about string and regex

[masterinex]

why is that when define regex_string there are three slashes \\\ infront the dot , however when i define str i only need two slashes \\ , would i not have to escape the dot "." when i define a regular string, so should i not use three slashes instead ?

also why don't i have escape the opening and closing parenthesis "(" and ")" in the regular string ?

	std::string str("\"\\.(cpp)\"");

	const std::string regex_string = R"!(\"\\\.\([[:alpha:][:digit:]]*\)\")!";
	const std::regex ext(regex_string);

	if (regex_match(str, ext))
	{
		cout << "regex" << endl; 
	}

[masterinex]

could someone enlighten me on this ??

[Chervil]

Quote from a previous thread:

When many backslashes are involved, using raw string literals would be easier: std::regex re( R"(\.cpp)" )
http://www.stroustrup.com/C++11FAQ.html#raw-strings

I'm not sure, are you asking about the parentheses used in the string constructor?

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#include <iostream> #include <string> int main() { std::string str("\"\\.(cpp)\""); std::cout << "contents of str = " << str << "\n"; std::string s2 = "\"\\.(cpp)\""; std::cout << "contents of s2 = " << s2 << "\n"; }

contents of str = "\.(cpp)" contents of s2 = "\.(cpp)"

[masterinex]

std::string str("\"\\.(cpp)\""); ---> two backslashes \\ in front the dot "."



const std::string regex_string = R"!(\"\\\.\([[:alpha:][:digit:]]*\)\")!"; ---> three slashes \\\ in front of the dot "."



std::string str("\"\\.(cpp)\""); ----> opening and closing parenthesis "(" and ")" are not escaped



const std::string regex_string = R"!(\"\\\.\([[:alpha:][:digit:]]*\)\")!"; ---> opening and closing parenthesis "(" and ")" are escaped


i am comparing the regular string with the construction of the raw string

[Chervil]

I think you need to distinguish between three separate concepts.

1. Which characters have a special meaning when used in a C++ string.

2. Which characters have a special meaning when used in a regular expression.

3. In the context of C++strings, the difference between traditional representation of a literal, and the raw-string approach to representing a literal.