1[/ 2 / Copyright (c) 2008 Eric Niebler 3 / 4 / Distributed under the Boost Software License, Version 1.0. (See accompanying 5 / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) 6 /] 7 8[section Accessing Results] 9 10[h2 Overview] 11 12Sometimes, it is not enough to know simply whether a _regex_match_ or _regex_search_ was successful or not. If 13you pass an object of type _match_results_ to _regex_match_ or _regex_search_, then after the algorithm has completed 14successfully the _match_results_ will contain extra information about which parts of the regex matched which parts 15of the sequence. In Perl, these sub-sequences are called ['back-references], and they are stored in the variables 16[^$1], [^$2], etc. In xpressive, they are objects of type _sub_match_, and they are stored in the _match_results_ 17structure, which acts as a vector of _sub_match_ objects. 18 19[h2 match_results] 20 21So, you've passed a _match_results_ object to a regex algorithm, and the algorithm has succeeded. Now you want 22to examine the results. Most of what you'll be doing with the _match_results_ object is indexing into it to access 23its internally stored _sub_match_ objects, but there are a few other things you can do with a _match_results_ 24object besides. 25 26The table below shows how to access the information stored in a _match_results_ object named `what`. 27 28[table match_results<> Accessors 29 [[Accessor] [Effects]] 30 [[`what.size()`] [Returns the number of sub-matches, which is always greater than zero after a successful match because the full match is stored in the zero-th sub-match.]] 31 [[`what[n]`] [Returns the ['n]-th sub-match.]] 32 [[`what.length(n)`] [Returns the length of the ['n]-th sub-match. Same as `what[n].length()`.]] 33 [[`what.position(n)`] [Returns the offset into the input sequence at which the ['n]-th sub-match begins.]] 34 [[`what.str(n)`] [Returns a `std::basic_string<>` constructed from the ['n]-th sub-match. Same as `what[n].str()`.]] 35 [[`what.prefix()`] [Returns a _sub_match_ object which represents the sub-sequence from the beginning of the input sequence to the start of the full match.]] 36 [[`what.suffix()`] [Returns a _sub_match_ object which represents the sub-sequence from the end of the full match to the end of the input sequence.]] 37 [[`what.regex_id()`] [Returns the `regex_id` of the _basic_regex_ object that was last used with this _match_results_ object.]] 38] 39 40There is more you can do with the _match_results_ object, but that will be covered when we talk about 41[link boost_xpressive.user_s_guide.grammars_and_nested_matches Grammars and Nested Matches]. 42 43[h2 sub_match] 44 45When you index into a _match_results_ object, you get back a _sub_match_ object. A _sub_match_ is basically a pair 46of iterators. It is defined like this: 47 48 template< class BidirectionalIterator > 49 struct sub_match 50 : std::pair< BidirectionalIterator, BidirectionalIterator > 51 { 52 bool matched; 53 // ... 54 }; 55 56Since it inherits publicaly from `std::pair<>`, _sub_match_ has `first` and `second` data members of type 57`BidirectionalIterator`. These are the beginning and end of the sub-sequence this _sub_match_ represents. 58_sub_match_ also has a Boolean `matched` data member, which is true if this _sub_match_ participated in the full 59match. 60 61The following table shows how you might access the information stored in a _sub_match_ object called `sub`. 62 63[table sub_match<> Accessors 64 [[Accessor] [Effects]] 65 [[`sub.length()`] [Returns the length of the sub-match. Same as `std::distance(sub.first,sub.second)`.]] 66 [[`sub.str()`] [Returns a `std::basic_string<>` constructed from the sub-match. Same as `std::basic_string<char_type>(sub.first,sub.second)`.]] 67 [[`sub.compare(str)`] [Performs a string comparison between the sub-match and `str`, where `str` can be a `std::basic_string<>`, C-style null-terminated string, or another sub-match. Same as `sub.str().compare(str)`.]] 68] 69 70[h2 __alert__ Results Invalidation __alert__] 71 72Results are stored as iterators into the input sequence. Anything which invalidates 73the input sequence will invalidate the match results. For instance, if you match a `std::string` object, 74the results are only valid until your next call to a non-const member function of that `std::string` object. 75After that, the results held by the _match_results_ object are invalid. Don't use them! 76 77[endsect] 78