Grammarly Markdown

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The idea of writing a context-free grammar (or CFG) forMarkdown keeps coming up. The assumption is that once it’s formalizedwith a grammar in BNF or EBNF, it serves as a formal specificationof the syntax, and it also becomes a source of parsers, possibly usingparser generators.

That idea doesn’t get far once you really take a look at the Markdownsyntax, which is not like the syntax of a programming language at all.The Markdown syntax is inherently ambiguous. (Yes, the syntax of C++ isambiguous too, but the points of ambiguity are known andlimited.)

To illustrate, consider the case of emphasis. *a* is em and **a** isstrong. If we wrote a (very simplified) CFG in an EBNF-like format forMarkdown text runs (i.e. contents of Markdown paragraphs), it would looklike this:

The book was published in multiple languages including English, consists of 240 pages and is available in Hardcover format. Duke-elder ophthalmology book pdf free download. The first edition of the novel was published in August 1st 2013, and was written by Kirby Larson. The main characters of this historical, historical fiction story are,.

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This already has ambiguities in that **a** can be interpreted by thisgrammar in two different ways: Just a strong run, or an em run insideanother em run.

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But it gets worse. The grammar we saw above would reject the input *a,which in Markdown should be interpreted as normal text. Same case fora*, **a and a**. If we wanted to handle these too, we need theseexpansions as well:

And with that addition, the amount of ambiguity shoots up significantlyand unmanageably.

If we take a closer look at why this happens, we can see that when a *token is encountered, the grammar cannot decide whether it should bepart of an em-qualifier or normal text (or sometimes, astrong-qualifier). That decision can be made only after scanning therest of the input till we find a matching closing *. If there is nomatching closing *, we will know that only if we scan until the end ofthe text-run’s string (which would be the end of the paragraph). So, forthe parser to know which rule to choose at a point, it might have tolook ahead an arbitrary number of tokens.

If Markdown was a format in which *a without a closing * is a syntaxerror, this wouldn’t be a problem to the parser - in that case, a single* can always be interpreted as part of an em-qualifier. But that’s nothow Markdown is, and if we change that, it wouldn’t be a very usefulwriting format.

So, that’s the reason why we won’t see a practical unambiguous CFG forMarkdown. The best we can do is just “.*” .

I’ve seen some suggestions that the indenting-based syntax for someconstructs in Markdown is what makes it hard to write a CFG for it. Idon’t think that’s the case because indentation can be detected attokenization stage and the grammar can refer them as INDENT and DEINDENTtokens. This, per my understanding, is how Python CFGs handleindentation.

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