PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1) #
PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)
NNAAMMEE #
perlvar - Perl predefined variables
DDEESSCCRRIIPPTTIIOONN #
TThhee SSyynnttaaxx ooff VVaarriiaabbllee NNaammeess 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 _p_a_c_k_a_g_e _q_u_a_l_i_f_i_e_r; see perlmod. A Unicode letter that is not ASCII is not considered to be a letter unless “use utf8” is in effect, and somewhat more complicated rules apply; see “Identifier parsing” in perldata for details.
Perl variable names may also be a sequence of digits, a single
punctuation character, or the two-character sequence: "^" (caret or
CIRCUMFLEX ACCENT) followed by any one of the characters "[][A-Z^_?\]".
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.
Since Perl v5.6.0, Perl variable names may also be alphanumeric strings
preceded by a caret. These must all be written using the demarcated
variable form using curly braces such as "${^Foo}"; the braces are nnoott
optional. "${^Foo}" denotes the scalar variable whose name is considered
to be a control-"F" followed by two "o"'s. (See "Demarcated variable
names using braces" in perldata for more information on this form of
spelling a variable name or specifying access to an element of an array
or a hash). These variables are reserved for future special uses by
Perl, except for the ones that begin with "^_" (caret-underscore). No
name that begins with "^_" will acquire a special meaning in any future
version of Perl; such names may therefore be used safely in programs.
$^_ itself, however, _i_s reserved.
Note that you also mmuusstt use the demarcated form to access subscripts of
variables of this type when interpolating, for instance to access the
first element of the "@{^CAPTURE}" variable inside of a double quoted
string you would write "${^CAPTURE[0]}" and NOT "${^CAPTURE}[0]" which
would mean to reference a scalar variable named "${^CAPTURE}" and not
index 0 of the magic "@{^CAPTURE}" array which is populated by the regex
engine.
Perl identifiers that begin with digits 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 "${^_XYZ}" variables are always taken to be in
package "main", regardless of any "package" declarations presently in
scope.
SSPPEECCIIAALL VVAARRIIAABBLLEESS #
The following names have special meaning to Perl. Most punctuation names
have reasonable mnemonics, or analogs in the shells. Nevertheless, if
you wish to use long variable names, you need only say:
use English;
at the top of your program. This aliases all the short names to the long
names in the current package. Some even have medium names, generally
borrowed from aawwkk. For more info, please see English.
Before you continue, note the sort order for variables. In general, we
first list the variables in case-insensitive, almost-lexigraphical order
(ignoring the "{" or "^" preceding words, as in "${^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.
GGeenneerraall VVaarriiaabblleess
$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:
• The following functions use $_ as a default argument:
abs, alarm, chomp, chop, chr, chroot, cos, defined, eval,
evalbytes, exp, fc, glob, hex, int, lc, lcfirst, length, log,
lstat, mkdir, oct, ord, pos, print, printf, quotemeta,
readlink, readpipe, ref, require, reverse (in scalar context
only), rmdir, say, sin, split (for its second argument), sqrt,
stat, study, uc, ucfirst, unlink, unpack.
• All file tests ("-f", "-d") except for "-t", which defaults to
STDIN. See "-X" in perlfunc
• 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 the next value or input record when a
"<FH>", "readline", "readdir" or "each" operation's result is
tested by itself as the sole criterion of a "while" test.
Outside a "while" test, this will not happen.
$_ is a global variable.
However, between perl v5.10.0 and v5.24.0, it could be used
lexically by writing "my $_". Making $_ refer to the global $_
in the same scope was then possible with "our $_". This
experimental feature was removed and is now a fatal error, but
you may encounter it in older code.
Mnemonic: underline is understood in certain operations.
@ARG #
@_ Within a subroutine the array @_ contains the parameters passed
to that subroutine. Inside a subroutine, @_ is the default array
for the array operators "pop" and "shift".
See perlsub.
$LIST_SEPARATOR #
$" When an array or an array slice is interpolated into a double-
quoted string or a similar context such as "/.../", its elements
are separated by this value. Default is a space. For example,
this:
print "The array is: @array\n";
is equivalent to this:
print "The array is: " . join($", @array) . "\n";
Mnemonic: works in double-quoted context.
$PROCESS_ID #
$PID #
$$ The process number of the Perl running this script. Though you
_c_a_n set this variable, doing so is generally discouraged,
although it can be invaluable for some testing purposes. It will
be reset automatically across "fork()" calls.
Note for Linux and Debian GNU/kFreeBSD users: Before Perl v5.16.0
perl would emulate POSIX semantics on Linux systems using
LinuxThreads, a partial implementation of POSIX Threads that has
since been superseded by the Native POSIX Thread Library (NPTL).
LinuxThreads is now obsolete on Linux, and caching "getpid()"
like this made embedding perl unnecessarily complex (since you'd
have to manually update the value of $$), so now $$ and
"getppid()" will always return the same values as the underlying
C library.
