Regex match the entire string [duplicate] - c++

Question

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?

Answer 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