Special Variables (Perl in a Nutshell, 2nd Edition)

4.4. Special Variables

Some variables have a predefined, special meaning in Perl. They use punctuation characters after the usual variable indicator ($, @, or %), such as $_. The explicit, long-form names are the variables' equivalents when you use the English module by including use English; at the top of your program.

4.4.1. Global Special Variables

The most common special variable is $_, which contains the default input and pattern-searching string. For example:

foreach ('hickory','dickory','doc') {
        print;
}

The first time the loop is executed, "hickory" is printed. The second time around, "dickory" is printed, and the third time, "doc" is printed. That's because in each iteration of the loop, the current string is placed in $_ and is used by default by print. Here are the places where Perl will assume $_, even if you don't specify it:

Various unary functions, including functions such as ord and int, as well as the all file tests (-f, -d), except for -t, which defaults to STDIN.
Various list functions such as print and unlink.
The pattern-matching operations m//, s///, and tr/// when used without an =~ operator.
The default iterator variable in a foreach loop if no other variable is supplied.
The implicit iterator variable in the grep and map functions.
The default place to put an input record when a line-input operation's result is tested by itself as the sole criterion of a while test (i.e., <filehandle>). Note that outside of a while test, this does not happen.

The following is a complete listing of global special variables:

$_
$ARG: The default input and pattern-searching space.
$.
$INPUT_LINE_NUMBER
$NR

4.4.2. Global Special Arrays and Hashes

@ARGV

4.4.3. Global Special Filehandles

ARGV

4.4.4. Global Special Constants

_ _END_ _

4.4.5. Regular Expression Special Variables

For more information on regular expressions, see Section 4.6, "Regular Expressions" later in this chapter.

$digit

Contains the text matched by the corresponding set of parentheses in the last pattern matched. For example, $1 matches whatever was contained in the first set of parentheses in the previous regular expression.

$&

$MATCH

The string matched by the last successful pattern match.

$'

$PREMATCH

The string preceding whatever was matched by the last successful pattern match.

$'

$POSTMATCH

The string following whatever was matched by the last successful pattern match.

$+

$LAST_PAREN_MATCH

The last bracket matched by the last search pattern. This is useful if you don't know which of a set of alternative patterns was matched. For example:

/Version: (.*)|Revision: (.*)/ && ($rev = $+);

$^N

The string matched by the most recently closed group. This is most useful inside (?{ . . . }) blocks for examining matched text. If you have multiple matches denoted by parentheses, $^N can be used in lieu of $1, $2, $3, etc., so you don't have to manually count the number of sets of parentheses that denote your matches. For example:

#!/usr/local/bin/perl -w

$words = "person|here";
$words =~ /(\w+)\|(\w+)/;
print $^N;   # Prints 'here'

4.4.6. Filehandle Special Variables

Most of these variables apply only when using formats. See Section 4.11, "Unicode" later in this chapter.

$|
$OUTPUT_AUTOFLUSH: If set to nonzero, forces an fflush(3) after every write or print on the currently selected output channel.
$%
$FORMAT_PAGE_NUMBER: The current page number of the currently selected output channel.
$=
$FORMAT_LINES_PER_PAGE: The current page length (printable lines) of the currently selected output channel. Default is 60.
$-
$FORMAT_LINES_LEFT: The number of lines left on the page of the currently selected output channel.
$~
$FORMAT_NAME: The name of the current report format for the currently selected output channel. Default is the name of the filehandle.
$^
$FORMAT_TOP_NAME: The name of the current top-of-page format for the currently selected output channel. Default is the name of the filehandle with _TOP appended.