How to reuse the number of group matches with regex?

Question

I'm using this regex ^x{3}y{3}$ to check for example this string: xxxyyy, where the number of x's is the same as the number of y's (in my example it is 3). But the problem is that I don't know the number of repetitions (it is in range from 0 to 10), so 3 has to be replaced with something abstract.

I can use ^x*y*$ or ^x{0,10}y{0,10}$, but this will not work when the number of x's is different from the number of y's.

I'm looking for something like \1, which is used for reusing of matched group, but in my case I want to reuse the number of group matches for x.

Answer 1

Since you've got a fixed upper limit, you could just list the matches out longhand:

^(x{10}y{10}|x{9}y{9}|x{8}y{8}|x{7}y{7}|x{6}y{6}|x{5}y{5}|x{4}y{4}|x{3}y{3}|x{2}y{2}|xy)?$

(The ? handles your "zero of each" case.)

Clearly if x and y are long in practice then there will be lots of repetition here.

Another approach would be to repeatedly replace xy with an empty string, and check that you end up with an empty string (since each time you remove a pair, you'll bring another pair together).