convert from sql to presto

This what I converted to but I am getting below

error - line 8:8: mismatched input 'else'. Expecting: ',',

SELECT 
  regdate, 
  date_format(regdate, '%Y-%m') as YearMonth, 
  date_format(regdate, '%m') as RegMonth, 
  date_format(regdate, '%Y') as RegYear --
  , 
  case when cast(
    year(RegDate) as varchar
  ) 
  and case when len(
    cast(
      month(RegDate) as varchar
    )
  ) = 1 then concat(
    '0', 
    cast(
      month(RegDate) as varchar
    ) else cast(
      month(RegDate) as varchar
    ) end < concat('20', code, '01') then 'Jan' when cast(
      year(RegDate) as varchar
    ) concat(
      case when len(
        cast(
          month(RegDate) as varchar
        )
      ) = 1 then concat(
        '0', 
        cast(
          month(RegDate) as varchar
        ) else cast(
          month(RegDate) as varchar
        ) end < concat('20', code, '12') then 'Dec' else Convert(
          varchar(3), 
          RegDate, 
          0
        ) end EditionMonth 
        FROM 
          details