from bisect import bisect_left, bisect_right
from itertools import chain, pairwise

def get_span(smaller_sentence, larger_sentence):
    stripped = larger_sentence.translate(str.maketrans('', '', '[]'))
    if (left := stripped.find(smaller_sentence)) == -1:
        raise ValueError(f"Span not found {smaller_sentence, larger_sentence}")
    right = left + len(smaller_sentence)
    count = last_index = 0
    lefts, rights = [], []
    while True:
        for bracket, indices in zip('[]', (lefts, rights)):
            index = larger_sentence.find(bracket, last_index)
            if index == -1:
                break
            indices.append(index - count)
            count += 1
            last_index = index + 1
        else:
            continue
        break
    start, end = bisect_left(lefts, left), bisect_right(rights, right)
    return ''.join(
        fragment for (fragment_start, fragment_end), bracket in zip(
            pairwise((
                left,
                *chain.from_iterable(zip(lefts[start:end], rights[start:end])),
                right
            )),
            (*'[]' * (end - start), '')
        )
        for fragment in (stripped[fragment_start:fragment_end], bracket)
    )

print(
    get_span(
        "quick brown fox jumps over",
        "The [quick brown fox] [jumps over] [the lazy dog]."
    )
)