perlvar - Perl predefined variables
Variable names in Perl can have several formats. Usually, they
must begin with a letter or underscore, in which case they can be
arbitrarily long (up to an internal limit of 251 characters) and
may contain letters, digits, underscores, or the special sequence
::
or '. In this case, the part before the last ::
or
' is taken to be a package qualifier; see perlmod.
Perl variable names may also be a sequence of digits or a single
punctuation or control character. These names are all reserved for
special uses by Perl; for example, the all-digits names are used
to hold data captured by backreferences after a regular expression
match. Perl has a special syntax for the single-control-character
names: It understands ^X (caret X
) to mean the control-X
character. For example, the notation $^W
(dollar-sign caret
W
) is the scalar variable whose name is the single character
control-W
. This is better than typing a literal control-W
into your program.
Since Perl 5.6, Perl variable names may be alphanumeric
strings that begin with control characters (or better yet, a caret).
These variables must be written in the form ${^Foo}
; the braces
are not optional. ${^Foo}
denotes the scalar variable whose
name is a control-F
followed by two o
's. These variables are
reserved for future special uses by Perl, except for the ones that
begin with ^_ (control-underscore or caret-underscore). No
control-character name that begins with ^_ will acquire a special
meaning in any future version of Perl; such names may therefore be
used safely in programs. $^_
itself, however, is reserved.
Perl identifiers that begin with digits, control characters, or
punctuation characters are exempt from the effects of the package
declaration and are always forced to be in package main
; they are
also exempt from strict 'vars'
errors. A few other names are also
exempt in these ways:
- ENV STDIN
- INC STDOUT
- ARGV STDERR
- ARGVOUT
- SIG
In particular, the special ${^_XYZ}
variables are always taken
to be in package main
, regardless of any package declarations
presently in scope.
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 awk. To avoid a performance hit, if you don't need the
$PREMATCH
, $MATCH
, or $POSTMATCH
it's best to use the English
module without them:
- use English '-no_match_vars';
Before you continue, note the sort order for variables. In general, we
first list the variables in case-insensitive, almost-lexigraphical
order (ignoring the { or ^ preceding words, as in ${^UNICODE}
or $^T
), although $_
and @_
move up to the top of the pile.
For variables with the same identifier, we list it in order of scalar,
array, hash, and bareword.
The default input and pattern-searching space. The following pairs are equivalent:
Here are the places where Perl will assume $_
even if you don't use it:
The following functions use $_
as a default argument:
abs, alarm, chomp, chop, chr, chroot, cos, defined, eval, exp, glob, hex, int, lc, lcfirst, length, log, lstat, mkdir, oct, ord, pos, print, quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only), rmdir, sin, split (on its second argument), sqrt, stat, study, uc, ucfirst, unlink, unpack.
All file tests (-f
, -d
) except for -t
, which defaults to STDIN.
See -X
The pattern matching operations m//, s/// and tr/// (aka y///)
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 implicit variable of given()
.
The default place to put an input record when a <FH>
operation's result is tested by itself as the sole criterion of a while
test. Outside a while
test, this will not happen.
As $_
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
$_
by declaring it in a file or in a block with my. Moreover,
declaring our $_
restores the global $_
in the current scope.
Mnemonic: underline is understood in certain operations.
Within a subroutine the array @_
contains the parameters passed to
that subroutine. Inside a subroutine, @_
is the default array for
the array operators push, pop, shift, and unshift.
See perlsub.
When an array or an array slice is interpolated into a double-quoted
string or a similar context such as /.../
, its elements are
separated by this value. Default is a space. For example, this:
- print "The array is: @array\n";
is equivalent to this:
Mnemonic: works in double-quoted context.
The process number of the Perl running this script. You should
consider this variable read-only, although it will be altered
across fork() calls.
Note for Linux users: on Linux, the C functions getpid()
and
getppid() return different values from different threads. In order to
be portable, this behavior is not reflected by $$
, whose value remains
consistent across threads. If you want to call the underlying getpid()
,
you may use the CPAN module Linux::Pid
.
Mnemonic: same as shells.
The real gid of this process. If you are on a machine that supports
membership in multiple groups simultaneously, gives a space separated
list of groups you are in. The first number is the one returned by
getgid()
, and the subsequent ones by getgroups()
, one of which may be
the same as the first number.
However, a value assigned to $(
must be a single number used to
set the real gid. So the value given by $(
should not be assigned
back to $(
without being forced numeric, such as by adding zero. Note
that this is different to the effective gid ($)
) which does take a
list.
You can change both the real gid and the effective gid at the same
time by using POSIX::setgid()
. Changes to $(
require a check to $!
to detect any possible errors after an attempted change.
Mnemonic: parentheses are used to group things. The real gid is the group you left, if you're running setgid.
The effective gid of this process. If you are on a machine that
supports membership in multiple groups simultaneously, gives a space
separated list of groups you are in. The first number is the one
returned by getegid()
, and the subsequent ones by getgroups()
,
one of which may be the same as the first number.
Similarly, a value assigned to $)
must also be a space-separated
list of numbers. The first number sets the effective gid, and
the rest (if any) are passed to setgroups()
. To get the effect of an
empty list for setgroups()
, just repeat the new effective gid; that is,
to force an effective gid of 5 and an effectively empty setgroups()
list, say $) = "5 5"
.
You can change both the effective gid and the real gid at the same
time by using POSIX::setgid()
(use only a single numeric argument).
