JavaScript - I get an error in Firebug

I am searching through a string trying to count the number of occurrences of @@ (two @ signs in a row). My code works well when the count is greater than 0, but I get an error in Firebug when the occurrence is zero.

This throws an error:

var str = "This string has no at signs!";
var pcount = str.match(/@@/g).length;
alert(pcount);

var str = "This string has @@ one pair of signs!";
var pcount = str.match(/@@/g).length;
alert(pcount);

var str = "This @@ string has @@ two pairs of signs!";
var pcount = str.match(/@@/g).length;
alert(pcount);

The error I get in Firebug is:

str.match(/@@/g) is null
[Break On This Error] var pcount = str.match(/@@/g).length;

What I want is for pcount to be zero if no @@ is found.

Can you help?

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ANSWER

I ended up using a combination of your answers. Thanks so much for your help!!!

var NewRows = getRowCount(ThisTextLen);
var Match = ThisText.match(/@@/g);
var ParagraphCount = 0;
if (Match != null) {
    ParagraphCount = ThisT开发者_如何学JAVAext.match(/@@/g).length;
    NewRows = NewRows + ParagraphCount;
}

Try:

var match = str.match(/@@/g); 
var pcount = 0;

if (match != null) { 
     pcount = str.match(/@@/g).length; 
}

alert(pcount);

You can write:

var pcount = (str.match(/@@/g) || []).length;

In case there is no match (the return value of str.match(/@@/g) will be null, which evaluates to false), the length property of the empty array will be accessed.

Try something like

var matched = str.match(/@@/g);

if(matched == null)
    pcount = 0;
else
    pcount = matched.length;

(or something more idiomatic like what Felix suggested, but this is the same idea)

Because no matches for /@@/, that is reason to return null object, so what you can do is

var str = "This  string has  two pairs of signs!";
var matches = str.match(/@@/gi);
var pcount = matches != null ? matches.length : 0;
alert(pcount);

this will work fine

var str = "This @@ string has @@ two pairs of signs!";
var pcount = 0;
if(str.match(/@@/g)!=null)
    pcount =str.match(/@@/g).length;
alert(pcount);