import re

a = 'This is a test, which uses commas for a bad IP Address. 54.128,5,5, 4.'

def stripBadCommas(s):
  newString = re.sub(r'(?<=[0-9]),(?=[0-9])', '.', s)
  return newString

a = 'This is a test, which uses commas for a bad IP Address. 54.128,5,5, 4.'
b = stripBadCommas(a)
print(a)
print(b)