from typing import Tuple
import re


def _lex_whitespace(header: str) -> Tuple[str, str]:
    """
    >>> _lex_whitespace(" \\n  a")
    ('', 'a')
    >>> _lex_whitespace(" abc")
    ('', 'abc')
    >>> _lex_whitespace("a ")
    ('', 'a ')
    """
    whitespace_match = re.match(r"\s+", header)
    if not whitespace_match:
        return "", header
    return "", header[whitespace_match.end() :]