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Working with Regular Expressions in Java

Joydip Kanjilal
Published: June 18, 2022

Java Developer Tutorials

A regular expression (RegEx) is a string search pattern that can be a character, a fixed string, or a complex expression comprising special characters that define the search pattern. Developers can take advantage of regular expressions in Java to search, edit and manipulate text data.

With Java, you can work with regular expressions using the Java.util.regex package. This package provides many classes and methods for manipulating regular expressions. This Java programming tutorial will examine how to work with regular expressions in Java.

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What are Regular Expressions in Java?

Regular expressions in Java are defined using a string of characters that constitute a search pattern and are used to identify patterns in your data. For example, you could use a regular expression to find all the occurrences of a certain word in a document. You can take advantage of regular expressions to check if a string is a valid email address or a valid SSN, etc.

Regular expressions are patterns that describe text. They can be used to search text based on a pattern, and replace text, validate input, and find patterns in text. A regular expression is represented as a string and should initially be compiled into an instance of the Pattern class. This pattern can then be used to construct a Matcher instance for performing matches.

What are Regular Expressions Used For?

Regular expressions are used in many different programming languages, such as C# and Python, and Java for parsing data from files or filtering out unwanted characters from a text.

Regular expressions can be used for searching data based on a pattern in several applications, including search engines. In Java, regular expressions are supported by the java.util.regex package.

Regular expressions can be used for:

  • Searching for a specific pattern in a body of text
  • Replacing all occurrences of a given pattern with another string
  • Extracting information from a body of text
  • Validating input

Anatomy of a Regular Expression

A regular expression is a sequence of characters that defines a search pattern. Usually, this search pattern is used to find certain substrings in other strings, or to replace those substrings with other character sequences. In Java, regular expressions are supported by the java.util.regex package.

There are three different parts to a regular expression in Java:

  • The pattern: This is the search pattern that you want to match against. It can be a simple String literal, or it can be more complex, using special characters to represent wildcards or alternative sequences.
  • The flags: These are optional settings that modify the way the regular expression is interpreted. For example, the “i” flag makes the search case-insensitive, so that it will match both “Java” and “java”.
  • The matcher: This is the object that actually performs the search operation on a String, using the pattern and flags.

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What is the java.util.regex Package

The java.util.regex package provides support for regular expressions in Java. This library makes it easy to find matches in strings, replace matching text, and split strings into smaller pieces based on a pattern.

To use this library with your Java programs, programmers need to import its namespace:

import java.util.regex.*;

The java.util.regex package comprises the MatchResult interface, and the Matcher, Pattern, and PatternSyntaxException classes. The following are the main classes in this package:

  • Pattern: A pattern is an immutable (i.e., unchangeable) representation of a regular expression. A Pattern object is used to create Matcher objects.
  • Matcher: A matcher class contains methods that can perform match operations, can interpret a pattern and then perform matches against an input string.

What are the Benefits of Using Regular Expressions in Java

You can take advantage of regular expressions for searching data based on a particular pattern. You can leverage regular expressions to find and replace text, validate data, and more. They can also make your code crisp, readable and easier to manage and maintain.

Use Cases of Regular Expressions

Regular expressions can be used to validate user input. For example, if you’re asking a user to enter their email address, you can use a regular expression to make sure that they have entered a valid address. This can help to prevent problems caused by incorrect data entry.

Another common use for regular expressions is to extract data. For example, assume that you have a string that contains information about an author, such as their name, address, and phone number. You could use regular expressions to extract this information and store it in separate variables. This can be helpful when working with large amounts of data, or when the format of the data is not known in advance.

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How to Program Regular Expressions in Java

In Java, you can use the Pattern and Matcher classes to work with regular expressions. The following Java code example illustrates how you can use regex to search for a pattern in Java:

import java.util.regex.Matcher;
import java.util.regex.Pattern;
 public class Main 
{
 public static void main(String[] args) 
 {
   Pattern pattern = Pattern.compile("India|USA|UK");
   Matcher matcher = pattern.matcher("I've been to USA once.");
     while (matcher.find()) {
            System.out.println("The start index is: " + matcher.start());
            System.out.println("\nThe end index: " + matcher.end() + " ");
        }
   }
}

Here are some common regular expression examples that you might find useful:

import java.util.regex.Matcher;
import java.util.regex.Pattern;
 public class Main 
{
 public static void main(String[] args) 
 {
    String regex = "Samsung";
    Pattern p = Pattern.compile(regex);
    Matcher m = p.matcher("Sansui");
    boolean b = m.matches();
    System.out.println(b);
 }
}

When you execute the above piece of code, the word false will be displayed at the console. You can perform case insensitive searches using regex expressions as well. The following code listing illustrates how this can be achieved:

import java.util.regex.Matcher;
import java.util.regex.Pattern;
 public class Main 
{
 public static void main(String[] args) 
 {
    String regex = "SONY";
    Pattern p = Pattern.compile(regex, Pattern.CASE_INSENSITIVE);
    Matcher m = p.matcher("sony");
    boolean b = m.matches(); // false
    System.out.println(b);
 }
}

When you execute the above piece of code, the word true will be displayed at the console.

Final Thoughts on RegEx in Java

Regular expressions are very versatile and can be used for many different purposes. They are a powerful tool to search and replace text as well as validate input. They can be very useful when working with strings in Java. They can help you match patterns, replace substrings, and split strings.

Read more Java programming tutorials and software development guides.

Source: www.developer.com