I have heard of regular expressions and only seen use cases for a few things so I don't think of using them very often. In the past I have done a couple of things and it has taken me hours to do. Later I talk to someone and they say "here is how to do it using a regular expression".
So what are things for which you have used Regular Expressions? If I get more examples then maybe I can begin to know when to look for and use them.
The last thing i used a regular expression for was to validate text input to make sure the input string conformed to certain rules, for example the second character had to be a '-'.
I read your question and immediately thought of this.
EDIT: Forgot to mention this an XKCD comic. http://xkcd.com/208/
In ASP.NET if you use user controls or master pages, even if you name your controls uniquely, they get mangled by the framework. I wrote a little wrapper around the prototype $ function to allow me to get mangled controls in javascript despite the name mangling. It uses a regular expression to search the DOM for controls that end with the appropriate name.
I also use it heavily in client/server-side validation of inputs that need to match specific input patterns.