Text Diff Checker

What this tool does

Aligns the two texts and marks every line that was kept, added or removed, with a similarity percentage. Useful for checking edits, redirects maps, robots.txt changes or rewritten copy.

Who should use it

Editors reviewing revisions, developers comparing config files, and SEOs checking what changed between two robots.txt or sitemap versions.

How it works

Uses a longest-common-subsequence algorithm on lines: it finds the largest set of lines that appear in both texts in the same order, and everything else is reported as added (+) or removed (−). Similarity = 2 × unchanged lines ÷ (lines in A + lines in B).

How to use it

1. Enter or paste original text in the form.
2. Adjust the options below the main box if you need to; the defaults suit most cases. Use "Load example" to see how input should look.
3. Press the run button. Processing happens instantly in your browser.
4. Review the result panel, then copy or download the output if the tool offers it. Use Reset to start over.

Practical example

Comparing "a\nb\nc" with "a\nc\nd" shows b removed, d added, and two lines unchanged (similarity 67%).

Understanding the results

Lines starting with "+" exist only in the changed text, "−" only in the original, and a leading space means unchanged. A changed word makes the whole line show as removed and added.

Limitations

Comparison is line based, not word based, and is limited to roughly 2,000 lines per side to stay fast in the browser.

Accuracy and report transparency

Results are computed in your browser from exactly what you enter. Deterministic steps (counting, parsing, encoding, validation) are verified by an automated test-suite with known answers. Where a method is a heuristic, the result panel says "estimated". No external data is used and nothing is invented.

Privacy

Your input never leaves your device for this tool. We count anonymous events (tool opened, run, copied) without storing what you typed. See the Privacy Policy.