What is the nicest way to parse this in C++?

Question

In my program, I have a list of "server address" in the following format:

host[:port]

The brackets here, indicate that the port is optional.

• host can be a hostname, an IPv4 or IPv6 address (possibly in "bracket-enclosed" notation).
• port, if present can be a numeric port number or a service string (like: "http" or "ssh").

If port is present and host is an IPv6 address, host must be in "bracket-enclosed" notation (Example: [::1])

Here are some valid examples:

localhost
localhost:11211
127.0.0.1:http
[::1]:11211
::1
[::1]

And an invalid example:

::1:80 // Invalid: Is this the IPv6 address ::1:80 and a default port, or the IPv6 address ::1 and the port 80 ?
::1:http // This is not ambigous, but for simplicity sake, let's consider this is forbidden as well.

My goal is to separate such entries in two parts (obviously host and port). I don't care if either the host or port are invalid as long as they don't contain a non-bracket-enclosed : (290.234.34.34.5 is ok for host, it will be rejected in the next process); I just want to separate the two parts, or if there is no port part, to know it somehow.

I tried to do something with std::stringstream but everything I come up to seems hacky and not really elegant.

How would you do this in C++ ?

I don't mind answers in C but C++ is prefered. Any boost solution is welcome as well.

Thank you.

Answer 1

Have you looked at boost::spirit? It might be overkill for your task, though.

Answer 2

Here's a simple class that uses boost::xpressive to do the job of verifying the type of IP address and then you can parse the rest to get the results.

Usage:

const std::string ip_address_str = "127.0.0.1:3282";
IpAddress ip_address = IpAddress::Parse(ip_address_str);
std::cout<<"Input String: "<<ip_address_str<<std::endl;
std::cout<<"Address Type: "<<IpAddress::TypeToString(ip_address.getType())<<std::endl;
if (ip_address.getType() != IpAddress::Unknown)
{
    std::cout<<"Host Address: "<<ip_address.getHostAddress()<<std::endl;
    if (ip_address.getPortNumber() != 0)
    {
        std::cout<<"Port Number: "<<ip_address.getPortNumber()<<std::endl;
    }
}

The header file of the class, IpAddress.h

#pragma once
#ifndef __IpAddress_H__
#define __IpAddress_H__


#include <string>

class IpAddress
{
public:
    enum Type
    {
        Unknown,
        IpV4,
        IpV6
    };
    ~IpAddress(void);

    /**
     * \brief   Gets the host address part of the IP address.
     * \author  Abi
     * \date    02/06/2010
     * \return  The host address part of the IP address.
    **/
    const std::string& getHostAddress() const;

    /**
     * \brief   Gets the port number part of the address if any.
     * \author  Abi
     * \date    02/06/2010
     * \return  The port number.
    **/
    unsigned short getPortNumber() const;

    /**
     * \brief   Gets the type of the IP address.
     * \author  Abi
     * \date    02/06/2010
     * \return  The type.
    **/
    IpAddress::Type getType() const;

    /**
     * \fn  static IpAddress Parse(const std::string& ip_address_str)
     *
     * \brief   Parses a given string to an IP address.
     * \author  Abi
     * \date    02/06/2010
     * \param   ip_address_str  The ip address string to be parsed.
     * \return  Returns the parsed IP address. If the IP address is
     *          invalid then the IpAddress instance returned will have its
     *          type set to IpAddress::Unknown
    **/
    static IpAddress Parse(const std::string& ip_address_str);

    /**
     * \brief   Converts the given type to string.
     * \author  Abi
     * \date    02/06/2010
     * \param   address_type    Type of the address to be converted to string.
     * \return  String form of the given address type.
    **/
    static std::string TypeToString(IpAddress::Type address_type);
private:
    IpAddress(void);

    Type m_type;
    std::string m_hostAddress;
    unsigned short m_portNumber;
};

#endif // __IpAddress_H__

The source file for the class, IpAddress.cpp

#include "IpAddress.h"
#include <boost/xpressive/xpressive.hpp>

namespace bxp = boost::xpressive;

static const std::string RegExIpV4_IpFormatHost = "^[0-9]{1,3}\\.[0-9]{1,3}\\.[0-9]{1,3}\\.[0-9]+(\\:[0-9]{1,5})?$";
static const std::string RegExIpV4_StringHost = "^[A-Za-z0-9]+(\\:[0-9]+)?$";