Changes to $)
require a check to $!
to detect any possible errors
after an attempted change.
$<
, $>
, $(
and $)
can be set only on
machines that support the corresponding set[re][ug]id() routine. $(
and $)
can be swapped only on machines supporting setregid()
.
Mnemonic: parentheses are used to group things. The effective gid is the group that's right for you, if you're running setgid.
Contains the name of the program being executed.
On some (but not all) operating systems assigning to $0
modifies
the argument area that the ps
program sees. On some platforms you
may have to use special ps
options or a different ps
to see the
changes. Modifying the $0
is more useful as a way of indicating the
current program state than it is for hiding the program you're
running.
Note that there are platform-specific limitations on the maximum
length of $0
. In the most extreme case it may be limited to the
space occupied by the original $0
.
In some platforms there may be arbitrary amount of padding, for
example space characters, after the modified name as shown by ps
.
In some platforms this padding may extend all the way to the original
length of the argument area, no matter what you do (this is the case
for example with Linux 2.2).
Note for BSD users: setting $0
does not completely remove "perl"
from the ps(1) output. For example, setting $0
to "foobar"
may
result in "perl: foobar (perl)"
(whether both the "perl: "
prefix
and the " (perl)" suffix are shown depends on your exact BSD variant
and version). This is an operating system feature, Perl cannot help it.
In multithreaded scripts Perl coordinates the threads so that any
thread may modify its copy of the $0
and the change becomes visible
to ps(1) (assuming the operating system plays along). Note that
the view of $0
the other threads have will not change since they
have their own copies of it.
If the program has been given to perl via the switches -e
or -E
,
$0
will contain the string "-e"
.
On Linux as of perl 5.14 the legacy process name will be set with
prctl(2)
, in addition to altering the POSIX name via argv[0]
as
perl has done since version 4.000. Now system utilities that read the
legacy process name such as ps, top and killall will recognize the
name you set when assigning to $0
. The string you supply will be
cut off at 16 bytes, this is a limitation imposed by Linux.
Mnemonic: same as sh and ksh.
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 awk. If your keys contain
binary data there might not be any safe value for $;
.
Consider using "real" multidimensional arrays as described in perllol.
Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
The real uid of this process. You can change both the real uid and the
effective uid at the same time by using POSIX::setuid()
. Since
changes to $<
require a system call, check $!
after a change
attempt to detect any possible errors.
Mnemonic: it's the uid you came from, if you're running setuid.
The effective uid of this process. For example:
- $< = $>; # set real to effective uid
- ($<,$>) = ($>,$<); # swap real and effective uids
You can change both the effective uid and the real uid at the same
time by using POSIX::setuid()
. Changes to $>
require a check
to $!
to detect any possible errors after an attempted change.
$<
and $>
can be swapped only on machines
supporting setreuid()
.
Mnemonic: it's the uid you went to, if you're running setuid.
Special package variables when using sort(), see sort.
Because of this specialness $a
and $b
don't need to be declared
(using use vars
, or our()) even when using the strict 'vars'
pragma. Don't lexicalize them with my $a
or my $b
if you want to
be able to use them in the sort() comparison block or function.
The current value of the flag associated with the -c switch.
Mainly of use with -MO=... to allow code to alter its behavior
when being compiled, such as for example to AUTOLOAD
at compile
time rather than normal, deferred loading. Setting
$^C = 1
is similar to calling B::minus_c
.
This variable was added in Perl 5.6.
The current value of the debugging flags. May be read or set. Like its
command-line equivalent, you can use numeric or symbolic values, eg
$^D = 10
or $^D = "st"
.
Mnemonic: value of -D switch.
The object reference to the Encode
object that is used to convert
the source code to Unicode. Thanks to this variable your Perl script
does not have to be written in UTF-8. Default is undef. The direct
manipulation of this variable is highly discouraged.
This variable was added in Perl 5.8.2.
The hash %ENV
contains your current environment. Setting a
value in ENV
changes the environment for any child processes
you subsequently fork() off.
The maximum system file descriptor, ordinarily 2. System file
descriptors are passed to exec()ed processes, while higher file
descriptors are not. Also, during an open(), system file descriptors are
preserved even if the open() fails (ordinary file descriptors are
closed before the open() is attempted). The close-on-exec
status of a file descriptor will be decided according to the value of
$^F
when the corresponding file, pipe, or socket was opened, not the
time of the exec().
The array @F
contains the fields of each line read in when autosplit
mode is turned on. See perlrun for the -a switch. This array
is package-specific, and must be declared or given a full package name
if not in package main when running under strict 'vars'
.
The current phase of the perl interpreter.
Possible values are:
The PerlInterpreter*
is being constructed via perl_construct
. This
value is mostly there for completeness and for use via the
underlying C variable PL_phase
. It's not really possible for Perl
code to be executed unless construction of the interpreter is
finished.
This is the global compile-time. That includes, basically, every
BEGIN
block executed directly or indirectly from during the
compile-time of the top-level program.
This phase is not called "BEGIN" to avoid confusion with
BEGIN
-blocks, as those are executed during compile-time of any
compilation unit, not just the top-level program. A new, localised
compile-time entered at run-time, for example by constructs as
eval "use SomeModule"
are not global interpreter phases, and
therefore aren't reflected by ${^GLOBAL_PHASE}
.
Execution of any CHECK
blocks.
Similar to "CHECK", but for INIT
-blocks, not CHECK
blocks.
The main run-time, i.e. the execution of PL_main_root
.
