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POCan you provide some examples of why it is hard to parse XML and HTML with a regex?
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  1. POCan you provide some examples of why it is hard to parse XML and HTML with a regex?
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    copied!<p>One mistake I see people making <a href="https://stackoverflow.com/questions/699708/variable-order-regex-syntax">over</a> and <a href="https://stackoverflow.com/questions/457015/regular-expression-help">over again</a> is trying to parse XML or HTML with a regex. Here are a few of the reasons parsing XML and HTML is hard:</p> <p>People want to treat a file as a sequence of lines, but this is valid:</p> <pre><code>&lt;tag attr="5" /&gt; </code></pre> <p>People want to treat &lt; or &lt;tag as the start of a tag, but stuff like this exists in the wild:</p> <pre><code>&lt;img src="imgtag.gif" alt="&lt;img&gt;" /&gt; </code></pre> <p>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):</p> <pre><code>&lt;span id="outer"&gt;&lt;span id="inner"&gt;foo&lt;/span&gt;&lt;/span&gt; </code></pre> <p>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):</p> <pre><code>&lt;span class="phonenum"&gt;(&lt;span class="area code"&gt;703&lt;/span&gt;) &lt;span class="prefix"&gt;348&lt;/span&gt;-&lt;span class="linenum"&gt;3020&lt;/span&gt;&lt;/span&gt; </code></pre> <p>Comments may contain poorly formatted or incomplete tags:</p> <pre><code>&lt;a href="foo"&gt;foo&lt;/a&gt; &lt;!-- FIXME: &lt;a href=" --&gt; &lt;a href="bar"&gt;bar&lt;/a&gt; </code></pre> <p>What other gotchas are you aware of?</p>
 

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