def regex_opt_inner(strings, open_paren):
    """Return a regex that matches any string in the sorted list of strings."""
    close_paren = open_paren and ')' or ''
    # print strings, repr(open_paren)
    if not strings:
        # print '-> nothing left'
        return ''
    first = strings[0]
    if len(strings) == 1:
        # print '-> only 1 string'
        return open_paren + escape(first) + close_paren
    if not first:
        # print '-> first string empty'
        return open_paren + regex_opt_inner(strings[1:], '(?:') \
            + '?' + close_paren
    if len(first) == 1:
        # multiple one-char strings? make a charset
        oneletter = []
        rest = []
        for s in strings:
            if len(s) == 1:
                oneletter.append(s)
            else:
                rest.append(s)
        if len(oneletter) > 1:  # do we have more than one oneletter string?
            if rest:
                # print '-> 1-character + rest'
                return open_paren + regex_opt_inner(rest, '') + '|' \
                    + make_charset(oneletter) + close_paren
            # print '-> only 1-character'
            return open_paren + make_charset(oneletter) + close_paren
    prefix = commonprefix(strings)
    if prefix:
        plen = len(prefix)
        # we have a prefix for all strings
        # print '-> prefix:', prefix
        return open_paren + escape(prefix) \
            + regex_opt_inner([s[plen:] for s in strings], '(?:') \
            + close_paren
    # is there a suffix?
    strings_rev = [s[::-1] for s in strings]
    suffix = commonprefix(strings_rev)
    if suffix:
        slen = len(suffix)
        # print '-> suffix:', suffix[::-1]
        return open_paren \
            + regex_opt_inner(sorted(s[:-slen] for s in strings), '(?:') \
            + escape(suffix[::-1]) + close_paren
    # recurse on common 1-string prefixes
    # print '-> last resort'
    return open_paren + \
        '|'.join(regex_opt_inner(list(group[1]), '')
                 for group in groupby(strings, lambda s: s[0] == first[0])) \
        + close_paren
Return a regex that matches any string in the sorted list of strings.