Execution of any END
blocks.
Global destruction.
Also note that there's no value for UNITCHECK-blocks. That's because those are run for each compilation unit individually, and therefore is not a global interpreter phase.
Not every program has to go through each of the possible phases, but transition from one phase to another can only happen in the order described in the above list.
An example of all of the phases Perl code can see:
- BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }
- INIT { print "init-time: ${^GLOBAL_PHASE}\n" }
- CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }
- {
- package Print::Phase;
- sub new {
- my ($class, $time) = @_;
- return bless \$time, $class;
- }
- sub DESTROY {
- my $self = shift;
- print "$$self: ${^GLOBAL_PHASE}\n";
- }
- }
- print "run-time: ${^GLOBAL_PHASE}\n";
- my $runtime = Print::Phase->new(
- "lexical variables are garbage collected before END"
- );
- END { print "end-time: ${^GLOBAL_PHASE}\n" }
- our $destruct = Print::Phase->new(
- "package variables are garbage collected after END"
- );
This will print out
This variable was added in Perl 5.14.0.
WARNING: This variable is strictly for internal use only. Its availability, behavior, and contents are subject to change without notice.
This variable contains compile-time hints for the Perl interpreter. At the end of compilation of a BLOCK the value of this variable is restored to the value when the interpreter started to compile the BLOCK.
When perl begins to parse any block construct that provides a lexical scope
(e.g., eval body, required file, subroutine body, loop body, or conditional
block), the existing value of $^H
is saved, but its value is left unchanged.
When the compilation of the block is completed, it regains the saved value.
Between the points where its value is saved and restored, code that
executes within BEGIN blocks is free to change the value of $^H
.
This behavior provides the semantic of lexical scoping, and is used in,
for instance, the use strict
pragma.
The contents should be an integer; different bits of it are used for different pragmatic flags. Here's an example:
Consider what happens during execution of the BEGIN block. At this point
the BEGIN block has already been compiled, but the body of foo()
is still
being compiled. The new value of $^H
will therefore be visible only while
the body of foo()
is being compiled.
Substitution of BEGIN { add_100() }
block with:
- BEGIN { require strict; strict->import('vars') }
demonstrates how use strict 'vars'
is implemented. Here's a conditional
version of the same lexical pragma:
- BEGIN { require strict; strict->import('vars') if $condition }
This variable was added in Perl 5.003.
The %^H
hash provides the same scoping semantic as $^H
. This makes it
useful for implementation of lexically scoped pragmas. See perlpragma.
This variable was added in Perl 5.6.
The array @INC
contains the list of places that the do EXPR
,
require, or use constructs look for their library files. It
initially consists of the arguments to any -I command-line
switches, followed by the default Perl library, probably
/usr/local/lib/perl, followed by ".", to represent the current
directory. ("." will not be appended if taint checks are enabled,
either by -T
or by -t
.) If you need to modify this at runtime,
you should use the use lib
pragma to get the machine-dependent
library properly loaded also:
You can also insert hooks into the file inclusion system by putting Perl
code directly into @INC
. Those hooks may be subroutine references, array
references or blessed objects. See require for details.
The hash %INC
contains entries for each filename included via the
do, require, or use operators. The key is the filename
you specified (with module names converted to pathnames), and the
value is the location of the file found. The require
operator uses this hash to determine whether a particular file has
already been included.
If the file was loaded via a hook (e.g. a subroutine reference, see
require for a description of these hooks), this hook is
by default inserted into %INC
in place of a filename. Note, however,
that the hook may have set the %INC
entry by itself to provide some more
specific info.
The current value of the inplace-edit extension. Use undef to disable
inplace editing.
Mnemonic: value of -i switch.
By default, running out of memory is an untrappable, fatal error.
However, if suitably built, Perl can use the contents of $^M
as an emergency memory pool after die()ing. Suppose that your Perl
were compiled with -DPERL_EMERGENCY_SBRK
and used Perl's malloc.
Then
- $^M = 'a' x (1 << 16);
would allocate a 64K buffer for use in an emergency. See the INSTALL file in the Perl distribution for information on how to add custom C compilation flags when compiling perl. To discourage casual use of this advanced feature, there is no English long name for this variable.
This variable was added in Perl 5.004.
The name of the operating system under which this copy of Perl was built, as determined during the configuration process. For examples see PLATFORMS in perlport.
The value is identical to $Config{'osname'}
. See also Config
and the -V command-line switch documented in perlrun.
In Windows platforms, $^O
is not very helpful: since it is always
MSWin32
, it doesn't tell the difference between
95/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName()
or
Win32::GetOSVersion() (see Win32 and perlport) to distinguish
between the variants.
This variable was added in Perl 5.003.
An internal variable used by PerlIO. A string in two parts, separated
by a \0
byte, the first part describes the input layers, the second
part describes the output layers.
This variable was added in Perl 5.8.2.
The internal variable for debugging support. The meanings of the various bits are subject to change, but currently indicate:
Debug subroutine enter/exit.
Line-by-line debugging. Causes DB::DB()
subroutine to be called for each
statement executed. Also causes saving source code lines (like 0x400).
Switch off optimizations.
Preserve more data for future interactive inspections.
Keep info about source lines on which a subroutine is defined.
Start with single-step on.
Use subroutine address instead of name when reporting.
Report goto &subroutine
as well.
Provide informative "file" names for evals based on the place they were compiled.
Provide informative names to anonymous subroutines based on the place they were compiled.
