Pidgin Parsers that parse keywords from surrounding text, but also keep the consumed text input.

keyword-plain-text.cs

var output = new System.Text.StringBuilder();

var parser = Map((text, keyword) =>
  {
      if (text.HasValue)
          output.Append(text.GetValueOrDefault());

      if (keyword.HasValue)
          output.Append(GetStringFromKeyword(keyword.GetValueOrDefault()));

      return Unit.Value;
  },
  Any.AtLeastOnceString().Optional(),
  OneOf(String("BeginA"), String("BeginB")) // etc...
      .Optional()
).SkipUntil(End());

// Do something with output.

keyword-with-octothorpe.cs

var output = new System.Text.StringBuilder();

var parser = Map((other, declaration) =>
  {
      if (other.HasValue)
          output.Append(other.GetValueOrDefault());

      if (declaration.HasValue)
      {
          var code = declaration.GetValueOrDefault();
          
          if (code.IsValid())
            // Runtime processing of declaration...
          else
            output.Append('#').Append(code);
      }
      return Unit.Value;
  },
  AnyCharExcept('#').AtLeastOnceString().Optional(),
  Token('#')
      .Then(Token(c => !char.IsWhiteSpace(c)).AtLeastOnceString())
      .Optional()
).SkipUntil(End());

// Do something with output.