=head1 DESCRIPTION
=head2 Variable names
+X<variable, name> X<variable name> X<data type> X<type>
Perl has three built-in data types: scalars, arrays of scalars, and
-associative arrays of scalars, known as "hashes". Normal arrays
-are ordered lists of scalars indexed by number, starting with 0 and with
-negative subscripts counting from the end. Hashes are unordered
-collections of scalar values indexed by their associated string key.
+associative arrays of scalars, known as "hashes". A scalar is a
+single string (of any size, limited only by the available memory),
+number, or a reference to something (which will be discussed
+in L<perlref>). Normal arrays are ordered lists of scalars indexed
+by number, starting with 0. Hashes are unordered collections of scalar
+values indexed by their associated string key.
Values are usually referred to by name, or through a named reference.
The first character of the name tells you to what sort of data
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>
Perl also has its own built-in variables whose names don't follow
these rules. They have strange names so they don't accidentally
parenthesized parts of a regular expression are saved under names
containing only digits after the C<$> (see L<perlop> and L<perlre>).
In addition, several special variables that provide windows into
-the inner working of Perl have names containing punctuation characters
-and control characters. These are documented in L<perlvar>.
+the inner working of Perl have names containing punctuation characters.
+These are documented in L<perlvar>.
+X<variable, built-in>
Scalar values are always named with '$', even when referring to a
scalar that is part of an array or a hash. The '$' symbol works
semantically like the English word "the" in that it indicates a
single value is expected.
+X<scalar>
$days # the simple scalar value "days"
$days[28] # the 29th element of array @days
$#days # the last index of array @days
Entire arrays (and slices of arrays and hashes) are denoted by '@',
-which works much like the word "these" or "those" does in English,
+which works much as the word "these" or "those" does in English,
in that it indicates multiple values are expected.
+X<array>
@days # ($days[0], $days[1],... $days[n])
@days[3,4,5] # same as ($days[3],$days[4],$days[5])
@days{'a','c'} # same as ($days{'a'},$days{'c'})
Entire hashes are denoted by '%':
+X<hash>
%days # (key1, val1, key2, val2 ...)
and @foo are two different variables. It also means that C<$foo[1]>
is a part of @foo, not a part of $foo. This may seem a bit weird,
but that's okay, because it is weird.
+X<namespace>
Because variable references always start with '$', '@', or '%', the
"reserved" words aren't in fact reserved with respect to variable
from conflict with future reserved words. Case I<is> significant--"FOO",
"Foo", and "foo" are all different names. Names that start with a
letter or underscore may also contain digits and underscores.
+X<identifier, case sensitivity>
+X<case>
It is possible to replace such an alphanumeric name with an expression
that returns a reference to the appropriate type. For a description
of this, see L<perlref>.
Names that start with a digit may contain only more digits. Names
-that do not start with a letter, underscore, or digit are limited to
-one character, e.g., C<$%> or C<$$>. (Most of these one character names
-have a predefined significance to Perl. For instance, C<$$> is the
-current process id.)
+that do not start with a letter, underscore, digit or a caret are
+limited to one character, e.g., C<$%> or
+C<$$>. (Most of these one character names have a predefined
+significance to Perl. For instance, C<$$> is the current process
+id. And all such names are reserved for Perl's possible use.)
+
+=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
+generic characters, 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 six forms (but don't use forms 5 and 6):
+
+=over
+
+=item 1.
+
+A sigil, followed solely by digits matching C<\p{POSIX_Digit}>, like
+C<$0>, C<$1>, or C<$10000>.
+
+=item 2.
+
+A sigil followed by a single character matching the C<\p{POSIX_Punct}>
+property, like C<$!> or C<%+>, except the character C<"{"> doesn't work.
+
+=item 3.
+
+A sigil, followed by a caret and any one of the characters
+C<[][A-Z^_?\]>, like C<$^V> or C<$^]>.
+
+=item 4.
+
+Similar to the above, a sigil, followed by bareword text in braces,
+where the first character is a caret. The next character is any one of
+the characters C<[][A-Z^_?\]>, followed by ASCII word characters. An
+example is C<${^GLOBAL_PHASE}>.
+
+=item 5.
