原公式：

if(

  len(lower(GETUSERDEPARTMENTS()))-len(substitute(GETUSERDEPARTMENTS(), ",", ""))=0,

  left(lower(GETUSERDEPARTMENTS()), 2),

  if(

    len(lower(GETUSERDEPARTMENTS()))-len(substitute(GETUSERDEPARTMENTS(), ",", ""))=1,

mid(lower(GETUSERDEPARTMENTS()),4,2),

mid(lower(GETUSERDEPARTMENTS()),find("@",substitute(lower(GETUSERDEPARTMENTS()),",","@",2))+1,100)

  )

)

效果：

想实现效果

各位大佬看看，怎么样进行修改，已更新问题描述