Debian GNU/kFreeBSD systems also used LinuxThreads up until and
including the 6.0 release, but after that moved to FreeBSD thread
semantics, which are POSIX-like.
To see if your system is affected by this discrepancy check if
"getconf GNU_LIBPTHREAD_VERSION | grep -q NPTL" returns a false
value. NTPL threads preserve the POSIX semantics.
Mnemonic: same as shells.
$PROGRAM_NAME #
$0 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 ppss(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 ppss(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 v5.14.0 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.
Wide characters are invalid in $0 values. For historical reasons,
though, Perl accepts them and encodes them to UTF-8. When this
happens a wide-character warning is triggered.
Mnemonic: same as sshh and kksshh.
$REAL_GROUP_ID #
$GID #
$( 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 _n_o_t 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 _g_r_o_u_p things. The real gid is
the group you _l_e_f_t, if you're running setgid.
$EFFECTIVE_GROUP_ID #
$EGID #
$) 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 _s_e_t_[_r_e_]_[_u_g_]_ii_dd_((_)) routine. $( and $) can be swapped
only on machines supporting "setregid()".
Mnemonic: parentheses are used to _g_r_o_u_p things. The effective
gid is the group that's _r_i_g_h_t for you, if you're running setgid.
$REAL_USER_ID #
$UID #
$< 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 _f_r_o_m, if you're running setuid.
$EFFECTIVE_USER_ID #
$EUID #
$> 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 _t_o, if you're running setuid.
$SUBSCRIPT_SEPARATOR #
$SUBSEP #
$; The subscript separator for multidimensional array emulation. If
you refer to a hash element as
$foo{$x,$y,$z}
it really means
$foo{join($;, $x, $y, $z)}
But don't put
@foo{$x,$y,$z} # a slice--note the @
which means
($foo{$x},$foo{$y},$foo{$z})
Default is "\034", the same as SUBSEP in aawwkk. 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.
$a
$b Special package variables when using "sort()", see "sort" in
perlfunc. 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.
%ENV The hash %ENV contains your current environment. Setting a value
in "ENV" changes the environment for any child processes you
subsequently "fork()" off.
As of v5.18.0, both keys and values stored in %ENV are
stringified.
my $foo = 1;
$ENV{'bar'} = \$foo;
if( ref $ENV{'bar'} ) {
say "Pre 5.18.0 Behaviour";
} else {
say "Post 5.18.0 Behaviour";
}
Previously, only child processes received stringified values:
my $foo = 1;
$ENV{'bar'} = \$foo;
# Always printed 'non ref'
system($^X, '-e',
q/print ( ref $ENV{'bar'} ? 'ref' : 'non ref' ) /);
This happens because you can't really share arbitrary data
structures with foreign processes.
$OLD_PERL_VERSION #
$] The revision, version, and subversion of the Perl interpreter,
represented as a decimal of the form 5.XXXYYY, where XXX is the
version / 1e3 and YYY is the subversion / 1e6. For example, Perl
v5.10.1 would be "5.010001".
This variable can be used to determine whether the Perl
interpreter executing a script is in the right range of versions:
warn "No PerlIO!\n" if "$]" < 5.008;
When comparing $], numeric comparison operators should be used,
but the variable should be stringified first to avoid issues
where its original numeric value is inaccurate.
See also the documentation of "use VERSION" and "require VERSION"
for a convenient way to fail if the running Perl interpreter is
too old.
See "$^V" for a representation of the Perl version as a version
object, which allows more flexible string comparisons.
The main advantage of $] over $^V is that it works the same on
any version of Perl. The disadvantages are that it can't easily
be compared to versions in other formats (e.g. literal v-strings,
"v1.2.3" or version objects) and numeric comparisons are subject
to the binary floating point representation; it's good for
numeric literal version checks and bad for comparing to a
variable that hasn't been sanity-checked.
The $OLD_PERL_VERSION form was added in Perl v5.20.0 for
historical reasons but its use is discouraged. (If your reason to
use $] is to run code on old perls then referring to it as
$OLD_PERL_VERSION would be self-defeating.)
Mnemonic: Is this version of perl in the right bracket?
$SYSTEM_FD_MAX #
$^F 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()".
@F The array @F contains the fields of each line read in when
autosplit mode is turned on. See perlrun for the --aa 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'".
@INC 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 --II command-line
switches, followed by the default Perl library, probably
_/_u_s_r_/_l_o_c_a_l_/_l_i_b_/_p_e_r_l. Prior to Perl 5.26, "." -which represents
the current directory, was included in @INC; it has been removed.
This change in behavior is documented in "PERL_USE_UNSAFE_INC"
and it is not recommended that "." be re-added to @INC. If you
need to modify @INC at runtime, you should use the "use lib"
pragma to get the machine-dependent library properly loaded as
well:
use lib '/mypath/libdir/';
use SomeMod;
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" in perlfunc for details.
%INC 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" in perlfunc 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.
$INPLACE_EDIT #
$^I The current value of the inplace-edit extension. Use "undef" to
disable inplace editing.
Mnemonic: value of --ii switch.
