Leading-zero detection is broken, it will not detect 0[1-9]+ numbers. For example this test will fail:
assertEquals(-1, Runes.solveExpression("2*1=?2"));
with:
java.lang.AssertionError: expected:<-1> but was:<0>

Instead of
for (final int i : new int[]{0,1,2,3,4,5,6,7,8,9})
you can do simply
for (int i = 0; i <= 9; i++)

+= operator on String is a bad coding practice, always use StringBuilder (or StringBuffer if you need synchronization, but 99.99% you don't).