如何检查用户输入的结尾是否以“？”结尾”。“”！“

Question

从用户处获得新消息。验证消息以确保它以大写字母开头，并以句点（'。'），问号（'？'）或感叹号（'！'）结尾。如果输入无效，请向用户显示错误消息以拒绝输入（请改用先前的消息）。否则，如果有效，请替换原始消息。

我检查了第一个有效的字符，现在要检查字符串的末尾，我将输入字符的长度加到输入的负-1处，但这会改变用户输入的第一个字符的值。

section .data
        text1 db "ENTER TEXT: "
        len equ $ - text1
        text2 db "THIS IS WHAT YOU ENTERED: "
        text3 db "invalid message, keeping curent "


section .bss
        character resb 255

section .text
        global main

main:
        mov r11, text2

        mov rax, 1
        mov rdi, 1
        mov rsi, text1
        mov rdx, 12
        syscall
        call validate


validate:

         mov rax, 0
        mov rdi, 0
        mov rsi, character
        mov rdx, 10
        syscall                                                                                                                                                                
        mov r8, rax

        mov rdi, character
        mov al, [character]
        mov ah, 'A'
        cmp al, ah
        jl invalid

        cmp al, 90
        jg invalid

        add rax, r8             ;                                                                                                                                                                           
        sub rax, 1

        cmp rax, '!'
        je valid
        cmp rax, '.'
        je valid
        cmp rax, '?'
        je valid
valid:

        mov r11, character

        mov rax, 1
        mov rdi, 1
        mov rsi, r11
        mov rdx, r8
        syscall
        jmp exit

invalid:

        mov rax, 1
        mov rdi, 1
        mov rsi, text3
        mov rdx, 12
        syscall
        jmp exit

exit:

        mov rax, 60
        xor rdi, rdi
        syscall

Answer 1

mov  cl, [rsi]      ; RSI already points to first character
cmp  cl, 'A'
jb  invalid
cmp  cl, 'Z'
ja  invalid

mov  al, [rsi+rax-2]    ; RAX = # of characters entered

cmp  al, '.'
jz  valid
cmp  al, '?'
jz  valid
cmp  al, '!'
jnz Error

现在，尚未更改前导字符，并且通过不必要地设置其他指针来节省一些字节。