may contain letters, digits, underscores, or the special sequence
C<::> or C<'>. In this case, the part before the last C<::> or
C<'> is taken to be a I<package qualifier>; see L<perlmod>.
-
-Perl variable names may also be a sequence of digits or a single
-punctuation or control character. These names are all reserved for
+A Unicode letter that is not ASCII is not considered to be a letter
+unless S<C<"use utf8">> is in effect, and somewhat more complicated
+rules apply; see L<perldata/Identifier parsing> for details.
+
+Perl variable names may also be a sequence of digits, a single
+punctuation character, or the two-character sequence: C<^> (caret or
+CIRCUMFLEX ACCENT) followed by any one of the characters C<[][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. Perl has a special syntax for the single-control-character
-names: It understands C<^X> (caret C<X>) to mean the control-C<X>
-character. For example, the notation C<$^W> (dollar-sign caret
-C<W>) is the scalar variable whose name is the single character
-control-C<W>. This is better than typing a literal control-C<W>
-into your program.
-
-Since Perl v5.6.0, Perl variable names may be alphanumeric
-strings that begin with control characters (or better yet, a caret).
-These variables must be written in the form C<${^Foo}>; the braces
-are not optional. C<${^Foo}> denotes the scalar variable whose
-name is a control-C<F> followed by two C<o>'s. These variables are
+match.
+
+Since Perl v5.6.0, Perl variable names may also be alphanumeric strings
+preceded by a caret. These must all be written in the form C<${^Foo}>;
+the braces are not optional. C<${^Foo}> denotes the scalar variable
+whose name is considered to be a control-C<F> followed by two C<o>'s.
+These variables are
reserved for future special uses by Perl, except for the ones that
-begin with C<^_> (control-underscore or caret-underscore). No
-control-character name that begins with C<^_> will acquire a special
+begin with C<^_> (caret-underscore). No
+name that begins with C<^_> will acquire a special
meaning in any future version of Perl; such names may therefore be
used safely in programs. C<$^_> itself, however, I<is> reserved.
-Perl identifiers that begin with digits, control characters, or
+Perl identifiers that begin with digits or
punctuation characters are exempt from the effects of the C<package>
declaration and are always forced to be in package C<main>; they are
also exempt from C<strict 'vars'> errors. A few other names are also
Within a subroutine the array C<@_> contains the parameters passed to
that subroutine. Inside a subroutine, C<@_> is the default array for
-the array operators C<push>, C<pop>, C<shift>, and C<unshift>.
+the array operators C<pop> and C<shift>.
See L<perlsub>.
The subscript separator for multidimensional array emulation. If you
refer to a hash element as
- $foo{$a,$b,$c}
+ $foo{$x,$y,$z}
it really means
- $foo{join($;, $a, $b, $c)}
+ $foo{join($;, $x, $y, $z)}
But don't put
- @foo{$a,$b,$c} # a slice--note the @
+ @foo{$x,$y,$z} # a slice--note the @
which means
- ($foo{$a},$foo{$b},$foo{$c})
+ ($foo{$x},$foo{$y},$foo{$z})
Default is "\034", the same as SUBSEP in B<awk>. If your keys contain
binary data there might not be any safe value for C<$;>.
value in C<ENV> changes the environment for any child processes
you subsequently C<fork()> off.
-As of 5.17.3, both keys and values stored in C<%ENV> are stringified.
+As of v5.18.0, both keys and values stored in C<%ENV> are stringified.
my $foo = 1;
$ENV{'bar'} = \$foo;
if( ref $ENV{'bar'} ) {
- say "Pre 5.17.3 Behaviour";
+ say "Pre 5.18.0 Behaviour";
} else {
- say "Post 5.17.3 Behaviour";
+ say "Post 5.18.0 Behaviour";
}
Previously, only child processes received stringified values:
$ENV{'bar'} = \$foo;
# Always printed 'non ref'
- system($^X,'-e',q/print ( ref $ENV{'bar'} ? 'ref' : 'non ref' ) /);
+ system($^X, '-e',
+ q/print ( ref $ENV{'bar'} ? 'ref' : 'non ref' ) /);
This happens because you can't really share arbitrary data structures with
foreign processes.