+
+A sigil, followed by any single character in the range C<[\xA1-\xAC\xAE-\xFF]>
+when not under C<S<"use utf8">>. (Under C<S<"use utf8">>, the normal
+identifier rules given earlier in this section apply.) Use of
+non-graphic characters (the C1 controls, the NO-BREAK SPACE, and the
+SOFT HYPHEN) has been disallowed since v5.26.0.
+The use of the other characters is unwise, as these are all
+reserved to have special meaning to Perl, and none of them currently
+do have special meaning, though this could change without notice.
+
+Note that an implication of this form is that there are identifiers only
+legal under C<S<"use utf8">>, and vice-versa, for example the identifier
+C<$E<233>tat> is legal under C<S<"use utf8">>, but is otherwise
+considered to be the single character variable C<$E<233>> followed by
+the bareword C<"tat">, the combination of which is a syntax error.
+
+=item 6.
+
+This is a combination of the previous two forms. It is valid only when
+not under S<C<"use utf8">> (normal identifier rules apply when under
+S<C<"use utf8">>). The form is a sigil, followed by text in braces,
+where the first character is any one of the characters in the range
+C<[\x80-\xFF]> followed by ASCII word characters up to the trailing
+brace.
+
+The same caveats as the previous form apply: The non-graphic
+characters are no longer allowed with S<"use utf8">, it is unwise
+to use this form at all, and utf8ness makes a big difference.
+
+=back
+
+Prior to Perl v5.24, non-graphical ASCII control characters were also
+allowed in some situations; this had been deprecated since v5.20.
=head2 Context
+X<context> X<scalar context> X<list context>
The interpretation of operations and values in Perl sometimes depends
on the requirements of the context around the operation or value.
scalar evaluates the right-hand side in scalar context, while
assignment to an array or hash evaluates the righthand side in list
context. Assignment to a list (or slice, which is just a list
-anyway) also evaluates the righthand side in list context.
+anyway) also evaluates the right-hand side in list context.
When you use the C<use warnings> pragma or Perl's B<-w> command-line
option, you may see warnings
context.
=head2 Scalar values
+X<scalar> X<number> X<string> X<reference>
All data in Perl is a scalar, an array of scalars, or a hash of
scalars. A scalar may contain one single value in any of three
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
+X<truth> X<falsehood> X<true> X<false> X<!> X<not> X<negation> X<0>
+X<boolean> X<bool>
+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.
+Negation of a true value by C<!> or C<not> returns a special false value.
+When evaluated as a string it is treated as C<"">, but as a number, it
+is treated as 0. Most Perl operators
+that return true or false behave this way.
There are actually two varieties of null strings (sometimes referred
to as "empty" strings), a defined one and an undefined one. The
use the defined() operator to determine whether a scalar value is
defined (this has no meaning on arrays or hashes), and the undef()
operator to produce an undefined value.
+X<defined> X<undefined> X<undef> X<null> X<string, null>
To find out whether a given string is a valid non-zero number, it's
sometimes enough to test it against both numeric 0 and also lexical
-"0" (although this will cause B<-w> noises). That's because strings
-that aren't numbers count as 0, just as they do in B<awk>:
+"0" (although this will cause noises if warnings are on). That's
+
because strings that aren't numbers count as 0, just as they do in B<awk>:
if ($str == 0 && $str ne "0") {
warn "That doesn't look like a number";
Assigning to C<$#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 miniscule measure of efficiency by pre-extending
+You can also gain some minuscule measure of efficiency by pre-extending
an array that is going to get big. You can also extend an array
by assigning to an element that is off the end of the array. You
can truncate an array down to nothing by assigning the null list
the last value, like the C comma operator, nor of built-in functions,
which return whatever they feel like returning.) The following is
always true:
-
- 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<array, length>
scalar(@whatever) == $#whatever + 1;
$element_count = scalar(@whatever);
-If you evaluate a hash in scalar context, it returns false if the
-hash is empty. If there are any key/value pairs, it returns true;
-more precisely, the value returned is a string consisting of the
+If you evaluate a hash in scalar context, it returns a false value if
+the hash is empty. If there are any key/value pairs, it returns a
+true value. A more precise definition is version dependent.
+
+Prior to Perl 5.25 the value returned was a string consisting of the
number of used buckets and the number of allocated buckets, separated
by a slash. This is pretty much useful only to find out whether
Perl's internal hashing algorithm is performing poorly on your data
of sixteen buckets has been touched, and presumably contains all
10,000 of your items. This isn't supposed to happen.
