I'm looking for feedback about anything and everything to do with this short program. For example, is there a risk in not sanitising the data, and if so, how would I do that given the need for non-alpha-numerics? How is the logic? Is my html up to date? Suggestions for styling? All input appreciated. I'm on a learning curve to become a professional, so need to be producing quality code.<?php/*PHP Regex testerAuthor: Robin Andrews 2016*/// Has form been reset?if (!empty($_POST['reset'])){    $_POST = array();    $msg = ;}// Check if fields completed and process if soif (!empty($_POST['pattern']) && !empty($_POST['string'])){    $pattern = $_POST['pattern'];    $string = $_POST['string'];    if (preg_match_all('~'.$pattern.'~', $string, $matches)){        $groups = implode(; , $matches[0]);        $count = count($matches[0]);        $msg =  $count .  Matches present:  . $groups;    } else {        $msg = Pattern is not present;    }    // clear $_POST data    $_POST = array();}?><!DOCTYPE html><html lang=en><head>    <meta charset=UTF-8>    <title>Regex Tester</title>    <style>        body{            font-family: arial;            font-size: 18px;        }        #wrapper{            width: 400px;            margin: 20px auto;            border: 2px solid blue;        }        h3{            text-align: center;        }        input, textarea{            font-size: inherit;            font-family: inherit;            box-sizing: border-box;            margin: 5px;            width:300px;            border-radius: 10px;            padding: 5px;        }        form{            width: 300px;            margin: 5px auto;        }        input[type=submit], #reset{            font-family: inherit;            font-size: 16px;            box-sizing: border-box;            margin: 5px 0;            width:100px;            border-radius: 10px;            padding: 2px;        }        #submit{            margin-left:5px;        }        #message{            margin: 10px;        }    </style></head><body>    <div id=wrapper>        <h3>Regex Tester</h3>        <form action= method=post>            Enter a string: <textarea name=string rows=4 autocomplete=off><?=isset($string)? $string : ''?></textarea><br>            Enter a pattern: <input type=text name=pattern autocomplete=off value=<?=isset($pattern)? $pattern : ''?>><br>            <input type=submit value=submit id=submit>            <input type=submit value=reset id=reset name=reset>        </form>        <div id=message>            <p><?=isset($msg)? $msg : '&nbsp'?></p>        </div><!-- message -->    </div><!--wrapper--></body></html>
Regex Tool in PHP
php;html;regex