preg_replace - understanding

I'm trying to understand how the code works within the preg_replace function
but the manual is just obscure as the examples are. Anyway, I am looking to
use it to replace "\\" in a string with "/" and I can not figure how how.
At first, I thought I could just do:

$filevalue = preg_replace("'\\', /", $filevalue);

but it doesn't do anything. If someone could relate what it is and what the
parts mean between the (), I would appreciate it.

thanks