swap is not a filesystem, it's not mounted, it's activated or deactivated (see man swapon). A mountpoint does not exist. In /etc/fstab the mountpoint is specified as none. Since swap is not mounted, I'd not expect the swap partition to appear in the output of mount or findmnt.
mount | grep sdb gives no output because the string sdb does not appear in the mount-output. Remember, /dev/sdb is your swap, it is not mounted.
mount | grep sd? gives no output because the string sd? does not appear in your mount-output. A drive named sd? does not exist, as simple as that. If you want grep to treat sd? as an extended regex, you'd use mount | grep -E sd? and your output wouldn't be empty but it probably wouldn't be what you expect since the meaning of the ? is very different in regex and globbing.
mount | grep "/dev/sd" is what gives you the output you desire here.
Some more details:
From info grep chapter 2.4 'grep' programms:
’-G’
‘--basic-regexp’
Interpret patterns as basic regular expressions (BREs). This is
the default.
From info grep chapter 3.6 Basic vs Extended Regular Expressions:
Basic vs Extended Regular Expressions
In basic regular expressions the meta-characters ?, +, {, |, (, and ) lose
their special meaning; instead use the backslashed versions
\?, \+, \{, \|, \(, and \).
Portable scripts should avoid the following constructs, as POSIX says
they produce undefined results:
...
• Basic regular expressions that use ‘\?’, ‘\+’, or ‘\|’.
...
But even when I use \? it does not work as expected for me, mount | grep sd\? still gives empty output which definitely shouldn't be the case. It only gives me expected output with the -E-option, with or without the backslash. Probably this is a bug.
