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## Function template before

boost::xpressive::before — Look-ahead assertion.

## Synopsis

    // In header: <boost/xpressive/regex_primitives.hpp>

    template<typename Expr> unspecified before(Expr const & expr);

## Description

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

Parameters: |

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