What is a Regular Expression?

A regular expression is a sequence of characters that forms a search pattern. When you search for data in a text, you can use this search pattern to describe what you are searching for.

A regular expression can be a single character or a more complicated pattern.

Regular expressions can be used to perform all types of text search and text replace operations.

Java does not have a built-in Regular Expression class, but we can import the java.util.regex package to work with regular expressions. The package includes the following classes:

• Pattern Class - Defines a pattern (to be used in a search)
• Matcher Class - Used to search for the pattern
• PatternSyntaxException Class - Indicates a syntax error in a regular expression pattern

Example

Find out if there are any occurrences of the word "Coding75" in a sentence:

import java.util.regex.Matcher;
import java.util.regex.Pattern;
 
public class Main {
  public static void main(String[] args) {
    Pattern pattern = Pattern.compile("Coding75", Pattern.CASE_INSENSITIVE);
    Matcher matcher = pattern.matcher("Visit Coding75!");
    boolean matchFound = matcher.find();
    if(matchFound) {
      System.out.println("Match found");
    } else {
      System.out.println("Match not found");
    }
  }
}

Output:

Match found

Example Explained

• The word "Coding75" is being searched for in a sentence.
• The Pattern.compile() method is used to create the pattern.
  • The first parameter specifies the pattern to search for.
  • The second parameter (Pattern.CASE_INSENSITIVE) makes the search case-insensitive (optional).
• The matcher() method searches for the pattern in the string and returns a Matcher object.
• The find() method returns true if the pattern is found; otherwise, it returns false.

Flags

Flags in the compile() method change how the search is performed:

• Pattern.CASE_INSENSITIVE - Ignores letter case when searching.
• Pattern.LITERAL - Treats special characters as ordinary characters.
• Pattern.UNICODE_CASE - Used with CASE_INSENSITIVE to ignore case in Unicode characters.

Regular Expression Patterns

The first parameter of the Pattern.compile() method is the pattern, which defines what to search for.

Brackets

Brackets are used to find a range of characters:

Metacharacters

Metacharacters are characters with a special meaning:

Quantifiers

Quantifiers define quantities:

This guide provides a basic understanding of Java Regular Expressions, their usage, and common patterns.