substitute

(substitute new old sequence &key test key)

Returns a new sequence in which every element matching old is replaced by new; all other elements are kept unchanged. The comparison is eql by default; the optional :test keyword takes a function designator to use a different comparison, and the optional :key keyword takes a selector function applied to each element before the comparison (the replacement value is new itself, unkeyed). The sequence may be a list or a string; a string yields a new string (with new a character). The original sequence is not modified; use nsubstitute for the destructive version (lists only).