[Bug libstdc++/58576] New: std::regex_match() reports mismatched braces on a valid regex

["galens at capaccess dot org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58576

            Bug ID: 58576
           Summary: std::regex_match() reports mismatched braces on a
                    valid regex
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: galens at capaccess dot org

Created attachment 30929
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30929&action=edit
Archive containing the g++ -v -save-temps compile log, the generated .ii file
and the original .cpp with the minimum-to-reproduce test case.

I attempted to use a regex to validate qualified hostnames.

However, when I used the regex from this thread (
http://stackoverflow.com/questions/1418423/the-hostname-regex ), with or
without replacing the [0-9A-Za-z] with [:alnum:] (and properly escaping the
backslashes), I get a regex_error exception thrown on std::regex_match() call,
with a regex_constants::error_brack as the reported code().


Using: An unmodified copy of gcc 4.8.1 20130603 from the Fedora 19 primary
repository (rpm ver: 4.8.1-1.fc19 )

(I use 4-spaces-per-tab in my source code, which isn't relevant for the code,
but might make hand-tracing of Parens, Braces, and Brackets I did in comments
more understandable.) 

(And, yes, I know replacing [0-9A-Za-z] with [:alnum:] isn't a legitimate
change WRT domain name validity, unless I force a 'C' locale.  It was just
easier to read when hunting down this issue.)