The standard answer is "don't". Just treat text is a blob, but the other question is what are you trying to accomplish?

- Are you trying to control the rendered length? In that case the perfect solution is actually rendering the string.

- Are you limiting storage size? Then you need to find a good split point that is <N bytes. This is probably done using extended grapheme clusters. (Although this also isn't perfect)

I'm sure there are other use cases as well. But at the end of the day try to avoid splitting text if it can be helped.