be a chain of identifiers, separated by C<::> (or by the slightly
archaic C<'>); all but the last are interpreted as names of packages,
to locate the namespace in which to look up the final identifier
-(see L<perlmod/Packages> for details). It's possible to substitute
-for a simple identifier, an expression that produces a reference
+(see L<perlmod/Packages> for details). For a more in-depth discussion
+on identifiers, see L<Identifier parsing>. It's possible to
+substitute for a simple identifier, an expression that produces a reference
to the value at runtime. This is described in more detail below
and in L<perlref>.
X<identifier>
significance to Perl. For instance, C<$$> is the current process
id.)
+=head2 Identifier parsing
+X<identifiers>
+
+Up until Perl 5.18, the actual rules of what a valid identifier
+was were a bit fuzzy. However, in general, anything defined here should
+work on previous versions of Perl, while the opposite -- edge cases
+that work in previous versions, but aren't defined here -- probably
+won't work on newer versions.
+As an important side note, please note that the following only applies
+to bareword identifiers as found in Perl source code, not identifiers
+introduced through symbolic references, which have much fewer
+restrictions.
+If working under the effect of the C<use utf8;> pragma, the following
+rules apply:
+
+ / (?[ ( \p{Word} & \p{XID_Start} ) + [_] ])
+ (?[ ( \p{Word} & \p{XID_Continue} ) ]) * /x
+
+That is, a "start" character followed by any number of "continue"
+characters. Perl requires every character in an identifier to also
+match C<\w> (this prevents some problematic cases); and Perl
+additionally accepts identfier names beginning with an underscore.
+
+If not under C<use utf8>, the source is treated as ASCII + 128 extra
+controls, and identifiers should match
+
+ / (?aa) (?!\d) \w+ /x
+
+That is, any word character in the ASCII range, as long as the first
+character is not a digit.
+
+There are two package separators in Perl: A double colon (C<::>) and a single
+quote (C<'>). Normal identifiers can start or end with a double colon, and
+can contain several parts delimited by double colons.
+Single quotes have similar rules, but with the exception that they are not
+legal at the end of an identifier: That is, C<$'foo> and C<$foo'bar> are
+legal, but C<$foo'bar'> is not.
+
+Additionally, if the identifier is preceded by a sigil --
+that is, if the identifier is part of a variable name -- it
+may optionally be enclosed in braces.
+
+While you can mix double colons with singles quotes, the quotes must come
+after the colons: C<$::::'foo> and C<$foo::'bar> are legal, but C<$::'::foo>
+and C<$foo'::bar> are not.
+
+Put together, a grammar to match a basic identifier becomes
+
+ /
+ (?(DEFINE)
+ (?<variable>
+ (?&sigil)
+ (?:
+ (?&normal_identifier)
+ | \{ \s* (?&normal_identifier) \s* \}
+ )
+ )
+ (?<normal_identifier>
+ (?: :: )* '?
+ (?&basic_identifier)
+ (?: (?= (?: :: )+ '? | (?: :: )* ' ) (?&normal_identifier) )?
+ (?: :: )*
+ )
+ (?<basic_identifier>
+ # is use utf8 on?
+ (?(?{ (caller(0))[8] & $utf8::hint_bits })
+ (?&Perl_XIDS) (?&Perl_XIDC)*
+ | (?aa) (?!\d) \w+
+ )
+ )
+ (?<sigil> [&*\$\@\%])
+ (?<Perl_XIDS> (?[ ( \p{Word} & \p{XID_Start} ) + [_] ]) )
+ (?<Perl_XIDC> (?[ \p{Word} & \p{XID_Continue} ]) )
+ )
+ /x
+
+Meanwhile, special identifiers don't follow the above rules; For the most
+part, all of the identifiers in this category have a special meaning given
+by Perl. Because they have special parsing rules, these generally can't be
+fully-qualified. They come in four forms:
+
+=over
+
+=item A sigil, followed solely by digits matching \p{POSIX_Digit}, like C<$0>,
+C<$1>, or C<$10000>.
+
+=item A sigil, followed by either a caret and a single POSIX uppercase letter,
+like C<$^V> or C<$^W>, or a sigil followed by a literal control character
+matching the C<\p{POSIX_Cntrl}> property. Due to a historical oddity, if not
+running under C<use utf8>, the 128 extra controls in the C<[0x80-0xff]> range
+may also be used in length one variables.
+
+=item Similar to the above, a sigil, followed by bareword text in brackets,
+where the first character is either a caret followed by an uppercase letter,
+or a literal control, like C<${^GLOBAL_PHASE}> or C<${\7LOBAL_PHASE}>.
+
+=item A sigil followed by a single character matching the C<\p{POSIX_Punct}>
+property, like C<$!> or C<%+>.
