What patch format does this accept?

Standard unified diffs as produced by git diff or diff -u: optional --- / +++ file headers followed by @@ -a,b +c,d @@ hunks whose lines are prefixed with a space (context), minus (removed) or plus (added).

How does it locate each hunk?

It first tries the line number in the @@ header. If the context and removed lines there do not match, it searches nearby lines for the exact context block, which lets a patch apply even when the file has shifted slightly.

What happens if a hunk does not match?

That hunk is reported as rejected and the surrounding source is left unchanged, while other hunks that do match are still applied. You can then fix the source or the patch and re-run.

No. The patch is parsed and applied entirely in your browser with JavaScript. Nothing is sent to a server, so you can safely apply patches to private source.

Can it reverse a patch?

This tool applies a patch in the forward direction. To reverse one, swap the minus and plus lines, or generate the inverse patch first; the applier always treats plus lines as additions and minus lines as removals.

Patch Applier — Gera Tools

A unified-diff patch describes a change as a set of hunks, each one a window of context lines plus the lines to remove (-) and add (+). The Patch Applier takes such a patch and your original text and produces the patched output in your browser — useful for applying a shared diff when you do not have a checkout to run git apply against.

How it works

The patch is parsed into hunks: each @@ -a,b +c,d @@ header starts a new hunk, and the following , -, + lines are collected.
For each hunk, the tool builds the expected old block (context + removed lines) and the new block (context + added lines).
It looks for the old block at the header’s line number; if that does not match exactly, it searches nearby lines for the same block (fuzzy location), so a small drift in the file still applies.
On a match it splices in the new block; on no match the hunk is rejected and the original lines are kept.

Example

Applying this hunk:

@@ -1,3 +1,3 @@
 .btn {
-  color: red;
+  color: blue;
 }

to a source whose first lines are .btn { / color: red; / } yields the same block with blue instead of red.

Notes

Hunks are applied independently, so a partly-stale patch still lands the hunks that match while clearly flagging those that do not. Line endings are normalised to \n during matching. Everything runs locally — your source and patch never leave the browser.