function openExternal()
{
    if (!document.getElementsByTagName)
        return;
    
    // get all <a xxx> tags
    var anchors = document.getElementsByTagName('a');
    var count = anchors.length;
    
    // loop through tags
    for (var i = 0; i < count; ++i)
    {
        var currentAnchor = anchors[i];

        // does tag contain href and rel?
        if (!currentAnchor.getAttribute('href') || !currentAnchor.getAttribute('rel'))
            continue;
        
        var content = currentAnchor.getAttribute('rel');
        
        // just a conversion to string from the unknown content type because search needs a string
        content = "n" + content;
            
        if (content.search("external") != -1)
            currentAnchor.target = '_blank';
    }
}

// run script on document load
window.onload = openExternal;