Use of "/" slash in mask

Hello all,

Just found out that the slash operator cannot be used solo in a mask.
hashcat -m 0 -a 6 dict.txt ?d\/  -> works
hashcat -m 0 -a 6 dict.txt \/?d  -> works
hashcat -m 0 -a 6 dict.txt \/        -> doesn't work
hashcat -m 0 -a 6 dict.txt '/'     -> doesn't work

I know adding a slash operato can be done in many different ways. Just wanted to point out / ask here why it is not accepted even with the backslash prior to it.