Java Regular Expression Tutorial with Examples

When I started my career in java, regular expressions were a nightmare for me. This tutorial is aimed to help you mastering java regular expression and for me to come back at regular interval to refresh my regular expressions learning.

What Are Regular Expressions?

A regular expression defines a pattern for a String. Regular Expressions can be used to search, edit or manipulate text. Regular expressions are not language specific but they differ slightly for each language. Java regular expressions are most similar to Perl.

Java Regular Expression classes are present in java.util.regex package that contains three classes: Pattern, Matcher and PatternSyntaxException.

1. Pattern object is the compiled version of the regular expression. It doesn’t have any public constructor and we use it’s public static method compile to create the pattern object by passing regular expression argument.

2. Matcher is the regex engine object that matches the input String pattern with the pattern object created. This class doesn’t have any public construtor and we get a Matcher object using pattern object matcher method that takes the input String as argument. We then use matches method that returns boolean result based on input String matches the regex pattern or not.

3. PatternSyntaxException is thrown if the regular expression syntax is not correct.

Let’s see all these classes in action with a simple example:

package com.journaldev.util;

import java.util.regex.*;

public class PatternExample {

	public static void main(String[] args) {
		Pattern pattern = Pattern.compile('.xx.');
		Matcher matcher = pattern.matcher('MxxY');
		System.out.println('Input String matches regex - '+matcher.matches());
		// bad regular expression
		pattern = Pattern.compile('*xx*');

	}

}

Output of the above program is:

Input String matches regex - true
Exception in thread 'main' java.util.regex.PatternSyntaxException: Dangling meta character '*' near index 0
*xx*
^
	at java.util.regex.Pattern.error(Pattern.java:1924)
	at java.util.regex.Pattern.sequence(Pattern.java:2090)
	at java.util.regex.Pattern.expr(Pattern.java:1964)
	at java.util.regex.Pattern.compile(Pattern.java:1665)
	at java.util.regex.Pattern.(Pattern.java:1337)
	at java.util.regex.Pattern.compile(Pattern.java:1022)
	at com.journaldev.util.PatternExample.main(PatternExample.java:13)

Since regular expressions are revolved around String, String class has been extended in Java 1.4 to provide a matches method that does pattern matching. Internally it uses Pattern and Matcher classes to do the processing but obviously it reduces the code lines.

Pattern class also contains matches method that takes regex and input String as argument and return boolean result after matching them.

So below code works fine for matching input String with regular expression.

		String str = 'bbb';
		System.out.println('Using String matches method: '+str.matches('.bb'));
		System.out.println('Using Pattern matches method: '+Pattern.matches('.bb', str));

So if your requirement is just to check if the input String matches with the pattern, you should save time by using simple String matches method. Use Pattern and Matches classes only when you need to manipulate the input String or you need to reuse the pattern.

Note that the pattern defined by regex is applied on the String from left to right and once a source character is used in a match, it can’t be reused.

For example, regex “121″ will match “31212142121″ only twice as “_121____121″.

Regular Expressions common matching symbols

 
Java Regular Expressions Metacharacters

There are two ways to use metacharacters as ordinary characters in regular expressions.

1. Precede the metacharacter with a backslash (\).
2. Keep metacharcter within \Q (which starts the quote) and \E (which ends it).

 
Regular Expression Quantifiers

Quantifiers specify the number of occurrence of a character to match against.

Quantifiers can be used with character classes and capturing groups also.

For example, [abc]+ means a, b or c one or more times.

(abc)+ means the group “abc” one more more times. We will discuss about Capturing Group now.

Regular Expression Capturing Groups

Capturing groups are used to treat multiple characters as a single unit. You can create a group using (). The portion of input String that matches the capturing group is saved into memory and can be recalled using Backreference.

You can use matcher.groupCount method to find out the number of capturing groups in a regex pattern. For example in ((a)(bc)) contains 3 capturing groups; ((a)(bc)), (a) and (bc) .

You can use Backreference in regular expression with backslash (\) and then the number of group to be recalled.

Capturing groups and Backreferences can be confusing, so let’s understand this with an example.

		System.out.println(Pattern.matches('(\\w\\d)\\1', 'a2a2')); //true
		System.out.println(Pattern.matches('(\\w\\d)\\1', 'a2b2')); //false
		System.out.println(Pattern.matches('(AB)(B\\d)\\2\\1', 'ABB2B2AB')); //true
		System.out.println(Pattern.matches('(AB)(B\\d)\\2\\1', 'ABB2B3AB')); //false

In the first example, at runtime first capturing group is (\w\d) which evaluates to “a2″ when matched with the input String “a2a2″ and saved in memory. So \1 is referring to “a2″ and hence it returns true. Due to same reason second statement prints false.
Try to understand this scenario for statement 3 and 4 yourself.

Now we will look at some important methods of Pattern and Matcher classes.

We can create a Pattern object with flags. For example Pattern.CASE_INSENSITIVE enables case insensitive matching.

Pattern class also provides split(String) that is similar to String class split() method.
Pattern class toString() method returns the regular expression String from which this pattern was compiled.

Matcher classes have start() and end() index methods that show precisely where the match was found in the input string.

Matcher class also provides String manipulation methods replaceAll(String replacement) and replaceFirst(String replacement).

Now we will see these common functions in action through a simple java class:

package com.journaldev.util;

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

public class RegexExamples {

	public static void main(String[] args) {
		// using pattern with flags
		Pattern pattern = Pattern.compile('ab', Pattern.CASE_INSENSITIVE);
		Matcher matcher = pattern.matcher('ABcabdAb');
		// using Matcher find(), group(), start() and end() methods
		while (matcher.find()) {
			System.out.println('Found the text \'' + matcher.group()
					+ '\' starting at ' + matcher.start()
					+ ' index and ending at index ' + matcher.end());
		}

		// using Pattern split() method
		pattern = Pattern.compile('\\W');
		String[] words = pattern.split('one@two#three:four$five');
		for (String s : words) {
			System.out.println('Split using Pattern.split(): ' + s);
		}

		// using Matcher.replaceFirst() and replaceAll() methods
		pattern = Pattern.compile('1*2');
		matcher = pattern.matcher('11234512678');
		System.out.println('Using replaceAll: ' + matcher.replaceAll('_'));
		System.out.println('Using replaceFirst: ' + matcher.replaceFirst('_'));
	}

}

Output of the above program is:

Found the text 'AB' starting at 0 index and ending at index 2
Found the text 'ab' starting at 3 index and ending at index 5
Found the text 'Ab' starting at 6 index and ending at index 8
Split using Pattern.split(): one
Split using Pattern.split(): two
Split using Pattern.split(): three
Split using Pattern.split(): four
Split using Pattern.split(): five
Using replaceAll: _345_678
Using replaceFirst: _34512678

Regular expressions are one of the area of java interview questions and in next few posts, I will provide some real life examples.
 

Reference: Java Regular Expression Tutorial with Examples from our JCG partner Pankaj Kumar at the Developer Recipes blog.

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