Changing the RegExp flags

Question

So basically I wrote myself this function so as to be able to count the number of occurances of a Substring in a String:

String.prototype.numberOf = function         


        

Answer 1

First, your current code does not work correctly when needle is a regex which does not match. i.e. The following line:

return this.match(needle).length;

The match method returns null when there is no match. A JavaScript error is then generated when the length property of null is (unsuccessfully) accessed. This is easily fixed like so:

var m = this.match(needle);
return m ? m.length : 0;

Now to the problem at hand. You are correct when you say that global, ignoreCase and multiline are read only properties. The only option is to create a new RegExp. This is easily done since the regex source string is stored in the re.source property. Here is a tested modified version of your function which corrects the problem above and creates a new RegExp object when needle does not already have its global flag set:

String.prototype.numberOf = function(needle) {
    var num = 0,
    lastIndex = 0;
    if (typeof needle === "string" || needle instanceof String) {
        while((lastIndex = this.indexOf(needle, lastIndex) + 1) > 0)
            {num++;} return num;
    } else if(needle instanceof RegExp) {
        if (!needle.global) {
            // If global flag not set, create new one.
            var flags = "g";
            if (needle.ignoreCase) flags += "i";
            if (needle.multiline) flags += "m";
            needle = RegExp(needle.source, flags);
        }
        var m = this.match(needle);
        return m ? m.length : 0;
    }
    return 0;
};

Answer 2

There isn't much you can do but I highly recommend you avoid using eval. You can extend the RegExp prototype to help you out.

RegExp.prototype.flags = function () {
    return (this.ignoreCase ? "i" : "")
        + (this.multiline ? "m" : "")
        + (this.global ? "g" : "");
};

var reg1 = /AAA/i;
var reg2 = new RegExp(reg1.source, reg1.flags() + 'g');

Answer 3

var globalRegex = new RegExp(needle.source, "g");

Live Demo EDIT: The m was only for the sake of demonstrating that you can set multiple modifiers

var regex = /find/;
var other = new RegExp(regex.source, "gm");
alert(other.global);
alert(other.multiline);

Answer 4

r = new Regexp(r.source, r.flags + (r.global ? "" : "g"));