How do you search a std::string for a substring in C++?

Question

I\'m trying to parse a simple string in C++. I know the string contains some text with a colon, followed immediately by a space, then a number. I\'d like to extract just t

Answer 1

std::string strInput = "Total disk space: 9852465";
std::string strNumber = "0";
size_t iIndex = strInput.rfind(": ");
if(iIndex != std::string::npos && strInput.length() >= 2)
{
  strNumber = strInput.substr(iIndex + 2, strInput.length() - iIndex - 2)
}

Answer 2

I'm surprised that no one mentioned regular expressions. They were added as part of TR1 and are included in Boost as well. Here's the solution using regex's

typedef std::tr1::match_results<std::string::const_iterator> Results;

std::tr1::regex re(":[[:space:]]+([[:digit:]]+)", std::tr1::regex::extended);
std::string     str("Sectors: 4095");
Results         res;

if (std::tr1::regex_search(str, res, re)) {
    std::cout << "Number found: " << res[1] << std::endl;
} else {
    std::cerr << "No number found." << std::endl;
}

It looks like a lot more work but you get more out of it IMHO.

Answer 3

I can't just tokenize on the space (using sstream and <<) because the text in front of the colon may or may not have spaces in it.

Right, but you can use std::getline:

string not_number;
int number;
if (not (getline(cin, not_number, ':') and cin >> number)) {
    cerr << "No number found." << endl;
}

Answer 4

Similar to Konrads answer, but using istream::ignore:

int number;
std::streamsize max = std::numeric_limits<std::streamsize>::max();
if (!(std::cin.ignore(max, ':') >> number)) {
    std::cerr << "No number found." << std::endl;
} else {
    std::cout << "Number found: " << number << std::endl;
}

Answer 5

For completeness, here's a simple solution in C:

int value;
if(sscanf(mystring.c_str(), "%*[^:]:%d", &value) == 1)
    // parsing succeeded
else
    // parsing failed

Explanation: the %*[^:] says to read in as many possible characters that aren't colons, and the * suppresses assignment. Then, the integer is read in, after the colon and any intervening white space.

Answer 6

const std::string pattern(": ");
std::string s("Sectors: 4095");
size_t num_start = s.find(pattern) + pattern.size();