Please feel free to suggest changes to make the code more efficient.

EDIT: for some reason, < is not showing up properly inside a code block. Kindly replace it with the right symbol when you test.

Python:

import sys

input_string = sys.argv[1]

matches = {
    "}": "{",
    "]": "[",
    ")": "("
}

bracket_matches_strict = []

def get_matching_substring_strict(string):
    substring = ''
    index_start = -1
    index_end = -1
    bracket_counts = {
        "{": 0,
        "[": 0,
        "(": 0
    }
    for index, letter in enumerate(string):
        if letter in matches.values():
            if index_start == -1:
                index_start = index
            substring += letter
            bracket_counts[letter] += 1
        if letter in matches.keys():
            if not substring:
                break
            if substring[-1] == matches[letter]:
                substring = substring[:-1]
                bracket_counts[matches[letter]] -= 1
                if not [cnt for cnt in bracket_counts.values() if cnt]:
                    index_end = index
                if [cnt for cnt in bracket_counts.values() if cnt < 0]:
                    break
            else:
                break

    if index_start != -1 and index_end != -1:
        matching_substring = string[index_start:index_end + 1]
        return len(matching_substring), matching_substring

for i in range(len(input_string)):
    bracket_substring_strict = get_matching_substring_strict(input_string[i:])
    if bracket_substring_strict:
        bracket_matches_strict.append(bracket_substring_strict)

print(sorted(bracket_matches_strict)[-1][1])