fails:MatchData#post_match returns the string after the match equiv. special var $'
fails:MatchData#post_match keeps taint status from the source string
fails:MatchData#post_match keeps untrusted status from the source string
fails:MatchData#post_match sets the encoding to the encoding of the source String
fails:MatchData#post_match sets an empty result to the encoding of the source String
