Learning Perl, Fourth Edition

Answers to Chapter 7 Exercises

1. Here’s one way to do it:

       while (<>) {
         if (/fred/) {
           print;
         }
       }

   This is pretty simple. The more important part of this exercise is trying it out on the sample strings. It doesn’t match Fred, showing that regular expressions are case-sensitive. (We’ll see how to change that later.) It does match frederick and Alfred since both of those strings contain the four-letter string fred. (Matching whole words only, so frederick and Alfred won’t match, is another feature we’ll see later.)

2. Here’s one way to do it: Change the pattern used in the first exercise’s answer to /[fF]red/. You could also have tried /(f|F)red/ or /fred|Fred/, but the character class is more efficient.

3. Here’s one way to do it: Change the pattern used in the first exercise’s answer to /\./. The backslash is needed because the dot is a metacharacter, or you could use a character class: /[.]/.

4. Here’s one way to do it: Change the pattern used in the first exercise’s answer to /[A-Z][a-z]+/.

5. Here’s one way to do it:

       while (<>) {
         if (/wilma/) {
           if (/fred/) {
             print;
           }
         }
       }

   This tests /fred/ after we find /wilma/ matches, but fred could appear before or after wilma in the line; each test is independent of the other.

   If you wanted to avoid the extra nested if test, you might have written something like this:^[371]

       while (<>) {
         if (/wilma.*fred|fred.*wilma/) {
           print;
         }
       }

   This works because we’ll either have wilma before fred or fred before wilma. If we had written /wilma.*fred/, that wouldn’t have matched ...