C++: How to split a string by ' ' (space), but not by '\ ' (backslash space)?

user_16969380 0
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I'm surprised there is no this question already despite how popular this task might be. Is it possible to do it without writing your own parser? Two or more whitespaces in a row must be skipped. Here's my solution if not

vector<string> split_with_backslash(const string &s)
{
    vector<string> ret;
    const char *ps = s.c_str();
    while (*ps++) {
        std::stringstream ss;
        switch (*ps) {
        default:
            ss << *ps;
            break;
        case ' ': case '\t': case '\n':
            ret.push_back(ss.str());
            ss.str(string());
            ss.clear();
            break;
        case '\\':
            if (!(*++ps == ' ' || *ps == '\t' || *ps == '\n'))
                --ps;
            break;
        }
    }
    return ret;
}

I'm bad with cpp iterators, so I've used c_str()

EDIT: Adding the wordexp example for @MarekR. It is relevant to the question, because wordexp nicely handles the quotation marks/backspaces, but not the special symbols below.

Here I'm preparing the pipeline for execution. This way I get rid of the '|' sign, which wordexp doesn't eat, but there's still <>& (I don't consider & for simplicity.

vector<string> subcommands;
boost::split(subcommands, cpp_buf, boost::is_any_of("|"));
wordexp_t res;
switch (wordexp(subcommands.back().c_str(), &res, 0)) {
    case 0:
        break;
    case WRDE_NOSPACE:
        wordfree(&res);
    default:
        exit(EXIT_FAILURE);
}
algorithm c++ split string
2 Answers

The code you have presented is so close to doing what you want it to do; just one subtle change required: move the std::stringstream ss; declaration to before (i.e. outside of) the while loop. As it stands, a new (empty) stringstream object is being created on each and every iteration of that loop.

However, there is a (possibly) easier way to achieve the same, by using an 'escape flag' to signal that any space is preceded by a backslash, and only pushing your substring to the result vector if that flag is false:

vector<string> split_with_backslash(const string& s)
{
    vector<string> ret;
    bool escape = false;
    std::stringstream ss{};
    for (auto ps : s) { // "ps" is now a simple "char" but I've kept the name
        switch (ps) {
            default:
                ss << ps;
                escape = false;
                break;
            case ' ': case '\t': case '\n':
                if (!escape) {
                    ret.push_back(ss.str());
                    ss.str(string());
                    ss.clear();
                }
                else {
                    ss << ps; // If you want the escaped space to be included in the string
                    escape = false;
                }
                break;
            case '\\':
                escape = true;
                break;
       // The following alternative "case '\\'" block will allow input of
       // escaped backslash characters (if that's required) ...
       /*   case '\\':
                if (escape) {
                    ss << ps;
                    escape = false;
                }
                else {
                    escape = true;
                }
                break; */
        }
    }
    if (!ss.str().empty()) ret.push_back(ss.str()); // Don't forget final sub-string!
    return ret;
}
user_108710730

Here's a version that lets you avoid allocation (except if you use split_all), and handles escaped backslashes as well, and includes testcases:

#include <string>
#include <vector>

// Needed for C++17 compatibility.
// In C++20, just use the string_view constructor.
template<class It>
std::string_view string_view_from_it(It b, It e)
{
    return std::string_view(&*b, e - b);
}

std::string_view skip_space(std::string_view s)
{
    auto it = s.begin();
    for (; it != s.end(); ++it)
    {
        if (!std::isspace(*it))
            break;
    }
    return string_view_from_it(it, s.end());
}
std::pair<std::string_view, std::string_view> split_one(std::string_view s)
{
    bool escaped = false;
    auto it = s.begin();
    for (; it != s.end(); ++it)
    {
        if (escaped)
        {
            escaped = false;
            continue;
        }
        if (*it == '\\')
            escaped = true;
        else if (std::isspace(*it))
            break;
    }
    return {string_view_from_it(s.begin(), it), string_view_from_it(it, s.end())};
}

std::vector<std::string_view> split_all(std::string_view s)
{
    std::vector<std::string_view> v;

    s = skip_space(s);
    while (!(s = skip_space(s)).empty())
    {
        auto pair = split_one(s);
        v.push_back(pair.first);
        s = pair.second;
    }
    return v;
}


#include <iostream>

int main()
{
    for (std::string_view s : {
            "",
            "  ",
            " a b ",
            "hello\\ world",
            " \\ \\\\ \\  \\",
    })
    {
        auto v = split_all(s);
        std::cout << "split('" << s << "') = [";

        bool first = true;
        for (auto s : v)
        {
            if (!first)
                std::cout << ", ";
            first = false;
            std::cout << "'" << s << "'";
        }
        std::cout << "]" << std::endl;
    }
}

That said, I would prefer a real lexer at this point, and actually handle the backslashes (including "backslash at end of input").

I also prefer to forbid all control characters other than newline - notably CR and TAB - or at least normalize them in an earlier phase.

user_14055880
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