Regular Expression in Java

A Regular Expression is a expression which representsa group of Strings according to a particular pattern.

 Example:

 We can write a Regular Expression to represent all valid mail ids.

 We can write a Regular Expression to represent all valid mobile numbers.

The main important application areas of Regular Expression are:

To implement validation logic.

To develop Pattern matching applications.

To develop translators like compilers,interpreters etc.

To develop digital circuts.

Example:

Import java.util.regex.*;

Class RegularExp

     {

             public static void main(String args[])

{

int count = 0;

Pattern p = Pattern.compile(“ab”)

            Matcher m = p.matcher(“abbbabbaba”);

while(m.find())

{

count++;

System.out.println(m.start()+”-----”+m.end()+”-    
-----”+m.group());

}

                  System.out.println(“no of occurences”+count);

            }

}

Output:

0------2------ab

4------6------ab

7------9------ab

The no of occurrences: 3

Pattern:

Pattern is a class present in java.util.regex.Pattern package.

It was implemented by Serializable Interface.

By using compile() method we can create an object for Pattern class.

Syntax: public static Patterncompile(String regex);

Example: Pattern  =  Pattern.compile(“ab”);

Matcher:

After creating Pattern object, It can be used to create a Matcher object.

This Matcher Object can match arbitrary character sequence against the regular expression.

Syntax: public Matchermatcher(String target);

Example: Matcher m=p.matcher("abbbabbaba");