Use regular expressions with preg_replace to remove letters other than alpha-numeric on every keyup and then check the length of a string.

Hope you get the desired result.

Solution:

Check this solution. I hope, it will work.

$(document).on('draw.dt','.dataTable', function() {

$(".dataTables_filter input").off().on("input keyup", function(e) { 
    // if value length is greater than 3 or user press enter (keycode = 13) on search we will search on datatable
    if(this.value.length >= 3 || e.keyCode == 13 || this.value == "") {
        var checkBackspaceEnter =  e.keyCode == 8 || e.keyCode == 13; // if user remove character from search we will search
        var checkAlphabet = (e.keyCode >= 58 && e.keyCode <= 90);   // check on alphabets
        var checkNum = (e.keyCode >= 48 && e.keyCode <= 57 || e.keyCode >= 96 && e.keyCode <= 105); // check on number values
        var ctrlCombine = (e.keyCode == 17 || ((e.keyCode == 65 || e.keyCode == 67 ) && e.ctrlKey)); // donot search when user hit Ctrl+A or Ctrl+C 
        checkAlphabet = ((ctrlCombine == false) ? checkAlphabet : false);   

        if ( checkAlphabet || checkNum || checkBackspaceEnter) {
            var dataTable = $('.table').DataTable();
            dataTable.search(this.value).draw();
        }
    }
}); 

});