perlsyn.pod

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=head1 NAME
X<syntax>

perlsyn - Perl syntax

=head1 DESCRIPTION

A Perl program consists of a sequence of declarations and statements
which run from the top to the bottom.  Loops, subroutines and other
control structures allow you to jump around within the code.

Perl is a B<free-form> language, you can format and indent it however
you like.  Whitespace mostly serves to separate tokens, unlike
languages like Python where it is an important part of the syntax.

Many of Perl's syntactic elements are B<optional>.  Rather than
requiring you to put parentheses around every function call and
declare every variable, you can often leave such explicit elements off
and Perl will figure out what you meant.  This is known as B<Do What I
Mean>, abbreviated B<DWIM>.  It allows programmers to be B<lazy> and to
code in a style with which they are comfortable.

Perl B<borrows syntax> and concepts from many languages: awk, sed, C,
Bourne Shell, Smalltalk, Lisp and even English.  Other
languages have borrowed syntax from Perl, particularly its regular
expression extensions.  So if you have programmed in another language
you will see familiar pieces in Perl.  They often work the same, but
see L<perltrap> for information about how they differ.

=head2 Declarations
X<declaration> X<undef> X<undefined> X<uninitialized>

The only things you need to declare in Perl are report formats and
subroutines (and sometimes not even subroutines).  A variable holds
the undefined value (C<undef>) until it has been assigned a defined
value, which is anything other than C<undef>.  When used as a number,
C<undef> is treated as C<0>; when used as a string, it is treated as
the empty string, C<"">; and when used as a reference that isn't being
assigned to, it is treated as an error.  If you enable warnings,
you'll be notified of an uninitialized value whenever you treat
C<undef> as a string or a number.  Well, usually.  Boolean contexts,
such as:

    my $a;
    if ($a) {}

are exempt from warnings (because they care about truth rather than
definedness).  Operators such as C<++>, C<-->, C<+=>,
C<-=>, and C<.=>, that operate on undefined left values such as:

    my $a;
    $a++;

are also always exempt from such warnings.

A declaration can be put anywhere a statement can, but has no effect on
the execution of the primary sequence of statements--declarations all
take effect at compile time.  Typically all the declarations are put at
the beginning or the end of the script.  However, if you're using
lexically-scoped private variables created with C<my()>, you'll
have to make sure
your format or subroutine definition is within the same block scope
as the my if you expect to be able to access those private variables.

Declaring a subroutine allows a subroutine name to be used as if it were a
list operator from that point forward in the program.  You can declare a
subroutine without defining it by saying C<sub name>, thus:
X<subroutine, declaration>

    sub myname;
    $me = myname $0 		or die "can't get myname";

Note that myname() functions as a list operator, not as a unary operator;
so be careful to use C<or> instead of C<||> in this case.  However, if
you were to declare the subroutine as C<sub myname ($)>, then
C<myname> would function as a unary operator, so either C<or> or
C<||> would work.

Subroutines declarations can also be loaded up with the C<require> statement
or both loaded and imported into your namespace with a C<use> statement.
See L<perlmod> for details on this.

A statement sequence may contain declarations of lexically-scoped
variables, but apart from declaring a variable name, the declaration acts
like an ordinary statement, and is elaborated within the sequence of
statements as if it were an ordinary statement.  That means it actually
has both compile-time and run-time effects.

=head2 Comments
X<comment> X<#>

Text from a C<"#"> character until the end of the line is a comment,
and is ignored.  Exceptions include C<"#"> inside a string or regular
expression.

=head2 Simple Statements
X<statement> X<semicolon> X<expression> X<;>

The only kind of simple statement is an expression evaluated for its
side effects.  Every simple statement must be terminated with a
semicolon, unless it is the final statement in a block, in which case
the semicolon is optional.  (A semicolon is still encouraged if the
block takes up more than one line, because you may eventually add
another line.)  Note that there are some operators like C<eval {}> and
C<do {}> that look like compound statements, but aren't (they're just
TERMs in an expression), and thus need an explicit termination if used
as the last item in a statement.

=head2 Truth and Falsehood
X<truth> X<falsehood> X<true> X<false> X<!> X<not> X<negation> X<0>

The number 0, the strings C<'0'> and C<''>, the empty list C<()>, and
C<undef> are all false in a boolean context. All other values are true.
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.

=head2 Statement Modifiers
X<statement modifier> X<modifier> X<if> X<unless> X<while>
X<until> X<foreach> X<for>

Any simple statement may optionally be followed by a I<SINGLE> modifier,
just before the terminating semicolon (or block ending).  The possible
modifiers are:

    if EXPR
    unless EXPR
    while EXPR
    until EXPR
    foreach LIST

The C<EXPR> following the modifier is referred to as the "condition".
Its truth or falsehood determines how the modifier will behave.

