Question: Prove that if A is a regular set whose symbols come from the alphabet I, then I* - A is a regular set. A number of programming languages define "regular expressions" somewhat differently than we have done in this chapter. In these instances, the regular expression is meant to describe a pattern for a set of strings so that an arbitrary string can be matched against the pattern to see whether the string belongs in the set. Examples are (a) searching a string to see whether it matches the format of a valid e-mail address and (b) extracting all instances of the form href = "..." from an HTML document. Perl (Practical Extraction and Report Language) is a language strong in text processing; following are some of the syntax rules for regular expressions in Perl
