By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
WITH yourTable AS (
SELECT '[<TV<standard#21.0#false#21.36#EUR>VT>]' AS Tax UNION ALL
SELECT '[<TV<standard#21.0#false#7.21#EUR>VT>]' UNION ALL
SELECT '[<TV<standard#17.0#false#5.17#EUR>VT>]'
)
SELECT
SUBSTRING(Tax,
CHARINDEX('#', Tax, PATINDEX('%[0-9]#%', Tax) + 3) + 1,
CHARINDEX('#', Tax, CHARINDEX('#', Tax, PATINDEX('%[0-9]#%', Tax) + 3) + 1) -
CHARINDEX('#', Tax, PATINDEX('%[0-9]#%', Tax) + 3) - 1) AS val
FROM yourTable;
| val |
|---|
| 21.36 |
| 7.21 |
| 5.17 |