A pure Ruby Markdown linter and formatter with zero external dependencies

Features • Quickstart • Installation • Usage • Configuration • Lint Rules • Formatting

• Pure Ruby, no C/Rust extensions or external dependencies
• Linting with configurable rules and clear violations
• Auto-formatting that follows mdformat conventions
• CLI for CI-friendly workflows and local formatting
• Simple YAML configuration file

gem install mdlint

# format in place
mdlint README.md docs/

# check-only (CI)
mdlint --check README.md

# show diffs
mdlint --diff README.md

With Bundler:

bundle add mdlint

Without Bundler:

gem install mdlint

Command line:

mdlint README.md docs/
mdlint --check README.md
mdlint --diff README.md

Options:

Usage: mdlint [options] [paths...]

Options:
    -c, --check              Check if files are formatted, exit with error if not
    -d, --diff               Show diff of changes
    -q, --quiet              Suppress output
    -e, --exclude PATTERN    Exclude files matching pattern
    -w, --wrap MODE          Paragraph wrapping: keep (default), no, or INTEGER
        --number             Use consecutive numbering for ordered lists
        --end-of-line MODE   End of line: lf (default), crlf, keep
    -v, --version            Show version
    -h, --help               Show help

Ruby API:

require "mdlint"

formatted = Mdlint.format("# Hello World")
Mdlint.format_file("README.md")

tokens = Mdlint.parse("# Hello World")

violations = Mdlint.lint("# Heading\n\n\n\nParagraph")
violations.each { |v| puts v }

violations = Mdlint.lint_file("README.md")

Create a .mdlint.yml file in your project root:

# Check mode (don't modify files)
check: false

# Quiet mode (suppress output)
quiet: false

# Exclude patterns
exclude:
  - "vendor/**/*.md"
  - "node_modules/**/*.md"

Key rules:

• Heading levels should increment by one
• Heading style should be consistent (ATX)
• No trailing spaces
• No multiple consecutive blank lines
• First line should be a top-level heading

Block elements:

• ATX headings (# Heading)
• Setext headings (converted to ATX)
• Paragraphs
• Bullet lists (-, *, +)
• Ordered lists (1., 2.)
• Blockquotes (>)
• Fenced code blocks (```)
• Indented code blocks (converted to fenced)
• Horizontal rules (---, ***, ___)
• HTML blocks
• Reference definitions

Inline elements:

• Bold (**text**, __text__)
• Italic (*text*, _text_)
• Inline code (`code`)
• Links ([text](url))
• Reference links ([text][ref])
• Images (![alt](src))
• Autolinks (<https://example.com>)
• Hard breaks (backslash + newline)

bin/setup
bundle exec rspec

Bug reports and pull requests are welcome on GitHub at https://github.com/ydah/mdlint.

MIT License. See LICENSE.txt for details.