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perlsynCurrent Perl documentation can be found at perldoc.perl.org. Here is our local, out-dated (pre-5.6) version:
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
Declarations
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 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 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 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.
Simple statementsThe 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 perlfunc:eval and perlfunc: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
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; }
Compound statementsIn 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: $!"; # a bit exotic, that last one
The
The
If there is a continue
BLOCK, it is always executed just before the
conditional is about to be evaluated again, just like the third part of a
Loop ControlThe next command is like the continue statement in C; it starts the next iteration of the loop:
LINE: while (<STDIN>) { next LINE if /^#/; # discard comments ... }
The last command is like the
LINE: while (<STDIN>) { last LINE if /^$/; # 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 $line }
Note that if there were a continue block on the above code, it would get executed even on discarded lines.
This is often used to reset line counters or
# inspired by :1,$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 $. reset if eof(); # reset ?pat? }
If the word
The loop control statements don't work in an
if (/pattern/) {{ next if /fred/; next if /barney/; # so something here }}
The form
For Loops
Perl's C-style
for ($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,
$on_a_tty = -t STDIN && -t STDOUT; sub prompt { print "yes? " if $on_a_tty } for ( prompt(); <STDIN>; prompt() ) { # do something }
Foreach Loops
The
The
If any part of
LIST is an array,
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
Basic BLOCKs and Switch StatementsA 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 perlfunc:eval, perlfunc:sub, or contrary to popular belief perlfunc: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: { $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: { /^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 { $nothing = 1 }
A common idiom for a
SWITCH: for ($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
#!/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
You might also consider writing a hash instead of synthesizing a
GotoAlthough 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
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")[$i];
The goto
-&NAME form is highly magical, and substitutes a
call to the named subroutine for the currently running subroutine. This is
used by
In almost all cases like this, it's usually a far, far better idea to use
the structured control flow mechanisms of next, last, or redo instead of resorting to a goto. For certain applications, the catch and throw pair of
perlfunc:eval and
PODs: Embedded DocumentationPerl has a mechanism for intermixing documentation with source code. While it's expecting the beginning of a new statement, if the compiler encounters a line that begins with an equal sign and a word, like this
=head1 Here There Be Pods!
Then that text and all remaining text up through and including a line
beginning with This allows you to intermix your source code and your documentation text freely, as in
=item snazzle($)
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 perlman:perlguts 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 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
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. Return to the Library |
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