var test = "LeftMessage is null and SentEmail is null and MetWith is null and SlsmanDist like 'KEE - kevin tang";

    alert(test.replace(/and SlsmanDist/g, function() {
            return "or SlsmanDist";
        }));

------解决方案--------------------
JScript code
var str="LeftMessage is null and SentEmail is null and MetWith is null and SlsmanDist like 'KEE - kevin tang%";
alert(str.replace(/and\s+(SlsmanDist)/gi,"Or $1"))

------解决方案--------------------
JScript code
var str = "LeftMessage is null and SentEmail is null and MetWith is null and  SlsmanDist";
alert(str.replace(/\band\b(?=\s*\bSlsmanDist\b)/img,"OR"));

------解决方案--------------------
看了上面几位，我不敢发言了...


------解决方案--------------------
探讨


JScript code
var str="LeftMessage is null and SentEmail is null and MetWith is null and SlsmanDist like 'KEE - kevin tang%";
alert(str.replace(/and\s+(SlsmanDist)/gi,"Or $1"))