It looks like this particular one has had some staying power, because it (or the PCRE version mentioned elsewhere in the comments) have been rather widely imple

It looks like this particular one has had some staying power, because it (or the PCRE version mentioned elsewhere in the comments) have been rather widely implemented. I didn't know about this and was a happy regular expression user without it, but this looks like a good feature for the specific use case of wanting other people to understand the structure of your regular expressions. And much more portable than I would have expected.