perlvar.pod

source of perl for linux application,

=head1 NAME

perlvar - Perl predefined variables

=head1 DESCRIPTION

=head2 Predefined Names

The following names have special meaning to Perl.  Most 
punctuation names have reasonable mnemonics, or analogs in the
shells.  Nevertheless, if you wish to use long variable names,
you need only say

    use English;

at the top of your program.  This will alias all the short names to the
long names in the current package.  Some even have medium names,
generally borrowed from B<awk>.

If you don't mind the performance hit, variables that depend on the
currently selected filehandle may instead be set by calling an
appropriate object method on the IO::Handle object.  (Summary lines
below for this contain the word HANDLE.)  First you must say

    use IO::Handle;

after which you may use either

    method HANDLE EXPR

or more safely,

    HANDLE->method(EXPR)

Each method returns the old value of the IO::Handle attribute.
The methods each take an optional EXPR, which if supplied specifies the
new value for the IO::Handle attribute in question.  If not supplied,
most methods do nothing to the current value--except for
autoflush(), which will assume a 1 for you, just to be different.
Because loading in the IO::Handle class is an expensive operation, you should
learn how to use the regular built-in variables.

A few of these variables are considered "read-only".  This means that if
you try to assign to this variable, either directly or indirectly through
a reference, you'll raise a run-time exception.

The following list is ordered by scalar variables first, then the
arrays, then the hashes.

=over 8

=item $ARG

=item $_

The default input and pattern-searching space.  The following pairs are
equivalent:

    while (<>) {...}	# equivalent only in while!
    while (defined($_ = <>)) {...}

    /^Subject:/
    $_ =~ /^Subject:/

    tr/a-z/A-Z/
    $_ =~ tr/a-z/A-Z/

    chomp
    chomp($_)

Here are the places where Perl will assume $_ even if you
don't use it:

=over 3

=item *

Various unary functions, including functions like ord() and int(), as well
as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
STDIN.

=item *

Various list functions like print() and unlink().

=item *

The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
without an C<=~> operator.

=item *

The default iterator variable in a C<foreach> loop if no other
variable is supplied.

=item *

The implicit iterator variable in the grep() and map() functions.

=item *

The default place to put an input record when a C<< <FH> >>
operation's result is tested by itself as the sole criterion of a C<while>
test.  Outside a C<while> test, this will not happen.

=back

(Mnemonic: underline is understood in certain operations.)

=back

=over 8

=item $<I<digits>>

Contains the subpattern from the corresponding set of capturing
parentheses from the last pattern match, not counting patterns
matched in nested blocks that have been exited already.  (Mnemonic:
like \digits.)  These variables are all read-only and dynamically
scoped to the current BLOCK.

=item $MATCH

=item $&

The string matched by the last successful pattern match (not counting
any matches hidden within a BLOCK or eval() enclosed by the current
BLOCK).  (Mnemonic: like & in some editors.)  This variable is read-only
and dynamically scoped to the current BLOCK.

The use of this variable anywhere in a program imposes a considerable
performance penalty on all regular expression matches.  See L<BUGS>.

=item $PREMATCH

=item $`

The string preceding whatever was matched by the last successful
pattern match (not counting any matches hidden within a BLOCK or eval
enclosed by the current BLOCK).  (Mnemonic: C<`> often precedes a quoted
string.)  This variable is read-only.

The use of this variable anywhere in a program imposes a considerable
performance penalty on all regular expression matches.  See L<BUGS>.

=item $POSTMATCH

=item $'

The string following whatever was matched by the last successful
pattern match (not counting any matches hidden within a BLOCK or eval()
enclosed by the current BLOCK).  (Mnemonic: C<'> often follows a quoted
string.)  Example:

    $_ = 'abcdefghi';
    /def/;
    print "$`:$&:$'\n";  	# prints abc:def:ghi

This variable is read-only and dynamically scoped to the current BLOCK.

The use of this variable anywhere in a program imposes a considerable
performance penalty on all regular expression matches.  See L<BUGS>.

