Text Diff Checker

Quick answer: Text Diff Checker is a free online tool that lets you paste two versions of any text to see line-by-line differences — added, removed, and changed lines highlighted in a clear comparison table. No account or install needed — it runs entirely in your browser, free.

Paste two versions of any text to see line-by-line differences — added, removed, and changed lines highlighted in a clear comparison table.

What is the Text Diff Checker?

A text diff checker compares two versions of a document and shows exactly what changed between them, line by line. Writers use it to review edits, developers use it to inspect config changes before deploying, students use it to check revisions of an essay, and support teams use it to spot what a customer altered in a template. The comparison here is powered by a longest-common-subsequence (LCS) algorithm — the same family of algorithms behind the diff tools built into Git and most code editors.

Lines that appear in both texts are marked unchanged; lines only in the first are marked removed; lines only in the second are marked added; and lines that were replaced show up side by side as changed. You get summary statistics (how many lines were added, removed, changed, or left alone) plus a full table pairing the original and modified text with line numbers, so nothing slips past. Whitespace-only differences are treated as real changes, which keeps the comparison honest for code and config files. As with every CoreAIWeb utility, the comparison runs entirely in your browser — your drafts and documents never leave your device.

How to use the Text Diff Checker

  1. Paste the original version into the first box and the modified version into the second box.
  2. Click Compare Texts to run the line-by-line diff.
  3. Read the stat cards first: they summarize how many lines were added, removed, changed, or unchanged.
  4. Scroll the comparison table to inspect each change with its line numbers and status label.
  5. For long documents, scan the Status column for changed and added rows to jump straight to what matters.

Text Diff Checker use cases

Reviewing document edits

See exactly what a colleague changed in a contract, report, or article before you approve it. The side-by-side table makes silent rewording impossible to miss, even in a fifty-page document.

Auditing config changes

Diff the old and new version of a server config to confirm only the intended lines were touched. One unexpected changed row can reveal a typo that would have taken down a service.

Checking student revisions

Compare draft one and draft two of an essay to verify feedback was actually addressed. Added and changed rows show where the writer engaged with your comments — and where they didn’t.

Debugging data files

Diff two CSV or JSON exports to find which records were added, removed, or modified between runs. Faster and far more reliable than eyeballing two spreadsheets, and the status labels make it obvious whether rows were inserted, deleted, or edited.

Plagiarism spot-checks

Compare a submitted text against a source to see how much was copied verbatim versus rewritten. Long runs of unchanged rows against the source are a strong signal worth investigating.

Tips

  • The comparison is line-based, so rewording within a single long line shows the whole line as changed — break text into shorter lines for finer detail.
  • Trailing spaces count as differences; normalize whitespace in both texts if you only care about content changes.
  • Very large documents (tens of thousands of lines) may take a moment — the algorithm compares every line against every other.
  • Line endings are normalized, so Windows (CRLF) versus Unix (LF) files will not produce false differences.

Frequently asked questions

How does the diff algorithm work?
The tool splits both texts into lines and computes their longest common subsequence (LCS) — the longest sequence of lines appearing in both, in order. Lines outside that shared sequence are classified as added, removed, or changed. It is the same core approach used by Git diff and classic Unix diff utilities.
What is the difference between changed and added/removed?
When a line is replaced by a different line at roughly the same position, it is reported as one changed row showing both versions side by side. Lines with no counterpart on the other side are reported as purely added or removed.
Is the comparison case-sensitive?
Yes. Hello and hello are treated as different lines, which keeps the diff honest for code where case always matters. If you need a case-insensitive comparison, convert both texts to the same case first with a case converter, then run the diff.
Can I compare code files?
Absolutely. The checker is language-agnostic — it compares raw text lines, so it works for code, configs, CSV data, HTML, or prose. It handles any language because it never tries to parse the code itself. For the finest detail on code, keep lines short rather than minified onto one line.
Why does moving a paragraph show as removed and added?
A line-based diff tracks content by position and matching, not by identity. A moved block does not match its old position, so it appears as removed from one place and added in another. The content is still easy to spot in the table.
Is there a size limit?
There is no hard limit, but the LCS algorithm's work grows with the product of the two line counts, so extremely large files (tens of thousands of lines each) can feel slow. For everyday documents and code files it runs instantly; very large inputs fall back to a positional comparison.
Are my texts uploaded to a server?
No. The entire comparison runs in JavaScript in your browser tab, and your documents are never transmitted, stored, or logged anywhere. You can safely diff confidential contracts, private source code, and personal drafts here, because absolutely nothing leaves your device.