@ISA Each package contains a special array called @ISA which contains
a list of that class's parent classes, if any. This array is
simply a list of scalars, each of which is a string that
corresponds to a package name. The array is examined when Perl
does method resolution, which is covered in perlobj.
To load packages while adding them to @ISA, see the parent
pragma. The discouraged base pragma does this as well, but should
not be used except when compatibility with the discouraged fields
pragma is required.
$^M 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
_I_N_S_T_A_L_L 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.
$OSNAME #
$^O 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 --VV 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
WWiinn3322::::GGeettOOSSVVeerrssiioonn(()) (see Win32 and perlport) to distinguish
between the variants.
This variable was added in Perl 5.003.
%SIG The hash %SIG contains signal handlers for signals. For example:
sub handler { # 1st argument is signal name
my($sig) = @_;
print "Caught a SIG$sig--shutting down\n";
close(LOG);
exit(0);
}
$SIG{'INT'} = \&handler;
$SIG{'QUIT'} = \&handler;
...
$SIG{'INT'} = 'DEFAULT'; # restore default action
$SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
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. Using an empty string or "undef" as the value
has the same effect as 'DEFAULT'.
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.
Using a string that doesn't correspond to any existing function
or a glob that doesn't contain a code slot is equivalent to
'IGNORE', but a warning is emitted when the handler is being
called (the warning is not emitted for the internal hooks
described below).
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 v5.8.0
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:
local $SIG{__WARN__} = sub { die $_[0] };
eval $proggie;
As the 'IGNORE' hook is not supported by "__WARN__", its effect
is the same as using 'DEFAULT'. You can disable warnings using
the empty subroutine:
local $SIG{__WARN__} = sub {};
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 &sub",
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__".
The $SIG{__DIE__} hook is called even inside an "eval()". It was
never intended to happen this way, but an implementation glitch
made this possible. This used to be deprecated, as it allowed
strange action at a distance like rewriting a pending exception
in $@. Plans to rectify this have been scrapped, as users found
that rewriting a pending exception is actually a useful feature,
and not a bug.
The $SIG{__DIE__} doesn't support 'IGNORE'; it has the same
effect as 'DEFAULT'.
"__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:
require Carp if defined $^S;
Carp::confess("Something wrong") if defined &Carp::confess;
die "Something wrong, but could not load Carp to give "
. "backtrace...\n\t"
. "To see backtrace try starting Perl with -MCarp switch";
Here the first line will load "Carp" _u_n_l_e_s_s 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" in perlfunc, "warn" in perlfunc, "eval" in perlfunc,
and warnings for additional information.
$BASETIME #
$^T The time at which the program began running, in seconds since the
epoch (beginning of 1970). The values returned by the --MM, --AA,
and --CC filetests are based on this value.
$PERL_VERSION #
$^V The revision, version, and subversion of the Perl interpreter,
represented as a version object.
This variable first appeared in perl v5.6.0; earlier versions of
perl will see an undefined value. Before perl v5.10.0 $^V was
represented as a v-string rather than a version object.
$^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
While version objects overload stringification, to portably
convert $^V into its string representation, use "sprintf()"'s
"%vd" conversion, which works for both v-strings or version
objects:
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 a decimal representation of the Perl version.
The main advantage of $^V over $] is that, for Perl v5.10.0 or
later, it overloads operators, allowing easy comparison against
other version representations (e.g. decimal, literal v-string,
"v1.2.3", or objects). The disadvantage is that prior to
v5.10.0, it was only a literal v-string, which can't be easily
printed or compared, whereas the behavior of $] is unchanged on
all versions of Perl.
Mnemonic: use ^V for a version object.
$EXECUTABLE_NAME #
$^X The name used to execute the current copy of Perl, from C's
"argv[0]" or (where supported) _/_p_r_o_c_/_s_e_l_f_/_e_x_e.
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:
# Build up a set of file names (not command names).
use Config;
my $this_perl = $^X;
if ($^O ne 'VMS') {
$this_perl .= $Config{_exe}
unless $this_perl =~ m/$Config{_exe}$/i;
}
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.
use Config;
my $secure_perl_path = $Config{perlpath};
if ($^O ne 'VMS') {
$secure_perl_path .= $Config{_exe}
unless $secure_perl_path =~ m/$Config{_exe}$/i;
}
VVaarriiaabblleess rreellaatteedd ttoo rreegguullaarr eexxpprreessssiioonnss 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:
my $outer = 'Wallace and Grommit';
my $inner = 'Mutt and Jeff';
my $pattern = qr/(\S+) and (\S+)/;
sub show_n { print "\$1 is $1; \$2 is $2\n" }
{
OUTER: #
show_n() if $outer =~ m/$pattern/;
INNER: { #
show_n() if $inner =~ m/$pattern/;
}
show_n();
}
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
_P_e_r_f_o_r_m_a_n_c_e _i_s_s_u_e_s
Traditionally in Perl, any use of any of the three variables "$`", $& or
"$'" (or their "use English" equivalents) anywhere in the code, caused
all subsequent successful pattern matches to make a copy of the matched
string, in case the code might subsequently access one of those
variables. This imposed a considerable performance penalty across the
whole program, so generally the use of these variables has been
discouraged.
