Perl Code Question

[Ed Eddington]

This is bugging me. I know that this works, but don't know why. Below is an 
example search/replace that swaps the first 2 terms separated by a space. 
The '^' here is performing some kind of "not" or "anything but" a space in 
both of the [] terms. I have used this in code, but have never found this 
usage of ^ in the Perl docs. Can someone explain what this '^' is doing?

s/([^ ]*) *([^ ]*)/$2 $1/;            # reverse 1st two fields




Thanks!
Ed