Using Regular Expression in Java

This article provides an introduction to usage of regular expressions in Java with some examples. Regular Expression or Regex in the computing terms refers to a sequence of symbols and characters expressing a string or pattern to be searched within a longer piece of text. There is a different syntax to regular expressions beyond the Java programming language. It is essential to understand the basics of regular expressions clearly to understand or create complex regular expressions. Regular Expression syntax in Java is more similar to the one used with Perl language.

Regex Constructs

In general regular expression include characters, character classes and quantifiers. The table below provides a reference to most common Regex constructs taken from the Java reference material.

Construct	Matches
Characters
x	The character x
\\	The backslash character
\t	The tab character
\n	The newline character
\r	The carriage-return character
Character Classes
[abc]	a, b, or c (simple class)
[^abc]	Any character except a, b, or c (negation)
[a-zA-Z]	a through z or A through Z, inclusive (range)
Predefined character classes
.	Any character (may or may not match line terminators)
\d	A digit: [0-9]
\s	A whitespace character: [ \t\n\x0B\f\r]
\S	A non-whitespace character: [^\s]
\w	A word character: [a-zA-Z_0-9]
\W	A non-word character: [^\w]
Boundary Matchers
^	The beginning of a line
$	The end of a line
\b	A word boundary
\B	A non-word boundary
Quantifiers
X?	X, once or not at all
X*	X, zero or more times
X+	X, one or more times
X{n}	X, exactly n times
X{n,}	X, at least n times
X{n,m}	X, at least n but not more than m times

Regex Java APIs

Java supports java.util.regex for regular expressions processing. 

1. Primarily there are 2 classes Pattern and Matcher.
2. Pattern is used to define a regular expression. Use the Pattern.compile() factory method to create a pattern.
3. Matcher is used to match the pattern against a input sequence. Use Pattern.matcher() to get create a Matcher.

Following example shows how to use the Pattern and Matcher.

package com.sourcetricks.regex;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class RegExTest {
 
 public static void main(String[] args) {

  // Matches the entire region
  String str = "java";
  Pattern pattern = Pattern.compile("java");
  Matcher matcher = pattern.matcher(str);
  if ( matcher.matches() ) {
   System.out.println("Pattern found.");
  }
  
  // Find if a pattern exists
  str = "This is a java tutorial";
  pattern = Pattern.compile("java");
  matcher = pattern.matcher(str);
  if ( matcher.find() ){
   System.out.println("Pattern found at location = " + matcher.start());
  }
  
  // Using flags to perform case insensitive match
  str = "This is a Java tutorial";
  pattern = Pattern.compile("java", Pattern.CASE_INSENSITIVE);
  matcher = pattern.matcher(str);
  if ( matcher.find() ){
   System.out.println("Pattern found at location = " + matcher.start());
  }
  
  // Split a string using a whitespace
  str = "This is a Java tutorial";
  pattern = Pattern.compile("\\s");
  String[] words = pattern.split(str);
  for ( String word : words )
   System.out.println(word);
  
  // Find all occurrences of a pattern
  str = "This is a Java tutorial. Java is programming language.";
  pattern = Pattern.compile("[jJ]ava");
  matcher = pattern.matcher(str);
  while ( matcher.find() ) {
   System.out.println("Pattern found at location = " + matcher.start());
  }
  // Replace all occurrences of a pattern
  String str1 = matcher.replaceAll("C++");
  System.out.println(str1);
 }
}

This example produces the following output.

Pattern found.
Pattern found at location = 10
Pattern found at location = 10
This
is
a
Java
tutorial
Pattern found at location = 10
Pattern found at location = 25
This is a C++ tutorial. C++ is programming language.