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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.

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:

(\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

$`

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.

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:

(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.

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

(?:(\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:


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