PERLFAQ7(1) Perl Programmers Reference Guide PERLFAQ7(1)NAMEperlfaq7 - General Perl Language Issues
DESCRIPTION
This section deals with general Perl language issues that
don't clearly fit into any of the other sections.
Can I get a BNF/yacc/RE for the Perl language?
There is no BNF, but you can paw your way through the yacc
grammar in perly.y in the source distribution if you're par-
ticularly brave. The grammar relies on very smart tokeniz-
ing code, so be prepared to venture into toke.c as well.
In the words of Chaim Frenkel: "Perl's grammar can not be
reduced to BNF. The work of parsing perl is distributed
between yacc, the lexer, smoke and mirrors."
What are all these $@%&* punctuation signs, and how do I
know when to use them?
They are type specifiers, as detailed in perldata:
$ for scalar values (number, string or reference)
@ for arrays
% for hashes (associative arrays)
& for subroutines (aka functions, procedures, methods)
* for all types of that symbol name. In version 4 you used them like
pointers, but in modern perls you can just use references.
There are couple of other symbols that you're likely to
encounter that aren't really type specifiers:
<> are used for inputting a record from a filehandle.
\ takes a reference to something.
Note that <FILE> is neither the type specifier for files nor
the name of the handle. It is the "<>" operator applied to
the handle FILE. It reads one line (well, record--see "$/"
in perlvar) from the handle FILE in scalar context, or all
lines in list context. When performing open, close, or any
other operation besides "<>" on files, or even when talking
about the handle, do not use the brackets. These are
correct: "eof(FH)", "seek(FH, 0, 2)" and "copying from STDIN
to FILE".
Do I always/never have to quote my strings or use semicolons
and commas?
Normally, a bareword doesn't need to be quoted, but in most
cases probably should be (and must be under "use strict").
But a hash key consisting of a simple word (that isn't the
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name of a defined subroutine) and the left-hand operand to
the "=>" operator both count as though they were quoted:
This is like this
---------------------------
$foo{line} $foo{'line'}
bar => stuff 'bar' => stuff
The final semicolon in a block is optional, as is the final
comma in a list. Good style (see perlstyle) says to put
them in except for one-liners:
if ($whoops) { exit 1 }
@nums = (1, 2, 3);
if ($whoops) {
exit 1;
}
@lines = (
"There Beren came from mountains cold",
"And lost he wandered under leaves",
);
How do I skip some return values?
One way is to treat the return values as a list and index
into it:
$dir = (getpwnam($user))[7];
Another way is to use undef as an element on the
left-hand-side:
($dev, $ino, undef, undef, $uid, $gid) = stat($file);
You can also use a list slice to select only the elements
that you need:
($dev, $ino, $uid, $gid) = ( stat($file) )[0,1,4,5];
How do I temporarily block warnings?
If you are running Perl 5.6.0 or better, the "use warnings"
pragma allows fine control of what warning are produced. See
perllexwarn for more details.
{
no warnings; # temporarily turn off warnings
$a = $b + $c; # I know these might be undef
}
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Additionally, you can enable and disable categories of warn-
ings. You turn off the categories you want to ignore and you
can still get other categories of warnings. See perllexwarn
for the complete details, including the category names and
hierarchy.
{
no warnings 'uninitialized';
$a = $b + $c;
}
If you have an older version of Perl, the $^W variable
(documented in perlvar) controls runtime warnings for a
block:
{
local $^W = 0; # temporarily turn off warnings
$a = $b + $c; # I know these might be undef
}
Note that like all the punctuation variables, you cannot
currently use my() on $^W, only local().
What's an extension?
An extension is a way of calling compiled C code from Perl.
Reading perlxstut is a good place to learn more about exten-
sions.
Why do Perl operators have different precedence than C
operators?
Actually, they don't. All C operators that Perl copies have
the same precedence in Perl as they do in C. The problem is
with operators that C doesn't have, especially functions
that give a list context to everything on their right, eg.
print, chmod, exec, and so on. Such functions are called
"list operators" and appear as such in the precedence table
in perlop.
