package com.journaldev.util;
 
import java.util.regex.Matcher;
import java.util.regex.Pattern;
 
public class RegexExamples {
 
  public static void main(String[] args) {
    // using pattern with flags
    Pattern pattern = Pattern.compile("ab", Pattern.CASE_INSENSITIVE);
    Matcher matcher = pattern.matcher("ABcabdAb");
    // using Matcher find(), group(), start() and end() methods
    while (matcher.find()) {
      System.out.println("Found the text \"" + matcher.group()
          + "\" starting at " + matcher.start()
          + " index and ending at index " + matcher.end());
    }
 
    // using Pattern split() method
    pattern = Pattern.compile("\\W");
    String[] words = pattern.split("one@two#three:four$five");
    for (String s : words) {
      System.out.println("Split using Pattern.split(): " + s);
    }
 
    // using Matcher.replaceFirst() and replaceAll() methods
    pattern = Pattern.compile("1*2");
    matcher = pattern.matcher("11234512678");
    System.out.println("Using replaceAll: " + matcher.replaceAll("_"));
    System.out.println("Using replaceFirst: " + matcher.replaceFirst("_"));
  }
 
}

String str = "bbb";
System.out.println("Using String matches method: "+str.matches(".bb"));
System.out.println("Using Pattern matches method: "+Pattern.matches(".bb", str));

System.out.println(Pattern.matches("(\\w\\d)\\1", "a2a2")); //true
  System.out.println(Pattern.matches("(\\w\\d)\\1", "a2b2")); //false
  System.out.println(Pattern.matches("(AB)(B\\d)\\2\\1", "ABB2B2AB")); //true
  System.out.println(Pattern.matches("(AB)(B\\d)\\2\\1", "ABB2B3AB")); //false

package com.journaldev.util;
 
import java.util.regex.Matcher;
import java.util.regex.Pattern;
 
public class RegexExamples {
 
  public static void main(String[] args) {
    // using pattern with flags
    Pattern pattern = Pattern.compile("ab", Pattern.CASE_INSENSITIVE);
    Matcher matcher = pattern.matcher("ABcabdAb");
    // using Matcher find(), group(), start() and end() methods
    while (matcher.find()) {
      System.out.println("Found the text \"" + matcher.group()
          + "\" starting at " + matcher.start()
          + " index and ending at index " + matcher.end());
    }
 
    // using Pattern split() method
    pattern = Pattern.compile("\\W");
    String[] words = pattern.split("one@two#three:four$five");
    for (String s : words) {
      System.out.println("Split using Pattern.split(): " + s);
    }
 
    // using Matcher.replaceFirst() and replaceAll() methods
    pattern = Pattern.compile("1*2");
    matcher = pattern.matcher("11234512678");
    System.out.println("Using replaceAll: " + matcher.replaceAll("_"));
    System.out.println("Using replaceFirst: " + matcher.replaceFirst("_"));
  }
 
}

Found the text "AB" starting at 0 index and ending at index 2
Found the text "ab" starting at 3 index and ending at index 5
Found the text "Ab" starting at 6 index and ending at index 8
Split using Pattern.split(): one
Split using Pattern.split(): two
Split using Pattern.split(): three
Split using Pattern.split(): four
Split using Pattern.split(): five
Using replaceAll: _345_678
Using replaceFirst: _34512678