Regex in Java

In Java Regex, It stand for Regular Expression it provides an API that define pattern for searching or manipulating strings.

It is manily used to define on strings such as password and email validation. Java Regex API provides 1 interface and 3 classes in java.util.regex package.

java.util.regex package

The java.util.regex package provides following classes and interfaces for Regex.

Matcher class
Pattern class
PatternSyntaxException class
MatchResult interface

Matcher class

It implements the MatchResult interface.

No.	Method	Description
1	boolean matches()	test whether the regular expression matches the pattern.
2	boolean find()	finds the next expression that matches the pattern.
3	boolean find(int start)	finds the next expression that matches the pattern from the given start number.
4	String group()	returns the matched subsequence.
5	int start()	returns the starting index of the matched subsequence.
6	int end()	returns the ending index of the matched subsequence.
7	int groupCount()	returns the total number of the matched subsequence.

Pattern class

It is used to define a pattern for the regex engine.

No.	Method	Description
1)	static Pattern compile(String regex)	compiles the given regex and returns the instance of the Pattern.
2)	Matcher matcher(CharSequence input)	creates a matcher that matches the given input with the pattern.
3)	static boolean matches(String regex, CharSequence input)	It works as the combination of compile and matcher methods. It compiles the regular expression and matches the given input with the pattern.
4)	String[] split(CharSequence input)	splits the given input string around matches of given pattern.
5)	String pattern()	returns the regex pattern.

Example of Java Regular Expressions

There are three ways to write the regex example in Java.

	import java.util.regex.*;
	public class Regex1{
	public static void main(String args[]){
	//1st way
	Pattern p = Pattern.compile(".s");//. represents single character
	Matcher m = p.matcher("as");
	boolean b = m.matches();
	
	//2nd way
	boolean b2=Pattern.compile(".s").matcher("as").matches();
	
	//3rd way
	boolean b3 = Pattern.matches(".s", "as");
	
	System.out.println(b+" "+b2+" "+b3);
	}}
	

Output

	true true true

Regular Expression . Example

The . (dot) represents a single character.

  import java.util.regex.*;  
   class Regex2{  
    public static void main(String args[]){  
     System.out.println(Pattern.matches(".s", "as"));//true (2nd char is s)  
     System.out.println(Pattern.matches(".s", "mk"));//false (2nd char is not s)  
     System.out.println(Pattern.matches(".s", "mst"));//false (has more than 2 char)  
     System.out.println(Pattern.matches(".s", "amms"));//false (has more than 2 char)  
    System.out.println(Pattern.matches("..s", "mas"));//true (3rd char is s)  
  }}  
	

Ourput:-

Regex Character classes

No.	Character Class	Description
1	[abc]	a, b, or c (simple class)
2	[^abc]	Any character except a, b, or c (negation)
3	[a-zA-Z]	a through z or A through Z, inclusive (range)
4	[a-d[m-p]]	a through d, or m through p: [a-dm-p] (union)
5	[a-z&&[def]]	d, e, or f (intersection)
6	[a-z&&[^bc]]	a through z, except for b and c: [ad-z] (subtraction)
7	[a-z&&[^m-p]]	a through z, and not m through p: [a-lq-z](subtraction)

Regular Expression Character classes Example

	import java.util.regex.*;
	class Regex3{
	public static void main(String args[]){
	System.out.println(Pattern.matches("[amn]", "abcd"));//false (not a or m or n)
	System.out.println(Pattern.matches("[amn]", "a"));//true (among a or m or n)
	System.out.println(Pattern.matches("[amn]", "ammmna"));//false (m and a comes more than once)
	}}
	

Ourput:-

Regex Quantifiers

The quantifiers specify the number of occurrences of a character.

Regex	Description
X?	X occurs once or not at all
X+	X occurs once or more times
X*	X occurs zero or more times
X{n}	X occurs n times only
X{n,}	X occurs n or more times
X{y,z}	X occurs at least y times but less than z times

Regular Expression Character classes and Quantifiers Example

	import java.util.regex.*;
	class Regex4{
	public static void main(String args[]){
	System.out.println("? quantifier ....");
	System.out.println(Pattern.matches("[amn]?", "a"));//true (a or m or n comes one time)
	System.out.println(Pattern.matches("[amn]?", "aaa"));//false (a comes more than one time)
	System.out.println(Pattern.matches("[amn]?", "aammmnn"));//false (a m and n comes more than one time)
	System.out.println(Pattern.matches("[amn]?", "aazzta"));//false (a comes more than one time)
	System.out.println(Pattern.matches("[amn]?", "am"));//false (a or m or n must come one time)
	
