xsl = /^(file|http):\/\/\S+\/\S+\.(xsl|xslt)$/i;
img = /^(file|http):\/\/\S+\/\S+\.(jpg|gif)$/i;

function validURL(myForm, URLType) {
    if (URLType == 'img') {
        isValid = validImage(myForm);
    }
    if (URLType == 'xsl') {
        isValid = validXSL(myForm);
    }
    if (!isValid) {
        alert("Invalid URL!");
        myForm.URL.focus();
        myForm.URL.select();
        return false;
    } else {
        myForm.submit();
    }
}

function validImage(myForm) {
    return img.test(myForm.URL.value);
}

function validXSL(myForm) {
    return xsl.test(myForm.URL.value);
}