That might work.

To avoid the overhead of the cache lookup I was thinking about something like

regex = None
def _has_surrogates(s):
    global regex
    if regex is None:
        regex = re.compile(short_regex)
    return regex.search(s)

but I have discarded it because it's not very pretty and still has the overhead of the function and an additional if.  Your version solves both the problems in a more elegant way.