# 整词匹配
    src = "a b ab ba aa bb aab baa ccc ddd"
    print "src, '%s'" % (src)
    # a > c
    old="a"
    new = "c"
    dst_1 = re.sub(r"\b%s\b" % (old), new, src)
    print "%s To %s, '%s'" % (old, new, dst_1)
    # aa > dd
    old="aa"
    new = "dd"
    dst_2 = re.sub(r"\b%s\b" % (old), new, src)
    print "%s To %s, '%s'" % (old, new, dst_2)
输出
src, 'a b ab ba aa bb aab baa ccc ddd'
a To c, 'c b ab ba aa bb aab baa ccc ddd'
aa To dd, 'a b ab ba dd bb aab baa ccc ddd'