# Regular Expression in Java

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ApplicationContext;
import java.util.concurrent.ExecutionException;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

@SpringBootApplication
public class DemoApplication {

	public static void main(String[] args) throws InterruptedException, ExecutionException {

		// Create the application context and start Spring Boot
		ApplicationContext appContext = SpringApplication.run(DemoApplication.class, args);

		// Defines a pattern (to be used in a search)
		Pattern p = Pattern.compile("aaa|Bbb", Pattern.CASE_INSENSITIVE);

		// Search for the pattern and keep some information regarding the search
		Matcher m = p.matcher("aaa fsasfa afasfas afasfa afasfsf bbbb aaa.");

		// There are no groups defined in RegEx
		System.out.println("Number of groups: " + m.groupCount());

		// For each pattern found in the string
		while (m.find()) {
			// m.group() shows the pattern found
			System.out.println(String.format("Pattern found: %s", m.group()));
			// m.start(), m.end() shows the start and the end position in the string for that finding
			System.out.println(String.format("Start and end position: %s %s \n", m.start(), m.end()));
		}

		String strWithEmails = " 39000 7900 8000 8900 8700";

		// Any number 4 character length which starts with 3,4 or 8 and has the last 3 characters 900.
		var regEx = "[348]900";

		// Remove all the numbers defined in regEx variable
		var newString1 = strWithEmails.replaceAll(regEx,"");
		System.out.println("newString1="+newString1);

		// Remove the pattern when it is found for the first time
		var newString2 = strWithEmails.replaceFirst(regEx,"");
		System.out.println("newString2="+newString2);

		// Split the initial string using a RegEx expression
		var stringArray = strWithEmails.split(regEx);
		System.out.print("values= ");
		for (String val1 : stringArray) {
			System.out.print(val1+",");
		}
		System.out.println("");

		// Test if the whole string follows the RegEx pattern
		var result = strWithEmails.matches(regEx);
		if (result) {
			System.out.println("The whole string follow the RegEx pattern");
		} else {
			System.out.println("The whole string DOES NOT follow the RegEx pattern");
		}
	}
}

Number of groups: 0
Pattern found: aaa
Start and end position: 0 3

Pattern found: bbb
Start and end position: 34 37

Pattern found: aaa
Start and end position: 39 42

newString1= 0 7900 8000  8700
newString2= 0 7900 8000 8900 8700
values=  ,0 7900 8000 , 8700,
The whole string DOES NOT follow the RegEx pattern

Process finished with exit code 0

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

@SpringBootApplication
public class Demo1Application {

	public static void main(String[] args) throws InterruptedException {
		SpringApplication.run(Demo1Application.class, args);

		String str1 = "a";
		String str2 = "aC";
		String str3 = "aCaaaCff";
		String str5 = "elephant10@hotmail.com";
		String str6 = "elephant10@hotmailcom";

		System.out.println("01. "+str1.matches("[abc]"));
		System.out.println("02. "+str2.matches("[.][C]"));
		System.out.println("03. "+str2.matches("[.C]"));
		System.out.println("04. "+str3.matches("aC[\\w]+"));

		String regex = "(\\w+)@([a-z]+)\\.([a-z]{3})";

		// Create a compiled representation of the regular expression
		Pattern p = Pattern.compile(regex);

		// Create a "search engine" to search in a string for the pre-defined pattern
		Matcher m1 = p.matcher(str5);
		Matcher m2 = p.matcher(str6);

		System.out.println("05. "+m1.find());
		System.out.println("06. "+m2.find());

		//Show a group from RegEx
		System.out.println("07. "+m1.group(2));

	}
}

01. true
02. false
03. false
04. true
05. true
06. false
07. hotmail