I was about to give you a "Best Practices" vote for using the modulo method instead of converting to a string.
But then you had a while(true)/break, so they cancel each other out.
Look into the do {} while(...) statement.

Here, not only twice, but 2 * str.lenght times.

You are totally right. Thanks for this comment. But a lot of time has passed since I wrote this. And it was just like "hmmm, can I write it in one line in a couple of minutes?". Now I'm working as a middle .Net developer and I would never do that in my production code. My Upper solution is extremely slow.

The regex is the completely wrong solution here. It's quite slow relative to the rest of the method, by at least one order of magnatude. Which is a high price for something which is ultimately going to fail -- the start of the pattern say "match semi-colon or anything in the range ..." which means bar, open bracket & close bracket will get through, along with any whitespace character aside from a blank (such as a tab).

You're doing the ToLower inside the loop, so you're doing the same thing (probably the most time consuming step in the whole function) over & over 26 times.

Sorry, but you're wrong. I win.

I'm aware of the quote. However, it's not particularly relevant here.

It mainly concerns itself with wasting lots of programmer time optimizing a minor part of the overall application.

Now granted, I did waste a lot of time running the benchmark, but I only did that for the second post, because I wanted real numbers for the message --- Just from looking at it, I knew the original version would be more than an order of magnitude slower.

Further, the orginal version offers no advantage (such as expandability, thread-saftey, or even readability), to justify it running slower. It just pointlessly wastes time.

Had the original been written as:
string GetPlanetName(int id)
{
Task.Sleep(20);
return planets[id - 1];
}
would you say removing the Sleep call was a premature optimization?

The entire point of this site is to train master programmers. However, the three points I made in the previous post are not about optimization or mastery, but rather basic competency. Knowing things like those (as well as appreciating the difference between things done at compile-time vs run-time) is a necessary skill of any developer.

ok read this https://stackify.com/premature-optimization-evil/

It's a valid solution as long as it only contains single words.
With a seperator you're basically back to an array again, except for speed / memory usage maybe.
Good solution in this case, but no best practice i'd guess. :)

you're kidding, right

Eeek.... Doing a Split() on every call --- just because you are too lazy to add the quotes & commas to make a proper array??