Mew-EX (BW: Dragons Exalted 46/124, 120/124; BW: Legendary Treasures RC24/RC32; XY: Black Star Promos XY126) is legal for Expanded and Standard Format play, the latter due to that final promo release.
Mew-ex (EX: Legend Maker 88/92) and Mew-ex (EX: Holon Phantoms 100/110) are only legal for Unlimited Format play.
Almost no one would mistake you as having meant that older card but I like being precise about such things. If you don't care, don't sweat it and happy gaming.