I have this expression in a code snippet i borrowed offline. It forces the new users to have a password that not only requires upper+lower+numbers but they must be in that order! If i enter lower+upper+numbers, it fails!

if (preg_match("/^.*(?=.{4,})(?=.*[0-9])(?=.*[a-z])(?=.*[A-Z]).*$/", $pw_clean, $matches)) {

Ive searched online but can't find a resource that tells me what some characters mean. I can see that the pattern is preg_match("/some expression/",yourstring,your match).

What do these mean?

1.  ^          -  ???
2.  .*         -  ???
3.  (?=.{4,})  -  requires 4 characters minimum
4.  (?.*[0-9]) -  requires it to have numbers
5.  (?=.*[a-z])-  requires it to have lowercase
6.  (?=.*[A-Z])-  requires it to have uppercase
7.  .*$        -  ???