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}}