Match and replace

Everybody else has pretty much nailed it with using capturing groups and substitutions, just wanted to provide a little more context:

The main two things that are used here are Named Capturing Groups and Substitutions

static void Main(string[] args) {

    var input = new[] {
        "\"formatter\": \"John\"", 
        "\"formatter\": \"Sue\"", 
        "\"formatter\": \"Greg\""
    };

    foreach (var s in input) {
        System.Console.Write("Original: [{0}]{1}", s, Environment.NewLine);
        System.Console.Write("Replaced: [{0}]{1}", ReFormat(s), Environment.NewLine);
        System.Console.WriteLine();
    }

    System.Console.ReadKey();
}

private static String ReFormat(String str) {
    //Use named capturing groups to make life easier
    var pattern = "(?<label>\"formatter\"): ([\"])(?<tag>.*)([\"])";

    //Create a substitution pattern for the Replace method
    var replacePattern = "${label}: ${tag}";

    return Regex.Replace(str, pattern, replacePattern, RegexOptions.IgnoreCase);
}