Regex not working in java 1.5

问题

public static final String PATTERN = "(?<=(^|,))(([^\",]+)|\"([^\"]*)\")(?=($|,))";
public static void main(String[] args) {
    String line = ",1234,ABC";
    Matcher matcher = Pattern.compile(PATTERN).matcher(line);
    while (matcher.find()) {
        if (matcher.group(3) != null) {
            System.out.println(matcher.group(3));
        } else {
            System.out.println(matcher.group(4));
        }
    }
}

I used the above program to parse the string ",1234,ABC". After parsing I should get 3 tokens as follows:

Empty String i.e ""
1234
ABC

It seems to work on Java 1.6 but its not working on Java 1.5.

Regex are in java since Java 1.4 then why am I facing such problem?

回答1:

This is a bug in the Java Class Library (Sun's implementation, taken over by Oracle), at least up to JRE 1.5 Update 18 and before JRE 1.6 Update 32 (the 2 versions I tested on).

After some testing, there are some bugs in the implementation of positive look-behind (?<=pattern) and also negative look-behind (?<!pattern)^1,2. Maybe it has something to do with how the engine backtracking when there are different width³ of the pattern separated by alternation |, inside a look-behind non-capturing group.

Swapping the order of items in the look-behind sometimes work⁴, but appendix 2 shows that it may not work all the time.

For now, it seems like extracting alternation out of the look-behind is a possible solution. For example: a look-behind with alternation (?<=pat1|pat2|pat3) is converted to (?:(?<=pat1)|(?<=pat2)|(?<=pat3)). Repeat until there is no | inside the look-behind. It seems to produce correct result for the test cases I used below.

So for the regex in question, this is the workaround (assuming the original one is correct):

"(?:^|(?<=,))(?:([^\",]+)|\"([^\"]*)\")(?:$|(?=,))"

Just in case there is problem with look-ahead, I also replace it with non-capturing group, since the result stays the same for your use case. (Testing has yet to reveal there is bug, but just in case.) Although I am not completely sure, I guess we can trust the engine to work correctly at least for (?<=,) and (?=,). I also take the liberty to reduce the number of capturing groups, so please recount them.

Appendix

Tested with input string ",abc,1234" and the regex "(?<=^|[,.])" and "(?<!^|[,.])". The results were different between JRE 1.5u18 and JRE 1.6u32. For positive look-behind "(?<=^|[,.])", the match at position 1 is missing from the output of JRE 1.5u18, compare to that of JRE 1.6u32. Instead, for JRE 1.5u18, position 1 appears in the result for negative look-behind "(?<!^|[,.])", while output of JRE 1.6u32 doesn't contain it.

It is not that much of a surprise to see this complementary behavior, as the positive and negative look-behind are exact opposite of each other.
Another test with input string ",abc,." and the regex "(?<=,abc|[,.])". The match at position 1 does not appear in the list of result for JRE 1.5u18, compare to JRE 1.6u32.

If we swap the alternation around: "(?<=[,.]|,abc)", the match at position 4 is missing from the result of JRE 1.5u18, compare to JRE 1.6u32.
May not limited to different width, but it is the case that I have tested.
I can make the regex in the question works on this input ",1234,ABC,\"sdfsdf,sdf\",sdfskhkf," by swapping ^ and , in the alternation, i.e. change (?<=(^|,)) to (?<=(,|^)).

回答2:

String line = ",1234,ABC";
String[]arr= line.split(",");
System.out.println("arr.length = " + arr.length);
for(String s : arr)
{
   System.out.println("s = \"" + s+"\"");
}

Output is:

arr.length = 3
s = ""
s = "1234"
s = "ABC"

来源：https://stackoverflow.com/questions/14414407/regex-not-working-in-java-1-5

标签

java

regex

jdk1.5