This is Info file perl.info, produced by Makeinfo version 1.68 from the input file bigperl.texi. settitle perl  File: perl.info, Node: perlnumber, Next: perlsec, Prev: perldiag, Up: Top semantics of numbers and numeric operations in Perl *************************************************** NAME ==== perlnumber - semantics of numbers and numeric operations in Perl SYNOPSIS ======== $n = 1234; # decimal integer $n = 0b1110011; # binary integer $n = 01234; # octal integer $n = 0x1234; # hexadecimal integer $n = 12.34e-56; # exponential notation $n = "-12.34e56"; # number specified as a string $n = "1234"; # number specified as a string $n = v49.50.51.52; # number specified as a string, which in # turn is specified in terms of numbers :-) DESCRIPTION =========== This document describes how Perl internally handles numeric values. Perl's operator overloading facility is completely ignored here. Operator overloading allows user-defined behaviors for numbers, such as operations over arbitrarily large integers, floating points numbers with arbitrary precision, operations over "exotic" numbers such as modular arithmetic or p-adic arithmetic, and so on. See *Note Overload: (pm.info)overload, for details. Storing numbers =============== Perl can internally represent numbers in 3 different ways: as native integers, as native floating point numbers, and as decimal strings. Decimal strings may have an exponential notation part, as in `"12.34e-56"'. *Native* here means "a format supported by the C compiler which was used to build perl". The term "native" does not mean quite as much when we talk about native integers, as it does when native floating point numbers are involved. The only implication of the term "native" on integers is that the limits for the maximal and the minimal supported true integral quantities are close to powers of 2. However, "native" floats have a most fundamental restriction: they may represent only those numbers which have a relatively "short" representation when converted to a binary fraction. For example, 0.9 cannot be respresented by a native float, since the binary fraction for 0.9 is infinite: binary0.1110011001100... with the sequence `1100' repeating again and again. In addition to this limitation, the exponent of the binary number is also restricted when it is represented as a floating point number. On typical hardware, floating point values can store numbers with up to 53 binary digits, and with binary exponents between -1024 and 1024. In decimal representation this is close to 16 decimal digits and decimal exponents in the range of -304..304. The upshot of all this is that Perl cannot store a number like 12345678901234567 as a floating point number on such architectures without loss of information. Similarly, decimal strings can represent only those numbers which have a finite decimal expansion. Being strings, and thus of arbitrary length, there is no practical limit for the exponent or number of decimal digits for these numbers. (But realize that what we are discussing the rules for just the storage of these numbers. The fact that you can store such "large" numbers does not mean that that the operations over these numbers will use all of the significant digits. See `"Numeric operators and numeric conversions"' in this node for details.) In fact numbers stored in the native integer format may be stored either in the signed native form, or in the unsigned native form. Thus the limits for Perl numbers stored as native integers would typically be -2**31..2**32-1, with appropriate modifications in the case of 64-bit integers. Again, this does not mean that Perl can do operations only over integers in this range: it is possible to store many more integers in floating point format. Summing up, Perl numeric values can store only those numbers which have a finite decimal expansion or a "short" binary expansion. Numeric operators and numeric conversions ========================================= As mentioned earlier, Perl can store a number in any one of three formats, but most operators typically understand only one of those formats. When a numeric value is passed as an argument to such an operator, it will be converted to the format understood by the operator. Six such conversions are possible: native integer --> native floating point (*) native integer --> decimal string native floating_point --> native integer (*) native floating_point --> decimal string (*) decimal string --> native integer decimal string --> native floating point (*) These conversions are governed by the following general rules: * If the source number can be represented in the target form, that representation is used. * If the source number is outside of the limits representable in the target form, a representation of the closest limit is used. (*Loss of information*) * If the source number is between two numbers representable in the target form, a representation of one of these numbers is used. (*Loss of information*) * In `< native floating point --' native integer >> conversions the magnitude of the result is less than or equal to the magnitude of the source. (*"Rounding to zero".*) * If the `< decimal string --' native integer >> conversion cannot be done without loss of information, the result is compatible with the conversion sequence `< decimal_string --' native_floating_point -> native_integer >>. In particular, rounding is strongly biased to 0, though a number like `"0.99999999999999999999"' has a chance of being rounded to 1. *RESTRICTION*: The conversions marked with `(*)' above involve steps performed by the C compiler. In particular, bugs/features of the compiler used may lead to breakage of some of the above rules. Flavors of Perl numeric operations ================================== Perl operations which take a numeric argument treat that argument in one of four different ways: they may force it to one of the integer/floating/ string formats, or they may behave differently depending on the format of the operand. Forcing a numeric value to a particular format does not change the number stored in the value. All the operators which need an argument in the integer format treat the argument as in modular arithmetic, e.g., `mod 2**32' on a 32-bit architecture. `sprintf "%u", -1' therefore provides the same result as `sprintf "%u", ~0'. Arithmetic operators except, `no integer' force the argument into the floating point format. Arithmetic operators except, `use integer' Bitwise operators, `no integer' force the argument into the integer format if it is not a string. Bitwise operators, `use integer' force the argument into the integer format Operators which expect an integer force the argument into the integer format. This is applicable to the third and fourth arguments of sysread, for example. Operators which expect a string force the argument into the string format. For example, this is applicable to `printf "%s", $value'. Though forcing an argument into a particular form does not change the stored number, Perl remembers the result of such conversions. In particular, though the first such conversion may be time-consuming, repeated operations will not need to redo the conversion. AUTHOR ====== Ilya Zakharevich `ilya@math.ohio-state.edu' Editorial adjustments by Gurusamy Sarathy SEE ALSO ======== *Note Overload: (pm.info)overload,  File: perl.info, Node: perlobj, Next: perltie, Prev: perltootc, Up: Top Perl objects ************ NAME ==== perlobj - Perl objects DESCRIPTION =========== First you need to understand what references are in Perl. See *Note Perlref: perlref, for that. Second, if you still find the following reference work too complicated, a tutorial on object-oriented programming in Perl can be found in *Note Perltoot: perltoot, and *Note Perltootc: perltootc,. If you're still with us, then here are three very simple definitions that you should find reassuring. 1. An object is simply a reference that happens to know which class it belongs to. 2. A class is simply a package that happens to provide methods to deal with object references. 