iterate chars in a string

< ^ > P N |<>|^_>< --- | ~ ...Help

Hi there!
I'm a ruby newbie, and I'm searching for a way to iterate every char in 
a string, but I cannot find any easy way. My problem is to look at every 
char in a string and match it with some known letter.
I use the String#each_byte iterator for now, but it still be a poor 
solution :/
Thanks,

shinya.