Python Case Insensitive Replace All of multiple strings

Question

I want to replace all occurrences of a set of strings in a text line. I came up with this approach, but I am sure there is a better way of doing this:

myDict = {}
test = re.compile(re.escape('pig'), re.IGNORECASE)
myDict['car'] = test
test = re.compile(re.escape('horse'), re.IGNORECASE)
myDict['airplane'] = test
test = re.compile(re.escape('cow'), re.IGNORECASE)
myDict['bus'] = test

mystring = 'I have this Pig and that pig with a hOrse and coW'

for key in myDict:      
    regex_obj = myDict[key]
    mystring = regex_obj.sub(key, mystring)

print mystring

I have this car and that car with a airplane and bus

Based on @Paul Rooney's answer below, ideally I would do this:

def init_regex():
    rd = {'pig': 'car', 'horse':'airplane', 'cow':'bus'}
    myDict = {}
    for key,value in rd.iteritems():
        pattern = re.compile(re.escape(key), re.IGNORECASE)
        myDict[value] = pattern

    return myDict

def strrep(mystring, patternDict):
    for key in patternDict:
        regex_obj = patternDict[key]
        mystring = regex_obj.sub(key, mystring)

    return mystring

Answer 1

Try

import itertools
import re

mystring = 'I have this Pig and that pig with a hOrse and coW'

rd = {'pig': 'car', 'horse':'airplane', 'cow':'bus'}

cachedict = {}

def strrep(orig, repdict):
    for k,v in repdict.iteritems():
        if k in cachedict:
            pattern = cachedict[k]
        else:
            pattern = re.compile(k, re.IGNORECASE)
            cachedict[k] = pattern
        orig = pattern.sub(v, orig)
    return orig

print strrep(mystring, rd)

This answer was initially written for python2, but for python 3 you would use repdict.items instead of repdict.iteritems .