Remove whitespace from within a regex capturing group

You can't return as single matching group. I think what are you looking for is non-capturing group (?: ).

There is explanation here

Maybe this regex would help you. It will exclude space character with non capturing group.

N.(\d{6})(?:\s?)(\d)

It will capture 123456 in group 1 and 7 in group 2.

What you want is probably this. It will return 1234567

"N.123456 7".replaceAll("N.(\\d{6})(?:\\s?)(\\d)", "$1$2")

Try this:

(?<=\d)\s+(?=\d+)

This should work.