Save source code lines into @{"_<$filename"}
.
Some bits may be relevant at compile-time only, some at run-time only. This is a new mechanism and the details may change. See also perldebguts.
The hash %SIG
contains signal handlers for signals. For example:
Using a value of 'IGNORE'
usually has the effect of ignoring the
signal, except for the CHLD
signal. See perlipc for more about
this special case.
Here are some other examples:
- $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
- $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
- $SIG{"PIPE"} = *Plumber; # somewhat esoteric
- $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
Be sure not to use a bareword as the name of a signal handler, lest you inadvertently call it.
If your system has the sigaction()
function then signal handlers
are installed using it. This means you get reliable signal handling.
The default delivery policy of signals changed in Perl 5.8.0 from immediate (also known as "unsafe") to deferred, also known as "safe signals". See perlipc for more information.
Certain internal hooks can be also set using the %SIG
hash. The
routine indicated by $SIG{__WARN__}
is called when a warning
message is about to be printed. The warning message is passed as the
first argument. The presence of a __WARN__
hook causes the
ordinary printing of warnings to STDERR
to be suppressed. You can
use this to save warnings in a variable, or turn warnings into fatal
errors, like this:
As the 'IGNORE'
hook is not supported by __WARN__
, you can
disable warnings using the empty subroutine:
The routine indicated by $SIG{__DIE__}
is called when a fatal
exception is about to be thrown. The error message is passed as the
first argument. When a __DIE__
hook routine returns, the exception
processing continues as it would have in the absence of the hook,
unless the hook routine itself exits via a goto, a loop exit, or a
die(). The __DIE__
handler is explicitly disabled during the
call, so that you can die from a __DIE__
handler. Similarly for
__WARN__
.
Due to an implementation glitch, the $SIG{__DIE__}
hook is called
even inside an eval(). Do not use this to rewrite a pending
exception in $@
, or as a bizarre substitute for overriding
CORE::GLOBAL::die()
. This strange action at a distance may be fixed
in a future release so that $SIG{__DIE__}
is only called if your
program is about to exit, as was the original intent. Any other use is
deprecated.
__DIE__
/__WARN__
handlers are very special in one respect: they
may be called to report (probable) errors found by the parser. In such
a case the parser may be in inconsistent state, so any attempt to
evaluate Perl code from such a handler will probably result in a
segfault. This means that warnings or errors that result from parsing
Perl should be used with extreme caution, like this:
Here the first line will load Carp
unless it is the parser who
called the handler. The second line will print backtrace and die if
Carp
was available. The third line will be executed only if Carp
was
not available.
Having to even think about the $^S
variable in your exception
handlers is simply wrong. $SIG{__DIE__}
as currently implemented
invites grievous and difficult to track down errors. Avoid it
and use an END{}
or CORE::GLOBAL::die override instead.
See die, warn, eval, and warnings for additional information.
The time at which the program began running, in seconds since the epoch (beginning of 1970). The values returned by the -M, -A, and -C filetests are based on this value.
Reflects if taint mode is on or off. 1 for on (the program was run with -T), 0 for off, -1 when only taint warnings are enabled (i.e. with -t or -TU).
This variable is read-only.
This variable was added in Perl 5.8.
Reflects certain Unicode settings of Perl. See perlrun
documentation for the -C
switch for more information about
the possible values.
This variable is set during Perl startup and is thereafter read-only.
This variable was added in Perl 5.8.2.
This variable controls the state of the internal UTF-8 offset caching code. 1 for on (the default), 0 for off, -1 to debug the caching code by checking all its results against linear scans, and panicking on any discrepancy.
This variable was added in Perl 5.8.9.
This variable indicates whether a UTF-8 locale was detected by perl at
startup. This information is used by perl when it's in
adjust-utf8ness-to-locale mode (as when run with the -CL
command-line
switch); see perlrun for more info on this.
This variable was added in Perl 5.8.8.
The revision, version, and subversion of the Perl interpreter,
represented as a version
object.
This variable first appeared in perl 5.6.0; earlier versions of perl
will see an undefined value. Before perl 5.10.0 $^V
was represented
as a v-string.
$^V
can be used to determine whether the Perl interpreter executing
a script is in the right range of versions. For example:
- warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
To convert $^V
into its string representation use sprintf()'s
"%vd"
conversion:
- printf "version is v%vd\n", $^V; # Perl's version
See the documentation of use VERSION
and require VERSION
for a convenient way to fail if the running Perl interpreter is too old.
See also $]
for an older representation of the Perl version.
This variable was added in Perl 5.6.
Mnemonic: use ^V for Version Control.
If this variable is set to a true value, then stat() on Windows will
not try to open the file. This means that the link count cannot be
determined and file attributes may be out of date if additional
hardlinks to the file exist. On the other hand, not opening the file
is considerably faster, especially for files on network drives.
This variable could be set in the sitecustomize.pl file to
configure the local Perl installation to use "sloppy" stat() by
default. See the documentation for -f in
perlrun for more information about site
customization.
This variable was added in Perl 5.10.
The name used to execute the current copy of Perl, from C's
argv[0]
or (where supported) /proc/self/exe.
Depending on the host operating system, the value of $^X
may be
a relative or absolute pathname of the perl program file, or may
be the string used to invoke perl but not the pathname of the
perl program file. Also, most operating systems permit invoking
programs that are not in the PATH environment variable, so there
is no guarantee that the value of $^X
is in PATH. For VMS, the
value may or may not include a version number.
