gistfile1.py

import re
import string

pronMap = {"i": "you",
           "you": "you",
           "they": "they",
           "we": "you",
           "he": "he",
           "she": "she",
           "it": "it"};

def couldShould(sentence) :
# (could should would won't can't)
    m = re.search("(I|you|they|we|he|she|it)\s+(could|should|would|won't|will not|can't|cannot|can not)(\s+have|'ve)\s+([^-.;,]*)", sentence, re.I);
    if (m) :
        return (pronMap[string.lower(m.group(1))] + " " + m.group(2) + " of " + m.group(4) + "!");
    return None

# testing...
print(couldShould("I should have done the shopping; but i forgot"));
print(couldShould("i should have done the shopping; but i forgot"));
print(couldShould("I should have done the shopping"));
print(couldShould("it should have been fixed - but it wasn't"));
print(couldShould("I should asd done the shopping"));
print(couldShould("we should have gone to the park...but it's raining"));
print(couldShould("we could've gone to the park...but it's raining"));
print(couldShould("she won't have heard of the new json datastore"));
print(couldShould("she won't've heard of the new json datastore"));
print(couldShould("she will not have heard of the new json datastore"));

gistfile2.txt

bash-3.2$ python ~/couldshould.py
you should of done the shopping!
you should of done the shopping!
you should of done the shopping!
it should of been fixed !
None
you should of gone to the park!
you could of gone to the park!
she won't of heard of the new json datastore!
she won't of heard of the new json datastore!
she will not of heard of the new json datastore!
bash-3.2$