import re

text = "the asbestos fiber <unk> <unk| is < unusually <unk once it enters the <<unk>$% with "
pattern = r"(?i)(?<!\S)(<unk>)(?!\S)|[^\w\s]+"

print(re.sub(r"\s{2,}", ' ', re.sub(pattern, r"\1", text)).lower().strip())