HTML Code

<!DOCTYPE html>

<html lang=en>

<head>

  <meta charset=UTF-8>

  <meta name=viewport content="width=device-width,initial-scale=1">

  <title>onkeypress — alphanumeric</title>

  <style>body{font-family:system-ui,sans-serif;margin:0;padding:1.5rem;background:#f8fafc;color:#334155;line-height:1.7}h1{margin:0 0 .75rem;font-size:1.25rem;color:#0f172a}label{display:block;font-size:.8125rem;font-weight:500;margin-bottom:.35rem}input{display:block;width:100%;max-width:280px;padding:.5rem .75rem;border:1px solid #cbd5e1;border-radius:8px;font-size:.875rem}.note{margin-top:.75rem;font-size:.8125rem;color:#64748b;max-width:420px}</style>

</head>

<body>

  <h1>Alphanumeric username</h1>

  <p>Symbols are blocked on keypress. Paste may still add other characters.</p>

  <label for=username>Username</label>

  <input id=username name=username onkeypress=validateUsername(event)>

  <p class=note>Legacy onkeypress demo — prefer oninput for new projects.</p>



  <script>

    function validateUsername(e) {

      if (e.key.length === 1 && !/^[a-z0-9]$/i.test(e.key)) {

        e.preventDefault();

      }

    }

  </script>

</body>

</html>

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HTML onkeypress Attribute — Alphanumeric filter