Validating Alphanumeric

What's the best way in Javascript to validate that a field is alphanumeric?

Code:

---

<script type="text/javascript">
//now tested:
var str = "skslkdrt492"
regexp1 = /\d/
regexp2 = /\w/
bool = true
for (i=0; i<str.length; i++)
    {
    if (str.charAt(i).search(regexp1) == -1 && str.charAt(i).search(regexp2) == -1)
        {
        bool = false
        }
    }
if (bool == false)
    {
    //not alpha-numeric.
    }
</script>

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