Your question is vague one. There're some issues which are still open:
- Do you accept leading spaces e.g.  "      Mazhar"?
- Do you accept trailing spaces e.g. "Mazhar      "?
- Do you accept double spaces e.g.   "Mazhar   123"?
- Can a valid string start with digit? e.g. "123 Mazhar"
In case answers are No, No, Yes, No you can put it like this
  // The pattern, you probably are looking for:
  string pattern = @"^[A-Za-z]+([A-Za-z0-9 ]*[A-Za-z0-9])*$";
  string[] tests = new string[] {
    // your test cases (valid strings)
    "Mazhar", "mazhar123", "mazhar khan1",
    // your test cases (invalid strings)
    "1233444", "@@@@@@",  "Mazhar@kkk",
    // my test cases (leading space, trailing space, double space, starts with digit)
    " Mazhar", "Mazhar ", "Mazhar    123", "123Mazhar"
  };
  var report = tests
    .Select(item => Regex.IsMatch(item, pattern)
      ? $"{item,15} is valid"
      : $"{item,15} is NOT valid");
  Console.Write(string.Join(Environment.NewLine, report));
The outcome is
         Mazhar is valid
      mazhar123 is valid
   mazhar khan1 is valid
        1233444 is NOT valid
         @@@@@@ is NOT valid
     Mazhar@kkk is NOT valid
         Mazhar is NOT valid  // leading spaces
        Mazhar  is NOT valid  // trailing spaces
  Mazhar    123 is valid      // we accept double spaces
      123Mazhar is NOT valid  // starts with digit