LOOPNE subtracts 1 from CX, leaving the flags alone, and jumps if CX ≠ 0 and ZF = 0. With a CMP just before it, the loop runs while the items do not match and stops on the first match or when the count is used up: a search. LOOPNZ is the same instruction (opcode E0h).

Prints 4. Search for "X" and it prints "-".
CLD
MOV     SI, OFFSET LETTERS1
MOV     CX, 5
SCAN1:
LODSB            ; AL = next letter (flags untouched)
CMP     AL, "D"
LOOPNE  SCAN1    ; on while not found and CX <> 0
JNE     NOTF1    ; ZF = 0: ran out without a match
MOV     AL, 5
SUB     AL, CL   ; position of the D: 4
ADD     AL, "0"  ; digit -> character
MOV     DL, AL
MOV     AH, 02h  ; DOS: print the character in DL
INT     21h
JMP     DONE1
NOTF1:
MOV     DL, "-"
MOV     AH, 02h  ; DOS: print the character in DL
INT     21h
DONE1:
 
LETTERS1 DB     "ABCDE"

Pitfalls

Decide found or not found with ZF (JE / JNE right after the loop). CX is 0 both when nothing matched and when the match was the last item. Anything between the CMP and the LOOPNE that changes ZF, such as INC SI, breaks the loop; that is why LODSB is convenient. Guard a count that may be 0 with JCXZ, and keep the body within the short range. For bytes in memory, REPNE SCASB performs the same search in one instruction.