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HTML

asasc

JavaScript

function incrementId(idString) {
    var pieces = idString.split("_");
    if (pieces[3]) {
        pieces[3] = parseInt(pieces[3]) + 1;
    } else {
        pieces[3] = "1";
    }
    return pieces.join("_");
}

var lastId = 'org_dropmenus_asdf_1';
var pieces = lastId.split("_");

if (pieces[3]) {
    pieces[3] = parseInt(pieces[3]) + 1;
} else {
    pieces[3] = "1";
}
document.write(incrementId('a'));
document.write('<br />');
document.write(incrementId('org_dropmenus_asdf_2'));
document.write('<br />');
document.write(incrementId('org_dropmenus_asdf_2'));
<input name="(incrementId('asc_1')" id="ss" readonly="readonly">