Is my text uploaded anywhere?

No. The comparison uses a longest-common-subsequence algorithm that runs entirely in your browser. Neither block of text is ever sent to a server.

Does it compare line by line or character by character?

It compares line by line, which is the most useful view for documents, config files, and code. Each whole line is marked as unchanged, added, or removed.

Can I ignore formatting differences?

Yes. Toggle Ignore case and Ignore leading/trailing whitespace so that only meaningful changes are highlighted while the original text is still displayed.

What does the longest-common-subsequence approach do?

It finds the largest set of lines that appear in the same order in both versions and marks them as unchanged. Anything not in that common sequence is shown as removed from the left or added on the right.

Why is a moved line shown as both removed and added?

A line-based diff has no concept of movement. If a line shifts to a new position, the algorithm records it as removed from its old spot and added at its new one.

Can it compare code or JSON?

Yes. Because it works on raw lines, it handles source code, JSON, CSV, configuration files and prose equally well. For pretty-printing JSON first, pair it with a JSON formatter.

Is there a size limit?

The line-by-line algorithm is efficient for typical documents and files. Extremely large inputs (tens of thousands of lines on each side) may slow down because the comparison cost grows with the product of the two line counts.

Text Diff Checker — Gera Tools

Text diff checker

The text diff checker compares two versions of text and shows exactly what changed, line by line — added lines in green with a plus sign and removed lines in red with a minus sign, just like a code-review diff. It suits proofreading edits, reviewing config changes, and spotting differences between two pasted documents.

How it works

Both inputs are split into lines and compared with a longest-common-subsequence (LCS) algorithm. The LCS is the largest sequence of lines that appears, in the same order, in both versions; those lines are marked unchanged. Walking through both texts, any line not part of the common sequence is emitted as a deletion (left only) or an insertion (right only), with original and new line numbers shown. Optional ignore case and ignore leading/trailing whitespace modes normalise lines before comparison, so cosmetic differences don’t register, while the original text is still what’s displayed.

Example

Left text:

alpha
beta
gamma

Right text:

alpha
delta
gamma

The diff keeps alpha and gamma as unchanged, marks - beta as removed and + delta as added — a count of 1 removed and 1 added.

The comparison runs entirely in your browser, so neither version is ever uploaded.