How to Match Nested Brackets Without Recursion or Balancing Groups in Regex?

Matching Nested Brackets Without Recursion or Balancing Groups

The problem of matching nested brackets without recursion or balancing groups arises when using regex flavors that lack these features. This task presents a unique challenge, as regular expressions are not typically well-suited for handling nested structures.

Solving the Puzzle Using Forward References

A solution to this problem involves using forward references to capture substrings within nested parentheses. The following regex accomplishes this:

(?=\()(?:(?=.*?\((?!.*?)(.*\)(?!.*).*))(?=.*?\)(?!.*?)(.*)).)+?.*?(?=)[^(]*(?=$)

Breaking Down the Expression

This complex expression consists of multiple components that work together to match nested groups of parentheses:

• Positive Lookahead for Opening Parenthesis: (?=(). This matches and consumes a '(' only if it exists.
• Group Iteration: (?:...) ?. This group iterates through the string, matching multiple balanced groups.
• Identifying Nested Opening Parentheses: (?=.*?((?!.*?1). This matches the next occurrence of '(' that is not preceded by the captured substring 1, ensuring that we don't match the same '(' again.
• Capturing Substring Inside Brackets: (.*)(?!.*2).*. This captures the portion of the string between the matched '(' and the next ')' that is not followed by the captured substring 2.
• Identifying Nested Closing Parentheses: (?=.*?)(?!.*?2). This matches the next occurrence of ')' that is not preceded by the captured substring 2, again ensuring non-redundant matching.
• Capturing Rest of Substring: (.*). This captures the remaining part of the string after the matched ')' and before the next '(' or end of the string.
• Matching and Consuming Characters: . This consumes a single character, allowing the group to continue matching.
• Final Validation: .*?(?=1)[^(]*(?=2$). This checks and validates that the last matched '(' and ')' form a balanced group.

By utilizing these components, the expression matches groups of nested parentheses at every iteration until the end of the string is reached.

Additional Note

The solution provided here is designed for flavors of regex that support forward references. For flavors that do not, such as JavaScript, this technique cannot be directly applied.

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