The pattern you are trying to match. For more information about regular expression syntax, see Regular expressions overview.
The offset into expression at which to start searching. start-offset is expressed as a positive integer, and reflects the number of characters to count when starting from the left side of the
string. The default is 1 (the start of the string).
For multiple matches within expression, specify an integer indicating the occurrence to locate. For example, 3 finds the third occurrence. The default is 1.
The escape character to use for regular-expression. The default is the backslash character (\).
REGEXP_SUBSTR returns NULL if regular-expression is not found.
Similar to the REGEXP search condition, the REGEXP_SUBSTR function uses code points for matching and range evaluation. This
means that database case sensitivity does not impact results. For more information on how REGEXP_SUBSTR performs matching
and set evaluation, see LIKE, REGEXP, and SIMILAR TO: Differences in character comparisons.
When matching against a character class that contains only a sub-character class, include the outer square brackets and the
square brackets for the sub-character class (for example, REGEXP_SUBSTR (expression, '[[:digit:]]')). For more on sub-character class matching, see Regular expressions: Special sub-character classes.
If start-offset is specified, that offset specifies the start of the expression to be matched. In particular, ^ matches the beginning of
the expression starting at start-offset.
Vendor extension. The corresponding function in the SQL/2008 standard is the SUBSTRING_REGEX function, which has similar
parameters. SUBSTRING_REGEX is part of optional SQL/2008 language feature F844.