+=item $OLD_PERL_VERSION
+
+=item $]
+X<$]> X<$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 $] lt '5.008';
+
+When comparing C<$]>, string comparison operators are B<highly
+recommended>. The inherent limitations of binary floating point
+representation can sometimes lead to incorrect comparisons for some
+numbers on some architectures.
+
+See also the documentation of C<use VERSION> and C<require VERSION>
+for a convenient way to fail if the running Perl interpreter is too old.
+
+See L</$^V> for a representation of the Perl version as a L<version>
+object, which allows more flexible string comparisons.
+
+The main advantage of C<$]> over C<$^V> is that it works the same on any
+version of Perl. The disadvantages are that it can't easily be compared
+to versions in other formats (e.g. literal v-strings, "v1.2.3" or
+version objects) and numeric comparisons can occasionally fail; it's good
+for string literal version checks and bad for comparing to a variable
+that hasn't been sanity-checked.
+
+The C<$OLD_PERL_VERSION> form was added in Perl v5.20.0 for historical
+reasons but its use is discouraged. (If your reason to use C<$]> is to
+run code on old perls then referring to it as C<$OLD_PERL_VERSION> would
+be self-defeating.)
+
+Mnemonic: Is this version of perl in the right bracket?
+
=item $SYSTEM_FD_MAX
=item $^F
X<$^V> X<$PERL_VERSION>
The revision, version, and subversion of the Perl interpreter,
-represented as a C<version> object.
+represented as a L<version> object.
This variable first appeared in perl v5.6.0; earlier versions of perl
will see an undefined value. Before perl v5.10.0 C<$^V> was represented
-as a v-string.
+as a v-string rather than a L<version> object.
C<$^V> can be used to determine whether the Perl interpreter executing
a script is in the right range of versions. For example:
warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
-To convert C<$^V> into its string representation use C<sprintf()>'s
-C<"%vd"> conversion:
+While version objects overload stringification, to portably convert
+C<$^V> into its string representation, use C<sprintf()>'s C<"%vd">
+conversion, which works for both v-strings or version objects:
printf "version is v%vd\n", $^V; # Perl's version
See the documentation of C<use VERSION> and C<require VERSION>
for a convenient way to fail if the running Perl interpreter is too old.
-See also C<$]> for an older representation of the Perl version.
+See also C<$]> for a decimal representation of the Perl version.
-This variable was added in Perl v5.6.0.
+The main advantage of C<$^V> over C<$]> 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.
-Mnemonic: use ^V for Version Control.
+Mnemonic: use ^V for a version object.
=item ${^WIN32_SLOPPY_STAT}
X<${^WIN32_SLOPPY_STAT}> X<sitecustomize> X<sitecustomize.pl>
$1 is Mutt; $2 is Jeff
$1 is Wallace; $2 is Grommit
-The C<Devel::NYTProf> and C<Devel::FindAmpersand>
-modules can help you find uses of these
-problematic match variables in your code.
+=head3 Performance issues
+
+Traditionally in Perl, any use of any of the three variables C<$`>, C<$&>
+or C<$'> (or their C<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 C<@-> and C<@+> dynamic arrays were introduced that
+supply the indices of successful matches. So you could for example do
+this:
+
+ $str =~ /pattern/;
+
+ print $`, $&, $'; # bad: perfomance hit
+
+ print # good: no perfomance hit
+ substr($str, 0, $-[0]),
+ substr($str, $-[0], $+[0]-$-[0]),
+ substr($str, $+[0]);
-Since Perl v5.10.0, you can use the C</p> match operator flag and the
-C<${^PREMATCH}>, C<${^MATCH}>, and C<${^POSTMATCH}> variables instead
-so you only suffer the performance penalties.
