# (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'

##</code><code>##

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'

##</code><code>##

perl -e 'printf "%s\n", "ab\nab\n" =~ /^((?:ab$)+)/m'