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Function template before

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.

[Note] Note

before(expr) is equivalent to the perl (?=...) extension. ~before(expr) is a negative look-ahead assertion, equivalent to the perl (?!...) extension.



The sub-expression to put in the look-ahead assertion.