Question

I have the need to check whether strings adhere to a particular ID format.

The format of the ID is as follows:

aBcDe-fghIj-KLmno-pQRsT-uVWxy

A sequence of five blocks of five letters upper case or lower case, separated by one dash.

I have the following regular expression that works:

string idFormat = "[a-zA-Z]{5}[-]{1}[a-zA-Z]{5}[-]{1}[a-zA-Z]{5}[-]{1}[a-zA-Z]{5}[-]{1}[a-zA-Z]{5}";

Note that there is no trailing dash, but the all of the blocks within the ID follow the same format. Therefore, I would like to be able to represent this sequence of four blocks with a trailing dash inside the regular expression and avoid the duplication.

I tried the following, but it doesn’t work:

string idFormat = "[[a-zA-Z]{5}[-]{1}]{4}[a-zA-Z]{5}";

How do I shorten this regular expression and get rid of the duplicated parts?

What is the best way to ensure that each block does also not contain any numbers?

---

Edit:

Thanks for the replies, I now understand the grouping in regular expressions.

I’m running a few tests against the regular expression, the following are relevant:

Test 1: aBcDe-fghIj-KLmno-pQRsT-uVWxy
Test 2: abcde-fghij-klmno-pqrst-uvwxy

With the following regular expression, both tests pass:

^([a-zA-Z]{5}-){4}[a-zA-Z]{5}$

With the next regular expression, test 1 fails:

^([a-z]{5}-){4}[a-z]{5}$

Several answers have said that it is OK to omit the A-Z when using a-z, but in this case it doesn’t seem to be working.

Answer 1

If you can set regex options to be case insensitive, you could replace all [a-zA-Z] with just plain [a-z]. Furthermore, [-]{1} can be written as -.

Your grouping should be done with (, ), not with [, ] (although you’re correctly using the latter in specifying character sets.

Depending on context, you probably want to throw in ^...$ which matches start and end of string, respectively, to verify that the entire string is a match (i.e. that there are no extra characters).

In javascript, something like this:

/^([a-z]{5}-){4}[a-z]{5}$/i