import re


def _validateStart(value, match):
    """
    Make sure that the characters preceding an ISBN number do not invalidate the match. The match is considered to
    be invalid if it is preceded by any alphanumeric character.
    
    :param value: String that was being searched
    :param match: MatchObject returned from regular expression function
    :returns: True if characters before match are valid
    :returns: False if characters before match are not valid
    """
    if match.start() > 0:
        preString = value[match.start()-1:match.start()]
        if re.match('\w', preString) is not None:
            return False
    return True