In Java string manipulation, searching for specific words is a fundamental task. Regular expressions (regex) offer a powerful and flexible approach to achieve this search. It matches the patterns simply by comparing substrings.

In this article, we will learn how to search for a particular word in a string using regex in Java.

Regular Expressions:

• Patterned expressions are used to represent sequences of characters and match them against text.
• Composed of special characters and metacharacters with specific meanings.

Word Boundaries:

• Ensures that matches occur as complete words rather than parts of other words.
• Represented by \b in regex patterns.

Step-by-Step Implementation

Step 1: Import java.util.regex

import java.util.regex.*;

Step 2: Compile the Regex Pattern

• Create a Pattern object using Pattern.compile().
• Enclose the word you want to search for (e.g., “Java”) within word boundaries \b.
• Optionally, use case-insensitive matching with Pattern.CASE_INSENSITIVE.

String word = "Java"; // Word to search for
Pattern pattern = Pattern.compile("\\b" + word + "\\b", Pattern.CASE_INSENSITIVE);

Step 3: Create a Matcher

• Instantiate a Matcher object using the compiled pattern and the string to search.
• find() attempts to find the first occurrence of the word.

String text = "This is a string containing the word Java.";
Matcher matcher = pattern.matcher(text);

Step 4: Iterate and Process Matches

• Use a loop to iterate through matches using find().
• Extract matched words:
  • start() returns the index of the match’s first character.
  • end() returns the index after the last character (non-inclusive).
  • Use substring extraction to get the matching word.

Example with output formatting:

while (matcher.find()) {
    int start = matcher.start();
    int end = matcher.end();
    String match = text.substring(start, end);
    System.out.println("Found word \"" + match + "\" at index [" + start + ", " + (end - 1) + "]");
}

Step 5: Handling No Matches

• If find() doesn’t find any matches, inform the user or handle it within your logic.

Program to Search a Particular Word in a String Using Regex in Java

Found word "Java" at index [37, 40]
Found word "Java" at index [55, 58]