[/ Copyright 2006-2007 John Maddock. Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt). ] [section:captures Understanding Marked Sub-Expressions and Captures] Captures are the iterator ranges that are "captured" by marked sub-expressions as a regular expression gets matched. Each marked sub-expression can result in more than one capture, if it is matched more than once. This document explains how captures and marked sub-expressions in Boost.Regex are represented and accessed. [h4 Marked sub-expressions] Every time a Perl regular expression contains a parenthesis group `()`, it spits out an extra field, known as a marked sub-expression, for example the expression: [pre (\w+)\W+(\w+)] Has two marked sub-expressions (known as $1 and $2 respectively), in addition the complete match is known as $&, everything before the first match as $\`, and everything after the match as $'. So if the above expression is searched for within `"@abc def--"`, then we obtain: [table [[Sub-expression][Text found]] [[$\`]["@"]] [[$&]["abc def"]] [[$1]["abc"]] [[$2]["def"]] [[$']["--"]] ] In Boost.Regex all these are accessible via the [match_results] class that gets filled in when calling one of the regular expression matching algorithms ([regex_search], [regex_match], or [regex_iterator]). So given: boost::match_results m; The Perl and Boost.Regex equivalents are as follows: [table [[Perl][Boost.Regex]] [[$\`][`m.prefix()`]] [[$&][`m[0]`]] [[$n][`m[n]`]] [[$\'][`m.suffix()`]] ] In Boost.Regex each sub-expression match is represented by a [sub_match] object, this is basically just a pair of iterators denoting the start and end position of the sub-expression match, but there are some additional operators provided so that objects of type [sub_match] behave a lot like a `std::basic_string`: for example they are implicitly convertible to a `basic_string`, they can be compared to a string, added to a string, or streamed out to an output stream. [h4 Unmatched Sub-Expressions] When a regular expression match is found there is no need for all of the marked sub-expressions to have participated in the match, for example the expression: [pre (abc)|(def)] can match either $1 or $2, but never both at the same time. In Boost.Regex you can determine which sub-expressions matched by accessing the `sub_match::matched` data member. [h4 Repeated Captures] When a marked sub-expression is repeated, then the sub-expression gets "captured" multiple times, however normally only the final capture is available, for example if [pre (?:(\w+)\W+)+] is matched against [pre one fine day] Then $1 will contain the string "day", and all the previous captures will have been forgotten. However, Boost.Regex has an experimental feature that allows all the capture information to be retained - this is accessed either via the `match_results::captures` member function or the `sub_match::captures` member function. These functions return a container that contains a sequence of all the captures obtained during the regular expression matching. The following example program shows how this information may be used: #include #include void print_captures(const std::string& regx, const std::string& text) { boost::regex e(regx); boost::smatch what; std::cout << "Expression: \"" << regx << "\"\n"; std::cout << "Text: \"" << text << "\"\n"; if(boost::regex_match(text, what, e, boost::match_extra)) { unsigned i, j; std::cout << "** Match found **\n Sub-Expressions:\n"; for(i = 0; i < what.size(); ++i) std::cout << " $" << i << " = \"" << what[i] << "\"\n"; std::cout << " Captures:\n"; for(i = 0; i < what.size(); ++i) { std::cout << " $" << i << " = {"; for(j = 0; j < what.captures(i).size(); ++j) { if(j) std::cout << ", "; else std::cout << " "; std::cout << "\"" << what.captures(i)[j] << "\""; } std::cout << " }\n"; } } else { std::cout << "** No Match found **\n"; } } int main(int , char* []) { print_captures("(([[:lower:]]+)|([[:upper:]]+))+", "aBBcccDDDDDeeeeeeee"); print_captures("(.*)bar|(.*)bah", "abcbar"); print_captures("(.*)bar|(.*)bah", "abcbah"); print_captures("^(?:(\\w+)|(?>\\W+))*$", "now is the time for all good men to come to the aid of the party"); return 0; } Which produces the following output: [pre Expression: "((\[\[:lower:\]\]+)|(\[\[:upper:\]\]+))+" Text: "aBBcccDDDDDeeeeeeee" '''**''' Match found '''**''' Sub-Expressions: $0 = "aBBcccDDDDDeeeeeeee" $1 = "eeeeeeee" $2 = "eeeeeeee" $3 = "DDDDD" Captures: $0 = { "aBBcccDDDDDeeeeeeee" } $1 = { "a", "BB", "ccc", "DDDDD", "eeeeeeee" } $2 = { "a", "ccc", "eeeeeeee" } $3 = { "BB", "DDDDD" } Expression: "(.'''*''')bar|(.'''*''')bah" Text: "abcbar" '''**''' Match found '''**''' Sub-Expressions: $0 = "abcbar" $1 = "abc" $2 = "" Captures: $0 = { "abcbar" } $1 = { "abc" } $2 = { } Expression: "(.'''*''')bar|(.'''*''')bah" Text: "abcbah" '''**''' Match found '''**''' Sub-Expressions: $0 = "abcbah" $1 = "" $2 = "abc" Captures: $0 = { "abcbah" } $1 = { } $2 = { "abc" } Expression: "^(?:(\w+)|(?>\W+))'''*$'''" Text: "now is the time for all good men to come to the aid of the party" '''**''' Match found '''**''' Sub-Expressions: $0 = "now is the time for all good men to come to the aid of the party" $1 = "party" Captures: $0 = { "now is the time for all good men to come to the aid of the party" } $1 = { "now", "is", "the", "time", "for", "all", "good", "men", "to", "come", "to", "the", "aid", "of", "the", "party" } ] Unfortunately enabling this feature has an impact on performance (even if you don't use it), and a much bigger impact if you do use it, therefore to use this feature you need to: * Define BOOST_REGEX_MATCH_EXTRA for all translation units including the library source (the best way to do this is to uncomment this define in boost/regex/user.hpp and then rebuild everything. * Pass the match_extra flag to the particular algorithms where you actually need the captures information (regex_search, regex_match, or regex_iterator). 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