A diff checker (short for 'difference checker') applies a diff algorithm , most commonly the longest common subsequence (LCS) algorithm , to identify the minimal set of changes between two texts.

The output is a color-coded view:

• Green , content added in the new version
• Red , content removed from the original
• No highlight , content unchanged in both

This is the same underlying mechanism that powers git diff, the Unix diff command, and code review views in GitHub, GitLab, and Bitbucket.

Different modes give you different levels of detail:

Lines mode Compares entire lines as the unit of change. Best for source code, config files, structured data (CSV, JSON, YAML). A changed line shows as a deletion of the old and an insertion of the new.

Words mode Breaks down to individual word changes within each line. Better for prose and articles , shows when a single word was replaced without flagging the entire surrounding sentence.

Characters mode The most granular option , highlights individual character changes within words. Useful for subtle differences like punctuation changes, spelling corrections, number typos (1000 vs 10000), or encoding artifacts that would be invisible at the word level.

Not every code comparison needs a full Git workflow. A diff checker is faster in many common situations:

• Quick check before committing , paste two versions of a function to verify your changes before staging
• Reviewing a shared snippet , compare a colleague's snippet against your current version without opening a PR
• Comparing config files , checking differences between staging and production configs
• Reviewing API responses , comparing two responses to find what changed between calls
• Validating generator output , checking that generated code matches an expected template

A browser-based diff checker handles any plain text , JavaScript, Python, SQL, YAML, JSON, HTML, CSS , without any setup.

Writers, editors, and content teams use diff checkers constantly:

• Draft revision review , comparing a first and second draft to see exactly what the editor changed
• Contract and legal document review , identifying precisely which clauses were modified between versions
• Academic work , verifying that a rewrite changed the wording substantially enough
• Translation review , comparing a source document against an updated translation
• Content localization , identifying which strings changed in a source file to know which translations need updating

In word mode, a diff checker surfaces rewrites and substitutions clearly , much faster than reading both versions in full.

Many diff checkers can export in unified patch format , the same format used by Git and Unix diff tools. It looks like this:

@@ -1,4 +1,4 @@
 The quick brown fox
-jumps over the lazy dog
+leaps over the sleeping cat
 and runs away

• Lines starting with - were in the original and are now removed
• Lines starting with + are in the new version and were added
• Lines with no prefix are unchanged (context lines)
• @@ -1,4 +1,4 @@ indicates line positions in the original and new files

Patch files can be applied with git apply or the patch command, shared with teammates for review, or used as documentation of what changed.