2. Numbers

#include <boost/regex.hpp>
#include <string>

std::string s("335635f");
boost::regex pattern("PATTERN");        // pattern given as a string-literal

if (boost::regex_match(s.begin(), s.end(), pattern)) {
        // string completely matches pattern
} else {
        // string does not match the pattern
}
// #---------------------------------
// Some PATTERNs in strings, escaped
// has nondigits                if     "\\D*"
// not a natural number"        unless "^\\d+$"             // rejects -3
// not an integer               unless "^-?\\d+$"           // rejects +3
// not an integer               unless "^[+-]?\\d+$"
// not a decimal number         unless "^-?\\d+\\.?\\d*$"     // rejects .2
// not a decimal number         unless "^-?(?:\\d+(?:\\.\\d*)?|\\.\\d+)$"

// A search of pattern in string is also possible.

#include <sstream>
#include <iomanip>

/*
 * equal(float num1, float num2, int accuracy) : returns true if
 * num1 and num2 are equal to accuracy number of decimal places
 */
bool equal(float num1, float num2, int accuracy) {
        std::stringstream s1, s2;
        // write num1 to s1, represented with as many digits
        // in the fraction part (fixed) as specified by the precision
        s1 << std::fixed << std::setprecision(accuracy) << num1;
        s2 << std::fixed << std::setprecision(accuracy) << num2;
        return s1.str() == s2.str();
}
// The approach of fcomparing formatted strings is not the most efficient.