for printf format:
 - %m will print strerror(errno)

for scanf format:
 - optional 'm' can be used with string conversions
 - [ is a valid conversion specifier

literal %:
 - do not match when literal % is used with other
   flags

known bug: the following is not be hilighted properly:
printf("%ms");
the 's' it not part of the conversion specifier in
this case but is when used with scanf.