Text Diff Checker
Compare two blocks of text line by line and see exactly what was added, removed, or left unchanged.
How it works
The comparison is a line-level diff built on the longest common subsequence (LCS) algorithm, the same family of logic behind git diff. Both texts are split into lines, then a dynamic-programming table works out the largest set of lines the two versions share in order. Walking that table back produces the operations you see: lines only in the left text become red removals, lines only in the right become green additions, shared lines stay neutral, each with its original line number.
Line-level diffing is a deliberate tradeoff. Character-level diffs can show that one word changed inside a line, but on real documents they degenerate into confetti: hundreds of tiny fragments that are harder to read than just seeing the old line removed and the new line added. Line-level output stays readable and maps directly onto how code review and version control present changes. The two checkboxes tune what "equal" means: ignore whitespace collapses runs of spaces and trims line ends before comparing (great for reformatted code), and ignore case lowercases both sides. The displayed lines keep their original formatting either way.
Because the DP table costs memory and time proportional to (lines in A) x (lines in B), the tool caps input at 4,000 lines per side; past that, the table alone would be tens of millions of cells and the tab would stutter. Within the cap, everything runs synchronously in your browser: both texts live only in your tab, no request fires when you type (check DevTools' Network panel), and confidential drafts or configs never touch a server.
Common questions
How does the comparison work?
It compares the two texts line by line and finds the longest common subsequence, the same core approach used by tools like git diff. Lines that only exist in one side are marked added or removed; everything else is shown unchanged.
Can I ignore whitespace or capitalization differences?
Yes, the two checkboxes above the input boxes let you ignore leading/trailing whitespace and repeated spaces, and ignore case, so only meaningful changes show up.
Is there a size limit?
Each side is capped at 4,000 lines to keep the comparison fast in your browser. For most code files, configs, and documents that's more than enough room.