using: http://grokdebug.herokuapp.com

input: a,b,c,d,e,f
pattern: %{WORD:x},%{WORD:x},
result:
{
  "x": [
    [
      "a",
      "b"
    ]
  ]
}

input: a,b,c,d,e,f
pattern: (?:%{WORD:x},){2}
result:
{
  "x": [
    [
      "b"
    ]
  ]
}

so something like (?:%{WORD:x},)*%{WORD:x} is a non-starter.
Thanks grok. Throk.