For a reference, here's how Java (tried w/ Oracle 1.8.0_31 and openjdk
1.7.0_65) Pattern.UNICODE_CHARACTER_CLASS works:
foo\uD834bar and foo\uDC00bar match ^foo[^a]bar$ and ^foo.bar$, so,
generally, lonely surrogates match /./.
Backreferences are allowed to consume the leading surrogate of a valid
surrogate pair:
Ex1: foo\uD834bar\uD834\uDC00 matches foo(.+)bar\1
But surprisingly:
Ex2: \uDC00foobar\uD834\uDC00foobar\uD834 doesn't match ^(.+)\1$
... So Ex2 works as if the input string was converted to UTF-32 before
matching, but Ex1 works as if it was def not. Idk what's the correct mental
model where both Ex1 and Ex2 would make sense.
java.util.regex.Pattern matches back references by comparing (Java) chars [1], but reads patterns as
a sequence of code points [2]. That should help to explain the differences between ex1 and ex2.
[1]
http://hg.openjdk.java.net/jdk8u/jdk8u/jdk/file/c46daef6edb5/src/share/classes/java/util/regex/Pattern.java#l4890
[2]
http://hg.openjdk.java.net/jdk8u/jdk8u/jdk/file/c46daef6edb5/src/share/classes/java/util/regex/Pattern.java#l1671
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