I have been searching Stack Overflow but cannot find the proper code for correcting e.g.

"hello! are you tired? no, not at all!"

Into:

"Hello! Are you tired? No, not at all!"

You can try this regex approach:

import re
re.sub("(^|[.?!])\s*([a-zA-Z])", lambda p: p.group(0).upper(), s)
# 'Hello! Are you tired? No, not at all!'

• (^|[.?!]) matches the start of the string ^ or .?! followed by optional spaces;
• [a-zA-Z] matches letter directly after the first pattern;
• use lambda function to convert the captured group to upper case;