isValidReg

/**
 * Returns a boolean if the registration plate input is one of the valid four, based on regular expressions.</br>
 * The valid four registration plate types are:<ul><li>Current Style: AA## AAA</li><li>Prefix Style: A### AAA</li><li>Suffix Style: AAA ###A</li><li>Dateless Style:<ul><li>#### AA or</li><li>AA #### or</li><li>### AAA or</li><li>AAA ### or</li><li>AAA #### (NI only)</li></ul></ul>
 * @param reg - String of registration plate to test.
 * @return true if the registration plate matches one of the styles of plate, false if it doesn't.
 * @author Zion Fox <a href="http://zionfox.net">(website)</a>
 */
public static boolean isValidReg(String reg) {
    reg = reg.toUpperCase(); //Force uppercase
    
    // return true on all styles of plate
    if(reg.matches("^[A-HJ-PR-Y]{2}(?!00|01|50)[0-9]{2} ?[A-HJ-PR-Z]{3}$") || // current style plate
       reg.matches("^[A-HJ-NPR-TV-Y](?!0)[0-9]{1,3} ?[A-HJ-PR-Y]{3}$") || // prefix style plate
       reg.matches("^[A-HJ-PR-Y]{3} ?(?!0)[0-9]{1,3}[A-HJ-NPR-TV-Y]$") || // suffix style plate
       reg.matches("(^[0-9]{1,4} [A-HJ-PR-Y]{1,2}$)|(^[0-9]{1,3} [A-HJ-PR-Y]{1,3}$)|(^[A-HJ-PR-Y]{1,2} [0-9]{1,4}$)|(^[A-HJ-PR-Y]{1,3} [0-9]{1,3}$)|(^[A-HJ-PR-Y][A-PR-Z]{2} ?[0-9]{1,4}$)") // dateless style plate
       ) return true;
    return false;
    
      ////////////////
     // Disclaimer //
    ////////////////
    /*
     * The above regular expressions were authored by Zion Fox in late 2018 during an assignment for Manchester Metropolitan University
     * Although this regex function above isn't perfect, it's the best and likely most accurate one we have available. 
     * They were coined after searches on the internet were providing results, but they were too simple and would allow completely invalid registration plates to pass.
     * Example simple for current style plate: "[A-Z]{2}[0-9]{2}[A-Z]{3}"
     * This would allow illegal characters, and therefore licence plates, to pass the test, and in the real world this wouldn't be allowed.
     */

    /* Regex Definition:
     * https://dvlaregistrations.dvla.gov.uk/help/#style_reg
     * https://assets.publishing.service.gov.uk/media/62554c78d3bf7f600782fd4f/inf104-vehicle-registration-numbers-and-number-plates.pdf
     * DVLA define UK registration plates into four groups:
     *   * Current style: AA## AAA
     *   * Prefix style: A### AAA
     *   * Suffix style: AAA ###A
     *   * Dateless style #### AA or AA #### or ### AAA or AAA ### or AAA #### (NI only)
     * 
     * The following regular expression is designed to cover all of these styles, and alternatives. These are split into groups contained with parenthesis, and separated by pipes.
     * Each group starts with ^ and ends in $, these are to signify the start or end of the test string, or lines within the string.
     * 
     * Group one: Current Style
     * regex literal: ^[A-HJ-PR-Y]{2}(?!00|01|50)[0-9]{2} ?[A-HJ-PR-Z]{3}$
     * Date Range: September 2001 to September 2049
     * Current style registration plates are two letters ranging from A to Y, followed by two numbers, followed by three letters. The numbers are the year marker.
     * DVLA disallow any letters to be I or Q, but specifically the leading letters to not be Z. The numbers also cannot be 00, 01, or 50.
     * The disallowed numbers use a technique called negative lookahead (contained in a subgroup), which will check the next characters for a match, and discard the result if it does.
     * 
     * Group two: Prefix Style
     * regex literal: ^[A-HJ-NPR-TV-Y](?!0)[0-9]{1,3} ?[A-HJ-PR-Y]{3}$
     * Date range: August 1983 to August 2001
     * Prefix style registration plates are one letter, one to three numbers, and three letters. The first letter is the year marker
     * Similar to the current style of registration plates, the DVLA disallow some characters, mostly I, Q, U and Z from all characters, but also the year marker disallows O from being shown.
     * Also, the number section must not start with a 0. 
     *  
     * Group three: Suffix Style
     * regex literal: ^[A-HJ-PR-Y]{3} ?(?!0)[0-9]{1,3}[A-HJ-NPR-TV-Y]$
     * Date range: February 1963 to July 1983
     * Suffix style registration plates are three letters, one to three numbers, and one letter. The last letter is the year marker.
     * Same restrictions as the Prefix style, but the whole format is reversed, where the year marker is at the end of the plate, not the start.
     *  
     * Group four: Dateless Style
     * regex literal: (^[0-9]{1,4} [A-HJ-PR-Y]{1,2}$)|(^[0-9]{1,3} [A-HJ-PR-Y]{1,3}$)|(^[A-HJ-PR-Y]{1,2} [0-9]{1,4}$)|(^[A-HJ-PR-Y]{1,3} [0-9]{1,3}$)|(^[A-HJ-PR-Y][A-PR-Z]{2} ?[0-9]{1,4}$)
     * Date range: up to February 1963
     * Dateless style registration plates are one to three/four numbers, and three/two letters. The format can be reversed, with the letters first and numbers last. Maximum number of characters is 6, the minimum is 2.
     * Northern Ireland limitations are only three letters, followed by one to four numbers. Letter range is unknown, but likely still restricts I (for being too similar to 1), and Q (for being too similar to a dirty O), however their example shows a Z (usually disallowed for being too similar to 2).
     *     Therefore I have assumed that I and Q are disallowed, but Z is allowed, but not as the first character.
     *     
     * Since being in contact with the DVLA for information on this topic:
     *   They have confirmed to me that the letter Q is used whenever the vehicle's date cannot be determined.
     *     I have decided to disallow this to be a case, as the vast majority of vehicles on the road will have a date of manufacture.
     *   In Nothern Ireland, Z and I are used in the plate, but never as the first letter. They must be the second or third letters.
     */
}