Regular expression for UK postal codes

Question

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 ().

Answer 1

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]

Answer 2

the ? means occurs 0 or 1 times and the brackets do grouping as you might expect, modifiers will work on groups. A regex tutorial is probably the best thing here

http://www.vogella.com/articles/JavaRegularExpressions/article.html

i had a brief look and it seems reasonable also for practice/play see this applet

http://www.cis.upenn.edu/~matuszek/General/RegexTester/regex-tester.html

simple example (ab)?

means 'ab' once or not at all