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<!DOCTYPE html>

<html lang="en">

<head>

  <meta charset="utf-8">

  <title>\d{3,}</title>

</head>

<body>

  <h1>At least three digits</h1>

  <pre id="out"></pre>

  <script>

    var samples = ["12", "123", "12345", "ab12"];

    document.getElementById("out").textContent = samples.map(function (s) {

      var m = s.match(/\d{3,}/);

      return s + " → " + (m ? m[0] : "null");

    }).join("\n");

  </script>

</body>

</html>

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JavaScript RegExp {X,} quantifier — three or more digits (Try It)