NAMEperlsyn - Perl syntax
DESCRIPTIONA 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
DeclarationsThe 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 ( 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 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 Subroutines declarations can also be loaded up with the 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 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 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 do {{ next if $x == $y; # do something here }} until $x++ > $z; For 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 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: $!"; # a bit exotic, that last one The The If there is a
Loop ControlThe LINE: while (<STDIN>) { next LINE if /^#/; # discard comments ... } The LINE: while (<STDIN>) { last LINE if /^$/; # exit when done with header ... } The 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 $line } Note that if there were a # inspired by :1,$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 $. reset if eof(); # reset ?pat? } If the word The loop control statements don't work in an if (/pattern/) {{ next if /fred/; next if /barney/; # so something here }} The form
For LoopsPerl's C-style for ($i = 1; $i < 10; $i++) { ... } is the same as this: $i = 1; while ($i < 10) { ... } continue { $i++; } There is one minor difference: if variables are declared with 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 LoopsThe The 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 If any part of LIST is an array,
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 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: 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 executed. The
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 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 $amode = do { if ($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 of subroutine references
instead of synthesizing a
GotoAlthough not for the faint of heart, Perl does support a The The goto(("FOO", "BAR", "GLARCH")[$i]); The In almost all cases like this, it's usually a far, far better idea to use the
structured control flow mechanisms of
PODs: Embedded DocumentationPerl has a mechanism for intermixing documentation with source code. While it's expecting the beginning of a new statement, if the compiler encounters a line that begins with an equal sign and a word, like this =head1 Here There Be Pods! Then that text and all remaining text up through and including a line
beginning with This allows you to intermix your source code and your documentation text freely, as in =item snazzle($) 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 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 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.
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