import re


def validate_period_format(period_str):
	"""validates a period in string format

	Args:
	    period_str (string): period in string format starting with a
	    number and ending with the letters m, h or d, for minutes, hour and day,
	    respectively.

	Returns:
	    bool: True if the period is valid, else False
	"""
	regex = r"^[0-9]+[hmd]$"
	result = re.search(regex, period_str, re.MULTILINE)
	return False if result is None else True