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 (i.e.
+a control character) 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.)
=head2 Context
int( <STDIN> )
-the integer operation provides scalar context for the E<lt><gt>
+the integer operation provides scalar context for the <>
operator, which responds by reading one line from STDIN and passing it
back to the integer operation, which will then find the integer value
of that line and return that. If, on the other hand, you say
sort( <STDIN> )
-then the sort operation provides list context for E<lt><gt>, which
+then the sort operation provides list context for <>, which
will proceed to read every line available up to the end of file, and
pass that list of lines back to the sort routine, which will then
sort those lines and return them as a list to whatever the context
context. Assignment to a list (or slice, which is just a list
anyway) also evaluates the righthand side in list context.
-When you use Perl's B<-w> command-line option, you may see warnings
+When you use the C<use warnings> pragma or Perl's B<-w> command-line
+option, you may see warnings
about useless uses of constants or functions in "void context".
Void context just means the value has been discarded, such as a
statement containing only C<"fred";> or C<getpwuid(0);>. It still
unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
The length of an array is a scalar value. You may find the length
-of array @days by evaluating C<$#days>, as in B<csh>. Technically
-speaking, this isn't the length of the array; it's the subscript
-of the last element, since there is ordinarily a 0th element.
+of array @days by evaluating C<$#days>, as in B<csh>. However, this
+isn't the length of the array; it's the subscript of the last element,
+which is a different value since there is ordinarily a 0th element.
Assigning to C<$#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.)
-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
10,000 of your items. This isn't supposed to happen.
You can preallocate space for a hash by assigning to the keys() function.
-This rounds up the allocated bucked to the next power of two:
+This rounds up the allocated buckets to the next power of two:
keys(%users) = 1000; # allocate 1024 buckets
12345
12345.67
.23E-10 # a very small number
- 4_294_967_296 # underline for legibility
+ 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
+You are allowed to use underscores (underbars) in numeric literals
+between digits for legibility. 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.
+
String literals are usually delimited by either single or double
quotes. They work much like quotes in the standard Unix shells:
double-quoted string literals are subject to backslash and variable
substitution; single-quoted strings are not (except for C<\'> and
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 Quotelike Operators"> for a list.
+forms. See L<perlop/"Quote and Quote-like Operators"> for a list.
Hexadecimal, octal, or binary, representations in string literals
(e.g. '0xff') are not automatically converted to their integer
print "The price is $Price.\n"; # interpreted
As in some shells, you can enclose the variable name in braces to
-disambiguate it from following alphanumerics. You must also do
+disambiguate it from following alphanumerics (and underscores).
+You must also do
this when interpolating a variable into a string to separate the
variable name from a following double-colon or an apostrophe, since
these would be otherwise treated as a package separator:
anything more complicated in the subscript will be interpreted as
an expression.
+A literal of the form C<v1.20.300.4000> is parsed as a string composed
+of characters with the specified ordinals. This form, known as
+v-strings, provides an alternative, more readable way to construct
+strings, rather than use the somewhat less readable interpolation form
+C<"\x{1}\x{14}\x{12c}\x{fa0}">. This is useful for representing
+Unicode strings, and for comparing version "numbers" using the string
+comparison operators, C<cmp>, C<gt>, C<lt> etc. If there are two or
+more dots in the literal, the leading C<v> may be omitted.
+
+ print v9786; # prints UTF-8 encoded SMILEY, "\x{263a}"
+ print v102.111.111; # prints "foo"
+ print 102.111.111; # same
+
+Such literals are accepted by both C<require> and C<use> for
+doing a version check. The C<$^V> special variable also contains the
+running Perl interpreter's version in this form. See L<perlvar/$^V>.
+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.
+
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
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 B<-w> switch, Perl will warn you about any
+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
say
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<E<lt>E<lt>> 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<E<lt>E<lt>> and
-the identifier. (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'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
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
interpolating an array with no elements is the same as if no
array had been interpolated at that point.
+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
+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
+we'd advise you to use this obfuscation.
+
A list value may also be subscripted like a normal array. You must
put the list in parentheses to avoid ambiguity. For example:
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,
which when assigned produces a 0, which is interpreted as FALSE.
-The final element may be an array or a hash:
+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:
+
+ $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
+context will translate that into the number of elements (here, the
+number of times the pattern matched) and assign that to $count. Note
+that simply using
+
+ $count = $string =~ /\d+/g;
+
+would not have worked, since a pattern match in scalar context will
+only return true or false, rather than a count of matches.
+
+The final element of a list assignment may be an array or a hash:
($a, $b, @rest) = split;
my($a, $b, %rest) = @_;
parameters lists and return lists from functions) always flatten out into
key/value pairs. That's why it's good to use references sometimes.
-It is often more readable to use the C<=E<gt>> operator between key/value
-pairs. The C<=E<gt>> operator is mostly just a more visually distinctive
+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:
%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',
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.
- $whoami = $ENV{"USER"}; # one element from the hash
- $parent = $ISA[0]; # one element from the array
- $dir = (getpwnam("daemon"))[7]; # likewise, but with list
+ $whoami = $ENV{"USER"}; # one element from the hash
+ $parent = $ISA[0]; # one element from the array
+ $dir = (getpwnam("daemon"))[7]; # likewise, but with list
A slice accesses several elements of a list, an array, or a hash
simultaneously using a list of subscripts. It's more convenient
than writing out the individual elements as a list of separate
scalar values.
- ($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'}, $colors{'green'})
- = (0xff0000, 0x0000ff, 0x00ff00);
+ = (0xff0000, 0x0000ff, 0x00ff00);
($folks[0], $folks[-1]) = ($folks[0], $folks[-1]);
Since changing a slice changes the original array or hash that it's
foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
foreach (@hash{keys %hash}) {
- s/^\s+//; # trim leading whitespace
- s/\s+$//; # trim trailing whitespace
- s/(\w+)/\u\L$1/g; # "titlecase" words
+ s/^\s+//; # trim leading whitespace
+ s/\s+$//; # trim trailing whitespace
+ s/(\w+)/\u\L$1/g; # "titlecase" words
}
A slice of an empty list is still an empty list. Thus:
is returned:
while ( ($home, $user) = (getpwent)[7,0]) {
- printf "%-8s %s\n", $user, $home;
+ printf "%-8s %s\n", $user, $home;
}
As noted earlier in this document, the scalar sense of list assignment
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');
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
+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
+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;
+ }
+
+ {
+ my $f = myopen("</etc/motd");
+ print <$f>;
+ # $f implicitly closed here
+ }
+
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
https://perl5.git.perl.org / perl5.git / blobdiff