Comment by tiagod

13 hours ago

I think even for single opening tags like asked there are impossible edge cases.

For example, this is perfectly valid XHTML:

    <a href="/" title="<a /> />"></a>

6 comments

tiagod

Reply
chungy  11 hours ago

No, that is not valid. The "<" and ">" characters in string values must always be escaped with &lt; and &gt;. The correct form would be:

    <a href="/" title="&lt;a /&gt; /&gt;"></a>

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.

  • kstrauser  11 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.