You usually can use the value of $^X
to re-invoke an independent
copy of the same perl that is currently running, e.g.,
- @first_run = `$^X -le "print int rand 100 for 1..100"`;
But recall that not all operating systems support forking or capturing of the output of commands, so this complex statement may not be portable.
It is not safe to use the value of $^X
as a path name of a file,
as some operating systems that have a mandatory suffix on
executable files do not require use of the suffix when invoking
a command. To convert the value of $^X
to a path name, use the
following statements:
Because many operating systems permit anyone with read access to
the Perl program file to make a copy of it, patch the copy, and
then execute the copy, the security-conscious Perl programmer
should take care to invoke the installed copy of perl, not the
copy referenced by $^X
. The following statements accomplish
this goal, and produce a pathname that can be invoked as a
command or referenced as a file.
Most of the special variables related to regular expressions are side effects. Perl sets these variables when it has a successful match, so you should check the match result before using them. For instance:
- if( /P(A)TT(ER)N/ ) {
- print "I found $1 and $2\n";
- }
These variables are read-only and dynamically-scoped, unless we note otherwise.
The dynamic nature of the regular expression variables means that their value is limited to the block that they are in, as demonstrated by this bit of code:
The output shows that while in the OUTER
block, the values of $1
and $2
are from the match against $outer
. Inside the INNER
block, the values of $1
and $2
are from the match against
$inner
, but only until the end of the block (i.e. the dynamic
scope). After the INNER
block completes, the values of $1
and
$2
return to the values for the match against $outer
even though
we have not made another match:
- $1 is Wallace; $2 is Grommit
- $1 is Mutt; $2 is Jeff
- $1 is Wallace; $2 is Grommit
Due to an unfortunate accident of Perl's implementation, use
English
imposes a considerable performance penalty on all regular
expression matches in a program because it uses the $`
, $&
, and
$'
, regardless of whether they occur in the scope of use
English
. For that reason, saying use English
in libraries is
strongly discouraged unless you import it without the match variables:
- use English '-no_match_vars'
The Devel::NYTProf
and Devel::FindAmpersand
modules can help you find uses of these
problematic match variables in your code.
Since Perl 5.10, you can use the /p match operator flag and the
${^PREMATCH}
, ${^MATCH}
, and ${^POSTMATCH}
variables instead
so you only suffer the performance penalties.
Contains the subpattern from the corresponding set of capturing parentheses from the last successful pattern match, not counting patterns matched in nested blocks that have been exited already.
These variables are read-only and dynamically-scoped.
Mnemonic: like \digits.
The string matched by the last successful pattern match (not counting
any matches hidden within a BLOCK or eval() enclosed by the current
BLOCK).
The use of this variable anywhere in a program imposes a considerable
performance penalty on all regular expression matches. To avoid this
penalty, you can extract the same substring by using @-. Starting
with Perl 5.10, you can use the </p> match flag and the ${^MATCH}
variable to do the same thing for particular match operations.
This variable is read-only and dynamically-scoped.
Mnemonic: like &
in some editors.
This is similar to $&
($MATCH
) 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 /p modifier.
This variable was added in Perl 5.10.
This variable is read-only and dynamically-scoped.
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.
The use of this variable anywhere in a program imposes a considerable
performance penalty on all regular expression matches. To avoid this
penalty, you can extract the same substring by using @-. Starting
with Perl 5.10, you can use the </p> match flag and the
${^PREMATCH}
variable to do the same thing for particular match
operations.
This variable is read-only and dynamically-scoped.
Mnemonic: ` often precedes a quoted string.
This is similar to $`
($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 /p modifier.
This variable was added in Perl 5.10
This variable is read-only and dynamically-scoped.
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). Example:
The use of this variable anywhere in a program imposes a considerable
performance penalty on all regular expression matches.
To avoid this penalty, you can extract the same substring by
using @-. Starting with Perl 5.10, you can use the </p> match flag
and the ${^POSTMATCH}
variable to do the same thing for particular
match operations.
This variable is read-only and dynamically-scoped.
Mnemonic: ' often follows a quoted string.
This is similar to $'
($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 /p modifier.
This variable was added in Perl 5.10.
This variable is read-only and dynamically-scoped.
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 = $+);
This variable is read-only and dynamically-scoped.
Mnemonic: be positive and forward looking.
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.
This is primarily used inside (?{...}) blocks for examining text
recently matched. For example, to effectively capture text to a variable
(in addition to $1
, $2
, etc.), replace (...)
with
- (?:(...)(?{ $var = $^N }))
By setting and then using $var
in this way relieves you from having to
worry about exactly which numbered set of parentheses they are.
This variable was added in Perl 5.8.
Mnemonic: the (possibly) Nested parenthesis that most recently closed.
This array holds the offsets of the ends of the last successful
submatches in the currently active dynamic scope. $+[0]
is
the offset into the string of the end of the entire match. This
is the same value as what the pos function returns when called
on the variable that was matched against. The nth element
of this array holds the offset of the nth submatch, so
$+[1]
is the offset past where $1
ends, $+[2]
the offset
past where $2
ends, and so on. You can use $#+
to determine
how many subgroups were in the last successful match. See the
examples given for the @-
variable.
This variable was added in Perl 5.6.
Similar to @+
, the %+
hash allows access to the named capture
buffers, should they exist, in the last successful match in the
currently active dynamic scope.
For example, $+{foo}
is equivalent to $1
after the following match:
- 'foo' =~ /(?<foo>foo)/;
The keys of the %+
hash list only the names of buffers that have
captured (and that are thus associated to defined values).
