position-if

(position-if predicate sequence &key key start end from-end)

Returns the 0-based index of the first element of sequence that satisfies predicate, or nil if none does. The sequence may be a list or a string (whose elements are characters). It returns the integer position rather than the element itself (compare find-if). :key selects what the predicate sees, :start/:end bound the scanned region, and with :from-end true the last satisfying element's index is returned.