scrbook chapter numbering

Question

I want chapter numbers prefixed by a letter like

M1: First chapter of "M" series
M1.1 First section in first chapter of "M" series
M2: Second chapter of "M" series
...

then change the prefix, e.g. to R

R1: First chapter of "R" series
R2: Second chapter of "R" series
...

I tried with some suggestions in scrguide like chapterlinesformat, but either the numbering did not work at all or was not resembled in the toc.

The TOC should look like

M1 chapterName1.................1
   M1.1 sectionname.............3
M2 chapterName..................5
R1 chapterName..................8
   R1.1 sectionname............10

etc.

Any suggestions are welcome

Sorry for not giving a MWE, since it is NOT working. one of my attemps follows: %Minimum NOT working example

\documentclass[
    a4paper, 
    10pt,
    pagesize,
    titlepage=true,
    numbers=noenddot,
]{scrbook}
% deutsches Sprachpaket
\usepackage[ngerman]{babel}


\newcommand{\expprefix}{}

\renewcommand\chapterformat{%
        \expprefix\thechapter %\chaptername
        \vspace{6pt}%
}


\begin{document}

\tableofcontents

\chapter{This is standard numbering}

% set chapter Prefix to M
\renewcommand{\expprefix}{M}
\setcounter{chapter}{0}

\chapter{This chapter should be numbered M1}
\section{Section1 of first}
\section{Section2 of first}

\chapter{Second chapter of M-series}

%set chapter prefix to R
\renewcommand{\expprefix}{R}
\setcounter{chapter}{0}

\chapter{This chapter should be numbered R1}
\section{Section1 of first R-chapter}
\section{Section2 of first R-chapter}

\chapter{Second chapter of "R" series}

\end{document}

Answer 1

Edit: I have stolen the idea by Christian Hupfer to redefine \thechapter and added the formatting of the TOC using the KOMA-Script-specific command \DeclareTOCStyleEntry. The command \chapterformat does not need to be changed in this case.

\documentclass[fontsize=10pt, paper=a4, numbers=noenddot]{scrbook}

\usepackage[ngerman]{babel}

\newcommand\expprefix{}
\let\latexorigthechapter\thechapter
\renewcommand{\thechapter}{\expprefix\latexorigthechapter}

\DeclareTOCStyleEntry[numwidth=2.3em]{default}{chapter}
\DeclareTOCStyleEntry[numwidth=3em, indent=2.3em]{default}{section}

\begin{document}

\tableofcontents

\chapter{This is standard numbering}

% set chapter Prefix to M
\renewcommand{\expprefix}{M}
\setcounter{chapter}{0}

\chapter{This chapter should be numbered M1}
\section{Section 1 of first M chapter}
\section{Section 2 of first M chapter}

\chapter{Second chapter of M-series}

% set chapter prefix to R
\renewcommand{\expprefix}{R}
\setcounter{chapter}{0}

\chapter{This chapter should be numbered R1}
\section{Section 1 of first R chapter}
\section{Section 2 of first R chapter}

\chapter{Second chapter of R series}

\end{document}

This is how the TOC looks like:

I did not bother to deal with chapter prefixes available in KOMA-Script; they could be added as needed. Furthermore, I removed the document class options that are set by default in order to simplify the MWE.

Answer 2

It seems that the chapter counter is resetted each time when the prefix is changed. So the prefix should be part of the chapter number.

Additionally you have to adjust tocnumwidth and tocindent for all section levels in TOC.

\documentclass[
  10pt,
  numbers=noenddot,
  ngerman
]{scrbook}
\usepackage{babel}

\newcommand*\expprefix{}
\newcommand*\setexpprefix[1]{%
  \setcounter{chapter}{0}%
  \def\expprefix{#1}%
}
\renewcommand\thechapter{\expprefix\arabic{chapter}}
\RedeclareSectionCommand[
  tocnumwidth=2em
]{chapter}
\RedeclareSectionCommand[
  tocindent=2em,
  tocnumwidth=2.8em
]{section}
\RedeclareSectionCommand[
  tocindent=4.8em,
  tocnumwidth=4em
]{subsection}

\usepackage{blindtext}% only for dummy text
%\usepackage[naturalnames]{hyperref}

\begin{document}
\tableofcontents
\chapter{This is standard numbering}

\setexpprefix{M}% set chapter Prefix to M
\chapter{This chapter should be numbered M1}
\section{Section1 of first}
\section{Section2 of first}
\chapter{Second chapter of M-series}
\blinddocument \Blindtext[2]

\setexpprefix{R}%set chapter prefix to R
\chapter{This chapter should be numbered R1}
\section{Section1 of first R-chapter}
\section{Section2 of first R-chapter}
\chapter{Second chapter of "R" series}
\end{document}

Result: