Why put a lookahead at the beginning, or a lookbehind at the end?

Question

I was trying to learn about positive/negative lookarounds, and I found this stack overflow answer that confused me:

Positive lookahead:

Syntax:
(?=REGEX_1)REGEX_2
Match only if REGEX_1 matches; after matching REGEX_1, the match is discarded and searching for REGEX_2 starts at the same position.

Maybe this is really straightforward and obvious, but I can't comprehend it. If the syntax were REGEX_2(?=REGEX_1), I'd understand, but not the way it's written.

Can someone explain what it means to have a lookahead at the beginning of a regex, how that differs from putting a lookahead at the end, and the value of doing so? I'd also like to ask the same question but about lookbehind at the end. I learn best from concrete examples. Thanks.

`(?=REGEX_1)` restricts a `REGEX_2` in `(?=REGEX_1)REGEX_2`, it means `REGEX_2` must start with `REGEX_1` pattern. — Wiktor Stribiżew, Apr 08 '21 at 21:22
[This thread](https://stackoverflow.com/questions/2126137/regex-lookahead-ordering) should explain this all. — Wiktor Stribiżew, Apr 08 '21 at 21:37
@WiktorStribiżew Yeah, that seems on target. Reading it now. Thanks — Daniel Kaplan, Apr 08 '21 at 21:37
@WiktorStribiżew that's definitely my question. I only partially understand the answer, so I left a comment. Thanks — Daniel Kaplan, Apr 08 '21 at 21:50
@WiktorStribiżew did you mean to say "Lookbehind" in both sentences? — Daniel Kaplan, Apr 08 '21 at 22:45
I see. Lookbehind asserts presence/absence of its pattern immediately to the left of the current location. Lookahead asserts presence/absence of its pattern immediately to the right of the current location. — Wiktor Stribiżew, Apr 08 '21 at 22:46

Why put a lookahead at the beginning, or a lookbehind at the end?

0 Answers0