Not able to understand python regex with ?=

Question

I am not able to understand how these regex works and when they are used. I haven't found concrete examples of these regex on python website. I know sed awk but haven't used these type of regex there

(?=...)
(?<=...)
(?(id/name)yes-pattern|no-pattern)

Answer 1

(?=...) is a positive lookahead assertion. It matches if there is the part in parentheses after ?= matches at the current position, but it will not consume any characters for the match. E.g. the regex a(?=b) will match an a followed by a b, but won't return the b as part of the match.

(?<=...) is the same, but a look behind, i.e. it looks backwards. Again, it doesn't consume anything.

(?(id/name)yes-pattern|no-pattern) is a conditional. If the named group id/name matched, then the string must match yes-pattern at this point, otherwise no-pattern.

To be honest, though, those are quite advanced features and I cannot remember ever having used a conditional. Lookaround is more common, but often very constrained by the regex engines, e.g. lookbehind can only be done with fixed-length strings in many cases.