import re


def is_well_formed_uuid(s):
    """Check if a string is a valid RFC 4122 UUID."""
    return re.match(r'^[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}$', s) != None