Operators for String Scalar Values

While not strictly an operator, it is important to understand the use of single and double quotation marks when working with strings, since these are essential when creating string values in a program. Single quotes (') provide a literal translation of the text they surround -- inside single quotes there are no special characters except for the single quote itself, which must be preceded by a backslash (\). Double quotes (") provide a very useful feature called variable interpolation. This means that, inside of your perl program, a variable name inside of double quotes will be replaced by a representation of the variable's value. This makes it very easy to create output containing variable values. For example

     $name = "Fred";
     print "Hello, $name";

will result in ``Hello, Fred'' being printed. This means that if, for example, you wanted to print a dollar sign, you would need to precede it with a backslash (\). The same is true for several other characters, including the at sign (@).

When variable interpolation is performed on an array, perl will insert spaces between the elements of the array, making it very useful for quickly checking the contents of an array. Note the difference in the output of the final two statements:

     @friends = ('Joe','Fred','Tom','Nancy');
     print @friends;     # prints JoeFredTomNancy 
     print "@friends";   # prints Joe Fred Tom Nancy

When you use double quotes, there are a number of escape sequences which have special meaning, summarized in Table 3.1.

Table 3.1: Escape Sequences in Double-Quoted Strings

Individual Symbols
Sequence	Meaning	Sequence	Meaning
\n	Newline	\r	Carriage Return
\t	Tab	\f	Formfeed
\b	Backspace	\a	Bell
\"	Double Quote	\\	Backslash
\nnn	Octal Character nnn	\xnn	Hex Character nn
Escape Operators
\l	Lowercase next letter
\L	Lowercase letters until \E
\u	Uppercase next letter
\U	Uppercase letters until \E
\Q	Backslash-quote non-alphanumerics until \E
\E	Terminate \L, \U or \Q

Perl is actually much more lenient than most languages regarding the use of quotes around string values. A general rule for perl is that if you have a so-called bareword (a string value without quotes) appearing in a context in which a quoted string could be used, perl will treat that bareword as if it were quoted, unless the bareword is the name of a function, in which case it will evaluate the function with its default value. (Section 1.4) You should use barewords in this way with the utmost care, because if one of the barewords happens to coincide with a function in perl, you will get very surprising results. Consider the following four variables:

     $first = "test";
     $second = test;
     $third = "chop";
     $fourth = chop;

As you might expect, the variables $first and $second will both have the value test. However, while $third will be equal to chop, $fourth will be undefined. This is because there happens to be a perl function called chop, which removes and returns the last character of a string value, and since its default value ($_) is undefined, it returns an undefined value.

When you use a numeric variable in a double-quoted string, perl silently creates a string equivalent and uses that. Perl will remember both the numeric version and the string version of the variable as long as the variable exists.

Beyond these capabilities, perl offers several more advanced quoting operators. (See Section 3.2).

Perl provides two other operators for string values. A single period (.) is perl's string concatenation operator. For example:

     $first = 'black';
     $second = 'jack';
     print $first . $second;

would result in ``blackjack'' being printed. The spaces surrounding the period in the above example are optional, and were added to make the code more readable. Note that no blanks are added between the concatenated strings. You could add them explicitly, that is:

     print $first . ' ' . $second;

would insert a blank between the two words. Notice that string variables and literal (single- or double-quoted) strings can be freely mixed when using the concatenation operator. In fact this is true for most functions and operators.

However, most perl programmers would take advantage of the variable extrapolation provided by double quotes, and simply use

     print "$first $second";

In either case, neither of the two strings which are concatenated is modified in any way. Besides printing these results, they could just as easily be stored as variables for later use:

     $thegame = "$first $second";

would set the scalar variable $thegame equal to the string value ``black jack''.

When perl performs variable interpolation, it will recognize non-alphanumeric characters (except for underline or double colons) as marking the end of a variable. So if you had a variable called $title with the value ``mypage'', perl would correctly interpolate the expression "$title.html" into ``mypage.html''. Now suppose we want to append the number ``1'' to the end of the string stored in the $title variable. Perl will interpret "$title1" as an entirely different variable. In cases like these, you need to surround the variable name with curly braces ({}) after the dollar sign, to let perl know when the variable name ends and the literal characters begin. In the current example, the string "${title}1" would correctly be interpolated into ``mypage1''.

The lower-case letter ``x'' serves as a repetition operator for string values in perl. The value on the right-hand side of the x should be a number, and the value on the left-hand side of the x will be repeated that many times, creating a new string value. If a number is used on the left-hand side of the x, perl will silently convert it to a string value. The following examples illustrate these points:

print '-' x 10;         # prints ----------
$x = 5;
$fives = $x x 4;        # sets $fives equal to '5555'
$y = 12/7;
print $y x 2;           # prints 1.714285714285711.71428571428571

In the last example, notice that perl's default string representation of a number may contain lots of decimal digits. To get more control over this, you can use the sprintf function. (See Section ).