Convert numbers in string to string

Question

I store some strings in a variable \def\nomVAR{A12Z4E} and I want to create some other command using the content of \nomVAR. LaTeX doesn't accept to declare variable with number in its name. So the idea will be to convert all the number in the string using \numberstringnum command (fmtcount package). The xstring can extract some specific string in a string but I have to precise the position of each one.

Do you think it is possible to write a function to convert each number to a string in a string and to rebuilt it?

Thus A12Z4E will become AonetwoZfourE.

MNWE

\documentclass{minimal}

\usepackage{xstring}
\usepackage{fmtcount}

\begin{document}

\def\nomVAR {A12Z4E}

\nomVAR

\numberstringnum{1}

\numberstringnum{\nomVAR} %not working

\end{document}

SOLUTION (thanks egreg)

\documentclass{minimal}

\usepackage{xstring}

\begin{document}

\def\nomVAR {A12Z4E}

\nomVAR

\numberstringnum{1}

\newcommand{\changestep}[2]{%
  \expandafter\StrSubstitute\expandafter{\x}{#1}{#2}[\x]%
}
\newcommand{\changeall}[1]{
 % suppress expansions made by xstring
\StrSubstitute{#1}{0}{zero}[\x]%
\x
\noexpandarg
\changestep{1}{one}
\changestep{2}{two}
\changestep{3}{three}
\changestep{4}{four}
\changestep{5}{five}
\changestep{6}{six}
\changestep{7}{seven}
\changestep{8}{eight}
\changestep{9}{nine}
\x
}

\changeall{\nomVAR}

\end{document}

Answer 1

You can do it expandably, provided the string only has alphanumeric ASCII characters:

\documentclass{article}

\makeatletter
\newcommand{\convertdigits}[1]{\expandafter\convert@digits#1\convert@digits}
\def\convert@digits#1{%
  \ifx#1\convert@digits
    \expandafter\@gobble
  \else
    \expandafter\@firstofone
  \fi
  {\convert@@digits#1}%
}
\def\convert@@digits#1{%
  \ifnum9=9#1%
    \expandafter\@gobble
  \else
    \expandafter\@firstofone
  \fi
  {\convert@@digit#1}\convert@digits
}
\def\convert@@digit#1{%
  \ifcase#1%
    zero\or
    one\or
    two\or
    three\or
    four\or
    five\or
    six\or
    seven\or
    eight\or
    nine\fi
}
\makeatother

\begin{document}

\def\myvar{A12Z4E}

\convertdigits{A12Z4E}

\convertdigits{\myvar}

\edef\myvartext{\convertdigits{\myvar}}
\texttt{\meaning\myvartext}

\end{document}

A different expl3 implementation: the argument to \convertdigits is fully expanded before being examined and scanned item by item.

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn

\NewExpandableDocumentCommand{\convertdigits}{m}
 {
  \guuk_convertdigits:e { #1 }
 }

\cs_new:Nn \guuk_convertdigits:n
 {
  \tl_map_function:nN { #1 } \guuk_digit:n
 }
\cs_generate_variant:Nn \guuk_convertdigits:n { e }

\cs_new:Nn \guuk_digit:n
 {
  \str_case:nnF { #1 }
   {
    {0}{zero}
    {1}{one}
    {2}{two}
    {3}{three}
    {4}{four}
    {5}{five}
    {6}{six}
    {7}{seven}
    {8}{eight}
    {9}{nine}
   }
   {#1}% not a digit
 }
\ExplSyntaxOff

\begin{document}

\def\myvar{A12Z4E}

\convertdigits{A12Z4E}

\convertdigits{\myvar}

\edef\myvartext{\convertdigits{\myvar}}
\texttt{\meaning\myvartext}

\end{document}

Answer 2

Here I set up a tokencycle to look (token by token) for letters (catcode 11) and numerical digits. Anything else is discarded. Digits are processed via \spelldigit, which executes a simple \ifcase.

\documentclass{article}
\usepackage{tokcycle}
\newcommand\numberstringnum[1]{%
  \resettokcycle
  \Characterdirective{\tctestifcatnx A##1{##1}{\spelldigit##1}}%
  \expandafter\tokcyclexpress\expandafter{#1}}
\newcommand\spelldigit[1]{%
  \ifcase\numexpr`#1-`0 zero\or one\or two\or three\or four\or five\or
  six\or seven\or eight\or nine\fi}
\begin{document}
\def\nomVAR {A12Z4E}

\nomVAR

\numberstringnum{\nomVAR} 

\numberstringnum{AB23z0BN56q}
\end{document}

If you need to be able to use the string to build a command, for example, this variant does not output the result, but instead stores it in \thestring, which can be subsequently used, as shown in this MWE.

\documentclass{article}
\usepackage{tokcycle}
\newcommand\numberstringnum[1]{%
  \Characterdirective{\tctestifcatnx A##1{\addcytoks{##1}}%
    {\addcytoks[x]{\spelldigit##1}}}%
  \Groupdirective{}%
  \Macrodirective{}%
  \Spacedirective{}%
  \expandafter\tokcyclexpress\expandafter{#1}%
  \edef\thestring{\the\cytoks}%
  \expandafter\def\csname\thestring\expandafter\endcsname\expandafter
    {#1}}
\newcommand\spelldigit[1]{%
  \ifcase\numexpr`#1-`0 zero\or one\or two\or three\or four\or five\or
  six\or seven\or eight\or nine\fi}
\begin{document}
\def\nomVAR {A12Z4E}

\numberstringnum{\nomVAR}
\textbackslash\thestring=\csname\thestring\endcsname

\numberstringnum{AB23Z0BN56Q}
\textbackslash\thestring=\csname\thestring\endcsname
\end{document}