reMaybeSpecial and custom syntax

[GarrettAlbright]

I'm upgrading the Markdown parsers on a client's site using CommonMark parsers on both the server and client side. The parser on the server side (in PHP) is the "official" one but we're doing previews on the client side and there were some inconsistencies with the previous scripts.

In the site's markup, there's some "custom" syntax elements which I'm trying to replicate under the CommonMark parsers/renderers as custom syntax/nodes (the previous scripts just did post-processing on the HTML after converting the Markdown). I've got this working on the PHP side, but I'm stuck on the JS side because the custom syntax uses the : character (specifically :::) to denote the start and end of a block similar to backtick fences, but there's an optimization which is stopping the parsing of custom blocks if they don't have a character in the reMaybeSpecial pattern:

https://github.com/commonmark/commonmark.js/blob/8c698a295f7cea58597769ccfaab2219add45e44/dist/commonmark.js#L8683

https://github.com/commonmark/commonmark.js/blob/8c698a295f7cea58597769ccfaab2219add45e44/dist/commonmark.js#L9438-L9444

If I use a debugger and set a breakpoint before line 9438 and then change reMaybeSpecial to include the : character, the rest of my code works more or less as expected…

Is there a way to work around this? I can't find a way to alter reMaybeSpecial from external code. It would be great if it were alterable somehow. (I'd try patching it myself, but I don't understand this newfangled JavaScript where require() is being used all over the place.)

[jgm]

Since the API allows customization, we should probably allow this to be customized too (e.g. by having a list of special characters).