HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>replace flags</title>
</head>
<body>
  <pre id="out"></pre>
  <script>
    var str = "Twas the night before Xmas...";
    document.getElementById("out").textContent =
      str.replace(/xmas/i, "Christmas");
  </script>
</body>
</html>

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String.replace() — ignore case (Try It)