# (not using 'say' because I wanted to try it on old perl versions)
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$ab)$/m'
perl -e 'printf "%s\n", "ab\nab\n" =~ /^(ab)$ab$/m'
perl -e 'printf "%s\n", "ab\nab\n" =~ /^(a)b$ab$/m'