Avoid literal `\geq`, `\\`, and `\empty` in output when using `filecontentsdef` and `expl3`

Question

Based on this answer, I am trying to define a newcases environment that automatically expands "if" into ", \quad & \text{if }" and expands "otherwise" into ", \quad & \text{otherwise}". My code and the output are included below. As you can see in the output, the result is incorrect with \geq, \\, and \empty rendered literally and fl inserted at each line break in the code.

\documentclass{article}
\usepackage{filecontentsdef}
\usepackage{amsmath}
\ExplSyntaxOn
\cs_generate_variant:Nn \tl_replace_all:Nnn {Nx}
\NewDocumentEnvironment {newcases} {} {
    % Use filecontentsdef to store everything until "\endfilecontentsdefmacro" 
    % into a macro named "\l__envcontents_tmp_tl".
    \filecontentsdefmacro \l__envcontents_tmp_tl
} {
    \endfilecontentsdefmacro
    \str_set:NV \l__envcontents_tmp_tl \l__envcontents_tmp_tl
\tl_replace_all:Nxn \l__envcontents_tmp_tl
  { \c_space_tl\tl_to_str:n{if} \c_space_tl} % Search
  {, \quad & \text{if~}}                     % Replace
\tl_replace_all:Nxn \l__envcontents_tmp_tl 
  {\tl_to_str:n{otherwise}}       % Search
  {, \quad \text{otherwise}}      % Replace

% Output the result.
\left\{\begin{array}{ll}
\l__envcontents_tmp_tl

\end{array}\right.
}
\ExplSyntaxOff
\begin{document}
Desired output:
\begin{equation}
  \begin{cases}
    a, \quad & \text{if } a \geq 0, \ 
    -a, \quad & \text{otherwise}.
  \end{cases}
\end{equation}
Actual output, using newcases environment:
\begin{equation}
  \begin{newcases}
    a if a \geq 0 \ 
    -a otherwise.
  \end{newcases}
\end{equation}
\end{document}

Answer 1

Solution without additional packages

Use a regex search and replace.

I'd not use newcases for the environment's name, because mathtools has a \newcases command.

\documentclass{article}
\usepackage{amsmath}
\ExplSyntaxOn
\tl_new:N \l__wintz_cases_tl
\NewDocumentEnvironment {autocases} {b}
  {
    \tl_set:Nn \l__wintz_cases_tl { #1 }
    % Search any control sequences immediately followed by "if".
    % If there is, insert a space between so the subsequent regex works. 
    \regex_replace_all:nnN { (\c{.*}) if } { \1\ if } \l__wintz_cases_tl
    % Search and replace "<space>if<space>" with ", \quad & \text{if }"
    \regex_replace_all:nnN { \s+if\s+ } { , \c{quad} &amp; \c{text}{if\ } } \l__wintz_cases_tl
    % Search and replace "otherwise" with ", \quad & \text{otherwise}"
    \regex_replace_all:nnN { otherwise } { , \c{quad} &amp; \c{text}{otherwise} } \l__wintz_cases_tl
    % Output the result.
    \begin{cases}
    \tl_use:N \l__wintz_cases_tl
    \end{cases}
  }{}
\ExplSyntaxOff
\begin{document}
Desired output:
\begin{equation}
  \begin{cases}
    a, \quad & \text{if } a \geq 0, \ 
    -a, \quad & \text{otherwise}.
  \end{cases}
\end{equation}
Actual output, using autocases environment:
\begin{equation}
  \begin{autocases}
    a if a \geq 0 \ 
    -a otherwise.
  \end{autocases}
\end{equation}
Actual output, using autocases environment:
\begin{equation}
  \begin{autocases}
    \infty if a \geq 0 \ 
    \oplus if a < -1 \
    0 otherwise.
  \end{autocases}
\end{equation}
\end{document}

With filecontentsdef (As in the original question)

Caveat. You should also be aware that autocases (or whatever name you prefer) cannot be used in amsmath alignment displays. For an environment that can be used with amsmathalignment displays, use the approach described above.

You want to rescan the string you get.

Don't mix strings with token lists: they're different data types. You also forgot an & and want to use cases, not array.

\documentclass{article}
\usepackage{filecontentsdef}
\usepackage{amsmath}
\ExplSyntaxOn
\str_new:N \l__envcontents_tmp_str
\cs_generate_variant:Nn \str_replace_all:Nnn {Nx}
\NewDocumentEnvironment {autocases} {} {
    % Use filecontentsdef to store everything until "\endfilecontentsdefmacro" 
    % into a string named "\l__envcontents_tmp_str".
    \filecontentsdefmacro \l__envcontents_tmp_str
} {
    \endfilecontentsdefmacro
\str_replace_all:Nxn \l__envcontents_tmp_str
  { \c_space_tl\tl_to_str:n{if} \c_space_tl} % search
  {, \quad & \text{if~}} % replace
\str_replace_all:Nxn \l__envcontents_tmp_str
  {\tl_to_str:n{otherwise}} % search
  {, \quad & \text{otherwise}} % replace

% Output the result.
\begin{cases}
\tl_rescan:nV {} \l__envcontents_tmp_str
\end{cases}

}
\ExplSyntaxOff
\begin{document}
Desired output:
\begin{equation}
  \begin{cases}
    a, \quad & \textup{if } a \geq 0, \ 
    -a, \quad & \textup{otherwise}.
  \end{cases}
\end{equation}
Actual output, using autocases environment:
\begin{equation}
  \begin{autocases}
    a if a \geq 0 \ 
    -a otherwise.
  \end{autocases}
\end{equation}
\end{document}