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.

What is the Text Diff Checker?

Free text diff checker that compares two blocks of text line by line, highlighting added and removed lines like a code diff. Optional ignore-case and ignore-whitespace modes. Runs entirely in your browser — nothing is uploaded. It runs free in your browser on Gera Tools, with nothing uploaded.

Text Diff Checker — Gera Tools

Name: Text Diff Checker
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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 is useful for proofreading edited documents, reviewing configuration file changes before deployment, comparing two versions of a contract clause, or quickly spotting what differs between two pasted paragraphs.

How the diff algorithm works

Both inputs are split into lines and compared using a longest-common-subsequence (LCS) algorithm. The LCS finds the largest set of lines that appears in the same order in both versions — these are the lines that didn’t change. Everything else is marked as a deletion (left text only) or an insertion (right text only), with line numbers shown for both sides.

Two optional modes adjust what counts as “the same”:

Ignore case — treats uppercase and lowercase as identical before comparing, so a line that only changed capitalisation does not show as modified.
Ignore leading/trailing whitespace — trims each line before comparison, hiding indentation-only changes. The original text is still what is displayed in the output.

Worked example

Left (original):

The meeting starts at 9am.
Agenda: budget review, roadmap.
Lunch break at noon.

Right (edited):

The meeting starts at 9am.
Agenda: budget review, Q3 roadmap.
Coffee break at noon.

The diff shows one unchanged line (The meeting starts at 9am.), then flags - Agenda: budget review, roadmap. as removed and + Agenda: budget review, Q3 roadmap. as added, and similarly for the lunch/coffee swap. Total: 2 removed, 2 added.

When to use each mode

Scenario	Suggested mode
Comparing code or config	Default (case-sensitive, whitespace matters)
Proofreading a document	Ignore case, ignore whitespace
Reviewing a copied-pasted contract clause	Ignore leading/trailing whitespace
Checking if two paragraphs are identical	Both ignore options on

Edge cases to know

A moved line is shown as both removed from its old position and added at its new one — the line-based algorithm has no concept of movement. Very large inputs (tens of thousands of lines per side) may be slower because comparison cost grows with the product of both line counts. Everything runs in your browser; neither version is ever uploaded.