On mobile, but here's a start: * The impact of "unicode by default." E.g., character classes and word boundaries. * Definition of line terminators. * Duplicate capture group names. * Legality of empty expressions. * Alternation match precedence, e.g., leftmost first (Perl) vs leftmost longest (POSIX). * Lack of certain features found in backtracking engines. * Class set operations. * Inconsequential escapes are not allowed. * Differences between match semantics of `^/$`. * Less strictness on nonsensical repetitions. See #765.