How to Use Negative Lookbehind in Java Regular Expressions?

How can I implement negative lookbehind in Java regular expressions?

String regex = "(?<!not)\bword\b"; // This matches 'word' only if it is not preceded by 'not'.

Negative lookbehind is a powerful feature in Java regular expressions (regex) that allows you to assert what should not precede a certain pattern. This is useful for matching strings under specific conditions, enhancing your search capabilities.

// Example of a negative lookbehind in action:
String input = "not a word, just a word";
String regex = "(?<!not\s)\bword\b";
Pattern pattern = Pattern.compile(regex);
Matcher matcher = pattern.matcher(input);
while (matcher.find()) {
    System.out.println("Matched: " + matcher.group());
} // Output will be 'word' only if 'not' does not precede it.

Causes

• Understanding the syntax of negative lookbehind: Lookbehind asserts that a given expression does not occur before the main pattern you are trying to match.
• The absence of negative lookbehind expressions in earlier versions of Java, which restricts their use unless you are using Java 9 or later.

Solutions

• Use the syntax `(?<!pattern)` to create a negative lookbehind, where `pattern` is the expression that must not be present before the main expression.
• Ensure you are using at least Java 9, as earlier versions do not support this feature.

Mistake: Using a negative lookbehind in an unsupported version of Java.

Solution: Ensure your Java version is 9 or later to utilize negative lookbehind.

Mistake: Incorrect regex syntax leading to unexpected matches or errors.

Solution: Double-check your regex pattern against official Java regex documentation to ensure proper syntax.