In Perl 5.6.0 the "@-" and "@+" dynamic arrays were introduced that
supply the indices of successful matches. So you could for example do
this:
$str =~ /pattern/;
print $`, $&, $'; # bad: performance hit
print # good: no performance hit
substr($str, 0, $-[0]),
substr($str, $-[0], $+[0]-$-[0]),
substr($str, $+[0]);
In Perl 5.10.0 the "/p" match operator flag and the "${^PREMATCH}",
"${^MATCH}", and "${^POSTMATCH}" variables were introduced, that allowed
you to suffer the penalties only on patterns marked with "/p".
In Perl 5.18.0 onwards, perl started noting the presence of each of the
three variables separately, and only copied that part of the string
required; so in
$`; $&; "abcdefgh" =~ /d/
perl would only copy the "abcd" part of the string. That could make a big
difference in something like
$str = 'x' x 1_000_000;
$&; # whoops
$str =~ /x/g # one char copied a million times, not a million chars
In Perl 5.20.0 a new copy-on-write system was enabled by default, which
finally fixes all performance issues with these three variables, and
makes them safe to use anywhere.
The "Devel::NYTProf" and "Devel::FindAmpersand" modules can help you find
uses of these problematic match variables in your code.
$<_d_i_g_i_t_s> ($1, $2, ...)
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.
Note there is a distinction between a capture buffer which
matches the empty string a capture buffer which is optional. Eg,
"(x?)" and "(x)?" The latter may be undef, the former not.
These variables are read-only and dynamically-scoped.
Mnemonic: like \digits.
@{^CAPTURE} #
An array which exposes the contents of the capture buffers, if
any, of the last successful pattern match, not counting patterns
matched in nested blocks that have been exited already.
Note that the 0 index of @{^CAPTURE} is equivalent to $1, the 1
index is equivalent to $2, etc.
if ("foal"=~/(.)(.)(.)(.)/) {
print join "-", @{^CAPTURE};
}
should output "f-o-a-l".
See also "$<_d_i_g_i_t_s> ($1, $2, ...)", "%{^CAPTURE}" and
“%{^CAPTURE_ALL}”. #
Note that unlike most other regex magic variables there is no
single letter equivalent to "@{^CAPTURE}". Also be aware that
when interpolating subscripts of this array you mmuusstt use the
demarcated variable form, for instance
print "${^CAPTURE[0]}"
see "Demarcated variable names using braces" in perldata for more
information on this form and its uses.
This variable was added in 5.25.7
$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).
See "Performance issues" above for the serious performance
implications of using this variable (even once) in your code.
This variable is read-only and dynamically-scoped.
Mnemonic: like "&" in some editors.
${^MATCH} #
This is similar to $& ($MATCH) except that it does not incur the
performance penalty associated with that variable.
See "Performance issues" above.
In Perl v5.18 and earlier, it is only guaranteed to return a
defined value when the pattern was compiled or executed with the
"/p" modifier. In Perl v5.20, the "/p" modifier does nothing, so
"${^MATCH}" does the same thing as $MATCH.
This variable was added in Perl v5.10.0.
This variable is read-only and dynamically-scoped.
$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.
See "Performance issues" above for the serious performance
implications of using this variable (even once) in your code.
This variable is read-only and dynamically-scoped.
Mnemonic: "`" often precedes a quoted string.
${^PREMATCH} #
This is similar to "$`" ($PREMATCH) except that it does not incur
the performance penalty associated with that variable.
See "Performance issues" above.
In Perl v5.18 and earlier, it is only guaranteed to return a
defined value when the pattern was compiled or executed with the
"/p" modifier. In Perl v5.20, the "/p" modifier does nothing, so
"${^PREMATCH}" does the same thing as $PREMATCH.
This variable was added in Perl v5.10.0.
This variable is read-only and dynamically-scoped.
$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). Example:
local $_ = 'abcdefghi';
/def/;
print "$`:$&:$'\n"; # prints abc:def:ghi
See "Performance issues" above for the serious performance
implications of using this variable (even once) in your code.
This variable is read-only and dynamically-scoped.
Mnemonic: "'" often follows a quoted string.
${^POSTMATCH} #
This is similar to "$'" ($POSTMATCH) except that it does not
incur the performance penalty associated with that variable.
See "Performance issues" above.
In Perl v5.18 and earlier, it is only guaranteed to return a
defined value when the pattern was compiled or executed with the
"/p" modifier. In Perl v5.20, the "/p" modifier does nothing, so
"${^POSTMATCH}" does the same thing as $POSTMATCH.
This variable was added in Perl v5.10.0.
This variable is read-only and dynamically-scoped.
$LAST_PAREN_MATCH #
$+ The text matched by the highest used capture group of the last
successful search pattern. It is logically equivalent to the
highest numbered capture variable ($1, $2, ...) which has a
defined value.
