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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.
      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:
    
(\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:
    
| 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<IteratorType> m;
The Perl and Boost.Regex equivalents are as follows:
| Perl | Boost.Regex | 
|---|---|
| $` | 
                 | 
| $& | 
                 | 
| $n | 
                 | 
| $' | 
                 | 
      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.
    
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:
(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.
    
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
(?:(\w+)\W+)+
is matched against
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 <boost/regex.hpp> #include <iostream> 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:
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: