code: update head comments

code/6/6.1.regex.cpp

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+//
+// 6.1.regex.cpp
+// chapter 06 regular expression
+// modern c++ tutorial
+//
+// created by changkun at changkun.de
+// https://github.com/changkun/modern-cpp-tutorial
+//
+
+#include <iostream>
+#include <string>
+#include <regex>
+
+int main() {
+    std::string fnames[] = {"foo.txt", "bar.txt", "test", "a0.txt", "AAA.txt"};
+    // In C++, `\` will be used as an escape character in the string. 
+    // In order for `\.` to be passed as a regular expression, 
+    // it is necessary to perform second escaping of `\`, thus we have `\\.`
+    std::regex txt_regex("[a-z]+\\.txt");
+    for (const auto &fname: fnames)
+        std::cout << fname << ": " << std::regex_match(fname, txt_regex) << std::endl;
+    
+    std::regex base_regex("([a-z]+)\\.txt");
+    std::smatch base_match;
+    for(const auto &fname: fnames) {
+        if (std::regex_match(fname, base_match, base_regex)) {
+            // the first element of std::smatch matches the entire string
+            // the second element of std::smatch matches the first expression with brackets
+            if (base_match.size() == 2) {
+                std::string base = base_match[1].str();
+                std::cout << "sub-match[0]: " << base_match[0].str() << std::endl;
+                std::cout << fname << " sub-match[1]: " << base << std::endl;
+            }
+        }
+    }
+    
+    return 0;
+}