3. A method is simply a subroutine that expects an object reference (or a package name, for class methods) as the first argument. We'll cover these points now in more depth. An Object is Simply a Reference ------------------------------- Unlike say C++, Perl doesn't provide any special syntax for constructors. A constructor is merely a subroutine that returns a reference to something "blessed" into a class, generally the class that the subroutine is defined in. Here is a typical constructor: package Critter; sub new { bless {} } That word new isn't special. You could have written a construct this way, too: package Critter; sub spawn { bless {} } This might even be preferable, because the C++ programmers won't be tricked into thinking that new works in Perl as it does in C++. It doesn't. We recommend that you name your constructors whatever makes sense in the context of the problem you're solving. For example, constructors in the Tk extension to Perl are named after the widgets they create. One thing that's different about Perl constructors compared with those in C++ is that in Perl, they have to allocate their own memory. (The other things is that they don't automatically call overridden base-class constructors.) The `{}' allocates an anonymous hash containing no key/value pairs, and returns it The bless() takes that reference and tells the object it references that it's now a Critter, and returns the reference. This is for convenience, because the referenced object itself knows that it has been blessed, and the reference to it could have been returned directly, like this: sub new { my $self = {}; bless $self; return $self; } You often see such a thing in more complicated constructors that wish to call methods in the class as part of the construction: sub new { my $self = {}; bless $self; $self->initialize(); return $self; } If you care about inheritance (and you should; see `"Modules: Creation, Use, and Abuse"', *Note Perlmodlib: perlmodlib,), then you want to use the two-arg form of bless so that your constructors may be inherited: sub new { my $class = shift; my $self = {}; bless $self, $class; $self->initialize(); return $self; } Or if you expect people to call not just `< CLASS-'new() >> but also `< $obj-'new() >>, then use something like this. The initialize() method used will be of whatever $class we blessed the object into: sub new { my $this = shift; my $class = ref($this) || $this; my $self = {}; bless $self, $class; $self->initialize(); return $self; } Within the class package, the methods will typically deal with the reference as an ordinary reference. Outside the class package, the reference is generally treated as an opaque value that may be accessed only through the class's methods. Although a constructor can in theory re-bless a referenced object currently belonging to another class, this is almost certainly going to get you into trouble. The new class is responsible for all cleanup later. The previous blessing is forgotten, as an object may belong to only one class at a time. (Although of course it's free to inherit methods from many classes.) If you find yourself having to do this, the parent class is probably misbehaving, though. A clarification: Perl objects are blessed. References are not. Objects know which package they belong to. References do not. The bless() function uses the reference to find the object. Consider the following example: $a = {}; $b = $a; bless $a, BLAH; print "\$b is a ", ref($b), "\n"; This reports $b as being a BLAH, so obviously bless() operated on the object and not on the reference. A Class is Simply a Package --------------------------- Unlike say C++, Perl doesn't provide any special syntax for class definitions. You use a package as a class by putting method definitions into the class. There is a special array within each package called @ISA, which says where else to look for a method if you can't find it in the current package. This is how Perl implements inheritance. Each element of the @ISA array is just the name of another package that happens to be a class package. The classes are searched (depth first) for missing methods in the order that they occur in @ISA. The classes accessible through @ISA are known as base classes of the current class. All classes implicitly inherit from class UNIVERSAL as their last base class. Several commonly used methods are automatically supplied in the UNIVERSAL class; see `"Default UNIVERSAL methods"' in this node for more details. If a missing method is found in a base class, it is cached in the current class for efficiency. Changing @ISA or defining new subroutines invalidates the cache and causes Perl to do the lookup again. If neither the current class, its named base classes, nor the UNIVERSAL class contains the requested method, these three places are searched all over again, this time looking for a method named AUTOLOAD(). If an AUTOLOAD is found, this method is called on behalf of the missing method, setting the package global $AUTOLOAD to be the fully qualified name of the method that was intended to be called. If none of that works, Perl finally gives up and complains. Perl classes do method inheritance only. Data inheritance is left up to the class itself. By and large, this is not a problem in Perl, because most classes model the attributes of their object using an anonymous hash, which serves as its own little namespace to be carved up by the various classes that might want to do something with the object. The only problem with this is that you can't sure that you aren't using a piece of the hash that isn't already used. A reasonable workaround is to prepend your fieldname in the hash with the package name. sub bump { my $self = shift; $self->{ __PACKAGE__ . ".count"}++; } A Method is Simply a Subroutine ------------------------------- Unlike say C++, Perl doesn't provide any special syntax for method definition. (It does provide a little syntax for method invocation though. More on that later.) A method expects its first argument to be the object (reference) or package (string) it is being invoked on. There are two ways of calling methods, which we'll call class methods and instance methods. A class method expects a class name as the first argument. It provides functionality for the class as a whole, not for any individual object belonging to the class. Constructors are often class methods, but see *Note Perltoot: perltoot, and *Note Perltootc: perltootc, for alternatives. Many class methods simply ignore their first argument, because they already know what package they're in and don't care what package they were invoked via. (These aren't necessarily the same, because class methods follow the inheritance tree just like ordinary instance methods.) Another typical use for class methods is to look up an object by name: sub find { my ($class, $name) = @_; $objtable{$name}; } An instance method expects an object reference as its first argument. Typically it shifts the first argument into a "self" or "this" variable, and then uses that as an ordinary reference. sub display { my $self = shift; my @keys = @_ ? @_ : sort keys %$self; foreach $key (@keys) { print "\t$key => $self->{$key}\n"; } } Method Invocation ----------------- There are two ways to invoke a method, one of which you're already familiar with, and the other of which will look familiar. Perl 4 already had an "indirect object" syntax that you use when you say print STDERR "help!!!\n"; This same syntax can be used to call either class or instance methods. We'll use the two methods defined above, the class method to lookup an object reference and the instance method to print out its attributes. $fred = find Critter "Fred"; display $fred 'Height', 'Weight'; These could be combined into one statement by using a BLOCK in the indirect object slot: display {find Critter "Fred"} 'Height', 'Weight'; For C++ fans, there's also a syntax using -> notation that does exactly the same thing. The parentheses are required if there are any arguments. $fred = Critter->find("Fred"); $fred->display('Height', 'Weight'); or in one statement, Critter->find("Fred")->display('Height', 'Weight'); There are times when one syntax is more readable, and times when the other syntax is more readable. The indirect object syntax is less cluttered, but it has the same ambiguity as ordinary list operators. Indirect object method calls are usually parsed using the same rule as list operators: "If it looks like a function, it is a function". (Presuming for the moment that you think two words in a row can look like a function name. C++ programmers seem to think so with some regularity, especially when the first word is "new".) Thus, the parentheses of new Critter ('Barney', 1.5, 70) are assumed to surround ALL the arguments of the method call, regardless of what comes after. Saying new Critter ('Bam' x 2), 1.4, 45 would be equivalent to Critter->new('Bam' x 2), 1.4, 45 which is unlikely to do what you want. Confusingly, however, this rule applies