Re: [Boost-users] which regex was matched

From: Eric Niebler (eric_at_[hidden])
Date: 2008-07-14 12:10:47

Peter Boot wrote:
> Is there functionality included in boost::xpressive::regex_search(...) 
> that would allow you to know which of an alternated group of expressions 
> e.g. "aaa|bbb|ccc" matched a substring in the input ?

Just as with Perl and with Boost.Regex, you could create backreferences 
and check to see which participated in the match:

  "(aaa)|(bbb)|(ccc)"

Then pass a match_results<> object to regex_search and check 
"what[N].matched" for N=1,2,3 (assuming your match_results<> object was 
named "what").

If the alternates are all string literals, then you can use an xpressive 
symbol table instead and discover which alternate matched more 
efficiently. For that you'd initialize a std::map with your alternates ...

std::map<string,int> syms;
syms["aaa"] = 1;
syms["bbb"] = 2;
syms["ccc"] = 3;
int which = 0;

// Match a symbol, write the corresponding
// integer into "which"
sregex rx = (a1= syms)[ ref(which) = a1 ];

HTH,

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com