HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>Filter [abc]</title>
</head>
<body>
  <h1>Filter and highlight</h1>
  <pre id="out"></pre>
  <script>
    var messy = "a1b!c2x";
    var allowed = messy.replace(/[^abc]/g, "");
    var highlighted = messy.replace(/[abc]/g, "[$&]");
    document.getElementById("out").textContent =
      allowed + "\n" + highlighted;
  </script>
</body>
</html>

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JavaScript RegExp [abc] — filter chars (Try It)