Implement regular expression matching with the following special characters:
. (period) which matches any single character
* (asterisk) which matches zero or more of the preceding element
That is, implement a function that takes in a string and a valid regular expression and returns whether or not the string matches the regular expression.
Examples:
>>> coding_problem_25('ra.', 'ray')
True
>>> coding_problem_25('ra.', 'raymond')
False
>>> coding_problem_25('.*at', 'chat')
True
>>> coding_problem_25('.*at', 'chats')
False