How to Match Nested Brackets Without Recursion or Balancing Groups?

Matching Nested Brackets Without Recursion or Balancing Groups

Challenge:

Match a set of arbitrarily nested brackets using a regex flavor that lacks recursion and balancing group support, such as Java's java.util.regex, capturing three outer groups within the given string:

(F(i(r(s)t))) ((S)(e)((c)(o))(n)d) (((((((Third)))))))

Solution: Forward References to the Rescue

Contrary to common belief, matching nested brackets without these advanced features is possible using forward references:

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

Break it Down:

This complex regex operates in two stages:

• Stage 1: Lookaheads identify sequences of balanced parentheses without consuming characters. This process continues until a complete outer group is found.
• Stage 2: A capturing group extracts the contents of the matched outer group.

How it Works:

• Checking for '(': Advance through the string until a '(' is encountered.
• Matching Balanced Parentheses: Use two lookaheads to ensure the next '(' and ')' are matched in a balanced way, capturing the inner content.
• Closing Bracket Check: Check for a closing ')' that matches the previously matched '(' and advance further.
• End-of-Outer-Group Check: Ensure that no more '(' appear before the closing ')' of the outer group.

Inner Group Matching Variant:

For matching inner groups, the strategy remains the same, but a capturing group is used to save the matched content within a balanced pair of parentheses:

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

Comprehensive Breakage:

A table summarizes the components and functionality of the regex:

Note	Component	Description
	(?=()	Look for '('
	(?:	Start group for iteration
	(?=.?((?!.?1))	Look for '(' not followed by 1, which contains the matched inner content
	(.)(?!.2).*))	Capture inner content and check for at least one more ')'
	(?=.?)(?!.?3))	Look for ')' not followed by 2, which contains the matched outer content
	(. )	Capture outer content
	.	Consume a character
	)	Close group
	?	Match as few times as possible
	.*?(?=1)	Match up to and including the last '('
	1*(?=2$)	Match up to the last ')' without encountering more '('

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