How to extract number from a string

I've a string like 'intercompany creditors {DEMO[[1]]}'. I want to extract only the numbers from the string, in example just '1'.

How to do this in Invantive SQL?

You should be able to do so with substr (get some piece of text from specific positions in the text) and instr (get the position from a specific piece of text inside some other text):

select substr
       ( d
       , instr(d, '[[') + 2
       , instr(d, ']]') - instr(d, '[[') - 2
       )
from   ( select 'intercompany creditors {DEMO[[1]]}' d 
         from dual@DataDictionary
       ) x