Text Diff Checker

In short A diff finds what changed between two texts using the longest common subsequence algorithm — the same approach the diff command and git use — and shows every added, removed and changed line. The algorithm works by finding the longest sequence of lines present in both versions in the same order; everything not in that sequence is an addition or a deletion. That is why a diff of a file where one paragraph moved shows it as a deletion in one place and an addition in another rather than as a move: the order changed, so it cannot be part of a common subsequence. Line-level diffing is the default and is right for code and structured text. Word-level diffing is better for prose, where a one-word change would otherwise mark the whole paragraph as rewritten, and the difference in readability is large on an edited document. Two things a diff cannot tell you: which version is correct, and whether a change was intentional. It tells you exactly what differs, which is a smaller and far more reliable claim.