So I have attempted to use a negative look back in a regexp statement and have looked online at other solutions but they don't seem to work for me so obviously I am doing something wrong-

I am looking for a return on the first line but the others should be null. Essentially I need CT CHEST or CT LUNG
Any assistance TIA 

```
with test (id, description) as (
  select 1, 'CT CHEST HIGH RESOLUTION, NO CONTRAST'                         from dual union all --want this
  select 2, 'INJECTION, THORACIC TRANSFORAMEN EPIDURAL, NON NEUROLYTIC W IMAGE GUIDANCE.'                 from dual union all   --do not want this
  select 3, 'The cow came back. But the dog went for a walk'    from dual) --do not want this 
  select id, description, regexp_substr(description, '(?<![a-z]ct).{1,20}(CHEST|THOR|LUNG)',1,1,'i') from test;

```