gistfile1.js

function make() {
    var text = "";
    var possible = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";

    for( var i=0; i < 25; i++ )
        text += possible.charAt(Math.floor(Math.random() * possible.length));

    return text;
}

var arr = [];
for(var i=0; i<20000; i++) {
    arr.push(make());
}

var searchFor = '123';

for(var i=0; i<arr.length; i++) {
    var candid = arr[i],
        index = arr[i].indexOf(searchFor);
    if (index >=0 ) {
        alert('found a match at index = ' + index + ', string was = ' + arr[i]);
    }
}