Regular Expression Lookarounds

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.