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 not replacements:
        # Edge case that'd produce a funny regex and cause a KeyError
        return string
    
    # 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)

Based on @mnesarco approach, I tried a functional one with support for one subgroup per expression:

import re
from typing import Dict, Union


def multireplace(table: Dict[str, str], string: str, flags: Union[int, re.RegexFlag] = 0):
    patterns = {
        f"_g{n}": pattern for n, pattern in enumerate(table)
    }

    def repl(match: re.Match):
        repkey = None
        groupkey = None

        for key, value in match.groupdict().items():
            if value is None:
                continue

            if key.startswith("_g"):
                repkey = key
            else:
                groupkey, groupval = key, value

        repval = table[patterns[repkey]]
        return repval if groupkey is None else repval.replace(fr"\g<{groupkey}>", groupval)

    regex = "|".join(fr"(?P<{group}>{rep})" for group, rep in patterns.items())
    return re.sub(regex, repl, string, flags=flags)

Test

table = {
    "aaa": "[This is three a]",
    "b+": "[This is one or more b]",
    r"(?<=<spam>).+(?=</spam>)": "[REDACTED]",
    r"</?\w+>": "[This is a tag]",
}

txt = multireplace(table, "whatever bb, aaa, <star> BBB <end> <tag>keep me</tag> and <spam>delete me</spam>", re.IGNORECASE)
print(txt)
# output: whatever [This is one or more b], [This is three a], [This is a tag] [This is one or more b] [This is a tag] [This is a tag]keep me[This is a tag] and [This is a tag][REDACTED][This is a tag]

table = {
    "aaa": "[This is three a]",
    "b+": "[This is one or more b]",
    r"<(?P<name>\w+)>(?P<value>.+)</(?P=name)>": r"[This is an HTML tag with text (\g<value>)]",
    r"</?\w+>": "[This is a tag]",
}

txt = multireplace(table, "whatever bb, aaa, <star> BBB <end> <tag>keep me</tag> and <spam>delete me</spam>", re.IGNORECASE)
print(txt)

# output: whatever [This is one or more b], [This is three a], [This is a tag] [This is one or more b] [This is a tag] [This is an HTML tag with text (keep me)] and [This is an HTML tag with text (delete me)]

It's still O(n), I don't know how priorities are being set inside the main regex, they should be based on the dictionary order, but when there is competition (eg r"<(?P<name>\w+)>(?P<value>.+)</(?P=name)>" versus r"(?<=<spam>).+(?=</spam>)") the first has precedence. Also, one cannot reference a group by its order, only by name.