c:\@Work\Perl\monks\OldChamp>perl -wMstrict -le
"use 5.010;
 ;;
 my %replacement = (
   'foo' => 'Black to move',
   'bar' => 'White to move',
   'zot' => 'Green to quit',
   );
 ;;
 my ($string2) =
   map  qr{ \b (?: $_) \b }xms,
   join ' | ',
   map  quotemeta,
   reverse sort
   keys %replacement
   ;
 print 'regex: ', $string2;
 ;;
 my $s = 'Event ? followed by some text and then foo';
 print qq{before: '$s'};
 ;;
 $s =~ s{ Event \s+ \K [?] (?= .*? ($string2)) }{$replacement{$1}}xms;
 print qq{after:  '$s'};
"
regex: (?msx-i: \b (?: zot | foo | bar) \b )
before: 'Event ? followed by some text and then foo'
after:  'Event Black to move followed by some text and then foo'