[regex] RegEx pattern any two letters followed by six numbers

Please assist with the proper RegEx matching. Any 2 letters followed by any combination of 6 whole numbers.

These would be valid: 
RJ123456
PY654321
DD321234

These would not
DDD12345
12DDD123

I depends on what is the regexp language you use, but informally, it would be:

[:alpha:][:alpha:][:digit:][:digit:][:digit:][:digit:][:digit:][:digit:]

where [:alpha:] = [a-zA-Z] and [:digit:] = [0-9]

If you use a regexp language that allows finite repetitions, that would look like:

[:alpha:]{2}[:digit:]{6}

The correct syntax depends on the particular language you're using, but that is the idea.

Everything you need here can be found in this quickstart guide. A straightforward solution would be [A-Za-z][A-Za-z]\d\d\d\d\d\d or [A-Za-z]{2}\d{6}.

If you want to accept only capital letters then replace [A-Za-z] with [A-Z].

Depending on if your regex flavor supports it, I might use:

\b[A-Z]{2}\d{6}\b    # Ensure there are "word boundaries" on either side, or

(?<![A-Z])[A-Z]{2}\d{6}(?!\d) # Ensure there isn't a uppercase letter before
                              # and that there is not a digit after

You could try something like this:

[a-zA-Z]{2}[0-9]{6}

Here is a break down of the expression:

[a-zA-Z]    # Match a single character present in the list below
               # A character in the range between “a” and “z”
               # A character in the range between “A” and “Z”
   {2}         # Exactly 2 times
[0-9]       # Match a single character in the range between “0” and “9”
   {6}         # Exactly 6 times

This will match anywhere in a subject. If you need boundaries around the subject then you could do either of the following:

^[a-zA-Z]{2}[0-9]{6}$

Which ensures that the whole subject matches. I.e there is nothing before or after the subject.

or

\b[a-zA-Z]{2}[0-9]{6}\b

which ensures there is a word boundary on each side of the subject.

As pointed out by @Phrogz, you could make the expression more terse by replacing the [0-9] for a \d as in some of the other answers.

[a-zA-Z]{2}\d{6}