Text diff checker online: compare two texts side by side

Compare any two blocks of text and highlight every difference instantly with ToolHQ's text diff checker, paste your original and revised versions, and see every addition, deletion, and change marked in colour.

A text diff checker is a comparison tool that takes two versions of a text and computes the minimal set of changes, additions and deletions, that transforms one into the other, then displays those changes highlighted so you can see exactly what is different between the two versions.

Reviewing document revisions, comparing translation outputs, checking code snippets, and validating edits all require knowing precisely what changed between two text versions. Scrolling through both versions manually and spot-checking differences is error-prone; a diff checker catches every change, including the single-character ones that human eyes slide past.

Key takeaways

• ToolHQ shows differences at both line level and character level within changed lines
• Additions appear in green, deletions in red, unchanged text is left unmarked
• Side-by-side and inline view modes let you choose how to read the diff
• Processed in your browser, no data is sent anywhere
• Whitespace-sensitive and whitespace-insensitive modes control whether spacing changes are flagged

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How text diff works

The algorithm behind most diff tools is based on the work of Eugene Myers, who published an efficient diff algorithm in 1986. The algorithm finds the longest common subsequence (LCS) between two texts, the longest sequence of lines or characters that appear in the same order in both versions. Everything not in the LCS is either an addition or a deletion.

According to Wikipedia's article on diff, the diff utility was first developed for Unix in the 1970s and the output format it defined, with + for additions and - for deletions, became the universal standard for displaying text differences, still used in version control systems like Git today.

The Myers diff algorithm, referenced in Wikipedia's article on the subject, is considered the standard implementation because it produces the shortest edit script (fewest changes) that transforms one text into another. This means the diff shows you only genuine changes, not spurious alternatives.

ToolHQ's diff checker applies this at two levels:

• Line level: entire added or removed lines are marked clearly
• Character level: within lines that changed, the specific characters that differ are highlighted, so you can see "colour" changed to "color" rather than just "this line changed"

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When you need a text diff checker

Document revision review. When a collaborator sends back an edited version of a document, a diff checker shows you exactly what they changed, every word, every punctuation mark, without requiring tracked changes to be enabled.

Code snippet comparison. Comparing two versions of a configuration file, a script, or a code block in plain text is faster with a diff tool than reading line by line. A single character difference in a config value is immediately visible.

Translation and localisation checking. When comparing a source text with its translated version, a diff can highlight structural differences, paragraphs added, sentences removed, that indicate translation coverage gaps.

Legal and contract review. Contract negotiations involve multiple redlined versions. A diff checker applied to plain text exports of two versions shows every change without relying on Word's track changes feature, which can be disabled or edited.

Content quality audit. When an article is revised between two publication dates, a diff between the old and new version shows every change made, useful for auditing whether a freelancer's revisions matched the brief.

Plagiarism spot-check. Pasting a student submission and an original source into a diff tool shows matching passages (unchanged text) and differences immediately, providing a quick visual scan before formal plagiarism detection tools are applied.

Version history reconstruction. When a file history is incomplete, comparing two saved snapshots of a document via diff gives you a readable record of what changed between those points, even without a version control system tracking the changes automatically.

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Mini-story: A project manager received two versions of a supplier contract, the version her company sent and the supplier's counter-signed version with requested changes. Her lawyer's fee for reviewing the differences was significant. She ran both versions through a diff checker first, identified the three changed clauses herself, and sent only those clauses to her lawyer for review. The targeted review took a fraction of the time and cost.

Try ToolHQ's text diff checker

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How to use the text diff checker

1. Paste the original text in the left input box.
2. Paste the revised text in the right input box. The "original" and "revised" labels are conventional; you can put either version on either side.
3. Choose your view. Side-by-side view shows original and revised in parallel columns. Inline view shows a single column with changes marked inline.
4. Set whitespace sensitivity. Turn on "ignore whitespace" mode if spacing differences should not count as changes. Leave it off if extra spaces and newlines matter.
5. Read the diff. Green highlights show text added in the revised version; red highlights show text removed from the original. Unmarked text is identical in both versions.
6. Copy or export. Some diff tools let you copy the diff output or save it. ToolHQ keeps the comparison in your browser session.

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Line-level vs character-level diff

Feature	Line-level only	Character-level
Detects added lines	Yes	Yes
Detects removed lines	Yes	Yes
Shows what changed within a line	No	Yes
Catches single-character typo	No	Yes
Speed on very long texts	Faster	Slightly slower

Character-level diff within changed lines is the significant quality improvement over basic line-level comparison. Without it, a diff checker might tell you "line 47 changed" without showing you that only the word "their" became "there." With character-level diff, the exact change is highlighted within the line.

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Mini-story: A developer was debugging a configuration issue where a production environment behaved differently from staging, but the config files looked identical at a glance. She ran both files through a text diff checker with character-level diff enabled. The tool immediately showed a trailing space after one value in the production config, invisible to the eye but causing the application to treat the value as including the space character. The fix was a five-second deletion; finding the problem had taken two hours of manual review.

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Frequently asked questions

What is the difference between a diff checker and a find-and-replace tool? A diff checker compares two texts and shows what is different between them, a read operation. Find and replace modifies text by substituting specific patterns, a write operation. Use diff to review changes; use find and replace to make them.

Can I diff code as well as prose? Yes. The tool handles any plain text: prose, code, configuration files, JSON, CSV, or any other plain-text format. It does not understand programming syntax but does detect every character difference correctly.

Does it work on very long documents? Yes. The tool handles large inputs in your browser. For very large documents (tens of thousands of lines), processing may take a second or two, but there is no server-side size limit.

Is the comparison case-sensitive? By default, yes. "Apple" and "apple" are treated as different strings. Most tools include a case-insensitive mode if needed.

Is my text stored or transmitted? No. Processed in your browser, no data is sent anywhere. Both text versions stay on your device.

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The short version

A text diff checker finds every difference between two versions of a text, additions in green, deletions in red, at line level and character level. It is faster and more accurate than manual comparison, and catches the single-character changes that human review misses.

Compare your texts now at ToolHQ, line and character level diff, side-by-side view, no account needed.

For related tools, try find and replace to make bulk edits, or sentence counter to analyse text length before and after revision.