PERLSYN(1) PERLSYN(1)
NAME
perlsyn - Perl syntax
DESCRIPTION
A Perl script consists of a sequence of declarations and statements. The
only things that need to be declared in 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. (Though you can get warnings about the use
of undefined values if you like.) The sequence of statements is executed
just once, unlike in sed and awk scripts, where the sequence of
statements 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 -n or -p switch. It's just not the mandatory default
like it is in sed and awk.)
Declarations
Perl is, for the most part, a free-form language. (The only exception to
this is format declarations, for obvious reasons.) Comments are indicated
by the "#" character, and extend to the end of the line. If you attempt
to use /* */ C-style comments, it will be interpreted either as division
or pattern matching, depending on the context, and C++ // comments just
look like a null regular expression, so don't do that.
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 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 sub name, thus:
sub myname;
$me = myname $0 or die "can't get myname";
Note that it functions as a list operator, not as a unary operator; so be
careful to use or instead of || in this case. However, if you were to
declare the subroutine as sub myname ($), then myname would functonion as
a unary operator, so either or or || would work.
Subroutines declarations can also be loaded up with the require statement
or both loaded and imported into your namespace with a use statement.
See the perlmod manpage for details on this.
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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.
Simple statements
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 there if the block takes
up more than one line, because you may eventually add another line.)
Note that there are some operators like eval {} and 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.
Any simple statement may optionally be followed by a SINGLE modifier,
just before the terminating semicolon (or block ending). The possible
modifiers are:
if EXPR
unless EXPR
while EXPR
until EXPR
The if and unless modifiers have the expected semantics, presuming you're
a speaker of English. The while and until modifiers also have the usual
"while loop" semantics (conditional evaluated first), except when applied
to a do-BLOCK (or to the now-deprecated 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 the do entry in the perlfunc manpage. Note also that the loop
control statements described later will NOT work in this construct,
because modifiers don't take loop labels. Sorry. You can always wrap
another block around it to do that sort of thing.
Compound statements
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).
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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 for (EXPR; EXPR; EXPR) BLOCK
LABEL foreach VAR (LIST) 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 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: $!";
# a bit exotic, that last one
The if statement is straightforward. Because BLOCKs are always bounded
by curly brackets, there is never any ambiguity about which if an else
goes with. If you use unless in place of if, the sense of the test is
reversed.
The while statement executes the block as long as the expression is true
(does not evaluate to the null string or 0 or "0"). The LABEL is
optional, and if present, consists of an identifier followed by a colon.
The LABEL identifies the loop for the loop control statements next, last,
and redo. If the LABEL is omitted, the loop control statement refers to
the innermost enclosing loop. This may include dynamically looking back
your call-stack at run time to find the LABEL. Such desperate behavior
triggers a warning if you use the -w flag.
If there is a continue BLOCK, it is always executed just before the
conditional is about to be evaluated again, just like the third part of a
for loop in C. Thus it can be used to increment a loop variable, even
when the loop has been continued via the next statement (which is similar
to the C continue statement).
Loop Control
The next command is like the continue statement in C; it starts the next
iteration of the loop:
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LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
...
}
The last command is like the break statement in C (as used in loops); it
immediately exits the loop in question. The continue block, if any, is
not executed:
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
...
}
The redo command restarts the loop block without evaluating the
conditional again. The continue block, if any, is 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 /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
}
Or here's a simpleminded Pascal comment stripper (warning: assumes no {
or } in strings).
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LINE: while (<STDIN>) {
while (s|({.*}.*){.*}|$1 |) {}
s|{.*}| |;
if (s|{.*| |) {
$front = $_;
while (<STDIN>) {
if (/}/) { # end of comment?
s|^|$front{|;
redo LINE;
}
}
}
print;
}
Note that if there were a continue block on the above code, it would get
executed even on discarded lines.
If the word while is replaced by the word until, the sense of the test is
reversed, but the conditional is still tested before the first iteration.
The form while/if BLOCK BLOCK, available in Perl 4, is no longer
available. Replace any occurrence of if BLOCK by if (do BLOCK).
For Loops
Perl's C-style for loop works exactly like the corresponding while loop;
that means that this:
for ($i = 1; $i < 10; $i++) {
...
