Look 0 of my work involves HTML, well maybe 1-2 percent does; however, about 60% of my work involves regular expressions, grammar, lexical scanning and syntactic parsing, so it still irks me, and will irk me beyond my grave, when people say shit like ‘Don’t parse HTML/Markdown/etc with regex! Use a parser generator!’

So this is stupid, because most people know that HTML and Markdown are not the type of languages that require a push-down parser, or even a simple LL(1) recursive-descent parser! Unless by ‘parser generator’ they mean ‘lexer generator’ or ‘PEG generator’, they are wrong, or at least, partly incorrect.

Like my diabetes, they are not grammatically Type 2 (Chomsky-wise, Context-Free); rather, they are Type 3 (Chomsky-wise, Regular).

It’s preferred if you don’t do a syntax-directed lexical translation of Markdown or HTML, and it’s best if you build a tree. I learned that making Mukette and I am currently using my implementation of ASDL to build a tree. But truth is, unlike Context-Free languages, like any non-markup language, it is ENTIRELY possible to do a syntax-directed translation of HTML and Markdown, using pre-compiled, or runtime-compiled regex.

You will have to introduce states to make it a proper Automata, but even that is not required. I once did a syntax-directed translation of Markdown to HTML in AWK! With just one extra state.

I don’t remember the copypasta that was talk of the town 10 years ago, I was a kid back then (17) and I could not dig it up. But it’s a troll that has stuck with me ever since.

Maybe, just maybe, a PEG paser generator could have been what they meant. But even then, PEG generators generate a recursive-descent parser most of the times.

In fact, I dare you to use Byacc, Btacc, Bison, Racc, PYLR, ANTLR, peg(1), leg(1), PackCC or any of these LALR or LL parser generators to parse a markup language. You’ll have a very bad time, it is not impossible, it’s just an overkill.

TL;DR: Most markup languages, like HTML or Markdown, are best lexed, not parsed! Even if you wish to make a tree out of it. But for syntax-directed translations, REs would do.

Thanks.

PS: If you translate a markup language into a tree, you can translate that tree into other markup languages. That’s what Pandoc does. Pandoc is hands-down the best piece of tool I have laid my hands on.

  • TehPers@beehaw.orgEnglish
    2·
    7 months ago

    This advice mostly applies to people who are less experienced and less familiar with just how complex HTML can be. As for other languages - if you’re doing regex on markdown, you’ll probably be fine (but you should verify if you’re writing something for the general case that must not fail). But in HTML’s case:

    • You have nested languages (CSS and JS)
    • You have tag-specific rules (img and link end in />, but div must end in a separate closing tag)
    • Browsers use error correction to try to make sense of invalid HTML, like inserting missing tags. Many websites rely on this behavior.

    If you’re trying to use Regex to parse a specific website’s HTML, you’ll be able to get what you want eventually, but as a general HTML parser, there will always be some website that breaks your assumptions.