regular expression ?<=

Positive lookahead

positive lookahead works

q(?=u)

matches a q that is follwed by a u

negative lookahaed  works

q(?!u)

matches a q that is not followed by a u

Positive lookbehind

Lookbehind has the same effect, but works backwards

positive lookbehind works

(?<=a)b

matches a b in cab but does not match  a in bed or debt

ref : https://www.princeton.edu/~mlovett/reference/Regular-Expressions.pdf