PERLSYN(1) Perl Programmers Reference Guide PERLSYN(1)NAMEperlsyn - Perl syntax
DESCRIPTION
A Perl script consists of a sequence of declarations and
statements. 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.) Text from a ""#"" character until the end of
the line is a comment, and is ignored. 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 regu
lar expression, so don't do that.
Declarations
The only things you need to declare in Perl are report
formats and subroutines--and even undefined subroutines
can be handled through AUTOLOAD. 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 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
("don't-care") contexts and operators such as "++", "--",
"+=", "-=", and ".=" are 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 lexi
cally-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 defin
ing it by saying "sub name", thus:
sub myname;
$me = myname $0 or die "can't get myname";
Note that myname() functions as a list operator, not as a
unary operator; so be careful to use "or" instead of "||"
in this case. However, if you were to declare the subrou
tine as "sub myname ($)", 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 the perlmod manpage
for details on this.
A statement sequence may contain declarations of lexi
cally-scoped variables, but apart from declaring a vari
able name, the declaration acts like an ordinary state
ment, and is elaborated within the sequence of statements
as if it were an ordinary statement. That means it actu
ally has both compile-time and run-time effects.
Simple statements
The only kind of simple statement is an expression evalu
ated for its side effects. Every simple statement must be
terminated with a semicolon, unless it is the final state
ment 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 SIN_
GLE modifier, just before the terminating semicolon (or
block ending). The possible modifiers are:
if EXPR
unless EXPR
while EXPR
until EXPR
foreach EXPR
The "if" and "unless" modifiers have the expected seman
tics, presuming you're a speaker of English. The "fore
ach" modifier is an iterator: For each value in EXPR, it
aliases "$_" to the value and executes the statement. 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"-SUBROU
TINE 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 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;
}
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 con
struct 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 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 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: $!";
# a bit exotic, that last one
The "if" statement is straightforward. Because BLOCKs are
always bounded by curly brackets, there is never any ambi
guity about which "if" an "else" goes with. If you use
"unless" in place of "if", the sense of the test is
reversed.
The "while" statement executes the block as long as the
expression is true (does not evaluate to the null string
"""" or "0" or ""0""). The LABEL is optional, and if pre
sent, consists of an identifier followed by a colon. The
LABEL identifies the loop for the loop control statements
"next", "last", and "redo". If the LABEL is omitted, the
loop control statement refers to the innermost enclosing
loop. This may include dynamically looking back your
call-stack at run time to find the LABEL. Such desperate
behavior triggers a warning if you use the "use warnings"
pragma or the -w flag. Unlike a "foreach" statement, a
"while" statement never implicitly localises any
variables.
If there is a "continue" BLOCK, it is always executed just
before the conditional is about to be evaluated again,
just like the third part of a "for" loop in C. Thus it
can be used to increment a loop variable, even when the
loop has been continued via the "next" statement (which is
similar to the C "continue" statement).
Loop Control
The "next" command is like the "continue" statement in C;
it starts the next iteration of the loop:
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
...
}
The "last" command is like the "break" statement in C (as
used in loops); it immediately exits the loop in question.
The "continue" block, if any, is not executed:
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
...
}
The "redo" command restarts the loop block without evalu
ating 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 $line
}
Note that if there were a "continue" block on the above
code, it would get executed even on discarded lines. This
is often used to reset line counters or "?pat?" one-time
matches.
# inspired by :1,$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 $.
reset if eof(); # reset ?pat?
}
If the word "while" is replaced by the word "until", the
sense of the test is reversed, but the conditional is
still tested before the first iteration.
The loop control statements don't work in an "if" or
"unless", since they aren't loops. You can double the
braces to make them such, though.
if (/pattern/) {{
next if /fred/;
next if /barney/;
# so something here
}}
The form "while/if BLOCK BLOCK", available in Perl 4, is
no longer available. Replace any occurrence of "if
BLOCK" by "if (do BLOCK)".
For Loops
Perl's C-style "for" loop works like the corresponding
"while" loop; that means that this:
for ($i = 1; $i < 10; $i++) {
...
}
is the same as this:
$i = 1;
while ($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 $on_a_tty }
for ( prompt(); <STDIN>; prompt() ) {
# do something
}
Foreach Loops
The "foreach" loop iterates over a normal list value and
sets the variable VAR to be each element of the list in
turn. If the variable is preceded with the keyword "my",
then it is lexically scoped, and is therefore visible only
within the loop. Otherwise, the variable is implicitly
local to the loop and regains its former value upon exit
ing the loop. If the variable was previously declared
with "my", it uses that variable instead of the global
one, but it's still localized to the loop.
The "foreach" keyword is actually a synonym for the "for"
keyword, so you can use "foreach" for readability or "for"
for brevity. (Or because the Bourne shell is more famil
iar to you than csh, so writing "for" comes more natu
rally.) If VAR is omitted, "$_" is set to each value.
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 ele
ment will fail. In other words, the "foreach" loop index
variable 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:
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
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 exe
cuted. 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 equiva
lent 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 real
ize 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
{ $nothing = 1 }
A common idiom for a "switch" statement is to use "fore
ach"'s aliasing to make a temporary assignment to "$_" 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 certainly that all the "&&" clauses are
true, you can use something like this, which "switches" on
the value of the "HTTP_USER_AGENT" environment variable.
#!/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 ref
erences 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
actually 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 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 bet
ter 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")[$i]);
The "goto"-&NAME form is highly magical, and substitutes a
call to the named subroutine for the currently running
subroutine. This is used by "AUTOLOAD()" subroutines that
wish to load another subroutine and then pretend that the
other subroutine had been called in the first place
(except that any modifications to "@_" in the current sub
routine are propagated to the other subroutine.) After
the "goto", not even "caller()" will be able to tell that
this routine was called first.
In almost all cases like this, it's usually a far, far
better idea to use the structured control flow mechanisms
of "next", "last", or "redo" instead of resorting to a
"goto". For certain applications, the catch and throw
pair of "eval{}" and die() for exception processing can
also be a prudent approach.
PODs: Embedded Documentation
Perl has a mechanism for intermixing documentation with
source code. While it's expecting the beginning of a new
statement, if the compiler encounters a line that begins
with an equal sign and a word, like this
=head1 Here There Be Pods!
Then that text and all remaining text up through and
including a line beginning with "=cut" will be ignored.
The format of the intervening text is described in the
perlpod manpage.
This allows you to intermix your source code and your doc
umentation text freely, as in
=item snazzle($)
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 $a\n";
You probably shouldn't rely upon the "warn()" being podded
out forever. Not all pod translators are well-behaved in
this regard, and perhaps the compiler will become pickier.
One may also use pod directives to quickly comment out a
section of code.
Plain Old Comments (Not!)
Much like the C preprocessor, Perl can process line direc
tives. Using this, one can control Perl's idea of file
names and line numbers in error or warning messages (espe
cially for strings that are processed with "eval()"). The
syntax for this mechanism is the same as for most C pre
processors: it matches the regular expression
"/^#\s*line\s+(\d+)\s*(?:\s"([^"]+)")?\s*$/" with "$1"
being the line number for the next line, and "$2" being
the optional filename (specified within 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
collisions 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.
% 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.
2001-03-18 perl v5.6.1 PERLSYN(1)