only when the indirect object is a bareword package name, not when it's a scalar, a BLOCK, or a `Package::' qualified package name. In those cases, the arguments are parsed in the same way as an indirect object list operator like print, so new Critter:: ('Bam' x 2), 1.4, 45 is the same as Critter::->new(('Bam' x 2), 1.4, 45) For more reasons why the indirect object syntax is ambiguous, see `"WARNING"' in this node below. There are times when you wish to specify which class's method to use. Here you can call your method as an ordinary subroutine call, being sure to pass the requisite first argument explicitly: $fred = MyCritter::find("Critter", "Fred"); MyCritter::display($fred, 'Height', 'Weight'); Unlike method calls, function calls don't consider inheritance. If you wish merely to specify that Perl should START looking for a method in a particular package, use an ordinary method call, but qualify the method name with the package like this: $fred = Critter->MyCritter::find("Fred"); $fred->MyCritter::display('Height', 'Weight'); If you're trying to control where the method search begins and you're executing in the class itself, then you may use the SUPER pseudo class, which says to start looking in your base class's @ISA list without having to name it explicitly: $self->SUPER::display('Height', 'Weight'); Please note that the `SUPER::' construct is meaningful *only* within the class. Sometimes you want to call a method when you don't know the method name ahead of time. You can use the arrow form, replacing the method name with a simple scalar variable containing the method name or a reference to the function. $method = $fast ? "findfirst" : "findbest"; $fred->$method(@args); # call by name if ($coderef = $fred->can($parent . "::findbest")) { $self->$coderef(@args); # call by coderef } WARNING ------- While indirect object syntax may well be appealing to English speakers and to C++ programmers, be not seduced! It suffers from two grave problems. The first problem is that an indirect object is limited to a name, a scalar variable, or a block, because it would have to do too much lookahead otherwise, just like any other postfix dereference in the language. (These are the same quirky rules as are used for the filehandle slot in functions like print and printf.) This can lead to horribly confusing precedence problems, as in these next two lines: move $obj->{FIELD}; # probably wrong! move $ary[$i]; # probably wrong! Those actually parse as the very surprising: $obj->move->{FIELD}; # Well, lookee here $ary->move([$i]); # Didn't expect this one, eh? Rather than what you might have expected: $obj->{FIELD}->move(); # You should be so lucky. $ary[$i]->move; # Yeah, sure. The left side of "->" is not so limited, because it's an infix operator, not a postfix operator. As if that weren't bad enough, think about this: Perl must guess *at compile time* whether name and move above are functions or methods. Usually Perl gets it right, but when it doesn't it, you get a function call compiled as a method, or vice versa. This can introduce subtle bugs that are hard to unravel. For example, calling a method new in indirect notation-as C++ programmers are so wont to do-can be miscompiled into a subroutine call if there's already a new function in scope. You'd end up calling the current package's new as a subroutine, rather than the desired class's method. The compiler tries to cheat by remembering bareword requires, but the grief if it messes up just isn't worth the years of debugging it would likely take you to track such subtle bugs down. The infix arrow notation using "`< -' >>" doesn't suffer from either of these disturbing ambiguities, so we recommend you use it exclusively. Default UNIVERSAL methods ------------------------- The UNIVERSAL package automatically contains the following methods that are inherited by all other classes: isa(CLASS) isa returns *true* if its object is blessed into a subclass of CLASS isa is also exportable and can be called as a sub with two arguments. This allows the ability to check what a reference points to. Example use UNIVERSAL qw(isa); if(isa($ref, 'ARRAY')) { #... } can(METHOD) can checks to see if its object has a method called METHOD, if it does then a reference to the sub is returned, if it does not then undef is returned. VERSION( [NEED] ) VERSION returns the version number of the class (package). If the NEED argument is given then it will check that the current version (as defined by the $VERSION variable in the given package) not less than NEED; it will die if this is not the case. This method is normally called as a class method. This method is called automatically by the VERSION form of use. use A 1.2 qw(some imported subs); # implies: A->VERSION(1.2); NOTE: can directly uses Perl's internal code for method lookup, and isa uses a very similar method and cache-ing strategy. This may cause strange effects if the Perl code dynamically changes @ISA in any package. You may add other methods to the UNIVERSAL class via Perl or XS code. You do not need to `use UNIVERSAL' to make these methods available to your program. This is necessary only if you wish to have isa available as a plain subroutine in the current package. Destructors ----------- When the last reference to an object goes away, the object is automatically destroyed. (This may even be after you exit, if you've stored references in global variables.) If you want to capture control just before the object is freed, you may define a DESTROY method in your class. It will automatically be called at the appropriate moment, and you can do any extra cleanup you need to do. Perl passes a reference to the object under destruction as the first (and only) argument. Beware that the reference is a read-only value, and cannot be modified by manipulating `$_[0]' within the destructor. The object itself (i.e. the thingy the reference points to, namely `${$_[0]}', `@{$_[0]}', `%{$_[0]}' etc.) is not similarly constrained. If you arrange to re-bless the reference before the destructor returns, perl will again call the DESTROY method for the re-blessed object after the current one returns. This can be used for clean delegation of object destruction, or for ensuring that destructors in the base classes of your choosing get called. Explicitly calling DESTROY is also possible, but is usually never needed. Do not confuse the previous discussion with how objects *CONTAINED* in the current one are destroyed. Such objects will be freed and destroyed automatically when the current object is freed, provided no other references to them exist elsewhere. Summary ------- That's about all there is to it. Now you need just to go off and buy a book about object-oriented design methodology, and bang your forehead with it for the next six months or so. Two-Phased Garbage Collection ----------------------------- For most purposes, Perl uses a fast and simple, reference-based garbage collection system. That means there's an extra dereference going on at some level, so if you haven't built your Perl executable using your C compiler's -O flag, performance will suffer. If you *have* built Perl with `cc -O', then this probably won't matter. A more serious concern is that unreachable memory with a non-zero reference count will not normally get freed. Therefore, this is a bad idea: { my $a; $a = \$a; } Even thought $a should go away, it can't. When building recursive data structures, you'll have to break the self-reference yourself explicitly if you don't care to leak. For example, here's a self-referential node such as one might use in a sophisticated tree structure: sub new_node { my $self = shift; my $class = ref($self) || $self; my $node = {}; $node->{LEFT} = $node->{RIGHT} = $node; $node->{DATA} = [ @_ ]; return bless $node => $class; } If you create nodes like that, they (currently) won't go away unless you break their self reference yourself. (In other words, this is not to be construed as a feature, and you shouldn't depend on it.) Almost. When an interpreter thread finally shuts down (usually when your program exits), then a rather costly but complete mark-and-sweep style of garbage collection is performed, and everything allocated by that thread gets destroyed. This is essential to support Perl as an embedded or a multithreadable language. For example, this program demonstrates Perl's two-phased garbage collection: #!