How do I match text only when it is preceded by a specific pattern in Vim regex?

Explanation

Vim's \@<= is a zero-width look-behind assertion. It requires the preceding group to match at the current position but does not consume any characters — so only target is matched and substituted while the prefix is left intact.

The general form is:

:%s/\(prefix\)\@<=target/replacement/g

How it works

• \(prefix\) — the look-behind group: text that must appear immediately before the match
• \@<= — asserts that the look-behind group precedes the current position (zero-width: not included in the match)
• target — the actual pattern to match and replace

In very magic mode (\v), escaping is reduced: \v(prefix)\@<=target

Example

Given:

function foo
function bar
var baz

To rename only identifiers after function , leaving var baz untouched:

:%s/\(function \)\@<=\w\+/handler/g

Result:

function handler
function handler
var baz

The prefix function is preserved; only the word following it was replaced.

Tips

• \@<! — negative look-behind: match text that is not preceded by a pattern.
• \@= / \@! — look-ahead and negative look-ahead assertions.
• Very magic reduces escaping: \v(function )\@<=\w+
• Unlike PCRE, Vim allows variable-length patterns in look-behind groups.