Java Regex Tutorial (With Examples)

 

Java Regex Tutorial (With Examples)

✅ What is Regex?

Regex (short for Regular Expression) is a sequence of characters that defines a search pattern. It is widely used for string pattern matching, validations, and data parsing.

In Java, regex is supported by the java.util.regex package.

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Key Classes in Java Regex

• Pattern: A compiled representation of a regular expression.
  
  

• Matcher: An engine that performs match operations on a character sequence using a Pattern.
  
  

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Basic Example

import java.util.regex.*;

public class RegexDemo {

    public static void main(String[] args) {

        Pattern pattern = Pattern.compile("Java");

        Matcher matcher = pattern.matcher("Java is fun");

        boolean matchFound = matcher.find();

        System.out.println("Match found? " + matchFound);  // Output: true

    }

}

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Common Regex Patterns

Pattern	Meaning	Example Match
.	Any character	"a", "1", "#"
\d	Any digit (0-9)	"3", "9"
\w	Any word char (a-z, A-Z, 0-9, _)	"a", "Z", "3", "_"
\s	Whitespace	" " (space), \t
*	0 or more	"a*", "", "aaa"
+	1 or more	"a+", "aaa"
?	0 or 1	"a?", "", "a"
[]	Character set	"[aeiou]" matches any vowel
^	Starts with	"^Java" matches "Java is..."
$	Ends with	"end$" matches "The end"

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Examples

✅ 1. Check if a string contains digits

String input = "My phone number is 12345";

boolean hasDigits = input.matches(".*\\d+.*");

System.out.println(hasDigits); // true

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✅ 2. Validate email address

String email = "test@example.com";

boolean isValid = email.matches("^[\\w.-]+@[\\w.-]+\\.[a-zA-Z]{2,6}$");

System.out.println(isValid); // true

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✅ 3. Extract numbers from a string

String input = "Order123, Invoice456";

Pattern pattern = Pattern.compile("\\d+");

Matcher matcher = pattern.matcher(input);

while (matcher.find()) {

    System.out.println("Found number: " + matcher.group());

}

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✅ 4. Replace all whitespaces

String messy = "Java   is \t awesome!";

String cleaned = messy.replaceAll("\\s+", " ");

System.out.println(cleaned); // Java is awesome!

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✅ 5. Validate phone number (e.g., US format)

String phone = "123-456-7890";

boolean isValid = phone.matches("\\d{3}-\\d{3}-\\d{4}");

System.out.println(isValid); // true

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Bonus: Pattern Flags

You can make patterns case-insensitive:

Pattern pattern = Pattern.compile("java", Pattern.CASE_INSENSITIVE);

Matcher matcher = pattern.matcher("I love Java");

System.out.println(matcher.find()); // true

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Tips for Automation Testers:

• Use regex in assertions with API responses.
  
  

• Clean dynamic values from logs before validation.
  
  

• Validate formats (email, timestamp, ID) using .matches().
  
  

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Practice Exercise for You:

1. Write a regex to validate dates in format dd/mm/yyyy.
   
   

2. Extract all words starting with capital letters from a paragraph.
   
   

3. Replace all HTML tags with an empty string (basic HTML cleanup).