I simulated it:
<?php
$text = '?pfel';
mb_internal_encoding( 'utf-8' );
printf( "%d\n", mb_eregi( '?pfel', $text ) ); // Output: 0
printf( "%d\n", mb_ereg( '?pfel', mb_strtolower( $text ) ) ); // Output: 1
printf( "%d\n", mb_eregi( '?pfel', mb_strtolower( $text ) ) ); // Output: 1
?>