Diff Checker
Compare two texts and see every difference highlighted line by line.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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Request a ToolHow to Use the Diff Checker
The Diff Checker compares two blocks of text and highlights every difference between them. It is useful for reviewing code changes, checking document revisions, comparing configuration files, or verifying that an edit introduced only the changes you intended.
- Paste the original text into the left textarea. This is the baseline version you want to compare against.
- Paste the modified text into the right textarea. This is the updated version with changes.
- Click Compare (or press Ctrl+Enter). The tool runs a line-by-line diff using a Longest Common Subsequence algorithm and displays the results below.
- Review the output. Added lines appear with a green background and a + prefix. Removed lines appear with a red background and a - prefix. Unchanged lines are shown for context. The summary shows the total number of changes, lines added, lines removed, and lines unchanged.
- Copy the result using the Copy button to share or save for reference.
The comparison runs entirely in your browser. Your text is never sent to a server, so it is safe for comparing sensitive code, credentials, or private documents.
About the Diff Checker
This tool performs a line-by-line comparison using the Longest Common Subsequence (LCS) algorithm, the same foundational technique used by diff on Unix systems and version control tools like Git. It identifies the longest sequence of lines common to both texts, then marks everything else as either added or removed.
Line-by-line diff is the most common approach for comparing structured text like source code, configuration files, CSV data, and written documents. For very large texts (thousands of lines), the comparison may take a moment since the algorithm runs in O(n*m) time, which is why this tool uses a Compare button rather than live-updating on every keystroke.
Frequently Asked Questions
Is my text sent to a server?
No. The entire comparison runs in your browser using JavaScript. Your text never leaves your device. This makes the tool safe for comparing code with API keys, passwords, or any other sensitive content.
What algorithm does this diff tool use?
The tool uses a Longest Common Subsequence (LCS) algorithm to perform line-by-line comparison. This is the same fundamental approach used by the Unix diff command and Git. It finds the longest set of matching lines between both texts, then identifies additions and removals relative to that common sequence.
Can I compare very large files?
The tool works well for texts up to a few thousand lines. For extremely large files (tens of thousands of lines), the comparison may take several seconds because the LCS algorithm scales with the product of line counts in both texts. For such cases, a dedicated desktop diff tool like VS Code or Meld may be more appropriate.
Does the diff checker show character-level changes?
This tool performs line-by-line comparison. If a single character changes on a line, the entire line is shown as removed (old version) and added (new version). This is the same behavior as a unified diff in Git. Character-level highlighting within changed lines is a possible future enhancement.