The Boost library is a collection of very useful C++ (template) libraries. However it's documentation is very complex and using the library straight-forward usually results in g++ scrolling endless pages of template instantiation errors.
This code snippet shows by example how to use the Boost.Regex library. It compiles and executes regular expressions on strings. Some test I ran showed that it is not as fast as pcre, however Boost.Regex it is easier and more elegant to use in C++ programs. The program must be linked with -lboost_regex.
#include <iostream>
#include <stdlib.h>
#include <boost/regex.hpp>
#include <boost/lexical_cast.hpp>
int main()
{
// This regex is compiled at start-up and matches YYYY-MM-DD dates. If it
// contains a syntax error, the program aborts at start-up with an
// exception.
static const boost::regex
date_regex("(199[0-9]|200[0-9])-([1-9]|0[1-9]|1[012])-([1-9]|[0-2][1-9]|3[01])");
// First example: char* c-style input strings use boost::cmatch results.
{
const char *input_cstr = "2007-03-14";
boost::cmatch char_matches;
if (boost::regex_match(input_cstr, char_matches, date_regex))
{
// Convert the parsed number using boost's lexical_cast library
int year = boost::lexical_cast<int>( char_matches[1] );
// Or use the old way: get the std::string object, then it's char*
int month = atoi( char_matches[2].str().c_str() );
std::cout << "First example:"
<< " year " << year
<< " month " << month
<< " day " << char_matches[3] << "\n";
}
else
{
std::cout << "First example should have matched the regex.\n";
}
}
// Second example: STL strings use boost::smatch results.
{
std::string input_stlstr = "2007-03-34";
boost::smatch str_matches;
if (boost::regex_match(input_stlstr, str_matches, date_regex))
{
std::cout << "Second example shouldn't have matched the regex.\n";
}
else
{
std::cout << "Second example didn't match the regex. This was intended.\n";
}
}
// Third example: Temporary regex object and no capture results needed.
{
if (boost::regex_match("2007", boost::regex("(199[0-9]|200[0-9])")))
{
std::cout << "Third example matched the temporary regex object.\n";
}
else
{
std::cout << "Third example should have matched the regex.\n";
}
}
// Fourth example: regex_match matches the whole string while regex_search
// matches substrings just like perl.
{
std::string input = "Today is 2007-03-14, how are you?";
if (boost::regex_match(input, date_regex))
{
std::cout << "Fourth example (regex_match) shouldn't match.\n";
}
else
{
std::cout << "As expected, the fourth example (regex_match) didn't match.\n";
}
if (boost::regex_search(input, date_regex))
{
std::cout << "While the fourth example using regex_search did matched.\n";
}
else
{
std::cout << "Fourth example using regex_search should have matched the regex.\n";
}
}
}