}
is the same as this:
$i = 1;
while ($i < 10) {
...
} continue {
$i++;
}
(There is one minor difference: The first form implies a lexical scope
for variables declared with my in the initialization expression.)
Besides the normal array index looping, for can lend itself to many other
interesting applications. Here's one that avoids the problem you get
into if you explicitly test for end-of-file on an interactive file
descriptor causing your program to appear to hang.
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$on_a_tty = -t STDIN && -t STDOUT;
sub prompt { print "yes? " if $on_a_tty }
for ( prompt(); <STDIN>; prompt() ) {
# do something
}
Foreach Loops
The 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 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 my, it uses that variable
instead of the global one, but it's still localized to the loop. (Note
that a lexically scoped variable can cause problems with you have
subroutine or format declarations.)
The foreach keyword is actually a synonym for the for keyword, so you can
use foreach for readability or for for brevity. If VAR is omitted, $_ is
set to each value. If LIST is an actual array (as opposed to an
expression returning a list value), you can modify each element of the
array by modifying VAR inside the loop. That's because the foreach loop
index variable is an implicit alias for each item in the list that you're
looping over.
Examples:
for (@ary) { s/foo/bar/ }
foreach 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:
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for (my $i = 0; $i < @ary1; $i++) {
for (my $j = 0; $j < @ary2; $j++) {
if ($ary1[$i] > $ary2[$j]) {
last; # can't go to outer :-(
}
$ary1[$i] += $ary2[$j];
}
# this is where that last takes me
}
Whereas here's how a Perl programmer more comfortable with the idiom
might do it:
OUTER: foreach my $wid (@ary1) {
INNER: foreach my $jet (@ary2) {
next OUTER if $wid > $jet;
$wid += $jet;
}
}
See how much easier this is? It's cleaner, safer, and faster. It's
cleaner because it's less noisy. It's safer because if code gets added
between the inner and outer loops later on, the new code won't be
accidentally executed. The next explicitly iterates the other loop
rather than merely terminating the inner one. And it's faster because
Perl executes a foreach statement more rapidly than it would the
equivalent for loop.
Basic BLOCKs and Switch Statements
A BLOCK by itself (labeled or not) is semantically 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. (Note that this is NOT true in
eval{}, sub{}, or contrary to popular belief do{} blocks, which do NOT
count as loops.) The continue block is optional.
The BLOCK construct is particularly nice for doing case structures.
SWITCH: {
if (/^abc/) { $abc = 1; last SWITCH; }
if (/^def/) { $def = 1; last SWITCH; }
if (/^xyz/) { $xyz = 1; last SWITCH; }
$nothing = 1;
}
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
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SWITCH: {
$abc = 1, last SWITCH if /^abc/;
$def = 1, last SWITCH if /^def/;
$xyz = 1, last SWITCH if /^xyz/;
$nothing = 1;
}
(That's actually not as strange as it looks once you realize that you can
use loop control "operators" within an expression, That's just the
normal C comma operator.)
or
SWITCH: {
/^abc/ && do { $abc = 1; last SWITCH; };
/^def/ && do { $def = 1; last SWITCH; };
/^xyz/ && do { $xyz = 1; last SWITCH; };
$nothing = 1;
}
or formatted so it stands out more as a "proper" switch statement:
SWITCH: {
/^abc/ && do {
$abc = 1;
last SWITCH;
};
/^def/ && do {
$def = 1;
last SWITCH;
};
/^xyz/ && do {
$xyz = 1;
last SWITCH;
};
$nothing = 1;
}
or
SWITCH: {
/^abc/ and $abc = 1, last SWITCH;
/^def/ and $def = 1, last SWITCH;
/^xyz/ and $xyz = 1, last SWITCH;
$nothing = 1;
}
or even, horrors,
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if (/^abc/)
{ $abc = 1 }
elsif (/^def/)
{ $def = 1 }
elsif (/^xyz/)
{ $xyz = 1 }
else
{ $nothing = 1 }
A common idiom for a switch statement is to use foreach's aliasing to
make a temporary assignment to $_ for convenient matching:
SWITCH: for ($where) {
/In Card Names/ && do { push @flags, '-e'; last; };
/Anywhere/ && do { push @flags, '-h'; last; };
/In Rulings/ && do { last; };
die "unknown value for form variable where: `$where'";
}
Another interesting approach to a switch statement is arrange for a do
block to return the proper value:
$amode = do {
if ($flag & O_RDONLY) { "r" }
elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" }
elsif ($flag & O_RDWR) {
if ($flag & O_CREAT) { "w+" }
else { ($flag & O_APPEND) ? "a+" : "r+" }
}
};
Goto
Although not for the faint of heart, Perl does support a goto statement.