C<if> executes the statement once I<if> and only if the condition is
true.  C<unless> is the opposite, it executes the statement I<unless>
the condition is true (i.e., if the condition is false).

    print "Basset hounds got long ears" if length $ear >= 10;
    go_outside() and play() unless $is_raining;

The C<foreach> modifier is an iterator: it executes the statement once
for each item in the LIST (with C<$_> aliased to each item in turn).

    print "Hello $_!\n" foreach qw(world Dolly nurse);

C<while> repeats the statement I<while> the condition is true.
C<until> does the opposite, it repeats the statement I<until> the
condition is true (or while the condition is false):

    # Both of these count from 0 to 10.
    print $i++ while $i <= 10;
    print $j++ until $j >  10;

The C<while> and C<until> modifiers have the usual "C<while> loop"
semantics (conditional evaluated first), except when applied to a
C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE statement), in
which case the block executes once before the conditional is
evaluated.  This is so that you can write loops like:

    do {
	$line = <STDIN>;
	...
    } until $line  eq ".\n";

See L<perlfunc/do>.  Note also that the loop control statements described
later will I<NOT> work in this construct, because modifiers don't take
loop labels.  Sorry.  You can always put another block inside of it
(for C<next>) or around it (for C<last>) to do that sort of thing.
For C<next>, just double the braces:
X<next> X<last> X<redo>

    do {{
	next if $x == $y;
	# do something here
    }} until $x++ > $z;

For C<last>, you have to be more elaborate:
X<last>

    LOOP: { 
	    do {
		last if $x = $y**2;
		# do something here
	    } while $x++ <= $z;
    }

B<NOTE:> The behaviour of a C<my> statement modified with a statement
modifier conditional or loop construct (e.g. C<my $x if ...>) is
B<undefined>.  The value of the C<my> variable may be C<undef>, any
previously assigned value, or possibly anything else.  Don't rely on
it.  Future versions of perl might do something different from the
version of perl you try it out on.  Here be dragons.
X<my>

=head2 Compound Statements
X<statement, compound> X<block> X<bracket, curly> X<curly bracket> X<brace>
X<{> X<}> X<if> X<unless> X<while> X<until> X<foreach> X<for> X<continue>

In Perl, a sequence of statements that defines a scope is called a block.
Sometimes a block is delimited by the file containing it (in the case
of a required file, or the program as a whole), and sometimes a block
is delimited by the extent of a string (in the case of an eval).

But generally, a block is delimited by curly brackets, also known as braces.
We will call this syntactic construct a BLOCK.

The following compound statements may be used to control flow:

    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 until (EXPR) BLOCK
    LABEL until (EXPR) BLOCK continue BLOCK
    LABEL for (EXPR; EXPR; EXPR) BLOCK
    LABEL foreach VAR (LIST) BLOCK
    LABEL foreach VAR (LIST) BLOCK continue BLOCK
    LABEL BLOCK continue BLOCK

Note that, unlike C and Pascal, these are defined in terms of BLOCKs,
not statements.  This means that the curly brackets are I<required>--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:

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

The C<if> statement is straightforward.  Because BLOCKs are always
bounded by curly brackets, there is never any ambiguity about which
C<if> an C<else> goes with.  If you use C<unless> in place of C<if>,
the sense of the test is reversed.

The C<while> statement executes the block as long as the expression is
L<true|/"Truth and Falsehood">.
The C<until> statement executes the block as long as the expression is
false.
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 C<next>, C<last>, and C<redo>.
If the LABEL is omitted, the loop control statement
refers to the innermost enclosing loop.  This may include dynamically
looking back your call-stack at run time to find the LABEL.  Such
desperate behavior triggers a warning if you use the C<use warnings>
pragma or the B<-w> flag.

If there is a C<continue> BLOCK, it is always executed just before the
conditional is about to be evaluated again.  Thus it can be used to
increment a loop variable, even when the loop has been continued via
the C<next> statement.

=head2 Loop Control
X<loop control> X<loop, control> X<next> X<last> X<redo> X<continue>

The C<next> command starts the next iteration of the loop:

    LINE: while (<STDIN>) {
	next LINE if /^#/;	# discard comments
	...
    }

The C<last> command immediately exits the loop in question.  The
C<continue> block, if any, is not executed:

    LINE: while (<STDIN>) {
	last LINE if /^$/;	# exit when done with header
	...
    }

The C<redo> command restarts the loop block without evaluating the
conditional again.  The C<continue> block, if any, is I<not> executed.
This command is normally used by programs that want to lie to themselves
about what was just input.

