Perl for Web Site Management

A Bit More About Quoting

So far, whenever we’ve needed to quote a string we’ve used double quotes (as in, "this is a double-quoted string"). In fact, Perl also supports the use of single quotes (as in, 'this is a single-quoted string'). It’s important for you to understand the difference between the two.

The difference is just this: when it processes a double-quoted string, Perl looks in it for things that look like variables and replaces them with the contents of those variables. This process is called variable interpolation . It also looks for certain sequences beginning with a backslash (\) and replaces them with special characters. The sequences are called backslash escapes, and the process of replacing them with special characters is called backslash interpretation.

When it’s processing a single-quoted string Perl doesn’t bother doing this. You get the string, just like it’s written. (Actually, Perl processes two backslash escapes within a single-quoted string: \', which it interprets as a literal single quote, and \\, which it interprets as a literal backslash. This lets you put literal single quotes and literal backslashes inside your string, which would otherwise be difficult to do.)

Let’s create a new script called quotes.plx (Example 2-2) to see how this works.

Example 2-2. A script to test how Perl treats single- and double-quoted strings

#!/usr/bin/perl # quotes.plx -- test handling of single- and double-quoted strings $veggies = 'rutabagas'; print "I like to eat ...