I recently came to write a regex to parse URIs. Now I wonder, did I miss something? Did I make a mistake or could I have written it cleaner? That's why I'm here.

In order to write the regex, I took the following paper as a reference http://www.ietf.org/rfc/rfc3986.txt (together with some Wikipedia articles about URIs).

    //According to http://www.ietf.org/rfc/rfc3986.txt
    private static final String URL_UNRESERVED_CHARS = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
    private static final String URL_UNRESERVED_SPECIAL_CHARS = "-._~";
    private static final String URL_UNRESERVED = URL_UNRESERVED_CHARS + URL_UNRESERVED_SPECIAL_CHARS;
    private static final String URL_RESERVED_GEN_DELIMS = ":/?#[]@";
    private static final String URL_RESERVED_SUB_DELIMS = "!$&'()*+,;=";
    private static final String URL_CHAR_ENCODING_SIGN = "%";
    
    public static final String URL_ALLOWED_CHARS = URL_UNRESERVED + URL_RESERVED_GEN_DELIMS + URL_RESERVED_SUB_DELIMS + URL_CHAR_ENCODING_SIGN;
    
    private static final String REGEX_SCHEME = "[A-Za-z][A-Za-z0-9+.-]*:"; //Also called 'protocol'
    private static final String REGEX_AUTHORATIVE_DECLARATION = "/{2}";
    private static final String REGEX_USERINFO = "(?:[A-Za-z0-9-._~]|%[A-Fa-f0-9]{2})+(?::(?:[A-Za-z0-9-._~]|[%][A-Fa-f0-9]{2})+)?@";
    private static final String REGEX_HOST = "(?:[A-Za-z0-9](?:[A-Za-z0-9-]*[A-Za-z0-9])?\\.){1,126}[A-Za-z0-9](?:[A-Za-z0-9-]*[A-Za-z0-9])?";
    private static final String REGEX_PORT = ":[0-9]+";
    private static final String REGEX_PATH = "/(?:[A-Za-z0-9-._~]|%[A-Fa-f0-9]{2})*";
    private static final String REGEX_QUERY = "\\?(?:[A-Za-z0-9-._~]+(?:=(?:[A-Za-z0-9-._~+]|%[A-Fa-f0-9]{2})+)?)(?:&|;[A-Za-z0-9-._~]+(?:=(?:[A-Za-z0-9-._~+]|%[A-Fa-f0-9]{2})+)?)*";
    //FRAGMENTs don't need to be parsed as they won't be sent to the server anyways
    
    public static final String REGEX_URL = "(?:" + REGEX_SCHEME + REGEX_AUTHORATIVE_DECLARATION + ")?(?:" + REGEX_USERINFO + ")?" + REGEX_HOST + "(?:" + REGEX_PORT + ")?(?:" + REGEX_PATH + ")*(?:" + REGEX_QUERY + ")?";

The above code snippet is written in java (which explains for the \\ as they need to be escaped in Strings in order to be taken as literals). I split the regex up into multiple parts in order to increase readability and maintenance (although it technically shouldn't require any).

That would be the regex in its true form as when encountered in the wild:

(?:[A-Za-z][A-Za-z0-9+.-]*:/{2})?(?:(?:[A-Za-z0-9-._~]|%[A-Fa-f0-9]{2})+(?::(?:[A-Za-z0-9-._~]|[%][A-Fa-f0-9]{2})+)?@)?(?:[A-Za-z0-9](?:[A-Za-z0-9-]*[A-Za-z0-9])?\.){1,126}[A-Za-z0-9](?:[A-Za-z0-9-]*[A-Za-z0-9])?(?::[0-9]+)?(?:/(?:[A-Za-z0-9-._~]|%[A-Fa-f0-9]{2})*)*(?:\?(?:[A-Za-z0-9-._~]+(?:=(?:[A-Za-z0-9-._~+]|%[A-Fa-f0-9]{2})+)?)(?:&|;[A-Za-z0-9-._~]+(?:=(?:[A-Za-z0-9-._~+]|%[A-Fa-f0-9]{2})+)?)*)?

TL;DR Regex above does validate a URI against the defined rules in http://www.ietf.org/rfc/rfc3986.txt | I couldn't break it yet, is it breakable? Can some of it be rewritten in a cleaner fashion?