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// A generic function for checking to see if an input element has a  URL contained in it

function checkURL(elem) {
	// Make sure that some text was entered, and that it's not the default http:// text
	return elem.value == '' || !elem.value == 'http://' || 
	// Make sure that it looks like a valid URL
	/^https?:\/\/([a-z0-9-]+\.)+[a-z0-9]{2,4}.*$/.test( elem.value );
}

// get an input element to check
var elem = document.getElementById("url");

// Check to see if the field is a valid URL
if (! checkURL(elem)) {
	alert("Field does not contain a URL.");
}