Text Diff Checker — Compare Two Texts Line by Line
Find exact line-by-line differences between texts using the LCS algorithm. Green for added lines, red for removed. Exact matching, no fuzzy compare.
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Unchanged
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Added
2
Removed
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One changed word marks the whole line
Most people expect word-level highlighting: change one word in a sentence, and that word is marked. This tool does line-level only. Change "Hello" to "Hello there" in a line, and the entire line is marked as removed and re-added. That is accurate for tracking bulk changes — paragraphs rewritten, functions replaced — but surprising in a single-sentence edit.
Exact line matching, nothing fuzzy
A diff finds the longest common sequence of identical lines. Two lines match only if they are character-for-character identical: trailing spaces, case, punctuation. There is no ignore-case option and no whitespace normalisation. A line with one extra space counts as a different line. That precision is useful when you need to know exactly which lines persist across versions and which are new.
Classic LCS, and what that costs
The diff engine uses the standard dynamic-programming implementation of Longest Common Subsequence. It compares every line of one text against every line of the other, so the work grows with the product of the two line counts, not their sum. That is instant for documents, reports and config files; two very large inputs of several thousand lines each will noticeably slow down. There is no hard cap — the browser simply takes longer.
What this tool does not do
No word-level or character-level highlighting. No ignore-case, no ignore-whitespace, no similarity score. No side-by-side view, no merge preview, no export. It is a reader, not an editor — for merging you want Git or an editor's merge UI. If your two texts differ only in capitalisation or spacing, normalise them first with the Text Case Converter or Text Utilities, then diff.
How to use the Text Diff Checker
Takes about a minute. No signup, no download, your data stays in your browser.
- 1Open the tool. Scroll up to the Text Diff Checker above — it loads instantly in your browser, no install needed.
- 2Enter your values. The fields come pre-filled with realistic defaults so you can see how it works — replace them with your own numbers.
- 3Read the result. The output updates instantly. Copy or share it — nothing is uploaded to a server, everything stays on your device.
Frequently asked questions
Common questions about the Text Diff Checker.
Does it highlight word-level changes within a line?
No. If you change one word in a line, the entire line is marked as removed and re-added. The tool diffs by line, not by word. That suits structural changes to documents, but for word-level precision you need a tool like Git or an editor with a built-in merge view.
Can I ignore whitespace or use case-insensitive matching?
No. The tool matches lines exactly as written, including trailing spaces and letter case. Every character must be identical for two lines to count as a match. If your text has inconsistent spacing or casing, normalise it with a text utility first, then run the diff.
How large can the text inputs be?
There is no hard limit, but the algorithm compares every line of one text against every line of the other, so time grows with the product of the two line counts. Documents, reports and config files are instant; two inputs of several thousand lines each will be noticeably slower.
What does the LCS algorithm do?
LCS (Longest Common Subsequence) finds the longest chain of lines that appear in both texts in the same order. Everything outside that chain is marked as added or removed. This handles out-of-order edits gracefully and is why moved blocks are reported as a deletion plus an insertion.
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