It is very simple. use re.sub, don't use replace.
import re
replacements = {r'\bthe\b':'a',
r'\bthis\b':'that'}
def replace_all(text, dic):
for i, j in dic.iteritems():
text = re.sub(i,j,text)
return text
replace_all("the cat has this thistle.", replacements)
It will print
a cat has that thistle.