Preprocess string to transform all diacritics and remove other special characters than appropriate
    
    :param string:
    :return:

def preprocess(string):
    """
    Preprocess string to transform all diacritics and remove other special characters than appropriate
    
    :param string:
    :return:
    """
    string = unicode(string, encoding="utf-8")
    # convert diacritics to simpler forms
    string = regex1.sub(lambda x: accents[x.group()], string)
    # remove all rest of the unwanted characters
    return regex2.sub('', string).encode('utf-8')