A common mistake is to write:
unlink $file || die "snafu";
This gets interpreted as:
unlink ($file || die "snafu");
To avoid this problem, either put in extra parentheses or
use the super low precedence "or" operator:
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(unlink $file) || die "snafu";
unlink $file or die "snafu";
The "English" operators ("and", "or", "xor", and "not")
deliberately have precedence lower than that of list opera-
tors for just such situations as the one above.
Another operator with surprising precedence is exponentia-
tion. It binds more tightly even than unary minus, making
"-2**2" product a negative not a positive four. It is also
right-associating, meaning that "2**3**2" is two raised to
the ninth power, not eight squared.
Although it has the same precedence as in C, Perl's "?:"
operator produces an lvalue. This assigns $x to either $a
or $b, depending on the trueness of $maybe:
($maybe ? $a : $b) = $x;
How do I declare/create a structure?
In general, you don't "declare" a structure. Just use a
(probably anonymous) hash reference. See perlref and
perldsc for details. Here's an example:
$person = {}; # new anonymous hash
$person->{AGE} = 24; # set field AGE to 24
$person->{NAME} = "Nat"; # set field NAME to "Nat"
If you're looking for something a bit more rigorous, try
perltoot.
How do I create a module?
(contributed by brian d foy)
perlmod, perlmodlib, perlmodstyle explain modules in all the
gory details. perlnewmod gives a brief overview of the pro-
cess along with a couple of suggestions about style.
If you need to include C code or C library interfaces in
your module, you'll need h2xs. h2xs will create the module
distribution structure and the initial interface files
you'll need. perlxs and perlxstut explain the details.
If you don't need to use C code, other tools such as
ExtUtils::ModuleMaker and Module::Starter, can help you
create a skeleton module distribution.
You may also want to see Sam Tregar's "Writing Perl Modules
for CPAN" ( http://apress.com/book/bookDisplay.html?bID=14 )
which is the best hands-on guide to creating module
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distributions.
How do I create a class?
See perltoot for an introduction to classes and objects, as
well as perlobj and perlbot.
How can I tell if a variable is tainted?
You can use the tainted() function of the Scalar::Util
module, available from CPAN (or included with Perl since
release 5.8.0). See also "Laundering and Detecting Tainted
Data" in perlsec.
What's a closure?
Closures are documented in perlref.
Closure is a computer science term with a precise but hard-
to-explain meaning. Closures are implemented in Perl as
anonymous subroutines with lasting references to lexical
variables outside their own scopes. These lexicals magi-
cally refer to the variables that were around when the sub-
routine was defined (deep binding).
Closures make sense in any programming language where you
can have the return value of a function be itself a func-
tion, as you can in Perl. Note that some languages provide
anonymous functions but are not capable of providing proper
closures: the Python language, for example. For more infor-
mation on closures, check out any textbook on functional
programming. Scheme is a language that not only supports
but encourages closures.
Here's a classic function-generating function:
sub add_function_generator {
return sub { shift() + shift() };
}
$add_sub = add_function_generator();
$sum = $add_sub->(4,5); # $sum is 9 now.
The closure works as a function template with some customi-
zation slots left out to be filled later. The anonymous
subroutine returned by add_function_generator() isn't techn-
ically a closure because it refers to no lexicals outside
its own scope.
Contrast this with the following make_adder() function, in
which the returned anonymous function contains a reference
to a lexical variable outside the scope of that function
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itself. Such a reference requires that Perl return a proper
closure, thus locking in for all time the value that the
lexical had when the function was created.
sub make_adder {
my $addpiece = shift;
return sub { shift() + $addpiece };
}
$f1 = make_adder(20);
$f2 = make_adder(555);
Now "&$f1($n)" is always 20 plus whatever $n you pass in,
whereas "&$f2($n)" is always 555 plus whatever $n you pass
in. The $addpiece in the closure sticks around.
Closures are often used for less esoteric purposes. For
example, when you want to pass in a bit of code into a func-
tion:
my $line;
timeout( 30, sub { $line = <STDIN> } );
If the code to execute had been passed in as a string,
'$line = <STDIN>', there would have been no way for the
hypothetical timeout() function to access the lexical vari-
able $line back in its caller's scope.
