Hello Gowthamraj
Looking at the various regular expressions can become confusing if you don’t actually use them. The ideal way to learn is to practice, and see how they behave as you apply them. The following lesson uses an example of a regular expression to apply a filter list to prevent a BGP transit AS situation:
You can also take a look at actual examples at the following Cisco documentation:
Finally, you can use the looking glass servers that Rene suggested in the lesson as well. He further explains how to do this in some of the posts above, including this one:
I hope this has been helpful!
Laz