<span class='spantype1_a'>text1</span>
<span class='spantype2_b'>text2</span>
<span class='spantype1_c'>text3</span>

$( "span" ).click(function() {
    var index = $('.' + $(this).attr('class')).index($(this));
    alert(index);
});

$( "span" ).click(function() {
    var aclass = $(this).attr('class');
    var back = aclass.lastIndexOf("_");
    var classtosearch = aclass.substring(0, back);
    alert($('.' + 'span[class^="' + classtosearch + '"]').index($(this)));
}

alert($('.' + 'span[class^="' + classtosearch + '"]').index($(this)));

alert($('span[class^="' + classtosearch + '"]').index($(this)));