Last updated 28 August 2026
How a line diff decides what changed
The comparison works by finding the longest sequence of lines that appears in both versions in the same order — the longest common subsequence. Everything in that sequence is unchanged; everything else is either a removal from the original or an addition in the new version. This is the same approach version control uses, and it explains a behaviour that sometimes surprises people: editing one word in a line produces a removed line and an added line rather than a marked-up single line, because the unit of comparison is the whole line. The two options above deal with the usual sources of false positives — trailing whitespace that no one can see, and capitalisation differences that may not matter for your purposes.
Common questions
Why does a small edit show as both a removal and an addition?
Because the comparison works on whole lines. If any part of a line differs, the old line is gone and a new one has taken its place. Most version-control tools report line diffs the same way.
Is my text uploaded anywhere?
No. Both texts stay in your browser and the comparison runs locally. Nothing is transmitted, which makes it safe for drafts, contracts or code you would not want to paste into a server-side tool.
What do the options do?
Ignoring leading and trailing spaces stops invisible whitespace from registering as a change. Ignoring case treats capitalisation differences as identical, which is useful when comparing prose but usually not what you want for code.
How large a text can it handle?
Comfortably up to a few thousand lines. The matching work grows with the size of both texts, so very large files will feel slow — for those, a desktop diff tool is the better instrument.