HTML Code

<!DOCTYPE html>

<html lang=en>

<head>

  <meta charset=UTF-8>

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

  <title>pattern — username</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;margin-bottom:.35rem;font-weight:500;font-size:.875rem}input{width:100%;max-width:320px;padding:.5rem .65rem;font:inherit;border:1px solid #cbd5e1;border-radius:6px}button{margin-top:.75rem;padding:.5rem 1rem;font:inherit;cursor:pointer;border-radius:6px;border:1px solid #cbd5e1;background:#fff}.hint{font-size:.8125rem;color:#64748b;margin-top:.5rem}#status{margin-top:.75rem;font-size:.875rem;font-weight:600}</style>

</head>

<body>

  <h1>Username pattern</h1>

  <form>

    <label for=username>Username:</label>

    <input id=username name=username pattern=[A-Za-z0-9_]{3,16} title="3–16 letters, numbers, or underscores" placeholder=user_12 required>

    <p class=hint>3–16 characters: letters, numbers, or underscore only.</p>

    <button type=button id=checkBtn>Check validity</button>

    <p id=status aria-live=polite>Try user_12 or ab (too short).</p>

  </form>



  <script>var username=document.getElementById("username"),status=document.getElementById("status");document.getElementById("checkBtn").addEventListener("click",(function(){status.textContent=username.checkValidity()?"Valid username: "+username.value:"Invalid — see title hint (3–16 letters, numbers, or _).",status.style.color=username.checkValidity()?"#166534":"#b45309"}))</script>

</body>

</html>

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HTML pattern Attribute — username rules