You can absolutely parse HTML with regex, so long as the document is finite in length. Every finite language is regular, hence can be parsed with regex's.
The more fundamental question for me is why the author didn't decide to make make code blocks non-breaking by default, or just add the class annotations when he writes the HTML?
You can't parse [X]HTML with regex. Because HTML can't be parsed by regex. Regex is not a tool that can be used to correctly parse HTML.
Some commenters are missing that this is a reference to https://stackoverflow.com/a/1732454.
Good ol' appeal to emotion devoid of technical argument.
You can absolutely parse HTML with regex, so long as the document is finite in length. Every finite language is regular, hence can be parsed with regex's.
Do a web search for the parent of your comment, read the Stackoverflow answer. It's a classic. Learn about Zalgo and Tony the pony, he comes.
I do not count substitution as "parsing"
You can parse a subset of it though, like if you're in control of the html yourself and avoid certain structures
Parsing HTML with a regex is never a good option, but it's sometimes the only option.
In the example from the article it certainly is an option. In Python you could either use a "soup" library or you could play around with a tool like https://www.w3.org/Tools/HTML-XML-utils/man1/hxpipe.html.
The more fundamental question for me is why the author didn't decide to make make code blocks non-breaking by default, or just add the class annotations when he writes the HTML?
What do you mean you can't. I do it all the time
You do some parts all the time.
Tony the pony, he comes.
Why even bother with the CSS class? Just apply text-wrap: nowrap; to all <code> elements and dispense with the fragile regex parsing.