/usr/bin/perl package Subtle; sub new { my $test; $test = \$test; warn "CREATING " . \$test; return bless \$test; } sub DESTROY { my $self = shift; warn "DESTROYING $self"; } package main; warn "starting program"; { my $a = Subtle->new; my $b = Subtle->new; $$a = 0; # break selfref warn "leaving block"; } warn "just exited block"; warn "time to die..."; exit; When run as `/tmp/test', the following output is produced: starting program at /tmp/test line 18. CREATING SCALAR(0x8e5b8) at /tmp/test line 7. CREATING SCALAR(0x8e57c) at /tmp/test line 7. leaving block at /tmp/test line 23. DESTROYING Subtle=SCALAR(0x8e5b8) at /tmp/test line 13. just exited block at /tmp/test line 26. time to die... at /tmp/test line 27. DESTROYING Subtle=SCALAR(0x8e57c) during global destruction. Notice that "global destruction" bit there? That's the thread garbage collector reaching the unreachable. Objects are always destructed, even when regular refs aren't. Objects are destructed in a separate pass before ordinary refs just to prevent object destructors from using refs that have been themselves destructed. Plain refs are only garbage-collected if the destruct level is greater than 0. You can test the higher levels of global destruction by setting the PERL_DESTRUCT_LEVEL environment variable, presuming `-DDEBUGGING' was enabled during perl build time. A more complete garbage collection strategy will be implemented at a future date. In the meantime, the best solution is to create a non-recursive container class that holds a pointer to the self-referential data structure. Define a DESTROY method for the containing object's class that manually breaks the circularities in the self-referential structure. SEE ALSO ======== A kinder, gentler tutorial on object-oriented programming in Perl can be found in *Note Perltoot: perltoot, and *Note Perltootc: perltootc,. You should also check out *Note Perlbot: perlbot, for other object tricks, traps, and tips, as well as *Note Perlmodlib: perlmodlib, for some style guides on constructing both modules and classes.  File: perl.info, Node: perlop, Next: perlre, Prev: perlsyn, Up: Top Perl operators and precedence ***************************** NAME ==== perlop - Perl operators and precedence SYNOPSIS ======== Perl operators have the following associativity and precedence, listed from highest precedence to lowest. Operators borrowed from C keep the same precedence relationship with each other, even where C's precedence is slightly screwy. (This makes learning Perl easier for C folks.) With very few exceptions, these all operate on scalar values only, not array values. left terms and list operators (leftward) left -> nonassoc ++ -- right ** right ! ~ \ and unary + and - left =~ !~ left * / % x left + - . left << >> nonassoc named unary operators nonassoc < > <= >= lt gt le ge nonassoc == != <=> eq ne cmp left & left | ^ left && left || nonassoc .. ... right ?: right = += -= *= etc. left , => nonassoc list operators (rightward) right not left and left or xor In the following sections, these operators are covered in precedence order. Many operators can be overloaded for objects. See *Note Overload: (pm.info)overload,. DESCRIPTION =========== Terms and List Operators (Leftward) ----------------------------------- A TERM has the highest precedence in Perl. They include variables, quote and quote-like operators, any expression in parentheses, and any function whose arguments are parenthesized. Actually, there aren't really functions in this sense, just list operators and unary operators behaving as functions because you put parentheses around the arguments. These are all documented in *Note Perlfunc: perlfunc,. If any list operator (print(), etc.) or any unary operator (chdir(), etc.) is followed by a left parenthesis as the next token, the operator and arguments within parentheses are taken to be of highest precedence, just like a normal function call. In the absence of parentheses, the precedence of list operators such as print, sort, or chmod is either very high or very low depending on whether you are looking at the left side or the right side of the operator. For example, in @ary = (1, 3, sort 4, 2); print @ary; # prints 1324 the commas on the right of the sort are evaluated before the sort, but the commas on the left are evaluated after. In other words, list operators tend to gobble up all arguments that follow, and then act like a simple TERM with regard to the preceding expression. Be careful with parentheses: # These evaluate exit before doing the print: print($foo, exit); # Obviously not what you want. print $foo, exit; # Nor is this. # These do the print before evaluating exit: (print $foo), exit; # This is what you want. print($foo), exit; # Or this. print ($foo), exit; # Or even this. Also note that print ($foo & 255) + 1, "\n"; probably doesn't do what you expect at first glance. See `Named Unary Operators' in this node for more discussion of this. Also parsed as terms are the `do {}' and `eval {}' constructs, as well as subroutine and method calls, and the anonymous constructors [] and `{}'. See also `Quote and Quote-like Operators' in this node toward the end of this section, as well as `"I' in this node. The Arrow Operator ------------------ "`< -' >>" is an infix dereference operator, just as it is in C and C++. If the right side is either a `[...]', `{...}', or a `(...)' subscript, then the left side must be either a hard or symbolic reference to an array, a hash, or a subroutine respectively. (Or technically speaking, a location capable of holding a hard reference, if it's an array or hash reference being used for assignment.) See *Note Perlreftut: perlreftut, and *Note Perlref: perlref,. Otherwise, the right side is a method name or a simple scalar variable containing either the method name or a subroutine reference, and the left side must be either an object (a blessed reference) or a class name (that is, a package name). See *Note Perlobj: perlobj,. Auto-increment and Auto-decrement --------------------------------- "++" and "-" work as in C. That is, if placed before a variable, they increment or decrement the variable before returning the value, and if placed after, increment or decrement the variable after returning the value. The auto-increment operator has a little extra builtin magic to it. If you increment a variable that is numeric, or that has ever been used in a numeric context, you get a normal increment. If, however, the variable has been used in only string contexts since it was set, and has a value that is not the empty string and matches the pattern `/^[a-zA-Z]*[0-9]*$/', the increment is done as a string, preserving each character within its range, with carry: print ++($foo = '99'); # prints '100' print ++($foo = 'a0'); # prints 'a1' print ++($foo = 'Az'); # prints 'Ba' print ++($foo = 'zz'); # prints 'aaa' The auto-decrement operator is not magical. Exponentiation -------------- Binary "**" is the exponentiation operator. It binds even more tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is implemented using C's pow(3) function, which actually works on doubles internally.) Symbolic Unary Operators ------------------------ Unary "!" performs logical negation, i.e., "not". See also not for a lower precedence version of this. Unary "-" performs arithmetic negation if the operand is numeric. If the operand is an identifier, a string consisting of a minus sign concatenated with the identifier is returned. Otherwise, if the string starts with a plus or minus, a string starting with the opposite sign is returned. One effect of these rules is that `-bareword' is equivalent to `"-bareword"'. Unary "~" performs bitwise negation, i.e., 1's complement. For example, `0666 & ~027' is 0640. (See also `Integer Arithmetic' in this node and `Bitwise String Operators' in this node.) Note that the width of the result is platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64 bits wide on a 64-bit platform, so if you are expecting a certain bit width, remember use the & operator to mask off the excess bits. Unary "+" has no effect whatsoever, even on strings. It is useful syntactically for separating a function name from a parenthesized expression that would otherwise be interpreted as the complete list of function arguments. (See examples above under `Terms and List Operators (Leftward)' in this node.) Unary "\" creates a reference to whatever follows it. See *Note Perlreftut: perlreftut, and *Note Perlref: perlref,. Do not confuse this behavior with the behavior of backslash within a string, although both forms do convey the notion of protecting the next thing from interpolation. Binding Operators ----------------- Binary "=~" binds a scalar expression to a pattern match. Certain operations search or modify the string $_ by default. This operator makes that kind of operation work on some other string. The right argument is a search pattern, substitution, or transliteration. The left argument is what is supposed to be searched, substituted, or transliterated instead of the default $_. When used in scalar context, the return value generally indicates the success of the operation. Behavior in list context depends on the particular operator. See `' in this node for details. If the right argument is an expression rather than a search pattern, substitution, or transliteration, it is interpreted as a search pattern at run time. This can be less efficient than an explicit search, because the pattern must be compiled every time the expression is evaluated. Binary "!~" is just like "=~" except the return value is negated in the logical sense. Multiplicative Operators ------------------------ Binary "*" multiplies two numbers. Binary "/" divides two numbers. Binary "%" computes the modulus of two numbers. Given integer operands `$a' and $b: If $b is positive, then `$a % $b' is `$a' minus the largest multiple of $b that is not greater than `$a'. If $b is negative, then `$a % $b' is `$a' minus the smallest multiple of $b that is not less than `$a' (i.e. the result will be less than or equal to zero). Note than when `use integer' is in scope, "%" give you direct access to the modulus operator as implemented by your C compiler. This operator is not as well defined for negative operands, but it will execute faster. Binary "x" is the repetition operator. In scalar context or if the left operand is not enclosed in parentheses, it returns a string consisting of the left operand repeated the number of times specified by the right operand. In list context, if the left operand is enclosed in parentheses, it repeats the list. print '-' x 80; # print row of dashes print "\t" x ($tab/8), ' ' x ($tab%8); # tab over @ones = (1) x 80; # a list of 80 1's @ones = (5) x @ones; # set all elements to 5 Additive Operators ------------------ Binary "+" returns the sum of two numbers. Binary "-" returns the difference of two numbers. Binary "." concatenates two strings. Shift Operators --------------- Binary "<<" returns the value of its left argument shifted left by the number of bits specified by the right argument. Arguments should be integers. (See also `Integer Arithmetic' in this node.) Binary ">>" returns the value of its left argument shifted right by the number of bits specified by the right argument. Arguments should be integers. (See also `Integer Arithmetic' in this node.) Named Unary Operators --------------------- The various named unary operators are treated as functions with one argument, with optional parentheses. These include the filetest operators, like -f, -M, etc. See *Note Perlfunc: perlfunc,. If any list operator (print(), etc.) or any unary operator (chdir(), etc.) is followed by a left parenthesis as the next token, the operator and arguments within parentheses are taken to be of highest precedence, just like a normal function call. Examples: chdir $foo || die; # (chdir $foo) || die chdir($foo) || die; # (chdir $foo) || die chdir ($foo) || die; # (chdir $foo) || die chdir +($foo) || die; # (chdir $foo) || die but, because * is higher precedence than ||: chdir $foo * 20; # chdir ($foo * 20) chdir($foo) * 20; # (chdir $foo) * 20 chdir ($foo) * 20; # (chdir $foo) * 20 chdir +($foo) * 20; # chdir ($foo * 20) rand 10 * 20; # rand (10 * 20) rand(10) * 20; # (rand 10) * 20 rand (10) * 20; # (rand 10) * 20 rand +(10) * 20; # rand (10 * 20) See also `"Terms and List Operators (Leftward)"' in this node. Relational Operators -------------------- Binary "<" returns true if the left argument is numerically less than the right argument. Binary ">" returns true if the left argument is numerically greater than the right argument. Binary "<=" returns true if the left argument is numerically less than or equal to the right argument. Binary ">=" returns true if the left argument is numerically greater than or equal to the right argument. Binary "lt" returns true if the left argument is stringwise less than the right argument. Binary "gt" returns true if the left argument is stringwise greater than the right argument. Binary "le" returns true if the left argument is stringwise less than or equal to the right argument. Binary "ge" returns true if the left argument is stringwise greater than or equal to the right argument. Equality Operators ------------------ Binary "==" returns true if the left argument is numerically equal to the right argument. Binary "!=" returns true if the left argument is numerically not equal to the right argument. Binary "<=>" returns -1, 0, or 1 depending on whether the left argument is numerically less than, equal to, or greater than the right argument. Binary "eq" returns true if the left argument is stringwise equal to the right argument. Binary "ne" returns true if the left argument is stringwise not equal to the right argument. Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise less than, equal to, or greater than the right argument. "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified by the current locale if `use locale' is in effect. See *Note Perllocale: perllocale,. Bitwise And ----------- Binary "&" returns its operators ANDed together bit by bit. (See also `Integer Arithmetic' in this node and `Bitwise String Operators' in this node.) Bitwise Or and Exclusive Or --------------------------- Binary "|" returns its operators ORed together bit by bit. (See also `Integer Arithmetic' in this node and `Bitwise String Operators' in this node.) Binary "^" returns its operators XORed together bit by bit. (See also `Integer Arithmetic' in this node and `Bitwise String Operators' in this node.) C-style Logical And ------------------- Binary "&&" performs a short-circuit logical AND operation. That is, if the left operand is false, the right operand is not even evaluated. Scalar or list context propagates down to the right operand if it is evaluated. C-style Logical Or ------------------ Binary "||" performs a short-circuit logical OR operation. That is, if the left operand is true, the right operand is not even evaluated. Scalar or list context propagates down to the right operand if it is evaluated. The `||' and `&&' operators differ from C's in that, rather than returning 0 or 1, they return the last value evaluated. Thus, a reasonably portable way to find out the home directory (assuming it's not "0") might be: $home = $ENV{'HOME'} || $ENV{'LOGDIR'} || (getpwuid($<))[7] || die "You're homeless!\n"; In particular, this means that you shouldn't use this for selecting between two aggregates for assignment: @a = @b || @c; # this is wrong @a = scalar(@b) || @c; # really meant this @a = @b ? @b : @c; # this works fine, though As more readable alternatives to `&&' and `||' when used for control flow, Perl provides and and or operators (see below). The short-circuit behavior is identical. The precedence of "and" and "or" is much lower, however, so that you can safely use them after a list operator without the need for parentheses: unlink "alpha", "beta", "gamma" or gripe(), next LINE; With the C-style operators that would have been written like this: unlink("alpha", "beta", "gamma") || (gripe(), next LINE); Using "or" for assignment is unlikely to do what you want; see below. Range Operators --------------- Binary ".." is the range operator, which is really two different operators depending on the context. In list context, it returns an array of values counting (up by ones) from the left value to the right value. If the left value is greater than the right value then it returns the empty array. The range operator is useful for writing `foreach (1..10)' loops and for doing slice operations on arrays. In the current implementation, no temporary array is created when the range operator is used as the expression in foreach loops, but older versions of Perl might burn a lot of memory when you write something like this: for (1 .. 