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javaregexpostal-code

Regular expression for UK postal codes

I'm making an application that asks the user to enter a postcode and outputs the postcode if it is valid.

I found the following pattern, which works correctly:

String pattern = "^([A-PR-UWYZ](([0-9](([0-9]|[A-HJKSTUW])?)?)|([A-HK-Y][0-9]([0-9]|[ABEHMNPRVWXY])?)) [0-9][ABD-HJLNP-UW-Z]{2})";

I don't know much about regex and it would be great if someone could talk me through this statement. I mainly don't understand the ? and use of ().

Solution

  • Your regex has the following:

    • ^ and $ - anchors for indicating start and end of matching input.
    • [A-PR-UWYZ] - Any character among A to P or R to U or W,Y,Z. Characters enclosed in square brackets form a character class, which allows any of the enclosed characters and - is for indicating a sequence of characters like [A-D] allowing A,B,C or D.
    • ([0-9]|[A-HJKSTUW])? - An optional character any of 0-9 or characters indicated by [A-HJKSTUW]. ? makes the preceding part optional. | is for an OR. The () combines the two parts to be ORed. Here you may use [0-9A-HJKSTUW] instead of this.
    • [ABD-HJLNP-UW-Z]{2} - Sequence of length 2 formed by characters allowed by the character class. {2} indicates the length 2. So [ABD-HJLNP-UW-Z]{2} is equivalent to [ABD-HJLNP-UW-Z][ABD-HJLNP-UW-Z]