mount a regular expression

Example search for at least 3 bars:

var expr = /\/(.*\/){2,}/;

alert(expr.test('http://exemplo.com')); // exibe false
alert(expr.test('http://exemplo.com/ola/')); // exibe true

This expression looks for a text with 3 bars with any number of characters between them (including none).

Editing: The expression above searches for the most extensive text possible, starting with a slash, ending with a slash and having at least 3 slashes. Following the philosophy in the comment of @mgibsonbr of better efficiency, which I agree, a more restrictive expression with less processing, finds only the first three bars with the fewest possible number of characters between them would be:

var expr = /\/.*?\/.*?\//;

As the questioner just wants to check, with regular expression, if the condition is true, the latter expression is more appropriate indeed.

If you want to check whether there is or is not, then you do not need regex. You can use the split which is faster.

function tem3Barras(url){
    return url.split('/').length > 3;
}

If you do not want to start with // then you can join two lines to check this and separate that part.

function tem3Barras(url){
    var doubleBar = url.indexOf('//');
    if (doubleBar != -1) url = url.slice(doubleBar + 2);
    return url.split('/').length > 2;
}

jsFiddle: link

Here's an example:

var array = [
    'http://exemplo.com',
    'http://exemplo.com/ola',
    'http://exemplo.com/ola/',
    'www.exemplo.com/ola/mundo/javascript',
    'www.exemplo.com/ola/mundo/javascript/'
];

array.forEach(function (elm) {

    if (elm.match(new RegExp('\/', 'g')).length > 3) {
        console.dir('possui mais de 3 barras: ' + elm);
    }

});

Here's a simple way to do a validation:

function checkBars(url) {
  if (url.indexOf('/') !== -1) {
     if (url.split('/').length > 3) {
        return true;      
     }
  }
return false;
}

if (checkBars('http://www.pt.stackoverflow.com/questions/91990/montar-uma-expressão-regular/92057')) {
  alert('possui mais de 3 barras');
} else {
  alert('possui menos de 3 barras');
}