Re: [boost] [xpressive] how to see how much of one's input that matched

From: Eric Niebler (eric_at_[hidden])
Date: 2008-02-05 11:26:42

Thorsten Ottosen wrote:
> I'm trying to build a parser, this time with boost.xpressive instead of 
> using boost.spirit.
> 
> In spirit I have this function that displays when parsing failed:
<snip>
> Can I get the same info with xpressive somehow?

Not directly, no. One possibility is to define an iterator adaptor that 
keeps track of the farthest position to which it's incremented. You can 
then use that iterator with xpressive and find out how much of the 
string gets eaten before the match fails.

Another possibility is to insert into your grammar strategically placed 
actions that throw the current position on parse failures. Something 
like ...

// untested!
struct parse_failure : std::exception
{
     parse_failure(std::string::const_iterator pos)
     { ... }
};

sregex ARule = this | that
                // this executes on parse failure
              | nil [ throw_<parse_failure>(second(_)) ];

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com