import re
s = '100 BROAD'
print re.sub('ROAD$', 'RD.', s)
print re.sub('\\bROAD$', 'RD.', s)
print re.sub(r'\bROAD$', 'RD.', s)
s = '100 BROAD ROAD APT. 3'
print re.sub(r'\bROAD$', 'RD.', s)
print re.sub(r'\bROAD\b', 'RD.', s)