For example, when processing a file like F</etc/termcap>.
If your input lines might end in backslashes to indicate continuation, you
want to skip ahead and get the next record.

    while (<>) {
	chomp;
	if (s/\\$//) {
	    $_ .= <>;
	    redo unless eof();
	}
	# now process $_
    }

which is Perl short-hand for the more explicitly written version:

    LINE: while (defined($line = <ARGV>)) {
	chomp($line);
	if ($line =~ s/\\$//) {
	    $line .= <ARGV>;
	    redo LINE unless eof(); # not eof(ARGV)!
	}
	# now process $line
    }

Note that if there were a C<continue> block on the above code, it would
get executed only on lines discarded by the regex (since redo skips the
continue block). A continue block is often used to reset line counters
or C<?pat?> one-time matches:

    # inspired by :1,$g/fred/s//WILMA/
    while (<>) {
	?(fred)?    && s//WILMA $1 WILMA/;
	?(barney)?  && s//BETTY $1 BETTY/;
	?(homer)?   && s//MARGE $1 MARGE/;
    } continue {
	print "$ARGV $.: $_";
	close ARGV  if eof();		# reset $.
	reset	    if eof();		# reset ?pat?
    }

If the word C<while> is replaced by the word C<until>, the sense of the
test is reversed, but the conditional is still tested before the first
iteration.

The loop control statements don't work in an C<if> or C<unless>, since
they aren't loops.  You can double the braces to make them such, though.

    if (/pattern/) {{
	last if /fred/;
	next if /barney/; # same effect as "last", but doesn't document as well
	# do something here
    }}

This is caused by the fact that a block by itself acts as a loop that
executes once, see L<"Basic BLOCKs">.

The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
available.   Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.

=head2 For Loops
X<for> X<foreach>

Perl's C-style C<for> loop works like the corresponding C<while> loop;
that means that this:

    for ($i = 1; $i < 10; $i++) {
	...
    }

is the same as this:

    $i = 1;
    while ($i < 10) {
	...
    } continue {
	$i++;
    }

There is one minor difference: if variables are declared with C<my>
in the initialization section of the C<for>, the lexical scope of
those variables is exactly the C<for> loop (the body of the loop
and the control sections).
X<my>

Besides the normal array index looping, C<for> can lend itself
to many other interesting applications.  Here's one that avoids the
problem you get into if you explicitly test for end-of-file on
an interactive file descriptor causing your program to appear to
hang.
X<eof> X<end-of-file> X<end of file>

    $on_a_tty = -t STDIN && -t STDOUT;
    sub prompt { print "yes? " if $on_a_tty }
    for ( prompt(); <STDIN>; prompt() ) {
	# do something
    }

Using C<readline> (or the operator form, C<< <EXPR> >>) as the
conditional of a C<for> loop is shorthand for the following.  This
behaviour is the same as a C<while> loop conditional.
X<readline> X<< <> >>

    for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
        # do something
    }

=head2 Foreach Loops
X<for> X<foreach>

The C<foreach> loop iterates over a normal list value and sets the
variable VAR to be each element of the list in turn.  If the variable
is preceded with the keyword C<my>, then it is lexically scoped, and
is therefore visible only within the loop.  Otherwise, the variable is
implicitly local to the loop and regains its former value upon exiting
the loop.  If the variable was previously declared with C<my>, it uses
that variable instead of the global one, but it's still localized to
the loop.  This implicit localisation occurs I<only> in a C<foreach>
loop.
X<my> X<local>

The C<foreach> keyword is actually a synonym for the C<for> keyword, so
you can use C<foreach> for readability or C<for> for brevity.  (Or because
the Bourne shell is more familiar to you than I<csh>, so writing C<for>
comes more naturally.)  If VAR is omitted, C<$_> is set to each value.
X<$_>

If any element of LIST is an lvalue, you can modify it by modifying
VAR inside the loop.  Conversely, if any element of LIST is NOT an
lvalue, any attempt to modify that element will fail.  In other words,
the C<foreach> loop index variable is an implicit alias for each item
in the list that you're looping over.
X<alias>

If any part of LIST is an array, C<foreach> will get very confused if
you add or remove elements within the loop body, for example with
C<splice>.   So don't do that.
X<splice>

C<foreach> probably won't do what you expect if VAR is a tied or other
special variable.   Don't do that either.

Examples:

    for (@ary) { s/foo/bar/ }

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

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

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

    foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
	print "Item: $item\n";
    }

Here's how a C programmer might code up a particular algorithm in Perl: