Text Diff Checker
Compare two texts side-by-side and highlight line-by-line differences.
About the Text Diff Checker
The Text Comparison Tool (also called a diff tool) compares two pieces of text line by line and highlights the differences. It shows which lines were added, removed, or changed between an original and a modified version. This is invaluable for reviewing edits, comparing document versions, verifying data transformations, and proofreading.
Diff tools are a staple of software development (Git diffs, pull request reviews) but are equally useful for writers, editors, lawyers, and anyone who works with versioned documents. If you have ever received a revised contract and needed to find what changed from the original, or wondered which paragraphs your co-author edited in a shared document, a diff tool gives you the answer in seconds.
This tool compares texts line by line. Lines that are identical in both versions are shown in gray (unchanged). Lines present only in the original are marked as removed; lines present only in the modified version are marked as added; lines that exist in both but differ are marked as changed. The summary at the top tells you the total counts at a glance.
How It Works
The comparison splits both texts into arrays of lines using split('\n'). It then iterates through both arrays in parallel, comparing the line at each index. If the lines match, they are marked as “same.” If one text has a line at that index but the other does not (because one is shorter), it is marked as added or removed. If both have a line at that index but the content differs, it is marked as changed.
This is a simple positional diff, not an LCS (Longest Common Subsequence) diff. It compares line N of the original to line N of the modified version. If a line was inserted in the middle of the modified text, every subsequent line would appear “changed” even if the content is the same — just shifted down. For smarter diffing, a tool like diff-match-patch (Google's library) uses the Myers algorithm to find the minimal set of changes.
Despite this limitation, the positional approach works well when comparing two versions of the same document where most lines are in the same position. It is fast (O(n) time), easy to understand, and produces a clear side-by-side view that humans can scan quickly.
Worked Examples
Compare the default original and modified texts. The tool shows line 1 (“The quick brown fox”) as same (gray), line 2 as changed (“jumps” → “leaps”), line 3 as same, and line 4 as changed (“This line will be removed” → “This line was added”). The summary reports 2 same, 2 changed, 0 added, 0 removed.
If you add a new line to the modified text (say, inserting “A new line here” between lines 2 and 3), the diff would show line 3 of the original (“This line is unchanged”) compared to line 3 of the modified (“A new line here”) — marked as changed even though the content is unchanged, just shifted. This is the limitation of positional diffing.
For comparing two versions of a contract, paste the original in the left field and the revised version in the right. Any line that was modified, added, or removed will be highlighted, making it easy to spot changes that might affect your obligations.
When to Use This Tool
- Reviewing edits made by a co-author or editor on a shared document.
- Comparing two versions of a contract or legal agreement to identify changes.
- Verifying that a data transformation (CSV export, JSON migration) preserved all records.
- Checking plagiarism by comparing a suspect text against the source.
- Reviewing configuration file changes between deployments.
- Comparing translation files to find untranslated or missing strings.
- Proofreading by comparing an original draft against a corrected version.
Limitations & Disclaimer
This tool uses positional line-by-line comparison, not the Myers diff algorithm. Inserting or deleting a line in the middle of the text will cause all subsequent lines to appear “changed.” For smart diffing that handles insertions and deletions correctly, use Git, VS Code, or a dedicated diff library like Google's diff-match-patch. The tool does not perform character-level highlighting within changed lines — it marks entire lines as same, changed, added, or removed. See our disclaimer for full terms. Related: Find and Replace and Remove Duplicate Lines.
Frequently Asked Questions
What is the difference between line-by-line and word-by-word comparison?
Line-by-line marks any line with a difference as changed. Word-by-word breaks each line into words and highlights individual word changes. Word-by-word is more granular but harder to read for heavily edited text.
Why does inserting one line cause many lines to show as changed?
This tool uses positional comparison (line N vs line N). Inserting a line shifts subsequent lines down by one, so every following line appears changed. Smart diff tools (like Git) use the Myers algorithm to find minimal changes, handling insertions and deletions correctly.
Can I compare files instead of pasting text?
Not directly. You need to open each file in a text editor, copy its contents, and paste into the two fields. For file-based diffing, use a desktop tool like VS Code's diff viewer, Meld, or Beyond Compare.
Does the tool handle different line endings?
The browser normalizes carriage-return-plus-newline (Windows) to a single newline (Unix) when reading from textareas, so mixed line endings should not cause false differences. If you see every line marked as changed, check for invisible character differences like trailing whitespace or BOM markers.
Is the comparison case-sensitive?
Yes. "Hello" and "hello" are treated as different lines. If you want case-insensitive comparison, convert both texts to the same case (using our Case Converter) before comparing.
Is my text uploaded anywhere?
No. All comparison happens in your browser. Your documents - including confidential contracts and unpublished drafts - never leave your device.
Last updated: September 9, 2026 · Author: HT99 Tools Editorial Team