How to check Regular expression in java Program -

import java.util.regex.*;

public class RegexExample1
{
    public static void main(String args[])
    {
        // 1st way
        Pattern p = Pattern.compile(".s"); // . represents single character
        Matcher m = p.matcher("as");
        boolean b = m.matches();

        // 2nd way
        boolean b2 = Pattern.compile(".s").matcher("as").matches();

        // 3rd way
        boolean b3 = Pattern.matches(".s", "as");

        System.out.println(b + " " + b2 + " " + b3);
    }
}

Step-by-Step Explanation

1. Importing the java.util.regex Package

import java.util.regex.*;

This program imports all classes from the java.util.regex package. The classes in this package are Pattern and Matcher, which enable matching strings according to regular expressions.

2. Class Declaration:

public class RegexExample1

A class named RegexExample1 is declared, and the main method is defined within it.

3. Main Method:

public static void main(String args[]) {

The main method is the entry point of the program. It will execute the logic defined inside it.

4. 1st Way to Use Regular Expressions:

Pattern p = Pattern.compile(\”.s\”);

Matcher m = p.matcher(\”as\”);

boolean b = m.matches();

Pattern.compile(\”.s\”) compiles a regex pattern (.s). By this pattern “.s”, meaning any single character (.) is followed by ‘s’.
The matcher(“as”) returns Matcher object to actually match the “as” to the pattern so compiled.
The m.matches() method checks whether the whole string “as” matches the pattern .s. Since “as” does match this pattern (any one character followed by s), the result will be true.
The value true is assigned to the variable b.

5. Using Alternating Form of Regex Patterns:

boolean b2 = Pattern.compile(“.s “).matcher(” as “).matches();

This is more concise than the first method. It compiles both pattern and matcher object in one line.

Pattern.compile(\\\”.s\\\”) compile the regex pattern.s.
matcher(\\\”as\\”) will generate Matcher object for string “as”.
matches() will check the string is matching the specified pattern and returns true since the string “as” is matched by the regex pattern.
The boolean variable b2 is assigned to the value output from the following statement.

6. 3rd Method by using Regular Expression:

boolean b3 = Pattern.matches(“.s”, “as”);

This is the very basic way that one can have for determining a string to see whether it follows a pattern which is provided in the form of regex in Java.
Pattern.matches(.s, as) directly searches for a string as whether this matches the.s pattern.
Since as fits the pattern-that is, any character followed by s-it becomes true and the result is stored in the variable b3.

7. Outputting the Results:

System.out.println(b + ” ” + b2 + ” ” + b3);

Finally, the program prints the values of b, b2, and b3. All three methods have found a successful match for the pattern.s with the string “as”, so all three values will be true.

The output will be:

true true true

Conclusion of the Program

The program demonstrates three different techniques for matching the regular expression against the string in Java.
It uses the pattern.s, meaning any character followed by the letter s.
It tests the string “as” against the pattern using all three methods: once with the Pattern and Matcher objects, and again just with the easier Pattern.matches() method.

Output: