Java Regex

The Regular expression is an API which defines a pattern for searching strings. Abbreviation for regular expression is called regex.

The search pattern is everything from a character, and a complex expression containing special characters that tell the pattern. The pattern that defined by regex may match one or several times or not at all for a given string. Regex is used to search, edit and manipulate text.

The process of modifying a text with a regex is to the string or text. The pattern is applied to the text from left to right. Once source character has been used in a match, it cannot be reused.

The regex is mainly used to define the constraint on strings such as email validation and password.

Java.util.regex package

Classes are for matching character sequences against patterns specified by regular expressions. This package provides classes and interfaces as following:

• MatchResult interface:

  It is the result of a match operation.

• Matcher class:

  It is an engine that performs match operations on a character sequence by interpreting a pattern.

• Pattern class:

  It is compiled representation of a regular expression and used to define a pattern for regex engine.

• PatternSyntaxException class:

  It is an unchecked exception thrown that indicate a syntax error in a regular expression pattern.

Matcher class

Pattern class

Example of Java Regex:

The Java regex can write in 3 ways:

1st way

import java.util.regex.*;  

public class Regex1{  
	public static void main(String args[]){  
		Pattern p = Pattern.compile(".e");
        //dot represents single character  
		Matcher m = p.matcher("be");  
		boolean ba = m.matches();  
  		System.out.println(ba);
	}   
} 

Output

true  

2nd way

import java.util.regex.*;  

public class Regex2{  
	public static void main(String args[]){  
		boolean ba=Pattern.compile(".e").matcher("be").matches();  
    	System.out.println(ba);
	}   
} 

Output

true 

3rd way

import java.util.regex.*;  

public class Regex3{  
	public static void main(String args[]){  
		boolean ba = Pattern.matches(".e", "be");  
		System.out.println(ba);
	}   
} 

Output

true 

Example

The dot represents a single character.

import java.util.regex.*;  

public class RegexExample{  
	public static void main(String args[]){  
		System.out.println(Pattern.matches(".e", "be"));
        //true  
		System.out.println(Pattern.matches(".e", "as"));
        //false  
		System.out.println(Pattern.matches(".e", "bet"));
        //false (It has more than 2 char)    
		System.out.println(Pattern.matches("..e", "tie"));
        //true (3rd char is e)  
	}
}