What is variable suicide and how can I prevent it?
This problem was fixed in perl 5.004_05, so preventing it
means upgrading your version of perl. ;)
Variable suicide is when you (temporarily or permanently)
lose the value of a variable. It is caused by scoping
through my() and local() interacting with either closures or
aliased foreach() iterator variables and subroutine argu-
ments. It used to be easy to inadvertently lose a
variable's value this way, but now it's much harder. Take
this code:
my $f = 'foo';
sub T {
while ($i++ < 3) { my $f = $f; $f .= $i; print $f, "\n" }
}
T;
print "Finally $f\n";
If you are experiencing variable suicide, that "my $f" in
the subroutine doesn't pick up a fresh copy of the $f whose
value is <foo>. The output shows that inside the subroutine
the value of $f leaks through when it shouldn't, as in this
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output:
foobar
foobarbar
foobarbarbar
Finally foo
The $f that has "bar" added to it three times should be a
new $f "my $f" should create a new lexical variable each
time through the loop. The expected output is:
foobar
foobar
foobar
Finally foo
How can I pass/return a {Function, FileHandle, Array, Hash,
Method, Regex}?
With the exception of regexes, you need to pass references
to these objects. See "Pass by Reference" in perlsub for
this particular question, and perlref for information on
references.
See "Passing Regexes", below, for information on passing
regular expressions.
Passing Variables and Functions
Regular variables and functions are quite easy to pass:
just pass in a reference to an existing or anonymous
variable or function:
func( \$some_scalar );
func( \@some_array );
func( [ 1 .. 10 ] );
func( \%some_hash );
func( { this => 10, that => 20 } );
func( \&some_func );
func( sub { $_[0] ** $_[1] } );
Passing Filehandles
As of Perl 5.6, you can represent filehandles with
scalar variables which you treat as any other scalar.
open my $fh, $filename or die "Cannot open $filename! $!";
func( $fh );
sub func {
my $passed_fh = shift;
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my $line = <$fh>;
}
Before Perl 5.6, you had to use the *FH or "\*FH" nota-
tions. These are "typeglobs"--see "Typeglobs and
Filehandles" in perldata and especially "Pass by Refer-
ence" in perlsub for more information.
Passing Regexes
To pass regexes around, you'll need to be using a
release of Perl sufficiently recent as to support the
"qr//" construct, pass around strings and use an
exception-trapping eval, or else be very, very clever.
Here's an example of how to pass in a string to be regex
compared using "qr//":
sub compare($$) {
my ($val1, $regex) = @_;
my $retval = $val1 =~ /$regex/;
return $retval;
}
$match = compare("old McDonald", qr/d.*D/i);
Notice how "qr//" allows flags at the end. That pattern
was compiled at compile time, although it was executed
later. The nifty "qr//" notation wasn't introduced
until the 5.005 release. Before that, you had to
approach this problem much less intuitively. For exam-
ple, here it is again if you don't have "qr//":
sub compare($$) {
my ($val1, $regex) = @_;
my $retval = eval { $val1 =~ /$regex/ };
die if $@;
return $retval;
}
$match = compare("old McDonald", q/($?i)d.*D/);
Make sure you never say something like this:
return eval "\$val =~ /$regex/"; # WRONG
or someone can sneak shell escapes into the regex due to
the double interpolation of the eval and the double-
quoted string. For example:
$pattern_of_evil = 'danger ${ system("rm -rf * &") } danger';
eval "\$string =~ /$pattern_of_evil/";
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Those preferring to be very, very clever might see the
O'Reilly book, Mastering Regular Expressions, by Jeffrey
Friedl. Page 273's Build_MatchMany_Function() is par-
ticularly interesting. A complete citation of this book
is given in perlfaq2.
Passing Methods
To pass an object method into a subroutine, you can do
this:
call_a_lot(10, $some_obj, "methname")
sub call_a_lot {
my ($count, $widget, $trick) = @_;
for (my $i = 0; $i < $count; $i++) {
$widget->$trick();
}
}
Or, you can use a closure to bundle up the object, its
method call, and arguments:
my $whatnot = sub { $some_obj->obfuscate(@args) };
func($whatnot);
sub func {
my $code = shift;
&$code();
}
You could also investigate the can() method in the
UNIVERSAL class (part of the standard perl distribu-
tion).
