Decode a JSON regex to bson.regex.Regex.

def _parse_canonical_regex(doc):
    """Decode a JSON regex to bson.regex.Regex."""
    regex = doc['$regularExpression']
    if len(doc) != 1:
        raise TypeError('Bad $regularExpression, extra field(s): %s' % (doc,))
    if len(regex) != 2:
        raise TypeError('Bad $regularExpression must include only "pattern"'
                        'and "options" components: %s' % (doc,))
    return Regex(regex['pattern'], regex['options'])