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Regular Expressions in Java

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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:

  1. Pattern Class
  2. Matcher Class
  3. PatternSyntaxException Class
  4. MatchResult Interface

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

Java Regex
S. No.Class/InterfaceDescription
1.Pattern ClassUsed for defining patterns
2.Matcher ClassUsed for performing match operations on text using patterns
3.PatternSyntaxException ClassUsed for indicating syntax error in a regular expression pattern
4.MatchResult InterfaceUsed 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. MethodDescription
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.MethodDescription
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

  1. We create a pattern object by calling Pattern.compile(), there is no constructor. compile() is a static method in the Pattern class.
  2. Like above, we create a Matcher object using matcher() on objects of the Pattern class.
  3. Pattern.matches() is also a static method that is used to check if a given text as a whole matches the pattern or not.
  4. find() is used to find multiple occurrences of patterns in the text.
  5. 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



Last Updated : 21 Sep, 2023
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