What is a diff checker?
A diff checker (short for "difference checker") is a tool that compares two pieces of text and highlights what has changed between them. It shows you lines that were added, removed, or remained the same. Diff tools are a fundamental part of software development and document management.
Line-by-line comparison explained
This tool uses the Longest Common Subsequence (LCS) algorithm to find the maximum number of lines that appear in both versions in the same order. Lines that exist only in the original are shown in red with a "−" prefix. Lines that exist only in the modified version are shown in green with a "+" prefix. Unchanged lines are shown in gray.
Common use cases
Diff checkers are used by developers to review code changes before committing, by writers to compare document revisions, by editors to track content edits, and by data teams to spot changes between data exports. They are also handy for comparing configuration files, contract versions, or any two pieces of text you need to audit.
How this tool works
Everything runs locally in your browser. The text is split into lines, the LCS algorithm computes the optimal alignment, and the result is rendered with color-coded highlighting. No data is sent to any server — your content remains completely private.