test.nf

params.index = 'testfile2.txt'

Channel
    .fromPath(params.index)
    .splitCsv(header:true, sep:"\t")
    .map{ row-> tuple(row.head1, row.head2, row.head4) }
    .into { samples_ch }

process transform {
  input:
  set head1, head2 from samples_ch

  output:
  set val { head1  - ~/\.(\d+)/ }, val( head1 ) into transformed

  script:
  """
  """
}

transformed
  .groupTuple()
  .set {tubbled}

  process receive_tuple {
    echo true

    input:
    set trans, org from tubbled

    script:
    """
    echo " ------------ "
    echo ${trans}
    echo ${org}  | sed  's/\\[/-I /g; s/\\]//g; s/,/ -I/g'
    """
  }