Hi Tag,
Yes, the lookahead and lookbehind operators are supported. The Scripting Language - Functions section in the User Guide provides a full list of the supported regular expression patterns for match_regex().
To check that the string contains the characters 'a' or 'b' as per your example you can use the following regular expression:
'^(?=.*[ab]).*'
If you are just trying to check for the existence of a set of characters, it is not necessary to use the lookahead operator; you could just use the following:
^.*[ab].*
Regards
Nigel