Recently I was asked where a novice would start learning how to read and write regular expressions. Below are two sites that I think would be useful.

Reading regular expressions

I think RegExr is a good site for explaining regular expressions.

Writing regular expressions

Learning to write regular expressions is quite hard. RegexOne provides a very good beginners tutorial to help people learn how to write regular expressions.

What resources have helped you to learn how to read/write regular expressions?