Hi, I'm new in JavaScript. How can I add regular expression validation to my form?
Help, please.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Regular Expression</title>
<link href="css/validationStyles.css" rel="stylesheet" type="text/css" />

<script type="text/javascript">
function validateThis(formObj)
{
var returnValue = true; //indicates if the form was completed properly


// validate security code input
if (formObj["security"].value.length == 0)
{
document.getElementById("err_msg").innerHTML = "Please enter your security code.";
returnValue = false;
}

// validate expiration year
if (formObj["year"].value == "Select")
{
document.getElementById("err_msg").innerHTML = "Please select the year from the menu.";
returnValue = false;
}

// validate expiration day
if (formObj["day"].value == "Select")
{
document.getElementById("err_msg").innerHTML = "Please select the day from the menu.";
returnValue = false;
}

// validate expiration month
if (formObj["month"].value == "Select")
{
document.getElementById("err_msg").innerHTML = "Please select the month from the menu.";
returnValue = false;
}

// validate name input
if (formObj["creditcard"].value.length == 0)
{
document.getElementById("err_msg").innerHTML = "Please enter your credit card number.";
returnValue = false;
}

// return variable determines form completion
return returnValue;
}
</script>
</head>

<body>
<form name="orderForm" method="POST">
<div class="field">
<span id="err_msg" class="help" style="color:red">&nbsp;</span> <!-- HELP TEXT -->

<br/>
<p>
<label for="name">Credit Card #:</label>
<input id="creditcard" name="creditcard" type="text" size="40" />
</p>
</div>

<div class="field">
<p>
<label for="month">Expiration:</label>
<select id="month" name="month" >
<option value="Select" selected="selected">Select Month</option>
<option value="JAN">January</option>
</select>

<label for="day"></label>
<select id="day" name="day" >
<option value="Select" selected="selected">Select Day</option>
<option value="01">01</option>
<option value="02">02</option>
<option value="03">03</option>
<option value="04">04</option>
</select>

<label for="year"></label>
<select id="year" name="year" >
<option value="Select" selected="selected">Select Year</option>
<option value="2012">2012</option>
<option value="2013">2013</option>
<option value="2014">2014</option>
</select>
</p>
</div>
<div class="field">
<p>
<label for="security">Security Code:</label>
<input id="security" name="security" type="text" size="10" />
</p>
</div>

<INPUT type="submit" name="submitButton" value="Submit Information" onClick="return validateThis(this.form);" />
</form>
</body>
</html>