JavaScript RegExp: Different results: Built pattern using string & using regexp “literal”?

Question

Is there any differences between using RegExp literals vs strings?

http://jsfiddle.net/yMMrk/

String.prototype.lastIndexOf = function(pattern) {
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Answer 1

You need to escape your \ in the string version as \\, like this:

String.prototype.lastIndexOf = function(pattern) {
    pattern = pattern + "(?![\\s\\S]*" + pattern + ")";
    var match = this.match(pattern);
    return (match == null) ? -1 : match.index;
}

function indexOfLastNewline(str) {
    var match = str.match(/\r?\n(?![\s\S]*(\r?\n))/);
    return (match == null) ? -1 : match.index;
}

var str = "Hello 1\nHello 2\nHello 3\nHello4";
alert(str.lastIndexOf("(\\r?\\n)")); // returns correctly (23)
alert(indexOfLastNewline(str)); // returns correctly (23)

You can test it out here.