make pattern for regex_string token less character-hungry

I think this fixes queries with two regexes, like so:

    select * from /.../ where x =~ /.../

The lexer would previously extract one big regex from the first to the last '/' character.

A problem that still remains is this:

    select a/2, b/2 from x
    ERROR: Error at 841508205:2713675. syntax error, unexpected REGEX_STRING, expecting FROM

Don't know how to fix that yet.