bug with RE

[Тучков Михаил <mtuchkov(at)ntrlab(dot)ru>]

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.