On safety: regular expressions and safemode

Hey, I have realized that we disallow use of regular expressions in templates, particularly =~ operator and others. In the safemode codebase I see this comment:

# not sure how secure ruby regexp is, so leave it out for now
:dregx, :dregx_once, :match2, :match3, :nth_ref, :back_ref,

However I can’t think of any security problems with that. Do you have anything in particular on mind?