[burntsushi]

Yes, it absolutely is.

And it's even okay beyond just starting with.

Search the regex crate repository for clones. There are a lot of them. Hell, Regex::new accepts a &str for a pattern, and one of the first things it does is convert it to a String.

[diarrhea]

Interesting. Most regexes being short, I reckon that copy is very cheap. Still I wonder, wouldn’t Cow be an acceptable middle ground, “best of both worlds” style (only copy when needed)?

[burntsushi]

No, because the clone is always a marginal cost, no matter how big the pattern is.

`Cow` would be a needless and gratuitously bad type to accept for Regex::new. There's no point. It would very likely suffer the same class problems as using Into<String>, because we'd need to use Into<Cow<'_, str>>, and thus it is susceptible to annoying inference failures.

It's always important to contextualize costs. In this case, regardless of the size of the pattern string, cloning it is always marginal relative to the other costs incurred. (One possible exception to this is if the pattern is just a simple literal with no regex meta characters. There in theory could be a fast path to side-step regex parsing and other things, but in practice there's not much need for that.)