[colm] regex subexpression capture

[adrian-thurston]

This may be finished. Need to investigate.

[adrian-thurston]

Experimented with a solution once, but certainly not complete in latest.

[adrian-thurston]

Would be great to have this when parsing Go. There we are using a regex to decide when to insert semi colons. Looks like:

    token insert_semi /
        ( 
            ( id - 'if' - 'then' - 'else' - 'end' )
        )
        [ \t]*
        ( line_comment | '\n' )
    /
    {
        parse BA: break_apart[$match_text]

        Prefix: str = input->pull( BA.pre_semi.data.length )
        input->push( ";" )
        input->push( Prefix )
    }

Note that we need to parse the match text again just to learn where to insert the semi. If we had subexpression match we could use the length of that match to decide how much to pull off before pushing the semi.