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.
$LAST_SUBMATCH_RESULT #
$^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. This is subtly different from $+. For
example in
"ab" =~ /^((.)(.))$/
we have
$1,$^N have the value "ab"
$2 has the value "a"
$3,$+ have the value "b"
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 v5.8.0.
Mnemonic: the (possibly) Nested parenthesis that most recently
closed.
@LAST_MATCH_END #
@+ 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 v5.6.0.
%{^CAPTURE} #
%LAST_PAREN_MATCH #
%+ 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).
If multiple distinct capture groups have the same name, then
$+{NAME} will refer to the leftmost defined group in the match.
The underlying behaviour of "%+" is provided by the
Tie::Hash::NamedCapture module.
NNoottee:: "%-" 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 v5.10.0. The "%{^CAPTURE}" alias
was added in 5.25.7.
This variable is read-only and dynamically-scoped.
@LAST_MATCH_START #
@- "$-[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 v5.6.0.
%{^CAPTURE_ALL} #
%- 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:
if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
foreach my $bufname (sort keys %-) {
my $ary = $-{$bufname};
foreach my $idx (0..$#$ary) {
print "\$-{$bufname}[$idx] : ",
(defined($ary->[$idx])
? "'$ary->[$idx]'"
: "undef"),
"\n";
}
}
}
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.
NNoottee:: "%-" 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 v5.10.0. The "%{^CAPTURE_ALL}"
alias was added in 5.25.7.
This variable is read-only and dynamically-scoped.
$LAST_REGEXP_CODE_RESULT #
$^R 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.
${^RE_COMPILE_RECURSION_LIMIT} #
The current value giving the maximum number of open but unclosed
parenthetical groups there may be at any point during a regular
expression compilation. The default is currently 1000 nested
groups. You may adjust it depending on your needs and the amount
of memory available.
This variable was added in Perl v5.30.0.
${^RE_DEBUG_FLAGS} #
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 v5.10.0.
${^RE_TRIE_MAXBUF} #
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 v5.10.0.
VVaarriiaabblleess rreellaatteedd ttoo ffiilleehhaannddlleess 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 $/ 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:
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..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.
$ARGV Contains the name of the current file when reading from "<>".
@ARGV 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, _n_o_t the program's
command name itself. See "$0" for the command name.
ARGV 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.
ARGVOUT The special filehandle that points to the currently open output
file when doing edit-in-place processing with --ii. Useful when
you have to do a lot of inserting and don't want to keep
modifying $_. See perlrun for the --ii switch.
IO::Handle->output_field_separator( EXPR )
$OUTPUT_FIELD_SEPARATOR #
$OFS #
$, The output field separator for the print operator. If defined,
this value is printed between each of print's arguments. Default
is "undef".
You cannot call "output_field_separator()" on a handle, only as a
static method. See IO::Handle.
Mnemonic: what is printed when there is a "," in your print
statement.
HANDLE->input_line_number( EXPR )
$INPUT_LINE_NUMBER #
$NR #
$. 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. _L_o_c_a_l_i_z_i_n_g _$_. _w_i_l_l _n_o_t _l_o_c_a_l_i_z_e
_t_h_e _f_i_l_e_h_a_n_d_l_e_'_s _l_i_n_e _c_o_u_n_t. Instead, it will localize perl's
notion of which filehandle $. is currently aliased to.
$. is reset when the filehandle is closed, but nnoott 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" in perlfunc).
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.
IO::Handle->input_record_separator( EXPR )
$INPUT_RECORD_SEPARATOR #
$RS #
$/ The input record separator, newline by default. This influences
Perl's idea of what a "line" is. Works like aawwkk'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.
local $/; # enable "slurp" mode
local $_ = <FH>; # whole file now here
s/\n[ \t]+/ /g;
Remember: the value of $/ is a string, not a regex. aawwkk has to
be better for something. :-)
Setting $/ to an empty string -- the so-called _p_a_r_a_g_r_a_p_h _m_o_d_e --
merits special attention. When $/ is set to "" and the entire
file is read in with that setting, any sequence of one or more
consecutive newlines at the beginning of the file is discarded.
With the exception of the final record in the file, each sequence
of characters ending in two or more newlines is treated as one
record and is read in to end in exactly two newlines. If the
last record in the file ends in zero or one consecutive newlines,
that record is read in with that number of newlines. If the last
record ends in two or more consecutive newlines, it is read in
with two newlines like all preceding records.
Suppose we wrote the following string to a file:
my $string = "\n\n\n";
$string .= "alpha beta\ngamma delta\n\n\n";
$string .= "epsilon zeta eta\n\n";
$string .= "theta\n";
my $file = 'simple_file.txt';
open my $OUT, '>', $file or die;
print $OUT $string;
close $OUT or die;
Now we read that file in paragraph mode:
local $/ = ""; # paragraph mode
open my $IN, '<', $file or die;
my @records = <$IN>;
close $IN or die;
@records will consist of these 3 strings:
(
"alpha beta\ngamma delta\n\n",
"epsilon zeta eta\n\n",
"theta\n",
)
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 number of characters. So this:
local $/ = \32768; # or \"32768", or \$var_containing_32768
open my $fh, "<", $myfile or die $!;
local $_ = <$fh>;
will read a record of no more than 32768 characters from $fh. 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 is deprecated and
will cause $/ to have the value of "undef", which will cause
reading in the (rest of the) whole file.
