T SQL : Filename with date string derivation with simple logic

DECLARE
 @Filename varchar(100)
 ,@FilenameSmall varchar(100)
 ,@BatchNameType varchar(50)

SET @BatchNameType = SPACE(0)
SET @Filename = '05 May 2017_FP.xlsx'
SET @Filename = '05 May 2017_FW.xlsx'
SET @Filename = '05 May 2017_HA.xlsx'
SET @Filename = '05 May 2017_WHO.xlsx'
SET @Filename = 'GENERICS-20170531_152713.xlsx'

SET @FilenameSmall = LOWER(@Filename)

SET @BatchNameType += CASE WHEN PATINDEX('%_fw.xlsx', @FilenameSmall) > 0 THEN 'FINALWAY-' ELSE SPACE(0) END
SET @BatchNameType += CASE WHEN PATINDEX('%_fp.xlsx', @FilenameSmall) > 0 THEN 'FINALWAY-' ELSE SPACE(0) END
SET @BatchNameType += CASE WHEN PATINDEX('%_ha.xlsx', @FilenameSmall) > 0 THEN 'HOMEALONE-' ELSE SPACE(0) END
SET @BatchNameType += CASE WHEN PATINDEX('%_who.xlsx', @FilenameSmall) > 0 THEN 'WHO-' ELSE SPACE(0) END

IF( LEN(@BatchNameType) > 0)
 BEGIN

  SELECT [@Filename] = @Filename, [@FilenameSmall] = @FilenameSmall,
   [BatchName] = @BatchNameType  + SUBSTRING(@FilenameSmall,8,4) + 
   CASE SUBSTRING(@FilenameSmall,4,3)
    WHEN 'jan' THEN '01'
    WHEN 'feb' THEN '02'
    WHEN 'mar' THEN '03'
    WHEN 'apr' THEN '04'
    WHEN 'may' THEN '05'
    WHEN 'jun' THEN '06'
    WHEN 'jul' THEN '07'
    WHEN 'aug' THEN '08'
    WHEN 'sep' THEN '09'
    WHEN 'oct' THEN '10'
    WHEN 'nov' THEN '11'
    WHEN 'dec' THEN '12' end
   + SUBSTRING(@FilenameSmall,1,2)
 END
ELSE
 BEGIN
  SELECT [@Filename] = @Filename, [@FilenameSmall] = @FilenameSmall,
   [BatchName] = LEFT(@Filename, PATINDEX('%.xlsx', @FilenameSmall) - 1)
 END