perl.man

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.fi
.PP
Array lists may be assigned to if and only if each element of the list
is an lvalue:
.nf

    ($a, $b, $c) = (1, 2, 3);

    ($map{\'red\'}, $map{\'blue\'}, $map{\'green\'}) = (0x00f, 0x0f0, 0xf00);

The final element may be an array or an associative array:

    ($a, $b, @rest) = split;
    local($a, $b, %rest) = @_;

.fi
You can actually put an array anywhere in the list, but the first array
in the list will soak up all the values, and anything after it will get
a null value.
This may be useful in a local().
.PP
An associative array literal contains pairs of values to be interpreted
as a key and a value:
.nf

.ne 2
    # same as map assignment above
    %map = ('red',0x00f,'blue',0x0f0,'green',0xf00);

.fi
Array assignment in a scalar context returns the number of elements
produced by the expression on the right side of the assignment:
.nf

	$x = (($foo,$bar) = (3,2,1));	# set $x to 3, not 2

.fi
.PP
There are several other pseudo-literals that you should know about.
If a string is enclosed by backticks (grave accents), it first undergoes
variable substitution just like a double quoted string.
It is then interpreted as a command, and the output of that command
is the value of the pseudo-literal, like in a shell.
In a scalar context, a single string consisting of all the output is
returned.
In an array context, an array of values is returned, one for each line
of output.
(You can set $/ to use a different line terminator.)
The command is executed each time the pseudo-literal is evaluated.
The status value of the command is returned in $? (see Predefined Names
for the interpretation of $?).
Unlike in \f2csh\f1, no translation is done on the return
data\*(--newlines remain newlines.
Unlike in any of the shells, single quotes do not hide variable names
in the command from interpretation.
To pass a $ through to the shell you need to hide it with a backslash.
.PP
Evaluating a filehandle in angle brackets yields the next line
from that file (newline included, so it's never false until EOF, at
which time an undefined value is returned).
Ordinarily you must assign that value to a variable,
but there is one situation where an automatic assignment happens.
If (and only if) the input symbol is the only thing inside the conditional of a
.I while
loop, the value is
automatically assigned to the variable \*(L"$_\*(R".
(This may seem like an odd thing to you, but you'll use the construct
in almost every
.I perl
script you write.)
Anyway, the following lines are equivalent to each other:
.nf

.ne 5
    while ($_ = <STDIN>) { print; }
    while (<STDIN>) { print; }
    for (\|;\|<STDIN>;\|) { print; }
    print while $_ = <STDIN>;
    print while <STDIN>;

.fi
The filehandles
.IR STDIN ,
.I STDOUT
and
.I STDERR
are predefined.
(The filehandles
.IR stdin ,
.I stdout
and
.I stderr
will also work except in packages, where they would be interpreted as
local identifiers rather than global.)
Additional filehandles may be created with the
.I open
function.
.PP
If a <FILEHANDLE> is used in a context that is looking for an array, an array
consisting of all the input lines is returned, one line per array element.
It's easy to make a LARGE data space this way, so use with care.
.PP
The null filehandle <> is special and can be used to emulate the behavior of
\fIsed\fR and \fIawk\fR.
Input from <> comes either from standard input, or from each file listed on
the command line.
Here's how it works: the first time <> is evaluated, the ARGV array is checked,
and if it is null, $ARGV[0] is set to \'-\', which when opened gives you standard
input.
The ARGV array is then processed as a list of filenames.
The loop
.nf

.ne 3
	while (<>) {
		.\|.\|.			# code for each line
	}

.ne 10
is equivalent to the following Perl-like pseudo code:

	unshift(@ARGV, \'\-\') \|if \|$#ARGV < $[;
	while ($ARGV = shift) {
		open(ARGV, $ARGV);
		while (<ARGV>) {
			.\|.\|.		# code for each line
		}
	}

