Java Regular Expression

Java Regex or Regular Expression is an API to define Pattern for searching or manipulating Strings, In simple word we can say that Regular expression is a representation of a group of String object according to a particular Pattern.
Java provides the java.util.regex package for pattern matching with regular expressions.
java.util.regex package consists of following three classes :

Pattern Class

• In Pattern class Pattern object is a compiled version of a regular expression.
• It contains a compile() method which is used to create a Pattern.
• Pattern class provides no Public constructors. 

           Public static Pattern compile(String)
Matcher Class

• Matcher object match the given pattern in the  target String
• Matcher object is created by matcher() method of Pattern Class

    Public Matcher matcher (String target)
Important Methods of Matcher class

S. No.	Method	Description
1	boolean find()	Returns boolean value after match
2	int start()	Returns the start index of the previous match.
3	int end()	Returns the offset after the last character matched.

Pattern Syntax Exception
A pattern checked Exception is an Unchecked Exception that indicates a syntax error in Regular Expression.
Character Classes

Classes	Description
[abc]	Either a or b or c
[^abc]	Except a,b and c
[a-z]	Any Lower case Alphabet Symbol
[A-Z]	Any Upper  case Alphabet Symbol
[a-zA-Z]	Any Alphabet Symbol
[0-9]	Any Digit from 0 to 9
[a-zA-Z0-9]	Any Alphabet Symbol
[^a-zA-Z0-9]	Except Alpha Numeric Symbols

import java.util.regex.*;

public class RegexDemo {
    public static void main(String a[]) {
        Pattern p = Pattern.compile("x");
        Matcher m = p.matcher("ba9j@z# 9");
        while (m.find()) {
            System.out.println(m.start() + "...................." + m.group());
        }
    }
}

Now, we will provide value of x which is our above character classes and after this we will obtain following outputs

x	Out Put
[abc]	0....................b
	1....................a
[^abc]	2....................9
	3....................j
	4....................@
	5....................z
	6....................#
	7....................
	8....................9
[a-z]	0....................b
	1....................a
	3....................j
	5....................z
[A-Z]
[a-zA-Z]	0....................b
	1....................a
	3....................j
	5....................z
[0-9]	2....................9
	8....................9
[a-zA-Z0-9]	0....................b
	1....................a
	2....................9
	3....................j
	5....................z
	8....................9
[^a-zA-Z0-9]	4....................@
	6....................#
	7....................

Pre-Defined Character Classes

Classes	Description
\s	Space Character
\S	Any Character Except Space
\d	Any Digit from [0-9]
\D	Any Character Except Digit
\w	Any word charatcer
\W	Except word Character
.	Any Character

Clear Concept with Example

import java.util.regex.*;
public class RegexDemo2 {

    public static void main(String[] args) {
        Pattern p = Pattern.compile("x");
        Matcher m = p.matcher("l3c J@8");
        while (m.find()) {
            System.out.println(m.start() + "...................." + m.group());
        }
    }
}
}

Here, value of x is continuously changing and w.r.t we obtained different values which is shown below in tabular form.

x	Out Put
\\s	3....................
\\S	0....................l
	1....................3
	2....................c
	4....................J
	5....................@
	6....................8
\\d	1....................3
	6....................8
\\D	0....................l
	2....................c
	3....................
	4....................J
	5....................@
\\w	0....................l
	1....................3
	2....................c
	4....................J
	6....................8
\\W	3....................
	5....................@
.	0....................l
	1....................3
	2....................c
	3....................
	4....................J
	5....................@
	6....................8

Quantifiers
Quantifiers are used to perform number of occurrence of a specific pattern. 

Classes	Description
a	Exactly one a
a+	At least one a
a*	Any number of a including Zero also
a?	At least one a