2.3.2.1 Numeric literals

Numeric literals are specified in any of several customary[8] floating point or integer formats:

[8] Customary in UNIX culture, that is. If you're from a different culture, welcome to ours!

12345               # integer
12345.67            # floating point
6.02E23             # scientific notation
0xffff              # hexadecimal
0377                # octal
4_294_967_296       # underline for legibility

Since Perl uses the comma as a list separator, you cannot use it to delimit the triples in a large number. To improve legibility, Perl does allow you to use an underscore character instead. The underscore only works within literal numbers specified in your program, not for strings functioning as numbers or data read from somewhere else. Similarly, the leading 0x for hex and 0 for octal work only for literals. The automatic conversion of a string to a number does not recognize these prefixes - you must do an explicit conversion[9] with the oct function (which works for hex-looking data, too, as it happens).

[9] Sometimes people think Perl should convert all incoming data for them. But there are far too many decimal numbers with leading zeroes in the world to make Perl do this automatically. For example, the zip code for O'Reilly & Associates' office in Cambridge, MA is 02140. The postmaster would get upset if your mailing label program turned 02140 into 1120 decimal.

2.3.2.2 String literals

String literals are usually delimited by either single or double quotes. They work much like UNIX shell quotes: double-quoted string literals are subject to backslash and variable interpolation; single-quoted strings are not (except for \' and \\, so that you can put single quotes and backslashes into single-quoted strings).

You can also embed newlines directly in your strings; that is, they can begin and end on different lines. This is nice for many reasons, but it also means that if you forget a trailing quote, the error will not be reported until Perl finds another line containing the quote character, which may be much further on in the script. Fortunately, this usually causes an immediate syntax error on the same line, and Perl is then smart enough to warn you that you might have a runaway string.

Note that a single-quoted string must be separated from a preceding word by a space, since a single quote is a valid (though deprecated) character in an identifier; see Chapter 5.

With double-quoted strings, the usual C-style backslash rules apply for inserting characters such as newline, tab, and so on. You may also specify characters in octal and hexadecimal, or as control characters:

In addition, there are escape sequences to modify the case of subsequent characters, as with the substitution operator in the vi editor:

Besides the backslash escapes listed above, double-quoted strings are subject to variable interpolation of scalar and list values. This means that you can insert the values of certain variables directly into a string literal. It's really just a handy form of string concatenation. Variable interpolation may only be done for scalar variables, entire arrays (but not hashes), single elements from an array or hash, or slices (multiple subscripts) of an array or hash. In other words, you may only interpolate expressions that begin with $ or @, because those are the two characters (along with backslash) that the string parser looks for.[10] Although a complete hash specified with a % may not be interpolated into the string, single hash values and hash slices are okay, because they begin with $ and @ respectively.

[10] Inside strings a literal @ that is not part of an array or slice identifier must be escaped with a backslash (\@), or else a compilation error will result. See Chapter 9, Diagnostic Messages.

The following code segment prints out: "The price is $100."

$Price = '$100';                    # not interpolated
print "The price is $Price.\n";     # interpolated

As in some shells, you can put braces around the identifier to distinguish it from following alphanumerics: "How ${verb}able!". In fact, an identifier within such braces is forced to be a string, as is any single identifier within a hash subscript. For example:

$days{'Feb'}

can be written as:

$days{Feb}

and the quotes will be assumed automatically. But anything more complicated in the subscript will be interpreted as an expression.

Apart from the subscripts of interpolated array and hash variables, there are no multiple levels of interpolation. In particular, contrary to the expectations of shell programmers, backquotes do not interpolate within double quotes, nor do single quotes impede evaluation of variables when used within double quotes.

2.3.2.3 Pick your own quotes

While we usually think of quotes as literal values, in Perl they function more like operators, providing various kinds of interpolating and pattern matching capabilities. Perl provides the customary quote characters for these behaviors, but also provides a way for you to choose your quote character for any of them.

Some of these are simply forms of "syntactic sugar" to let you avoid putting too many backslashes into quoted strings. Any non-alphanumeric, non-whitespace delimiter can be used in place of /.[11] If the delimiters are single quotes, no variable interpolation is done on the pattern. If the opening delimiter is a parenthesis, bracket, brace, or angle bracket, the closing delimiter will be the matching construct. (Embedded occurrences of the delimiters must match in pairs.) Examples:

[11] In particular, the newline and space characters are not allowed as delimiters. (Ancient versions of Perl allowed this.)

