How do I use zero-width lookbehind assertions in Vim search patterns to match text only when preceded (or not preceded) by a pattern?

Explanation

Vim's \@<= and \@<! atoms let you write zero-width lookbehind assertions — they check what comes before the match position without consuming characters. This unlocks highly precise search and substitution patterns that would otherwise require capturing groups and complex replacement logic.

How it works

Both atoms are placed immediately after a pattern they test against:

• \(LOOKBEHIND\)\@<=MATCH — matches MATCH only if preceded by LOOKBEHIND
• \(LOOKBEHIND\)\@<!MATCH — matches MATCH only if not preceded by LOOKBEHIND

The lookbehind pattern is wrapped in \(\) so Vim knows its extent. Neither the lookbehind text nor any position is consumed — the match starts exactly at MATCH.

Example

Given:

return foobar
call foobar

To find foobar only when preceded by return :

/\(return \)\@<=foobar

This matches only the first foobar, leaving the second untouched. Use in a substitution:

:%s/\(return \)\@<=foobar/baz/g

Result:

return baz
call foobar

Tips

• The lookahead counterpart is \@= (positive) and \@! (negative) — placed after the pattern they test
• For very magic mode (\v), write %(LOOKBEHIND)@<=MATCH without extra backslashes
• Variable-length lookbehinds are allowed in Vim's regex engine (unlike some other engines)
• Combine with \@<= in substitutions to prepend or append text without re-capturing the prefix