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Boost.RegexUnderstanding Captures |
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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:
$`
"@" $& "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 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 possition 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:
Revised 12 Dec 2003
© Copyright John Maddock 2003
Use, modification and distribution are subject to 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)