The Java Regex or Regular Expression is an API to define a pattern for searching or manipulating strings.
It is widely used to define the constraint on strings such as password and email validation. After learning Java regex section, you will be able to test your regular expressions by the Java Regex Tester Tool.
Java Regex API provides 1 interface and 3 classes in java.util.regex package.
java.util.regex package
The Matcher and Pattern classes provide the facility of Java regular expression. The java.util.regex package provides following classes and interfaces for regular expressions.
1. MatchResult interface
2. Matcher class
3. Pattern class
4. PatternSyntaxException class

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Table of Contents

Matcher class

It implements the MatchResult interface. It is a regex engine which is used to perform match operations on a character sequence.

No.	Method	Description
1	booleanmatches()	test whether the regular expression matches the pattern.
2	booleanfind()	finds the next expression that matches the pattern.
3	booleanfind(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 the compiled version of a regular expression. 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 booleanmatches(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.

1. import java.util.regex.*; 
2. public class RegexExample1{ 
3. public static void main(String args[]){ 
4. //1st way 
5. Pattern p = Pattern.compile(".s");//. represents single character 
6. Matcher m = p.matcher("as"); 
7. boolean b = m.matches(); 
8. 
9. //2nd way 
10. boolean b2=Pattern.compile(".s").matcher("as").matches(); 
11. 
12. //3rd way 
13. boolean b3 = Pattern.matches(".s", "as"); 
14. 
15. System.out.println(b+" "+b2+" "+b3); 
16. }}

Output
true truetrue

Regular Expression . Example

The . (dot) represents a single character.

1. import java.util.regex.*; 
2. class RegexExample2{ 
3. public static void main(String args[]){ 
4. System.out.println(Pattern.matches(".s", "as"));//true (2nd char is s) 
5. System.out.println(Pattern.matches(".s", "mk"));//false (2nd char is not s) 
6. System.out.println(Pattern.matches(".s", "mst"));//false (has more than 2 char) 
7. System.out.println(Pattern.matches(".s", "amms"));//false (has more than 2 char) 
8. System.out.println(Pattern.matches("..s", "mas"));//true (3rd char is s) 
9. }}

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

1. import java.util.regex.*; 
2. class RegexExample3{ 
3. public static void main(String args[]){ 
4. System.out.println(Pattern.matches("[amn]", "abcd"));//false (not a or m or n) 
5. System.out.println(Pattern.matches("[amn]", "a"));//true (among a or m or n) 
6. System.out.println(Pattern.matches("[amn]", "ammmna"));//false (m and a comes more than once) 
7. }}

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

1. import java.util.regex.*; 
2. class RegexExample4{ 
3. public static void main(String args[]){ 
4. System.out.println("? quantifier ...."); 
5. System.out.println(Pattern.matches("[amn]?", "a"));//true (a or m or n comes one time) 
6. System.out.println(Pattern.matches("[amn]?", "aaa"));//false (a comes more than one time) 
7. System.out.println(Pattern.matches("[amn]?", "aammmnn"));//false (a m and n comes more than one time) 
8. System.out.println(Pattern.matches("[amn]?", "aazzta"));//false (a comes more than one time) 
9. System.out.println(Pattern.matches("[amn]?", "am"));//false (a or m or n must come one time) 
10. 
11. System.out.println("+ quantifier ...."); 
12. System.out.println(Pattern.matches("[amn]+", "a"));//true (a or m or n once or more times) 
13. System.out.println(Pattern.matches("[amn]+", "aaa"));//true (a comes more than one time) 
14. System.out.println(Pattern.matches("[amn]+", "aammmnn"));//true (a or m or n comes more than once) 
15. System.out.println(Pattern.matches("[amn]+", "aazzta"));//false (z and t are not matching pattern) 
16. 
17. System.out.println("* quantifier ...."); 
18. System.out.println(Pattern.matches("[amn]*", "ammmna"));//true (a or m or n may come zero or more times) 
19. 
20. }}

