IE5 And Look Ahead Expressions

I am using a JS form validator that uses the following regex to test whether a string contains at least one unsigned int and one letter.

var reAlphanumeric = /^(?=.*[0-9]+.*)(?=.*[a-zA-Z]+.*)([0-9a-zA-Z])+$/;

function checkAlphaNum( theField ) {
if ( isWhitespace( theField.value ) || !reAlphanumeric.test( theField.value ) ) {
return false;
} else {
return true;
}}

Since IE5 doesn't support look ahead expressions, I am using the following function to get the same result in IE5.

var reUnsignedInteger = /^[+]?[0-9]+$/;
var reAlphabetic = /^[a-zA-Z]+$/;

function IE_AlphaNum( s ) {
var isInt = false;
var isLet = false;
var isOp = false;
var len;
var setStr = strArray( s );

for ( var i = 0; i < setStr.length; i++ ) {
if ( reAlphabetic.test( setStr[i] ) ) {
isLet = true;
break;
}
}

for ( var i = 0; i < setStr.length; i++ ) {
if ( reUnsignedInteger.test( setStr[i] ) ) {
isInt = true;
break;
}
}

for ( var i = 0; i < setStr.length; i++ ) {
if ( !reAlphabetic.test( setStr[i] ) && !reUnsignedInteger.test( setStr[i] ) ) {
isOp = true;
break;
}
}

if ( isInt && isLet && !isOp )
return true;
else
return false;
}

Can a regex be used that will satisify the one int/one letter rule that will work in IE5 or if not, can the above function be refactored w/o having to interate through the string array each time? Neither method works in Opera 6. Also, is there any condition except for validating perhaps a password where such a rule would be needed?