Regex for UK registration number

Question

I've been playing with creating a regular expression for UK registration numbers but have hit a wall when it comes to restricting overall length of the string in question. I currently have the following:

^(([a-zA-Z]?){1,3}(\d){1,3}([a-zA-Z]?){1,3})

This allows for an optional string (lower or upper case) of between 1 and 3 characters, followed by a mandatory numeric of between 1 and 3 characters and finally, a mandatory string (lower or upper case) of between 1 and 3 characters.

This works fine but I then want to apply a max length of 7 characters to the entire string but this is where I'm failing. I tried adding a 1,7 restriction to the end of the regex but the three 1,3 checks are superseding it and therefore allowing a max length of 9 characters.

Examples of registration numbers that need to pass are as follows:

• A1
• AAA111
• AA11AAA
• A1AAA
• A11AAA
• A111AAA

In the examples above, the A's represents any letter, upper or lower case and the 1's represent any number. The max length is the only restriction that appears not to be working. I disable the entry of a space so they can be assumed as never present in the string.

Answer 1

If you know what lengths you are after, I'd recommend you use the .length property which some languages expose for string length. If this is not an option, you could try using something like so: ^(?=.{1,7})(([a-zA-Z]?){1,3}(\d){1,3}([a-zA-Z]?){1,3})$, example here.