Trac rejected my comment because it has links in it, so here's the relevant information. This issue arose as of [http://gcc.gnu.org/viewcvs/gcc?view=revision&revision=197248 GCC SVN commit 197248] which implements [http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3582.html N3582]. It occurs because the [http://gcc.gnu.org/viewcvs/gcc/trunk/gcc/cp/parser.c?r1=197248&r2=197247&pathrev=197248 change on line 4109] preserves parentheses at the front end (with that change reverted, gcc works correctly). This is now [http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59681 PR C++/59681]. The fix in GCC is not trivial, so I've just made [https://github.com/boostorg/regex/pull/1 pull #1] so the trivial fix for boost (remove the parens) can be applied.