using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

            var regex = new Regex(@"""number"":\s*\""([\d\s]{10,})""", RegexOptions.IgnoreCase);

            var json = @"

                {

                    ""email"": ""abc@gmail.com"",

                    ""value"": 7984,

                    ""currency"": ""EUR"",

                    ""customerIp"": ""127.0.0.1"",

                    ""trackId"": ""028594896802"",

                    ""autoCapture"": ""Y"",

                    ""card"": {

                        ""billingDetails"": {

                            ""addressLine1"": ""The Road"",

                            ""addressLine2"": null,

                            ""postcode"": ""LN6 3DN"",

                            ""country"": ""GB"",

                            ""city"": ""Hastings"",

                            ""state"": null,

                            ""phone"": {

                                ""number"": ""07097011122"",

                                ""countryCode"": ""44""

                            }

                        },

                        ""name"": ""monique prevost"",

                        ""number"": ""4242424242424242"",

                        ""expiryMonth"": ""06"",

                        ""expiryYear"": ""2018"",

                        ""cvv"": ""123""

                    }

                }

            ";

​

        Console.WriteLine(regex.Replace(json, match => "xxx"));

    }

}