ADDITIONAL SYSTEM INFORMATION :
Win 10 / Java 1.8.0_181

A DESCRIPTION OF THE PROBLEM :
Negation on nested character classes does not work
Pattern.compile("[^[a]]").matcher("a").matches(); 
provides true, but should provide false

(same for more complicated expressions) 
if this should not be supported I would expect a proper exception for an invalid syntax

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
        // shows version 1.8.0_181
        System.out.println(System.getProperty("java.version"));

        // OK: runs through => matches a => fine
        assert Pattern.compile("[[a]]").matcher("a").matches();

        // OK: does not match a 
        assert ! Pattern.compile("[^a]").matcher("a").matches();

        // NOT OK: does still match a = > assertion fails
        assert ! Pattern.compile("[^[a]]").matcher("a").matches();


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
expected

[^[a]] should have same semantic as [a]

=>
Pattern.compile("[^[a]]").matcher("a").matches(); 
should result in false, not true
ACTUAL -
Pattern.compile("[^[a]]").matcher("a").matches();  

=> false

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s. above
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CUSTOMER SUBMITTED WORKAROUND :
avoid nested character classes and negation

FREQUENCY : always