+As of Perl 5.25 the return was changed to be the count of keys in the
+hash. If you need access to the old behavior you can use
+C<Hash::Util::bucket_ratio()> instead.
+
+If a tied hash is evaluated in scalar context, the C<SCALAR> method is
+called (with a fallback to C<FIRSTKEY>).
+X<hash, scalar context> X<hash, bucket> X<bucket>
+
You can preallocate space for a hash by assigning to the keys() function.
This rounds up the allocated buckets to the next power of two:
keys(%users) = 1000; # allocate 1024 buckets
=head2 Scalar value constructors
+X<scalar, literal> X<scalar, constant>
Numeric literals are specified in any of the following floating point or
integer formats:
- 12345
- 12345.67
- .23E-10 # a very small number
- 3.14_15_92 # a very important number
- 4_294_967_296 # underscore for legibility
- 0xff # hex
- 0xdead_beef # more hex
- 0377 # octal
- 0b011011 # binary
+ 12345
+ 12345.67
+ .23E-10 # a very small number
+ 3.14_15_92 # a very important number
+ 4_294_967_296 # underscore for legibility
+ 0xff # hex
+ 0xdead_beef # more hex
+ 0377 # octal (only numbers, begins with 0)
+ 0b011011 # binary
+ 0x1.999ap-4 # hexadecimal floating point (the 'p' is required)
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>
String literals are usually delimited by either single or double
quotes. They work much like quotes in the standard Unix shells:
C<\\>). The usual C-style backslash rules apply for making
characters such as newline, tab, etc., as well as some more exotic
forms. See L<perlop/"Quote and Quote-like Operators"> for a list.
+X<string, literal>
Hexadecimal, octal, or binary, representations in string literals
(e.g. '0xff') are not automatically converted to their integer
representation. The hex() and oct() functions make these conversions
for you. See L<perlfunc/hex> and L<perlfunc/oct> for more details.
+Hexadecimal floating point can start just like a hexadecimal literal,
+and it can be followed by an optional fractional hexadecimal part,
+but it must be followed by C<p>, an optional sign, and a power of two.
+The format is useful for accurately presenting floating point values,
+avoiding conversions to or from decimal floating point, and therefore
+avoiding possible loss in precision. Notice that while most current
+platforms use the 64-bit IEEE 754 floating point, not all do. Another
+potential source of (low-order) differences are the floating point
+rounding modes, which can differ between CPUs, operating systems,
+and compilers, and which Perl doesn't control.
+
You can also embed newlines directly in your strings, i.e., they can end
on a different line than they begin. This is nice, but if you forget
your trailing quote, the error will not be reported until Perl finds
names beginning with $ or @, followed by an optional bracketed
expression as a subscript.) The following code segment prints out "The
price is $Z<>100."
+X<interpolation>
+
+ $Price = '$100'; # not interpolated
+ print "The price is $Price.\n"; # interpolated
+
+There is no double interpolation in Perl, so the C<$100> is left as is.
- $Price = '$100'; # not interpreted
- print "The price is $Price.\n"; # interpreted
+By default floating point numbers substituted inside strings use the
+dot (".") as the decimal separator. If C<use locale> is in effect,
+and POSIX::setlocale() has been called, the character used for the
+decimal separator is affected by the LC_NUMERIC locale.
+See L<perllocale> and L<POSIX>.
As in some shells, you can enclose the variable name in braces to
disambiguate it from following alphanumerics (and underscores).
this when interpolating a variable into a string to separate the
variable name from a following double-colon or an apostrophe, since
these would be otherwise treated as a package separator:
+X<interpolation>
$who = "Larry";
print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
$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<$days{Feb}> and the quotes will be assumed automatically. But
-anything more complicated in the subscript will be interpreted as
-an expression.
+anything more complicated in the subscript will be interpreted as an
+expression. This means for example that C<$version{2.0}++> is
+equivalent to C<$version{2}++>, not to C<$version{'2.0'}++>.
+
+=head3 Special floating point: infinity (Inf) and not-a-number (NaN)
+
+Floating point values include the special values C<Inf> and C<NaN>,
+for infinity and not-a-number. The infinity can be also negative.
