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Asked: 2026-05-11T15:46:17+00:00

One mistake I see people making over and over again is trying to parse

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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:

<tag attr='5' />

People want to treat < or <tag as the start of a tag, but stuff like this exists in the wild:

<img src='imgtag.gif' alt='<img>' />

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):

<span id='outer'><span id='inner'>foo</span></span>

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):

<span class='phonenum'>(<span class='area code'>703</span>) <span class='prefix'>348</span>-<span class='linenum'>3020</span></span>

Comments may contain poorly formatted or incomplete tags:

<a href='foo'>foo</a> <!-- FIXME:     <a href=' --> <a href='bar'>bar</a>

What other gotchas are you aware of?

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1 Answer

  1. Here’s some fun valid XML for you:

    <!DOCTYPE x [ <!ENTITY y 'a]>b'> ]> <x>     <a b='&y;>' />     <![CDATA[[a>b <a>b <a]]>     <?x <a> <!-- <b> ?> c --> d </x>

    And this little bundle of joy is valid HTML:

    <!DOCTYPE html PUBLIC '-//W3C//DTD HTML 4.01 Transitional//EN' 'http://www.w3.org/TR/html4/loose.dtd' [     <!ENTITY % e 'href='hello''>     <!ENTITY e '<a %e;>'> ]>     <title>x</TITLE> </head>     <p id  =  a:b center>     <span / hello </span>     &amp<br left>     <!---- >t<!---> < -->     &e link </a> </body>

    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:

    <!DOCTYPE HTML PUBLIC '-//W3C//DTD HTML 4.01//EN'   'http://www.w3.org/TR/html4/strict.dtd'>  <HTML/   <HEAD/     <TITLE/>/     <P/>
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