Nsryjdtyk

I have the code below this:

#include <string>

#include <regex>



int main(int argc, char const *argv) {

  std::string s = "_apple_";



  std::regex r1("_(\s|\S)+_");

  std::regex r2("_[\s\S]+_");

  std::regex r3("_.+_");

  std::regex r4("_[pale]+_");



  std::smatch sm;

  printf("r1:%d r2:%d r3:%d r4:%dn", 

        std::regex_match(s, sm, r1), 

        std::regex_match(s, sm, r2), 

        std::regex_match(s, sm, r3), 

        std::regex_match(s, sm, r4));



  return 0;

}

output:r1:1 r2:0 r3:1 r4:1

I can not understand why r2 is not match?

My environment is:

Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 10.0.0 (clang-1000.11.45.5)
Target: x86_64-apple-darwin17.7.0
Thread model: posix
InstalledDir:/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin.

The clang regex flavor is POSIX ERE acc. to clang-format regex syntax reference. In POSIX bracket expressions, the usual regex escape sequences, like s, d, w, and even ], are not supported.

The [sS] is the same as [\sS], and matches a backslash, s and S chars.

However, in POSIX regex standard, . matches any chars including line break chars thus there is no need using [sS] workaround.