1_000_000) { # code } In scalar context, ".." returns a boolean value. The operator is bistable, like a flip-flop, and emulates the line-range (comma) operator of *sed*, *awk*, and various editors. Each ".." operator maintains its own boolean state. It is false as long as its left operand is false. Once the left operand is true, the range operator stays true until the right operand is true, *AFTER* which the range operator becomes false again. It doesn't become false till the next time the range operator is evaluated. It can test the right operand and become false on the same evaluation it became true (as in *awk*), but it still returns true once. If you don't want it to test the right operand till the next evaluation, as in *sed*, just use three dots ("...") instead of two. In all other regards, "..." behaves just like ".." does. The right operand is not evaluated while the operator is in the "false" state, and the left operand is not evaluated while the operator is in the "true" state. The precedence is a little lower than || and &&. The value returned is either the empty string for false, or a sequence number (beginning with 1) for true. The sequence number is reset for each range encountered. The final sequence number in a range has the string "E0" appended to it, which doesn't affect its numeric value, but gives you something to search for if you want to exclude the endpoint. You can exclude the beginning point by waiting for the sequence number to be greater than 1. If either operand of scalar ".." is a constant expression, that operand is implicitly compared to the $. variable, the current line number. Examples: As a scalar operator: if (101 .. 200) { print; } # print 2nd hundred lines next line if (1 .. /^$/); # skip header lines s/^/> / if (/^$/ .. eof()); # quote body # parse mail messages while (<>) { $in_header = 1 .. /^$/; $in_body = /^$/ .. eof(); # do something based on those } continue { close ARGV if eof; # reset $. each file } As a list operator: for (101 .. 200) { print; } # print $_ 100 times @foo = @foo[0 .. $#foo]; # an expensive no-op @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items The range operator (in list context) makes use of the magical auto-increment algorithm if the operands are strings. You can say @alphabet = ('A' .. 'Z'); to get all normal letters of the alphabet, or $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15]; to get a hexadecimal digit, or @z2 = ('01' .. '31'); print $z2[$mday]; to get dates with leading zeros. If the final value specified is not in the sequence that the magical increment would produce, the sequence goes until the next value would be longer than the final value specified. Conditional Operator -------------------- Ternary "?:" is the conditional operator, just as in C. It works much like an if-then-else. If the argument before the ? is true, the argument before the : is returned, otherwise the argument after the : is returned. For example: printf "I have %d dog%s.\n", $n, ($n == 1) ? '' : "s"; Scalar or list context propagates downward into the 2nd or 3rd argument, whichever is selected. $a = $ok ? $b : $c; # get a scalar @a = $ok ? @b : @c; # get an array $a = $ok ? @b : @c; # oops, that's just a count! The operator may be assigned to if both the 2nd and 3rd arguments are legal lvalues (meaning that you can assign to them): ($a_or_b ? $a : $b) = $c; Because this operator produces an assignable result, using assignments without parentheses will get you in trouble. For example, this: $a % 2 ? $a += 10 : $a += 2 Really means this: (($a % 2) ? ($a += 10) : $a) += 2 Rather than this: ($a % 2) ? ($a += 10) : ($a += 2) That should probably be written more simply as: $a += ($a % 2) ? 10 : 2; Assignment Operators -------------------- "=" is the ordinary assignment operator. Assignment operators work as in C. That is, $a += 2; is equivalent to $a = $a + 2; although without duplicating any side effects that dereferencing the lvalue might trigger, such as from tie(). Other assignment operators work similarly. The following are recognized: **= += *= &= <<= &&= -= /= |= >>= ||= .= %= ^= x= Although these are grouped by family, they all have the precedence of assignment. Unlike in C, the scalar assignment operator produces a valid lvalue. Modifying an assignment is equivalent to doing the assignment and then modifying the variable that was assigned to. This is useful for modifying a copy of something, like this: ($tmp = $global) =~ tr [A-Z] [a-z]; Likewise, ($a += 2) *= 3; is equivalent to $a += 2; $a *= 3; Similarly, a list assignment in list context produces the list of lvalues assigned to, and a list assignment in scalar context returns the number of elements produced by the expression on the right hand side of the assignment. Comma Operator -------------- Binary "," is the comma operator. In scalar context it evaluates its left argument, throws that value away, then evaluates its right argument and returns that value. This is just like C's comma operator. In list context, it's just the list argument separator, and inserts both its arguments into the list. The => digraph is mostly just a synonym for the comma operator. It's useful for documenting arguments that come in pairs. As of release 5.001, it also forces any word to the left of it to be interpreted as a string. List Operators (Rightward) -------------------------- On the right side of a list operator, it has very low precedence, such that it controls all comma-separated expressions found there. The only operators with lower precedence are the logical operators "and", "or", and "not", which may be used to evaluate calls to list operators without the need for extra parentheses: open HANDLE, "filename" or die "Can't open: $!\n"; See also discussion of list operators in `Terms and List Operators (Leftward)' in this node. Logical Not ----------- Unary "not" returns the logical negation of the expression to its right. It's the equivalent of "!" except for the very low precedence. Logical And ----------- Binary "and" returns the logical conjunction of the two surrounding expressions. It's equivalent to && except for the very low precedence. This means that it short-circuits: i.e., the right expression is evaluated only if the left expression is true. Logical or and Exclusive Or --------------------------- Binary "or" returns the logical disjunction of the two surrounding expressions. It's equivalent to || except for the very low precedence. This makes it useful for control flow print FH $data or die "Can't write to FH: $!"; This means that it short-circuits: i.e., the right expression is evaluated only if the left expression is false. Due to its precedence, you should probably avoid using this for assignment, only for control flow. $a = $b or $c; # bug: this is wrong ($a = $b) or $c; # really means this $a = $b || $c; # better written this way However, when it's a list-context assignment and you're trying to use "||" for control flow, you probably need "or" so that the assignment takes higher precedence. @info = stat($file) || die; # oops, scalar sense of stat! @info = stat($file) or die; # better, now @info gets its due Then again, you could always use parentheses. Binary "xor" returns the exclusive-OR of the two surrounding expressions. It cannot short circuit, of course. C Operators Missing From Perl ----------------------------- Here is what C has that Perl doesn't: unary & Address-of operator. (But see the "\" operator for taking a reference.) unary * Dereference-address operator. (Perl's prefix dereferencing operators are typed: $, @, %, and &.) (TYPE) Type-casting operator. Quote and Quote-like Operators ------------------------------ While we usually think of quotes as literal values, in Perl they function as operators, providing various kinds of interpolating and pattern matching capabilities. Perl provides customary quote characters for these behaviors, but also provides a way for you to choose your quote character for any of them. In the following table, a `{}' represents any pair of delimiters you choose. Customary Generic Meaning Interpolates '' q{} Literal no "" qq{} Literal yes `` qx{} Command yes (unless '' is delimiter) qw{} Word list no // m{} Pattern match yes (unless '' is delimiter) qr{} Pattern yes (unless '' is delimiter) s{}{} Substitution yes (unless '' is delimiter) tr{}{} Transliteration no (but see below) Non-bracketing delimiters use the same character fore and aft, but the four sorts of brackets (round, angle, square, curly) will all nest, which means that q{foo{bar}baz} is the same as 'foo{bar}baz' Note, however, that this does not always work for quoting Perl code: $s = q{ if($a eq "}") ... }; # WRONG is a syntax error. The `Text::Balanced' module on CPAN is able to do this properly. There can be whitespace between the operator and the quoting characters, except when `#' is being used as the quoting