What Is a Text Diff?
A text diff (short for "difference") is a comparison between two pieces of text that shows exactly what changed between them. Diff tools highlight added lines, removed lines, and unchanged lines so you can quickly understand modifications without reading both texts in full. The concept originates from the Unix diff utility created in the early 1970s, which became a foundational tool for software development and version control systems.
Common Use Cases
Text diff tools are essential across many workflows. Developers use them to review code changes before committing to version control systems like Git. Technical writers compare document revisions to track edits and ensure accuracy. System administrators diff configuration files to identify unintended changes after updates. Students compare essay drafts to see how their writing evolved. Legal professionals compare contract versions to spot altered clauses. Quality assurance teams diff API responses to detect regressions. Whether you are comparing two versions of a configuration file, reviewing a pull request, or checking how a document was edited, a diff tool saves time and reduces the chance of missing important changes.
How Diff Algorithms Work
Most diff algorithms are based on the Longest Common Subsequence (LCS) problem. The algorithm finds the longest sequence of lines (or characters) that appear in both texts in the same order. Lines not part of this common subsequence are marked as either additions or deletions. The classic dynamic programming approach to LCS runs in O(n*m) time where n and m are the lengths of the two inputs. More advanced algorithms like Myers' diff algorithm optimize this by focusing on the shortest edit script, finding the minimal number of insertions and deletions needed to transform one text into the other. This tool implements a standard LCS-based approach that works well for most comparison tasks, providing both line-level and character-level diffing to give you maximum visibility into what changed.
Features of This Tool
This online text diff tool runs entirely in your browser. No data is sent to any server, making it safe for comparing sensitive content. You can choose between inline diff mode, which shows changes in a unified view with character-level highlighting, and side-by-side mode, which aligns the original and modified text in two columns for easy visual scanning. Additional options include ignoring whitespace differences and case differences, which are useful when you want to focus on meaningful content changes rather than formatting. The tool provides statistics showing the number of lines added, removed, and unchanged, giving you a quick summary of the scope of changes.