hello is there a cleaner/fastest way to update PAI header user part other than using subst

i come up with 

subst('/^P-Asserted-Identity:(.*)(sip|sips|tel|tels):([^@]*)@([a-zA-Z0-9.]+)(.*)$/P-Asserted-Identity:\1\2:$var(newPAIUser)@\4\5/ig');

Thanks.