How to write a definition with variants?

A listofitems approach. Also, listofitems is available in Plain TeX.

\documentclass{article}
\usepackage{listofitems}
\def\variant#1.{%
  \setsepchar{is||has}%
  \greadlist*\varinput{#1}
  \varinput[1] & \varinputsep[1] & \varinput[2].
}
\begin{document}
\begin{tabular}{|c|c|c|}
\variant Jim Jones is a fictional character.\\
\variant Tim Smith has two brothers.
\end{tabular}
\end{document}

Similarly done in plain TeX

\input listofitems
\def\variant#1.{%
  \setsepchar{is||has}%
  \greadlist*\varinput{#1}
  \varinput[1] \& \varinputsep[1] \& \varinput[2].
}
\variant Jim Jones is a fictional character.

\variant Tim Smith has two brothers.

\bye

The \replacestrings macro from OPmac can be used:

%from OPmac code:
\bgroup \catcode`!=3 \catcode`?=3
\gdef\replacestrings#1#2{\long\def\replacestringsA##1#1{\def\tmpb{##1}\replacestringsB}%
   \long\def\replacestringsB##1#1{\ifx!##1\relax \else\addto\tmpb{#2##1}%
      \expandafter\replacestringsB\fi}%     improved version <May 2016> inspired 
   \expandafter\replacestringsA\tmpb?#1!#1% 
   \long\def\replacestringsA##1?{\def\tmpb{##1}}\expandafter\replacestringsA\tmpb
}
\egroup
\long\def\addto#1#2{\expandafter\def\expandafter#1\expandafter{#1#2}}

%variant definition:
\def\variant #1.{\def\tmpb{#1}%
  \replacestrings{ is }{ \tabsep is \tabsep }%
  \replacestrings{ has }{ \tabsep has \tabsep }%
  \tmpb
}
\def\tabsep{&}

%% test:
\halign{#\hfil\vrule\strut\ &#\hfil\vrule\ &# \hfil\cr
%
  \variant Jim Jones is a fictional character. \cr
  \variant Tim Smith has two brothers. \cr
}

\end

With expl3, of course. ;-)

\input expl3-generic

\ExplSyntaxOn

\cs_new_protected:Npn \3 #1.
 {
  \tl_set:Nx \l_tmpa_tl { \tl_trim_spaces:n { #1 } }
  \regex_replace_once:nnN { \s*(is|has)\s* } { \cT\& \1 \cT\& } \l_tmpa_tl
  \l_tmpa_tl. \cr
 }

\ExplSyntaxOff

\halign{#\hfil&\ \hfil#\hfil\ &# \hfil\cr
  \3 Jim Jones is a fictional character and is funny.
  \3 Tim Smith has two brothers.
}

\bye