IpAddress::IpAddress(void)
:m_type(Unknown)
,m_portNumber(0)
{
}

IpAddress::~IpAddress(void)
{
}

IpAddress IpAddress::Parse( const std::string& ip_address_str )
{
    IpAddress ipaddress;
    bxp::sregex ip_regex = bxp::sregex::compile(RegExIpV4_IpFormatHost);
    bxp::sregex str_regex = bxp::sregex::compile(RegExIpV4_StringHost);
    bxp::smatch match;
    if (bxp::regex_match(ip_address_str, match, ip_regex) || bxp::regex_match(ip_address_str, match, str_regex))
    {
        ipaddress.m_type = IpV4;
        // Anything before the last ':' (if any) is the host address
        std::string::size_type colon_index = ip_address_str.find_last_of(':');
        if (std::string::npos == colon_index)
        {
            ipaddress.m_portNumber = 0;
            ipaddress.m_hostAddress = ip_address_str;
        }else{
            ipaddress.m_hostAddress = ip_address_str.substr(0, colon_index);
            ipaddress.m_portNumber = atoi(ip_address_str.substr(colon_index+1).c_str());
        }
    }
    return ipaddress;
}

std::string IpAddress::TypeToString( Type address_type )
{
    std::string result = "Unknown";
    switch(address_type)
    {
    case IpV4:
        result = "IP Address Version 4";
        break;
    case IpV6:
        result = "IP Address Version 6";
        break;
    }
    return result;
}

const std::string& IpAddress::getHostAddress() const
{
    return m_hostAddress;
}

unsigned short IpAddress::getPortNumber() const
{
    return m_portNumber;
}

IpAddress::Type IpAddress::getType() const
{
    return m_type;
}

I have only set the rules for IPv4 because I don't know the proper format for IPv6. But I'm pretty sure it's not hard to implement it. Boost Xpressive is just a template based solution and hence do not require any .lib files to be compiled into your exe, which I believe makes is a plus.

By the way just to break down the format of regex in a nutshell...
^ = start of string
$ = end of string
[] = a group of letters or digits that can appear
[0-9] = any single-digit between 0 and 9
[0-9]+ = one or more digits between 0 and 9
the '.' has a special meaning for regex but since our format has 1 dot in an ip-address format we need to specify that we want a '.' between digits by using '\.'. But since C++ needs an escape sequence for '\' we'll have to use "\\."
? = optional component

So, in short, "^[0-9]+$" represents a regex, which is true for an integer.
"^[0-9]+\.$" means an integer that ends with a '.'
"^[0-9]+\.[0-9]?$" is either an integer that ends with a '.' or a decimal.
For an integer or a real number, the regex would be "^[0-9]+(\.[0-9]*)?$".
RegEx an integer that is between 2 and 3 numbers is "^[0-9]{2,3}$".

Now to break down the format of the ip address:

"^[0-9]{1,3}\\.[0-9]{1,3}\\.[0-9]{1,3}\\.[0-9]+(\\:[0-9]{1,5})?$"

This is synonymous to: "^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]+(\:[0-9]{1,5})?$", which means:

[start of string][1-3 digits].[1-3 digits].[1-3 digits].[1-3 digits]<:[1-5 digits]>[end of string]
Where, [] are mandatory and <> are optional

The second RegEx is simpler than this. It's just a combination of a alpha-numeric value followed by an optional colon and port-number.

By the way, if you would like to test out RegEx you can use this site.

Edit: I failed to notice that you optionally had http instead of port number. For that you can change the expression to:

"^[0-9]{1,3}\\.[0-9]{1,3}\\.[0-9]{1,3}\\.[0-9]+(\\:([0-9]{1,5}|http|ftp|smtp))?$"

This accepts formats like:
127.0.0.1
127.0.0.1:3282
127.0.0.1:http
217.0.0.1:ftp
18.123.2.1:smtp