How to use External tables to load a file which has multiple record formats  and requires Multi-table insert

I have an input  file with format as below
A header record with fixed position Fields (Header1)
A Sub header record with fixed position fields (Sub Header1)
A Detail record with fixed position fields (Multiple Detail records)
A sub trailer record with fixed position fields (Sub Trailer1)
A Sub header record with fixed position fields (Sub Header2)
A Detail record with fixed position fields (Multiple Detail records)
A sub trailer record with fixed position fields (Sub Trailer2)
A trailer record with fixed position Fields (Trailer1)
The above all repeats for another header : trailer record.  I want all the headers and trailers to go into a table, sub headers and subtrailers go into another table and all detail into another table. Total 3 tables.

Example:

H234234TEST  ojrfsj234242434302{sfl;mk;  (Header record)
S32wrsfwr23424 234242423 23423424234 (Sub Header record 1 )
D32wrsfwr23424 234242423 234234242342324 (Detail record)
X342342 223424v 452353453535355  (Sub trailer record1)
S5645764357646457457 (Sub Header record 2)
D32wrsfwr23424 234242423 234234242342324 (Detail record)
X342342 223424v 452353453535355  (Sub trailer record2)
T234252 43523346 56346435734747 (Trailer record)
Another Header to trailer batch would start ..like wise about 20 to 30 header to trailer in one file.

The first character represents whether it is a header or trailer or sub header or sub trailer or detail record.
All the header records are of fixed position format.
Like wise all the trailer records are of fixed position format.
Like wise all the sub header records are of fixed position format.
Like wise all the sub trailer records are of fixed position format.
Like wise all the detail records are of fixed position format.

In addition to above, in the data there might be some fields, where I would like to call a function to transform the data while loading.

Bascially all the header and trailer records need to go into a table,
Sub header and Sub trailer into another table
and Detail into another table.

I would need to create external tables to load such kind of data , basically the multi record format file to be loaded into multiple tables.  [Oracle database]
Could some one please let me know how to go about this?

Thanks a lot.