+
+=back
+
=head2 Context
X<context> X<scalar context> X<list context>
references are strongly-typed, uncastable pointers with builtin
reference-counting and destructor invocation.
-A scalar value is interpreted as TRUE in the Boolean sense if it is not
-the null string or the number 0 (or its string equivalent, "0"). The
+A scalar value is interpreted as FALSE in the Boolean sense
+if it is undefined, the null string or the number 0 (or its
+string equivalent, "0"), and TRUE if it is anything else. The
Boolean context is just a special kind of scalar context where no
conversion to a string or a number is ever performed.
X<boolean> X<bool> X<true> X<false> X<truth>
Assigning to C<$#days> actually changes the length of the array.
Shortening an array this way destroys intervening values. Lengthening
an array that was previously shortened does not recover values
-that were in those elements. (It used to do so in Perl 4, but we
-had to break this to make sure destructors were called when expected.)
+that were in those elements.
X<$#> X<array, length>
You can also gain some minuscule measure of efficiency by pre-extending
always true:
X<array, length>
- scalar(@whatever) == $#whatever - $[ + 1;
-
-Version 5 of Perl changed the semantics of C<$[>: files that don't set
-the value of C<$[> no longer need to worry about whether another
-file changed its value. (In other words, use of C<$[> is deprecated.)
-So in general you can assume that
-X<$[>
-
scalar(@whatever) == $#whatever + 1;
Some programmers choose to use an explicit conversion so as to
0b011011 # binary
You are allowed to use underscores (underbars) in numeric literals
-between digits for legibility. You could, for example, group binary
+between digits for legibility (but not multiple underscores in a row:
+C<23__500> is not legal; C<23_500> is).
+You could, for example, group binary
digits by threes (as for a Unix-style mode argument such as 0b110_100_100)
or by fours (to represent nibbles, as in 0b1010_0110) or in other groups.
X<number, literal>
$0 and the $s variables in the (presumably) non-existent package
C<who>.
-In fact, an identifier within such curlies is forced to be a string,
-as is any simple identifier within a hash subscript. Neither need
+In fact, a simple identifier within such curlies is forced to be
+a string, and likewise within a hash subscript. Neither need
quoting. Our earlier example, C<$days{'Feb'}> can be written as
C<$days{Feb}> and the quotes will be assumed automatically. But
anything more complicated in the subscript will be interpreted as an
The special literals __FILE__, __LINE__, and __PACKAGE__
represent the current filename, line number, and package name at that
-point in your program. They may be used only as separate tokens; they
+point in your program. __SUB__ gives a reference to the current
+subroutine. They may be used only as separate tokens; they
will not be interpolated into strings. If there is no current package
(due to an empty C<package;> directive), __PACKAGE__ is the undefined
-value. (But the empty C<package;> is no longer supported, as of version
-5.10.)
-X<__FILE__> X<__LINE__> X<__PACKAGE__> X<line> X<file> X<package>
+value. (But the empty C<package;> is no longer supported, as of version
+5.10.) Outside of a subroutine, __SUB__ is the undefined value. __SUB__
+is only available in 5.16 or higher, and only with a C<use v5.16> or
+C<use feature "current_sub"> declaration.
+X<__FILE__> X<__LINE__> X<__PACKAGE__> X<__SUB__>
+X<line> X<file> X<package>
The two control characters ^D and ^Z, and the tokens __END__ and __DATA__
may be used to indicate the logical end of the script before the actual
Text after __DATA__ may be read via the filehandle C<PACKNAME::DATA>,
where C<PACKNAME> is the package that was current when the __DATA__
token was encountered. The filehandle is left open pointing to the
-contents after __DATA__. It is the program's responsibility to
-C<close DATA> when it is done reading from it. For compatibility with
-older scripts written before __DATA__ was introduced, __END__ behaves
-like __DATA__ in the top level script (but not in files loaded with
-C<require> or C<do>) and leaves the remaining contents of the
-file accessible via C<main::DATA>.
+line after __DATA__. The program should C<close DATA> when it is done
+reading from it. (Leaving it open leaks filehandles if the module is
+reloaded for any reason, so it's a safer practice to close it.) For
+compatibility with older scripts written before __DATA__ was
+introduced, __END__ behaves like __DATA__ in the top level script (but
+not in files loaded with C<require> or C<do>) and leaves the remaining
+contents of the file accessible via C<main::DATA>.