=item $LAST_PAREN_MATCH

=item $+

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

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

(Mnemonic: be positive and forward looking.)
This variable is read-only and dynamically scoped to the current BLOCK.

=item @LAST_MATCH_END

=item @+

This array holds the offsets of the ends of the last successful
submatches in the currently active dynamic scope.  C<$+[0]> is
the offset into the string of the end of the entire match.  This
is the same value as what the C<pos> function returns when called
on the variable that was matched against.  The I<n>th element
of this array holds the offset of the I<n>th submatch, so
C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
past where $2 ends, and so on.  You can use C<$#+> to determine
how many subgroups were in the last successful match.  See the
examples given for the C<@-> variable.

=item $MULTILINE_MATCHING

=item $*

Set to a non-zero integer value to do multi-line matching within a
string, 0 (or undefined) to tell Perl that it can assume that strings
contain a single line, for the purpose of optimizing pattern matches.
Pattern matches on strings containing multiple newlines can produce
confusing results when C<$*> is 0 or undefined. Default is undefined.
(Mnemonic: * matches multiple things.) This variable influences the
interpretation of only C<^> and C<$>. A literal newline can be searched
for even when C<$* == 0>.

Use of C<$*> is deprecated in modern Perl, supplanted by 
the C</s> and C</m> modifiers on pattern matching.

Assigning a non-numerical value to C<$*> triggers a warning (and makes
C<$*> act if C<$* == 0>), while assigning a numerical value to C<$*>
makes that an implicit C<int> is applied on the value.

=item input_line_number HANDLE EXPR

=item $INPUT_LINE_NUMBER

=item $NR

=item $.

The current input record number for the last file handle from which
you just read() (or called a C<seek> or C<tell> on).  The value
may be different from the actual physical line number in the file,
depending on what notion of "line" is in effect--see C<$/> on how
to change that.  An explicit close on a filehandle resets the line
number.  Because C<< <> >> never does an explicit close, line
numbers increase across ARGV files (but see examples in L<perlfunc/eof>).
Consider this variable read-only: setting it does not reposition
the seek pointer; you'll have to do that on your own.  Localizing C<$.>
has the effect of also localizing Perl's notion of "the last read
filehandle".  (Mnemonic: many programs use "." to mean the current line
number.)

=item input_record_separator HANDLE EXPR

=item $INPUT_RECORD_SEPARATOR

=item $RS

=item $/

The input record separator, newline by default.  This 
influences Perl's idea of what a "line" is.  Works like B<awk>'s RS
variable, including treating empty lines as a terminator if set to
the null string.  (An empty line cannot contain any spaces
or tabs.)  You may set it to a multi-character string to match a
multi-character terminator, or to C<undef> to read through the end
of file.  Setting it to C<"\n\n"> means something slightly
different than setting to C<"">, if the file contains consecutive
empty lines.  Setting to C<""> will treat two or more consecutive
empty lines as a single empty line.  Setting to C<"\n\n"> will
blindly assume that the next input character belongs to the next
paragraph, even if it's a newline.  (Mnemonic: / delimits
line boundaries when quoting poetry.)

    undef $/;		# enable "slurp" mode
    $_ = <FH>;		# whole file now here
    s/\n[ \t]+/ /g;

Remember: the value of C<$/> is a string, not a regex.  B<awk> has to be
better for something. :-)

Setting C<$/> to a reference to an integer, scalar containing an integer, or
scalar that's convertible to an integer will attempt to read records
instead of lines, with the maximum record size being the referenced
integer.  So this:

    $/ = \32768; # or \"32768", or \$var_containing_32768
    open(FILE, $myfile);
    $_ = <FILE>;

will read a record of no more than 32768 bytes from FILE.  If you're
not reading from a record-oriented file (or your OS doesn't have
record-oriented files), then you'll likely get a full chunk of data
with every read.  If a record is larger than the record size you've
set, you'll get the record back in pieces.

