1. A regular expression is a method used in programming for pattern matching. Regular expressions provide a flexible and concise means to match strings of text.
2. Following is the answer:

import redef is_allowed_specific_char(string):
    charRe = re.compile(r'[^a-zA-Z0-9.]')
    string = charRe.search(string)
    return not bool(string)
 print(is_allowed_specific_char("ABCDEFabcdef123450"))
 print(is_allowed_specific_char("*&%@#!}{"))

3. Answer: a.
   re is a part of the standard library and can be imported using import re.

4. Answer: a.
   It will look for the pattern at the beginning and return None if it isn't found.
5. Answer: d.
   This function returns the entire match.