A loop's LABEL is not actually a valid target for a goto; it's just the
name of the loop. There are three forms: goto-LABEL, goto-EXPR, and
goto-&NAME.
The goto-LABEL form finds the statement labeled with LABEL and resumes
execution there. It may not be used to go into any construct that
requires initialization, such as a subroutine or a foreach loop. It also
can't be used to go into a construct that is optimized away. It can be
used to go almost anywhere else within the dynamic scope, including out
of subroutines, but it's usually better to use some other construct such
as last or die. The author of Perl has never felt the need to use this
form of goto (in Perl, that is--C is another matter).
The goto-EXPR form expects a label name, whose scope will be resolved
dynamically. This allows for computed gotos per FORTRAN, but isn't
necessarily recommended if you're optimizing for maintainability:
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goto ("FOO", "BAR", "GLARCH")[$i];
The goto-&NAME form is highly magical, and substitutes a call to the
named subroutine for the currently running subroutine. This is used by
AUTOLOAD() subroutines that wish to load another subroutine and then
pretend that the other subroutine had been called in the first place
(except that any modifications to @_ in the current subroutine are
propagated to the other subroutine.) After the goto, not even caller()
will be able to tell that this routine was called first.
In almost all cases like this, it's usually a far, far better idea to use
the structured control flow mechanisms of next, last, or redo instead of
resorting to a goto. For certain applications, the catch and throw pair
of eval{} and die() for exception processing can also be a prudent
approach.
PODs: Embedded Documentation
Perl has a mechanism for intermixing documentation with source code.
While it's expecting the beginning of a new statement, if the compiler
encounters a line that begins with an equal sign and a word, like this
=head1 Here There Be Pods!
Then that text and all remaining text up through and including a line
beginning with =cut will be ignored. The format of the intervening text
is described in the perlpod manpage.
This allows you to intermix your source code and your documentation text
freely, as in
=item snazzle($)
The snazzle() function will behave in the most spectacular
form that you can possibly imagine, not even excepting
cybernetic pyrotechnics.
=cut back to the compiler, nuff of this pod stuff!
sub snazzle($) {
my $thingie = shift;
.........
}
Note that pod translators should look at only paragraphs beginning with a
pod directive (it makes parsing easier), whereas the compiler actually
knows to look for pod escapes even in the middle of a paragraph. This
means that the following secret stuff will be ignored by both the
compiler and the translators.
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$a=3;
=secret stuff
warn "Neither POD nor CODE!?"
=cut back
print "got $a\n";
You probably shouldn't rely upon the warn() being podded out forever.
Not all pod translators are well-behaved in this regard, and perhaps the
compiler will become pickier.
One may also use pod directives to quickly comment out a section of code.
Plain Old Comments (Not!)
Much like the C preprocessor, perl can process line directives. Using
this, one can control perl's idea of filenames and line numbers in error
or warning messages (especially for strings that are processed with
eval()). The syntax for this mechanism is the same as for most C
preprocessors: it matches the regular expression
/^#\s*line\s+(\d+)\s*(?:\s"([^"]*)")?/ with $1 being the line number for
the next line, and $2 being the optional filename (specified within
quotes).
Here are some examples that you should be able to type into your command
shell:
% perl
# line 200 "bzzzt"
# the `#' on the previous line must be the first char on line
die 'foo';
__END__
foo at bzzzt line 201.
% perl
# line 200 "bzzzt"
eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
__END__
foo at - line 2001.
% perl
eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
__END__
foo at foo bar line 200.
% perl
# line 345 "goop"
eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
print $@;
__END__
foo at goop line 345.
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