As of 5.19.9 setting $/ to any other form of reference will throw
a fatal exception. This is in preparation for supporting new ways
to set $/ in the future.
On VMS only, record reads bypass PerlIO layers and any associated
buffering, so you must not mix record and non-record reads on the
same filehandle. Record mode mixes with line mode only when the
same buffering layer is in use for both modes.
You cannot call "input_record_separator()" on a handle, only as a
static method. See IO::Handle.
See also "Newlines" in perlport. Also see "$.".
Mnemonic: / delimits line boundaries when quoting poetry.
IO::Handle->output_record_separator( EXPR )
$OUTPUT_RECORD_SEPARATOR #
$ORS #
$\ The output record separator for the print operator. If defined,
this value is printed after the last of print's arguments.
Default is "undef".
You cannot call "output_record_separator()" on a handle, only as
a static method. See IO::Handle.
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.
HANDLE->autoflush( EXPR )
$OUTPUT_AUTOFLUSH #
$| 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 rrsshh and want to see the output as it's
happening. This has no effect on input buffering. See "getc" in
perlfunc for that. See "select" in perlfunc on how to select the
output channel. See also IO::Handle.
Mnemonic: when you want your pipes to be piping hot.
${^LAST_FH} #
This read-only variable contains a reference to the last-read
filehandle. This is set by "<HANDLE>", "readline", "tell", "eof"
and "seek". This is the same handle that $. and "tell" and "eof"
without arguments use. It is also the handle used when Perl
appends ", <STDIN> line 1" to an error or warning message.
This variable was added in Perl v5.18.0.
_V_a_r_i_a_b_l_e_s _r_e_l_a_t_e_d _t_o _f_o_r_m_a_t_s
The special variables for formats are a subset of those for filehandles.
See perlform for more information about Perl's formats.
$ACCUMULATOR #
$^A 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" in
perlfunc.
IO::Handle->format_formfeed(EXPR)
$FORMAT_FORMFEED #
$^L What formats output as a form feed. The default is "\f".
You cannot call "format_formfeed()" on a handle, only as a static
method. See IO::Handle.
HANDLE->format_page_number(EXPR)
$FORMAT_PAGE_NUMBER #
$% The current page number of the currently selected output channel.
Mnemonic: "%" is page number in nnrrooffff.
HANDLE->format_lines_left(EXPR)
$FORMAT_LINES_LEFT #
$- The number of lines left on the page of the currently selected
output channel.
Mnemonic: lines_on_page - lines_printed.
IO::Handle->format_line_break_characters EXPR
$FORMAT_LINE_BREAK_CHARACTERS #
$: 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.
You cannot call "format_line_break_characters()" on a handle,
only as a static method. See IO::Handle.
Mnemonic: a "colon" in poetry is a part of a line.
HANDLE->format_lines_per_page(EXPR)
$FORMAT_LINES_PER_PAGE #
$= The current page length (printable lines) of the currently
selected output channel. The default is 60.
Mnemonic: = has horizontal lines.
HANDLE->format_top_name(EXPR)
$FORMAT_TOP_NAME #
$^ 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" filehandle is "STDOUT_TOP".
Mnemonic: points to top of page.
HANDLE->format_name(EXPR)
$FORMAT_NAME #
$~ 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 $^.
EErrrroorr VVaarriiaabblleess 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 a non-0 value if the external program
_/_c_d_r_o_m_/_i_n_s_t_a_l_l 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 wwaaiitt(2) for details. In contrast to $! and $^E,
which are set only if an 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 $?.
${^CHILD_ERROR_NATIVE} #
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, and WSTOPSIG 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 v5.10.0.
$EXTENDED_OS_ERROR #
$^E 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 EEVVMMSSEERRRR.
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.
$EXCEPTIONS_BEING_CAUGHT #
$^S Current state of the interpreter.
$^S State
--------- -------------------------------------
undef Parsing module, eval, or main program
true (1) Executing an eval or try block
false (0) Otherwise
The first state may happen in $SIG{__DIE__} and $SIG{__WARN__}
handlers.
The English name $EXCEPTIONS_BEING_CAUGHT is slightly misleading,
because the "undef" value does not indicate whether exceptions
are being caught, since compilation of the main program does not
catch exceptions.
This variable was added in Perl 5.004.
$WARNING #
$^W The current value of the warning switch, initially true if --ww was
used, false otherwise, but directly modifiable.
See also warnings.
Mnemonic: related to the --ww switch.
${^WARNING_BITS} #
The current set of warning checks enabled by the "use warnings"
pragma. It has the same scoping as the $^H and "%^H" variables.
The exact values are considered internal to the warnings pragma
and may change between versions of Perl.
Each time a statement completes being compiled, the current value
of "${^WARNING_BITS}" is stored with that statement, and can
later be retrieved via "(caller($level))[9]".
This variable was added in Perl v5.6.0.
$OS_ERROR #
$ERRNO #
$! When referenced, $! retrieves the current value of the C "errno"
integer variable. If $! is assigned a numerical value, that
value is stored in "errno". When referenced as a string, $!
yields the system error string corresponding to "errno".
Many system or library calls set "errno" if they fail, to
indicate the cause of failure. They usually do nnoott set "errno"
to zero if they succeed and may set "errno" to a non-zero value
on success. This means "errno", hence $!, is meaningful only
_i_m_m_e_d_i_a_t_e_l_y after a ffaaiilluurree:
if (open my $fh, "<", $filename) {
# Here $! is meaningless.
...
}
else {
# ONLY here is $! meaningful.
...
# Already here $! might be meaningless.
}
# Since here we might have either success or failure,
# $! is meaningless.
Here, _m_e_a_n_i_n_g_l_e_s_s means that $! may be unrelated to the outcome
of the "open()" operator. Assignment to $! is similarly
ephemeral. It can be used immediately before invoking the
"die()" operator, to set the exit value, or to inspect the system
error string corresponding to error _n, or to restore $! to a
meaningful state.
Perl itself may set "errno" to a non-zero on failure even if no
system call is performed.
Mnemonic: What just went bang?
%OS_ERROR #
%ERRNO #
%! 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). The specific true value is not guaranteed,
but in the past has generally been the numeric value of $!. 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.
$CHILD_ERROR #
$? 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 sshh and kksshh.
$EVAL_ERROR #
$@ The Perl error from the last "eval" operator, i.e. the last
exception that was caught. For "eval BLOCK", this is either a
runtime error message or the string or reference "die" was called
with. The "eval STRING" form also catches syntax errors and
other compile time exceptions.
If no error occurs, "eval" sets $@ to the empty string.
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 error "at"?
VVaarriiaabblleess rreellaatteedd ttoo tthhee iinntteerrpprreetteerr ssttaattee These variables provide information about the current interpreter state.
$COMPILING #
$^C The current value of the flag associated with the --cc switch.
Mainly of use with --MMOO==...... 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 v5.6.0.
$DEBUGGING #
$^D The current value of the debugging flags. May be read or set.
Like its command-line equivalent, you can use numeric or symbolic
values, e.g. "$^D = 10" or "$^D = "st"". See "--DD_n_u_m_b_e_r" in
perlrun. The contents of this variable also affects the debugger
operation. See "Debugger Internals" in perldebguts.
Mnemonic: value of --DD switch.
${^GLOBAL_PHASE} #
The current phase of the perl interpreter.
Possible values are:
CONSTRUCT #
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.
START 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}".
CHECK Execution of any "CHECK" blocks.
INIT Similar to "CHECK", but for "INIT"-blocks, not "CHECK"
blocks.
RUN The main run-time, i.e. the execution of "PL_main_root".
END Execution of any "END" blocks.
DESTRUCT #
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
compile-time: START
check-time: CHECK
init-time: INIT
run-time: RUN
lexical variables are garbage collected before END: RUN
end-time: END
package variables are garbage collected after END: DESTRUCT
This variable was added in Perl 5.14.0.
$^H 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.
Each time a statement completes being compiled, the current value
of $^H is stored with that statement, and can later be retrieved
via "(caller($level))[8]".
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:
sub add_100 { $^H |= 0x100 }
sub foo {
BEGIN { add_100() }
bar->baz($boon);
}
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.
%^H The "%^H" hash provides the same scoping semantic as $^H. This
makes it useful for implementation of lexically scoped pragmas.
See perlpragma. All the entries are stringified when accessed
at runtime, so only simple values can be accommodated. This
means no pointers to objects, for example.
Each time a statement completes being compiled, the current value
of "%^H" is stored with that statement, and can later be
retrieved via "(caller($level))[10]".
When putting items into "%^H", in order to avoid conflicting with
other users of the hash there is a convention regarding which
keys to use. A module should use only keys that begin with the
module's name (the name of its main package) and a "/" character.
For example, a module "Foo::Bar" should use keys such as
"Foo::Bar/baz".
This variable was added in Perl v5.6.0.
${^OPEN} #
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 is the mechanism that applies the lexical effects of the
open pragma, and the main program scope effects of the "io" or
"D" options for the -C command-line switch and PERL_UNICODE
environment variable.
The functions "accept()", "open()", "pipe()", "readpipe()" (as
well as the related "qx" and "`STRING`" operators), "socket()",
"socketpair()", and "sysopen()" are affected by the lexical value
of this variable. The implicit "ARGV" handle opened by
"readline()" (or the related "<>" and "<<>>" operators) on passed
filenames is also affected (but not if it opens "STDIN"). If
this variable is not set, these functions will set the default
layers as described in "Defaults and how to override them" in
PerlIO.
