Java String replace(), replaceAll() and replaceFirst()

package io.thecodeforge.strings;

public class StringReplaceExample {

    public static void main(String[] args) {
        String url = "https://thecodeforge.io/payment/service/docs";

        // replace(char, char) — single character replacement
        String noSlashes = url.replace('/', '-');
        System.out.println(noSlashes);
        // https:--thecodeforge.io-payment-service-docs

        // replace(String, String) — literal string, all occurrences
        String http = url.replace("https", "http");
        System.out.println(http);
        // http://thecodeforge.io/payment/service/docs

        // replaceAll(regex, replacement) — regex-powered, all occurrences
        String camelCase = "paymentRetryService";
        // Insert underscore before each uppercase letter
        String snakeCase = camelCase.replaceAll("([A-Z])", "_$1").toLowerCase();
        System.out.println(snakeCase); // payment_retry_service

        // replaceAll with DOT — common bug
        String dotted = "192.168.1.1";
        System.out.println(dotted.replaceAll(".", "X"));   // XXXXXXXXXX — dot = any char!
        System.out.println(dotted.replaceAll("\\.", "X")); // 192X168X1X1 — escaped dot
        System.out.println(dotted.replace(".", "X"));      // 192X168X1X1 — safer for literal

        // replaceFirst() — only the first match
        String log = "ERROR:101 ERROR:102 ERROR:103";
        System.out.println(log.replaceFirst("ERROR", "WARN"));
        // WARN:101 ERROR:102 ERROR:103

        // replaceAll for removing whitespace
        String messy = "  payment   service  ";  
        String clean = messy.replaceAll("\\s+", " ").trim();
        System.out.println(clean); // payment service

        // Replacing special regex chars in replacement string: use \\$ for literal $
        String price = "Amount: $100";
        System.out.println(price.replaceAll("\\$", "USD ")); // Amount: USD 100
    }
}