+
+The infinity is the result of certain math operations that overflow
+the floating point range, like 9**9**9. The not-a-number is the
+result when the result is undefined or unrepresentable. Though note
+that you cannot get C<NaN> from some common "undefined" or
+"out-of-range" operations like dividing by zero, or square root of
+a negative number, since Perl generates fatal errors for those.
+
+The infinity and not-a-number have their own special arithmetic rules.
+The general rule is that they are "contagious": C<Inf> plus one is
+C<Inf>, and C<NaN> plus one is C<NaN>. Where things get interesting
+is when you combine infinities and not-a-numbers: C<Inf> minus C<Inf>
+and C<Inf> divided by C<Inf> are C<NaN> (while C<Inf> plus C<Inf> is
+C<Inf> and C<Inf> times C<Inf> is C<Inf>). C<NaN> is also curious
+in that it does not equal any number, I<including> itself:
+C<NaN> != C<NaN>.
+
+Perl doesn't understand C<Inf> and C<NaN> as numeric literals, but
+you can have them as strings, and Perl will convert them as needed:
+"Inf" + 1. (You can, however, import them from the POSIX extension;
+C<use POSIX qw(Inf NaN);> and then use them as literals.)
+
+Note that on input (string to number) Perl accepts C<Inf> and C<NaN>
+in many forms. Case is ignored, and the Win32-specific forms like
+C<1.#INF> are understood, but on output the values are normalized to
+C<Inf> and C<NaN>.
+
+=head3 Version Strings
+X<version string> X<vstring> X<v-string>
A literal of the form C<v1.20.300.4000> is parsed as a string composed
-of characters with the specified ordinals. This provides an alternative,
-more readable way to construct strings, rather than use the somewhat less
-readable interpolation form C<"\x{1}\x{14}\x{12c}\x{fa0}">. This is useful
-for representing Unicode strings, and for comparing version "numbers"
-using the string comparison operators, C<cmp>, C<gt>, C<lt> etc.
-If there are two or more dots in the literal, the leading C<v> may be
-omitted.
-
- print v9786; # prints UTF-8 encoded SMILEY, "\x{263a}"
+of characters with the specified ordinals. This form, known as
+v-strings, provides an alternative, more readable way to construct
+strings, rather than use the somewhat less readable interpolation form
+C<"\x{1}\x{14}\x{12c}\x{fa0}">. This is useful for representing
+Unicode strings, and for comparing version "numbers" using the string
+comparison operators, C<cmp>, C<gt>, C<lt> etc. If there are two or
+more dots in the literal, the leading C<v> may be omitted.
+
+ print v9786; # prints SMILEY, "\x{263a}"
print v102.111.111; # prints "foo"
print 102.111.111; # same
Such literals are accepted by both C<require> and C<use> for
-doing a version check. The C<$^V> special variable also contains the
-running Perl interpreter's version in this form. See L<perlvar/$^V>.
+doing a version check. Note that using the v-strings for IPv4
+addresses is not portable unless you also use the
+inet_aton()/inet_ntoa() routines of the Socket package.
+
+Note that since Perl 5.8.1 the single-number v-strings (like C<v65>)
+are not v-strings before the C<< => >> operator (which is usually used
+to separate a hash key from a hash value); instead they are interpreted
+as literal strings ('v65'). They were v-strings from Perl 5.6.0 to
+Perl 5.8.0, but that caused more confusion and breakage than good.
+Multi-number v-strings like C<v65.66> and C<65.66.67> continue to
+be v-strings always.
+
+=head3 Special Literals
+X<special literal> X<__END__> X<__DATA__> X<END> X<DATA>
+X<end> X<data> X<^D> X<^Z>
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.
+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
end of file. Any following text is ignored.
-Text after __DATA__
but may be read via the filehandle C<PACKNAME::DATA>,
+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 toplevel 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>.
+
+The C<DATA> file handle by default has whatever PerlIO layers were
+in place when Perl read the file to parse the source. Normally that
+means that the file is being read bytewise, as if it were encoded in
+Latin-1, but there are two major ways for it to be otherwise. Firstly,
+if the C<__END__>/C<__DATA__> token is in the scope of a C<use utf8>
+pragma then the C<DATA> handle will be in UTF-8 mode. And secondly,
+if the source is being read from perl's standard input then the C<DATA>
+file handle is actually aliased to the C<STDIN> file handle, and may
+be in UTF-8 mode because of the C<PERL_UNICODE> environment variable or
+perl's command-line switches.
See L<SelfLoader> for more description of __DATA__, and
an example of its use. Note that you cannot read from the DATA
as it is seen (during compilation), at which point the corresponding
__DATA__ (or __END__) token has not yet been seen.
+=head3 Barewords
+X<bareword>
+
A word that has no other interpretation in the grammar will
be treated as if it were a quoted string. These are known as
"barewords". As with filehandles and labels, a bareword that consists
entirely of lowercase letters risks conflict with future reserved
words, and if you use the C<use warnings> pragma or the B<-w> switch,
-Perl will warn you about any
-such words. Some people may wish to outlaw barewords entirely. If you
+Perl will warn you about any such words. Perl limits barewords (like
+identifiers) to about 250 characters. Future versions of Perl are likely
+to eliminate these arbitrary limitations.
+
+Some people may wish to outlaw barewords entirely. If you
say
use strict 'subs';
end of the enclosing block. An inner block may countermand this
by saying C<no strict 'subs'>.
+=head3 Array Interpolation
+X<array, interpolation> X<interpolation, array> X<$">
+
Arrays and slices are interpolated into double-quoted strings
by joining the elements with the delimiter specified in the C<$">
-variable (C<$LIST_SEPARATOR> in English), space by default. The
-following are equivalent:
+variable (C<$LIST_SEPARATOR> if "use English;" is specified),
+space by default. The following are equivalent:
$temp = join($", @ARGV);
system "echo $temp";
plain paranoid, you can force the correct interpretation with curly
braces as above.
-A line-oriented form of quoting is based on the shell "here-document"
-syntax. Following a C<< << >> you specify a string to terminate
-the quoted material, and all lines following the current line down to
-the terminating string are the value of the item. The terminating
-string may be either an identifier (a word), or some quoted text. If
-quoted, the type of quotes you use determines the treatment of the
-text, just as in regular quoting. An unquoted identifier works like
-double quotes. There must be no space between the C<< << >> and
-the identifier, unless the identifier is quoted. (If you put a space it
-will be treated as a null identifier, which is valid, and matches the first
-empty line.) The terminating string must appear by itself (unquoted and
-with no surrounding whitespace) on the terminating line.
-
- print <<EOF;
- The price is $Price.
- EOF
-
- print << "EOF"; # same as above
- The price is $Price.
- EOF
-
- print << `EOC`; # execute commands
- echo hi there
- echo lo there
- EOC
-
- print <<"foo", <<"bar"; # you can stack them
- I said foo.
- foo
- I said bar.
- bar
-
- myfunc(<< "THIS", 23, <<'THAT');
- Here's a line
- or two.
- THIS
- and here's another.
- THAT
-
-Just don't forget that you have to put a semicolon on the end
-to finish the statement, as Perl doesn't know you're not going to
-try to do this:
-
- print <<ABC
- 179231
- ABC
- + 20;
-
-If you want your here-docs to be indented with the
-rest of the code, you'll need to remove leading whitespace
-from each line manually:
-
- ($quote = <<'FINIS') =~ s/^\s+//gm;
- The Road goes ever on and on,
- down from the door where it began.
- FINIS
-
-If you use a here-doc within a delimited construct, such as in C<s///eg>,
-the quoted material must come on the lines following the final delimiter.
-So instead of
-
- s/this/<<E . 'that'
- the other
- E
- . 'more '/eg;
-
-you have to write
-
- s/this/<<E . 'that'
- . 'more '/eg;
- the other
- E
-
-If the terminating identifier is on the last line of the program, you
-must be sure there is a newline after it; otherwise, Perl will give the
-warning B<Can't find string terminator "END" anywhere before EOF...>.
-
-Additionally, the quoting rules for the identifier are not related to
-Perl's quoting rules -- C<q()>, C<qq()>, and the like are not supported
-in place of C<''> and C<"">, and the only interpolation is for backslashing
-the quoting character:
-
- print << "abc\"def";
- testing...
- abc"def
-
-Finally, quoted strings cannot span multiple lines. The general rule is
-that the identifier must be a string literal. Stick with that, and you
-should be safe.
+If you're looking for the information on how to use here-documents,
+which used to be here, that's been moved to
+L<perlop/Quote and Quote-like Operators>.
