关于java正则表达式的问题

public class TestString {
public static void main(String args[]) {

String str="For my money, the important thing about the meeting was bridge- buildingf";
String regEx="a*f";
Pattern p=Pattern.compile(regEx);
Matcher m=p.matcher(str);
System.out.println(m.replaceAll("1"));
}

}

问：我如何找到上述字符串，以a标记开始，f标记结束的子，a，f之间是任意字符的子串