Python: Replace lower case to upper, and vice versa, simultaneously via ReGex lib

I open this question although a duplicate of it exists in C# lang. (without the relevant resolution needed).

Im trying to replace all the lower case characters in a given string, with upper case chars, and vice versa. This should be done simultaneously with the least time complexity (because of use in high volume of verbal translations).

The IO:

input:
str_1 = "Www.GooGle.com"

output:
"wWW.gOOgLE.COM"

The code:

import re                   # import RegEx lib

str_1 = "Www.GooGle.com"    # variable tested

def swapLowerUpper(source):
    # takes source string
    # returns group 0 regex to lower and group 1 regex to upper, by source
    return re.sub(r'([A-Z]+)|([a-z]+)', lambda x: x.group(0).lower(), source)

# check the output
print(swapLowerUpper(str_1)

The Question:

I have a hard time in triggering the second group (which index is 1) and apply the attribute ".upper()" on it. My attempt was to open it as {x: x.group(0).lower(), x: x.group(1).upper()} which failed.

>Solution :

Check whether x.group(1) or x.group(2) was matched, and return the appropriate replacement.

def swapLowerUpper(source):
    return re.sub(r'([A-Z]+)|([a-z]+)', lambda x: x.group(1).lower() if x.group(1) else x.group(2).upper(), source)