Java RegEx

What is a Regular Expression?

A regular expression, comprised of characters, serves as a pattern for searching within text data, allowing you to specify the criteria for your search.

A regular expression can range from a single character to a more intricate pattern.

Regular expressions enable various types of text search and replace operations.

In Java, there isn’t a dedicated Regular Expression class, but we can utilize the java.util.regex package to handle regular expressions, which includes several classes.

• The Pattern Class: defines a pattern for use in searches.
• The Matcher Class: conducts the search based on the pattern.
• The PatternSyntaxException Class: This class flags syntax errors within a regular expression pattern.

Example

Determine if the word “code7school” appears 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("code7chool", Pattern.CASE_INSENSITIVE);
    Matcher matcher = pattern.matcher("Visit W3Schools!");
    boolean matchFound = matcher.find();
    if(matchFound) {
      System.out.println("Match found");
    } else {
      System.out.println("Match not found");
    }
  }
}

Example Explained

In this instance, the sentence is being scanned for the presence of the term “code7school”.

Initially, the pattern is established using the Pattern.compile() method. The first argument specifies the pattern sought, while the second argument, an optional flag, signifies a case-insensitive search.

The matcher() method is employed to seek the pattern within a string. It yields a Matcher object that encapsulates details regarding the executed search.

The find() method yields true if the pattern is discovered within the string, and false if it is not.

Flags

Flags passed into the compile() method modify the behavior of the search operation. Here are some examples:

1. Pattern.CASE_INSENSITIVE: means that the search will ignore the case of letters.
2. Pattern.LITERAL: indicates that special characters in the pattern will be treated as ordinary characters without any special meaning during a search.
3. Pattern.UNICODE_CASE: When used alongside the CASE_INSENSITIVE flag, it enables the ignoring of letter case even for characters outside the English alphabet during a search.

Regular Expression Patterns

Brackets in the Pattern.compile() method’s first parameter denote a pattern, specifying what is sought during a search.

They are utilized to identify a range of characters.

Metacharacters

Metacharacters are characters endowed with special significance.

Quantifiers

Quantifiers specify quantities.