The underlying behaviour of %+
is provided by the
Tie::Hash::NamedCapture module.
Note: %-
and %+
are tied views into a common internal hash
associated with the last successful regular expression. Therefore mixing
iterative access to them via each may have unpredictable results.
Likewise, if the last successful match changes, then the results may be
surprising.
This variable was added in Perl 5.10.
This variable is read-only and dynamically-scoped.
$-[0]
is the offset of the start of the last successful match.
$-[n] is the offset of the start of the substring matched by
n-th subpattern, or undef if the subpattern did not match.
Thus, after a match against $_
, $&
coincides with substr $_, $-[0],
$+[0] - $-[0]
. Similarly, $n coincides with substr $_, $-[n],
$+[n] - $-[n]
if $-[n]
is defined, and $+ coincides with
substr $_, $-[$#-], $+[$#-] - $-[$#-]
. One can use $#-
to find the last
matched subgroup in the last successful match. Contrast with
$#+
, the number of subgroups in the regular expression. Compare
with @+
.
This array holds the offsets of the beginnings of the last
successful submatches in the currently active dynamic scope.
$-[0]
is the offset into the string of the beginning of the
entire match. The nth element of this array holds the offset
of the nth submatch, so $-[1]
is the offset where $1
begins, $-[2]
the offset where $2
begins, and so on.
After a match against some variable $var
:
$`
is the same as substr($var, 0, $-[0])
$&
is the same as substr($var, $-[0], $+[0] - $-[0])
$'
is the same as substr($var, $+[0])
$1
is the same as substr($var, $-[1], $+[1] - $-[1])
$2
is the same as substr($var, $-[2], $+[2] - $-[2])
$3
is the same as substr($var, $-[3], $+[3] - $-[3])
This variable was added in Perl 5.6.
Similar to %+
, this variable allows access to the named capture groups
in the last successful match in the currently active dynamic scope. To
each capture group name found in the regular expression, it associates a
reference to an array containing the list of values captured by all
buffers with that name (should there be several of them), in the order
where they appear.
Here's an example:
would print out:
- $-{A}[0] : '1'
- $-{A}[1] : '3'
- $-{B}[0] : '2'
- $-{B}[1] : '4'
The keys of the %-
hash correspond to all buffer names found in
the regular expression.
The behaviour of %-
is implemented via the
Tie::Hash::NamedCapture module.
Note: %-
and %+
are tied views into a common internal hash
associated with the last successful regular expression. Therefore mixing
iterative access to them via each may have unpredictable results.
Likewise, if the last successful match changes, then the results may be
surprising.
This variable was added in Perl 5.10
This variable is read-only and dynamically-scoped.
The result of evaluation of the last successful (?{ code })
regular expression assertion (see perlre). May be written to.
This variable was added in Perl 5.005.
The current value of the regex debugging flags. Set to 0 for no debug output
even when the re 'debug'
module is loaded. See re for details.
This variable was added in Perl 5.10.
Controls how certain regex optimisations are applied and how much memory they utilize. This value by default is 65536 which corresponds to a 512kB temporary cache. Set this to a higher value to trade memory for speed when matching large alternations. Set it to a lower value if you want the optimisations to be as conservative of memory as possible but still occur, and set it to a negative value to prevent the optimisation and conserve the most memory. Under normal situations this variable should be of no interest to you.
This variable was added in Perl 5.10.
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:
But the following code is quite bad:
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 $/
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 {}
block, you should create one yourself. For
example:
Here is an example of how your own code can go broken:
- for ( 1..3 ){
- $\ = "\r\n";
- nasty_break();
- print "$_";
- }
- sub nasty_break {
- $\ = "\f";
- # do something with $_
- }
You probably expect this code to print the equivalent of
- "1\r\n2\r\n3\r\n"
but instead you get:
- "1\f2\f3\f"
Why? Because nasty_break()
modifies $\
without localizing it
first. The value you set in nasty_break()
is still there when you
return. The fix is to add local() so the value doesn't leak out of
nasty_break()
:
- local $\ = "\f";
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.
Contains the name of the current file when reading from <>
.
The array @ARGV
contains the command-line arguments intended for
the script. $#ARGV
is generally the number of arguments minus
one, because $ARGV[0]
is the first argument, not the program's
command name itself. See $0
for the command name.
The special filehandle that iterates over command-line filenames in
@ARGV
. Usually written as the null filehandle in the angle operator
<>
. Note that currently ARGV
only has its magical effect
within the <>
operator; elsewhere it is just a plain filehandle
corresponding to the last file opened by <>
. In particular,
passing \*ARGV
as a parameter to a function that expects a filehandle
may not cause your function to automatically read the contents of all the
files in @ARGV
.
The special filehandle that points to the currently open output file
when doing edit-in-place processing with -i. Useful when you have
to do a lot of inserting and don't want to keep modifying $_
. See
perlrun for the -i switch.
The output field separator for the print operator. If defined, this
value is printed between each of print's arguments. Default is undef.
Mnemonic: what is printed when there is a "," in your print statement.
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 $/
, Perl's idea of what
constitutes a line may not match yours.) When a line is read from a
filehandle (via readline() or <>
), or when tell() or
seek() is called on it, $.
becomes an alias to the line counter
for that filehandle.
You can adjust the counter by assigning to $.
