I'm trying to fix my site to meet WCAG 2.0. This means that all links in my site must have a title. To do it right and not miss any
Generally, it is not a good idea to use regular expressions on a complex system like a DOM. However, in your (simple) example, you might get along with:
This ignores links with a title attribute, regardless where they are. See a demo on regex101.com.
<a href="http://python.org" data="Here comes a title">This one fails</a>not matching it in the way you intented.