====== Non-unique Elements ======
{{ :markow:pasted:20170905-223117.png?300}}
You are given a non-empty list of integers (X). For this task, you should return a list consisting of only the non-unique elements in this list. To do so you will need to remove all unique elements (elements which are contained in a given list only once). When solving this task, do not change the order of the list. Example: ''[1, 2, 3, 1, 3]'' ''1'' and ''3'' non-unique elements and result will be ''[1, 3, 1, 3]''.

== Examples ==
^ Input ^ Output |
| ''[1, 2, 3, 1, 3]'' | ''[1, 3, 1, 3]'' |
| ''[1, 2, 3, 4, 5]'' | ''[]'' |
| ''[5, 5, 5, 5, 5]'' | ''[5, 5, 5, 5, 5]'' |
| ''[10, 9, 10, 10, 9, 8]'' | ''[10, 9, 10, 10, 9]'' |

== Solution ==
<code text>
/ζ(\d+)(.*)ζ\1/ζ$1$2λ$1/
/ζ(\d+)(.*)λ\1/λ$1$2λ$1/
/ζ(\d+)(, )?//
/, ]/]/
/[ζλ]//g !
/(\d+)/ζ$1/g 
</code>

== Reference ==
  * https://js.checkio.org/mission/non-unique-elements/