Regular Expression's Jungle

Hello,
I want to add some regular expression; when i get some data in any other
format just convert it to my own. like if i have a date like 02/23/2008 then
convert it to 02-23-2008. i have surfed the web but cannot find any simple
example that can fit in. I found many complex examples all the way, which
just passed over me...lolz. Is there any simple example available or help
through which i can understand the basic working of regex, and can define my
own regular expressions.
Thanks
Regards,
Saqib Javed