A simple (but not very flexible) solution would be to just use a .filter() with a function:

$('div').filter(function() {
return /^show|\sshow/.test(this.className);
});

You could also write a fairly simple selector plugin. Here is one that you can use with any attribute, not just a class:

(function($) {
$.extend($.expr[':'], {
attrStart: function(element, index, matches, set) {
matches = matches[3].split('|');

var attr = matches.length > 1 ? matches.shift() : 'className',
val = matches.join(''),
re = new RegExp('^' + val + '|\\s' + val);

attr = $(element).attr( attr );

return !!attr && re.test(attr);
}

});
})(jQuery);

Usage would be something like this, using a "pipe" delimiter between the attribute name and the value you're looking for:

$('div:attrStart(class|show)')

The plugin makes "class" the default attribute, so you could also do this:

$('div:attrStart(show)')