Regular Expressions in Java

Last Updated : 21 Sep, 2023

In Java, Regular Expressions or Regex (in short) in Java is an API for defining String patterns that can be used for searching, manipulating, and editing a string in Java. Email validation and passwords are a few areas of strings where Regex is widely used to define the constraints. Regular Expressions in Java are provided under java.util.regex package. This consists of 3 classes and 1 interface. The java.util.regex package primarily consists of the following three classes as depicted below in tabular format as follows:

Regex Classes and Interfaces

Regex in Java provides 3 classes and 1 interface which are as follows:

More understanding can be interpreted from the image provided below as follows:

Java Regex

S. No.	Class/Interface	Description
1.	Pattern Class	Used for defining patterns
2.	Matcher Class	Used for performing match operations on text using patterns
3.	PatternSyntaxException Class	Used for indicating syntax error in a regular expression pattern
4.	MatchResult Interface	Used for representing the result of a match operation

Pattern Class

This class is a compilation of regular expressions that can be used to define various types of patterns, providing no public constructors. This can be created by invoking the compile() method which accepts a regular expression as the first argument, thus returning a pattern after execution.

S. No.	Method	Description
1.	compile(String regex)	It is used to compile the given regular expression into a pattern.
2.	compile(String regex, int flags)	It is used to compile the given regular expression into a pattern with the given flags.
3.	flags()	It is used to return this pattern’s match flags.
4.	matcher(CharSequence input)	It is used to create a matcher that will match the given input against this pattern.
5.	matches(String regex, CharSequence input)	It is used to compile the given regular expression and attempts to match the given input against it.
6.	pattern()	It is used to return the regular expression from which this pattern was compiled.
7.	quote(String s)	It is used to return a literal pattern String for the specified String.
8.	split(CharSequence input)	It is used to split the given input sequence around matches of this pattern.
9.	split(CharSequence input, int limit)	It is used to split the given input sequence around matches of this pattern. The limit parameter controls the number of times the pattern is applied.
10.	toString()	It is used to return the string representation of this pattern.

Example: Pattern class

Java

// Java Program Demonstrating Working of matches() Method
// Pattern class
 
// Importing Pattern class from java.util.regex package
import java.util.regex.Pattern;
 
// Main class
class GFG {
 
    // Main driver method
    public static void main(String args[])
    {
 
        // Following line prints "true" because the whole
        // text "geeksforgeeks" matches pattern
        // "geeksforge*ks"
        System.out.println(Pattern.matches(
            "geeksforge*ks", "geeksforgeeks"));
 
        // Following line prints "false" because the whole
        // text "geeksfor" doesn't match pattern "g*geeks*"
        System.out.println(
            Pattern.matches("g*geeks*", "geeksfor"));
    }
}

Output

true
false

Matcher class

This object is used to perform match operations for an input string in Java, thus interpreting the previously explained patterns. This too defines no public constructors. This can be implemented by invoking a matcher() on any pattern object.

S. No.	Method	Description
1.	find()	It is mainly used for searching multiple occurrences of the regular expressions in the text.
2.	find(int start)	It is used for searching occurrences of the regular expressions in the text starting from the given index.
3.	start()	It is used for getting the start index of a match that is being found using find() method.
4.	end()	It is used for getting the end index of a match that is being found using find() method. It returns the index of the character next to the last matching character.
5.	groupCount()	It is used to find the total number of the matched subsequence.
6.	group()	It is used to find the matched subsequence.
7.	matches()	It is used to test whether the regular expression matches the pattern.

Note: T Pattern.matches() checks if the whole text matches with a pattern or not. Other methods (demonstrated below) are mainly used to find multiple occurrences of patterns in the text.

Let us do discuss a few sample programs as we did for the Pattern class. Here we will be discussing a few Java programs that demonstrate the workings of compile(), find(), start(), end(), and split() in order to get a better understanding of the Matcher class.

Example 1: Pattern Searching

Java

// Java program to demonstrate working of
// String matching in Java
 
// Importing Matcher and Pattern class
import java.util.regex.Matcher;
import java.util.regex.Pattern;
 
// Main class
class GFG {
 
    // Main driver method
    public static void main(String args[])
    {
 
        // Create a pattern to be searched
        // Custom pattern
        Pattern pattern = Pattern.compile("geeks");
 
        // Search above pattern in "geeksforgeeks.org"
        Matcher m = pattern.matcher("geeksforgeeks.org");
 
        // Finding string using find() method
        while (m.find())
 
            // Print starting and ending indexes
            // of the pattern in the text
            // using this functionality of this class
            System.out.println("Pattern found from "
                               + m.start() + " to "
                               + (m.end() - 1));
    }
}

Output

Pattern found from 0 to 4
Pattern found from 8 to 12

Regex Character classes

Character Class	Description
[xyz]	x,y or z
[^xyz]	Any characters other than x,y or z
[a-zA-Z]	characters from range a to z or A to Z.
[a-f[m-t]]	Union of a to f and m to t.
[a-z && p-y]	All the range of elements intersection between two ranges
[a-z && [^bc]]	a to z union with except b and c
[a-z && [^m-p]]	a to z union with except range m to p

Below is the implementation of the above topic:

Java

// Java Program to check on Regex
import java.io.*;
import java.util.regex.*;
 
// Driver class
class GFG {
    // Main function
    public static void main(String[] args)
    {
        // Checks if the string matches with the regex
        // Should be single character a to z
        System.out.println(Pattern.matches("[a-z]", "g"));
 
        // Check if the element is range a to z or A to Z
        System.out.println(
            Pattern.matches("[a-zA-Z]", "Gfg"));
    }
}

Output

true
false

Regex Metacharacters

Regex	Description
X?	X appears once or not
X+	X appears once or more than once
X*	X appears zero or not once
X{n}	X appears n times
X{n,}	X appears n times or more than n
X{n,m}	X appears greater than equal to n times and less than m times.

