What is regex recursion and (?(DEFINE)) in Perl?

Regex recursion in Perl allows for the matching of nested patterns using the "subroutine" technique, where part of a regular expression can call itself. This is particularly useful for matching cases like nested parentheses or balanced tags.

The `(?(DEFINE))` construct in Perl regex is used to define sub-patterns that can be referenced later within the same expression. This technique is useful for creating more complex patterns without cluttering the main regex with repetitive code.

Example of Regex Recursion and (?(DEFINE))


        // Example of regex recursion to match nested parentheses
        $pattern = qr{
            (?(DEFINE)
                (?&open)  # Define open parenthesis
                \(        # Match an open parenthesis
            )
            (?(DEFINE)
                (?&close) # Define close parenthesis
                \)        # Match a close parenthesis
            )
            (?&open)       # Start matching an open parenthesis
            (?:           # Non-capturing group for contents
                (?:      # Non-capturing group for recursive matching
                    (?:   # Match either a close or another open parenthesis
                        (?&open) | (?&close)
                    )?     # Match it zero or more times
                )*       # Repeat to match nested patterns
            )
            (?&close)      # Ensure to close with a corresponding close parenthesis
        }x;

        $string = "((abc)(def(g(h))))";
        if ($string =~ $pattern) {
            echo "Matched!";
        } else {
            echo "Did not match.";
        }

regex recursion perl (?(DEFINE)) nested patterns regular expressions

What is regex recursion and (?(DEFINE)) in Perl?

Example of Regex Recursion and (?(DEFINE))

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What is regex recursion and (?(DEFINE)) in Perl?

Example of Regex Recursion and (?(DEFINE))

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