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Java Regex

The Java Regex or Regular Expression is an API to define pattern for searching or manipulating strings.

It is widely used to define constraint on strings such as password and email validation. After learning java regex tutorial, you will be able to test your own 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

It provides following classes and interface for regular expressions. The Matcher and Pattern classes are widely used in java regular expression.

  1. MatchResult interface
  2. Matcher class
  3. Pattern class
  4. PatternSyntaxException class

Matcher class

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

No.MethodDescription
1boolean matches()test whether the regular expression matches the pattern.
2boolean find()finds the next expression that matches the pattern.
3boolean find(int start)finds the next expression that matches the pattern from the given start number.
4String group()returns the matched subsequence.
5int start()returns the starting index of the matched subsequence.
6int end()returns the ending index of the matched subsequence.
7int 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.MethodDescription
1static Pattern compile(String regex)compiles the given regex and return the instance of pattern.
2Matcher matcher(CharSequence input)creates a matcher that matches the given input with pattern.
3static 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.
4String[] split(CharSequence input)splits the given input string around matches of given pattern.
5String 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 ClassDescription
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.

RegexDescription
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 a short codes.

RegexDescription
.Any character (may or may not match terminator)
\dAny digits, short of [0-9]
\DAny non-digit, short for [^0-9]
\sAny whitespace character, short for [\t\n\x0B\f\r]
\SAny non-whitespace character, short for [^\s]
\wAny word character, short for [a-zA-Z_0-9]
\WAny non-word character, short for [^\w]
\bA word boundary
\BA 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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