Java Regular Expression Tutorial - Java Regex Boundaries

To the match the beginning of a line, or match whole word, not part of any word, we have to set the boundary for matchers.

The following table lists Boundary Matchers Inside Regular Expressions

Boundary MatchersMeaning
^The beginning of a line
$The end of a line
\bA word boundary
\BA non-word boundary
\A The beginning of the input
\GThe end of previous match
\ZThe end of the input but for the final terminator, if any
\z The end of the input


The following code demonstrates how to match a word boundary using a regular expression.

//from  w w  w. j a v a 2 s  .  c  om
public class Main {
  public static void main(String[] args) {
    // \\b to get \b inside the string literal.
    String regex = "\\bJava\\b";
    String replacementStr = "XML";
    String inputStr = "Java and Javascript";
    String newStr = inputStr.replaceAll(regex, replacementStr);

    System.out.println("Regular  Expression: " + regex);
    System.out.println("Input String: " + inputStr);
    System.out.println("Replacement String:  " + replacementStr);
    System.out.println("New String:  " + newStr);

The code above generates the following result.