See L<SelfLoader> for more description of __DATA__, and
an example of its use. Note that you cannot read from the DATA
This interpolation combines with the facts that the opening
and closing parentheses are optional (except when necessary for
precedence) and lists may end with an optional comma to mean that
-multiple commas within lists are legal syntax. The list C<1,,3> is a
+multiple commas within lists are legal syntax. The list C<1,,3> is a
concatenation of two lists, C<1,> and C<3>, the first of which ends
with that optional comma. C<1,,3> is C<(1,),(3)> is C<1,3> (And
similarly for C<1,,,3> is C<(1,),(,),3> is C<1,3> and so on.) Not that
It's also the source of a useful idiom for executing a function or
performing an operation in list context and then counting the number of
return values, by assigning to an empty list and then using that
-assignment in scalar context. For example, this code:
+assignment in scalar context. For example, this code:
$count = () = $string =~ /\d+/g;
will place into $count the number of digit groups found in $string.
This happens because the pattern match is in list context (since it
is being assigned to the empty list), and will therefore return a list
-of all matching parts of the string. The list assignment in scalar
+of all matching parts of the string. The list assignment in scalar
context will translate that into the number of elements (here, the
-number of times the pattern matched) and assign that to $count. Note
+number of times the pattern matched) and assign that to $count. Note
that simply using
$count = $string =~ /\d+/g;
pairs. The C<< => >> operator is mostly just a more visually distinctive
synonym for a comma, but it also arranges for its left-hand operand to be
interpreted as a string if it's a bareword that would be a legal simple
-identifier. C<< => >> doesn't quote compound identifiers, that contain
-double colons. This makes it nice for initializing hashes:
+identifier. C<< => >> doesn't quote compound identifiers, that contain
+double colons. This makes it nice for initializing hashes:
%map = (
red => 0x00f,
mean that it comes out in that order. See L<perlfunc/sort> for examples
of how to arrange for an output ordering.
+If a key appears more than once in the initializer list of a hash, the last
+occurrence wins:
+
+ %circle = (
+ center => [5, 10],
+ center => [27, 9],
+ radius => 100,
+ color => [0xDF, 0xFF, 0x00],
+ radius => 54,
+ );
+
+ # same as
+ %circle = (
+ center => [27, 9],
+ color => [0xDF, 0xFF, 0x00],
+ radius => 54,
+ );
+
+This can be used to provide overridable configuration defaults:
+
+ # values in %args take priority over %config_defaults
+ %config = (%config_defaults, %args);
+
=head2 Subscripts
An array can be accessed one scalar at a
@myarray = (5, 50, 500, 5000);
print "The Third Element is", $myarray[2], "\n";
-The array indices start with 0. A negative subscript retrieves its
+The array indices start with 0. A negative subscript retrieves its
value from the end. In our example, C<$myarray[-1]> would have been
5000, and C<$myarray[-2]> would have been 500.
Hash subscripts are similar, only instead of square brackets curly brackets
-are used. For example:
+are used. For example:
%scientists =
(
$dir = (getpwnam("daemon"))[7];
+=head2 Multi-dimensional array emulation
+
+Multidimensional arrays may be emulated by subscripting a hash with a
+list. The elements of the list are joined with the subscript separator
+(see L<perlvar/$;>).
+
+ $foo{$a,$b,$c}
+
+is equivalent to
+
+ $foo{join($;, $a, $b, $c)}
+
+The default subscript separator is "\034", the same as SUBSEP in B<awk>.
+
=head2 Slices
X<slice> X<array, slice> X<hash, slice>
@a = ()[1,0]; # @a has no elements
@b = (@a)[0,1]; # @b has no elements
- @c = (0,1)[2,3]; # @c has no elements
But:
@a = (1)[1,0]; # @a has two elements
@b = (1,undef)[1,0,2]; # @b has three elements
+More generally, a slice yields the empty list if it indexes only
+beyond the end of a list:
+
+ @a = (1)[ 1,2]; # @a has no elements
+ @b = (1)[0,1,2]; # @b has three elements
+
This makes it easy to write loops that terminate when a null list
is returned:
Now that we have the C<*foo{THING}> notation, typeglobs aren't used as much
for filehandle manipulations, although they're still needed to pass brand
-new file and directory handles into or out of functions. That's because
+new file and directory handles into or out of functions. That's because
C<*HANDLE{IO}> only works if HANDLE has already been used as a handle.
In other words, C<*FH> must be used to create new symbol table entries;
C<*foo{THING}> cannot. When in doubt, use C<*FH>.
All functions that are capable of creating filehandles (open(),
opendir(), pipe(), socketpair(), sysopen(), socket(), and accept())
automatically create an anonymous filehandle if the handle passed to
-them is an uninitialized scalar variable. This allows the constructs
+them is an uninitialized scalar variable. This allows the constructs
such as C<open(my $fh, ...)> and C<open(local $fh,...)> to be used to
create filehandles that will conveniently be closed automatically when
-the scope ends, provided there are no other references to them. This
+the scope ends, provided there are no other references to them. This
largely eliminates the need for typeglobs when opening filehandles
that must be passed around, as in the following example:
https://perl5.git.perl.org / perl5.git / blobdiff