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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def rule_to_regex(cls, rule):
 """ Convert AdBlock rule to a regular expression. """

        if not rule:
            return rule

        # Check if the rule isn't already regexp
        if rule.startswith('/') and rule.endswith('/'):
            if len(rule) > 1:
                rule = rule[1:-1]
            else:
                raise AdblockParsingError('Invalid rule')
            return rule

        # escape special regex characters
        rule = re.sub(r"([.$+?{}()\[\]\\])", r"\\\1", rule)

        # XXX: the resulting regex must use non-capturing groups (?:
        # for performance reasons; also, there is a limit on number
        # of capturing groups, no using them would prevent building
        # a single regex out of several rules.

        # Separator character ^ matches anything but a letter, a digit, or
        # one of the following: _ - . %. The end of the address is also
        # accepted as separator.
        rule = rule.replace("^", "(?:[^\w\d_\-.%]|$)")

        # * symbol
        rule = rule.replace("*", ".*")

        # | in the end means the end of the address
        if rule[-1] == '|':
            rule = rule[:-1] + '$'

        # || in the beginning means beginning of the domain name
        if rule[:2] == '||':
            # XXX: it is better to use urlparse for such things,
            # but urlparse doesn't give us a single regex.
            # Regex is based on http://tools.ietf.org/html/rfc3986#appendix-B
            if len(rule) > 2:
                #          |            | complete part     |
                #          |  scheme    | of the domain     |
                rule = r"^(?:[^:/?#]+:)?(?://(?:[^/?#]*\.)?)?" + rule[2:]

        elif rule[0] == '|':
            # | in the beginning means start of the address
            rule = '^' + rule[1:]

        # other | symbols should be escaped
        # we have "|$" in our regexp - do not touch it
        rule = re.sub("(\|)[^$]", r"\|", rule)

        return rule