$field = '9\\9';
print("[$field] => ");
    $field =~ s/(?<!\d)     # not preceeded by digits
                (?:\\|\/)   # back or forward slash
                (?!\d)      # not succeeded by digits
            / & /xg;        # replace with '&' (globally)
print("[$field]\n");

$field = '9\\9';
print("[$field] => ");
    $field =~ s/
                (?<!C)      # not preceeded by "C"
                (?:\\|\/)   # back or forward slash
                (?!O)       # not succeeded by "O"
                |
                (?<!\d)     # not preceeded by digits
                (?:\\|\/)   # back or forward slash
                (?!\d)      # not succeeded by digits
            / & /xg;        # replace with '&' (globally)
print("[$field]\n");

__END__

[9\9] => [9\9]
[9\9] => [9 & 9]