Regular expressions without a doubt are one of the best things you can learn to use properly. There are only two books you need to really unlock everything you need to know about them:

• Introducing Regular Expressions
• Mastering Regular Expressions

Get them both. Read them cover to cover (something I very rarely do with technical books!), and you will know kung fu.

Edit: also, FYI, regular expressions seem really complicated from the outside, but once you get to grips with them you realise they're not really that complicated at all - don't be put off :-)

I have used a desktop application : RegExpBuddy for years. That is where I test my expressions and learn to understand what happens when for example choose greedy or not. It gives the reg expression in actual code for many different languages like javascript, php, powershell etc . It has helped me immensely. Its like a pocket calculator for regular expressions. Can also recommend the Mastering Regular Expression book mention by Rowley . I once took a pluralsight course that took the subject seriously; not just quick fix recipes.

Code School's Regex Course