perlsyn - Perl syntax
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. 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 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 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.
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
foreach EXPR
The if and unless modifiers have the expected semantics, presuming you're a speaker of
English. The foreach 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 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 do. 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;
}
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: $!";
# a bit exotic, that last one
The if statement is straightforward. Because BLOCKs are always bounded by curly
brackets, there is never any ambiguity about which
if an else goes with. If you use unless in place of if, the sense of the test is reversed.
The while statement executes the block as long as the expression is true (does not
evaluate to the null string ("") or 0 or "0"). The
LABEL is optional, and if present, consists of an identifier followed by a colon. The
LABEL identifies the loop for the loop control statements
next,
last, and redo. If the
LABEL is omitted, the loop control statement refers to the innermost enclosing loop. This may include dynamically looking back your call-stack at run time to find the
LABEL. Such desperate behavior triggers a warning if you use the
-w flag.
If there is a continue
BLOCK, it is always executed just before the
conditional is about to be evaluated again, just like the third part of a
for loop in
C. Thus it can be used to increment a loop variable,
even when the loop has been continued via the next statement (which is similar to the
C continue statement).
The next command is like the continue statement in C; it starts the next iteration of the loop:
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
...
}
The last command is like the break statement in
C (as used in loops); it immediately exits the loop in
question. The
continue block, if any, is not executed:
LINE: while (<STDIN>) {
last LINE if /^$/; # 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 $line
}
Note that if there were a continue block on the above code, it would get executed even on discarded lines.
This is often used to reset line counters or ?pat? one-time matches.
# inspired by :1,$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 $.
reset if eof(); # reset ?pat?
}
If the word while is replaced by the word until, the sense of the test is reversed, but the conditional is still tested
before the first iteration.
The loop control statements don't work in an if or unless, since they aren't loops. You can double the braces to make them such,
though.
if (/pattern/) {{
next if /fred/;
next if /barney/;
# so something here
}}
The form while/if BLOCK BLOCK, available in Perl 4, is no longer available. Replace any occurrence of if BLOCK by if (do BLOCK).
Perl's C-style for loop works exactly like the corresponding while loop; that means that this:
for ($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
}
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 if
you have subroutine or format declarations within the loop which refer to
it.)
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. If any element of
LIST is an lvalue, you can modify it 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.
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/ }
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.
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" } # 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 envariable.
#!/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 instead of synthesizing a switch
statement.
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")[$i];
The goto
-&NAME form is highly magical, and substitutes a
call to the named subroutine for the currently running subroutine. This is
used by
AUTOLOAD() subroutines that wish to load another subroutine and then pretend that the
other subroutine had been called in the first place (except that any
modifications to @_ in the current subroutine are propagated to the other subroutine.) After
the goto, not even caller()
will be able to tell that this routine was called first.
In almost all cases like this, it's usually a far, far better idea to use
the structured control flow mechanisms of next, last, or redo instead of resorting to a goto. For certain applications, the catch and throw pair of
eval{} and
die() for exception processing
can also be a prudent approach.
Perl has a mechanism for intermixing documentation with source code. While it's expecting the beginning of a new statement, if the compiler encounters a line that begins with an equal sign and a word, like this
=head1 Here There Be Pods!
Then that text and all remaining text up through and including a line
beginning with =cut will be ignored. The format of the intervening text is described in the perlpod manpage.
This allows you to intermix your source code and your documentation text freely, as in
=item snazzle($)
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.
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.
If rather than formatting bugs, you encounter substantive content errors in these documents, such as mistakes in the explanations or code, please use the perlbug utility included with the Perl distribution.