, but this will not
actually move the seek pointer. Localizing $.
will not localize
the filehandle's line count. Instead, it will localize perl's notion
of which filehandle $.
is currently aliased to.
$.
is reset when the filehandle is closed, but not when an open
filehandle is reopened without an intervening close(). For more
details, see I/O Operators in perlop. Because <>
never does
an explicit close, line numbers increase across ARGV
files (but see
examples in eof).
You can also use 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.
The input record separator, newline by default. This influences Perl's
idea of what a "line" is. Works like 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
undef to read through the end of file. Setting it to "\n\n"
means something slightly different than setting to ""
, if the file
contains consecutive empty lines. Setting to ""
will treat two or
more consecutive empty lines as a single empty line. Setting to
"\n\n"
will blindly assume that the next input character belongs to
the next paragraph, even if it's a newline.
Remember: the value of $/
is a string, not a regex. awk has to
be better for something. :-)
Setting $/
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:
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. Trying to set the record size to zero or less will cause reading in the (rest of the) whole file.
On VMS, record reads are done with the equivalent of 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 Newlines in perlport. Also see $.
.
Mnemonic: / delimits line boundaries when quoting poetry.
The output record separator for the print operator. If defined, this
value is printed after the last of print's arguments. Default is undef.
Mnemonic: you set $\
instead of adding "\n" at the end of the print.
Also, it's just like $/
, but it's what you get "back" from Perl.
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; $|
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 rsh and
want to see the output as it's happening. This has no effect on input
buffering. See getc for that. See select on
how to select the output channel. See also IO::Handle.
Mnemonic: when you want your pipes to be piping hot.
The special variables for formats are a subset of those for filehandles. See perlform for more information about Perl's formats.
The current value of the write() accumulator for format() lines.
A format contains formline() calls that put their result into
$^A
. After calling its format, write() prints out the contents
of $^A
and empties. So you never really see the contents of $^A
unless you call formline() yourself and then look at it. See
perlform and formline PICTURE,LIST.
What formats output as a form feed. The default is \f
.
The current page number of the currently selected output channel.
Mnemonic: %
is page number in nroff.
The number of lines left on the page of the currently selected output channel.
Mnemonic: lines_on_page - lines_printed.
The current set of characters after which a string may be broken to
fill continuation fields (starting with ^) in a format. The default is
" \n-", to break on a space, newline, or a hyphen.
Mnemonic: a "colon" in poetry is a part of a line.
The current page length (printable lines) of the currently selected output channel. The default is 60.
Mnemonic: = has horizontal lines.
The name of the current top-of-page format for the currently selected
output channel. The default is the name of the filehandle with _TOP
appended. For example, the default format top name for the STDOUT
filehanlde is STDOUT_TOP
.
Mnemonic: points to top of page.
The name of the current report format for the currently selected
output channel. The default format name is the same as the filehandle
name. For example, the default format name for the STDOUT
filehandle is just STDOUT
.
Mnemonic: brother to $^
.
The variables $@
, $!
, $^E
, and $?
contain information
about different types of error conditions that may appear during
execution of a Perl program. The variables are shown ordered by
the "distance" between the subsystem which reported the error and
the Perl process. They correspond to errors detected by the Perl
interpreter, C library, operating system, or an external program,
respectively.
To illustrate the differences between these variables, consider the following Perl expression, which uses a single-quoted string. After execution of this statement, perl may have set all four special error variables:
- eval q{
- open my $pipe, "/cdrom/install |" or die $!;
- my @res = <$pipe>;
- close $pipe or die "bad pipe: $?, $!";
- };
When perl executes the eval() expression, it translates the
open(), <PIPE>
, and close calls in the C run-time library
and thence to the operating system kernel. perl sets $!
to
the C library's errno
if one of these calls fails.
$@
is set if the string to be eval-ed did not compile (this may
happen if open or close were imported with bad prototypes), or
if Perl code executed during evaluation die()d. In these cases the
value of $@
is the compile error, or the argument to die (which
will interpolate $!
and $?
). (See also Fatal, though.)
Under a few operating systems, $^E
may contain a more verbose error
indicator, such as in this case, "CDROM tray not closed." Systems that
do not support extended error messages leave $^E
the same as $!
.
Finally, $?
may be set to non-0 value if the external program
/cdrom/install fails. The upper eight bits reflect specific error
conditions encountered by the program (the program's exit() value).
The lower eight bits reflect mode of failure, like signal death and
core dump information. See wait(2) for details. In contrast to
$!
and $^E
, which are set only if error condition is detected,
the variable $?
is set on each wait or pipe close,
overwriting the old value. This is more like $@
, which on every
eval() is always set on failure and cleared on success.
For more details, see the individual descriptions at $@
, $!
,
$^E
, and $?
.
The native status returned by the last pipe close, backtick (``
)
command, successful call to wait() or waitpid(), or from the
system() operator. On POSIX-like systems this value can be decoded
with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED,
WSTOPSIG and WIFCONTINUED functions provided by the POSIX module.
Under VMS this reflects the actual VMS exit status; i.e. it is the
same as $?
when the pragma use vmsish 'status'
is in effect.
This variable was added in Perl 5.8.9.
Error information specific to the current operating system. At the
moment, this differs from $!
under only VMS, OS/2, and Win32 (and
for MacPerl). On all other platforms, $^E
is always just the same
as $!
.
Under VMS, $^E
provides the VMS status value from the last system
error. This is more specific information about the last system error
than that provided by $!
. This is particularly important when $!
is set to EVMSERR.
Under OS/2, $^E
is set to the error code of the last call to OS/2
API either via CRT, or directly from perl.
Under Win32, $^E
always returns the last error information reported
by the Win32 call GetLastError()
which describes the last error
from within the Win32 API. Most Win32-specific code will report errors
via $^E
. ANSI C and Unix-like calls set errno
and so most
portable Perl code will report errors via $!
.
Caveats mentioned in the description of $!
generally apply to
$^E
, also.
This variable was added in Perl 5.003.
Mnemonic: Extra error explanation.
Current state of the interpreter.
The first state may happen in $SIG{__DIE__}
and $SIG{__WARN__}
handlers.
This variable was added in Perl 5.004.
The current value of the warning switch, initially true if -w was used, false otherwise, but directly modifiable.
See also warnings.
Mnemonic: related to the -w switch.
The current set of warning checks enabled by the use warnings
pragma.
See the documentation of warnings
for more details.
This variable was added in Perl 5.10.
If used numerically, yields the current value of the C errno
variable, or in other words, if a system or library call fails, it
sets this variable. This means that the value of $!
is meaningful
only immediately after a failure:
The meaningless stands for anything: zero, non-zero,
undef. A successful system or library call does not set the
variable to zero.
If used as a string, yields the corresponding system error string. You
can assign a number to $!
to set errno if, for instance, you
want "$!"
to return the string for error n, or you want to set
the exit value for the die() operator.
Mnemonic: What just went bang?
Each element of %!
has a true value only if $!
is set to that
value. For example, $!{ENOENT}
is true if and only if the current
value of $!
is ENOENT
; that is, if the most recent error was "No
such file or directory" (or its moral equivalent: not all operating
systems give that exact error, and certainly not all languages). To
check if a particular key is meaningful on your system, use exists
$!{the_key}
; for a list of legal keys, use keys %!
. See Errno
for more information, and also see $!.
This variable was added in Perl 5.005.
The status returned by the last pipe close, backtick (``
) command,
successful call to wait() or waitpid(), or from the system()
operator. This is just the 16-bit status word returned by the
traditional Unix wait() system call (or else is made up to look
like it). Thus, the exit value of the subprocess is really ($?>>
8
), and $? & 127
gives which signal, if any, the process died
from, and $? & 128
reports whether there was a core dump.
Additionally, if the h_errno
variable is supported in C, its value
is returned via $?
if any gethost*()
function fails.
If you have installed a signal handler for SIGCHLD
, the
value of $?
will usually be wrong outside that handler.
Inside an END
subroutine $?
contains the value that is going to be
given to exit(). You can modify $?
in an END
subroutine to
change the exit status of your program. For example:
- END {
- $? = 1 if $? == 255; # die would make it 255
- }
Under VMS, the pragma use vmsish 'status'
makes $?
reflect the
actual VMS exit status, instead of the default emulation of POSIX
status; see $? in perlvms for details.
Mnemonic: similar to sh and ksh.
The Perl syntax error message from the last eval() operator. If $@
is
the null string, the last eval() parsed and executed correctly
(although the operations you invoked may have failed in the normal
fashion).
Warning messages are not collected in this variable. You can, however,
set up a routine to process warnings by setting $SIG{__WARN__}
as
described in %SIG.
Mnemonic: Where was the syntax error "at"?
Deprecating a variable announces the intent of the perl maintainers to eventually remove the variable from the langauge. It may still be available despite its status. Using a deprecated variable triggers a warning.
Once a variable is removed, its use triggers an error telling you the variable is unsupported.
See perldiag for details about error messages.
$#
was a variable that you could be use to format printed numbers.
After a deprecation cycle, its magic was removed in Perl 5.10 and
using it now triggers a warning: $# is no longer supported.
This is not the sigil you use in front of an array name to get the
last index, like $#array
. That's still how you get the last index
of an array in Perl. The two have nothing to do with each other.
Deprecated in Perl 5.
Removed in Perl 5.10.
$*
was a variable that you could use to enable multiline matching.
After a deprecation cycle, its magic was removed in Perl 5.10.
Using it now triggers a warning: $* is no longer supported.
You should use the /s and /m regexp modifiers instead.
Deprecated in Perl 5.
Removed in Perl 5.10.
This variable stores the index of the first element in an array, and of the first character in a substring. The default is 0, but you could theoretically set it to 1 to make Perl behave more like awk (or Fortran) when subscripting and when evaluating the index() and substr() functions.
As of release 5 of Perl, assignment to $[
is treated as a compiler
directive, and cannot influence the behavior of any other file.
(That's why you can only assign compile-time constants to it.)
Its use is highly discouraged.
Prior to Perl 5.10, assignment to $[
could be seen from outer lexical
scopes in the same file, unlike other compile-time directives (such as
strict). Using local() on it would bind its value strictly to a lexical
block. Now it is always lexically scoped.
Mnemonic: [ begins subscripts.
Deprecated in Perl 5.12.
See $^V
for a more modern representation of the Perl version that allows
accurate string comparisons.
The version + patchlevel / 1000 of the Perl interpreter. This variable can be used to determine whether the Perl interpreter executing a script is in the right range of versions:
- warn "No checksumming!\n" if $] < 3.019;
The floating point representation can sometimes lead to inaccurate numeric comparisons.
See also the documentation of use VERSION
and require VERSION
for a convenient way to fail if the running Perl interpreter is too old.
Mnemonic: Is this version of perl in the right bracket?
Deprecated in Perl 5.6.