Problems getting subexpressions in regex_search
Hi all, This is the test program: #include <boost/regex.hpp> #include <iostream> #include <string> int main() { static const boost::regex regexRedirectedSection( "\\[(.+?) \\s*=\\s*<(.+?):(.+?)>\\]", boost::regbase::perl ); std::string input = "[MY_SECTION = <FILE:SECTION>]"; boost::cmatch what; if ( boost::regex_search( input, what, regexRedirectedSection, boost::regbase::perl ) ) { cerr << "Matched new style section redirection!!!! Input is: " << input << endl; cerr << "0 gives: --" << what[0].first << "-- and second: " << what[0].second << endl; cerr << "1 gives: --" << what[1].first << "-- and second: " << what[1].second << endl; cerr << "2 gives: --" << what[2].first << "-- and second: " << what[2].second << endl; cerr << "3 gives: --" << what[3].first << "-- and second: " << what[3].second << endl; } return 0; } This is the output: [anibal@anibal ControlFile]$ ./match Matched new style section redirection!!!! Input is: [MY_SECTION = <FILE:SECTION>] 0 gives: --[MY_SECTION = <FILE:SECTION>]-- and second: 1 gives: --MY_SECTION = <FILE:SECTION>]-- and second: = <FILE:SECTION>] 2 gives: --FILE:SECTION>]-- and second: :SECTION>] 3 gives: --SECTION>]-- and second: >] I don't understand why $1 is not giving my MY_SECTION only. It looks like the non-greedy operator is not working as I expect it. This same expression works in Perl and jakarta's regex engine in Java. I basically need to extract MY_SECTION, FILE, and SECTION from the input string. Any pointers as to what's going on will be greatly appreciated. -Anibal
On Saturday, June 29, 2002, at 11:29 AM, soleani wrote:
boost::cmatch what; if ( boost::regex_search( input, what, regexRedirectedSection, boost::regbase::perl ) ) { cerr << "Matched new style section redirection!!!! Input is: " << input << endl; cerr << "0 gives: --" << what[0].first << "-- and second: " << what[0].second << endl; cerr << "1 gives: --" << what[1].first << "-- and second: " << what[1].second << endl; cerr << "2 gives: --" << what[2].first << "-- and second: " << what[2].second << endl; cerr << "3 gives: --" << what[3].first << "-- and second: " << what[3].second << endl;
[...]
Matched new style section redirection!!!! Input is: [MY_SECTION = <FILE:SECTION>] 0 gives: --[MY_SECTION = <FILE:SECTION>]-- and second: 1 gives: --MY_SECTION = <FILE:SECTION>]-- and second: = <FILE:SECTION>] 2 gives: --FILE:SECTION>]-- and second: :SECTION>] 3 gives: --SECTION>]-- and second: >]
I don't understand why $1 is not giving my MY_SECTION only. It looks like the non-greedy operator is not working as I expect it. This same expression works in Perl and jakarta's regex engine in Java.
Your misunderstanding here is the nature of "first" and "second". These are iterators that can be used to pinpoint where in the original string you matched. The fact that you can stream them out to cerr without a compiler error is a sort of "accident", and the output you are getting is from the point where the iterator is pointing to the end of the string. If you simply used the matches as strings, you'd see the results you'd expect. cerr << "0 gives: --" << what[0] << endl; cerr << "1 gives: --" << what[1] << endl; cerr << "2 gives: --" << what[2] << endl; cerr << "3 gives: --" << what[3] << endl; You only want to use what[0].first and what[0].second when you have a reason to want to locate the part of the original string that was matched, and even then, you won't want to use them on their own, but rather together. Aside to John Maddock: Perhaps these should be named something more like begin and end rather than first and second? -- Darin
participants (3)
-
Darin Adler -
soleani