Parse Cron Fields from String

Problem

Given a string containing a simplified cron-style schedule with exactly five space-separated fields (minute hour day month weekday), write a function that validates and parses it into a dictionary. Each field may be either * or a non-negative integer string. Return a dictionary with the five parsed fields if valid; otherwise return an empty dictionary.

Constraints

• 1 <= len(expr) <= 10^4
• Fields are separated by one or more spaces
• A valid field is either "*" or a string of digits 0-9
• No range validation is required beyond checking the field format