RedQuilt

A modern Markdown document processor in pure Ruby, with an arena-style AST. Passes the full CommonMark spec test suite, and generally faster than kramdown.

Installation

Add this line to Gemfile:

gem "red_quilt"

Quick Start

Parsing and rendering

require "red_quilt"

# Parse Markdown to a document
doc = RedQuilt.parse("# Hello\n\nThis is **bold**.")
html = doc.to_html
# => "<h1>Hello</h1>\n<p>This is <strong>bold</strong>.</p>\n"

# Or render directly (without building AST)
html = RedQuilt.render_html("# Hello\n\n**bold**")

HTML is safe by default

RedQuilt.render_html("Hi <em>tag</em>")
# => "<p>Hi &lt;em&gt;tag&lt;/em&gt;</p>\n"

RedQuilt.render_html("Hi <em>tag</em>", allow_html: true)
# => "<p>Hi <em>tag</em></p>\n"

Options

RedQuilt.parse and RedQuilt.render_html accept:

Option	Default	Effect
`allow_html:`	`false`	Pass raw HTML through instead of escaping it
`disallow_raw_html:`	`false`	With `allow_html`, still neutralize GFM's dangerous tags (`<script>`, `<iframe>`, …)
`extended_autolinks:`	`false`	GFM: linkify bare `http(s)://` / `www.` / email addresses
`footnotes:`	`false`	GFM footnotes (see below)
`lint:`	`false`	Collect lint diagnostics (empty links, missing image alt, heading-level skips)

Footnotes (opt-in)

RedQuilt.render_html(<<~MD, footnotes: true)
  Here is a reference.[^1]

  [^1]: And the footnote text.
MD
# The reference becomes a superscript link, and a trailing
# <section class="footnotes"> lists the referenced definitions (in
# first-reference order) with backrefs.

Diagnostics

Parsing never raises on malformed input; warnings are collected on the document.

doc = RedQuilt.parse("[x](javascript:alert(1))", lint: true)
doc.diagnostics.map(&:rule)   # => [:unsafe_url]
doc.diagnostics.first.severity # => :warning

Heading anchors (opt-in)

render_html / to_html accept heading_ids: to give every heading a slugified id for anchor links. Slugs follow GitHub's scheme but keep Unicode intact, so Japanese headings stay readable; duplicates get -1, -2 suffixes.

RedQuilt.render_html("# Hello World\n\n## はじめに", heading_ids: true)
# => "<h1 id=\"hello-world\">Hello World</h1>\n<h2 id=\"はじめに\">はじめに</h2>\n"

Tilt integration

RedQuilt ships a Tilt adapter. NOTE: It is not loaded by default; require it explicitly and add tilt to your own bundle:

require "red_quilt/tilt"

Tilt.new("page.md").render          # => HTML
Tilt.new("page.md", footnotes: true).render

Native options (allow_html:, footnotes:, …) pass straight through; Tilt's escape_html: convention is also honored.

Documentation

API reference — Document / NodeRef / SourceSpan, supported syntax, and usage examples
Architecture overview (日本語)
Arena usage guide (日本語)
CommonMark conformance notes (日本語)

CommonMark Compatibility

RedQuilt achieves 100% compliance with the CommonMark v0.31.2 specification. See the conformance notes for GFM extensions and intentional deviations.

Command-line Tool

RedQuilt ships with a redquilt CLI for converting Markdown files to HTML or inspecting the AST.

Basic usage

# Convert Markdown file to HTML
redquilt input.md > output.html

# Convert from stdin
echo "# Hello" | redquilt

# Output as AST (for debugging)
redquilt --format ast input.md

# Output as MDAST-compatible JSON (for external tools)
redquilt --format json input.md

# Standalone HTML document with title
redquilt --standalone --title "My Document" input.md

# Enable GFM extended autolinks / footnotes
redquilt --extended-autolinks --footnotes input.md

# Standalone page with the bare template (no embedded CSS)
redquilt --theme none input.md

Options

--format FORMAT          Output format: html (default), ast, json
--allow-html             Pass raw HTML through to the output
--disallow-raw-html      With --allow-html, filter GFM's dangerous tags
--extended-autolinks     Linkify bare URLs and email addresses (GFM)
--footnotes              Enable GFM footnotes
--lint                   Collect lint diagnostics
--[no-]standalone        Wrap HTML in full document (default: on)
--auto-title             Use the first heading's text as <title>
--title TITLE            Explicit <title> text
--lang LANG              html lang attribute (default: "en")
--css URL                Add a stylesheet link
--theme THEME            Embedded stylesheet: default (default) or none
--diagnostics            Print diagnostics to stderr
--diagnostics-only       Print diagnostics only (suppress output)
-h, --help               Show help
-v, --version            Show version

Exit code is 0 on success, 1 if errors are detected.

Safe-by-Default HTML Rendering

Security model

RedQuilt prioritizes security by default:

# Default: All HTML is escaped, dangerous URLs blocked
RedQuilt.render_html("<script>alert('xss')</script>")
# => "<p>&lt;script&gt;alert('xss')&lt;/script&gt;</p>"

RedQuilt.render_html("[click](javascript:alert(1))")
# => "<p><a href=\"\">click</a></p>"

Allowed URL schemes

In link/image destinations, only these schemes are permitted:

Absolute: http://, https://, ftp://, tel:, ssh://
Relative: /path, #anchor, path/to/file
Special: mailto: (autolinks only)

All other schemes (javascript:, data:, vbscript:, etc.) are blocked by replacing the URL with an empty string.

Autolinks (<scheme:...>) follow CommonMark and allow arbitrary schemes, so they use a denylist instead: only the script-executing schemes javascript:, vbscript:, and data: are blocked.

Opting into HTML pass-through

# Allow raw HTML (use with trusted input only)
RedQuilt.render_html(user_markdown, allow_html: true)

# This passes HTML blocks and inline tags through unchanged

Development

Running tests

bundle exec rake spec

Runs 70+ CommonMark compatibility and feature tests.

Benchmark

ruby spec/bench_inline.rb
ruby spec/bench_block.rb

Profiles parse performance on various Markdown patterns.

Performance (v0.6.1, Ruby 4.0.5)

Comparison against kramdown on arm64-darwin (Apple Silicon), measured with spec/bench_vs_kramdown.rb (benchmark-ips):

Fixture	Size	RedQuilt (i/s)	kramdown (i/s)	RedQuilt vs kramdown
short_paragraph	49 B	26,531	5,416	4.90x faster
long_paragraph	1.4 KB	981	926	within error
nested_emphasis	1.4 KB	999	689	1.45x faster
many_links	2.0 KB	1,131	794	1.43x faster
mixed_markup	1.8 KB	1,028	729	1.41x faster
deep_nesting	800 B	827	349	2.37x faster
cmark_spec	205 KB	39.1	30.2	1.30x faster

Reproduce locally:

bundle exec ruby spec/bench_vs_kramdown.rb

License

MIT