how to extract variable lengh of strings from file in datasatage using substring or index function

Question 1

if its in pl/sql then you can do this:

SQL> declare
  2    v_str varchar2(2000) := 'start1
  3  david@gmail.com
  4  david@gmail.com
  5  david@gmail.com
  6  david@gmail.com
  7  david@gmail.com
  8  david@gmail.com
  9  david@gmail.com
 10  start2';
 11    v_newstr varchar2(2000);
 12    v_start_delim varchar2(10) := 'start1';
 13    v_end_delim varchar2(10) := 'start2';
 14  begin
 15    v_newstr := substr(v_str, instr(v_str, v_start_delim)+length(v_start_delim),
 16                       instr(v_str, v_end_delim) - instr(v_str, v_start_delim) - length(v_start_delim));
 17
 18    -- remove leading/trailing breaks.
 19    v_newstr := trim(both chr(10) from v_newstr);
 20    dbms_output.put_line(v_newstr);
 21
 22  end;
 23  /
david@gmail.com
david@gmail.com
david@gmail.com
david@gmail.com
david@gmail.com
david@gmail.com
david@gmail.com

PL/SQL procedure successfully completed.

or if you had the string in SQL (assuming the string is all in one row)

SQL> select trim(both chr(10) from
 2           substr(str, instr(str, 'start1')+length('start1'),
 3                instr(str, 'start2') - instr(str, 'start1') - length('start1'))
 4         ) newstr
 5    from data;

NEWSTR
-----------------------
david@gmail.com
david@gmail.com
david@gmail.com
david@gmail.com
david@gmail.com
david@gmail.com
david@gmail.com

Question 2

First of all you must use an order by. By the rules of SQL, if you do not use an order by, then a database can return the results in any order, even different than the last time you did the same query.

Assuming you do an order by id, and the field name you are querying is name

select name
from table1
where id > (select id from table1 where name = 'start1')
  and id < (select id from table1 where name = 'start2')
order by id

Question 3

I'm guessing here as it is impossible to understand what is required:

SELECT TRIM(SUBSTR(str, start1+1, btwn_starts)) final_str
  FROM
  (
  SELECT 'start1 '||' some data in between '||' start2' str
        , Length('start1') start1
        , Length('start2') start2 
        , Length(' some data in between ') btwn_starts
    FROM dual
  )
 /

Output string: some data in between