var wholeStr = "This is my string.";
//var re = /\bis\b/gi;
var oldStr = "is";
var re=new RegExp("\\b"+oldStr+"\\b","gi");
//var oldStr = "is";
//var re=new RegExp("/\b"+oldStr+"\b/","gi");
var wholeStr = wholeStr.replace(re, "is not");
alert(wholeStr)
//var re = /\bis\b/gi;
var oldStr = "is";
var re=new RegExp("\\b"+oldStr+"\\b","gi");
//var oldStr = "is";
//var re=new RegExp("/\b"+oldStr+"\b/","gi");
var wholeStr = wholeStr.replace(re, "is not");
alert(wholeStr)
var oldStr = "is";
var re=new RegExp("\\b"+oldStr+"\\b","gi");
var wholeStr = wholeStr.replace(re, "is notss");
alert(wholeStr)