Text Diff Checker

How Text Diff Checker Works

A text diff checker lines up two versions of the same text and highlights exactly which words were added, removed, or left unchanged between them. It's the same idea behind "track changes" in a word processor or a code diff, but for any plain text you paste in -- drafts, emails, contracts, or article revisions.

Formula & Method

This tool splits both texts into word-level tokens (keeping whitespace as its own tokens so spacing is preserved) and runs a classic longest-common-subsequence (LCS) diff algorithm to find the smallest set of insertions and deletions that turns the original into the changed version. Matching runs are shown as plain text, removed tokens are struck through in red, and added tokens are highlighted in green. A "Text Similarity" percentage is also computed as 2 × matched words ÷ (words in original + words in changed) × 100, and to keep the comparison fast in-browser it's capped at roughly 2,500 words per side.

Worked Example

Comparing "The quick brown fox jumps over the lazy dog." against "The quick brown fox leaps over the lazy dog and runs away." the tool matches 7 words (The, quick, brown, fox, over, the, lazy), treats "jumps" → "leaps" and "dog." → "dog" as one removal and one addition each, and flags "and runs away." as 3 more additions -- for a total of 2 words removed and 5 words added. That works out to a similarity score of 66.7%.

Frequently Asked Questions

Does the similarity percentage account for word order?

Yes, indirectly -- the underlying algorithm looks for the longest sequence of words that appears in the same order in both texts, so words that merely moved to a different position (rather than staying in matching sequence) are typically counted as a removal plus an addition rather than as unchanged.

Why is punctuation sometimes shown as part of a "changed" word?

Tokens are split on whitespace, not punctuation, so a word with trailing punctuation (like "dog.") is a different token than the same word without it (like "dog"). If punctuation changes between versions, the word attached to it will show as removed and re-added even though the word itself didn't change.

Is there a limit to how much text I can compare?

Yes -- each side is capped at about 2,500 words. Beyond that the diff is skipped and a message is shown instead, since the comparison runs entirely in your browser and very long texts would make it slow.

Does my text get uploaded anywhere?

No -- the comparison runs entirely client-side in JavaScript. Nothing you paste in is sent to a server.