Is it possible to emulate possessive quantifiers (.NET doesn’t support it) using atomic grouping (or in other way)?
Note. I found that (x+x+)++y can be replaced with (?>(x+x+)+)y, but this is just an example and I don’t know whether always {something}@+ equals to (?>{something}@) (where @ is a quantifier).
Yup. May I quote the master himself, Jeffrey Friedl, from page 142 of his classic Mastering Regular Expressions (3rd Edition):
"In one sense, possessive quantifiers are just syntactic sugar, as they can be mimicked with atomic grouping. Something like
.++has exactly the same result as(?>.+), although a smart implementation can optimize possessive quantifiers more than atomic grouping."
Nope, that's all there is to it. Possessive quantifiers are just a convenient shorthand for atomic groups.
Now, if you were using a flavor that doesn't support atomic groups either (like JavaScript and Python), you could use a lookahead to get the same effect:
(?=((x+x+)+))\1y
A lookahead works just like an atomic group except that it doesn't consume what it matches. So you wrap its contents in a capturing group, then use a backreference to do the consuming.