"open()" ignores this variable (and the default layers) when
called with 3 arguments and explicit layers are specified.
Indirect calls to these functions via modules like IO::Handle are
not affected as they occur in a different lexical scope.
Directory handles such as opened by "opendir()" are not currently
affected.
This variable was added in Perl v5.8.0.
$PERLDB #
$^P The internal variable for debugging support. The meanings of the
various bits are subject to change, but currently indicate:
0x01 Debug subroutine enter/exit.
0x02 Line-by-line debugging. Causes "DB::DB()" subroutine to be
called for each statement executed. Also causes saving
source code lines (like 0x400).
0x04 Switch off optimizations.
0x08 Preserve more data for future interactive inspections.
0x10 Keep info about source lines on which a subroutine is
defined.
0x20 Start with single-step on.
0x40 Use subroutine address instead of name when reporting.
0x80 Report "goto &subroutine" as well.
0x100 Provide informative "file" names for evals based on the
place they were compiled.
0x200 Provide informative names to anonymous subroutines based on
the place they were compiled.
0x400 Save source code lines into "@{"_<$filename"}".
0x800 When saving source, include evals that generate no
subroutines.
0x1000
When saving source, include source that did not compile.
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.
${^TAINT} #
Reflects if taint mode is on or off. 1 for on (the program was
run with --TT), 0 for off, -1 when only taint warnings are enabled
(i.e. with --tt or --TTUU).
Note: if your perl was built without taint support (see perlsec),
then "${^TAINT}" will always be 0, even if the program was run
with --TT).
This variable is read-only.
This variable was added in Perl v5.8.0.
${^SAFE_LOCALES} #
Reflects if safe locale operations are available to this perl
(when the value is 1) or not (the value is 0). This variable is
always 1 if the perl has been compiled without threads. It is
also 1 if this perl is using thread-safe locale operations. Note
that an individual thread may choose to use the global locale
(generally unsafe) by calling "switch_to_global_locale" in
perlapi. This variable currently is still set to 1 in such
threads.
This variable is read-only.
This variable was added in Perl v5.28.0.
${^UNICODE} #
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 v5.8.2.
${^UTF8CACHE} #
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 v5.8.9. It is subject to change
or removal without notice, but is currently used to avoid
recalculating the boundaries of multi-byte UTF-8-encoded
characters.
${^UTF8LOCALE} #
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 v5.8.8.
DDeepprreeccaatteedd aanndd rreemmoovveedd vvaarriiaabblleess Deprecating a variable announces the intent of the perl maintainers to eventually remove the variable from the language. 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.
$# $# was a variable that could be used to format printed numbers.
After a deprecation cycle, its magic was removed in Perl v5.10.0
and using it now triggers a warning: "$# 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 v5.10.0.
$* $* was a variable that you could use to enable multiline
matching. After a deprecation cycle, its magic was removed in
Perl v5.10.0. Using it now triggers a warning: "$* is no longer
supported". You should use the "/s" and "/m" regexp modifiers
instead.
Deprecated in Perl 5.
Removed in Perl v5.10.0.
$[ 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
aawwkk (or Fortran) when subscripting and when evaluating the
iinnddeexx(()) and ssuubbssttrr(()) 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 v5.10.0, assignment to $[ could be seen from outer
lexical scopes in the same file, unlike other compile-time
directives (such as strict). Using llooccaall(()) on it would bind its
value strictly to a lexical block. Now it is always lexically
scoped.
As of Perl v5.16.0, it is implemented by the arybase module.
As of Perl v5.30.0, or under "use v5.16", or "no feature
"array_base"", $[ no longer has any effect, and always contains
0. Assigning 0 to it is permitted, but any other value will
produce an error.
Mnemonic: [ begins subscripts.
Deprecated in Perl v5.12.0.
${^ENCODING} #
This variable is no longer supported.
It used to hold the _o_b_j_e_c_t _r_e_f_e_r_e_n_c_e to the "Encode" object that
was used to convert the source code to Unicode.
Its purpose was to allow your non-ASCII Perl scripts not to have
to be written in UTF-8; this was useful before editors that
worked on UTF-8 encoded text were common, but that was long ago.
It caused problems, such as affecting the operation of other
modules that weren't expecting it, causing general mayhem.
If you need something like this functionality, it is recommended
that use you a simple source filter, such as Filter::Encoding.
If you are coming here because code of yours is being adversely
affected by someone's use of this variable, you can usually work
around it by doing this:
local ${^ENCODING};
near the beginning of the functions that are getting broken.
This undefines the variable during the scope of execution of the
including function.
This variable was added in Perl 5.8.2 and removed in 5.26.0.
Setting it to anything other than "undef" was made fatal in Perl
5.28.0.
${^WIN32_SLOPPY_STAT} #
This variable no longer has any function.
This variable was added in Perl v5.10.0 and removed in Perl
v5.34.0.
perl v5.36.3 2023-02-15 PERLVAR(1)