$single = q!I said, "You said, 'She said it.'"!;
$double = qq(Can't we get some "good" $variable?);
$chunk_of_code = q {
    if ($condition) {
        print "Gotcha!";
    }
};

Finally, for two-string constructs like s/// and tr///, if the first pair of quotes is a bracketing pair, then the second part gets its own starting quote character, which needn't be the same as the first pair. So you can write things like s{foo}(bar) or tr[a-z][A-Z]. Whitespace is allowed between the two inner quote characters, so you could even write that last one as:

tr [a-z]
   [A-Z];

2.3.2.4 Or leave the quotes out entirely

A word that has no other interpretation in the grammar will be treated as if it were a quoted string. These are known as barewords.[12] For example:

[12] As with filehandles and labels, a bareword that consists entirely of lowercase letters risks conflict with future reserved words. If you use the -w switch, Perl will warn you about barewords.

@days = (Mon,Tue,Wed,Thu,Fri);
print STDOUT hello, ' ', world, "\n";

sets the array @days to the short form of the weekdays and prints hello world followed by a newline on STDOUT. If you leave the filehandle out, Perl tries to interpret hello as a filehandle, resulting in a syntax error. Because this is so error-prone, some people may wish to outlaw barewords entirely. If you say:

use strict 'subs';

then any bareword that would not be interpreted as a subroutine call produces a compile-time error instead. The restriction lasts to the end of the enclosing block. An inner block may countermand this by saying:

no strict 'subs';

Note that the bare identifiers in constructs like:

"${verb}able"
$days{Feb}

are not considered barewords, since they're allowed by explicit rule rather than by having "no other interpretation in the grammar".

2.3.2.5 Interpolating array values

Array variables are interpolated into double-quoted strings by joining all the elements of the array with the delimiter specified in the $" variable[13] (which is a space by default). The following are equivalent:

[13] $LIST_SEPARATOR if you use the English library module. See Chapter 7, The Standard Perl Library.

$temp = join($",@ARGV);
print $temp;

print "@ARGV";

Within search patterns (which also undergo double-quotish interpolation) there is a bad ambiguity: Is /$foo[bar]/ to be interpreted as /${foo}[bar]/ (where [bar] is a character class for the regular expression) or as /${foo[bar]}/ (where [bar] is the subscript to array @foo)? If @foo doesn't otherwise exist, then it's obviously a character class. If @foo exists, Perl takes a good guess about [bar], and is almost always right.[14] If it does guess wrong, or if you're just plain paranoid, you can force the correct interpretation with braces as above. Even if you're merely prudent, it's probably not a bad idea.

[14] The guesser is too boring to describe in full, but basically takes a weighted average of all the things that look like character classes (a-z, \w, initial ^) versus things that look like expressions (variables or reserved words).

2.3.2.6 "Here" documents

A line-oriented form of quoting is based on the shell's here-document syntax.[15] Following a << you specify a string to terminate the quoted material, and all lines following the current line down to the terminating string are quoted. The terminating string may be either an identifier (a word), or some quoted text. If quoted, the type of quote you use determines the treatment of the text, just as in regular quoting. An unquoted identifier works like double quotes. There must be no space between the << and the identifier. (If you insert a space, it will be treated as a null identifier, which is valid but deprecated, and matches the first blank line - see the first Hurrah! example below.) The terminating string must appear by itself (unquoted and with no surrounding whitespace) on the terminating line.

[15] It's line-oriented in the sense that delimiters are lines rather than characters. The starting delimiter is the current line, and the terminating delimiter is a line consisting of the string you specify.

    print <<EOF;    # same as earlier example    
The price is $Price.
EOF

    print <<"EOF";  # same as above, with explicit quotes
The price is $Price.
EOF

    print <<'EOF';    # single-quoted quote
All things (e.g. a camel's journey through
A needle's eye) are possible, it's true.
But picture how the camel feels, squeezed out
In one long bloody thread, from tail to snout.
                                -- C.S. Lewis
EOF

    print << x 10;    # print next line 10 times
The camels are coming!  Hurrah!  Hurrah!

    print <<"" x 10;  # the preferred way to write that
The camels are coming!  Hurrah!  Hurrah!

    print <<`EOC`;    # execute commands
echo hi there
echo lo there
EOC

    print <<"dromedary", <<"camelid"; # you can stack them
I said bactrian.
dromedary
She said llama.
camelid

Just don't forget that you have to put a semicolon on the end to finish the statement, as Perl doesn't know you're not going to try to do this:

print <<ABC
179231
ABC
    + 20;   # prints 179251

2.3.2.7 Other literal tokens

Two special literals are __LINE__ and __FILE__, which represent the current line number and filename at that point in your program. They may only be used as separate tokens; they will not be interpolated into strings. In addition, the token __END__ may be used to indicate the logical end of the script before the actual end of file. Any following text is ignored, but may be read via the DATA filehandle.

The __DATA__ token functions similarly to the __END__ token, but opens the DATA filehandle within the current package's namespace, so that required files can each have their own DATA filehandles open simultaneously. For more information, see Chapter 5.

2.3.3.1 Scalar and list context

Every operation[16] that you invoke in a Perl script is evaluated in a specific context, and how that operation behaves may depend on the requirements of that context. There are two major contexts: scalar and list. For example, assignment to a scalar variable evaluates the right-hand side in a scalar context, while assignment to an array or a hash (or slice of either) evaluates the right-hand side in a list context. Assignment to a list of scalars would also provide a list context to the right-hand side.

[16] Here we use the term "operation" loosely to mean either an operator or a term. The two concepts fuzz into each other when you start talking about functions that parse like terms but look like unary operators.

You will be miserable until you learn the difference between scalar and list context, because certain operators know which context they are in, and return lists in contexts wanting a list, and scalar values in contexts wanting a scalar. (If this is true of an operation, it will be mentioned in the documentation for that operation.) In computer lingo, the functions are overloaded on the type of their return value. But it's a very simple kind of overloading, based only on the distinction between singular and plural values, and nothing else.

Other operations supply the list contexts to their operands, and you can tell which ones they are because they all have LIST in their syntactic descriptions. Generally it's quite intuitive.[17] If necessary, you can force a scalar context in the middle of a LIST by using the scalar function. (Perl provides no way to force a list context in a scalar context, because anywhere you would want a list context it's already provided by the LIST of some controlling function.)

[17] Note, however, that the list context of a LIST can propagate down through subroutine calls, so it's not always obvious by inspection whether a given simple statement is going to be evaluated in a scalar or list context. The program can find out its context within a subroutine by using the wantarray function.

Scalar context can be further classified into string context, numeric context, and don't-care context. Unlike the scalar versus list distinction we just made, operations never know which scalar context they're in. They simply return whatever kind of scalar value they want to, and let Perl translate numbers to strings in string context, and strings to numbers in numeric context. Some scalar contexts don't care whether a string or number is returned, so no conversion will happen. (This happens, for example, when you are assigning the value to another variable. The new variable just takes on the same subtype as the old value.)

2.3.3.2 Boolean context

One special scalar context is called Boolean context. Boolean context is simply any place where an expression is being evaluated to see whether it's true or false. We sometimes write true and false when we mean the technical definition that Perl uses: a scalar value is true if it is not the null string or the number 0 (or its string equivalent, "0"). References are always true.

A Boolean context is a don't-care context in the sense that it never causes any conversions to happen (at least, no conversions beyond what scalar context would impose).

We said that a null string is false, but there are actually two varieties of null scalars: defined and undefined. Boolean context doesn't distinguish between defined and undefined scalars. Undefined null scalars are returned when there is no real value for something, such as when there was an error, or at end of file, or when you refer to an uninitialized variable or element of an array. An undefined null scalar may become defined the first time you use it as if it were defined, but prior to that you can use the defined operator to determine whether the value is defined or not. (The return value of defined is always defined, but not always true.)

2.3.3.3 Void context

Another peculiar kind of scalar context is the void context. This context not only doesn't care what the return value is, it doesn't even want a return value. From the standpoint of how functions work, it's no different from an ordinary scalar context. But if you use the -w command-line switch, the Perl compiler will warn you if you use an expression with no side effects in a place that doesn't want a value, such as in a statement that doesn't return a value. For example, if you use a string as a statement:

"Camel Lot";

you may get a warning like this:

Useless use of a constant in void context in myprog line 123;

2.3.3.4 Interpolative context

We mentioned that double-quoted literal strings do backslash interpretation and variable interpolation, but the interpolative context (often called "double-quote context") applies to more than just double-quoted strings. Some other double-quotish constructs are the generalized backtick operator qx//, the pattern match operator m//, and the substitution operator s///. In fact, the substitution operator does interpolation on its left side before doing a pattern match, and then does interpolation on its right side each time the left side matches.

