CustomExpandTokenProcessorExample.cs

using Sitecore.Diagnostics;
using Sitecore.Pipelines.ExpandInitialFieldValue;

namespace CustomExpandTokenProcessors.Pipelines
{
  public class CustomTokenReplacer : ExpandInitialFieldValueProcessor
  {
    private const string Token = "$customtoken";

    public override void Process(ExpandInitialFieldValueArgs args)
    {
      Assert.ArgumentNotNull(args, "args");

      // TODO: Add your own custom token replacement logic here...
    }
  }
}