REGEXP_SUBSTR() select before string

Question

I selected string with uppercase RED and this works good

SQL> WITH DATA AS
  2   ( SELECT 'we saw a RED apple on the big tree' str FROM dual)
  3  SELECT str,
  4  REGEXP_SUBSTR(str, '[^(RED]+') before_str
  5  FROM data;
STR                                BEFORE_STR
---------------------------------- ----------------------------------------------------------------------------------------------------------------------------------------
we saw a RED apple on the big tree we saw a

But when I selected with lowercase, I do not get the results I want

SQL> WITH DATA AS
  2  ( SELECT 'we saw a red apple on the big tree' str FROM dual)
  3  SELECT str,
  4  REGEXP_SUBSTR(str, '[^(red]+') before_str
  5  FROM data;

STR                                BEFORE_STR
---------------------------------- ----------------------------------------------------------------------------------------------------------------------------------------
we saw a red apple on the big tree w

How can I get results that I would not use in capital letters?

If I wanted to use the function on some rows in the table, I would not get the desired results

Answer 1

You need to specify 'i' for case-insensitive matching. Having said that, your REGEXP is incorrect... [^(RED]+ will match all characters until one of ( , R , E , D is found.

You could use REGEXP_INSTR to locate the position of match and SUBSTR to extract the substring:

WITH DATA AS( 
    SELECT 'we saw a red apple on the big tree' str FROM dual UNION ALL
    SELECT 'we saw a RED apple on the big tree' str FROM dual UNION ALL
    SELECT 'we saw something' str FROM dual
)
SELECT str, SUBSTR(str, 1, REGEXP_INSTR(str, 'RED', 1, 1, 0, 'i') - 1) before_str
FROM data;

Result:

| STR                                | BEFORE_STR |
|------------------------------------|------------|
| we saw a red apple on the big tree | we saw a   |
| we saw a RED apple on the big tree | we saw a   |
| we saw something                   | NULL       |

Answer 2

Make sure to add a space before and after your keyword else you'll get BEFORE_STR returned where it is only PART of a word. Here a capture group is used to get the first part where all characters are followed by the case-insensitive keyword surrounded by spaces. Note REGEXP_SUBSTR returns the original string if the match is not found.

SQL> WITH DATA(str) AS(
       SELECT 'we saw a red apple on the big tree' FROM dual UNION ALL
       SELECT 'we saw a RED apple on the big tree' FROM dual UNION ALL
       SELECT 'we saw something'                   FROM dual UNION ALL
       SELECT 'we saw a redbird on the big tree'   FROM dual
   )
   SELECT str, REGEXP_SUBSTR(str, '^(.*?)( RED )', 1, 1, 'i', 1) before_str
   FROM data;

STR                                BEFORE_STR
---------------------------------- ----------------------------------
we saw a red apple on the big tree we saw a
we saw a RED apple on the big tree we saw a
we saw something
we saw a redbird on the big tree

SQL>