Java suppression lettres dans String

package test.regex;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class ExpReg {
	public static void main(String[] args) {
		Pattern p = Pattern.compile("[0-9]*");
		Matcher m = p.matcher("12zer45aaz43");
		StringBuffer sb = new StringBuffer();
		while (m.find()) {
			String texte = m.group();
			if (!texte.equals(""))
				sb.append(texte);
		}
		System.out.println(sb.toString());
	}
}

char c;
int castC ;
string motfinal;
for(int i=0 ; i<string_depart.length;i++){
c= string_depart.chartAt(i);
castC =(int)c;//on recupere son code ascii 
...

string motfinal;
for(int i=0 ; i<string_depart.length;i++){
char c= string_depart.chartAt(i);
int castC =(int)c;//on recupere son code ascii 
if(castC<=057 && castC>=48)//le code ascii des entiers 0 a 9
{motfinal = motfinal+c;//ajout du caractere dans la chaine
}
}