javascript - How to not allow values that are not in the database in an Autocomplete form with PHP & JS

I have an autocomplete function with a form where the name of the city is type into. All possible values are in a database and are pull out of the database and store them in the variable $list

Then I have my form with the input and the button

<form name="form1" method="post" action="searchresults.php" >
    <input id="hero-demo" autofocus type="search" name="search" placeholder="City" >
    <input type="submit" name="submit" value="Search">
</form>
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and the autocomplete function:

   <script>
    $(function(){
        $('#hero-demo').autoComplete({
            minChars: 1,
            source: function(term, suggest){
                term = term.toLowerCase();
                var choices = [<?= $list ?>];
                var suggestions = [];
                for (i=0;i<choices.length;i++)
                    if (~choices[i].toLowerCase().indexOf(term)) suggestions.push(choices[i]);
                suggest(suggestions);
            }
        });

    });

</script>
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How can I do, so if for instance the user introduce the value "London" and that value is not in my database ($list) then the Search button doesn't work.

Thank you very much