So one would have to specify the constraint like this when wishing to use the new reg-exp, right:

@URL(regexp=RFC_3987_REGEXP, regexpOnly=true)

I don't know, but this really looks too verbose to me. As a user, I'd then rather use @Pattern:

@Pattern(regexp=RFC_3987_REGEXP)

Switching between current and new behavior is something I'd want to do once for all @URL constraints, not for each single constraint instance.

How about one of the following:

• Offer a global configuration property for changing the validation behavior from current to the new reg-exp based approach
• Offer a new constraint, e.g. @Url2, @Rfc3987Url etc.