Regex using condition on match results of another regex - regex

Question

I have a long string S and a string-to-string map M, where keys in M are the results of a regex match on S. I want to do a find-and-replace on S where, whenever one of the matches from that same regex is exactly one of my keys K in M, I replace it with its value M[K].
In order to do this I think I'd need to access the result of regex matches within a regex. If I try to store the result of a match and test equality outside a regex, I can't do my replace because I no longer know where the match was. How do I accomplish my goal?
Examples:
S = "abcd_a", regex = "[a-z]", M = {a:b}
result: "bbcd_b" because the regex would match the a's and replace them with b's
S = "abcd_a", regex = "[a-z]*", M = {a:b}
result: "abcd_b" because the regex would match "abcd" (but not replace it because it is not exactly "a") and the final 'a' (which it would replace because it is exactly "a")

Answer 1

EDIT Thanks for AlanMoore's suggestion. The code is now simpler.
I tried using python (2.7x) to solve this simple example, but it can be achieved with any other language. What's important is the approach (algorithm). Hope it helps:
import re
from itertools import cycle
S = "abcd_a"
REGEX = "[a-z]"
M = {'a':'b'}
def ReplaceWithDict(pattern):
# split by match group and map the match against map dict
return ''.join([M[v] if v and v in M else v for v in re.split(pattern, S)])
print ReplaceWithDict('([a-z])')
print ReplaceWithDict('([a-z]*)')
Output:
bbcd_b
abcd_b