escape-regex.js

String.prototype.escapeToRegex = function (){
    var parts = [];
    for (x=0;x<this.length;x++){parts.push("["+this[x]+"]");}
    return parts.join("");
}
String.prototype.escapeToRegexNegative = function (){
    var last = "", parts = [];
    for (x=0;x<this.length;x++){
        parts.push(last.escapeToRegex() + "[^"+this[x]+"]");
        last += this[x];
    }
    return "(" + parts.join("|") + "|^|$)";
}

/* testing */

var got = "gab".escapeToRegex();
var expected = "[g][a][b]";
if (got != expected) {
    throw got + " should be equals to " + expected;
}
var got = "gab".escapeToRegexNegative();
var expected = "([^g]|[g][^a]|[g][a][^b]|^|$)";
if (got != expected) {
    throw got + " should be equals to " + expected;
}
alert("all tests passed!");

Cuidado que o x tá virando variável global boy