如何使用正则表达式准确匹配三个连续数字？

import redata = [    '123',    '123u',    'u123',    'x123y',    '1234',    'x1234y',    '12',    '123y456',]for s in data:    print(re.findall(r'(?<!d)(d{3})(?!d)', s))

['123']['123']['123']['123'][][][]['123', '456']

['123', '123u', 'u123', 'x123y', '1234', '12'].map(s => s.match(/^d*(d{3})d*$/)?.[1])

['123', '123', '123', '123', undefined, undefined]