Text Diff Checker
Paste or type two versions of text to see what changed. Additions are highlighted in green, deletions in red.
Reviewed by the ToolNestr Editorial Team — July 2026
How text diff checking works
The diff checker uses the Longest Common Subsequence (LCS) algorithm to identify which characters are the same between two texts. The LCS finds the longest sequence of characters that appears in both texts in the same order, without requiring them to be consecutive. Everything outside this common subsequence is flagged as added or deleted.
Characters present in the new text but absent from the old are highlighted as additions (green). Characters in the old but missing from the new are highlighted as deletions (red). Unchanged characters are shown in neutral styling. This character-level diff provides the highest precision, showing exactly which characters changed between versions.
Worked example
What different diff use cases mean
Text diffing is used across many fields. Here is how the usage breaks down.
Developer
Reviews code changes before commits, compares config files across environments, and debunks unexpected output by diffing against expected results.
Writer & Editor
Tracks revisions between drafts, reviews suggested edits, and ensures no content was accidentally lost during the editing process.
Data Analyst
Compares CSV exports, validates data transformations, and identifies discrepancies between two datasets or database dumps.
Student
Checks for plagiarism between sources, compares essay drafts, and reviews corrected assignments against originals to understand mistakes.
| Algorithm | Granularity | Speed | Best For |
|---|---|---|---|
| Myers Diff | Line / word | Fast | Code version control (Git) |
| LCS (Longest Common Subsequence) | Character | Moderate | Precision text comparison |
| Word diff | Word | Fast | Document editing, prose |
| Line diff | Line | Very fast | Code, structured data |
How to use the diff checker
Paste both versions
Enter or paste the old (original) text in the left box and the new (changed) text in the right box.
Click compare
Press the Compare button to see additions highlighted in green and deletions in red.
Review and copy
Review the highlighted differences. Use the stats to see how many changes were made.
Tips for using diff checker
Compare similar-length texts
The diff works best when the two versions have roughly the same length. Very different texts produce noisy results with many changes.
Use swap to reverse perspectives
Click Swap texts to exchange old and new. This is useful when you accidentally paste them in the wrong order.
Check before and after editing
Always diff before and after making significant edits to ensure no unintended changes or deletions occurred during the process.
What is diff?
Diff is a comparison tool that shows the differences between two texts. Originally developed for code version control, diff algorithms identify added, removed or changed sections between two versions. The most common use is reviewing code changes before committing them, but diff is equally useful for documents, data and any text-based content.
How diff works
The LCS (Longest Common Subsequence) algorithm finds the longest sequence of characters that appears in both texts in the same order. Everything outside the LCS is flagged as an addition or deletion. Characters present in the new text but not the old are additions (green). Characters in the old text but absent in the new are deletions (red). Characters in both are shown as unchanged.
Applications of diff
Diff tools are used in version control systems (Git), collaborative document editing, plagiarism detection, data validation, configuration management, and legal document review. Any situation where you need to know exactly what changed between two versions of text benefits from a diff checker.
When to use text diff checking
Text diff checking is valuable in a wide range of scenarios beyond basic code review. Writers and editors use diff tools to track revisions between document drafts, ensuring no content is accidentally lost during the editing process. When collaborating on shared documents, a diff checker reveals exactly what each collaborator changed, eliminating the guesswork from version tracking. Data analysts use diff comparisons to validate data transformations, comparing CSV exports before and after processing to identify unexpected discrepancies. Configuration managers rely on diff tools to track changes in server configuration files, database connection strings, and deployment scripts across environments. Legal professionals use diff checkers to review contract revisions, highlighting exactly which clauses were added, removed, or modified between versions. Students use diff tools to compare their work against source materials for proper attribution and to understand corrections made by instructors. In quality assurance, testers diff expected output against actual output to catch regressions and unexpected behavior. Web developers diff HTML, CSS, and JavaScript files to understand what changed between deploys. The common thread across all these use cases is the need for precise, character-level visibility into what changed between two pieces of text.
Limitations and edge cases
Text diff checkers using the Longest Common Subsequence algorithm have several limitations to be aware of. The LCS algorithm has quadratic time complexity, meaning comparison time grows with the product of the two text lengths. Very large texts over 10,000 characters may cause noticeable slowdown in the browser. For comparing entire codebases or large documents, dedicated diff tools like Git diff or specialized desktop applications are more appropriate. The character-level approach also produces noisy results when comparing texts that are substantially different. If the old and new texts share very little common content, most characters will be flagged as additions and deletions, making the output difficult to interpret. In such cases, a word-level or line-level diff may be more useful. The tool also treats all characters equally, including whitespace. A change from two spaces to one space, or an added trailing newline, will be highlighted as a change. This can be helpful for precise work but distracting for prose comparison. Users comparing natural language should be aware that the diff shows every character change, which may include minor punctuation edits that do not affect meaning. For best results, ensure both texts are roughly similar in length and content before comparing.
Frequently asked questions
How does the diff algorithm work?
The tool uses a character-level LCS (Longest Common Subsequence) algorithm to identify which characters were added, removed or unchanged between the two texts.
Is there a size limit?
Very large texts (over 10,000 characters) may slow down the browser. For most everyday comparisons the tool works instantly.
Can I compare files?
You can copy and paste file contents into the two text areas. There is no file upload feature, but paste works for any text content.
Is my text private?
Yes. Every comparison runs entirely in your browser — nothing is uploaded to any server.
What do the colours mean?
Green highlights show text that was added in the new version. Red highlights show text that was removed from the old version. Unchanged text stays neutral.
Can I copy the diff result?
The result is displayed as formatted text. You can select and copy it or use the copy button to copy the entire output.
Does it work on mobile?
Yes. The two text areas stack vertically on small screens for easy comparison on any device.
What is the difference between word and character diff?
Character diff shows every single character change. Word diff groups changes by whole words. This tool uses character-level diff for maximum precision.
Can I compare code with this tool?
Yes. It works well for comparing code snippets, configuration files, JSON and other text-based data.
Why would I need a diff checker?
Common uses include reviewing edits in documents, comparing code changes, checking for plagiarism, validating data transformations, and tracking content revisions.