Edit in JSFiddle

var availableTags = [
            "ActionScript",
            "AppleScript",
            "Asp",
            "BASIC",
            "C",
            "C++",
            "Clojure",
            "COBOL",
            "ColdFusion",
            "Erlang",
            "Fortran",
            "Groovy",
            "Haskell",
            "Java",
            "JavaScript",
            "Lisp",
            "Perl",
            "PHP",
            "Python",
            "Ruby",
            "Scala",
            "Scheme"
        ];

$("#auto").autocomplete({
    source: availableTags,
    change: function (event, ui) {
        $("input[type='submit']").prop("disabled", !ui.item);
    }
});
<form>
<input type="text" id="auto" />
    <input type="submit" disabled="disabled" />
</form>

              

External resources loaded into this fiddle: