Learning Perl, 5th 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 that frederick and Alfred wouldn’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:

   Change the pattern used in the first exercise’s answer to /(\S)\1/. The \S character class matches the nonwhitespace character, and the parentheses allow you to use the back reference \1 to match the same character immediately following it.

6. Here’s one way to do it:

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

   This tests /fred/ only 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 ...