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Java Regex | Regular Expression

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Java Regex

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 tutorial, 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

Java Regex API

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 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 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 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.

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Output

true true true 

Regular Expression . Example

The . (dot) represents a single character.

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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

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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

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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 [tnx0Bfr]
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

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Regular Expression Question 1


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Regular Expression Question 2

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Java Regex Finder Example

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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