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

1. import java.util.regex.*; 
2. class RegexExample5{ 
3. public static void main(String args[]){ 
4. System.out.println("metacharacters d....");\\d means digit 
5. 
6. System.out.println(Pattern.matches("\\d", "abc"));//false (non-digit) 
7. System.out.println(Pattern.matches("\\d", "1"));//true (digit and comes once) 
8. System.out.println(Pattern.matches("\\d", "4443"));//false (digit but comes more than once) 
9. System.out.println(Pattern.matches("\\d", "323abc"));//false (digit and char) 
10. 
11. System.out.println("metacharacters D....");\\D means non-digit 
12. 
13. System.out.println(Pattern.matches("\\D", "abc"));//false (non-digit but comes more than once) 
14. System.out.println(Pattern.matches("\\D", "1"));//false (digit) 
15. System.out.println(Pattern.matches("\\D", "4443"));//false (digit) 
16. System.out.println(Pattern.matches("\\D", "323abc"));//false (digit and char) 
17. System.out.println(Pattern.matches("\\D", "m"));//true (non-digit and comes once) 
18. 
19. System.out.println("metacharacters D with quantifier...."); 
20. System.out.println(Pattern.matches("\\D*", "mak"));//true (non-digit and may come 0 or more times) 
21. 
22. }}

Regular Expression Question 1

1. /*Create a regular expression that accepts alphanumeric characters only. 
2. Its length must be six characters long only.*/ 
3. 
4. import java.util.regex.*; 
5. class RegexExample6{ 
6. public static void main(String args[]){ 
7. System.out.println(Pattern.matches("[a-zA-Z0-9]{6}", "arun32"));//true 
8. System.out.println(Pattern.matches("[a-zA-Z0-9]{6}", "kkvarun32"));//false (more than 6 char) 
9. System.out.println(Pattern.matches("[a-zA-Z0-9]{6}", "JA2Uk2"));//true 
10. System.out.println(Pattern.matches("[a-zA-Z0-9]{6}", "arun$2"));//false ($ is not matched) 
11. }}

Regular Expression Question 2

1. /*Create a regular expression that accepts 10 digit numeric characters 
2. starting with 7, 8 or 9 only.*/ 
3. 
4. import java.util.regex.*; 
5. class RegexExample7{ 
6. public static void main(String args[]){ 
7. System.out.println("by character classes and quantifiers ..."); 
8. System.out.println(Pattern.matches("[789]{1}[0-9]{9}", "9953038949"));//true 
9. System.out.println(Pattern.matches("[789][0-9]{9}", "9953038949"));//true 
10. 
11. System.out.println(Pattern.matches("[789][0-9]{9}", "99530389490"));//false (11 characters) 
12. System.out.println(Pattern.matches("[789][0-9]{9}", "6953038949"));//false (starts from 6) 
13. System.out.println(Pattern.matches("[789][0-9]{9}", "8853038949"));//true 
14. 
15. System.out.println("by metacharacters ..."); 
16. System.out.println(Pattern.matches("[789]{1}\\d{9}", "8853038949"));//true 
17. System.out.println(Pattern.matches("[789]{1}\\d{9}", "3853038949"));//false (starts from 3) 
18. 
19. }}

Java Regex Finder Example

1. import java.util.regex.Pattern; 
2. import java.util.Scanner; 
3. import java.util.regex.Matcher; 
4. public class RegexExample8{ 
5. public static void main(String[] args){ 
6. Scanner sc=new Scanner(System.in); 
7. while (true) { 
8. System.out.println("Enter regex pattern:"); 
9. Pattern pattern = Pattern.compile(sc.nextLine()); 
10. System.out.println("Enter text:"); 
11. Matcher matcher = pattern.matcher(sc.nextLine()); 
12. boolean found = false; 
13. while (matcher.find()) { 
14. System.out.println("I found the text "+matcher.group()+" starting at index "+ 
15. matcher.start()+" and ending at index "+matcher.end()); 
16. found = true; 
17. } 
18. if(!found){ 
19. System.out.println("No match found."); 
20. } 
21. } 
22. } 
23. }

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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