Question

I would like to extend the Javascript built-in Array object by addind a new method: searchByRegexp

Basically the method will get a regexp as input and will return:
_ a string representing the element of the array that matches the regexp (if more than one match, the first one will be returned)
_ an empty string if none of the elements matches the regular expression.

Answer 1

Quick and dirty (fiddle for your fiddling pleasure):

Array.prototype.searchByRegexp = function (rx) {
    for(var i = 0, ln = this.length; i < ln; i++) {
        // test item i
        if(rx.test(this[i])) {
            return this[i] + "";
        }
    }
    // return an empty string if no matches are found
    return "";
}

However, you may wish to implement more general methods instead…

Array.prototype.find = function(delegate) {   
    for(var i = 0, ln = this.length; i < ln; i++) {
        if(delegate(this[i])){
            return this[i];   
        }
    }
    return null;
}

Array.prototype.findAll = function(delegate) {   
    var results = [];
    for(var i = 0, ln = this.length; i < ln; i++) {
        if(delegate(this[i])){
            results.push(this[i]);   
        }
    }
    return results ;
}

Array.prototype.each = function (delegate) {
    for (var i = 0, ln = this.length; i < ln; i++) {
        delegate(this[i], i);
    }
}

… which could then perform regex comparison like so:

// test values
var sizes = ["Small", "Medium", "Large", "Extra-Large", "Extra-Extra-Large"];

// Print single value or blank
document.write(sizes.find(function(size){
    return /large/i.test(size);
}) || "");     

// horizontal rule to separate our results
document.write("<hr/>");

// Print all matches
sizes.findAll(function(size){
    return /large/i.test(size);
}).each(function(size){
    document.write(size + "<br/>"); 
});