documentclass{scrartcl}

usepackage{lmodern}

usepackage{array}
usepackage{booktabs}
usepackage{tabu}

% First attempt:

usepackage{xstring}
newcommand{tosfescapestep}[2]{%
    expandafterStrSubstituteexpandafter{x}{#1}{#2}[x]%
}
newcommand{tosfescapehelper}[1]{{%
        noexpandarg % suppress expansions made by xstring
        StrSubstitute{#1}{0}{oldstylenums{1}}[x]% first step
        tosfescapestep{1}{oldstylenums{1}}%
        tosfescapestep{2}{oldstylenums{2}}%
        tosfescapestep{3}{oldstylenums{3}}%
        tosfescapestep{4}{oldstylenums{4}}%
        tosfescapestep{5}{oldstylenums{5}}%
        tosfescapestep{6}{oldstylenums{6}}%
        tosfescapestep{7}{oldstylenums{7}}%
        tosfescapestep{8}{oldstylenums{8}}%
        tosfescapestep{9}{oldstylenums{9}}%
        x}}
newcommand{tosfescape}[1]{tosfescapehelper{#1}}

% Second attempt:

usepackage{xparse}
usepackage{expl3}
%usepackage{l3regex}

ExplSyntaxOn

NewDocumentCommand tosfdemo { m }%
    { tosfdemo_helper:n { #1 } }

tl_new:N l_tosfdemo_tl
cs_new:Npn tosfdemo_helper:n #1 {
    tl_set:Nn l_tosfdemo_tl {#1}
    regex_replace_all:nnN { (d+) } { c{oldstylenums}cB{ 1 cE} } l_tosfdemo_tl
    tl_use:N l_tosfdemo_tl
}

ExplSyntaxOff

begin{document}

noindent Braces -- bold, first and second attempt to texttt{oldstylenums} textbf{work}:\[medskipamount]
textbf{bold 1 23}\
tosfescape{first 1 23}\
tosfdemo{second 1 23}bigskip

noindenttexttt{textbackslash bgroup} & texttt{textbackslash egroup}  -- as in table~ref{tab:1}, bold textbf{works}, first and second attempt to texttt{oldstylenums} textbf{does not work}:\[medskipamount]
textbfbgroup bold 1 23egroup\
tosfescapebgroup first 1 23egroup\
tosfdemobgroup second 1 23egroup

begin{table}[h]
    centering
    caption{Bold textbf{works}, first and second attempt to texttt{oldstylenums} textbf{does not work}}
    label{tab:1}
    begin{tabu}{c >{textbfbgroup}c<{egroup} >{tosfescapebgroup}c<{egroup} >{tosfdemobgroup}c<{egroup}}
        %toprule
        rowfont{itshapebfseries} no change    & texttt{textbackslash textbf} & texttt{textbackslash tosfescape} & texttt{textbackslash tosfdemo} \
        midrule
                                    regular 1 23 & bold 1 23                      & first 1 23                         & second 1 23 \
        %bottomrule
    end{tabu}
end{table}

end{document}