import re


def is_valid_hair_color(value):
    """Validate a # followed by exactly six characters 0-9 or a-f.

    >>> is_valid_hair_color("#123abc")
    True
    >>> is_valid_hair_color("#123abz")
    False
    >>> is_valid_hair_color("123abc")
    False
    """
    return bool(re.match(r"^#[0-9a-f]{6}$", value))