name of the subexpression
pointer to location where to store the start position, or %NULL
pointer to location where to store the end position, or %NULL
%TRUE if the position was fetched, %FALSE otherwise. If the position cannot be fetched, @start_pos and @end_pos are left unchanged.
2.14
Retrieves the position in bytes of the capturing parentheses named @name.
If @name is a valid sub pattern name but it didn't match anything (e.g. sub pattern "X", matching "b" against "(?P<X>a)?b") then @start_pos and @end_pos are set to -1 and %TRUE is returned.