add pattern modifiers support to regexes

[bdecarne]

[hafriedlander]

Hi. I can see why you'd want to allowed some of the modifiers, but at least A and D shouldn't be allowed - they won't work, and allowing them gives a false sense that they will. Likewise, S and x are always set, even if not provided in the rule's regex.

Is there a specific modifier you want to enable?