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="https://stackoverflow.com/questions/701166/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="https://stackoverflow.com/questions/701166/foo">foo</a>
<!-- FIXME:
<a href="
-->
<a href="bar">bar</a>
What other gotchas are you aware of?