POSIX stands for Portable Operating System Interface (yeah, the X sort of confuses me too), and it is a specific group of tools all designed to operate a computer in a standard way. I would send you to a source where you can get the whole standard, but its copyrighted by the IEEE. So you'll have to settle for a similar, but free set of documents
Almost every operating system has a POSIX compliant toolset built in - a few don't, but even on most of those, it can be installed.
This is A VERY GOOD IDEA. They put a lot of thought into these tools, and they work really well for doing any number of tasks.
Method #1: Cygwin
The most notable place that the posix standard is missing is
Windows. The POSIX standard for Windows is implemented by Cygwin (install at your own risk).
Method #2: Linux
If you've got a PC, another way to be able to play with POSIX stuff is to get a version of Linux (a free operating system available for PCs which implements the POSIX standard). If you want to try this, go download Mandrake Linux. Its the easiest version of Linux to use. BE VERY CAREFUL Improper installation may make you lose everything on your computer.
Well...in general, I'd say the best was is to go get OSX. Its already POSIX compliant. Other than that, you can install Mandrake Linux or many other forms of Linux on it as well...but its not as well supported as it is for the PC.
One of the keys to many things that you can do with strings (files, letters, etc.) involves the use of regular expressions.
This guy can tell you about it a lot better than I can.
I use regular expressions to sort my mail into categories.
Go here to test your regular expressions.
Do you use the tilde shortcut like you should? Learn how.