>perl -wMstrict -le
"my $s = 'a X bar XX c XXX d XXXX e XXXXX foo XXXXXX g';
 ;;
 my @tokens = qw(a bar c d e foo g);
 my ($token) = map qr{ \b $_ \b }xms, join q{|}, @tokens;
 my $max = 3;
 ;;
 while ($s =~ m{ ( .{0,$max} $token) (?= (.{0,$max})) }xmsg) {
   print qq{'$1$2'};
   }
"
'a X '
' X bar XX'
'XX c XX'
'XX d XX'
'XX e XX'
'XX foo XX'
'XX g'