Preg split not splitting some unicodes about doc-en HOT 4 OPEN

These are Unicode surrogate code points. They don't correspond to a valid Unicode character. This is mojibake. You can't split them meaningfully into characters if they are not characters in the first place. What exactly is your expectation here?

from doc-en.

From the PCRE docs:

In addition to checking the format of the string, there is a check to ensure that all code points lie in the range U+0 to U+10FFFF, excluding the surrogate area. The so-called "non-character" code points are not excluded because Unicode corrigendum # 9 makes it clear that they should not be.

Characters in the "Surrogate Area" of Unicode are reserved for use by UTF-16, where they are used in pairs to encode code points with values greater than 0xFFFF. The code points that are encoded by UTF-16 pairs are available independently in the UTF-8 and UTF-32 encodings. (In other words, the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up UTF-8 and UTF-32.)

We may want to document, that surrogates are not supported; converting to Utf-8 first may yield the desired result.

from doc-en.

from doc-en.

Java works with UTF-16, PHP' PCRE with UTF-8.

from doc-en.