Convert GitHub-Flavored Markdown to CommonMark

A README with all three convertible constructs

A real README mixes a task list, struck text, and a bare support URL. One pass rewrites all three; the heading and code fence are untouched.

Input:
# Roadmap
- [x] Ship v1
- [ ] ~~Old plan~~ rewrite docs
Support: https://example.com/help

Output:
# Roadmap
- Ship v1
- <del>Old plan</del> rewrite docs
Support: <https://example.com/help>

Strikethrough survives as visible struck text

CommonMark has no native strikethrough, so the tool emits raw HTML. Any renderer that allows HTML (most do) shows the text struck through.

Input:
Price: ~~$99~~ $79 this week.

Output:
Price: <del>$99</del> $79 this week.

Bare URL gets an angle-bracket autolink

Strict CommonMark only autolinks URLs wrapped in <>. The tool adds them so the link stays clickable after conversion.

Input:
Docs live at https://docs.example.com and the API at https://api.example.com.

Output:
Docs live at <https://docs.example.com> and the API at <https://api.example.com>.

Existing markdown links are left alone

A URL already inside a [text](url) link is preceded by (, which the autolink regex excludes — so inline links are never double-wrapped.

Input:
Read the [guide](https://example.com/guide) first.

Output:
Read the [guide](https://example.com/guide) first.
(unchanged)

Tables pass through untouched

This tool does not convert pipe tables. They remain GFM pipe tables in the output; if your renderer needs a table extension, enable it there or keep the table as-is.

Input:
| Feature | Status |
|---------|--------|
| Tables  | kept   |

Output (identical):
| Feature | Status |
|---------|--------|
| Tables  | kept   |

Despite a common assumption, this tool does NOT convert pipe tables to <table> HTML. The engine has exactly three regex passes (task lists, strikethrough, autolinks) and tables are not one of them. Pipe tables pass through verbatim. Most CommonMark renderers accept pipe tables via a table extension, or render them as plain text; if you need real HTML tables, render the Markdown with the Markdown to HTML tool instead.

The task-list regex matches only [ ] and lowercase [x] (the character class is [ x]). An uppercase - [X] done is left exactly as written. GitHub itself accepts uppercase X, so source files sometimes contain it. Lowercase your [X] checkboxes before converting if you want them stripped.

The pattern only matches bullet markers (-, *, +). An ordered task item like 1. [ ] step keeps its [ ] because GFM task lists are defined on bullet lists only. If you have ordered checkboxes, convert the marker to a bullet first or remove the [ ] manually.

The strikethrough regex is ~~([^~]+)~~ — it matches only runs with no internal ~. So ~~strike with ~ tilde~~ is left unconverted because the inner ~ breaks the [^~]+ group. Remove or escape stray tildes inside struck spans if you need them converted.

The autolink pass does not skip inline code spans. A backtick span like ` curl https://api.example.com becomes curl <https://api.example.com> — the inserted > lands after the closing backtick and breaks the code span. Wrap example URLs in <>` yourself, or keep them in fenced blocks (the autolink regex still matches inside fences too, so prefer pre-wrapping).

The URL pattern is greedy up to whitespace or ), >, ]. A sentence-final URL like See https://example.com. becomes <https://example.com.> — the period is inside the brackets. Some renderers then 404 on the trailing dot. Put a space or wrap the URL manually if the trailing character matters.

Because the regex stops at ), a Wikipedia-style URL like https://en.wikipedia.org/wiki/Markdown_(disambiguation) is wrapped as <https://en.wikipedia.org/wiki/Markdown_(disambiguation>) — the trailing ) falls outside the brackets, breaking the link. Pre-wrap such URLs in <> before running the tool.

:smile: shortcodes and alert blockquotes (> [!NOTE], > [!WARNING]) are GFM-specific but are NOT part of this transform — they pass through unchanged. To remove emoji shortcodes use the Emoji Remover. Alerts will render as ordinary blockquotes in a strict renderer because the [!NOTE] marker is just text to it.

Free tier rejects files over 1 MB or 500,000 characters — and the character limit is checked separately from byte size, so a dense ASCII file can hit 500,000 chars well under 1 MB. Upgrade to Pro (10 MB / 5,000,000 chars) for large documentation sets, or split first with a splitter tool.

There is no reverse mode. To go from CommonMark back to GFM you would re-add ~~ around <del> spans, restore [ ] checkboxes, and remove autolink brackets by hand or with a find-replace tool. The converter is one-directional.