REGEXP_SUBSTR(expr, pat[, pos[, occurrence[, match_type]]])

Returns the substring of the string expr that matches the regular
expression specified by the pattern pat, NULL if there is no match. If
expr or pat is NULL, the return value is NULL.

REGEXP_SUBSTR() takes these optional arguments:

o pos: The position in expr at which to start the search. If omitted,
  the default is 1.

o occurrence: Which occurrence of a match to search for. If omitted,
  the default is 1.

o match_type: A string that specifies how to perform matching. The
  meaning is as described for REGEXP_LIKE().

The result returned by this function uses the character set and
collation of the expression searched for matches.

For additional information about how matching occurs, see the
description for REGEXP_LIKE().

URL: https://dev.mysql.com/doc/refman/8.4/en/regexp.html

