How do you replace groups in a regular expression?

Question

How, exactly, do you replace groups while appending them to a string buffer? For Example: (a)(b)(c) How can you replace group 1 with d, group 2 with e and so on? I'm working with the Java regex engine. Thanks in advance.

Answer 1

You could use Matcher's appendReplacement

Here is an example sample using:

input: "hello bob How is your cat?"

regular expression: "(bob|cat)"

output: "hello alice How is your dog"

public static void main(String[] args) {
    Pattern p = Pattern.compile("(bob|cat)");
    Matcher m = p.matcher("hello bob How is your cat?");
    StringBuffer s = new StringBuffer();
    while (m.find()) {
        m.appendReplacement(s, doReplace(m.group(1)));
    }
    m.appendTail(s);
    System.out.println(s.toString());
}


public static String doReplace(String s) {
    if(s.equals("bob")) {
        return "alice";
    }
    if(s.equals("cat")) {
        return "dog";
    }

    return "";
}

Answer 2

Are you looking for something like this?

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

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

public class Program1 {
    public static void main(String[] args) {
        Pattern p = Pattern.compile("(a)(b)(c)");
        String str = "111abc222abc333";
        String out = null;
        Matcher m = p.matcher(str);
        out = m.replaceAll("z$3y$2x$1");
        System.out.println(out);
    }
}

This gives 111zcybxa222zcybxa333 as output.
I guess you will see what this example does.

But OK, I think there's no ready built-in
method through which you can say e.g.:
- replace group 3 with zzz
- replace group 2 with yyy
- replace group 1 with xxx

Answer 3

You could use Matcher#start(group) and Matcher#end(group) to build a generic replacement method:

public static String replaceGroup(String regex, String source, int groupToReplace, String replacement) {
    return replaceGroup(regex, source, groupToReplace, 1, replacement);
}

public static String replaceGroup(String regex, String source, int groupToReplace, int groupOccurrence, String replacement) {
    Matcher m = Pattern.compile(regex).matcher(source);
    for (int i = 0; i < groupOccurrence; i++)
        if (!m.find()) return source; // pattern not met, may also throw an exception here
    return new StringBuilder(source).replace(m.start(groupToReplace), m.end(groupToReplace), replacement).toString();
}

public static void main(String[] args) {
    // replace with "%" what was matched by group 1 
    // input: aaa123ccc
    // output: %123ccc
    System.out.println(replaceGroup("([a-z]+)([0-9]+)([a-z]+)", "aaa123ccc", 1, "%"));

    // replace with "!!!" what was matched the 4th time by the group 2
    // input: a1b2c3d4e5
    // output: a1b2c3d!!!e5
    System.out.println(replaceGroup("([a-z])(\\d)", "a1b2c3d4e5", 2, 4, "!!!"));
}

Check online demo here.