Text Diff Checker

What is Diff Checker?

Free online diff checker. Compare two blocks of text and see the differences highlighted with added, removed, and unchanged lines. Developer utilities built for web developers, software engineers, and programmers. TekPulse's Diff Checker is completely free — no signup, no ads in results, and no data stored on our servers.

How to Use Diff Checker Online

1. Paste the original text in the left panel and the modified text in the right panel
2. Click "Compare" — added lines are highlighted in green, removed lines in red, unchanged lines in white
3. The summary shows the total lines added, removed, and unchanged
4. Use the word-level diff toggle to see character-level changes within each line
5. Download the diff report as a text file for code review or documentation purposes

Diff Checker Formula & Calculation

This tool compares two texts line by line using the classic LCS algorithm: it builds an (m+1)×(n+1) table where each cell dp[i][j] holds the length of the longest sequence of lines common to both texts up through line i of text A and line j of text B. Matching lines extend the diagonal (dp[i-1][j-1]+1); non-matching lines take the larger of the cell above or to the left. Walking this table backwards from the bottom-right corner reconstructs the actual sequence of kept, added, and removed lines.

This is the same core algorithm behind the Unix `diff` command and `git diff` (which use a refined variant called Myers diff for speed) — it finds the minimal set of insertions/deletions that transforms text A into text B, rather than naively comparing line N of A to line N of B.

Key Factors That Affect Your Result

1. Line-based, not character-based — This tool diffs whole lines, not individual characters/words — a single changed word anywhere in a line marks the entire line as removed+added. For word-level diffing you need a different granularity of comparison.
2. Ignore case / ignore whitespace — These options normalize each line (lowercase it, or collapse repeated whitespace) before comparison, so formatting-only or capitalization-only differences don't show up as changes — useful when comparing code after a reformat.
3. Quadratic cost on large inputs — Because the DP table is m×n, diffing two 10,000-line files means a 100-million-cell table. Browser-based diff tools like this one are best for files up to a few thousand lines.

Why Use TekPulse Diff Checker?