Text Diff Checker
Compare two texts and highlight added and removed lines
Compares two blocks of text side by side and highlights exactly which lines were added, removed, or stayed the same — paste both, and the comparison updates live.
How it works
The comparison works line by line using a standard longest-common-subsequence algorithm: lines present in both texts are marked unchanged, while lines only in the first or second are marked removed or added respectively. Very large inputs (millions of line-pairs to compare) are rejected rather than risking the page freezing while it computes.
Frequently asked questions
Does it compare word by word, or only whole lines?
Whole lines — if a single line changes even slightly (one word edited), the entire line shows as removed from the original and added in the new version, rather than highlighting just the changed word within it.
Why did it say my text is too large to compare?
The comparison algorithm's cost grows with the number of lines in both texts multiplied together, so extremely large inputs are rejected before attempting the comparison rather than risking the page becoming unresponsive while it runs.
What do the + and - symbols mean?
A line starting with + only appears in the second (right-hand) text; a line starting with - only appears in the first (left-hand) text. Lines with neither prefix are identical in both and shown for context.
Does line order matter?
Yes — moving a block of unchanged lines to a different position in the text is still detected as removal from the old position and addition at the new one, since the comparison doesn't specifically detect reordering as its own operation.
Is my text sent anywhere?
No — the entire comparison runs locally in your browser using JavaScript. Nothing is uploaded.