public static void main(String[] args) {
        String s ="abc11,cd123 f7895";
        String s2 = s.replaceAll("(?<!f)[0-9]+?", "*");

        
        System.out.println(s);
        System.out.println(s2);
    }

  String s2 = s.replaceAll("(?<!e)[0-9]+?", "*");

------解决方案--------------------
分开来吧，一次性的正则不容易

------解决方案--------------------
这样吧
Java code

         String s ="abc11,cd123 f7895";
         String[] s2 = s.split("(?<!\\w)");
         StringBuffer sb = new StringBuffer();
           for(int i = 1; i < s2.length;i++) {
              if(s2[i].charAt(0)!='f') {
                  sb.append(s2[i].replaceAll("\\d","*"));
              }
              else {
                  sb.append(s2[i]);
              }
           }
           
            System.out.println(s);
            System.out.println(sb);

------解决方案--------------------
f\d+

------解决方案--------------------
Java code
String s = "abc11,cd123 f7895 abcd1234 ef1234 abc123";
Pattern p = Pattern.compile(".*?\\d+");
Matcher m = p.matcher(s);
StringBuffer sb = new StringBuffer();
while (m.find()) {
    if (!m.group().matches("\\W*f\\d+")) { // 如果是f后跟数字不替换就用 .*?f\\d+
        m.appendReplacement(sb, m.group().replaceAll("\\d+", ""));
    }
}
m.appendTail(sb);
System.out.println(sb);

------解决方案--------------------
Java code
String str="abc11,cd123 f7895";
        str=" "+str+" ";
        Pattern pat=Pattern.compile("[^f0-9][0-9]+(?=[^0-9])");
        Matcher mcr=pat.matcher(str);
        StringBuffer sb=new StringBuffer();
        while(mcr.find()){
            mcr.appendReplacement(sb, mcr.group().replaceAll("[0-9]", "*"));
        }
        mcr.appendTail(sb);
        str=sb.toString().replaceAll("^ | $","");
        System.out.println(str);

------解决方案--------------------
就看LZ什么需求了
Java code
String s = "abc11,cd123 f7895 abcd1234 ef1234 abc123";
s = s.replaceAll("([^f\\d]+)\\d+", "$1");
System.out.println(s);