(1) The following program does not compile:

public class X {

    public static void main(String args[]) {
        Object o = null;
        if (o __matches (42 + 0)) {
        }
    }
}

/home/srikanth/tmp/X.java:5: error: ')' expected
        if (o __matches (42 + 0)) {
             ^
/home/srikanth/tmp/X.java:5: error: ';' expected
        if (o __matches (42 + 0)) {
                                ^
2 errors

(2) The following program is expected to compile but fail at runtime with Arithmetic exception, but fails to compile:

public class X {

    public static void main(String args[]) {
        Object o = null;
        if (o __matches 42 / 0) {
        }
    }
}

Javac message:
/home/srikanth/tmp/X.java:5: error: constant expression required
        if (o __matches 42 / 0) {
                        ^
1 error

(Compare with int x = 42/0; which fails with Arithmetic exception)

(3) This program shows some odd behavior:

public class X {

    public static void main(String args[]) {
        Object o = null;
        if (o __matches String x __matches true) {} // This compiles
        if (o __matches true __matches false) {} // This does not - Why ?
    }
}

compiler's complaint being:
/home/srikanth/tmp/X.java:6: error: constant expression required
        if (o __matches true __matches false) {} // This does not - Why ?
                        ^
1 error