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
can truncate an array down to nothing by assigning the null list
() to it. The following are equivalent:
- my @whatever = ();
+ @whatever = ();
$#whatever = -1;
If you evaluate an array in scalar context, it returns the length
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
+
scalar(@whatever) == $#whatever + 1;
Some programmers choose to use an explicit conversion so as to
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:
- my %users = ();
keys(%users) = 1000; # allocate 1024 buckets
=head2 Scalar value constructors
expression as a subscript.) The following code segment prints out "The
price is $Z<>100."
- my $Price = '$100'; # not interpolated
- print "The price is $Price.\n"; # interpolated
+ $Price = '$100'; # not interpreted
+ 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 (and underscores).
variable name from a following double-colon or an apostrophe, since
these would be otherwise treated as a package separator:
- my $who = "Larry";
+ $who = "Larry";
print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
print "We use ${who}speak when ${who}'s here.\n";
variable (C<$LIST_SEPARATOR> in English), space by default. The
following are equivalent:
- my $temp = join($", @ARGV);
+ $temp = join($", @ARGV);
system "echo $temp";
system "echo @ARGV";
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:
-
- (my $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> in the section on
+L<Quote and Quote-like Operators>.
=head2 List value constructors
to be a list literal is simply the value of the final element, as
with the C comma operator. For example,
- my @foo = ('cc', '-E', $bar);
+ @foo = ('cc', '-E', $bar);
assigns the entire list value to array @foo, but
- my $foo = ('cc', '-E', $bar);
+ $foo = ('cc', '-E', $bar);
assigns the value of variable $bar to the scalar variable $foo.
Note that the value of an actual array in scalar context is the
length of the array; the following assigns the value 3 to $foo:
- my @foo = ('cc', '-E', $bar);
- my $foo = @foo; # $foo gets 3
+ @foo = ('cc', '-E', $bar);
+ $foo = @foo; # $foo gets 3
You may have an optional comma before the closing parenthesis of a
list literal, so that you can say:
- my @foo = (
- 1,
- 2,
- 3,
+ @foo = (
+ 1,
+ 2,
+ 3,
);
To use a here-document to assign an array, one line per element,
you might use an approach like this:
- my @sauces = <<End_Lines =~ m/(\S.*\S)/g;
- normal tomato
- spicy tomato
- green chile
- pesto
- white wine
+ @sauces = <<End_Lines =~ m/(\S.*\S)/g;
+ normal tomato
+ spicy tomato
+ green chile
+ pesto
+ white wine
End_Lines
LISTs do automatic interpolation of sublists. That is, when a LIST is
put the list in parentheses to avoid ambiguity. For example:
# Stat returns list value.
- my $time = (stat($file))[8];
+ $time = (stat($file))[8];
# SYNTAX ERROR HERE.
- my $time = stat($file)[8]; # OOPS, FORGOT PARENTHESES
+ $time = stat($file)[8]; # OOPS, FORGOT PARENTHESES
# Find a hex digit.
- my $hexdigit = ('a','b','c','d','e','f')[$digit-10];
+ $hexdigit = ('a','b','c','d','e','f')[$digit-10];
# A "reverse comma operator".
return (pop(@foo),pop(@foo))[0];
Lists may be assigned to only when each element of the list
is itself legal to assign to:
- my($a, $b, $c) = (1, 2, 3);
+ ($a, $b, $c) = (1, 2, 3);
- ($map{red}, $map{blue}, $map{green}) = (0x00f, 0x0f0, 0xf00);
+ ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
An exception to this is that you may assign to C<undef> in a list.
This is useful for throwing away some of the return values of a
function:
- my($dev, $ino, undef, undef, $uid, $gid) = stat($file);
+ ($dev, $ino, undef, undef, $uid, $gid) = stat($file);
List assignment in scalar context returns the number of elements
produced by the expression on the right side of the assignment:
- my $x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
- my $x = (($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,
return values, by assigning to an empty list and then using that
assignment in scalar context. For example, this code:
- my $count = () = $string =~ /\d+/g;
+ $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
number of times the pattern matched) and assign that to $count. Note
that simply using
- my $count = $string =~ /\d+/g;
+ $count = $string =~ /\d+/g;
would not have worked, since a pattern match in scalar context will
only return true or false, rather than a count of matches.
The final element of a list assignment may be an array or a hash:
- my($a, $b, @rest) = split;
- # or
+ ($a, $b, @rest) = split;
my($a, $b, %rest) = @_;
You can actually put an array or hash anywhere in the list, but the first one
items to be interpreted as a key and a value:
# same as map assignment above
- my %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
+ %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
While literal lists and named arrays are often interchangeable, that's
not the case for hashes. Just because you can subscript a list value like
interpreted as a string--if it's a bareword that would be a legal identifier.
