Try this:
string result = Regex.Match(input, "\"text\":\"(?<txt>[^,\"]+)\"").Groups["txt"].Value;
If you have more than one
text
variable in your input, try this:
string[] results = Regex.Matches(input, "\"text\":\"(?<txt>[^,\"]+)\"").Cast<Match>().Select(x => x.Groups["txt"].Value).ToArray();
Hope this helps.