Q: Lonely surrogates and unicode regexps

[André Bargull]

> 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