Read How to parse HTML...∞ and Bring Me Your Regexs! I Will Create HTML To Break Them!∞.
Here is a short list of HTML parsers for popular languages. XML parsers seem to be easy enough to find.

• El-Kabong HTML∞ (C)
• HTML Tidy∞ (C)
• HTML::Parser∞ (Perl)
• HTMLParser∞ (Python)
• GNU Wget∞ wget/src/html-parse.c (C)
• libxml2∞ (C)
• Libwww - the W3C Protocol Library∞ (C)
• PHP: DOM Functions∞
• PHP: Tidy Functions∞
• PHP: SimpleXML∞

For UNIX command line tools (awk, sed, etc), consider converting HTML to XHTML using Tidy∞ and then to PYX using XMLStarlet∞.

Ideally, you'll want to use the features of your language or application software to do this. Here are some examples:
Perl:
PHP:
sed:
vi:
mod_rewrite:
grep:
If you cannot use such a technique because your application (e.g. a text editor) does not allow that level of programmability, you may be able to get by with an expression such as: Note however that this may be much slower than the equivalent negated expression.

This is another of those situations where regular expressions alone are not enough. The best way is to match the line against multiple patterns:
Perl:
PHP:
sed:
grep:
Again, if you cannot use such a technique, try