Quickstart

redlines compares two strings and produces structured output showing their differences. Changes are represented with strike-throughs and highlights, in the manner of Microsoft Word’s track changes, and the output carries change information, positions and statistics for programmatic use.

Install

pip install redlines

Python 3.10 to 3.14 are supported.

Optional extras

Extra	Install	What it adds
pdf	pip install redlines[pdf]	Comparing PDF files
nupunkt	pip install redlines[nupunkt]	Sentence boundary detection that handles abbreviations, citations and URLs (Python 3.11+)
levenshtein	pip install redlines[levenshtein]	Levenshtein distance in the statistics

Compare from the command line

JSON is the default output, so a bare invocation is enough:

redlines "The quick brown fox jumps over the lazy dog." "The quick brown fox walks past the lazy dog."

Files work the same way, and --pretty makes the JSON readable:

redlines --pretty old_version.txt new_version.txt

Compare from Python

from redlines import Redlines

test = Redlines(
    "The quick brown fox jumps over the lazy dog.",
    "The quick brown fox walks past the lazy dog.",
    markdown_style="none",
)
print(test.output_markdown)
# The quick brown fox <del>jumps over </del><ins>walks past </ins>the lazy dog.

Other output formats are available on the same object: output_json for structured changes and statistics, output_rich for terminal display, and compare(markdown_style=...) for the six markdown styles.

Where to go next

• The agent integration guide covers invocation from agents and automation, the JSON structure, error handling and integration patterns.
• The API reference is generated from the docstrings and documents every class and method.
• The decision records explain why the library is shaped the way it is, and where it is going.

Reading this site as an agent

Every page is available as plain markdown by appending .md to its path — /guides/agent-guide.md, for instance. /llms.txt indexes the site, /llms-small.txt is the usage documentation in one file, and /llms-full.txt adds the planning documents and decision records.