Strikethrough by double tildes

[blackst0ne]

We want to migrate from redcarpet to github/cmark.
Unfortunately, we have two major blockers. One of them is strikethrough.

In redcarpet double tildes starts strikethrough while cmark does that with just one tilde.

From redcarpet's readme:

:strikethrough: parse strikethrough, PHP-Markdown style. Two ~ characters mark the start of a strikethrough, e.g. this is ~~good~~ bad.

Is there any chance to update cmark somehow to let it use ~~ instead of ~ (maybe as an option)?

Thank you! 🙂

/cc @kivikakk

[kivikakk]

Hey! 👋 cmark does it with any number of (matched) tildes, e.g.:

~hi~ ~~hello~~ ~~~hey~~~

Produces:

hi hello ~~~hey~~~

Does this help you?

[blackst0ne]

@kivikakk

Sorry if I wasn't clear. 🙂
I meant to parse only double ~

Example:

one~two and three~four

should not produce any strikethrough, but

one~~two and three~~four

should act like it does now:

onetwo and threefour

I understand that you might not be very interested in this feature but it would really help people to migrate from redcarpet to cmark which is wa-a-ay better. 🙂

[kivikakk]

Okay, you're totally clear now 😅 I'm on holiday now for the next week, but when I'm back, I'll investigate if this would be a breaking change (how many people currently use ~…~ across GitHub? what percentage of strikethrough uses look like this?). If not, I'd be happy to make the change. ❤️

[kivikakk]

Unfortunately, it looks like about fully 50% of strikethrough uses on GitHub.com are with a single tilde on either side 😞 I'm sorry, but we'll be unable to make this change.

[foonathan]

The OPs issue seems to be similar to using underscores for italic text_not italic_text. Maybe adding the same rules for ~ would help?

[blackst0ne]

@kivikakk is there any chance to have this proposal as an option?

So on github.com by default it'd be as it is now, but if one wants to use cmark-gfm as a gem, their could use an option to use double tildes. 🙂

[kivikakk]

Ah, of course! There totally is. Would you like to submit a PR? I'd be happy to accept it. Once done we'll need to also get the plumbing for passing the option through in commonmarker, which I'd also be happy to help with.

[blackst0ne]

@kivikakk I'm not familiar with the codebase. Could you give some tips, please? 🙂

[kivikakk]

Unfortunately I'm short on time, so I can't give too much assistance (otherwise I'd do it myself); start by looking over /extensions/strikethrough.c. We use cmark_inline_parser_scan_delimiters to count the number of delimeters when matching an opening strikethrough block; try returning NULL after there if delims < 2. After that, it's just a matter of making it into an option (see /src/cmark.h for existing CMARK_OPT_* defines).

[blackst0ne]

@kivikakk thank you!

Can we keep this issue open then? 🙂

[kivikakk]

Sure thing!

[kivikakk]

I've added CMARK_OPT_STRIKETHROUGH_DOUBLE_TILDE. I'll get this into the Ruby gem shortly. 👍