The [GEDCOM X JSON format](https://github.com/FamilySearch/gedcomx/blob/master/specifications/json-format-specification.md#uri) defines URIs according to [RFC 3986](http://www.ietf.org/rfc/rfc3986.txt). The RFC provides a regex for parsing, not validating, that appears to match any string. Other regexes I've seen are enormous http://stackoverflow.com/a/17813893.