Text Diff Checker — Compare Two Texts, Highlight Changes Online
Paste two versions of a document and see every added, removed, and changed line highlighted. Word-level diff, unified or side-by-side, browser-only.
About Diff Checker
A text diff checker compares two versions of the same writing and marks exactly what changed between them — added lines in green, removed lines in red, and modified lines with the specific words that differ highlighted inside them. The ZTools Text Diff Checker runs a real line-level longest-common-subsequence pass and then a word-level pass inside each modified line, so a one-word edit in a long paragraph shows as one word changed, not the whole line rewritten. It offers a unified view and a side-by-side view, optional ignore-case and ignore-whitespace matching, and export of a patch-style unified diff. Everything runs in your browser via a `useMemo`/`useDeferredValue` pipeline — no text is uploaded, no CDN is called, and typing stays responsive on large inputs. It is built for prose and plain text — drafts, essays, translations, contract redlines, config files — where you care about wording rather than code review workflows.
Use cases
- Comparing two drafts of a document. Paste yesterday's draft on the left and today's on the right to see every sentence you added, cut, or reworded. Word-level highlighting means a single reworded phrase stands out instead of the whole paragraph turning red, so you can review the actual edits at a glance before sending the newer version on.
- Checking what an editor or collaborator changed. When a document comes back from a co-author or an editor, drop the original and the returned copy in to surface every silent change. This catches edits that a "track changes" pass missed — reordered lines, a swapped number, a quietly deleted clause — without needing the file to be in any particular format.
- Reviewing a translation against its source revision. Compare two revisions of a translated text to confirm that a source-text update was actually carried through. Ignore-whitespace matching keeps reflowed line breaks from showing as false changes, so you only see the wording that genuinely moved.
- Diffing config or note files without git. For a plain-text config, an `.env` sketch, or a set of meeting notes that is not under version control, paste the two states to get a clean unified diff you can copy into a ticket or a message — a lightweight stand-in for `git diff` when the file never lived in a repository.
How it works
- Paste the original text. Put the earlier version in the left (or top) panel. There is no length field to configure — the tool sizes itself to whatever you paste.
- Paste the changed text. Put the newer version in the right (or bottom) panel. The diff recomputes automatically as you type or paste; there is no "compare" button to wait on.
- Choose unified or side-by-side. Switch views with the segmented control. Unified stacks changes in one column; side-by-side aligns the two versions row-for-row for line-by-line reading.
- Tune the matching. Toggle ignore-case and ignore-whitespace when trivial differences (capitalisation, trailing spaces, reflowed breaks) are adding noise you do not care about.
- Read the highlighted result. Additions, deletions, and modifications are colour-coded, with the exact changed words marked inside modified lines. A running count summarises how many lines were added, removed, and changed.
- Copy or download the diff. Export a patch-style unified diff to paste into a ticket, a code review, or a message — the full result is always available even when only a capped number of rows is rendered on screen.
Examples
Input: Left: "Sugar is sweet"
Right: "Honey is sweet"
Output: One modified line; the word "Sugar" marked removed and "Honey" marked added, the rest of the line unchanged.
Word-level highlighting isolates the single changed word instead of flagging the whole line.
Input: Left: 4 lines · Right: 5 lines (one line inserted in the middle)
Output: Four unchanged lines plus one green "added" line at the insertion point — not four "changed" lines below it.
The LCS pass aligns the shared lines so an insert does not cascade into every line after it.
Frequently asked questions
What is the difference between this and the Diff Checker?
They share the same underlying line-and-word diff engine. The Text Diff Checker is framed for prose and plain text — drafts, translations, notes, contract wording — while the Diff Checker leans toward code, JSON, and git-style review. Use whichever matches how you think about your comparison; the output is the same quality either way.
Is my text uploaded anywhere?
No. The comparison runs entirely in your browser — nothing is sent to a server and no third-party script is loaded. You can disconnect from the internet and it still works.
Does word-level highlighting work inside long lines?
Yes. Within each modified line the tool runs a second, word-level pass, so a single reworded word in a long paragraph is highlighted on its own rather than marking the entire line as changed.
Can I ignore capitalisation or whitespace differences?
Yes. Ignore-case and ignore-whitespace toggles let you suppress differences you do not care about, which is useful for comparing text that was reflowed or re-cased without meaningful edits.
Can I export the result?
Yes — you can copy or download a patch-style unified diff. That format pastes cleanly into tickets, code reviews, and chat.
Is there a size limit?
There is no hard cap you configure. Very large inputs render a limited number of rows on screen for performance, but the complete diff is always available through copy and download.
Pro tips
- Turn on ignore-whitespace first when comparing text that was re-wrapped — it removes the most common source of false "changes".
- Use side-by-side view for reading two versions in parallel, and unified view for a compact change list to paste elsewhere.
- For a quick sanity check, load the built-in sample texts to see how additions, deletions, and word-level edits are rendered.
- Copy the unified diff into a ticket instead of screenshotting — reviewers can read the exact wording change rather than a picture of it.
Reviewed by Ahsan Mahmood · Last updated 2026-07-23 · Part of ZTools.
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