I actually pull values from the user defined type of field that allows
you to input your own regular expressions for validation. I haven't
had any issues pulling from this field and have used it heavily in a
9.0.0.27 build. Seems like a simple solution would be to simply
create your own user defined field and put whatever regexp you want in
there and then go to town with your render tags. You should even be
able to change existing fields of the different types to user defined
without much issue and have it all start working. So really you can
have a standard field of any pattern in your pages without issue. Hope
that helps.