can not recognize the look beind regex
From the tutorial https://www.regular-expressions.info/lookaround.html
(?<!a)b
and (?<=a)b
both should be valid regex syntax, but they can not be recognized.
https://regexper.com/#%28%3F%3C%3Da%29b%20
https://regexper.com/#%28%3F%3C!a%29b
Error: Line 1: expected "?"