MySQL insert() and instr() equiv

When using MySQL I have one select that uses MySQL functions so I am
wondering about the best way to go to end up with the same result in
PostgreSQL.

uid is an email address stored in the passwd table as user@domain.com
and this construct allows an incoming username such as "user.domain.com"
to be compared to the stored "user@domain.com".

SELECT wpath FROM passwd WHERE uid="\L" OR insert(uid,instr(uid,'@'),1,'.')="\L"

--markc