Can you provide some examples of why it is hard to parse XML and HTML with a regex?

One mistake I see people making over and over again is trying to parse XML or HTML with a regex. Here are a few of the reasons parsing XML and HTML is hard:

People want to treat a file as a sequence of lines, but this is valid:

attr="5"
/>

People want to treat < or

People often want to match starting tags to ending tags, but XML and HTML allow tags to contain themselves (which traditional regexes cannot handle at all):

foo

People often want to match against the content of a document (such as the famous "find all phone numbers on a given page" problem), but the data may be marked up (even if it appears to be normal when viewed):

(703)
348-3020

Comments may contain poorly formatted or incomplete tags:

foo

bar

What other gotchas are you aware of?

Answer

Here's some fun valid XML for you:

b"> ]>

    
    b b 
      d

And this little bundle of joy is valid HTML:

    
    ">
]>
    x

    
    
    &

     < -->
    &e link

Not to mention all the browser-specific parsing for invalid constructs.

Good luck pitting regex against that!

EDIT (Jörg W Mittag): Here is another nice piece of well-formed, valid HTML 4.01:

  "http://www.w3.org/TR/html4/strict.dtd"> 
      /<br/>    <p/><br/></code></pre><br/>    </div>
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Thursday, 29 September 2016

Can you provide some examples of why it is hard to parse XML and HTML with a regex?