org 100h

Start:

Greeting:
        mov     ah, $09
        mov     dx, greeting
        int 21h

Input:
        mov     ah, $0A
        mov     dx, inputString
        int 21h

        ;go next line
        mov     ah, $09
        mov     dx, nextLine
        int 21h

CheckLength:
        cmp     [inputString + 1], 4
        jle     PrintNo

CheckEquality:
        mov     dh, [inputString + 3]   ;dh!
        cmp     dh, [inputString + 5]   ;dh!
        jne     PrintNo

CheckLetter:
        mov     ch, [inputString + 3]
        cmp     ch, 'A'
        jb      PrintNo
        cmp     ch, 'z'
        jg      PrintNo
        cmp     ch, 'Z'
        jb      CheckDigit
        cmp     ch, 'a'
        jg      CheckDigit

CheckDigit:
        movsx   si, [inputString + 1]   ;store length here
        mov     ch, [inputString + si]
        cmp     ch, '0'
        jb      PrintNo
        cmp     ch, '9'
        jg      PrintNo

        ;if everything was right
        jmp     PrintYes

PrintNo:
        mov     ah, $09
        mov     dx, ansNo
        int 21h
        jmp Exit

PrintYes:
        mov     ah, $09
        mov     dx, ansYes
        int 21h
        jmp Exit

Exit:
        mov     ah, $09
        mov     dx, pressAnyKey
        int 21h
        mov     ah, $08
        int 21h
        ret

greeting        db "Enter your string:", 13, 10, "$"
inputString     db 11, 0, 10 dup (?)
nextLine        db 13, 10, "$"
ansYes          db "Yes!", 13, 10, "$"
ansNo           db "No!", 13, 10, "$"
pressAnyKey     db 13, 10, "Press any key to exit...$"