string exp= "12345678";
  string regEx1="^([-+]?[(\d+\.?(\d+)?)]{1,8})($)";
  Regex topRowRegx = new Regex(regEx1, RegexOptions.IgnoreCase | RegexOptions.Multiline);
  Match matchResult = topRowRegx.Match(exp.Trim());

string exp = "text> -9.9999999 "some text here"";
   var resultNumber = Regex.Match(exp, "[-+]?(\d{1,8}|(?=\d\d*\.\d+?$)[\d\.]{1,9})");
   ("Result:"+resultNumber.ToString()).Dump();

^[-+]?(d{1,8}|(?=dd*.d+?$)[d.]{1,9})$

^[-+]?(\d{1,8}|(?=\d\d*\.\d+?$)[\d\.]{1,9})$

^[+-]?(?:(?=d+.d+$)[d.]{3,9}|(?=d+$)d{1,8})$