boost::xpressive::before — Look-ahead assertion.
// In header: <boost/xpressive/regex_primitives.hpp> template<typename Expr> unspecified before(Expr const & expr);
before(expr) succeeds if the expr sub-expression would match at the current position in the sequence, but expr is not included in the match. For instance, before("foo") succeeds if we are before a "foo". Look-ahead assertions can be negated with the bit-compliment operator.
before(expr) is equivalent to the perl (?=...) extension. ~before(expr) is a negative look-ahead assertion, equivalent to the perl (?!...) extension.