Different behavior in C regex vs C++ regex using extended POSIX grammar

Question

I am seeing different results when using the C POSIX regex library and the C++ standard library implementation. Here is my code:

string pattern = "\\s";
string testString = " ";

regex_t cre;
int status = regcomp(&cre, pattern.c_str(), REG_EXTENDED);
int result = (regexec(&cre, testString.c_str(), 0, 0, 0) == 0);
cout << "C: " << result << endl;

regex re(pattern, regex_constants::extended);
smatch sm;
cout << "C++: " << regex_search(testString, sm, re) << endl;

The C portion successfully matches the whitespace, but the C++ one throws this error:

terminate called after throwing an instance of 'std::regex_error'
what(): Unexpected escape character.

I understand that the string literal is escaped meaning that the actual regex that is used in pattern matching should be \s. I also only see this issue when using POSIX extended grammar. In the C++ version, if I do not specify POSIX extended grammar when constructing the regex, it defaults to ECMAScript grammar and is able to parse correctly.

What is going on here?

Answer 1

regex_constants::extended triggers the POSIX ERE regex syntax that does not support shorthand character classes. Note the C regex.h module supports \s as a non-standard extension.

To match any whitespace in regex_constants::extended enabled POSIX ERE flavor, you need to use string pattern = "[[:space:]]".

However, you should just rely on the default ECMAScript flavor, and use

regex re(pattern);
// or
regex re(pattern, std::regex::ECMAScript);