tagchecker.py

import re


# Events - start tag, end tag
START = 'start'
END = 'end'


def regex_lexer(markup, rgx_tag="<(?P<event>/?)(?P<tag>[A-Z])>"):
    """ Regex-based lexer.

    Usage:
    >>> list(regex_lexer(""))
    []

    >>> list(regex_lexer("<A></B>"))
    [('start', 'A'), ('end', 'B')]

    :param markup: Input text
    :type markup: str
    :param rgx_tag: Regular expression to match a single tag.
                    Must contain named capturing groups "event" and "tag".
    :type rgx_tag: str
    :return: [(event, tag)]
    :rtype: [(str, str)]
    """
    matches = re.finditer(rgx_tag, markup)
    for match in matches:
        tag = match.group('tag')
        event = END if match.group('event') == '/' else START
        yield event, tag


def tag_checker(markup, lexer=regex_lexer):
    """ Check markup tags correctness, return success or error message

    :param markup: Input text
    :type markup: str
    :param lexer: Lexer callable
    :return: Error or Success message
    :rtype: str

    Usage:
    >>> tag_checker("")
    'Correctly tagged paragraph'

    >>> tag_checker(" ")
    'Correctly tagged paragraph'

    >>> tag_checker("<A><B></B></A>")
    'Correctly tagged paragraph'

    >>> tag_checker("<A><B></B>")
    'Expected </A> found #'

    >>> tag_checker("<A><B></A>")
    'Expected </B> found </A>'

    >>> tag_checker("The following text<C><B>is centred and in boldface</B></C>")
    'Correctly tagged paragraph'

    >>> tag_checker("<B>This <\g>is <B>boldface</B> in <<*> a</B> <\6> <<d>sentence")
    'Correctly tagged paragraph'

    >>> tag_checker("<B><C> This should be centred and in boldface, but the tags are wrongly nested </B></C>")
    'Expected </C> found </B>'

    >>> tag_checker("<B>This should be in boldface, but there is an extra closing tag</B></C>")
    'Expected # found </C>'

    >>> tag_checker("<B><C>This should be centred and in boldface, but there is a missing closing tag</C>")
    'Expected </B> found #'

    """
    lexemes = lexer(markup)
    stack = []
    for (event, tag) in lexemes:
        if event == START:
            stack.append(tag)
        elif event == END:
            if len(stack) == 0:
                return "Expected # found </{}>".format(tag)
            expected_tag = stack.pop()
            if tag != expected_tag:
                return "Expected </{}> found </{}>".format(expected_tag, tag)
    if len(stack) > 0:
        expected_tag = stack.pop()
        return "Expected </{}> found #".format(expected_tag)
    return "Correctly tagged paragraph"


if __name__ == "__main__":
    import doctest
    doctest.testmod()