5th July 2023, 2 min read

Having fun with string literal suffixes in C++

The C++11 standard introduced user-defined string suffixes. It also added regular expressions to the C++ language as a standard feature. I wanted to have fun and see whether we could combine these features.

Regular expressions are useful to check whether a given string matches a pattern. For example, the expression \d+ checks that the string is made of one or more digits. Unfortunately, the backlash character needs to be escaped in C++, so the string \d+ may need to be written as "\d+" or you may use a raw string: a raw string literal starts with R"( and ends in )" so you can write R"(\d+)". For complicated expressions, a raw string might be better.

A user-defined string literal is a way to specialize a string literal according to your own needs. It is effectively a convenient way to design your own “string types”. You can code it up as:

myclass operator"" _mysuffix(const char *str, size_t len) {
  return myclass(str, len);
}

And once it is defined, instead of writing myclass("mystring", 8), you can write "mystring"_mysuffix.

In any case, we would like to have a syntax such as this:

bool is_digit = "\d+"_re("123");

I can start with a user-defined string suffix:

convenience_matcher operator "" _re(const char *str, size_t) {
return convenience_matcher(str);
}

I want my convenience_matcher to construct a regular expression instance, and to call the matching function whenever a parameter is passed in parenthesis. The following class might work:

#include <regex>
struct convenience_matcher {
  convenience_matcher(const char *str) : re(str) {}
  bool match(const std::string &s) {
    std::smatch base_match;
    return std::regex_match(s, base_match, re);
  }
  bool operator()(const std::string &s) { return match(s); }
  std::regex re;
};

And that is all. The following expressions will then return a Boolean value indicating whether we have the required pattern:

 "\\d+"_re("123") // true
 "\\d+"_re("a23") // false
 R"(\d+)"_re("123") // true
 R"(\d+)"_re("a23") // false

I have posted a complete example. It is just for illustration and I do not recommend using this code for anything serious. I am sure that you can do better!