perlvar.pod

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=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 aliases all the short names to the long
names in the current package. Some even have medium names, generally
borrowed from B<awk>. In general, it's best to use the

    use English '-no_match_vars';

invocation if you don't need $PREMATCH, $MATCH, or $POSTMATCH, as it avoids
a certain performance hit with the use of regular expressions. See
L<English>.

Variables that depend on the currently selected filehandle may be set by
calling an appropriate object method on the IO::Handle object, although
this is less efficient than using the regular built-in variables. (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.

You should be very careful when modifying the default values of most
special variables described in this document. In most cases you want
to localize these variables before changing them, since if you don't,
the change may affect other modules which rely on the default values
of the special variables that you have changed. This is one of the
correct ways to read the whole file at once:

    open my $fh, "foo" or die $!;
    local $/; # enable localized slurp mode
    my $content = <$fh>;
    close $fh;

But the following code is quite bad:

    open my $fh, "foo" or die $!;
    undef $/; # enable slurp mode
    my $content = <$fh>;
    close $fh;

since some other module, may want to read data from some file in the
default "line mode", so if the code we have just presented has been
executed, the global value of C<$/> is now changed for any other code
running inside the same Perl interpreter.

Usually when a variable is localized you want to make sure that this
change affects the shortest scope possible. So unless you are already
inside some short C<{}> block, you should create one yourself. For
example:

    my $content = '';
    open my $fh, "foo" or die $!;
    {
        local $/;
        $content = <$fh>;
    }
    close $fh;

Here is an example of how your own code can go broken:

    for (1..5){
        nasty_break();
        print "$_ ";
    }
    sub nasty_break {
        $_ = 5;
        # do something with $_
    }

You probably expect this code to print:

    1 2 3 4 5

but instead you get:

    5 5 5 5 5

Why? Because nasty_break() modifies C<$_> without localizing it
first. The fix is to add local():

        local $_ = 5;

It's easy to notice the problem in such a short example, but in more
complicated code you are looking for trouble if you don't localize
changes to the special variables.

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

=over 8

=item $ARG

=item $_
X<$_> X<$ARG>

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

As C<$_> is a global variable, this may lead in some cases to unwanted
side-effects.  As of perl 5.9.1, you can now use a lexical version of
C<$_> by declaring it in a file or in a block with C<my>.  Moreover,
declaring C<our $_> restores the global C<$_> in the current scope.

(Mnemonic: underline is understood in certain operations.)

=back

=over 8

=item $a

=item $b
X<$a> X<$b>

Special package variables when using sort(), see L<perlfunc/sort>.
Because of this specialness $a and $b don't need to be declared
(using use vars, or our()) even when using the C<strict 'vars'> pragma.
Don't lexicalize them with C<my $a> or C<my $b> if you want to be
able to use them in the sort() comparison block or function.

=back

=over 8

=item $<I<digits>>
X<$1> X<$2> X<$3>

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 $&
X<$&> X<$MATCH>

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>.

See L</@-> for a replacement.

=item ${^MATCH}
X<${^MATCH}>

This is similar to C<$&> (C<$POSTMATCH>) except that it does not incur the
performance penalty associated with that variable, and is only guaranteed
to return a defined value when the pattern was compiled or executed with
the C</p> modifier.

=item $PREMATCH

=item $`
X<$`> X<$PREMATCH>

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>.

See L</@-> for a replacement.

=item ${^PREMATCH}
X<${^PREMATCH}>

This is similar to C<$`> ($PREMATCH) except that it does not incur the
performance penalty associated with that variable, and is only guaranteed
to return a defined value when the pattern was compiled or executed with
the C</p> modifier.

=item $POSTMATCH

=item $'
X<$'> X<$POSTMATCH>

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:

    local $_ = 'abcdefghi';
    /def/;
    print "$`:$&:$'\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>.

See L</@-> for a replacement.

=item ${^POSTMATCH}
X<${^POSTMATCH}>

This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
performance penalty associated with that variable, and is only guaranteed
to return a defined value when the pattern was compiled or executed with
the C</p> modifier.

=item $LAST_PAREN_MATCH

=item $+
X<$+> X<$LAST_PAREN_MATCH>

The text matched by the last bracket of the last successful 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_SUBMATCH_RESULT

=item $^N
X<$^N>

The text matched by the used group most-recently closed (i.e. the group
with the rightmost closing parenthesis) of the last successful search
pattern.  (Mnemonic: the (possibly) Nested parenthesis that most
recently closed.)

This is primarily used inside C<(?{...})> blocks for examining text
recently matched. For example, to effectively capture text to a variable
(in addition to C<$1>, C<$2>, etc.), replace C<(...)> with

     (?:(...)(?{ $var = $^N }))

By setting and then using C<$var> in this way relieves you from having to
worry about exactly which numbered set of parentheses they are.

This variable is dynamically scoped to the current BLOCK.

=item @LAST_MATCH_END

=item @+
X<@+> X<@LAST_MATCH_END>

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 %+
X<%+>

Similar to C<@+>, the C<%+> hash allows access to the named capture
buffers, should they exist, in the last successful match in the
currently active dynamic scope.

For example, C<$+{foo}> is equivalent to C<$1> after the following match:

  'foo' =~ /(?<foo>foo)/;

The keys of the C<%+> hash list only the names of buffers that have
captured (and that are thus associated to defined values).

The underlying behaviour of C<%+> is provided by the
L<Tie::Hash::NamedCapture> module.

B<Note:> C<%-> and C<%+> are tied views into a common internal hash
associated with the last successful regular expression. Therefore mixing
iterative access to them via C<each> may have unpredictable results.
Likewise, if the last successful match changes, then the results may be
surprising.

=item HANDLE->input_line_number(EXPR)

=item $INPUT_LINE_NUMBER

=item $NR

=item $.
X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>

Current line number for the last filehandle accessed.

Each filehandle in Perl counts the number of lines that have been read
from it.  (Depending on the value of C<$/>, Perl's idea of what
constitutes a line may not match yours.)  When a line is read from a
filehandle (via readline() or C<< <> >>), or when tell() or seek() is
called on it, C<$.> becomes an alias to the line counter for that
filehandle.

You can adjust the counter by assigning to C<$.>, but this will not
actually move the seek pointer.  I<Localizing C<$.> will not localize
the filehandle's line count>.  Instead, it will localize perl's notion
of which filehandle C<$.> is currently aliased to.

C<$.> is reset when the filehandle is closed, but B<not> when an open
filehandle is reopened without an intervening close().  For more
details, see L<perlop/"IE<sol>O Operators">.  Because C<< <> >> never does
an explicit close, line numbers increase across ARGV files (but see
examples in L<perlfunc/eof>).

You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
line counter for a given filehandle without having to worry about
which handle you last accessed.

(Mnemonic: many programs use "." to mean the current line number.)

=item IO::Handle->input_record_separator(EXPR)

=item $INPUT_RECORD_SEPARATOR

=item $RS

=item $/
X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR>

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.)

    local $/;           # enable "slurp" mode
    local $_ = <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