using System;

public class Program

{

    public static void Main()

    {

        var validation ="required|sffdfdsfsdf|custom_validation[^\\d{5}[10]or[100](-\\d{4})?$###Invalid zipCode format. US format zip code formats (e.g., '94105-0011' or '94105')]";

        var afterTheSplit = validation.Split('|');

        foreach(var field in afterTheSplit){

            if(field.StartsWith("custom_validation")){  

                string regx = field.Substring(0,field.IndexOf("###")).Replace("custom_validation[",""); 

​

                //Replace 'Or' with '|'  and '\\' with '\' 

                regx =regx.Replace("or","|").Replace("\\",@"\");

                Console.WriteLine(regx);

            }

        }

    }

}