perl -e 'printf "%s\n", "ab\nab\n" =~ /^(ab$a)b$/m'
perl -e 'printf "%s\n", "ab\nab\n" =~ /^(ab$)ab$/m'
perl -e 'printf "%s\n", "ab\nab\n" =~ /^ab$(ab)$/m'
perl -e 'printf "%s\n", "ab\nab\n" =~ /^(ab$){2}/m'
perl -e 'printf "%s\n", "ab\nab\n" =~ /(ab$){2}/m'
perl -e 'printf "%s\n", "ab\nab\n" =~ /(?^:^(?^m:(ab$){2}))/m'