Python string multireplacement

multireplace.py

def multireplace(string, replacements, ignore_case=False):
    """
    Given a string and a replacement map, it returns the replaced string.

    :param str string: string to execute replacements on
    :param dict replacements: replacement dictionary {value to find: value to replace}
    :param bool ignore_case: whether the match should be case insensitive
    :rtype: str

    """
    # If case insensitive, we need to normalize the old string so that later a replacement
    # can be found. For instance with {"HEY": "lol"} we should match and find a replacement for "hey",
    # "HEY", "hEy", etc.
    if ignore_case:
        def normalize_old(s):
            return s.lower()

        re_mode = re.IGNORECASE

    else:
        def normalize_old(s):
            return s

        re_mode = 0

    replacements = {normalize_old(key): val for key, val in replacements.items()}
    
    # Place longer ones first to keep shorter substrings from matching where the longer ones should take place
    # For instance given the replacements {'ab': 'AB', 'abc': 'ABC'} against the string 'hey abc', it should produce
    # 'hey ABC' and not 'hey ABc'
    rep_sorted = sorted(replacements, key=len, reverse=True)
    rep_escaped = map(re.escape, rep_sorted)
    
    # Create a big OR regex that matches any of the substrings to replace
    pattern = re.compile("|".join(rep_escaped), re_mode)
    
    # For each match, look up the new string in the replacements, being the key the normalized old string
    return pattern.sub(lambda match: replacements[normalize_old(match.group(0))], string)