Informatica does not have this documented but you could use SUBSTR function to go from Right to Left as well. Just use a negative start.
IIF(SUBSTR(FILENM,-4)='.PDF', 'PORTABLE')
of what I learned today
IIF(SUBSTR(FILENM,-4)='.PDF', 'PORTABLE')
REG_EXTRACT('Fiction__JK_ROWLING__HARRY_POTTER__MAX07','(\w+)(__\w+__)(\w+)(__.*)',3)
HARRY_POTTER