=head2 List value constructors
+X<list>
List values are denoted by separating individual values by commas
(and enclosing the list in parentheses where precedence requires it):
length of the array; the following assigns the value 3 to $foo:
@foo = ('cc', '-E', $bar);
- $foo = @foo; # $foo gets 3
+ $foo = @foo; # $foo gets 3
You may have an optional comma before the closing parenthesis of a
list literal, so that you can say:
@foo = (
- 1,
- 2,
- 3,
+ 1,
+ 2,
+ 3,
);
To use a here-document to assign an array, one line per element,
you might use an approach like this:
@sauces = <<End_Lines =~ m/(\S.*\S)/g;
- normal tomato
- spicy tomato
- green chile
- pesto
- white wine
+ normal tomato
+ spicy tomato
+ green chile
+ pesto
+ white wine
End_Lines
LISTs do automatic interpolation of sublists. That is, when a LIST is
This interpolation combines with the facts that the opening
and closing parentheses are optional (except when necessary for
precedence) and lists may end with an optional comma to mean that
-multiple commas within lists are legal syntax. The list C<1,,3> is a
+multiple commas within lists are legal syntax. The list C<1,,3> is a
concatenation of two lists, C<1,> and C<3>, the first of which ends
with that optional comma. C<1,,3> is C<(1,),(3)> is C<1,3> (And
similarly for C<1,,,3> is C<(1,),(,),3> is C<1,3> and so on.) Not that
($dev, $ino, undef, undef, $uid, $gid) = stat($file);
+As of Perl 5.22, you can also use C<(undef)x2> instead of C<undef, undef>.
+(You can also do C<($x) x 2>, which is less useful, because it assigns to
+the same variable twice, clobbering the first value assigned.)
+
+When you assign a list of scalars to an array, all previous values in that
+array are wiped out and the number of elements in the array will now be equal to
+the number of elements in the right-hand list -- the list from which
+assignment was made. The array will automatically resize itself to precisely
+accommodate each element in the right-hand list.
+
+ use warnings;
+ my (@xyz, $x, $y, $z);
+
+ @xyz = (1, 2, 3);
+ print "@xyz\n"; # 1 2 3
+
+ @xyz = ('al', 'be', 'ga', 'de');
+ print "@xyz\n"; # al be ga de
+
+ @xyz = (101, 102);
+ print "@xyz\n"; # 101 102
+
+When, however, you assign a list of scalars to another list of scalars, the
+results differ according to whether the left-hand list -- the list being
+assigned to -- has the same, more or fewer elements than the right-hand list.
+
+ ($x, $y, $z) = (1, 2, 3);
+ print "$x $y $z\n"; # 1 2 3
+
+ ($x, $y, $z) = ('al', 'be', 'ga', 'de');
+ print "$x $y $z\n"; # al be ga
+
+ ($x, $y, $z) = (101, 102);
+ print "$x $y $z\n"; # 101 102
+ # Use of uninitialized value $z in concatenation (.)
+ # or string at [program] line [line number].
+
+If the number of scalars in the left-hand list is less than that in the
+right-hand list, the "extra" scalars in the right-hand list will simply not be
+assigned.
+
+If the number of scalars in the left-hand list is greater than that in the
+left-hand list, the "missing" scalars will become undefined.
+
+ ($x, $y, $z) = (101, 102);
+ for my $el ($x, $y, $z) {
+ (defined $el) ? print "$el " : print "<undef>";
+ }
+ print "\n";
+ # 101 102 <undef>
+
List assignment in scalar context returns the number of elements
produced by the expression on the right side of the assignment:
- $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
- $x = (($foo,$bar) = f()); # set $x to f()'s return count
+ $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
+ $x = (($foo,$bar) = f()); # set $x to f()'s return count
This is handy when you want to do a list assignment in a Boolean
context, because most list functions return a null list when finished,
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;
It is often more readable to use the C<< => >> operator between key/value
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 identifier.
-This makes it nice for initializing hashes:
+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:
%map = (
- red => 0x00f,
- blue => 0x0f0,
- green => 0xf00,
+ red => 0x00f,
+ blue => 0x0f0,
+ green => 0xf00,
);
or for initializing hash references to be used as records:
$rec = {
- witch => 'Mable the Merciless',
- cat => 'Fluffy the Ferocious',
- date => '10/31/1776',
+ witch => 'Mable the Merciless',
+ cat => 'Fluffy the Ferocious',
+ date => '10/31/1776',
};
or for using call-by-named-parameter to complicated functions:
$field = $query->radio_group(
- name => 'group_name',
+ name => 'group_name',
values => ['eenie','meenie','minie'],
default => 'meenie',
linebreak => 'true',
mean that it comes out in that order. See L<perlfunc/sort> for examples
of how to arrange for an output ordering.
