Verify New Password Meets Format Conditions

I'm working on a site which is password protected using ASP and an Access database.

I need to require that new passwords be at least 8 characters and contain at least one character from upper case, lower case, number and special characters. I've searched but haven't found anything that looks like it would work - only scripts to prohibit certain characters.

function mt_valForm(val)
{
var passed = true;
var mt_error = '';
if (val.txtusername.value == ""){passed=false; mt_error+="Please provide a user name
"}

if (val.txtpassword.value == ""){passed=false; mt_error+="Please provide a password
"}

if (val.txtemail.value.indexOf("@") == -1 || val.txtemail.value.indexOf(".") == -1){passed=false;
mt_error+="Please provide a valid email address
"}

if (mt_error!= ''){alert ("There were errors - please correct these:

" + mt_error)}
return passed;