Description:
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According to http://docs.php.net/manual/en/regexp.reference.php PCRE functions should be able to match surrogates in Unicode mode. However, it is my understanding that surrogates are not allowed in UTF-8, which is the encoding used by the Unicode mode. That would explain why preg_match() and preg_replace() fail when operating on UTF-8-encoded surrogates.

Note that both functions fail in a different way. preg_match() returns 0 whereas preg_replace() returns NULL.

I'm not sure what the fix should be. Being able to match surrogates would make my life easier, but if it's not valid UTF-8 then it might be more consistent (albeit in a twisted way) to return NULL, as that's what PCRE functions do on invalid UTF-8.

Reproduce code:
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// \xED\xA0\x80 is character 0xD800 in UTF-8
var_dump(preg_match('#.#u', ".\xED\xA0\x80"));
var_dump(preg_replace('#\p{Cs}#u', '', ".\xED\xA0\x80"));

Expected result:
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int(1)
string(1) "."

Actual result:
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int(0)
NULL