.fi
except that it isn't as cumbersome to say, and will actually work.
It really does shift array ARGV and put the current filename into
variable ARGV.
It also uses filehandle ARGV internally\*(--<> is just a synonym for
<ARGV>, which is magical.
(The pseudo code above doesn't work because it treats <ARGV> as non-magical.)
.PP
You can modify @ARGV before the first <> as long as the array ends up
containing the list of filenames you really want.
Line numbers ($.) continue as if the input was one big happy file.
(But see example under eof for how to reset line numbers on each file.)
.PP
.ne 5
If you want to set @ARGV to your own list of files, go right ahead.
If you want to pass switches into your script, you can
put a loop on the front like this:
.nf

.ne 10
	while ($_ = $ARGV[0], /\|^\-/\|) {
		shift;
	    last if /\|^\-\|\-$\|/\|;
		/\|^\-D\|(.*\|)/ \|&& \|($debug = $1);
		/\|^\-v\|/ \|&& \|$verbose++;
		.\|.\|.		# other switches
	}
	while (<>) {
		.\|.\|.		# code for each line
	}

.fi
The <> symbol will return FALSE only once.
If you call it again after this it will assume you are processing another
@ARGV list, and if you haven't set @ARGV, will input from
.IR STDIN .
.PP
If the string inside the angle brackets is a reference to a scalar variable
(e.g. <$foo>),
then that variable contains the name of the filehandle to input from.
.PP
If the string inside angle brackets is not a filehandle, it is interpreted
as a filename pattern to be globbed, and either an array of filenames or the
next filename in the list is returned, depending on context.
One level of $ interpretation is done first, but you can't say <$foo>
because that's an indirect filehandle as explained in the previous
paragraph.
You could insert curly brackets to force interpretation as a
filename glob: <${foo}>.
Example:
.nf

.ne 3
	while (<*.c>) {
		chmod 0644, $_;
	}

is equivalent to

.ne 5
	open(foo, "echo *.c | tr \-s \' \et\er\ef\' \'\e\e012\e\e012\e\e012\e\e012\'|");
	while (<foo>) {
		chop;
		chmod 0644, $_;
	}

.fi
In fact, it's currently implemented that way.
(Which means it will not work on filenames with spaces in them unless
you have /bin/csh on your machine.)
Of course, the shortest way to do the above is:
.nf

	chmod 0644, <*.c>;

.fi
.Sh "Syntax"
.PP
A
.I perl
script consists of a sequence of declarations and commands.
The only things that need to be declared in
.I perl
are report formats and subroutines.
See the sections below for more information on those declarations.
All uninitialized user-created objects are assumed to
start with a null or 0 value until they
are defined by some explicit operation such as assignment.
The sequence of commands is executed just once, unlike in
.I sed
and
.I awk
scripts, where the sequence of commands is executed for each input line.
While this means that you must explicitly loop over the lines of your input file
(or files), it also means you have much more control over which files and which
lines you look at.
(Actually, I'm lying\*(--it is possible to do an implicit loop with either the
.B \-n
or
.B \-p
switch.)
.PP
A declaration can be put anywhere a command can, but has no effect on the
execution of the primary sequence of commands\*(--declarations all take effect
at compile time.
Typically all the declarations are put at the beginning or the end of the script.
.PP
.I Perl
is, for the most part, a free-form language.
(The only exception to this is format declarations, for fairly obvious reasons.)
Comments are indicated by the # character, and extend to the end of the line.
If you attempt to use /* */ C comments, it will be interpreted either as
division or pattern matching, depending on the context.
So don't do that.
.Sh "Compound statements"
In
.IR perl ,
a sequence of commands may be treated as one command by enclosing it
in curly brackets.
We will call this a BLOCK.
.PP
The following compound commands may be used to control flow:
.nf

.ne 4
	if (EXPR) BLOCK
	if (EXPR) BLOCK else BLOCK
	if (EXPR) BLOCK elsif (EXPR) BLOCK .\|.\|. else BLOCK
	LABEL while (EXPR) BLOCK
	LABEL while (EXPR) BLOCK continue BLOCK
	LABEL for (EXPR; EXPR; EXPR) BLOCK
	LABEL foreach VAR (ARRAY) BLOCK
	LABEL BLOCK continue BLOCK

