原来是这样Regex m4 = new Regex("<img src=(?(.))">");
改后是这样Regex m4 = new Regex("<img src=(?(.?))(?=[>])"); 在最后加一个？=
Getunit(result40, "(?<=(开始字符串))[.\s\S]*?(?=(结束字符串))")

string rsp = @'{\r\n \"mof4BZanVTH5cXEagkQbGiPKiNypZ44a8L\": {\r\n \"purpose\": \"receive\"\r\n },\r\n \"2N9vzLfHeiWe6iDWocJEZy9BnEBszNmbDcc\": {\r\n \"purpose\": \"receive\"\r\n },\r\n \"bcrt1qt9ynl3q2824sx3ywevpsn0t7rkmhaksyy4s8zz\": {\r\n \"purpose\": \"receive\"\r\n }\r\n}'
string s= Json(rsp).Content.ToString();
Regex reg = new Regex("(?<=(\r\n \"))[.\\s\\S]*?(?=(\\\": {\r\n \\\"purpose))");
var matchs = reg.Matches(s);
value[0].addresslist.Add(matchs[1].Groups[0].Value);
莫名奇妙，正则匹配后断头少尾的。