character. `q#foo#' is parsed as the string foo, while `q #foo#' is the operator q followed by a comment. Its argument will be taken from the next line. This allows you to write: s {foo} # Replace foo {bar} # with bar. For constructs that do interpolate, variables beginning with "`$'" or "`@'" are interpolated, as are the following escape sequences. Within a transliteration, the first eleven of these sequences may be used. \t tab (HT, TAB) \n newline (NL) \r return (CR) \f form feed (FF) \b backspace (BS) \a alarm (bell) (BEL) \e escape (ESC) \033 octal char (ESC) \x1b hex char (ESC) \x{263a} wide hex char (SMILEY) \c[ control char (ESC) \N{name} named char \l lowercase next char \u uppercase next char \L lowercase till \E \U uppercase till \E \E end case modification \Q quote non-word characters till \E If `use locale' is in effect, the case map used by `\l', `\L', `\u' and `\U' is taken from the current locale. See *Note Perllocale: perllocale,. For documentation of `\N{name}', see *Note Charnames: (pm.info)charnames,. All systems use the virtual `"\n"' to represent a line terminator, called a "newline". There is no such thing as an unvarying, physical newline character. It is only an illusion that the operating system, device drivers, C libraries, and Perl all conspire to preserve. Not all systems read `"\r"' as ASCII CR and `"\n"' as ASCII LF. For example, on a Mac, these are reversed, and on systems without line terminator, printing `"\n"' may emit no actual data. In general, use `"\n"' when you mean a "newline" for your system, but use the literal ASCII when you need an exact character. For example, most networking protocols expect and prefer a CR+LF (`"\012\015"' or `"\cJ\cM"') for line terminators, and although they often accept just `"\012"', they seldom tolerate just `"\015"'. If you get in the habit of using `"\n"' for networking, you may be burned some day. You cannot include a literal `$' or `@' within a `\Q' sequence. An unescaped `$' or `@' interpolates the corresponding variable, while escaping will cause the literal string `\$' to be inserted. You'll need to write something like `m/\Quser\E\@\Qhost/'. Patterns are subject to an additional level of interpretation as a regular expression. This is done as a second pass, after variables are interpolated, so that regular expressions may be incorporated into the pattern from the variables. If this is not what you want, use `\Q' to interpolate a variable literally. Apart from the behavior described above, Perl does not expand multiple levels of interpolation. In particular, contrary to the expectations of shell programmers, back-quotes do *NOT* interpolate within double quotes, nor do single quotes impede evaluation of variables when used within double quotes. Regexp Quote-Like Operators --------------------------- Here are the quote-like operators that apply to pattern matching and related activities. ?PATTERN? This is just like the /pattern/ search, except that it matches only once between calls to the reset() operator. This is a useful optimization when you want to see only the first occurrence of something in each file of a set of files, for instance. Only `??' patterns local to the current package are reset. while (<>) { if (?^$?) { # blank line between header and body } } continue { reset if eof; # clear ?? status for next file } This usage is vaguely depreciated, which means it just might possibly be removed in some distant future version of Perl, perhaps somewhere around the year 2168. m/PATTERN/cgimosx /PATTERN/cgimosx Searches a string for a pattern match, and in scalar context returns true if it succeeds, false if it fails. If no string is specified via the `=~' or `!~' operator, the $_ string is searched. (The string specified with `=~' need not be an lvalue-it may be the result of an expression evaluation, but remember the `=~' binds rather tightly.) See also *Note Perlre: perlre,. See *Note Perllocale: perllocale, for discussion of additional considerations that apply when `use locale' is in effect. Options are: c Do not reset search position on a failed match when /g is in effect. g Match globally, i.e., find all occurrences. i Do case-insensitive pattern matching. m Treat string as multiple lines. o Compile pattern only once. s Treat string as single line. x Use extended regular expressions. If "/" is the delimiter then the initial m is optional. With the m you can use any pair of non-alphanumeric, non-whitespace characters as delimiters. This is particularly useful for matching path names that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is the delimiter, then the match-only-once rule of ?PATTERN? applies. If "'" is the delimiter, no interpolation is performed on the PATTERN. PATTERN may contain variables, which will be interpolated (and the pattern recompiled) every time the pattern search is evaluated, except for when the delimiter is a single quote. (Note that $) and $| might not be interpolated because they look like end-of-string tests.) If you want such a pattern to be compiled only once, add a `/o' after the trailing delimiter. This avoids expensive run-time recompilations, and is useful when the value you are interpolating won't change over the life of the script. However, mentioning `/o' constitutes a promise that you won't change the variables in the pattern. If you change them, Perl won't even notice. See also `"qr' in this node. If the PATTERN evaluates to the empty string, the last *successfully* matched regular expression is used instead. If the `/g' option is not used, m// in list context returns a list consisting of the subexpressions matched by the parentheses in the pattern, i.e., ($1, $2, $3...). (Note that here $1 etc. are also set, and that this differs from Perl 4's behavior.) When there are no parentheses in the pattern, the return value is the list (1) for success. With or without parentheses, an empty list is returned upon failure. Examples: open(TTY, '/dev/tty'); =~ /^y/i && foo(); # do foo if desired if (/Version: *([0-9.]*)/) { $version = $1; } next if m#^/usr/spool/uucp#; # poor man's grep $arg = shift; while (<>) { print if /$arg/o; # compile only once } if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) This last example splits $foo into the first two words and the remainder of the line, and assigns those three fields to $F1, $F2, and $Etc. The conditional is true if any variables were assigned, i.e., if the pattern matched. The `/g' modifier specifies global pattern matching-that is, matching as many times as possible within the string. How it behaves depends on the context. In list context, it returns a list of the substrings matched by any capturing parentheses in the regular expression. If there are no parentheses, it returns a list of all the matched strings, as if there were parentheses around the whole pattern. In scalar context, each execution of `m//g' finds the next match, returning true if it matches, and false if there is no further match. The position after the last match can be read or set using the pos() function; see `pos', *Note Perlfunc: perlfunc,. A failed match normally resets the search position to the beginning of the string, but you can avoid that by adding the `/c' modifier (e.g. `m//gc'). Modifying the target string also resets the search position. You can intermix `m//g' matches with `m/\G.../g', where `\G' is a zero-width assertion that matches the exact position where the previous `m//g', if any, left off. The `\G' assertion is not supported without the `/g' modifier. (Currently, without `/g', `\G' behaves just like `\A', but that's accidental and may change in the future.) Examples: # list context ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g); # scalar context $/ = ""; $* = 1; # $* deprecated in modern perls while (defined($paragraph = <>)) { while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) { $sentences++; } } print "$sentences\n"; # using m//gc with \G $_ = "ppooqppqq"; while ($i++ < 2) { print "1: '"; print $1 while /(o)/gc; print "', pos=", pos, "\n"; print "2: '"; print $1 if /\G(q)/gc; print "', pos=", pos, "\n"; print "3: '"; print $1 while /(p)/gc; print "', pos=", pos, "\n"; } The last example should print: 1: 'oo', pos=4 2: 'q', pos=5 3: 'pp', pos=7 1: '', pos=7 2: 'q', pos=8 3: '', pos=8 A useful idiom for lex-like scanners is `/\G.../gc'. You can combine several regexps like this to process a string part-by-part, doing different actions depending on which regexp matched. Each regexp tries to match where the previous one leaves off. $_ = <<'EOL'; $url = new URI::URL "http://www/"; die if $url eq "xXx"; EOL LOOP: { print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc; print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc; print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc; print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc; print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc; print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc; print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc; print ". That's all!