Simplified Common Lisp reference
position
Symbol class: Sequences (Lists, Strings) and Arrays
Syntax:
Symbol type: function
positionitemsequencetest(keyword)from-end(keyword)start(keyword)end(keyword)key(keyword) => index or NIL
Argument description:
item an item to be found
sequence a sequence to be searched
test function key and item comparison
from-end direction of search, default is NIL - forward
start starting position for search, default is 0
end final position for search, default is NIL - end of sequence
key function for extracting value before test

POSITION function searches for an element (item) satisfying the test. Return value is index of such item or NIL if item is not found. Index is relative to start of the sequence regardless of arguments. See also POSITION-IF, FIND, FIND-IF and MEMBER. 

(position #\s "Some sequence") => 5
(position #\s "Some sequence" :key #'char-downcase) => 0
(position #\s "Some sequence" :key #'char-downcase :start 1) => 5
(position #\x "Some sequence") => NIL
(position '(1 2) #(9 3 (1 2) 6 7 8)) => NIL
(position '(1 2) #(9 3 (1 2) 6 7 8) :test #'equal) => 2
(position 1 #(0 1 0 0 0 1 0) :from-end t) => 5
Function indexFull documentation for position (HyperSpec)