[php-src] Issue #9970: mb_ereg_search_pos fails test "start of string" when using mb_ereg_search_setpos

Issue: https://github.com/php/php-src/issues/9970 Author: dashiad ### Description The following code: ```php <?php mb_ereg_search_init("qaaoiuu"); mb_ereg_search_setpos(1); var_dump(mb_ereg_search_pos("^a{2}(..)u{2}")); ``` Resulted in this output: ``` bool(false) ``` But I expected this output instead: ``` array(2) { [0]=> int(1) [1]=> int(6) } ``` The "^" modifier is not applying to the start of the whole string either (independently of "setpos"), as : ```php <?php mb_ereg_search_init("aaoiuu"); mb_ereg_search_setpos(1); var_dump(mb_ereg_search_pos("^a{2}(..)u{2}")); ``` Returns false too (this should be expected). ``` bool(false) ``` Does "^" have a different meaning in the mb_ereg_search functions? ### PHP Version PHP 8.1.12 and previous versions ### Operating System _No response_