This makes it nice for initializing hashes:
- my %map = (
- red => 0x00f,
- blue => 0x0f0,
- green => 0xf00,
+ %map = (
+ red => 0x00f,
+ blue => 0x0f0,
+ green => 0xf00,
);
or for initializing hash references to be used as records:
- my $rec = {
- witch => 'Mable the Merciless',
- cat => 'Fluffy the Ferocious',
- date => '10/31/1776',
+ $rec = {
+ witch => 'Mable the Merciless',
+ cat => 'Fluffy the Ferocious',
+ date => '10/31/1776',
};
or for using call-by-named-parameter to complicated functions:
- use CGI;
- my $query = CGI->new;
- my $field = $query->radio_group(
- name => 'group_name',
+ $field = $query->radio_group(
+ name => 'group_name',
values => ['eenie','meenie','minie'],
default => 'meenie',
linebreak => 'true',
- labels => \%labels,
+ labels => \%labels
);
Note that just because a hash is initialized in that order doesn't
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.
- my $whoami = $ENV{"USER"}; # one element from the hash
- my $parent = $ISA[0]; # one element from the array
- my $dir = (getpwnam("daemon"))[7]; # likewise, but with list
+ $whoami = $ENV{"USER"}; # one element from the hash
+ $parent = $ISA[0]; # one element from the array
+ $dir = (getpwnam("daemon"))[7]; # likewise, but with list
A slice accesses several elements of a list, an array, or a hash
simultaneously using a list of subscripts. It's more convenient
than writing out the individual elements as a list of separate
scalar values.
- my($him, $her) = @folks[0,-1]; # array slice
- my @them = @folks[0 .. 3]; # array slice
- my($who, $home) = @ENV{"USER", "HOME"}; # hash slice
- my($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, $dir) = (getpwnam("daemon"))[2,7]; # list slice
Since you can assign to a list of variables, you can also assign to
an array or hash slice.
- my( @days, %colors, @folks );
- @days[3..5] = qw(Wed Thu Fri);
+ @days[3..5] = qw/Wed Thu Fri/;
@colors{'red','blue','green'}
- = (0xff0000, 0x0000ff, 0x00ff00);
+ = (0xff0000, 0x0000ff, 0x00ff00);
@folks[0, -1] = @folks[-1, 0];
The previous assignments are exactly equivalent to
- my( @days, %colors, @folks );
- ($days[3], $days[4], $days[5]) = qw(Wed Thu Fri);
- ($colors{red}, $colors{blue}, $colors{green})
- = (0xff0000, 0x0000ff, 0x00ff00);
- ($folks[0], $folks[-1]) = ($folks[-1], $folks[0]);
+ ($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], $folks[-1]);
Since changing a slice changes the original array or hash that it's
slicing, a C<foreach> construct will alter some--or even all--of the
foreach (@array[ 4 .. 10 ]) { s/peter/paul/ }
foreach (@hash{keys %hash}) {
- s/^\s+//; # trim leading whitespace
- s/\s+$//; # trim trailing whitespace
- s/(\w+)/\u\L$1/g; # "titlecase" words
+ s/^\s+//; # trim leading whitespace
+ s/\s+$//; # trim trailing whitespace
+ s/(\w+)/\u\L$1/g; # "titlecase" words
}
A slice of an empty list is still an empty list. Thus:
- my @a = ()[1,0]; # @a has no elements
- my @b = (@a)[0,1]; # @b has no elements
- my @c = (0,1)[2,3]; # @c has no elements
+ @a = ()[1,0]; # @a has no elements
+ @b = (@a)[0,1]; # @b has no elements
+ @c = (0,1)[2,3]; # @c has no elements
But:
- my @a = (1)[1,0]; # @a has two elements
- my @b = (1,undef)[1,0,2]; # @b has three elements
+ @a = (1)[1,0]; # @a has two elements
+ @b = (1,undef)[1,0,2]; # @b has three elements
This makes it easy to write loops that terminate when a null list
is returned:
- while ( my($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 main use of typeglobs in modern Perl is create symbol table aliases.
This assignment:
- {
-
*this = *that;
makes $this an alias for $that, @this an alias for @that, %this an alias
make @Here::blue an alias for @There::green, or %Here::blue an alias for
%There::green, etc. See L<perlmod/"Symbol Tables"> for more examples
of this. Strange though this may seem, this is the basis for the whole
-module import/export system. And none of it works under
-C<use strict 'vars'>.
+module import/export system.
Another use for typeglobs is to pass filehandles into a function or
to create new filehandles. If you need to use a typeglob to save away
a filehandle, do it this way:
- my $fh = *STDOUT;
+ $fh = *STDOUT;
or perhaps as a real reference, like this:
- my $fh = \*STDOUT;
+ $fh = \*STDOUT;
See L<perlsub> for examples of using these as indirect filehandles
in functions.
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;
}
- my $fh = newopen('/etc/passwd');
+ $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
that must be passed around, as in the following example:
sub myopen {
- my $filename = shift;
- open my $fh, $filename
- or die "Can't open '$filename': $!";
- return $fh;
+ open my $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
}
Another way to create anonymous filehandles is with the Symbol