Parse a rule and return it as generator. Each iteration yields tuples
    in the form ``(converter, arguments, variable)``. If the converter is
    `None` it's a static url part, otherwise it's a dynamic one.

    :internal:

def parse_rule(rule):
    """Parse a rule and return it as generator. Each iteration yields tuples
    in the form ``(converter, arguments, variable)``. If the converter is
    `None` it's a static url part, otherwise it's a dynamic one.

    :internal:
    """
    pos = 0
    end = len(rule)
    do_match = _rule_re.match
    used_names = set()
    while pos < end:
        m = do_match(rule, pos)
        if m is None:
            break
        data = m.groupdict()
        if data["static"]:
            yield None, None, data["static"]
        variable = data["variable"]
        converter = data["converter"] or "default"
        if variable in used_names:
            raise ValueError("variable name %r used twice." % variable)
        used_names.add(variable)
        yield converter, data["args"] or None, variable
        pos = m.end()
    if pos < end:
        remaining = rule[pos:]
        if ">" in remaining or "<" in remaining:
            raise ValueError("malformed url rule: %r" % rule)
        yield None, None, remaining