> It seems as if something like *[6] works but not *[6]*

Remember that *[6]* will match anything, because *[6] 
litterally means "any number of characters equal to '6'" ; 
and 0 is included in "any number".

(the regexp equivalent would be [6]*.* ; which is identical 
to .*)

There isn't yet any '[6]+' equivalent ; but this may be 
added in the future