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POThe fundamental reason why regex and HTML don't mix? The theory behind it?
 

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  1. POThe fundamental reason why regex and HTML don't mix? The theory behind it?
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    <p>To start with, I cannot do anything but refer to what I believe is the most famous SO post ever:</p> <p><a href="https://stackoverflow.com/questions/1732348/regex-match-open-tags-except-xhtml-self-contained-tags">RegEx match open tags except XHTML self-contained tags</a></p> <p>Now, is it even a question for StackOverflow? I don't know, but I'll try...</p> <p>I'll speak from a personal point of view. While I've never had to do that, I know that the day I have to parse HTML, I will certainly not go with regexes; I'll try and find an HTML parsing library. Fine.</p> <p>But I don't know why.</p> <p>At one point, I decided to do CSS validation in Java. I knew "by the guts" that regexes wouldn't cut it, so I used <a href="https://github.com/sirthias/parboiled/wiki" rel="nofollow noreferrer">Parboiled</a>.</p> <p>And I don't know why.</p> <p>The "why" troubles me. I am no newbie with regexes at all. I just can't put a clear line between what regex engines can, and cannot do.</p> <p>My question is the following: what is this clear line? What fundamental characteristic of an input must exist so that it is mathematically demonstrated that any regex engine cannot reliably determine success and failure?</p> <p>Can you give a simple, theoretical input which would spell failure as to a regex engine's ability to give a reliable "match/no match" answer? If yes, what is the defining characteristic of such an input?</p> <p><strong>EDIT</strong> For the sake of this discussion, I'll add a task suggested by a post on SO (which I can't find the link to at the moment, sorry) which is simpler than HTML, but for which I won't use regexes: shell command line parsing.</p> <p>As far as the shell is concerned, those are equivalent:</p> <pre><code>alias ll="ls -l" alias ll=ls\ -l alias l"l"=ls' -'l "alia"s l"l= "ls\ -l </code></pre> <p>Shell quoting mechanisms are so numerous that I'll just create a Parboiled grammar in this case... But this is "out of my guts". Because I find it easier probably... But that doesn't prove that this is not feasible with regexes.</p>
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    1. COThe simple answer is: regular expressions, in the formal sense, are not capable of parsing texts that may contain arbitrary levels of nesting. In HTML/XML, you can have arbitrary nesting of tags. So if you are parsing a page whose template never ever changes, you can use regex to extract a fairly static portion of it, but to account for different levels of nesting, you have to use another solution.
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    2. CO@Anorov see my comment on the given answer. With PCRE and .NET regular expressions you *can* parse nested structures.
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      1. POThe fundamental reason why regex and HTML don't mix? The theory behind it?
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    3. CO@m.buettner Note I said "regular expressions, *in the formal sense*".
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