/^[\da-f][\da-f]:
  [\da-f][\da-f]:
  [\da-f][\da-f]:
  [\da-f][\da-f]:
  [\da-f][\da-f]:
  [\da-f][\da-f]$/ix
[download]

Or with less typing:

/^
  (?:
     [\da-f]{2}:
   ){5}
  [\da-f]{2}
$/ix;
[download]

I used the /x modifier to allow for freeform whitespace within the RE. I used the /i modifier so that hex digits can be represented in either lower or upper case. And in the second example, quantifiers were used to factor out the unnecessary repetition.

Update: Fixed quantifier on last term.