-=head2 Slices
+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:
-A common way to access an array or a hash is one scalar element at a
-time. You can also subscript a list to get a single element from it.
+ # values in %args take priority over %config_defaults
+ %config = (%config_defaults, %args);
- $whoami = $ENV{"USER"}; # one element from the hash
- $parent = $ISA[0]; # one element from the array
- $dir = (getpwnam("daemon"))[7]; # likewise, but with list
+=head2 Subscripts
+
+An array can be accessed one scalar at a
+time by specifying a dollar sign (C<$>), then the
+name of the array (without the leading C<@>), then the subscript inside
+square brackets. For example:
+
+ @myarray = (5, 50, 500, 5000);
+ print "The Third Element is", $myarray[2], "\n";
+
+The array indices start with 0. A negative subscript retrieves its
+value from the end. In our example, C<$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:
+
+ %scientists =
+ (
+ "Newton" => "Isaac",
+ "Einstein" => "Albert",
+ "Darwin" => "Charles",
+ "Feynman" => "Richard",
+ );
+
+ print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";
+
+You can also subscript a list to get a single element from it:
+
+ $dir = (getpwnam("daemon"))[7];
+
+=head2 Multi-dimensional array emulation
+
+Multidimensional arrays may be emulated by subscripting a hash with a
+list. The elements of the list are joined with the subscript separator
+(see L<perlvar/$;>).
+
+ $foo{$a,$b,$c}
+
+is equivalent to
+
+ $foo{join($;, $a, $b, $c)}
+
+The default subscript separator is "\034", the same as SUBSEP in B<awk>.
+
+=head2 Slices
+X<slice> X<array, slice> X<hash, slice>
A slice accesses several elements of a list, an array, or a hash
simultaneously using a list of subscripts. It's more convenient
than writing out the individual elements as a list of separate
scalar values.
- ($him, $her) = @folks[0,-1]; # array slice
- @them = @folks[0 .. 3]; # array slice
- ($who, $home) = @ENV{"USER", "HOME"}; # hash slice
- ($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice
+ ($him, $her) = @folks[0,-1]; # array slice
+ @them = @folks[0 .. 3]; # array slice
+ ($who, $home) = @ENV{"USER", "HOME"}; # hash slice
+ ($uid, $dir) = (getpwnam("daemon"))[2,7]; # list slice
Since you can assign to a list of variables, you can also assign to
an array or hash slice.
@days[3..5] = qw/Wed Thu Fri/;
@colors{'red','blue','green'}
- = (0xff0000, 0x0000ff, 0x00ff00);
+ = (0xff0000, 0x0000ff, 0x00ff00);
@folks[0, -1] = @folks[-1, 0];
The previous assignments are exactly equivalent to
($days[3], $days[4], $days[5]) = qw/Wed Thu Fri/;
($colors{'red'}, $colors{'blue'}, $colors{'green'})
- = (0xff0000, 0x0000ff, 0x00ff00);
- ($folks[0], $folks[-1]) = ($folks[0], $folks[-1]);
+ = (0xff0000, 0x0000ff, 0x00ff00);
+ ($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);
Since changing a slice changes the original array or hash that it's
slicing, a C<foreach> construct will alter some--or even all--of the
foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
- foreach (@hash{keys %hash}) {
- s/^\s+//; # trim leading whitespace
- s/\s+$//; # trim trailing whitespace
- s/(\w+)/\u\L$1/g; # "titlecase" words
+ foreach (@hash{qw[key1 key2]}) {
+ s/^\s+//; # trim leading whitespace
+ s/\s+$//; # trim trailing whitespace
+ s/(\w+)/\u\L$1/g; # "titlecase" words
}
-A slice of an empty list is still an empty list. Thus:
-
- @a = ()[1,0]; # @a has no elements
- @b = (@a)[0,1]; # @b has no elements
- @c = (0,1)[2,3]; # @c has no elements
-
-But:
+As a special exception, when you slice a list (but not an array or a hash),
+if the list evaluates to empty, then taking a slice of that empty list will
+always yield the empty list in turn. Thus:
- @a = (1)[1,0]; # @a has two elements
- @b = (1,undef)[1,0,2]; # @b has three elements
+ @a = ()[0,1]; # @a has no elements
+ @b = (@a)[0,1]; # @b has no elements
+ @c = (sub{}->())[0,1]; # @c has no elements
+ @d = ('a','b')[0,1]; # @d has two elements
+ @e = (@d)[0,1,8,9]; # @e has four elements
+ @f = (@d)[8,9]; # @f has two elements
This makes it easy to write loops that terminate when a null list
is returned:
- while ( ($home, $user) = (getpwent)[7,0]) {
- printf "%-8s %s\n", $user, $home;
+ while ( ($home, $user) = (getpwent)[7,0] ) {
+ printf "%-8s %s\n", $user, $home;
}
As noted earlier in this document, the scalar sense of list assignment
The null list contains no elements, so when the password file is
exhausted, the result is 0, not 2.
