BUG #3487: regular exp

The following bug has been logged online:

Bug reference: 3487
Logged by: Mikhail
Email address: Tuchkov(dot)Michail(at)gmail(dot)com
PostgreSQL version: 8.1
Operating system: windows 2003 SP2
Description: regular exp
Details:

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.