Regex expression in C++ and double backslash

问题

I'm reading a text file in the form of

People list
[Jane]
Female
31
...

and for each line I want to loop through and find the line that contains "[...]" For example, [Jane]

I came up with the regex expression

"(^[\w+]$)"

which I tested that it works using regex101.com. However, when I try to use that in my code, it fails to match with anything. Here's my code:

void Jane::JaneProfile() {
    // read each line, for each [title], add the next lines into its array
    std::smatch matches;
    for(int i = 0; i < m_numberOfLines; i++) { // #lines in text file
        std::regex pat ("(^\[\w+\]$)");
        if(regex_search(m_lines.at(i), matches, pat)) {
            std::cout << "smatch " << matches.str(0) << std::endl;
            std::cout << "smatch.size() = " << matches.size() << std::endl;

        } else
            std::cout << "wth" << std::endl;
    }
}

When I run this code, all the lines go to the else loop and nothing matches...

I searched up for answers, but I got confused when I saw that for C++ you have to use double backslashes instead one backslash to escape... But it didn't work for my code even when I used double backslashes... Where did I go wrong?

By the way, I'm using Qt Creator 3.6.0 Based on (Desktop) Qt 5.5.1 (Clang 6.1 (Apple), 64 bit)

---Edit----

I tried doing:

std::regex pat (R"(^\[\\w+\]$)");

But I get an error saying

Use of undeclared identifier 'R'

I already have #include <regex> but do I need to include something else?

回答1:

Either escape the backslashes or use the raw character version with a prefix that won't appear in the regex:

escaped:

std::regex pat("^\\[\\w+\\]$");

raw character string:

std::regex pat(R"regex(^\[\w+\]$)regex");

working demo (adapted from OPs posted code):

#include <iostream>
#include <regex>
#include <sstream>
#include <string>
#include <vector>

int main()
{
    auto test_data =
    "People list\n"
    "[Jane]\n"
    "Female\n"
    "31";

    // initialise test data
    std::istringstream source(test_data);
    std::string buffer;
    std::vector<std::string> lines;
    while (std::getline(source, buffer)) {
        lines.push_back(std::move(buffer));
    }

    // test the regex

    // read each line, for each [title], add the next lines into its array
    std::smatch matches;
    for(int i = 0; i < lines.size(); ++i) { // #lines in text file
        static const std::regex pat ("(^\\[\\w+\\]$)");
        if(regex_search(lines.at(i), matches, pat)) {
            std::cout << "smatch " << matches.str() << std::endl;
            std::cout << "smatch.size() = " << matches.size() << std::endl;
        } else
            std::cout << "wth" << std::endl;
    }

    return 0;
}

expected output:

wth
smatch [Jane]
smatch.size() = 2
wth
wth

来源：https://stackoverflow.com/questions/36345626/regex-expression-in-c-and-double-backslash

标签

c++

regex

qt-creator

backslash