+In Perl 5.10.0 the C</p> match operator flag and the C<${^PREMATCH}>,
+C<${^MATCH}>, and C<${^POSTMATCH}> variables were introduced, that allowed
+you to suffer the penalties only on patterns marked with C</p>.
-If you are using Perl v5.20.0 or higher, you do not need to worry about
-this, as the three naughty variables are no longer naughty.
+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
+ $str =~ /x/g # one char copied a million times, not a million chars
+
+In Perl 5.20.0 a new copy-on-write system was enabled by default, which
+finally fixes all performance issues with these three variables, and makes
+them safe to use anywhere.
+
+The C<Devel::NYTProf> and C<Devel::FindAmpersand> modules can help you
+find uses of these problematic match variables in your code.
=over 8
any matches hidden within a BLOCK or C<eval()> enclosed by the current
BLOCK).
-In Perl v5.18 and earlier, the use of this variable
-anywhere in a program imposes a considerable
-performance penalty on all regular expression matches. To avoid this
-penalty, you can extract the same substring by using L</@->. Starting
-with Perl v5.10.0, you can use the C</p> match flag and the C<${^MATCH}>
-variable to do the same thing for particular match operations.
+See L</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.
This is similar to C<$&> (C<$MATCH>) except that it does not incur the
performance penalty associated with that variable.
+
+See L</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 C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
pattern match, not counting any matches hidden within a BLOCK or C<eval>
enclosed by the current BLOCK.
-In Perl v5.18 and earlier, 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 L</@->. Starting
-with Perl v5.10.0, you can use the C</p> match flag and the
-C<${^PREMATCH}> variable to do the same thing for particular match
-operations.
+See L</Performance issues> above for the serious performance implications
+of using this variable (even once) in your code.
This variable is read-only and dynamically-scoped.
This is similar to C<$`> ($PREMATCH) except that it does not incur the
performance penalty associated with that variable.
+
+See L</Performance issues> above.
+
In Perl v5.18 and earlier, it is only guaranteed
to return a defined value when the pattern was compiled or executed with
the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
C<${^PREMATCH}> does the same thing as C<$PREMATCH>.
-This variable was added in Perl v5.10.0
+This variable was added in Perl v5.10.0.
This variable is read-only and dynamically-scoped.
/def/;
print "$`:$&:$'\n"; # prints abc:def:ghi
-In Perl v5.18 and earlier, 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 L</@->. Starting with Perl v5.10.0, you can use the C</p> match flag
-and the C<${^POSTMATCH}> variable to do the same thing for particular
-match operations.
+See L</Performance issues> above for the serious performance implications
+of using this variable (even once) in your code.
This variable is read-only and dynamically-scoped.
This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
performance penalty associated with that variable.
+
+See L</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 C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
This variable was added in Perl v5.6.0.
-=item %LAST_MATCH_START
-
=item %-
-X<%-> X<%LAST_MATCH_START>
+X<%->
Similar to C<%+>, this variable allows access to the named capture groups
in the last successful match in the currently active dynamic scope. To
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.
+size to zero or less is deprecated and will cause $/ to have the value
+of "undef", which will cause reading in the (rest of the) whole file.
+
+As of 5.19.9 setting C<$/> to any other form of reference will throw a
+fatal exception. This is in preparation for supporting new ways to set
+C<$/> in the future.
On VMS only, record reads bypass PerlIO layers and any associated
buffering, so you must not mix record and non-record reads on the
indicator, such as in this case, "CDROM tray not closed." Systems that
do not support extended error messages leave C<$^E> the same as C<$!>.
-Finally, C<$?> may be set to non-0 value if the external program
+Finally, C<$?> may be set to a non-0 value if the external program
F</cdrom/install> fails. The upper eight bits reflect specific error
conditions encountered by the program (the program's C<exit()> value).
