I have worked for 5 years mainly in java desktop applications accessing Oracle databases and I have never used regular expressions. Now I enter Stack Overflow and I see a lot of
If you want to learn about regular expressions, I recommend Mastering Regular Expressions. It goes all the way from the very basic concepts, all the way up to talking about how different engines work underneath. The last 4 chapters also gives a dedicated chapter to each of PHP, .Net, Perl, and Java. I learned a lot from it, and still use it as a reference.