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April 11, 2020

Java Joy: Using Functions To Replace Values In Strings

Since Java 9 we can use a function as argument for the Matcher.replaceAll method. The function is invoked with a single argument of type MatchResult and must return a String value. The MatchResult object contains a found match we can get using the group method. If there are capturing groups in the regular expression used for replacing a value we can use group method with the capturing group index as argument.

In the following example we use the replaceAll method and we use a regular expression without and with capturing groups:

package mrhaki.pattern;

import java.util.function.Function;
import java.util.regex.MatchResult;
import java.util.regex.Pattern;

public class Replace {
    public static void main(String[] args) {
        // Define a pattern to find text between brackets.
        Pattern admonition = Pattern.compile("\\[\\w+\\]");
        // Sample text.
        String text = "[note] Pay attention. [info]
        // Function to turn a found result for regular expression to upper case.
        Function<MatchResult, String> bigAdmonition = match ->;
        assert admonition.matcher(text).replaceAll(bigAdmonition).equals("[NOTE] Pay attention. [INFO] Read more.");

        // Pattern for capturing numbers from string like: run20=390ms.
        Pattern boundaries = Pattern.compile("run(\\d+)=(\\d+)ms");

        // Function to calculate seconds from milliseconds.
        Function<MatchResult, String> runResult = match -> {
            double time = Double.parseDouble(;
            return "Execution " + + " took " + (time / 1000) + " seconds.";

        assert boundaries.matcher("run20=390ms").replaceAll(runResult).equals("Execution 20 took 0.39 seconds.");

Written with Java 14.