Empty regexp replaceall and surrogate pairs results in corrupted utf16.

[Dawid Weiss]

I guess a lot depends on the point of view. From historical point of
view (where a char[] and a String are basically unsigned values) that
pattern should simply process every value (index) and work like you
say. But from a practical point of view I think it is a bug -- it
corrupts the string, transforming legal unicode into invalid values.

I checked with Python (3) and the behavior there is the expected one
(it work at the unicode codepoint level rather than surrogate level).

Where is the behavior of "" that you mention defined? I admit I
couldn't find any reference to this in the documentation:

> Using an empty String "" as a regex for the replaceAll() takes the
> advantage of the special meaning of "", in which it is interpreted as
> it can match any possible zero-width position of the target String

I'm not saying you're wrong (and that pattern is definitely not common
so it's probably academic discussion) but I'd like some concrete
reference as to how an empty pattern should behave. To me consistency
with the rest of the Pattern specification would be that it operates
at "zero width position between unicode characters" not between any
char[] value, even an incorrect one or in the middle of a surrogate.

Dawid

On Fri, Jun 8, 2012 at 12:46 AM, Xueming Shen <xueming.shen at oracle.com> wrote:
> Personally I don't think it is a bug. A j.l.String represents a sequence of
> UTF-16 chars. While
> a pair of surrogates represents a supplementary character, a single
> surrogate itself is still
> a "legal" independent entity inside a String object and length of a String
> is still defined as
> the total number of char unit and an index value between a high surrogate
> and a low
> surrogate is still a legal index value that can be used to access the char
> at that particular
> position. Using an empty String "" as a regex for the replaceAll() takes the
> advantage of the
> special meaning of "", in which it is interpreted as it can match any
> possible zero-width
> position of the target String, it does  not imply anything regarding
> "character"  or
> "characters" around it, so I would not interpret it as a zero-with character
> boundary,
> therefor a "position" in between a pair surrogates is still a good "found"
> for replacing.
>
> -Sherman
>
>
> On 6/7/2012 1:07 PM, Dawid Weiss wrote:
>>
>> Hi, I'm a committer to the Apache Lucene project. We have randomized
>> tests and one seed hit the following (simplified) scenario:
>>
>>    String s1 = "AB\uD840\uDC00C";
>>    String s2 = s1.replaceAll("", "X");
>>
>> the input contains an extended unicode character (any surrogate pair
>> will do). The pattern is an empty string (in fact, it was randomized
>> as "]|" but it's the same problem so I omit the details). The problem
>> is that after applying this pattern, replaceAll inserts X in between
>> the surrogate pair characters and this results in invalid UTF-16:
>>
>> AB��C
>> XAXBX?X?XCX
>>
>> I believe this is a bug in the regexp implementation (sorry, don't
>> have a patch for it) but I'd like to confirm it's not something known.
>> Pointers appreciated.
>>
>> Dawid