If we want to represent a group of strings according to a particular pattern, then we should go for regular expression.

e.g1. We can write a regular expression to represent all mobile numbers.

e.g2. We can write a regular expression to represent all mail ids.

The main application area of regular expressions are :

• • To develop validation frameworks.
• • To develop pattern matching applications (ctrl+f in windows, grep in UNIX).
• • To develop translators like compilers, interpreters, assemblers etc.
• • To develop digital circuits like moore and melay machines.
• • To develop communication protocol like TCP/IP, UDP etc.

Pattern:

A Pattern object represents a compiled version of regular expression. We can create a Pattern object by using compile() method of Pattern class.

public static Pattern compile(String regularExpression);

for exa; Pattern p=Pattern.compile("ab");

here "ab" reprsents a particular pattern.

Matcher:

We can use Matcher object to match the given pattern in the target String. We can create Matcher object using matcher() method of Pattern class.

public Matcher matcher(String target);

for exa; Matcher m=p.matcher("ababbaba");

here "ababbaba" is the target String.

Methods of Matcher class:

• • public boolean find() : It attempts to find match and returns true if it is available, otherwise returns false.
• • public int start() : returns start index of the match.
• • public int end() : returns end+1 index of the match.
• • public String group() : returns the matched pattern.

Note : Pattern and Matcher class present in java.util.regex package and introduced in 1.4 versions of Java.

Let's see a demo program for better understanding:

import java.util.regex.*;
class RegexDemo {
	public static void main(String[] args) {
		// create a Pattern object
		Pattern p = Pattern.compile("ab");
		//create a Matcher object
		Matcher m = p.matcher("ababbaba");
		while (m.find()) {
			System.out.println(m.start() + "...." + m.end() + "...." + m.group());
		}
	}
}

Output
0....2....ab
2....4....ab
5....7....ab

Let's see another program to find the occurrences of "ab" in "ababbaba"

import java.util.regex.*;
class RegexDemo1 {
	public static void main(String[] args) {
		int count = 0;
		//create a Pattern object
		Pattern p = Pattern.compile("ab");
		//create a Matcher object
		Matcher m = p.matcher("ababbaba");
		while (m.find()) {
			count++;
			System.out.println(m.start() + "...." + m.end() + "...." + m.group());
		}
		System.out.println("the no. of occurrences" + count);
	}
}

Output
0....2....ab
2....4....ab
5....7....ab
the no. of occurences3