PHP：正则表达式获取name =“”的值

I'm still very much learning Regex and I suck at it :(

I am getting an external webpage, saving it to a local file. The page contains a form, now I need to find all the form's input's names and put it into an array, for example:

<input type="text" name="firstName">

or

<input type="text" name="firstName" class="something">

from the above I just need firstName to go into my array.

To complicate matters just a little bit, I have noticed people writing it a bit different sometimes because of the space character, like so:

name= "firstName"
name = "firstName"
name ="firstName"

As I am still learning, I would really appreciate it if you could explain your regex

Quick example:

<?PHP

$myDocument = new DOMDocument;
$myDocument->loadHTMLfile('example_page.html');

// Get form item(s)
$formItems = $myDocument->getElementsByTagName('form');

// Get input items from first form item
$inputItems = $formItems->items(0)->getElementsByTagName('input');

// Name attributes
foreach ($inputItems as $inputName) {
     echo $inputName->getAttribute('name') . "<br />";
}

?>

You can use something like

$xmldoc = new DOMDocument();
$xmldoc->load('REQUIRED_URL');
foreach ($xmldoc->getElementsByTagName('form') as $formitem) {
    $inputnodes = $formitem->getElementsByTagName('input');
    $name = $inputnodes->item(0)->getAttribute('name');
    echo $name;
}