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Avoid Testing Empty Lines with Regex MULTILINE Flag without Checking String Emptiness

Medium
maintainabilityerror-prone

What is it?​

This practice is triggered by the use of regular expressions with the MULTILINE flag to detect empty lines without checking if the entire string is empty. The use of ^$ with the MULTILINE flag does not match the string end for empty checks, which can lead to missing out empty lines at the end of inputs.

Why apply it?​

Using regex with the MULTILINE flag along with ^$ can lead to incorrect assumptions about empty lines since it fails to match empty strings at the end of input. By ensuring that we check if a string is completely empty, we can accurately detect empty lines.

How to fix it?​

When searching for empty lines, combine the use of the ^$ pattern with a check for emptiness using isEmpty() method of the string.

Examples​

Example 1:​

Negative​

The negative example only uses the ^$ regex with the MULTILINE flag without verifying if the entire input string is empty.

import java.util.regex.Pattern;

public class EmptyLineChecker {
    static final Pattern p = Pattern.compile("^$", Pattern.MULTILINE);  /* Noncompliant */

    public boolean containsEmptyLines(String str) {
        return p.matcher(str).find();
    }

    // Testing the method
    public static void main(String[] args) {
        EmptyLineChecker checker = new EmptyLineChecker();
        System.out.println(checker.containsEmptyLines("line1\n\nline2"));  // Correctly prints 'true'
        System.out.println(checker.containsEmptyLines(""));                // Incorrectly prints 'false'
    }
}

Example 2:​

Positive​

The positive example correctly combines regex pattern matching for empty lines with a check that the input string itself may be empty.

import java.util.regex.Pattern;

public class EmptyLineChecker {
    static final Pattern p = Pattern.compile("^$", Pattern.MULTILINE);  /* Compliant */

    public boolean containsEmptyLines(String str) {
        return p.matcher(str).find() || str.isEmpty();
    }

    // Testing the method
    public static void main(String[] args) {
        EmptyLineChecker checker = new EmptyLineChecker();
        System.out.println(checker.containsEmptyLines("line1\n\nline2"));  // Prints 'true'
        System.out.println(checker.containsEmptyLines(""));                // Prints 'true'
    }
}

Negative​

The negative example misses checking if the string itself is empty, thus failing to identify scenarios where the entire input is empty.

import java.util.regex.Pattern;

public class ContentAnalyzer {
    private static final Pattern emptyLinePattern = Pattern.compile("^$", Pattern.MULTILINE);  /* Noncompliant */

    public boolean hasEmptyLines(String content) {
        return emptyLinePattern.matcher(content).find();
    }

    public static void main(String[] args) {
        ContentAnalyzer analyzer = new ContentAnalyzer();
        System.out.println(analyzer.hasEmptyLines("text\n\nmore text"));  // Correctly prints 'true'
        System.out.println(analyzer.hasEmptyLines(""));                   // Incorrectly prints 'false'
        System.out.println(analyzer.hasEmptyLines("\nonly line"));        // Correctly prints 'true'
    }
}

Example 3:​

Positive​

The positive example utilizes both the regex and a direct isEmpty() check to correctly identify empty lines or if the string itself is empty.

import java.util.regex.Pattern;

public class ContentAnalyzer {
    private static final Pattern emptyLinePattern = Pattern.compile("^$", Pattern.MULTILINE);  /* Compliant */

    public boolean hasEmptyLinesOrString(String content) {
        return emptyLinePattern.matcher(content).find() || content.isEmpty();
    }

    public static void main(String[] args) {
        ContentAnalyzer analyzer = new ContentAnalyzer();
        System.out.println(analyzer.hasEmptyLinesOrString("text\n\nmore text"));  // Prints 'true'
        System.out.println(analyzer.hasEmptyLinesOrString(""));                   // Prints 'true'
        System.out.println(analyzer.hasEmptyLinesOrString("\nonly line"));        // Prints 'true'
    }
}