On VMS, record reads are done with the equivalent of C<sysread>,
so it's best not to mix record and non-record reads on the same
file.  (This is unlikely to be a problem, because any file you'd
want to read in record mode is probably unusable in line mode.)
Non-VMS systems do normal I/O, so it's safe to mix record and
non-record reads of a file.

See also L<perlport/"Newlines">.  Also see C<$.>.

=item autoflush HANDLE EXPR

=item $OUTPUT_AUTOFLUSH

=item $|

If set to nonzero, forces a flush right away and after every write
or print on the currently selected output channel.  Default is 0
(regardless of whether the channel is really buffered by the
system or not; C<$|> tells you only whether you've asked Perl
explicitly to flush after each write).  STDOUT will
typically be line buffered if output is to the terminal and block
buffered otherwise.  Setting this variable is useful primarily when
you are outputting to a pipe or socket, such as when you are running
a Perl program under B<rsh> and want to see the output as it's
happening.  This has no effect on input buffering.  See L<perlfunc/getc>
for that.  (Mnemonic: when you want your pipes to be piping hot.)

=item output_field_separator HANDLE EXPR

=item $OUTPUT_FIELD_SEPARATOR

=item $OFS

=item $,

The output field separator for the print operator.  Ordinarily the
print operator simply prints out its arguments without further
adornment.  To get behavior more like B<awk>, set this variable as
you would set B<awk>'s OFS variable to specify what is printed
between fields.  (Mnemonic: what is printed when there is a "," in
your print statement.)

=item output_record_separator HANDLE EXPR

=item $OUTPUT_RECORD_SEPARATOR

=item $ORS

=item $\

The output record separator for the print operator.  Ordinarily the
print operator simply prints out its arguments as is, with no
trailing newline or other end-of-record string added.  To get
behavior more like B<awk>, set this variable as you would set
B<awk>'s ORS variable to specify what is printed at the end of the
print.  (Mnemonic: you set C<$\> instead of adding "\n" at the
end of the print.  Also, it's just like C<$/>, but it's what you
get "back" from Perl.)

=item $LIST_SEPARATOR

=item $"

This is like C<$,> except that it applies to array and slice values
interpolated into a double-quoted string (or similar interpreted
string).  Default is a space.  (Mnemonic: obvious, I think.)

=item $SUBSCRIPT_SEPARATOR

=item $SUBSEP

=item $;

The subscript separator for multidimensional array emulation.  If you
refer to a hash element as

    $foo{$a,$b,$c}

it really means

    $foo{join($;, $a, $b, $c)}

But don't put

    @foo{$a,$b,$c}	# a slice--note the @

which means

    ($foo{$a},$foo{$b},$foo{$c})

Default is "\034", the same as SUBSEP in B<awk>.  If your
keys contain binary data there might not be any safe value for C<$;>.
(Mnemonic: comma (the syntactic subscript separator) is a
semi-semicolon.  Yeah, I know, it's pretty lame, but C<$,> is already
taken for something more important.)

Consider using "real" multidimensional arrays as described
in L<perllol>.

=item $OFMT

=item $#

The output format for printed numbers.  This variable is a half-hearted
attempt to emulate B<awk>'s OFMT variable.  There are times, however,
when B<awk> and Perl have differing notions of what counts as 
numeric.  The initial value is "%.I<n>g", where I<n> is the value
of the macro DBL_DIG from your system's F<float.h>.  This is different from
B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#>
explicitly to get B<awk>'s value.  (Mnemonic: # is the number sign.)

Use of C<$#> is deprecated.

=item format_page_number HANDLE EXPR

=item $FORMAT_PAGE_NUMBER

=item $%

The current page number of the currently selected output channel.
Used with formats.
(Mnemonic: % is page number in B<nroff>.)

=item format_lines_per_page HANDLE EXPR

=item $FORMAT_LINES_PER_PAGE

=item $=

The current page length (printable lines) of the currently selected
output channel.  Default is 60.  
Used with formats.
(Mnemonic: = has horizontal lines.)

=item format_lines_left HANDLE EXPR