PERLSYN(1) Perl Programmers Reference Guide PERLSYN(1)NAMEperlsyn - Perl syntax
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 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 impor-
tant part of the syntax.
Many of Perl's syntactic elements are optional. Rather than
requiring you to put parentheses around every function call
and declare every variable, you can often leave such expli-
cit elements off and Perl will figure out what you meant.
This is known as Do What I Mean, abbreviated DWIM. It
allows programmers to be lazy and to code in a style with
which they are comfortable.
Perl 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 pro-
grammed in another language you will see familiar pieces in
Perl. They often work the same, but see perltrap for infor-
mation about how they differ.
Declarations
The only things you need to declare in Perl are report for-
mats and subroutines (and sometimes not even subroutines).
A variable holds the undefined value ("undef") until it has
been assigned a defined value, which is anything other than
"undef". When used as a number, "undef" is treated as 0;
when used as a string, it is treated as the empty string,
""; 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 "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 "++", "--",
"+=", "-=", and ".=", that operate on undefined left values
such as:
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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 "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 $0 or die "can't get myname";
Note that myname() functions as a list operator, not as a
unary operator; so be careful to use "or" instead of "||" in
this case. However, if you were to declare the subroutine
as "sub myname ($)", then "myname" would function 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 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 ela-
borated within the sequence of statements as if it were an
ordinary statement. That means it actually has both
compile-time and run-time effects.
Comments
Text from a "#" character until the end of the line is a
comment, and is ignored. Exceptions include "#" inside a
string or regular expression.
Simple Statements
The only kind of simple statement is an expression evaluated
for its side effects. Every simple statement must be ter-
minated with a semicolon, unless it is the final statement
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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 "eval {}" and "do
{}" that look like compound statements, but aren't (they're
just TERMs in an expression), and thus need an explicit ter-
mination if used as the last item in a statement.
Truth and Falsehood
The number 0, the strings '0' and '', the empty list "()",
and "undef" are all false in a boolean context. All other
values are true. Negation of a true value by "!" or "not"
returns a special false value. When evaluated as a string it
is treated as '', but as a number, it is treated as 0.
Statement Modifiers
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
foreach LIST
The "EXPR" following the modifier is referred to as the
"condition". Its truth or falsehood determines how the
modifier will behave.
"if" executes the statement once if and only if the condi-
tion is true. "unless" is the opposite, it executes the
statement unless the condition is true (i.e., if the condi-
tion is false).
print "Basset hounds got long ears" if length $ear >= 10;
go_outside() and play() unless $is_raining;
The "foreach" modifier is an iterator: it executes the
statement once for each item in the LIST (with $_ aliased to
each item in turn).
print "Hello $_!\n" foreach qw(world Dolly nurse);
"while" repeats the statement while the condition is true.
"until" does the opposite, it repeats the statement until
the condition is true (or while the condition is false):
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# Both of these count from 0 to 10.
print $i++ while $i <= 10;
print $j++ until $j > 10;
The "while" and "until" modifiers have the usual ""while"
loop" semantics (conditional evaluated first), except when
applied to a "do"-BLOCK (or to the 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 "do" in perlfunc. 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 put another block inside of it (for "next") or around
it (for "last") to do that sort of thing. For "next", just
double the braces:
do {{
next if $x == $y;
# do something here
}} until $x++ > $z;
For "last", you have to be more elaborate:
LOOP: {
do {
last if $x = $y**2;
# do something here
} while $x++ <= $z;
}
NOTE: The behaviour of a "my" statement modified with a
statement modifier conditional or loop construct (e.g. "my
$x if ...") is undefined. The value of the "my" variable
may be "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.
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 pro-
gram 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 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 brack-
ets 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: $!";
# a bit exotic, that last one
The "if" statement is straightforward. Because BLOCKs are
always bounded by curly brackets, there is never any ambi-
guity about which "if" an "else" goes with. If you use
"unless" in place of "if", the sense of the test is
reversed.
The "while" statement executes the block as long as the
expression is true (does not evaluate to the null string ""
or 0 or "0"). The "until" statement executes the block as
long as the expression is false. The LABEL is optional, and
if present, consists of an identifier followed by a colon.
The LABEL identifies the loop for the loop control state-
ments "next", "last", and "redo". If the LABEL is omitted,
the loop control statement refers to the innermost enclosing
loop. This may include dynamically looking back your call-
stack at run time to find the LABEL. Such desperate
behavior triggers a warning if you use the "use warnings"
pragma or the -w flag.
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If there is a "continue" BLOCK, it is always executed just
before the conditional is about to be evaluated again. Thus
it can be used to increment a loop variable, even when the
loop has been continued via the "next" statement.
Loop Control
The "next" command starts the next iteration of the loop:
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
...
}
The "last" command immediately exits the loop in question.
The "continue" block, if any, is not executed:
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
...
}
The "redo" command restarts the loop block without evaluat-
ing 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 con-
tinuation, 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 $line
}
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Note that if there were a "continue" block on the above
code, it would get executed only on lines discarded by the
regex (since redo skips the continue block). A continue
block is often used to reset line counters or "?pat?" one-
time matches:
# inspired by :1,$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 $.
reset if eof(); # reset ?pat?
}
If the word "while" is replaced by the word "until", the
sense of the test is reversed, but the conditional is still
tested before the first iteration.
The loop control statements don't work in an "if" or
"unless", since they aren't loops. You can double the
braces to make them such, though.
if (/pattern/) {{
last if /fred/;
next if /barney/; # same effect as "last", but doesn't document as well
# do something here
}}
This is caused by the fact that a block by itself acts as a
loop that executes once, see "Basic BLOCKs and Switch State-
ments".
