Sub Articulo 125.1

This is really not too difficult,
as the followin fully expandable solution shows:
\def\replacespaces #1#2{\start {#2}#1 {\replacespaces} }
\def\take #1#2 {\ifx \replacespaces#2\expandafter\stop
    \else #1#2\expandafter\take\fi{#1}}
\def\start #1#2 {\ifx \replacespaces \expandafter\stop
    \else #2\expandafter\take\fi{#1}}
\def\stop #1{}
% Some examples
\message {\replacespaces {Eins Zwei Drei Vier}_} 
\message {\replacespaces {Eins Zwei Drei Vier }_} 
\message {\replacespaces {Eins Zwei Drei Vier}{}} 
\message {[\replacespaces {Eins}{_}]} 
\message {[\replacespaces { }{_}]} 
\message {[\replacespaces { Eins }{_}]} 
\message {[\replacespaces {}{_}]} 
\end
The \ifx in \start is incomplete (I think, you don't 
want to compare \replacespaces with \expandafter).
It seems to be rubbish of a previous version, because
the "end" check is done in \take.
\def\start #1#2 {#2\take{#1}}