Below is the implementation of Regex Metacharacters:

Java

// Java Program to check on regex
import java.io.*;
import java.util.regex.*;
 
// Driver class
class GFG {
    // Main function
    public static void main(String[] args)
    {
        // Checking all the strings using regex
        System.out.println(Pattern.matches("[b-z]?", "a"));
 
        // Check if all the elements are in range a to z
        // or A to Z
        System.out.println(
            Pattern.matches("[a-zA-Z]+", "GfgTestCase"));
 
        // Check if elements is not in range a to z
        System.out.println(Pattern.matches("[^a-z]?", "g"));
 
        // Check if all the elements are either g,e,k or s
        System.out.println(
            Pattern.matches("[geks]*", "geeksgeeks"));
    }
}

Output

false
true
false
true

Java Regex Finder Example

Regex	Description
.	Any character
\d	Any digits, [0-9]
\D	Any non-digit, [^0-9]
\s	Whitespace character, [\t\n\x0B\f\r]
\S	Non-whitespace character, [^\s]
\w	Word character, [a-zA-Z_0-9]
\W	Non-word character, [^\w]
\b	Word boundary
\B	Non -Word boundary

Below is the implementation of the Java Regex Finder:

Java

// Java Program to implement regex
import java.io.*;
import java.util.regex.*;
 
// Driver Class
class GFG {
    // Main Function
    public static void main(String[] args)
    {
        // Check if all elements are numbers
        System.out.println(Pattern.matches("\\d+", "1234"));
 
        // Check if all elements are non-numbers
        System.out.println(Pattern.matches("\\D+", "1234"));
 
        // Check if all the elements are non-numbers
        System.out.println(Pattern.matches("\\D+", "Gfg"));
 
        // Check if all the elements are non-spaces
        System.out.println(Pattern.matches("\\S+", "gfg"));
    }
}

Output

true
false
true
true

Conclusion

Lastly, let us do discuss some of the important observations as retrieved from the above article

We create a pattern object by calling Pattern.compile(), there is no constructor. compile() is a static method in the Pattern class.
Like above, we create a Matcher object using matcher() on objects of the Pattern class.
Pattern.matches() is also a static method that is used to check if a given text as a whole matches the pattern or not.
find() is used to find multiple occurrences of patterns in the text.
We can split a text based on a delimiter pattern using the split() method

FAQs in Java Regex

Q1. What are regular expressions in Java?

Ans:

Regular Expressions in java are used for string patterns that can be used for searching, manipulating, and editing a string in Java.

Q2. What is a simple example of regular expression in Java?

Ans:

A simple example of a regular expression in java is mentioned below:
Java
// Java Program to check on Regex

import java.io.*;

import java.util.regex.*;

// Driver class

class GFG {

    // Main function

    public static void main(String[] args)

    {

        // Checks if the string matches with the regex

        // Should be single character a to z

        System.out.println(Pattern.matches("[a-z]", "g"));

         // Check if all the elements are non-numbers

        System.out.println(Pattern.matches("\\D+", "Gfg"));

        // Check if all the elements are non-spaces

        System.out.println(Pattern.matches("\\S+", "gfg"));

    }

}
Output
true
true
true

Suggest improvement

Java.io.RandomAccessFile Class Method | Set 1

Regex Tutorial - How to write Regular Expressions?

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Overview of Java

Basics of Java

Input/Output in Java

Flow Control in Java

Operators in Java

Strings in Java

Arrays in Java

OOPS in Java

Inheritance in Java

Abstraction in Java

Encapsulation in Java

Polymorphism in Java

Constructors in Java

Methods in Java

Interfaces in Java

Wrapper Classes in Java

Keywords in Java

Access Modifiers in Java

Memory Allocation in Java

Classes of Java

Packages in Java

Exception Handling in Java

Multithreading in Java

Synchronization in Java

File Handling in Java

Java Regex

Java IO

Java Networking

JDBC - Java Database Connectivity

Overview of Java

Basics of Java

Input/Output in Java

Flow Control in Java

Operators in Java

Strings in Java

Arrays in Java

OOPS in Java

Inheritance in Java

Abstraction in Java

Encapsulation in Java

Polymorphism in Java

Constructors in Java

Methods in Java

Interfaces in Java

Wrapper Classes in Java

Keywords in Java

Access Modifiers in Java

Memory Allocation in Java

Classes of Java

Packages in Java

Exception Handling in Java

Multithreading in Java

Synchronization in Java

File Handling in Java

Java Regex

Java IO

Java Networking

JDBC - Java Database Connectivity

Regular Expressions in Java

Regex Classes and Interfaces

Pattern Class

Java

Matcher class

Java

Regex Character classes

Java

Regex Metacharacters

Java

Java Regex Finder Example

Java

Conclusion

FAQs in Java Regex

Q1. What are regular expressions in Java?

Q2. What is a simple example of regular expression in Java?

Java

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