[texhax] (no subject)

[Philip TAYLOR]

Zalman Rubinstein wrote:

> Dear Sirs,
>  
> I am a TUG member and have a technical question in Plain TeX programming.

OK, after clarification from Alex Scorpan, here is what
I think you want :

% Split-XY.tex

\input XY-Preamble

\newtoks \t
\newtoks \xpart
\newtoks \ypart
\newtoks \leftpart
\newtoks \rightpart
\newif \ifmoretodo

\t={ab...xy.....cd....xy....ef....xy....}

Original text : \the \t

\t = \expandafter {\the \t xy} % append a sentinel

\def \Split  #1 on #2#3
     {\def \parse ##1#2#3##2\sentinel
         {\leftpart = {##1}%
          \rightpart = {##2}%
         }%
      \def \pivot {#2#3}%
      \parse #1\sentinel
       Whole : [#1]\endgraf
       Left part : [\the \leftpart]\endgraf
       Right part : [\the \rightpart]\endgraf
      \edef \head {\the \leftpart}
      \edef \tail {\the \rightpart}
      \xpart = \expandafter \expandafter \expandafter \expandafter \expandafter \expandafter \expandafter {\expandafter \expandafter \expandafter \the \expandafter \xpart \the \leftpart}%
      \ypart = \expandafter \expandafter \expandafter \expandafter \expandafter \expandafter \expandafter {\expandafter \expandafter \expandafter \the \expandafter \ypart \the \leftpart}%
      \ifx \tail \empty
         \moretodofalse
          More to do : false\endgraf
      \else
         \xpart = \expandafter {\the \xpart #2}
         \ypart = \expandafter {\the \ypart #3}
         \moretodotrue
          More to do : true\endgraf
      \fi
       X part : \the \xpart \endgraf
       Y part : \the \ypart \endgraf
     }

\count 255 = 0
\loop
     \advance \count 255 by 1
     \expandafter \expandafter \expandafter \Split \expandafter {\the \t} on xy
     \ifnum \count 255 > 255 \moretodofalse \fi % catch runaways
\ifmoretodo
     \t = \rightpart
\repeat


\end