How do I create a static variable?
(contributed by brian d foy)
Perl doesn't have "static" variables, which can only be
accessed from the function in which they are declared. You
can get the same effect with lexical variables, though.
You can fake a static variable by using a lexical variable
which goes out of scope. In this example, you define the
subroutine "counter", and it uses the lexical variable
$count. Since you wrap this in a BEGIN block, $count is
defined at compile-time, but also goes out of scope at the
end of the BEGIN block. The BEGIN block also ensures that
the subroutine and the value it uses is defined at compile-
time so the subroutine is ready to use just like any other
subroutine, and you can put this code in the same place as
other subroutines in the program text (i.e. at the end of
the code, typically). The subroutine "counter" still has a
reference to the data, and is the only way you can access
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the value (and each time you do, you increment the value).
The data in chunk of memory defined by $count is private to
"counter".
BEGIN {
my $count = 1;
sub counter { $count++ }
}
my $start = count();
.... # code that calls count();
my $end = count();
In the previous example, you created a function-private
variable because only one function remembered its reference.
You could define multiple functions while the variable is in
scope, and each function can share the "private" variable.
It's not really "static" because you can access it outside
the function while the lexical variable is in scope, and
even create references to it. In this example,
"increment_count" and "return_count" share the variable. One
function adds to the value and the other simply returns the
value. They can both access $count, and since it has gone
out of scope, there is no other way to access it.
BEGIN {
my $count = 1;
sub increment_count { $count++ }
sub return_count { $count }
}
To declare a file-private variable, you still use a lexical
variable. A file is also a scope, so a lexical variable
defined in the file cannot be seen from any other file.
See "Persistent Private Variables" in perlsub for more
information. The discussion of closures in perlref may help
you even though we did not use anonymous subroutines in this
answer. See "Persistent Private Variables" in perlsub for
details.
What's the difference between dynamic and lexical (static)
scoping? Between local() and my()?
"local($x)" saves away the old value of the global variable
$x and assigns a new value for the duration of the subrou-
tine which is visible in other functions called from that
subroutine. This is done at run-time, so is called dynamic
scoping. local() always affects global variables, also
called package variables or dynamic variables.
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"my($x)" creates a new variable that is only visible in the
current subroutine. This is done at compile-time, so it is
called lexical or static scoping. my() always affects
private variables, also called lexical variables or (improp-
erly) static(ly scoped) variables.
For instance:
sub visible {
print "var has value $var\n";
}
sub dynamic {
local $var = 'local'; # new temporary value for the still-global
visible(); # variable called $var
}
sub lexical {
my $var = 'private'; # new private variable, $var
visible(); # (invisible outside of sub scope)
}
$var = 'global';
visible(); # prints global
dynamic(); # prints local
lexical(); # prints global
Notice how at no point does the value "private" get printed.
That's because $var only has that value within the block of
the lexical() function, and it is hidden from called subrou-
tine.
In summary, local() doesn't make what you think of as
private, local variables. It gives a global variable a tem-
porary value. my() is what you're looking for if you want
private variables.
See "Private Variables via my()" in perlsub and "Temporary
Values via local()" in perlsub for excruciating details.
How can I access a dynamic variable while a similarly named
lexical is in scope?
If you know your package, you can just mention it expli-
citly, as in $Some_Pack::var. Note that the notation $::var
is not the dynamic $var in the current package, but rather
the one in the "main" package, as though you had written
$main::var.
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use vars '$var';
local $var = "global";
my $var = "lexical";
print "lexical is $var\n";
print "global is $main::var\n";
Alternatively you can use the compiler directive our() to
bring a dynamic variable into the current lexical scope.
require 5.006; # our() did not exist before 5.6
use vars '$var';
local $var = "global";
my $var = "lexical";
print "lexical is $var\n";
{
our $var;
print "global is $var\n";
}
What's the difference between deep and shallow binding?
In deep binding, lexical variables mentioned in anonymous
subroutines are the same ones that were in scope when the
subroutine was created. In shallow binding, they are which-
ever variables with the same names happen to be in scope
when the subroutine is called. Perl always uses deep bind-
ing of lexical variables (i.e., those created with my()).
However, dynamic variables (aka global, local, or package
variables) are effectively shallowly bound. Consider this
just one more reason not to use them. See the answer to
"What's a closure?".
Why doesn't "my($foo) = <FILE>;" work right?
"my()" and "local()" give list context to the right hand
side of "=". The <FH> read operation, like so many of
Perl's functions and operators, can tell which context it
was called in and behaves appropriately. In general, the
scalar() function can help. This function does nothing to
the data itself (contrary to popular myth) but rather tells
its argument to behave in whatever its scalar fashion is. If
that function doesn't have a defined scalar behavior, this
of course doesn't help you (such as with sort()).
To enforce scalar context in this particular case, however,
you need merely omit the parentheses:
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local($foo) = <FILE>; # WRONG
local($foo) = scalar(<FILE>); # ok
local $foo = <FILE>; # right
You should probably be using lexical variables anyway,
although the issue is the same here:
my($foo) = <FILE>; # WRONG
my $foo = <FILE>; # right
How do I redefine a builtin function, operator, or method?
Why do you want to do that? :-)
If you want to override a predefined function, such as
open(), then you'll have to import the new definition from a
different module. See "Overriding Built-in Functions" in
perlsub. There's also an example in "Class::Template" in
perltoot.
If you want to overload a Perl operator, such as "+" or
"**", then you'll want to use the "use overload" pragma,
documented in overload.
If you're talking about obscuring method calls in parent
classes, see "Overridden Methods" in perltoot.
What's the difference between calling a function as &foo and
foo()?
When you call a function as &foo, you allow that function
access to your current @_ values, and you bypass prototypes.
The function doesn't get an empty @_--it gets yours! While
not strictly speaking a bug (it's documented that way in
perlsub), it would be hard to consider this a feature in
most cases.
When you call your function as "&foo()", then you do get a
new @_, but prototyping is still circumvented.
Normally, you want to call a function using "foo()". You
may only omit the parentheses if the function is already
known to the compiler because it already saw the definition
("use" but not "require"), or via a forward reference or
"use subs" declaration. Even in this case, you get a clean
@_ without any of the old values leaking through where they
don't belong.
How do I create a switch or case statement?
This is explained in more depth in the perlsyn. Briefly,
there's no official case statement, because of the variety
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of tests possible in Perl (numeric comparison, string com-
parison, glob comparison, regex matching, overloaded com-
parisons, ...). Larry couldn't decide how best to do this,
so he left it out, even though it's been on the wish list
since perl1.
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.
But if one wants to use pure Perl, the general answer is to
write a construct like this:
for ($variable_to_test) {
if (/pat1/) { } # do something
elsif (/pat2/) { } # do something else
elsif (/pat3/) { } # do something else
else { } # default
}
Here's a simple example of a switch based on pattern match-
ing, this time lined up in a way to make it look more like a
switch statement. We'll do a multiway conditional based on
the type of reference stored in $whatchamacallit:
SWITCH: for (ref $whatchamacallit) {
/^$/ && die "not a reference";
/SCALAR/ && do {
print_scalar($$ref);
last SWITCH;
};
/ARRAY/ && do {
print_array(@$ref);
last SWITCH;
};
/HASH/ && do {
print_hash(%$ref);
last SWITCH;
};
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/CODE/ && do {
warn "can't print function ref";
last SWITCH;
};
# DEFAULT
warn "User defined type skipped";
}
See "perlsyn/"Basic BLOCKs and Switch Statements"" for many
other examples in this style.
