How to extract date from text string - sql

Question

I need to extract the date (08-01-2021) from the below string that has no whitespace
select 'Date-08-01-2021-Trans-1000008-PH.0000-BA-CR-9999.21' from dual
I tried to apply the REGEXP_SUBSTR function as shown below, but using this query I just removed 'Date-'
with x as
(select 'Date-08-01-2021-Trans-1000008-PH.0000-BA-CR-9999.21' as str
from dual)
SELECT REGEXP_SUBSTR(STR, 'Date-([^ ]+)',1,1,'i',1)
FROM x;
Please advise

Answer 1

You are zero-padding the date values so each term has a fixed length and have a fixed prefix so you do not need to use (slow) regular expressions and can just use simple string functions:
SELECT TO_DATE(SUBSTR(value, 6, 10), 'DD-MM-YYYY')
FROM table_name;
(Note: if you still want it as a string, rather than as a date, then just use SUBSTR without wrapping it in TO_DATE.)
For example:
WITH table_name ( value ) AS (
SELECT 'Date-08-01-2021-Trans-1000008-PH.0000-BA-CR-9999.21' FROM DUAL
)
SELECT TO_DATE(SUBSTR(value, 6, 10), 'DD-MM-YYYY') AS date_value
FROM table_name;
Outputs:
DATE_VALUE
08-JAN-21
db<>fiddle here
If the Date- prefix is not going to always be at the start then use INSTR to find it:
WITH table_name ( value ) AS (
SELECT 'Date-08-01-2021-Trans-1000008-PH.0000-BA-CR-9999.21' FROM DUAL UNION ALL
SELECT 'Trans-1000008-Date-08-02-2021-PH.0000-BA-CR-9999.21' FROM DUAL
)
SELECT TO_DATE(SUBSTR(value, INSTR(value, 'Date-') + 5, 10), 'DD-MM-YYYY') AS date_value
FROM table_name;
Which outputs:
DATE_VALUE
08-JAN-21
08-FEB-21
If you can have multiple Date- substrings and you want to find the one that is either at the start of the string or has a - prefix then you may need regular expressions:
WITH table_name ( value ) AS (
SELECT 'Date-08-01-2021-Trans-1000008-PH.0000-BA-CR-9999.21' FROM DUAL UNION ALL
SELECT 'TransDate-1000008-Date-08-02-2021-PH.0000-BA-CR-9999.21' FROM DUAL
)
SELECT TO_DATE(
REGEXP_SUBSTR(value, '(^|-)Date-(\d\d-\d\d-\d{4})([-.]|$)', 1, 1, 'i', 2),
'DD-MM-YYYY'
) AS date_value
FROM table_name;
db<>fiddle here

Answer 2

Just use a more precise regular expression:
SELECT REGEXP_SUBSTR(STR, 'Date-([0-9]{2}-[0-9]{2}-[0-9]{4})', 1, 1, 'i', 1)
FROM x;
Or for less accuracy but more conciseness:
SELECT REGEXP_SUBSTR(STR, 'Date-([-0-9]{10})', 1, 1, 'i', 1)