The lower eight bits reflect mode of failure, like signal death and
core dump information. See L<wait(2)> for details. In contrast to
-C<$!> and C<$^E>, which are set only if error condition is detected,
+C<$!> and C<$^E>, which are set only if an error condition is detected,
the variable C<$?> is set on each C<wait> or pipe C<close>,
overwriting the old value. This is more like C<$@>, which on every
C<eval()> is always set on failure and cleared on success.
value. For example, C<$!{ENOENT}> is true if and only if the current
value of C<$!> is C<ENOENT>; that is, if the most recent error was "No
such file or directory" (or its moral equivalent: not all operating
-systems give that exact error, and certainly not all languages). To
-check if a particular key is meaningful on your system, use C<exists
-$!{the_key}>; for a list of legal keys, use C<keys %!>. See L<Errno>
-for more information, and also see L</$!>.
+systems give that exact error, and certainly not all languages). The
+specific true value is not guaranteed, but in the past has generally
+been the numeric value of C<$!>. To check if a particular key is
+meaningful on your system, use C<exists $!{the_key}>; for a list of legal
+keys, use C<keys %!>. See L<Errno> for more information, and also see
+L</$!>.
This variable was added in Perl 5.005.
=item $@
X<$@> X<$EVAL_ERROR>
-The Perl syntax error message from the
-last C<eval()> operator. If C<$@> is
-the null string, the last C<eval()> parsed and executed correctly
-(although the operations you invoked may have failed in the normal
-fashion).
+The Perl error from the last C<eval> operator, i.e. the last exception that
+was caught. For C<eval BLOCK>, this is either a runtime error message or the
+string or reference C<die> was called with. The C<eval STRING> form also
+catches syntax errors and other compile time exceptions.
+
+If no error occurs, C<eval> sets C<$@> to the empty string.
Warning messages are not collected in this variable. You can, however,
set up a routine to process warnings by setting C<$SIG{__WARN__}> as
described in L</%SIG>.
-Mnemonic: Where was the syntax error "at"?
+Mnemonic: Where was the error "at"?
=back
=item ${^ENCODING}
X<${^ENCODING}>
+DEPRECATED!!!
+
The I<object reference> to the C<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 I<undef>. The direct
-manipulation of this variable is highly discouraged.
+does not have to be written in UTF-8. Default is C<undef>.
+
+Setting this variable to any other value than C<undef> is deprecated due
+to fundamental defects in its design and implementation. It is planned
+to remove it from a future Perl version. Its purpose was to allow your
+non-ASCII Perl scripts to not 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 causes problems, such as affecting the operation
+of other modules that aren't expecting it, causing general mayhem. Its
+use can lead to segfaults.
+
+If you need something like this functionality, you should use the
+L<encoding> pragma, which is also deprecated, but has fewer nasty side
+effects.
+
+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 ${^ENCODING};
+
+near the beginning of the functions that are getting broken. This
+undefines the variable during the scope of execution of the including
+function.
This variable was added in Perl 5.8.2.
The C<%^H> hash provides the same scoping semantic as C<$^H>. This makes
it useful for implementation of lexically scoped pragmas. See
-L<perlpragma>.
+L<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.
When putting items into C<%^H>, in order to avoid conflicting with other
users of the hash there is a convention regarding which keys to use.
Save source code lines into C<@{"_<$filename"}>.
+=item 0x800
+
+When saving source, include evals that generate no subroutines.
+
+=item 0x1000
+
+When saving source, include source that did not compile.
+
=back
Some bits may be relevant at compile-time only, some at
=over 8
-=item $OFMT
-
=item $#
-X<$#> X<$OFMT>
+X<$#>
C<$#> 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
Removed in Perl v5.10.0.
-=item $ARRAY_BASE
-
=item $[
-X<$[> X<$ARRAY_BASE>
+X<$[>
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
Deprecated in Perl v5.12.0.
-=item $OLD_PERL_VERSION
-
-=item $]
-X<$]> X<$OLD_PERL_VERSION>
-
-See L</$^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 C<use VERSION> and C<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?
-
=back
=cut
https://perl5.git.perl.org / perl5.git / blobdiff