PERLSYN(1)
NNAAMMEE
perlsyn - Perl syntax
DDEESSCCRRIIPPTTIIOONN
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 sseedd and aawwkk 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 --nn or --pp switch. It's just not the
mandatory default like it is in sseedd and aawwkk.)
DDeeccllaarraattiioonnss
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.
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.
SSiimmppllee ssttaatteemmeennttss
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.
CCoommppoouunndd ssttaatteemmeennttss
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 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: $!";
# a bit exotic, that last one
The if statement is straightforward. Because BLOCKs are
always bounded by curly brackets, there is never any
ambiguity about which if an else goes with. If you use
unless in place of if, the sense of the test is reversed.
The while statement executes the block as long as the
expression is true (does not evaluate to the null string
or 0 or "0"). The LABEL is optional, and if present,
consists of an identifier followed by a colon. The LABEL
identifies the loop for the loop control statements next,
last, and redo. If the LABEL is omitted, the loop control
statement refers to the innermost enclosing loop. This
may include dynamically looking back your call-stack at
run time to find the LABEL. Such desperate behavior
triggers a warning if you use the --ww flag.
If there is a continue BLOCK, it is always executed just
before the conditional is about to be evaluated again,
just like the third part of a for loop in C. Thus it can
be used to increment a loop variable, even when the loop
has been continued via the next statement (which is
similar to the C continue statement).
LLoooopp CCoonnttrrooll
The next command is like the continue statement in C; it
starts the next iteration of the loop:
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
...
}
The last command is like the break statement in C (as used
in loops); it immediately exits the loop in question. The
continue block, if any, is not executed:
LINE: while (<STDIN>) {
last LINE if /^$/; # 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).
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).
FFoorr LLooooppss
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.
$on_a_tty = -t STDIN && -t STDOUT;
sub prompt { print "yes? " if $on_a_tty }
for ( prompt(); <STDIN>; prompt() ) {
# do something
}
FFoorreeaacchh LLooooppss
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:
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.
BBaassiicc BBLLOOCCKKss aanndd SSwwiittcchh SSttaatteemmeennttss
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
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,
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 { ($flag & O_APPEND) ? "a+" : "r+" }
}
};
GGoottoo
Although not for the faint of heart, Perl does support a
goto statement. A loop's LABEL is not actually a valid
target for a goto; it's just the name of the loop. There
are three forms: goto-LABEL, goto-EXPR, and goto-&NAME.
The goto-LABEL form finds the statement labeled with LABEL
and resumes execution there. It may not be used to go
into any construct that requires initialization, such as a
subroutine or a foreach loop. It also can't be used to go
into a construct that is optimized away. It can be used
to go almost anywhere else within the dynamic scope,
including out of subroutines, but it's usually better to
use some other construct such as last or die. The author
of Perl has never felt the need to use this form of goto
(in Perl, that is--C is another matter).
The goto-EXPR form expects a label name, whose scope will
be resolved dynamically. This allows for computed gotos
per FORTRAN, but isn't necessarily recommended if you're
optimizing for maintainability:
goto ("FOO", "BAR", "GLARCH")[$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.
PPOODDss:: EEmmbbeeddddeedd DDooccuummeennttaattiioonn
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.
$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.
PPllaaiinn OOlldd CCoommmmeennttss ((NNoott!!))
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.