Let’s keep this simple.
(?=pattern)positive lookahead(?!pattern)negative lookahead(?<=pattern)positive lookbehind(?<!pattern)negative lookbehind
Lookarounds can be nested, and lookbehinds can generally only be of a fixed length.
Where available, you can use lookarounds with grep -P. The -o flag will only output the match, instead of the full matching line.
That’s all.