Input要素の<code>value</code>

<input type="text" id="pattern" value="\d" />

<script type="text/javascript">
var p1 = '\d';
var p2 = document.getElementById('pattern').value;
console.log('p1 is ' + p1);             // → p1 is d
console.log('p2 is ' + p2);             // → p2 is \d

console.log(new RegExp(p1).test(123));  // → false
console.log(new RegExp(p2).test(123));  // → true
</script>