for my $field ( map $_, 'C/O', '2/4', 'C/2', 'd/l', '2/', '/2') {
  print "$field...";
  $field =~ s/
    (?<!C(?=.O))   # Look back a character and make sure it's not C-something-O from there
    (?<!\d(?=.\d)) # Make sure it's also not a digit-something-digit
    (?:\\|\/)      # The something is a slash or backslash
  / & /xgi;   
  print "$field\n";
}