In our C++ code we have a macro that is used to report an identifier that someone can use to look things up.

For example, let us say it’s a Nato Stock Number (NSN). We might have a function or macro like:

NSN("3139-00-121-6210");

We want the NSN to be legibale in the code and we don’t want to allow something like:

NSN("nonsense string");

While this could be caught run time, it would be nicer to let a user know when at compile time that it’s not correct.

Compile Time Regular Expressions

Having listened to CppCast Episode 171: Compile Time Regular Expressions with Hana Dusíková, I was aware that there was as compile time regular expression library. However I didn’t pay as much attention to the episode as I thought…

What better way to make up for my lack of attention by attempting to use the library myself to try and enforce valid NSNs.

The Library

Like most code these days Hana has graciously shared the code on gitub, https://github.com/hanickadot/compile-time-regular-expressions.

I decided to try out the ctre.hpp single header option mentioned in the readme.

The readme also has a nice example snippet to get started (I’m stuck at C++17):

static constexpr auto pattern = ctll::fixed_string{ "h.*" };

constexpr auto match(std::string_view sv) noexcept {
    return ctre::match<pattern>(sv);
}

The Code Implementation

Modifying the example provided by the libary’s readme I came up with:

#include "ctre.hpp"

static constexpr auto nsn = ctll::fixed_string{ "\\d{4}-\\d{2}-\\d{3}-\\d{4}" };

constexpr auto match(std::string_view sv) noexcept {
    // For those unfamiliar with "!!", it will negate twice.  This will convert
    // a value/object to bool if it supports the bool() operator or similar.
    return !!ctre::match<nsn>(sv);
}

int main(){
    printf("Fake nsn doesn't match: %d\n", match("wrong_string"));
    printf("Real nsn matches: %d\n", match("3139-00-121-6210"));
    return 0;
}

And this printed out:

Fake nsn doesn't match: 0
Real nsn matches: 1

This is a run time example though, so now it’s time to try a compile time.

First Failed Compile Time Implementation

My first attempt at trying to convert this to compile time was to use an if constexpr.

constexpr auto match(std::string_view sv) noexcept {
    if constexpr(ctre::match<nsn>(sv)){
        return true;
    } else {
        return false;
    }
}

Admitedly I don’t fully understand constexper other than it can result in compile time evaluation.

This failed to compile on MSVC with compiler error C2131.

Successful Compile Time Implementation

After a few different failures, I was able to come upon this stackoverflow answer, https://stackoverflow.com/a/66815881/4866781.

So I reverted the match function and converted the printf() statements to static_asserts().

#include "ctre.hpp"

static constexpr auto nsn = ctll::fixed_string{ "\\d{4}-\\d{2}-\\d{3}-\\d{4}" };

constexpr auto match(std::string_view sv) noexcept {
    return !!ctre::match<nsn>(sv);
}

int main(){
    static_assert(match("wrong_string"));
    static_assert(match("3139-00-121-6210"));
    return 0;
}

Though the "wrong_string" will cause a compiler failure, you can comment it out to see that the "3139-00-121-6210" succesfully compiles.