Password Validation With Reg Exp

I need to validate a password to only allow A-Z, a-z and 0-9. I
thought this would work:

function Validate()
{
var passphrase = document.all.challenge.value;

var re = /[a-zA-Z0-9]/;
if (!re.test(passphrase))
{
alert("Please enter a valid password!");
return false;
}
else
{
return true;
}
}

however if the following is entered 045# - this is accepted a being
valid. What am i doing wrong?