import re
def repl(m):
return "{0}{1}{2}".format(m.group(1), re.sub(r'\w+', 'Aa', m.group(2)), m.group(3))
string = "one,two,three,"
print(re.sub(r'(,?)(.*?)(,+)',repl, string))
print(re.sub(r'(,?)(.*?)(,+)', lambda m: "{0}{1}{2}".format(m.group(1), re.sub(r'\w+', 'Aa', m.group(2)), m.group(3)), string))
aW1wb3J0IHJlCmRlZiByZXBsKG0pOgoJcmV0dXJuICJ7MH17MX17Mn0iLmZvcm1hdChtLmdyb3VwKDEpLCByZS5zdWIocidcdysnLCAnQWEnLCBtLmdyb3VwKDIpKSwgbS5ncm91cCgzKSkKc3RyaW5nID0gIm9uZSx0d28sdGhyZWUsIgpwcmludChyZS5zdWIocicoLD8pKC4qPykoLCspJyxyZXBsLCBzdHJpbmcpKQpwcmludChyZS5zdWIocicoLD8pKC4qPykoLCspJywgbGFtYmRhIG06ICJ7MH17MX17Mn0iLmZvcm1hdChtLmdyb3VwKDEpLCByZS5zdWIocidcdysnLCAnQWEnLCBtLmdyb3VwKDIpKSwgbS5ncm91cCgzKSksIHN0cmluZykpCg==