import copy
import re


def sanitize_sentences(article: list):
    """Function removes all non alphabet symbols from articles.
    
    Args:
        article (list): List of sentences
    
    Returns:
        list: List of list with sanitized sentences.
    """
    sanitized_article = copy.deepcopy(article)
    for i in range(len(article)):
        sanitized_article[i] = re.sub(r'[^a-zA-Z]', ' ', sanitized_article[i])
        sanitized_article[i] = re.sub(r'\[[0-9]*\]', ' ', sanitized_article[i])
        sanitized_article[i] = re.sub(r'\s+', ' ', sanitized_article[i])
        sanitized_article[i] = sanitized_article[i].lower()
    return sanitized_article