How to combine-concat charset in Hashcat?

[greywinged]

Example:
min password length is 6
c | charset [a-z A-Z 0-9] is equal ( l + u + d )

?c?c?c?c?c?c

Code:

- [ Built-in Charsets ] -

  ? | Charset
===+=========
  l | abcdefghijklmnopqrstuvwxyz [a-z]
  u | ABCDEFGHIJKLMNOPQRSTUVWXYZ [A-Z]
  d | 0123456789                [0-9]
  h | 0123456789abcdef          [0-9a-f]
  H | 0123456789ABCDEF          [0-9A-F]
  s |  !"#$%&'()*+,-./:;<=>?@[\]^_`{|}~
  a | ?l?u?d?s
  b | 0x00 - 0xff