Text Diff Checker

What the diff checker does

When two versions of a document disagree, the hard part is finding the three lines that actually changed among the hundreds that did not. A diff checker does that for you: it lines the two texts up, works out the smallest set of edits that turns one into the other, and colors the result — green for what was added, red for what was removed.

It is the same idea developers use to review code, applied to any text: contracts, essays, translations, published articles and their drafts.

How to use it

1. Paste the original version into the left box and the changed version into the right box.
2. Choose how to compare. Line is best for documents and code. Word highlights small edits inside a paragraph. Character catches a single changed letter or a moved comma.
3. Turn on Hide unchanged lines to focus on just the edits in a long document.
4. Read the summary counts for a quick sense of how much changed, then scan the colored output for the detail.
5. Use Swap to flip which text is treated as the original.

How the comparison works

The tool splits both texts into units — lines, words or characters, depending on your choice — and runs a standard shortest-edit-script algorithm (the same approach behind the classic diff command). It finds the longest sequence the two versions share, then marks everything else as an addition or a deletion. Identical opening and closing sections are matched first, which is why comparing two nearly identical documents is fast.

In word mode, additions and deletions are shown inline, so you can read a changed sentence with the old wording struck through and the new wording underlined in place. In line mode, each line is labeled with a + for added, − for removed, or a blank for unchanged, along with a line number.

To keep the browser responsive, very large comparisons are capped: beyond a certain amount of divergence the tool stops searching for the absolute minimum edit and reports the differing block as a straight replacement. This only affects pathologically different inputs; ordinary edits are always shown at full detail. Character mode on very long texts is the most demanding, so switch to line mode if you are comparing whole books.

Examples

Comparing The meeting is on Tuesday at noon. with The meeting is on Wednesday at noon. in word mode highlights just one change: Tuesday removed, Wednesday added. Everything else is shown as unchanged, so you can be confident nothing else moved.

In line mode, comparing two versions of a list:

Original          Changed
------------      ------------
milk              milk
eggs              eggs
bread             butter
                  bread

the tool marks bread as unchanged, butter as added, and shows that the order shifted — rather than reporting every line after the insertion as different, which a naive comparison would do.

Common use cases

• Reviewing edits. See exactly what a co-author or editor changed in your draft.
• Comparing contract versions. Find the clause that moved between two rounds of negotiation.
• Checking a published page against your source. Confirm the CMS did not silently alter anything.
• Proofreading translations. Compare two translation passes to see what the reviewer touched.
• Debugging config files. Spot the one setting that differs between a working and a broken file.
• Tracking your own revisions when you do not have version control.

Privacy and security

Both texts are compared in your browser. Neither version is uploaded, logged or stored, which matters when you are diffing contracts, unreleased content or confidential configuration.

Only your comparison settings are saved locally. The output is written with safe DOM methods, so even text that contains HTML tags is displayed as plain characters and never executed. See the privacy policy for more.