How to extract numbers from a string

Question

String a = \"sin(23)+cos(4)+2!+3!+44!\";
a.replaceAll(\"\\D\"); //Not working it is only extracting Digits 

I want to extract the numbers which are wit

Answer 1

First thing: Strings are immutable. You code you tried should be more like

a = a.replaceAll("\\D",""); 

Second, if you are sure that you will not have more complext expressions like ((1+2)!+3)! then you can use appendReplacement and appendTail methods from Matcher class.

String a = "sin(23)+cos(4)+2!+3!+44!";

StringBuffer sb = new StringBuffer();
Pattern p = Pattern.compile("(\\d+)!");
Matcher m = p.matcher(a);
while(m.find()){
    String number = m.group(1);//only part in parenthesis, without "!"
    m.appendReplacement(sb, calculatePower(m.group(number )));
}
m.appendTail(sb);
a = sb.toString();