splitting input words into an array

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I have little experience with PHP and regular expressions. I created a simple html input form and a submit button. i would like to do the following:
When the button is clicked, regexp splits the input string into an array of words:
  1. that are all lowercase (with all ã changed to a)
  2. and are all 10 symbols long.
Should it be like that?
$search_string = $_GET['keywords'];

$regexp = preg_split(^[a-zA-Z]{1,10}+$, $search_string);

$regexp = strtolower($search_string);
This is my simple html code:
<div>
       <label for="keywords">keywords:</label>
       <input type="text" name="keywords">
</div> 
<input type="submit" value="Add Records">
Can you help me improve my regexp code? It doesn't seem to be working.
This is my code:
   <!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>
<body>
    <form action="htmlform.php" method="POST">
        keywords <input type="text" name="keywords">
        <input type="submit" value="submit">
    </form>
    <?php 

        $search_string = $_GET['keywords'];

        $regexp = preg_split("/^[a-zA-Z]{1,10}+$/", $search_string);

         $regexp = strtolower($search_string);

     ?>
</body>
</html>

Answer:

Maybe it is because your form method is POST but you are looking at $_GET?
$search_string = $_POST['keywords'];

at edit
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