Question

I am trying to write a regex which would match a (not necessarily repeating) sequence of text blocks, e.g.:

foo,bar,foo,bar

My initial thought was to use backreferences, something like

(foo|bar)(,\1)*

But it turns out that this regex only matches foo,foo or bar,bar but not foo,bar or bar,foo (and so on).

Is there any other way to refer to a part of a pattern?

In the real world, foo and bar are 50+ character long regexes and I simply want to avoid copy pasting them to define a sequence.

Answer 1

With a decent regex flavor you could use (foo|bar)(?:,(?-1))* or the like.
But Java does not support subpattern calls.

So you end up having a choice of doing String replace/format like in ajx’s answer, or you could condition the comma if you know when it should be present and when not. For example:

(?:(?:foo|bar)(?:,(?!$|\s)|))+