How to find the id of input type text and if not present then add it in c-#

Question

How to find the id of <input type= text> and if not present then add it in C#

I have considered the above link for my solution, but my problem is a bit different. Suppose in future I added a new textbox (id= "Answer4") to my string prior to the 1st textbox (id= "Answer1") like:

string Question="<input data-size="0" data-type="0" id="Answer4" placeholder="Answer4" type="text" /> 
<br/>
<br/>
<input data-size="0" data-type="0" id="Answer1" placeholder="Answer1" type="text" /> 
<br/>
<br/>
<input data-size="0" data-type="0" placeholder="Answer2" type="text" /><br/>
<br/>
<input data-size="0" data-type="0" id="Answer3" placeholder="Answer3" type="text" /><br/>
<br/>
&nbsp;";

and id is missing from the 2nd textbox. How can I add its id. I have done the code like :

string textboxTagPattern= @"(<input)([^>]*)(type=\"")(text)(\"")([^>]*)(/>)";
Regex rgx = new Regex(textboxTagPattern, RegexOptions.IgnoreCase);
MatchCollection questionInputText = rgx.Matches(Question);

var arr = Question.Replace("<input", "^<input").Split('^');
        string ques = "";
        int count = 0;
        for (int i = 0; i < arr.Length; i++)
        {
            //If it is of <input type="text"> format and the id is not present
            if (arr[i].IndexOf("type=\"text\"") != -1)
            {
                count++;
                if (arr[i].IndexOf("id") == -1)
                {
                    arr[i] = arr[i].Insert(arr[i].IndexOf("input ") + "input ".Length, "id=\"Answer" 
                            + count + "\" ");
                     // According to this logic the 2nd and 3rd textboxes will have the same 
                      //id(id=Answer3), which is wrong. How can I correct it?
                }

            }
            ques = ques + arr[i].Replace("\n", "");
        }