Now that we know how to use pattern matching with regular expressions, we can use PHP to check the information that the user as entered into a form. For this example, I will keep the form fields very simple: we want to check that the user’s first name, postal code, email address, and province have been filled out correctly.

For the purposes of clarity, I will also do the form validation on a separate page from the form itself. The form will be on a page called form.html, and the receiving page, the action of the form, will be formhandler.php This would typically not be the case: it's much more common to do all of these operations on a single page, but that can be a little confusing at first, and is an approach I will cover in a later entry.

First, the code for the form, which is a simple HTML document:

<form method="post" action="formhandler.php">
	<fieldset>
		<legend>Please enter your information</legend>
			<label for="firstname" accesskey="f">First name</label>
			<input type="text" name="firstname" id=firstname" size="30">
			<label for="postalcode" accesskey="p">Postal code</label>
			<input type="text" name="postalcode" id=postalcode" size="9">
			<label for="email" accesskey="e">eMail address</label>
			<input type="email" name="email" id=email" size="50">
			<label for="province" accesskey="t">Province / Territory</label>
			<select name="province" id=province">
				<option value="" selected>-- select one --
				<option value="AB">Alberta
				<option value="BC">British Columbia
				<option value="MB">Manitoba
				<option value="NB">New Brunswick
				<option value="NL">Newfoundland and Labrador
				<option value="NS">Nova Scotia
				<option value="NT">Northwest Territories
				<option value="NU">Nunavut
				<option value="ON">Ontario
				<option value="PE">Prince Edward Island
				<option value="QC">Québec
				<option value="SK">Saskatchewan
				<option value="YT">Yukon
			</select>
		<input type="submit" value="Go">
</fieldset>
</form>

Next, the page that is the action for this form, formhandler.php. The first thing to do is simplify the variables received, just to make life slightly easier, as well as creating some variables that we will use later:

$firstname = $_POST['firstname'];
$postalcode = $_POST['postalcode'];
$email = $_POST['email'];
$province = $_POST['province'];
$errorflag = false;
$errorfirstname = false;
$errorpostalcode = false;
$erroremail = false;
$errorprovince = false;

I’m then going to set up the patterns for the fields:

$namepattern = “/^[[:alpha:].’ -]{2,15}$/”;
$postalpattern = "/[A-Z][0-9][A-Z][0-9][A-Z][0-9]/";
$emailpattern =
"/^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]{2,63}(\.[a-z0-9-]+)*(\.[a-z]{2,6})$/";

As the user may have entered a hyphen or a space when entering her postal code, I’m going to create an array of those values. I’ll then remove any occurrences of hyphens or spaces with nothing at all (effectively removing them) and place the result, converted to uppercase, in a variable:

$rem_array = array(" ","-");
$temp_postalcode = strtoupper(str_replace($rem_array,"", $postalcode));

Finally, we check that the entered information matches the expected patterns or values. If they do not, we want to do two things:

1. Record a specific error for that form field;
2. Record that there is something wrong with the form as a whole.

Naturally, there are more efficient ways of doing this, but I find this method to be the easiest to understand to start. The tests would be something like the following:

if (!preg_match($namepattern, $firstname)) { 
	$errorflag = true; $errorfirstname = true;
}
if (!preg_match($emailpattern, $email)) { 
	$errorflag = true; $erroremail = true;
}

Note that we test our cleaned-up $temp_postalcode variable, not the original postal code:

if (!preg_match($postalpattern, $temp_postalcode)) { 
	$errorflag = true; $errorpostalcode = true;
}

There’s no need to pattern-test $province, as that will always be a known set of values; if the user has not chosen a province or territory, $province will be blank:

if (!$province) { 
	$errorflag = true; $errorprovince = true;
}

Now we have $errorflag recording if there is something wrong with our form. We can now decide whether to process the information, or to alert the user that she has made an error:

if ($errorflag) {
	/* show the form again */
} else {
	/* process the form information */
}

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