c:\@Work\Perl\monks>perl -wMstrict -le
"my $string = '/.*u\/ba$/';
 ;;
 my $flag = qr{ [msix] }xms;
 ;;
 my $convertable =
 my ($pattern, $modifiers) = $string =~ m{
   \A \s* / (.*?) / ($flag*) \z
   }xms;
 die qq{bad rx: '$string'} unless $convertable;
 ;;
 my $rx = qr{(?$modifiers)$pattern};
 print $rx;
 print 'A: match' if 'u/ba' =~ $rx;
 ;;
 my $ry = qr{ \A foo $rx }xms;
 print $ry
 ;;
 print 'B: match' if 'foolubatu/ba' =~ $ry;
 print 'C: match' if 'Foolubatu/ba' =~ $ry;
"
(?-xism:(?).*u\/ba$)
A: match
(?msx-i: \A foo (?-xism:(?).*u\/ba$) )
B: match