Extract Single Quote Enclosed String Using Regex in Java

Regex or Regular Expression is the language used for pattern-matching and string manipulation. It consists of a sequence of characters that define a search pattern and can be used for performing actions like search, replace, and even validate on text input. A regular expression consists of a series of characters and symbols that amount to form a search pattern.

In this article, we are going to see how to write a Java program to extract a single quote-enclosed string from a larger string using Regex.

Java provides support for regex from the java.util.regex package. The pattern class represents a compiled regular expression and the matcher class can be used for matching a pattern against a given input string.

Problem Statement

Write a Java program to extract one or more substrings enclosed in single quotes from a larger string using regular expressions. The program should be able to handle both single and multiple occurrences of such substrings.

Input

input = "This is a 'single quote' enclosed string"

Output

single quote

Single Substring Enclosed in Single Quotes

Following are the steps to extract a single quote enclosed string from a larger string using Regex ?

• Import the necessary classes.
  
• Declare a string variable containing the input text.
• We will create a Regex pattern by using the Pattern.compile() method to define a Regex pattern that matches text within single quotes.
• Create the matcher object by instantiating a matcher object using the Pattern.matcher() method with the input string.
• By using the find() method to search for the pattern in the input string. If found, extract the substring using the group(1) method.
• Print the extracted substring.

Example

In the example below, we will first be defining the input string as well as the regex pattern we want to match. The pattern ?(_+?)' matches any sequence of characters enclosed within single quotes and the part _*? Matches any character 0 or more times but as few times as possible in order to allow the rest of the pattern to match.

We then create a Matcher object from the pattern to apply to the input string with the help of the find method. In the event that the pattern matches, we extract the matched string using the group() method with a parameter of 1 which is representative to the 1st capture group in the pattern. This is the drawback of this method that it does not capture all groups of single quotes enclosed substrings.

import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class StringExtractor {
   public static void main(String[] args) {
      String input = "This is a 'single quote' enclosed string";
      Pattern pattern = Pattern.compile("'(.*?)'");
      Matcher matcher = pattern.matcher(input);
        
      if (matcher.find()) {
         String extractedString = matcher.group(1);
         System.out.println(extractedString);
      }
   }
}

Output

single quote

Multiple Single Quote Enclosed Substring

Following are the steps to extract a single quote enclosed string from a larger string using Regex ?

• Import the necessary classes.
• Declare a string variable with the input text.
• Use Pattern.compile() to define a regex pattern that matches text within single quotes.
• Instantiate a Matcher object with the input string.
• Create a list to store all matched substrings.
• Use a while loop to find all matches in the input string using the find() method. Extract each match using group(1) and add it to the list.
• Iterate through the list and print each extracted substring.

Example

The above method had 1 major drawback that it was too simple and could not extract multiple single quote enclosed substrings from the input string and extracted only the 1st occurrence. This is an updated and advanced version of the previous method as it is capable of extracting multiple occurrences. We make use of a while loop to iterate and keep searching for matches till none are left in the input string. The matches list is used to store all the extracted strings and is returned by the method. The main method demonstrates how to make use of the updated extractStringsWithRegex() method for extracting all single quote enclosed strings.

import java.util.regex.Matcher;
import java.util.regex.Pattern;
import java.util.ArrayList;
import java.util.List;
public class StringExtractor {    
   public static List<String> extractStringsWithRegex(String input) {
      // This function takes string as input, iterates over to search for regex matches
      // and stores them in a List named matches which is finally returned in the end
      Pattern pattern = Pattern.compile("'(.*?)'");
      Matcher matcher = pattern.matcher(input);
      List<String> matches = new ArrayList<>();
      while (matcher.find()) {
         matches.add(matcher.group(1));
      }
      return matches;
   }   
   public static void main(String[] args) {
      String input = "This is a 'test' string with 'multiple' 'single quote' enclosed 'words'";
      List<String> matches = extractStringsWithRegex(input);
      for (String match : matches) {
         System.out.println(match);
      }
   }
}

Output

test
multiple
single quote
words

The Java program to extract a single quote enclosed string from a larger string using regex has some advantages and disadvantages which are as follows.

Advantages

• Regex is highly powerful and allows the matching of single quote enclosed strings and even more complicated patterns to be matched.

• The Matcher class provides us with additional methods for working with the matched string like finding the start and end indices of the match.

Disadvantages

• Writing and understanding regex can be more difficult to understand as compared to other methods.

• Regex may be slower as compared to other methods, especially for large input strings or complex patterns.

Conclusion

There are several ways to extract strings enclosed in single quotes, with the most common being regex, split(), and substring() methods. Regex is a powerful and flexible option, especially for complex patterns, though it can be slower for large strings. The Pattern class defines the regex pattern, while the Matcher class applies it to the input string and extracts the matching text. Regex is widely used for tasks like validating user input and manipulating text, but it's crucial to design and test patterns carefully to handle all edge cases effectively.