Customize subsection counter

Question

I am compiling a solution file with exercises from different sections of a book, so I hope the exercises are numbered as sectionnumber.subsectionnumber.exercisenumber. The issue arises since the book's subsection labeling is particular: it is a combination of numbers and alphabets, so after subsection 1.1, 1.2, for example, it would then start alphabetically like subsections 1.A and 1.B (I assume to illustrate unimportant materials). I am looking for a way to customize the counter so that the subsections and exercises are numbered correspondingly. Thanks!

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Update: Just saw this post, which solves most of the issue. I am still hoping to find a more ''automatic'' way of doing this, like using counter, etc.

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For a working example, say

\documentclass[12pt]{article}
\usepackage[letterpaper, margin=1in]{geometry}
\usepackage{amsmath, amssymb, mathtools, tikz-cd, enumitem, fancyhdr, mleftright, wrapfig, scrextend,mathrsfs,amsthm}
\theoremstyle{definition}
\newtheorem{exercise}{Exercise}[subsection]
\newtheorem{corollary}{Corollary}[exercise]
\newtheorem{lemma}{Lemma}[exercise]
\newtheorem{proposition}{Proposition}[exercise]
\newtheorem{claim}{Claim}[exercise]
\newtheorem{remark}{Remark}[exercise]
\begin{document}
\section{Section 1}
\begin{exercise} % 1.0.1
\end{exercise}
\subsection{Subsection 1.1}
\begin{exercise} % 1.1.1
\end{exercise}
\subsection{Subsection 1.2}
\begin{exercise} % 1.2.1
\end{exercise}
\subsection{Subsection 1.A}
\begin{exercise} % 1.A.1
\end{exercise}
\subsection{Subsection 1.B}
\begin{exercise} % 1.B.1
\end{exercise}
\section{Section 2}
\subsection{Subsection 2.1}
\begin{exercise} % 2.1.1
\end{exercise}
\subsection{Subsection 2.2}
\end{document}

Answer 1

You could define a commend, that changes the subsection numbering style redefining \thesubsection and also stores the last value of the previous numbering style and optionally restores this value. In the example below \subsectionnumbering{<style>} would restore the previously last number of the style, but the star variant \subsectionnumber*{<style>} would always start with 1.

\documentclass[12pt]{article}
\usepackage[letterpaper, margin=1in]{geometry}
\usepackage{amsmath, amssymb, mathtools, tikz-cd, enumitem, fancyhdr, mleftright, wrapfig, scrextend,mathrsfs,amsthm}
\theoremstyle{definition}
\newtheorem{exercise}{Exercise}[subsection]
\newtheorem{corollary}{Corollary}[exercise]
\newtheorem{lemma}{Lemma}[exercise]
\newtheorem{proposition}{Proposition}[exercise]
\newtheorem{claim}{Claim}[exercise]
\newtheorem{remark}{Remark}[exercise]
\errorcontextlines\maxdimen
\makeatletter
\newcommand{\currentsubsectionnumbering}{arabic}
\NewDocumentCommand{\subsectionnumbering}{sm}{%
  \expandafter\edef\csname lastsubsection\currentsubsectionnumbering number\endcsname
    {\expandafter\the\value{subsection}}%
  \renewcommand{\thesubsection}{\thesection.@nameuse{#2}{subsection}}%
  \setcounter{subsection}{0}%
  \IfBooleanF{#1}{% star variant don't set it to the last used number of the numbering style
    @ifundefined{lastsubsection#2number}{}{%
      \setcounter{subsection}{@nameuse{lastsubsection#2number}}%
    }%
  }
  @namedef{currentsubsectionnumbering}{#2}%
}
\makeatother
\begin{document}
\section{Section 1}
\begin{exercise} % 1.0.1
\end{exercise}
\subsection{Subsection 1.1}
\begin{exercise} % 1.1.1
\end{exercise}
\subsection{Subsection 1.2}
\begin{exercise} % 1.2.1
\end{exercise}
\subsectionnumbering{Alph}
\subsection{Subsection 1.A}
\begin{exercise} % 1.A.1
\end{exercise}
\subsection{Subsection 1.B}
\begin{exercise} % 1.B.1
\end{exercise}
\subsectionnumbering{arabic}
\subsection{Subsection 1.3}
\begin{exercise} % 1.1.1
\end{exercise}
\subsectionnumbering{Alph}
\subsection{Subsection 1.C}
\begin{exercise} % 1.C.1
\end{exercise}
\subsectionnumbering{arabic}% should be BEFORE \section
\section{Section 2}
%\subsectionnumbering*{arabic}% alternative: use the star variant to start
                              % again with subsection number 1
\subsection{Subsection 2.1}
\begin{exercise} % 2.1.1
\end{exercise}
\subsectionnumbering*{Alph}% Here we don't want to use the previous upper case
                           % letter value, but want to start new.
\subsection{Subsection 2.A}
\begin{exercise} % 2.A.1
\end{exercise}
\end{document}