.fi
Note that, unlike C and Pascal, these are defined in terms of BLOCKs, not
statements.
This means that the curly brackets are \fIrequired\fR\*(--no dangling statements allowed.
If you want to write conditionals without curly brackets there are several
other ways to do it.
The following all do the same thing:
.nf

.ne 5
	if (!open(foo)) { die "Can't open $foo: $!"; }
	die "Can't open $foo: $!" unless open(foo);
	open(foo) || die "Can't open $foo: $!";	# foo or bust!
	open(foo) ? \'hi mom\' : die "Can't open $foo: $!";
				# a bit exotic, that last one

.fi
.PP
The
.I if
statement is straightforward.
Since BLOCKs are always bounded by curly brackets, there is never any
ambiguity about which
.I if
an
.I else
goes with.
If you use
.I unless
in place of
.IR if ,
the sense of the test is reversed.
.PP
The
.I while
statement executes the block as long as the expression is true
(does not evaluate to the null string or 0).
The LABEL is optional, and if present, consists of an identifier followed by
a colon.
The LABEL identifies the loop for the loop control statements
.IR next ,
.IR last ,
and
.I redo
(see below).
If there is a
.I continue
BLOCK, it is always executed just before
the conditional is about to be evaluated again, similarly to the third part
of a
.I for
loop in C.
Thus it can be used to increment a loop variable, even when the loop has
been continued via the
.I next
statement (similar to the C \*(L"continue\*(R" statement).
.PP
If the word
.I while
is replaced by the word
.IR until ,
the sense of the test is reversed, but the conditional is still tested before
the first iteration.
.PP
In either the
.I if
or the
.I while
statement, you may replace \*(L"(EXPR)\*(R" with a BLOCK, and the conditional
is true if the value of the last command in that block is true.
.PP
The
.I for
loop works exactly like the corresponding
.I while
loop:
.nf

.ne 12
	for ($i = 1; $i < 10; $i++) {
		.\|.\|.
	}

is the same as

	$i = 1;
	while ($i < 10) {
		.\|.\|.
	} continue {
		$i++;
	}
.fi
.PP
The foreach loop iterates over a normal array value and sets the variable
VAR to be each element of the array in turn.
The variable is implicitly local to the loop, and regains its former value
upon exiting the loop.
The \*(L"foreach\*(R" keyword is actually identical to the \*(L"for\*(R" keyword,
so you can use \*(L"foreach\*(R" for readability or \*(L"for\*(R" for brevity.
If VAR is omitted, $_ is set to each value.
If ARRAY is an actual array (as opposed to an expression returning an array
value), you can modify each element of the array
by modifying VAR inside the loop.
Examples:
.nf

.ne 5
	for (@ary) { s/foo/bar/; }

	foreach $elem (@elements) {
		$elem *= 2;
	}

.ne 3
	for ((10,9,8,7,6,5,4,3,2,1,\'BOOM\')) {
		print $_, "\en"; sleep(1);
	}

	for (1..15) { print "Merry Christmas\en"; }

.ne 3
	foreach $item (split(/:[\e\e\en:]*/, $ENV{\'TERMCAP\'})) {
		print "Item: $item\en";
	}

.fi
.PP
The BLOCK by itself (labeled or not) is equivalent to a loop that executes
once.
Thus you can use any of the loop control statements in it to leave or
restart the block.
The
.I continue
block is optional.
This construct is particularly nice for doing case structures.
.nf

.ne 6
	foo: {
		if (/^abc/) { $abc = 1; last foo; }
		if (/^def/) { $def = 1; last foo; }
		if (/^xyz/) { $xyz = 1; last foo; }
		$nothing = 1;
	}

.fi
There is no official switch statement in perl, because there
are already several ways to write the equivalent.
In addition to the above, you could write
.nf

.ne 6
	foo: {
		$abc = 1, last foo  if /^abc/;
		$def = 1, last foo  if /^def/;
		$xyz = 1, last foo  if /^xyz/;