DCG-compatible interface

[mndrix]

Regular expressions can be useful as building blocks for DCGs. For example,

foo -->
    game_console(Console),
    regex_dcg('games?').

I considered making a regex/5 predicate whose argument order is consistent with DCGs, but that would require one to always use regex//3 even if options and captures aren't needed. As in the example above, one often needs only a pattern.

I decided that a separate DCG version is the most appropriate. This is probably the most fundamental regular expression predicate. regex/4 might be built on top of it with something like:

regex(Pattern, Opts, Text, Captures) :-
    regex_dcg(Pattern, Opts, Captures, Text, _).

The DCG interface should only match a regular expression anchored at the first character of Text. That's consistent with the way that DCGs consume only from the head. If a DCG wants the regex to match anywhere, it can add its own logic for discarding characters that don't match.