C++ Regular Expressions with Boost Regex

Question

I am trying to take a string in C++ and find all IP addresses contained inside, and put them into a new vector string.

I've read a lot of documentation on regex, but I just can't seem to understand how to do this simple function.

I believe I can use this Perl expression to find any IP address:

re("\b(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\b");

But I am still stumped on how to do the rest.

Hematology

Did you try the Boost Regex tutorial and documentation? Got some code so far to share with us? – Trixy

what exactly are you trying to match with that regex? First try to match a single IP address – Conde

Have a look at John D Cook's excellent tutorial Getting started with C++ TR1 regular expressions. It's designed for those who already understand RegEx but can't figure out how to make it do stuff in C++. – Anglaangle

That re has bugs: It allows 00 and it does not work with left justified or right justified IP addresses. It is also not syntactically factored for maximum speed. The correct one is at #5804953. – Vantage

Answer 1

Perhaps you're looking for something like this. It uses regex_iterator to get all matches of the current pattern. See reference.

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

int main()
{
    std::string text(" 192.168.0.1 abc 10.0.0.255 10.5.1 1.2.3.4a 5.4.3.2 ");
    const char* pattern =
        "\\b(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)"
        "\\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)"
        "\\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)"
        "\\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\\b";
    boost::regex ip_regex(pattern);

    boost::sregex_iterator it(text.begin(), text.end(), ip_regex);
    boost::sregex_iterator end;
    for (; it != end; ++it) {
        std::cout << it->str() << "\n";
        // v.push_back(it->str()); or something similar     
    }
}

Output:

192.168.0.1
10.0.0.255
5.4.3.2

Side note: you probably meant \\b instead of \b; I doubt you watnted to match backspace character.

Nosewheel

Iterator "end" is not initialized. Is that OK? – Fleta

@truthseeker: It is initialized by default constructor. – Nosewheel

The end is default constructed, but this algorithm has bugs (adjacency. left justification, right justification) and the regex is wrong (and otherwise slow). – Vantage

Answer 2

The offered solution is quite good, thanks for it. Though I found a slight mistake in the pattern itself.

For example, something like 49.000.00.01 would be taken as a valid IPv4 address and from my understanding, it shouldn't be (just happened to me during some dump processing).

I suggest to improve the patter into:

"\\b(25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9][0-9]?|0)"
"\\.(25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9][0-9]?|0)"
"\\.(25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9][0-9]?|0)"
"\\.(25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9][0-9]?|0)\\b";

This should allow only 0.0.0.0 as the all-zero-in, which I suppose to be correct and it will eliminate all .00. .000. etc.

Torquemada

Answer 3

#include <string>
#include <list>
#include <boost/regex.hpp>
typedef std::string::const_iterator ConstIt;

int main()
{
    // input text, expected result, & proper address pattern
    const std::string sInput
    (
            "192.168.0.1 10.0.0.255 abc 10.5.1.00"
            " 1.2.3.4a 168.72.0 0.0.0.0 5.4.3.2"
    );
    const std::string asExpected[] =
    {
        "192.168.0.1",
        "10.0.0.255",
        "0.0.0.0",
        "5.4.3.2"
    };
    boost::regex regexIPs
    (
        "(^|[ \t])("
        "(2(5[0-5]|[0-4][0-9])|1[0-9][0-9]|[1-9]?[0-9])[.]"
        "(2(5[0-5]|[0-4][0-9])|1[0-9][0-9]|[1-9]?[0-9])[.]"
        "(2(5[0-5]|[0-4][0-9])|1[0-9][0-9]|[1-9]?[0-9])[.]"
        "(2(5[0-5]|[0-4][0-9])|1[0-9][0-9]|[1-9]?[0-9])"
        ")($|[ \t])"
    );

    // parse, check results, and return error count
    boost::smatch what;
    std::list<std::string> ns;
    ConstIt end = sInput.end();
    for (ConstIt begin = sInput.begin();
                boost::regex_search(begin, end, what, regexIPs);
                begin = what[0].second)
    {
        ns.push_back(std::string(what[2].first, what[2].second));
    }

    // check results and return number of errors (zero)
    int iErrors = 0;
    int i = 0;
    for (std::string & s : ns)
        if (s != asExpected[i ++])
            ++ iErrors;
    return iErrors;
}

Vantage