+Slices in scalar context return the last item of the slice.
+
+ @a = qw/first second third/;
+ %h = (first => 'A', second => 'B');
+ $t = @a[0, 1]; # $t is now 'second'
+ $u = @h{'first', 'second'}; # $u is now 'B'
+
If you're confused about why you use an '@' there on a hash slice
instead of a '%', think of it like this. The type of bracket (square
or curly) governs whether it's an array or a hash being looked at.
hash indicates whether you are getting back a singular value (a
scalar) or a plural one (a list).
+=head3 Key/Value Hash Slices
+
+Starting in Perl 5.20, a hash slice operation
+with the % symbol is a variant of slice operation
+returning a list of key/value pairs rather than just values:
+
+ %h = (blonk => 2, foo => 3, squink => 5, bar => 8);
+ %subset = %h{'foo', 'bar'}; # key/value hash slice
+ # %subset is now (foo => 3, bar => 8)
+ %removed = delete %h{'foo', 'bar'};
+ # %removed is now (foo => 3, bar => 8)
+ # %h is now (blonk => 2, squink => 5)
+
+However, the result of such a slice cannot be localized or used
+in assignment. These are otherwise very much consistent with hash slices
+using the @ symbol.
+
+=head3 Index/Value Array Slices
+
+Similar to key/value hash slices (and also introduced
+in Perl 5.20), the % array slice syntax returns a list
+of index/value pairs:
+
+ @a = "a".."z";
+ @list = %a[3,4,6];
+ # @list is now (3, "d", 4, "e", 6, "g")
+ @removed = delete %a[3,4,6]
+ # @removed is now (3, "d", 4, "e", 6, "g")
+ # @list[3,4,6] are now undef
+
+Note that calling L<C<delete>|perlfunc/delete EXPR> on array values is
+strongly discouraged.
+
=head2 Typeglobs and Filehandles
+X<typeglob> X<filehandle> X<*>
Perl uses an internal type called a I<typeglob> to hold an entire
symbol table entry. The type prefix of a typeglob is a C<*>, because
For example:
sub newopen {
- my $path = shift;
- local *FH; # not my!
- open (FH, $path) or return undef;
- return *FH;
+ my $path = shift;
+ local *FH; # not my!
+ open (FH, $path) or return undef;
+ return *FH;
}
$fh = newopen('/etc/passwd');
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:
sub myopen {
open my $fh, "@_"
- or die "Can't open '@_': $!";
- return $fh;
+ or die "Can't open '@_': $!";
+ return $fh;
}
{
my $f = myopen("</etc/motd");
- print <$f>;
- # $f implicitly closed here
+ print <$f>;
+ # $f implicitly closed here
}
+Note that if an initialized scalar variable is used instead the
+result is different: C<my $fh='zzz'; open($fh, ...)> is equivalent
+to C<open( *{'zzz'}, ...)>.
+C<use strict 'refs'> forbids such practice.
+
Another way to create anonymous filehandles is with the Symbol
module or with the IO::Handle module and its ilk. These modules
have the advantage of not hiding different types of the same name
-during the local(). See the bottom of L<perlfunc/open()> for an
+during the local(). See the bottom of L<perlfunc/open> for an
example.
=head1 SEE ALSO
https://perl5.git.perl.org / perl5.git / blobdiff