Sometimes you should change the positions of the constant
and the variable. For example, let's say you wanted to test
which of many answers you were given, but in a case-
insensitive way that also allows abbreviations. You can use
the following technique if the strings all start with dif-
ferent characters or if you want to arrange the matches so
that one takes precedence over another, as "SEND" has pre-
cedence over "STOP" here:
chomp($answer = <>);
if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" }
elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" }
elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" }
elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" }
elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" }
A totally different approach is to create a hash of function
references.
my %commands = (
"happy" => \&joy,
"sad", => \&sullen,
"done" => sub { die "See ya!" },
"mad" => \&angry,
);
print "How are you? ";
chomp($string = <STDIN>);
if ($commands{$string}) {
$commands{$string}->();
} else {
print "No such command: $string\n";
}
How can I catch accesses to undefined variables, functions,
or methods?
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The AUTOLOAD method, discussed in "Autoloading" in perlsub
and "AUTOLOAD: Proxy Methods" in perltoot, lets you capture
calls to undefined functions and methods.
When it comes to undefined variables that would trigger a
warning under "use warnings", you can promote the warning to
an error.
use warnings FATAL => qw(uninitialized);
Why can't a method included in this same file be found?
Some possible reasons: your inheritance is getting confused,
you've misspelled the method name, or the object is of the
wrong type. Check out perltoot for details about any of the
above cases. You may also use "print ref($object)" to find
out the class $object was blessed into.
Another possible reason for problems is because you've used
the indirect object syntax (eg, "find Guru "Samy"") on a
class name before Perl has seen that such a package exists.
It's wisest to make sure your packages are all defined
before you start using them, which will be taken care of if
you use the "use" statement instead of "require". If not,
make sure to use arrow notation (eg., "Guru->find("Samy")")
instead. Object notation is explained in perlobj.
Make sure to read about creating modules in perlmod and the
perils of indirect objects in "Method Invocation" in per-
lobj.
How can I find out my current package?
If you're just a random program, you can do this to find out
what the currently compiled package is:
my $packname = __PACKAGE__;
But, if you're a method and you want to print an error mes-
sage that includes the kind of object you were called on
(which is not necessarily the same as the one in which you
were compiled):
sub amethod {
my $self = shift;
my $class = ref($self) || $self;
warn "called me from a $class object";
}
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How can I comment out a large block of perl code?
You can use embedded POD to discard it. Enclose the blocks
you want to comment out in POD markers. The <=begin> direc-
tive marks a section for a specific formatter. Use the
"comment" format, which no formatter should claim to under-
stand (by policy). Mark the end of the block with <=end>.
# program is here
=begin comment
all of this stuff
here will be ignored
by everyone
=end comment
=cut
# program continues
The pod directives cannot go just anywhere. You must put a
pod directive where the parser is expecting a new statement,
not just in the middle of an expression or some other arbi-
trary grammar production.
See perlpod for more details.
How do I clear a package?
Use this code, provided by Mark-Jason Dominus:
sub scrub_package {
no strict 'refs';
my $pack = shift;
die "Shouldn't delete main package"
if $pack eq "" || $pack eq "main";
my $stash = *{$pack . '::'}{HASH};
my $name;
foreach $name (keys %$stash) {
my $fullname = $pack . '::' . $name;
# Get rid of everything with that name.
undef $$fullname;
undef @$fullname;
undef %$fullname;
undef &$fullname;
undef *$fullname;
}
}
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Or, if you're using a recent release of Perl, you can just
use the Symbol::delete_package() function instead.
How can I use a variable as a variable name?
Beginners often think they want to have a variable contain
the name of a variable.
$fred = 23;
$varname = "fred";
++$$varname; # $fred now 24
This works sometimes, but it is a very bad idea for two rea-
sons.
The first reason is that this technique only works on global
variables. That means that if $fred is a lexical variable
created with my() in the above example, the code wouldn't
work at all: you'd accidentally access the global and skip
right over the private lexical altogether. Global variables
are bad because they can easily collide accidentally and in
general make for non-scalable and confusing code.
Symbolic references are forbidden under the "use strict"
pragma. They are not true references and consequently are
not reference counted or garbage collected.
The other reason why using a variable to hold the name of
another variable is a bad idea is that the question often
stems from a lack of understanding of Perl data structures,
particularly hashes. By using symbolic references, you are
just using the package's symbol-table hash (like %main::)
instead of a user-defined hash. The solution is to use your
own hash or a real reference instead.
