Hi Sherman, ok so I'll admit to reading through to the end of your note
and finding it interesting ;-)
Some comments in-lined.
On 03/Jun/2011 22:55, Xueming Shen wrote:
I'm sure everybody understands what "negated character classes" [^...]
in j.u.regex means.
You would never have doubt about
[^c] does NOT match "c"
[^0-9] does NOT match "8"
[^a-z] does NOT match "b"
[^a-bc-d] does NOT match 'c"
But how about
does [^[c]] match "c"?
does [^[0-9]] match "8"?
does [^[a-z]] match "b"?
does [^a-b[c-d]] match "c"?
I was wrong on all of them when was asked first time and it took me
a while to figure out what is going on behind it. Oh, btw, the answer
is "yes" for all 4, yes, the
[^[c]] matches "c"
[^[0-9]] matches "8"
[^[a-z]] matches "b".
[^a-b[c-d]] matches "c" (while [^a-bc-d] does NOT match "c")
I would not have known the right answer to this quiz either; it seems
that the use of nested character sets is sufficiently rare that we've
not had to learn how these behave.
Another interesting sample is
[^a-b[c-d]e-f] matches "c" but does NOT match "e" (so the "e-f" part after
the nested character class [c-d] does back to "normal").
It appears the "negation" of the "outer" character class does not
affect its nested character class,
I think the easiest way to explain the situation is not to consider the
negation separately, but that [^ is the syntax of a negated character
set. Having a normal character set inside a negated character set then
seems ok to me.
so [^X] is always opposite from
[^[X]], "X" to be any character class.
Same "strange" thing seems to be true for "intersection operation&&"
as well, so both [a-d&&c-f] and [^a-d&&c-f] do NOT match "a".
This does not sound correct, at least for me.
This case is, I agree, a gotcha which is hard to justify through the
syntax rather than the implementation.
The source code suggests that we are treating the nested/embedded
[...] character class and the "intersection&&" specially, so
[^[X] is interpreted as [^] union [X]
[^X[Y]] is interpreted as [^X] union [Y]
[^X[Y]Z] is interpreted as [^XZ] union [Y]
[^X&&Y] is interpreted as [^X]&& Y
What I meant "treating...specially" is that we do NOT do the same
thing for other "embedded character classes", so while [^[a-z]] does
match "c", [^\p{Lower}] actually does NOT match "c", which I would
expect.
The j.u.regex.Pattern APIs do NOT help. All the samples given for
"Character classes"[1] section are "simple" negation, no "nested"
sample is given. And neither "^" nor "[^...]" appear in the operator
precedence table. The behaviors in other regex engines, such as Perl
and POSIX, don't help, as "nested" character class is not supported
there.
I did check with the original author who wrote this part of the code.
It appears the current implementation is indeed what he intended to
do back then, so this behavior is NOT an implementation bug but by
design.
Personally I don't feel this design is not correct.
More mind tricks ;-) ? You think the design *is* wrong?
Ideally, I would assume the spec either specifies [^...] as a
separate "group operator" to be the "complement" of [...], or "^" as
the "negation operator" with the lowest precedence, such as (from
lowest to highest)
(1) Negation ^ (only at the beginning of the [...])
(2) Intersection&&
(3) Range -
(4) nested class []
or as I suggested above
(5) negated class [^ ... ]
and the understanding that nested classes do not 'inherit' the negation
property of their parent.
So
[^X[Y]] would be the "complement" of [X<union>[Y]]
[^X[Y]Z] would be the "complement" of [X<union>[Y]<union>Z]
[^X&&Y] would be the "complement" of [X&&Y]
for example, if I dump the regex internal logic node tree for the sample
regex
[^a-b[c-d]e-f], the jdk7 and jdk8 results would look like
/home/sherman/TL/regex$
/export/sherman/Workspace/jdk7/jdk/build/linux-i586/bin/java RegEx -flag
"1000" "[^a-b[c-d]e-f]" "c"
Pattern=<[^a-b[c-d]e-f]>
Input =<c>
1:<Difference> (7)
2:<Union> (0)
3:<Complement> (0)
4:<Range [a-b]> (0)
5:<Range [c-d]> (0)
6:<Range [e-f]> (0)
7:<END> (0)
-------------------------------
match:true
groupCount=0
/home/sherman/TL/regex$
/export/sherman/Workspace/jdk8/jdk/build/linux-i586/bin/java RegEx -flag
"1000" "[^a-b[c-d]e-f]" "c"
Pattern=<[^a-b[c-d]e-f]>
Input =<c>
1:<Complement> (7)
2:<Union> (0)
3:<Union> (0)
4:<Range [a-b]> (0)
5:<Range [c-d]> (0)
6:<Range [e-f]> (0)
7:<END> (0)
-------------------------------
match:false
I know, most of people might not be interested, but if you have read
this far, means you are interested:-) and might have some opinions.
It is definitely an incompatible change, given this has been the
behavior from the very beginning of Java regex and has been there for
almost a decade, I would appreciate any comment/opinion, especially
if you agree that the existing behavior is NOT "correct" and therefor
we need to fix, OR you think the existing one is just fine (the fact
I only heard one complain the past 5 -6 years:-) so far), OR even the
existing behavior is not "ideal", but given the compatibility
concern, we might just leave it alone.
Since it does not seem to be causing a great problem, I would be
inclined to document it and leave it alone.
Should the future include subtractive character classes then writing,
e.g. [^a-z-[safe]] would seem more natural than [^a-z-[^safe]].
Regards,
Tim
The fix/change itself is relatively easy, as showed at
http://cr.openjdk.java.net/~sherman/pattern_cc/
<http://cr.openjdk.java.net/%7Esherman/pattern_cc/>
Thanks
-Sherman
[1] http://download.java.net/jdk7/docs/api/java/util/regex/Pattern.html#cc
