程序代码：

DEFINE CLASS iptextbox as TextBox
    inputmask="###.###.###.###"
    
    PROCEDURE init
        this.SelStart=0
        this.SelLength=3
    ENDPROC 
    
    PROCEDURE keypress
    LPARAMETERS nKeyCode, nShiftAltCtrl

        IF nkeycode=32 OR nkeycode=9 OR nkeycode=46 OR nkeycode=13
            IF this.selstart>AT(".",this.value,3)
                IF nkeycode=32 OR nkeycode=46 OR nkeycode=13
                    NODEFAULT 
                    KEYBOARD '{TAB}'
                ENDIF     
            ELSE 
                NODEFAULT 
                ctext=SUBSTR(this.Value,1,this.selstart)
                ndot=OCCURS(".",ctext) &&有几个“.”
                ndot=IIF(ndot=3,3,ndot+1)
                ndotpos=AT(".",this.value,ndot)
                this.SelStart=ndotpos
                this.SelLength=3
            ENDIF
        ENDIF
    ENDPROC 
    
    PROCEDURE valid
        n=ALINES(atemp,this.Value,5,".")
        IF n#4
            SET MESSAGE TO "每个地址在0~255之间，不能为空"
            RELEASE atemp
            RETURN .f.
        ENDIF
    ENDPROC 
             
ENDDEFINE