Wanted solution using regex, here you are:
>>> import re
>>> s = "StringText someText Wwwwww SomeOtherText OtherText SOMETextMore etc etc etc"
>>> def replacement(match):
... return match.group(1).lower()
>>> re.sub(r'([sS]\w+)', replacement, s)
'stringtext sometext Wwwwww someothertext OtherText sometextmore etc etc etc'
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solved Python re: how to lowercase some words [closed]