$USER_VARS{"fred"} = 23;
$varname = "fred";
$USER_VARS{$varname}++; # not $$varname++
There we're using the %USER_VARS hash instead of symbolic
references. Sometimes this comes up in reading strings from
the user with variable references and wanting to expand them
to the values of your perl program's variables. This is
also a bad idea because it conflates the program-addressable
namespace and the user-addressable one. Instead of reading
a string and expanding it to the actual contents of your
program's own variables:
$str = 'this has a $fred and $barney in it';
$str =~ s/(\$\w+)/$1/eeg; # need double eval
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it would be better to keep a hash around like %USER_VARS and
have variable references actually refer to entries in that
hash:
$str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all
That's faster, cleaner, and safer than the previous
approach. Of course, you don't need to use a dollar sign.
You could use your own scheme to make it less confusing,
like bracketed percent symbols, etc.
$str = 'this has a %fred% and %barney% in it';
$str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all
Another reason that folks sometimes think they want a vari-
able to contain the name of a variable is because they don't
know how to build proper data structures using hashes. For
example, let's say they wanted two hashes in their program:
%fred and %barney, and that they wanted to use another
scalar variable to refer to those by name.
$name = "fred";
$$name{WIFE} = "wilma"; # set %fred
$name = "barney";
$$name{WIFE} = "betty"; # set %barney
This is still a symbolic reference, and is still saddled
with the problems enumerated above. It would be far better
to write:
$folks{"fred"}{WIFE} = "wilma";
$folks{"barney"}{WIFE} = "betty";
And just use a multilevel hash to start with.
The only times that you absolutely must use symbolic refer-
ences are when you really must refer to the symbol table.
This may be because it's something that can't take a real
reference to, such as a format name. Doing so may also be
important for method calls, since these always go through
the symbol table for resolution.
In those cases, you would turn off "strict 'refs'" tem-
porarily so you can play around with the symbol table. For
example:
@colors = qw(red blue green yellow orange purple violet);
for my $name (@colors) {
no strict 'refs'; # renege for the block
*$name = sub { "<FONT COLOR='$name'>@_</FONT>" };
}
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All those functions (red(), blue(), green(), etc.) appear to
be separate, but the real code in the closure actually was
compiled only once.
So, sometimes you might want to use symbolic references to
directly manipulate the symbol table. This doesn't matter
for formats, handles, and subroutines, because they are
always global--you can't use my() on them. For scalars,
arrays, and hashes, though--and usually for subroutines--
you probably only want to use hard references.
What does "bad interpreter" mean?
(contributed by brian d foy)
The "bad interpreter" message comes from the shell, not
perl. The actual message may vary depending on your plat-
form, shell, and locale settings.
If you see "bad interpreter - no such file or directory",
the first line in your perl script (the "shebang" line) does
not contain the right path to perl (or any other program
capable of running scripts). Sometimes this happens when you
move the script from one machine to another and each machine
has a different path to perl---/usr/bin/perl versus
/usr/local/bin/perl for instance. It may also indicate that
the source machine has CRLF line terminators and the desti-
nation machine has LF only: the shell tries to find
/usr/bin/perl<CR>, but can't.
If you see "bad interpreter: Permission denied", you need to
make your script executable.
In either case, you should still be able to run the scripts
with perl explicitly:
% perl script.pl
If you get a message like "perl: command not found", perl is
not in your PATH, which might also mean that the location of
perl is not where you expect it so you need to adjust your
shebang line.
AUTHOR AND COPYRIGHT
Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington,
and other authors as noted. All rights reserved.
This documentation is free; you can redistribute it and/or
modify it under the same terms as Perl itself.
Irrespective of its distribution, all code examples in this
file are hereby placed into the public domain. You are
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PERLFAQ7(1) Perl Programmers Reference Guide PERLFAQ7(1)
permitted and encouraged to use this code in your own pro-
grams for fun or for profit as you see fit. A simple com-
ment in the code giving credit would be courteous but is not
required.
perl v5.8.8 2006-06-30 21
