Some problems I see with your regex:
There’s no need for the |[\r\n] sequences in your regex; a negated character class like [^*] matches everything except *, including line separators. It’s only the . (dot) metacharacter that doesn’t match those.
Once you’re inside the comment, the only character you have to look for is an asterisk; as long as you don’t see one of those, you can gobble up as many characters you want. That means it makes no sense to use [^*] when you can use [^*]+ instead. In fact, you might as well put that in an atomic group — (?>[^*]+) — because you’ll never have any reason to give up any of those not-asterisks once you’ve matched them.
Filtering out extraneous junk, the final alternative inside your outermost parens is \*+[^*/], which means “one or more asterisks, followed by a character that isn’t an asterisk or a slash”. That will always match the asterisk at the end of the comment, and it will always have to give it up again because the next character is a slash. In fact, if there are twenty asterisks leading up to the final slash, that part of your regex will match them all, then it will give them all up, one by one. Then the final part — \*+/ — will match them for keeps.
For maximum performance, I would use this regex:
/\*(?>(?:(?>[^*]+)|\*(?!/))*)\*/
This will match a well-formed comment very quickly, but more importantly, if it starts to match something that isn’t a valid comment, it will fail as quickly as possible.
Courtesy of David, here’s a version that matches nested comments with any level of nesting:
(?s)/\*(?>/\*(?<LEVEL>)|\*/(?<-LEVEL>)|(?!/\*|\*/).)+(?(LEVEL)(?!))\*/
It uses .NET’s Balancing Groups, so it won’t work in any other flavor. For the sake of completeness, here’s another version (from RegexBuddy’s Library) that uses the Recursive Groups syntax supported by Perl, PCRE and Oniguruma/Onigmo:
/\*(?>[^*/]+|\*[^/]|/[^*])*(?>(?R)(?>[^*/]+|\*[^/]|/[^*])*)*\*/