bug with RE

Please, help me!
My code:
"IF char_length(substring(str from '^[a-zA-Z]*://'))=0 THEN  str:= 'http://'||str;
END IF;"

If str= http://www.msn.com then nothing happen
(all is OK), but if str= just www.msn.com then nothing happen too!

Why?

Then I experimen with that:
I change '=' to '<>'results:
str=http://www.msn.com -> http://http://www.msn.com
str=www.msn.com -> http://www.msn.com

Thank you.