Regular Expression in Java - Java Regex Example

Welcome to Regular Expression in Java. It’s also called Regex in Java. When I started programming, java regular expression was a nightmare for me. This tutorial is aimed to help you master Regular Expression in Java. I will also come back here to refresh my Java Regex learning.

Regular Expression in Java

The regular expression in java defines a pattern for a String. Regular Expression can be used to search, edit or manipulate text. A regular expression is not language specific but they differ slightly for each language. Regular Expression in Java is most similar to Perl. Java Regex classes are present in java.util.regex package that contains three classes:

1. Pattern: Pattern object is the compiled version of the regular expression. Pattern class 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: Matcher is the java regex engine object that matches the input String pattern with the pattern object created. Matcher class doesn’t have any public constructor 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: PatternSyntaxException is thrown if the regular expression syntax is not correct.

Let’s have a look at Java Regex example program.

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*");

	}

}

When we run this java regex example program, we get below output.

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 java regular expression revolves around String, String class has been extended in Java 1.4 to provide a matches method that does regex pattern matching. Internally it uses Pattern and Matcher java regex 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 a regular expression in Java.

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 and lines of code by using simple String matches method. You should 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 Expression in Java - common matching symbols

Java Regex Metacharacters

We have some meta characters in Java regex, it’s like shortcodes for common matching patterns.

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 in Java - Quantifiers

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

Java Regex 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 in Java - Capturing Groups

Regular Expression in Java Capturing groups is 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 java regex pattern. For example, ((a)(bc)) contains 3 capturing groups - ((a)(bc)), (a) and (bc) . You can use Backreference in the regular expression with a backslash (\) and then the number of the 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 the same reason the 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.

1. We can create a Pattern object with flags. For example Pattern.CASE_INSENSITIVE enables case insensitive matching.
2. Pattern class also provides split(String) method that is similar to String class split() method.
3. Pattern class toString() method returns the regular expression String from which this pattern was compiled.
4. Matcher classes have start() and end() index methods that show precisely where the match was found in the input string.
5. Matcher class also provides String manipulation methods replaceAll(String replacement) and replaceFirst(String replacement).

Let’s look at these java regex methods in a simple example program.

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("_"));
	}

}

The output of the above java regex example 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

That’s all for Regular expressions in Java. Java Regex seems hard at first, but if you work with them for some time, it’s easy to learn and use.

You can checkout complete code and more regular expressions examples from our GitHub Repository.