The form "while/if BLOCK BLOCK", available in Perl 4, is no
longer available. Replace any occurrence of "if BLOCK" by
"if (do BLOCK)".
For Loops
Perl's C-style "for" loop works like the corresponding
"while" loop; that means that this:
for ($i = 1; $i < 10; $i++) {
...
}
is the same as this:
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$i = 1;
while ($i < 10) {
...
} continue {
$i++;
}
There is one minor difference: if variables are declared
with "my" in the initialization section of the "for", the
lexical scope of those variables is exactly the "for" loop
(the body of the loop and the control sections).
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
}
Using "readline" (or the operator form, "<EXPR>") as the
conditional of a "for" loop is shorthand for the following.
This behaviour is the same as a "while" loop conditional.
for ( prompt(); defined( $_ = <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. This implicit localisa-
tion occurs only in a "foreach" loop.
The "foreach" keyword is actually a synonym for the "for"
keyword, so you can use "foreach" for readability or "for"
for brevity. (Or because the Bourne shell is more familiar
to you than csh, so writing "for" comes more naturally.) If
VAR is omitted, $_ is set to each value.
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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 "foreach" loop index vari-
able is an implicit alias for each item in the list that
you're looping over.
If any part of LIST is an array, "foreach" will get very
confused if you add or remove elements within the loop body,
for example with "splice". So don't do that.
"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 algo-
rithm 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:
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OUTER: for my $wid (@ary1) {
INNER: for my $jet (@ary2) {
next OUTER if $wid > $jet;
$wid += $jet;
}
}
See how much easier this is? It's cleaner, safer, and fas-
ter. 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.
However, starting from Perl 5.8 to get switch and case one
can use the Switch extension and say:
use Switch;
after which one has switch and case. It is not as fast as
it could be because it's not really part of the language
(it's done using source filters) but it is available, and
it's very flexible.
In addition to the above BLOCK construct, 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 expres-
sion. That's just the binary comma operator in scalar con-
text. See "Comma Operator" in perlop.)
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;
}
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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" } # XXX: isn't this 0?
elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" }
elsif ($flag & O_RDWR) {
if ($flag & O_CREAT) { "w+" }
else { ($flag & O_APPEND) ? "a+" : "r+" }
}
};
Or
print do {
($flags & O_WRONLY) ? "write-only" :
($flags & O_RDWR) ? "read-write" :
"read-only";
};
Or if you are certain that all the "&&" clauses are true,
you can use something like this, which "switches" on the
value of the "HTTP_USER_AGENT" environment variable.
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#!/usr/bin/perl
# pick out jargon file page based on browser
$dir = 'http://www.wins.uva.nl/~mes/jargon';
for ($ENV{HTTP_USER_AGENT}) {
$page = /Mac/ && 'm/Macintrash.html'
|| /Win(dows )?NT/ && 'e/evilandrude.html'
|| /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html'
|| /Linux/ && 'l/Linux.html'
|| /HP-UX/ && 'h/HP-SUX.html'
|| /SunOS/ && 's/ScumOS.html'
|| 'a/AppendixB.html';
}
print "Location: $dir/$page\015\012\015\012";
That kind of switch statement only works when you know the
"&&" clauses will be true. If you don't, the previous "?:"
example should be used.
You might also consider writing a hash of subroutine refer-
ences instead of synthesizing a "switch" statement.
Goto
Although not for the faint of heart, Perl does support a
"goto" statement. There are three forms: "goto"-LABEL,
"goto"-EXPR, and "goto"-&NAME. A loop's LABEL is not actu-
ally a valid target for a "goto"; it's just the name of the
loop.
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 sub-
routine 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 "goto"s
per FORTRAN, but isn't necessarily recommended if you're
optimizing for maintainability:
goto(("FOO", "BAR", "GLARCH")[$i]);
The "goto"-&NAME form is highly magical, and substitutes a
call to the named subroutine for the currently running sub-
routine. This is used by "AUTOLOAD()" subroutines that wish
to load another subroutine and then pretend that the other
subroutine had been called in the first place (except that
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any modifications to @_ in the current subroutine are pro-
pagated to the other subroutine.) After the "goto", not
even "caller()" will be able to tell that this routine was
called first.
In almost all cases like this, it's usually a far, far
better idea to use the structured control flow mechanisms of
"next", "last", or "redo" instead of resorting to a "goto".
For certain applications, the catch and throw pair of
"eval{}" and die() for exception processing can also be a
prudent approach.
PODs: Embedded Documentation
Perl has a mechanism for intermixing documentation with
source code. While it's expecting the beginning of a new
statement, if the compiler encounters a line that begins
with an equal sign and a word, like this
=head1 Here There Be Pods!
Then that text and all remaining text up through and includ-
ing a line beginning with "=cut" will be ignored. The for-
mat of the intervening text is described in perlpod.
This allows you to intermix your source code and your docu-
mentation text freely, as in
=item snazzle($)
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 $thingie = shift;
.........
}
Note that pod translators should look at only paragraphs
beginning with a pod directive (it makes parsing easier),
whereas the compiler actually knows to look for pod escapes
even in the middle of a paragraph. This means that the fol-
lowing secret stuff will be ignored by both the compiler and
the translators.
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$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!)
Perl can process line directives, much like the C preproces-
sor. 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
# example: '# line 42 "new_filename.plx"'
/^\# \s*
line \s+ (\d+) \s*
(?:\s("?)([^"]+)\2)? \s*
$/x
with $1 being the line number for the next line, and $3
being the optional filename (specified with or without
quotes).
There is a fairly obvious gotcha included with the line
directive: Debuggers and profilers will only show the last
source line to appear at a particular line number in a given
file. Care should be taken not to cause line number colli-
sions in code you'd like to debug later.
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.
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% 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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