1. What does \d stand for if used in a Perl regular expression?
  2. What does the operator =~ mean if used in a Perl expression, such as $a =~ /World/;.
  3. If I want to perform a substitution in a Perl expression, what operator will I use (e.g., in the expression $a =~ ?/a/b/;, what replaces ?).
  4. How can I write a regular expression to show that I want 0 or more repetitions of the character 'a'?