?3)* )";               # E?
my $g1 = "( u | uau(?2) | u(?2)(?3) )";     # e

my $re = qr/ $g1 (?(DEFINE) $g2 $g3 ) /x;

warn "$re\n";
/^$re$/ and print while <>;