Use a dict to replace a group of text in a string in python

gistfile1.py

import re

s = "string to replace with KEY1 and KEY2, but also KEY3. But KEY5 is not defined in the replacements dict."
pattern = re.compile(r'\b(' + '|'.join(replacements.keys()) + r')\b')

replacements = {
    'KEY': 'VALUE',
    'KEY2': 'VALUE2',
    'KEY3': 'VALUE3',
    'KEY4': 'VALUE4'
}

print pattern.sub(lambda x: replacements[x.group()], s)