Can I replace groups in Java regex?

Question

I have this code, and I want to know, if I can replace only groups (not all pattern) in Java regex. Code:

 //...
 Pattern p = Pattern.compile("(\d).*(\d)");
    String input = "6 example input 4";
    Matcher m = p.matcher(input);
    if (m.find()) {

        //Now I want replace group one ( (\d) ) with number 
       //and group two (too (\d) ) with 1, but I don't know how.

    }

Answer 1

Use $n (where n is a digit) to refer to captured subsequences in replaceFirst(...). I'm assuming you wanted to replace the first group with the literal string "number" and the second group with the value of the first group.

Pattern p = Pattern.compile("(\d)(.*)(\d)");
String input = "6 example input 4";
Matcher m = p.matcher(input);
if (m.find()) {
    // replace first number with "number" and second number with the first
    String output = m.replaceFirst("number $3$1");  // number 46
}

Consider (D+) for the second group instead of (.*). * is a greedy matcher, and will at first consume the last digit. The matcher will then have to backtrack when it realizes the final (d) has nothing to match, before it can match to the final digit.