The interpolative context only happens inside quotes, or things that work like quotes, so perhaps it's not fair to call it a context in the same sense as scalar and list context. (Then again, maybe it is.)

2.3.7.1 Command input (backtick) operator

First of all, we have the command input operator, also known as the backticks operator, because it looks like this:

$info = `finger $user`;

A string enclosed by backticks (grave accents) first undergoes variable interpolation just like a double-quoted string. The result of that is then interpreted as a command by the shell, and the output of that command becomes the value of the pseudo-literal. (This is modeled after a similar operator in some of the UNIX shells.) In scalar context, a single string consisting of all the output is returned. In list context, a list of values is returned, one for each line of output. (You can set $/ to use a different line terminator.)

The command is executed each time the pseudo-literal is evaluated. The numeric status value of the command is saved in $? (see the section "Special Variables" later in this chapter for the interpretation of $?). Unlike the csh version of this command, no translation is done on the return data - newlines remain newlines. Unlike any of the shells, single quotes do not hide variable names in the command from interpretation. To pass a $ through to the shell you need to hide it with a backslash. The $user in our example above is interpolated by Perl, not by the shell. (Because the command undergoes shell processing, see Chapter 6, Social Engineering, for security concerns.)

The generalized form of backticks is qx// (for "quoted execution"), but the operator works exactly the same way as ordinary backticks. You just get to pick your quote characters.

2.3.7.2 Line input (angle) operator

The most heavily used input operator is the line input operator, also known as the angle operator. Evaluating a filehandle in angle brackets (<STDIN>, for example) yields the next line from the associated file. (The newline is included, so according to Perl's criteria for truth, a freshly input line is always true, up until end of file, at which point an undefined value is returned, which is false.) Ordinarily you would assign the input value to a variable, but there is one situation where an automatic assignment happens. If and only if the line input operator is the only thing inside the conditional of a while loop, the value is automatically assigned to the special variable $_. The assigned value is then tested to see whether it is defined. (This may seem like an odd thing to you, but you'll use the construct in almost every Perl script you write.) Anyway, the following lines are equivalent to each other:

while (defined($_ = <STDIN>)) { print $_; }   # the long way
while (<STDIN>) { print; }                    # the short way
for (;<STDIN>;) { print; }                    # while loop in disguise
print $_ while defined($_ = <STDIN>);         # long statement modifier
print while <STDIN>;                          # short statement modifier

Remember that this special magic requires a while loop. If you use the input operator anywhere else, you must assign the result explicitly if you want to keep the value:

if (<STDIN>)      { print; }   # WRONG, prints old value of $_
if ($_ = <STDIN>) { print; }   # okay

The filehandles STDIN, STDOUT, and STDERR are predefined and pre-opened.[21] Additional filehandles may be created with the open function. See the open entry in Chapter 3 for details on this. Some object modules also create object references that can be used as filehandles. See the FileHandle module in Chapter 7.

[21] The filehandles stdin, stdout, and stderr will also work except in packages, where they would be interpreted as local identifiers rather than global. They're only there for compatibility with very old scripts, so use the uppercase versions.

In the while loops above, we were evaluating the line input operator in a scalar context, so it returned each line separately. However, if you use it in a list context, a list consisting of all the remaining input lines is returned, one line per list element. It's easy to make a large data space this way, so use this feature with care:

$one_line = <MYFILE>;   # Get first line.
@all_lines = <MYFILE>;  # Get the rest of the lines.

There is no while magic associated with the list form of the input operator, because the condition of a while loop is always a scalar context (as is any conditional).

Using the null filehandle within the angle operator is special and can be used to emulate the command-line behavior of typical UNIX filter programs such as sed and awk. When you read lines from <>, it magically gives you all the lines from all the files mentioned on the command line. If no files were mentioned, it gives you standard input instead, so your program is easy to insert into the middle of a pipeline of processes.

Here's how it works: the first time <> is evaluated, the @ARGV array is checked, and if it is null, $ARGV[0] is set to "-", which when opened gives you standard input. The @ARGV array is then processed as a list of filenames. The loop:

while (<>) {
    ...                     # code for each line
}

is equivalent to the following Perl-like pseudocode:

@ARGV = ('-') unless @ARGV;
while ($ARGV = shift) {
    open(ARGV, $ARGV) or warn "Can't open $ARGV: $!\n";
    while (<ARGV>) {
        ...         # code for each line
    }
}

except that it isn't so cumbersome to say, and will actually work. It really does shift array @ARGV and put the current filename into variable $ARGV. It also uses filehandle ARGV internally - <> is just a synonym for <ARGV>, which is magical. (The pseudocode above doesn't work because it treats <ARGV> as non-magical.)

