substitution

Hello.
How do you substitute if you have /b[a]b/ and you want [bbb]?
This does not seem to work:
        String patt = " [.,]";

        Perl5Matcher patternMatcher;
        Perl5Compiler compiler = new Perl5Compiler();
        patternMatcher  = new Perl5Matcher();
        try {
        Perl5Pattern pattern = (Perl5Pattern) compiler.compile(patt);

        Perl5Substitution subs = new Perl5Substitution("[.,]");
        return Util.substitute(patternMatcher,pattern,subs,inputS,Util.SUBSTITUTE_ALL);
        } catch(Exception e){
            e.printStackTrace();
    }
Thanks,
A