no preceding characters in regexp statement - regex

Question

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;

Answer 1

regexp_substr(description,'([^A-Z]|^)[CT].{1,20}(CHEST|THOR|LUNG)',1,1,'i')
works

Answer 2

Leverage Oracle Subexrpession Parameter to Check for CT
I would leverage the use of subexpressions to use a pattern like this:
'regexp_substr(description, '(^| )((ct ).*((CHEST)|(THOR)|(LUNG)))', 1, 1,'i', 2)`
-subexpression 1 to look for beginning of line or a space: (^| )
-subexpression 3 to look for 'CT': (ct )
-allow for other characters: .*
-subexressions 5,6,7: (CHEST)|(THOR)|(LUNG)
-subexpression 2 which contain subexpression 3 an subexprssion 4
I use the last optional parameter to identify that I want subexpression 2.
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, '(^| )((ct ).*((CHEST)|(THOR)|(LUNG)))', 1, 1,'i', 2)
FROM test;