You can modify @ARGV before the first <> as long as the array ends up containing the list of filenames you really want. Line numbers ($.) continue as if the input were one big happy file. (But see the example under eof for how to reset line numbers on each file.)

If you want to set @ARGV to your own list of files, go right ahead. If you want to pass switches into your script, you can use one of the Getopts modules or put a loop on the front like this:

while ($_ = $ARGV[0], /^-/) {
    shift;
    last if /^--$/;
    if (/^-D(.*)/) { $debug = $1 }
    if (/^-v/)     { $verbose++  }
    ...             # other switches
}
while (<>) {
    ...             # code for each line
}

The <> symbol will return false only once. If you call it again after this it will assume you are processing another @ARGV list, and if you haven't set @ARGV, it will input from STDIN.

If the string inside the angle brackets is a scalar variable (for example, <$foo>), then that variable contains the name of the filehandle to input from, or a reference to the same. For example:

$fh = \*STDIN;
$line = <$fh>;

2.3.7.3 Filename globbing operator

You might wonder what happens to a line input operator if you put something fancier inside the angle brackets. What happens is that it mutates into a different operator. If the string inside the angle brackets is anything other than a filehandle name or a scalar variable (even if there are just extra spaces), it is interpreted as a filename pattern to be "globbed".[22] The pattern is matched against the files in the current directory (or the directory specified as part of the glob pattern), and the filenames so matched are returned by the operator. As with line input, the names are returned one at a time in scalar context, or all at once in list context. In fact, the latter usage is more prevalent. You generally see things like:

[22] This has nothing to do with the previously mentioned typeglobs, other than that they both use the * character in a wildcard fashion. The * character has the nickname "glob" when used like this. With typeglobs you're globbing symbols with the same name from the symbol table. With a filename glob, you're doing wildcard matching on the filenames in a directory, just as the various shells do.

my @files = <*.html>;

As with other kinds of pseudo-literals, one level of variable interpolation is done first, but you can't say <$foo> because that's an indirect filehandle as explained earlier. (In older versions of Perl, programmers would insert braces to force interpretation as a filename glob: <${foo}>. These days, it's considered cleaner to call the internal function directly as glob($foo), which is probably the right way to have invented it in the first place.)

Whether you use the glob function or the old angle-bracket form, the globbing operator also does while magic like the line input operator, and assigns the result to $_. For example:

while (<*.c>) {
    chmod 0644, $_;
}

is equivalent to:

open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
while (<FOO>) {
    chop;
    chmod 0644, $_;
}

In fact, it's currently implemented that way, more or less. (Which means it will not work on filenames with spaces in them unless you have csh(1) on your machine.) Of course, the shortest way to do the above is:

chmod 0644, <*.c>;

Because globbing invokes a subshell, it's often faster to call readdir yourself and just do your own grep on the filenames. Furthermore, due to its current implementation of using a shell, the glob routine may get "Arg list too long" errors (unless you've installed tcsh(1) as /bin/csh).

A glob evaluates its (embedded) argument only when it is starting a new list. All values must be read before it will start over. In a list context this isn't important, because you automatically get them all anyway. In a scalar context, however, the operator returns the next value each time it is called, or a false value if you've just run out. Again, false is returned only once. So if you're expecting a single value from a glob, it is much better to say:

($file) = <blurch*>;  # list context

than to say:

$file = <blurch*>;    # scalar context

because the former slurps all the matched filenames and resets the operator, while the latter will alternate between returning a filename and returning false.

It you're trying to do variable interpolation, it's definitely better to use the glob operator, because the older notation can cause people to become confused with the indirect filehandle notation. But with things like this, it begins to become apparent that the borderline between terms and operators is a bit mushy:

@files = glob("$dir/*.[ch]");   # call glob as function
@files = glob $some_pattern;    # call glob as operator

We left the parentheses off of the second example to illustrate that glob can be used as a unary operator; that is, a prefix operator that takes a single argument. The glob operator is an example of a named unary operator, which is just one of the kinds of operators we'll talk about in the section "Operators" later in this chapter. But first we're going to talk about pattern matching operations, which also parse like terms but operate like operators.