	System.out.println("+ quantifier ....");
	System.out.println(Pattern.matches("[amn]+", "a"));//true (a or m or n once or more times)
	System.out.println(Pattern.matches("[amn]+", "aaa"));//true (a comes more than one time)
	System.out.println(Pattern.matches("[amn]+", "aammmnn"));//true (a or m or n comes more than once)
	System.out.println(Pattern.matches("[amn]+", "aazzta"));//false (z and t are not matching pattern)
	
	System.out.println("* quantifier ....");
	System.out.println(Pattern.matches("[amn]*", "ammmna"));//true (a or m or n may come zero or more times)
	
	}}
	

Ourput:-

Regex Metacharacters

The regular expression metacharacters work as shortcodes.

Regex	Description
.	Any character (may or may not match terminator)
\d	Any digits, short of [0-9]
\D	Any non-digit, short for [^0-9]
\s	Any whitespace character, short for [\t\n\x0B\f\r]
\S	Any non-whitespace character, short for [^\s]
\w	Any word character, short for [a-zA-Z_0-9]
\W	Any non-word character, short for [^\w]
\b	A word boundary
\B	A non word boundary

Regular Expression Metacharacters Example

	  import java.util.regex.*;  
class Regex5{  
public static void main(String args[]){  
System.out.println("metacharacters d...."); 
System.out.println(Pattern.matches("\\d", "abc"));   
System.out.println(Pattern.matches("\\d", "1"));   
System.out.println(Pattern.matches("\\d", "4443"));   
System.out.println(Pattern.matches("\\d", "323abc")); 
System.out.println("metacharacters D....");   
System.out.println(Pattern.matches("\\D", "abc"));   
System.out.println(Pattern.matches("\\D", "1"));   
System.out.println(Pattern.matches("\\D", "4443"));   
System.out.println(Pattern.matches("\\D", "323abc"));   
System.out.println(Pattern.matches("\\D", "m"));   
System.out.println("metacharacters D with quantifier....");  
System.out.println(Pattern.matches("\\D*", "mak"));   
  
}}  

Ourput:-

Regular Expression Question 1

	  import java.util.regex.*;  
 class Regex6{  
 public static void main(String args[]){  
 System.out.println(Pattern.matches("[a-zA-Z0-9]{6}", "arun32"));//true  
 System.out.println(Pattern.matches("[a-zA-Z0-9]{6}", "kkvarun32"));//false (more than 6 char)  
 System.out.println(Pattern.matches("[a-zA-Z0-9]{6}", "JA2Uk2"));//true  
 System.out.println(Pattern.matches("[a-zA-Z0-9]{6}", "arun$2"));//false ($ is not matched)  
 }}  
	

Ourput:-

Regular Expression Question 2

	  import java.util.regex.*;  
class Regex7{  
 public static void main(String args[]){  
System.out.println("by character classes and quantifiers ...");  
System.out.println(Pattern.matches("[789]{1}[0-9]{9}", "9953038949"));//true  
System.out.println(Pattern.matches("[789][0-9]{9}", "9953038949"));//true  
  
System.out.println(Pattern.matches("[789][0-9]{9}", "99530389490"));//false (11 characters)  
System.out.println(Pattern.matches("[789][0-9]{9}", "6953038949"));//false (starts from 6)  
System.out.println(Pattern.matches("[789][0-9]{9}", "8853038949"));//true  
  
System.out.println("by metacharacters ...");  
System.out.println(Pattern.matches("[789]{1}\\d{9}", "8853038949"));//true  
System.out.println(Pattern.matches("[789]{1}\\d{9}", "3853038949"));//false (starts from 3)  
  
}}  
	

Ourput:-

Java Regex Finder Example

	import java.util.regex.Pattern;
	import java.util.Scanner;
	import java.util.regex.Matcher; 
	public class Regex8{ 
	public static void main(String[] args){ 
	Scanner sc=new Scanner(System.in);
	while (true) { 
	System.out.println("Enter regex pattern:");
	Pattern pattern = Pattern.compile(sc.nextLine()); 
	System.out.println("Enter text:");
	Matcher matcher = pattern.matcher(sc.nextLine()); 
	boolean found = false; 
	while (matcher.find()) { 
	System.out.println("I found the text "+matcher.group()+" starting at index "+ 
	matcher.start()+" and ending at index "+matcher.end()); 
	found = true; 
	} 
	if(!found){ 
	System.out.println("No match found."); 
	} 
	} 
	} 
	} 
	

Output:

	Enter regex pattern: java
	Enter text: this is java, do you know java
	I found the text java starting at index 8 and ending at index 12
	I found the text java starting at index 26 and ending at index 30

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