FULL PRODUCT VERSION :
java version "1.6.0_14"
Java(TM) SE Runtime Environment (build 1.6.0_14-b08)
Java HotSpot(TM) Client VM (build 14.0-b16, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows XP [Version 5.1.2600]

A DESCRIPTION OF THE PROBLEM :
The following Java code will fail, because the CANON_EQ pattern flag produces the wrong code and therefore causes a PatterSyntaxException. It seems that the wrong code is created due to the parenthesis:

import java.util.regex.Pattern;
public class test {
public static void main(String[] args) {
Pattern patternThatIsGonnaBug=Pattern.compile("(ë)",Pattern.CANON_EQ);
}
}

Output:
Exception in thread "main" java.util.regex.PatternSyntaxException: Unmatched closing ')' near index 11
((?:e?)|ë)|e)?)
           ^
at java.util.regex.Pattern.error(Pattern.java:1713)
at java.util.regex.Pattern.compile(Pattern.java:1464)
at java.util.regex.Pattern.<init>(Pattern.java:1133)
at java.util.regex.Pattern.compile(Pattern.java:847)
at test.main(test.java:4)

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
run the code as described above

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
a valid regular expression with matching group should be created
ACTUAL -
a broken regular expression is created

ERROR MESSAGES/STACK TRACES THAT OCCUR :
Exception in thread "main" java.util.regex.PatternSyntaxException: Unmatched closing ')' near index 11
((?:e?)|ë)|e)?)
           ^
at java.util.regex.Pattern.error(Pattern.java:1713)
at java.util.regex.Pattern.compile(Pattern.java:1464)
at java.util.regex.Pattern.<init>(Pattern.java:1133)
at java.util.regex.Pattern.compile(Pattern.java:847)
at test.main(test.java:4)

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.util.regex.Pattern;
public class test {
public static void main(String[] args) {
Pattern patternThatIsGonnaBug=Pattern.compile("(ë)",Pattern.CANON_EQ);
}
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
A workaround would be to create a function which does the work of the CANON_EQ flag, but that would be very painful