@ucheng for ACF 5 you need to use the acf-validate_value filter. I wrote a plugin called Validated Field that wraps other field types to provide server side validation, uniqueness queries, etc.
It has just been updated to support ACF 5, could use some help testing it if you want to give the latest beta version a try: https://wordpress.org/support/topic/validated-field-34-beta
Edit: ACF 5 Support and many new features available from https://wordpress.org/plugins/validated-field-for-acf/
