Comment by comex

13 hours ago

If you already know where the start of the opening tag is, then I think a regex is capable of finding the end of that same opening tag, even in cases like yours. In that sense, it’s possible to use a regex to parse a single tag. What’s not possible is finding opening tags within a larger fragment of HTML.

4 comments

comex

kstrauser 12 hours ago

For any given regex, an opponent can craft a string which is valid HTML but that the regex cannot parse. There are a million edge cases like:

  <!—- Don't count <hr> this! -—> but do count <hr> this -->

and

  <!-- <!-- Ignore <ht> this --> but do count <hr> this —->

Now your regex has to include balanced comment markers. Solve that

You need a context-free grammar to correctly parse HTML with its quoting rules, and escaping, and embedded scripts and CDATA, etc. etc. etc. I don't think any common regex libraries are as powerful as CFGs.

Basically, you can get pretty far with regexes, but it's provably (like in a rigorous compsci kinda way) impossible to correctly parse all valid HTML with only regular expressions.

marcosdumay 10 hours ago
HTML comments do not nest. The obvious tokenizer you can create with regular expressions is the correct one.
- kstrauser 10 hours ago
  
  If you're talking about tokenizers, then you're no longer parsing HTML with a regex. You're tokenizing it with a regex and processing it with an actual parser.
  
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