The (?X) flag setting is special in that it must always occur earlier in the pattern than any of the additional features it turns on,The (?X) flag setting is special in that it must always occur earlier in the pattern than any of the additional features it turns on, even when it is at top level.
L'option (?X) est particulière, car elle doit toujours intervenir avant toutes les autres options, même au niveau du masque entier.