int pcre2_substring_number_from_name(const pcre2_code *code, PCRE2_SPTR name);
This convenience function finds the number of a named substring capturing parenthesis in a compiled pattern, provided that it is a unique name. The function arguments are:
code Compiled regular expression
name Name whose number is required
The yield of the function is the number of the parenthesis if the name is found, or PCRE2_ERROR_NOSUBSTRING if it is not found. When duplicate names are allowed (PCRE2_DUPNAMES is set), if the name is not unique, PCRE2_ERROR_NOUNIQUESUBSTRING is returned. You can obtain the list of numbers with the same name by calling pcre2_substring_nametable_scan().
There is a complete description of the PCRE2 native API in the pcre2api page and a description of the POSIX API in the pcre2posix page.