Beginning Web Development With Perl : From Novice to ... - Nabo

284APPENDIX ■ PERL BASICSKeywordsA keyword is a term in Perl that has a predefined meaning. One example is the term use as wesaw in the statementuse warnings;Other types of keywords include built-in functions such as print() and control flowconstructs such as if and while. We will talk about many built-in functions and control flowconstructs in detail as we progress in our discussion of Perl.It’s a good idea to respect keywords and not give anything else the same name as one. Forexample, a little later on you’ll learn that you can create and name a variable, and that callingyour variable $print is perfectly allowable. The problem with this is that it leads to confusingand uninformative statements like print $print. It is always a good idea to give a variablea meaningful name, one that relates to its content in a logical manner—for example, $my_name,@telephone_numbers, %account_info, and so on, rather than $a, @b, and %c.Statements and Statement BlocksIf functions are the verbs of Perl, then statements are the sentences. Instead of a period,a statement in Perl usually ends with a semicolon, as shown earlier:print "Hello, world!\ n";To print some more text, we can add another statement:print "Hello, world!\ n";print "Goodbye, world!\ n";Escape SequencesUTF8 gives us 65,536 characters, and ASCII gives us 256 characters, but on the average keyboard,there’s only a hundred or so keys. Even using the Shift keys, there will still be somecharacters that you aren’t going to be able to type. There will also be some things that youdon’t want to stick in the middle of your program, because they would make it messy or confusing.However, you’ll want to refer to some of these characters in strings that you output.Perl provides us with mechanisms called escape sequences as an alternative way of getting tothem. You’ve already seen the use of \ n to start a new line. Table A-1 lists the more commonescape sequences.Table A-1. Escape SequencesEscape Sequence Meaning\ t Tab\ n Start a new line (usually called newline)\ r Carriage return\ b Back up one character (backspace)\ a Alarm (rings the system bell)\ x{ 1F18} Unicode character