regex lookbehind
# Positive lookahead
q(?=u) # Matches the letter "q" if "q" is followed by the letter "u"
# Negative lookahead
q(?!u) # Matches the letter "q" if "q" is NOT followed by the letter "u"
# Positive lookbehind
(?<=a)b # Matches the letter "b" if "b" is preceded by the letter "a"
# Negative lookbehind
(?<!a)b # Matches the letter "b" if "b" is NOT preceded by the letter "a"