this is my textbox auto complete function. its loading fine but i want to load suggestions only user enter this " [ " sign. user can enter multiple times in one textbox. like this

hello [world] this is [my] sample.

function getOnChangeText() { 
    var str = $("#txtMyText").val();   
   if (str = "[") {
        var strText = str.replace("[", "");
    $.ajax({
        url: '../../Page/TextSearch',
        type: 'POST',
        dataType: 'json',
        data: JSON.stringify({ translateText: strText }), 
        contentType: 'application/json; charset=utf-8',
        success: function (data) {
              $("#txtMyText").autocomplete({
                  source: data
            });
            }
        });
        $("#txtDisplayText").focus();
    } 
}