PERLSYN(1)PERLSYN(1)NAMEperlsyn - 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.)
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.
As of Perl 5, 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
just
sub myname;
$me = myname $0 or die "can't get myname";
Note that it functions as a list operator though, not a unary operator,
so be careful to use or instead of ⎪⎪ there.
Subroutines declarations can also be imported by a use statement.
Also as of Perl 5, 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.
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, since 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 {
$_ = <STDIN>;
...
} until $_ 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,
since 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).
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: $!";
# a bit exotic, that last one
The if statement is straightforward. Since BLOCKs are always bounded
by curly brackets, there is never any ambiguity about which if an else
goes with. If you use unless in place of if, the sense of the test is
reversed.
The while statement executes the block as long as the expression is
true (does not evaluate to the null string or 0 or "0"). The LABEL is
optional, and if present, consists of an identifier followed by a
colon. The LABEL identifies the loop for the loop control statements
next, last, and redo (see below). If there is a continue BLOCK, it is
always executed just before the conditional is about to be evaluated
again, just like the third part of a for loop in C. Thus it can be
used to increment a loop variable, even when the loop has been
continued via the next statement (which is similar to the C continue
statement).
If the word while is replaced by the word until, the sense of the test
is reversed, but the conditional is still tested before the first
iteration.
In either the if or the while statement, you may replace "(EXPR)" with
a BLOCK, and the conditional is true if the value of the last statement
in that block is true. (This feature continues to work in Perl 5 but
is deprecated. Please change any occurrences of "if BLOCK" to "if (do
BLOCK)".)
The C-style for loop works exactly like the corresponding while loop:
for ($i = 1; $i < 10; $i++) {
...
}
is the same as
$i = 1;
while ($i < 10) {
...
} continue {
$i++;
}
The foreach loop iterates over a normal list value and sets the
variable VAR to be each element of the list in turn. 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.) 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. Examples:
for (@ary) { s/foo/bar/; }
foreach $elem (@elements) {
$elem *= 2;
}
for ((10,9,8,7,6,5,4,3,2,1,'BOOM')) {
print $_, "\n"; sleep(1);
}
for (1..15) { print "Merry Christmas\n"; }
foreach $item (split(/:[\\\n:]*/, $ENV{'TERMCAP'})) {
print "Item: $item\n";
}
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. The continue block is
optional. This 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 one 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 }
3rd Berkeley DistributionPERLSYN(1)
