def regex_opt_inner(strings, open_paren):
    close_paren = open_paren and ')' or ''
    if not strings:
        return ''
    first = strings[0]
    if len(strings) == 1:
        return open_paren + escape(first) + close_paren
    if not first:
        return open_paren + regex_opt_inner(strings[1:], '(?:') \
            + '?' + close_paren
    if len(first) == 1:
        oneletter = []
        rest = []
        for s in strings:
            if len(s) == 1:
                oneletter.append(s)
            else:
                rest.append(s)
        if len(oneletter) > 1:
            if rest:
                return open_paren + regex_opt_inner(rest, '') + '|' \
                    + make_charset(oneletter) + close_paren
            return open_paren + make_charset(oneletter) + close_paren
    prefix = commonprefix(strings)
    if prefix:
        plen = len(prefix)
        return open_paren + escape(prefix) \
            + regex_opt_inner([s[plen:] for s in strings], '(?:') \
            + close_paren
    strings_rev = [s[::-1] for s in strings]
    suffix = commonprefix(strings_rev)
    if suffix:
        slen = len(suffix)
        return open_paren \
            + regex_opt_inner(sorted(s[:-slen] for s in strings), '(?:') \
            + escape(suffix[::-1]) + close_paren
    return open_paren + \
        '|'.join(regex_opt_inner(list(group[1]), '')
                 for group in groupby(strings, lambda s: s[0] == first[0])) \
        + close_paren