Function template keep
boost::xpressive::keep — Create an independent sub-expression. 
 
Synopsis
template<typename Expr> unspecified keep(Expr const & expr);
Description
Turn off back-tracking for a sub-expression. Any branches or repeats within the sub-expression will match only one way, and no other alternatives are tried.
| ![[Note]](../../../../doc/src/images/note.png) | Note | 
| keep(expr) is equivalent to the perl (?>...) extension. | 
| Parameters: | 
| expr
 | The sub-expression to modify.  |  |