perlsyn - Perl syntax |
perlsyn - Perl syntax
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 regular
expression, so don't do that.
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 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:
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 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 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 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; }
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 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: $!"; # a bit exotic, that last one
The if
statement is straightforward. Because BLOCKs are always
bounded by curly brackets, there is never any ambiguity about which
if
an else
goes with. If you use unless
in place of if
,
the sense of the test is reversed.
The while
statement executes the block as long as the expression is
true (does not evaluate to the null string ""
or 0
or "0"
).
The LABEL is optional, and if present, consists of an identifier followed
by a colon. The LABEL identifies the loop for the loop control
statements next
, last
, and redo
.
If the LABEL is omitted, the loop control statement
refers to the innermost enclosing loop. This may include dynamically
looking back your call-stack at run time to find the LABEL. Such
desperate behavior triggers a warning if you use the use warnings
pragma or the -w flag.
Unlike a foreach
statement, a while
statement never implicitly
localises any variables.
If there is a continue
BLOCK, it is always executed just before the
conditional is about to be evaluated again, just like the third part of a
for
loop in C. Thus it can be used to increment a loop variable, even
when the loop has been continued via the next
statement (which is
similar to the C continue
statement).
The next
command is like the continue
statement in C; it starts
the next iteration of the loop:
LINE: while (<STDIN>) { next LINE if /^#/; # discard comments ... }
The last
command is like the break
statement in C (as used in
loops); it immediately exits the loop in question. The
continue
block, if any, is not executed:
LINE: while (<STDIN>) { last LINE if /^$/; # 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 $line }
Note that if there were a continue
block on the above code, it would
get executed only on lines discarded by the regex (since redo skips the
continue block). A continue block is often used to reset line counters
or ?pat?
one-time matches:
# inspired by :1,$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 $. reset if eof(); # reset ?pat? }
If the word while
is replaced by the word until
, the sense of the
test is reversed, but the conditional is still tested before the first
iteration.
The loop control statements don't work in an if
or unless
, since
they aren't loops. You can double the braces to make them such, though.
if (/pattern/) {{ last if /fred/; next if /barney/; # same effect as "last", but doesn't document as well # do something here }}
This is caused by the fact that a block by itself acts as a loop that executes once, see Basic BLOCKs and Switch Statements.
The form while/if BLOCK BLOCK
, available in Perl 4, is no longer
available. Replace any occurrence of if BLOCK
by if (do BLOCK)
.
Perl's C-style for
loop works like the corresponding while
loop;
that means that this:
for ($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 }
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.
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. 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 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 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 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.
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
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 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.
#!/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 references
instead of synthesizing a switch
statement.
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 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")[$i]);
The goto
-&NAME form is highly magical, and substitutes a call to the
named subroutine for the currently running subroutine. This is used by
AUTOLOAD()
subroutines that wish to load another subroutine and then
pretend that the other subroutine had been called in the first place
(except that any modifications to @_
in the current subroutine are
propagated to the other subroutine.) After the goto
, not even caller()
will be able to tell that this routine was called first.
In almost all cases like this, it's usually a far, far better idea to use the
structured control flow mechanisms of next
, last
, or redo
instead of
resorting to a goto
. For certain applications, the catch and throw pair of
eval{}
and die()
for exception processing can also be a prudent approach.
Perl has a mechanism for intermixing documentation with source code. While it's expecting the beginning of a new statement, if the compiler encounters a line that begins with an equal sign and a word, like this
=head1 Here There Be Pods!
Then that text and all remaining text up through and including a line
beginning with =cut
will be ignored. The format of the intervening
text is described in the perlpod manpage.
This allows you to intermix your source code and your documentation text freely, as in
=item snazzle($)
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 $a\n";
You probably shouldn't rely upon the warn()
being podded out forever.
Not all pod translators are well-behaved in this regard, and perhaps
the compiler will become pickier.
One may also use pod directives to quickly comment out a section of code.
Much like the C preprocessor, Perl can process line directives. Using
this, one can control Perl's idea of filenames and line numbers in
error or warning messages (especially for strings that are processed
with eval()
). The syntax for this mechanism is the same as for most
C preprocessors: it matches the regular expression
/^#\s*line\s+(\d+)\s*(?:\s"([^"]+)")?\s*$/
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.
perlsyn - Perl syntax |