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Pattern class of Regular Expression in Java

April 17, 2010 2 comments

Introduction:

This article makes you bit more knowledge in java regular expressions. For manage the regular expressions, the java have the three classes in java.util.regex package. But in this article we focused only Pattern class.

A regular expression is a pattern of characters that describes a set of strings. We use the regular expressions to find, display, or modify some or all of the occurrences of a pattern in an input sequence.

Java.util.regex classes:

The package java.util.regex contains three classes such as,

  • Pattern Class
  • Matcher Class
  • PatternSyntaxExcpetion

Let us take a look at Pattern class.

Pattern Class:

A regular expression which is specified as a string that should be first compiled into an instance of Pattern class. The resulting pattern can be used to create an instance of  Matcher class which contains various in-built methods that helps in performing a match against the regular expression. Many Matcher objects can share the same Pattern object.

Create Pattern using compile():

Pattern class doesn’t have a public constructor. So by using the static method compile we can create the pattern.

Pattern p = Pattern.compile("my regexp");

For Regular expression symbols,click here

Important Note:

The backslash is an escape character in Java Strings. i.e., backslash has a predefine meaning in Java. You have to use “\\” instead of “\”.

If you want to define “\w” then you must be using “\\w” in your regex like this.

Pattern r = Pattern.compile(“\\w+”); //Place your pattern here

“\w“ represents a word character, i.e., short for [a-zA-Z_0-9]

We can create the Pattern with flags.

Syntax:

Pattern pattern=Pattern.compile(regex,flags);

For ex, If we want to neglect the case sensitive we can achieve by using below one,

Pattern pattern = Pattern.compile(“\\w+”,Pattern.CASE_INSENSITIVE);

Validate pattern using matches():

The matches() method is used to check whether the given input is match with the pattern. This method returns true only if the entire input text matches the pattern.

boolean isMatch = Pattern.matches(“\\w+”,”Welcome to java world”);

Get the Pattern using pattern():

The pattern() method is used for find out the pattern of the given string. This method returns the regular expression as a string from which this pattern was compiled.

Pattern p=input.pattern();

Split input using split():

The split() method is used to split the given input text based on the given pattern. It returns a String array. There are two forms of split() method,

  • split(String input)
  • split(String input, int limit)

In the second form, we have an argument called limit which is used to specify the limit i.e. the number of resultant strings that have to be obtained by split() method.

String[] str = pattern.split(input,3);

Sample code for Pattern class and methods:

MailID Validation.java:

import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 *
 * @author Home
 */
class MailIDValidation
{
 public static void main(String args[])
 {
 //Input the string for validation
 String email = "mymail@gmail.com";

 //Set the email pattern string
 Pattern p = Pattern.compile(".+@.+\\.[a-z]+");

 //Match the given string with the pattern
 Matcher m = p.matcher(email);

 //check whether match is found
 boolean matchFound = m.matches();

 if (matchFound)
 System.out.println("Valid Email Id.");
 else
 System.out.println("Invalid Email Id.");
 }
}

I hope it offer bit more knowledge in Regular Expressions. The next article makes you clear in Matcher class in regular expression.Please leave your footprints (comments) here.