Please explain this e-mail validation regular expression:
<p>I have this script uses regular expressions to check that a form field contains a valid email address.Please explain me from declare</p>

<pre><code>var emailfilter=/^\w+[\+\.\w-]*@([\w-]+\.)*\w+[\w-]*\.([a-z]{2,4}|\d+)$/i;
</code></pre>

<p>Thank you</p>

<p>Source:</p>

<pre><code>&lt;script type="text/javascript"&gt;

/***********************************************
* Email Validation script- © Dynamic Drive (www.dynamicdrive.com)
* This notice must stay intact for legal use.
* Visit http://www.dynamicdrive.com/ for full source code
***********************************************/

var emailfilter=/^\w+[\+\.\w-]*@([\w-]+\.)*\w+[\w-]*\.([a-z]{2,4}|\d+)$/i

function checkmail(e){
var returnval=emailfilter.test(e.value)
if (returnval==false){
alert("Please enter a valid email address.")
e.select()
}
return returnval
}

&lt;/script&gt;

&lt;form&gt;
&lt;input name="myemail" type="text" style="width: 270px"&gt; &lt;input type="submit" onClick="return checkmail(this.form.myemail)" value="Submit" /&gt;

&lt;/form&gt;
</code></pre>
