#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

no warnings 'uninitialized';
say for 'abA' =~ /^( (?{warn "* [$1,$2,$3]\n"})
                      ([ab]) (?{warn "\tL [$1,$2,$3]\n"})
                      |
                      ([ab]) (?{warn "\t\tR [$1,$2,$3]\n"})
                  )*
                  ((?{ warn "\t\t\t\\3 [$1,$2,$3]\n"})
                      \2
                  )
                  $/xi;