Including numbers but not whitespace:
"Stack Me 123 Heppa1 oeu".replaceAll("\\W","").toCharArray();
=> S, t, a, c, k, M, e, 1, 2, 3, H, e, p, p, a, 1, o, e, u
Without numbers and whitespace:
"Stack Me 123 Heppa1 oeu".replaceAll("[^a-z^A-Z]","").toCharArray()
=> S, t, a, c, k, M, e, H, e, p, p, a, o, e, u