\n"; } Here is the output (split into several lines): line-noise lowercase line-noise lowercase UPPERCASE line-noise UPPERCASE line-noise lowercase line-noise lowercase line-noise lowercase lowercase line-noise lowercase lowercase line-noise MiXeD line-noise. That's all! q/STRING/ `'STRING'' A single-quoted, literal string. A backslash represents a backslash unless followed by the delimiter or another backslash, in which case the delimiter or backslash is interpolated. $foo = q!I said, "You said, 'She said it.'"!; $bar = q('This is it.'); $baz = '\n'; # a two-character string qq/STRING/ "STRING" A double-quoted, interpolated string. $_ .= qq (*** The previous line contains the naughty word "$1".\n) if /\b(tcl|java|python)\b/i; # :-) $baz = "\n"; # a one-character string qr/STRING/imosx This operators quotes-and compiles-its STRING as a regular expression. STRING is interpolated the same way as PATTERN in `m/PATTERN/'. If "'" is used as the delimiter, no interpolation is done. Returns a Perl value which may be used instead of the corresponding `/STRING/imosx' expression. For example, $rex = qr/my.STRING/is; s/$rex/foo/; is equivalent to s/my.STRING/foo/is; The result may be used as a subpattern in a match: $re = qr/$pattern/; $string =~ /foo${re}bar/; # can be interpolated in other patterns $string =~ $re; # or used standalone $string =~ /$re/; # or this way Since Perl may compile the pattern at the moment of execution of qr() operator, using qr() may have speed advantages in some situations, notably if the result of qr() is used standalone: sub match { my $patterns = shift; my @compiled = map qr/$_/i, @$patterns; grep { my $success = 0; foreach my $pat (@compiled) { $success = 1, last if /$pat/; } $success; } @_; } Precompilation of the pattern into an internal representation at the moment of qr() avoids a need to recompile the pattern every time a match `/$pat/' is attempted. (Perl has many other internal optimizations, but none would be triggered in the above example if we did not use qr() operator.) Options are: i Do case-insensitive pattern matching. m Treat string as multiple lines. o Compile pattern only once. s Treat string as single line. x Use extended regular expressions. See *Note Perlre: perlre, for additional information on valid syntax for STRING, and for a detailed look at the semantics of regular expressions. qx/STRING/ `STRING` A string which is (possibly) interpolated and then executed as a system command with `/bin/sh' or its equivalent. Shell wildcards, pipes, and redirections will be honored. The collected standard output of the command is returned; standard error is unaffected. In scalar context, it comes back as a single (potentially multi-line) string. In list context, returns a list of lines (however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR). Because backticks do not affect standard error, use shell file descriptor syntax (assuming the shell supports this) if you care to address this. To capture a command's STDERR and STDOUT together: $output = `cmd 2>&1`; To capture a command's STDOUT but discard its STDERR: $output = `cmd 2>/dev/null`; To capture a command's STDERR but discard its STDOUT (ordering is important here): $output = `cmd 2>&1 1>/dev/null`; To exchange a command's STDOUT and STDERR in order to capture the STDERR but leave its STDOUT to come out the old STDERR: $output = `cmd 3>&1 1>&2 2>&3 3>&-`; To read both a command's STDOUT and its STDERR separately, it's easiest and safest to redirect them separately to files, and then read from those files when the program is done: system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr"); Using single-quote as a delimiter protects the command from Perl's double-quote interpolation, passing it on to the shell instead: $perl_info = qx(ps $$); # that's Perl's $$ $shell_info = qx'ps $$'; # that's the new shell's $$ How that string gets evaluated is entirely subject to the command interpreter on your system. On most platforms, you will have to protect shell metacharacters if you want them treated literally. This is in practice difficult to do, as it's unclear how to escape which characters. See *Note Perlsec: perlsec, for a clean and safe example of a manual fork() and exec() to emulate backticks safely. On some platforms (notably DOS-like ones), the shell may not be capable of dealing with multiline commands, so putting newlines in the string may not get you what you want. You may be able to evaluate multiple commands in a single line by separating them with the command separator character, if your shell supports that (e.g. `;' on many Unix shells; & on the Windows NT cmd shell). Beginning with v5.6.0, Perl will attempt to flush all files opened for output before starting the child process, but this may not be supported on some platforms (see *Note Perlport: perlport,). To be safe, you may need to set $| ($AUTOFLUSH in English) or call the autoflush() method of IO::Handle on any open handles. Beware that some command shells may place restrictions on the length of the command line. You must ensure your strings don't exceed this limit after any necessary interpolations. See the platform-specific release notes for more details about your particular environment. Using this operator can lead to programs that are difficult to port, because the shell commands called vary between systems, and may in fact not be present at all. As one example, the type command under the POSIX shell is very different from the type command under DOS. That doesn't mean you should go out of your way to avoid backticks when they're the right way to get something done. Perl was made to be a glue language, and one of the things it glues together is commands. Just understand what you're getting yourself into. See `"I' in this node for more discussion. qw/STRING/ Evaluates to a list of the words extracted out of STRING, using embedded whitespace as the word delimiters. It can be understood as being roughly equivalent to: split(' ', q/STRING/); the difference being that it generates a real list at compile time. So this expression: qw(foo bar baz) is semantically equivalent to the list: 'foo', 'bar', 'baz' Some frequently seen examples: use POSIX qw( setlocale localeconv ) @EXPORT = qw( foo bar baz ); A common mistake is to try to separate the words with comma or to put comments into a multi-line `qw'-string. For this reason, the `use warnings' pragma and the -w switch (that is, the $^W variable) produces warnings if the STRING contains the "," or the "#" character. s/PATTERN/REPLACEMENT/egimosx Searches a string for a pattern, and if found, replaces that pattern with the replacement text and returns the number of substitutions made. Otherwise it returns false (specifically, the empty string). If no string is specified via the `=~' or `!~' operator, the $_ variable is searched and modified. (The string specified with `=~' must be scalar variable, an array element, a hash element, or an assignment to one of those, i.e., an lvalue.) If the delimiter chosen is a single quote, no interpolation is done on either the PATTERN or the REPLACEMENT. Otherwise, if the PATTERN contains a $ that looks like a variable rather than an end-of-string test, the variable will be interpolated into the pattern at run-time. If you want the pattern compiled only once the first time the variable is interpolated, use the `/o' option. If the pattern evaluates to the empty string, the last successfully executed regular expression is used instead. See *Note Perlre: perlre, for further explanation on these. See *Note Perllocale: perllocale, for discussion of additional considerations that apply when `use locale' is in effect. Options are: e Evaluate the right side as an expression. g Replace globally, i.e., all occurrences. i Do case-insensitive pattern matching. m Treat string as multiple lines. o Compile pattern only once. s Treat string as single line. x Use extended regular expressions. Any non-alphanumeric, non-whitespace delimiter may replace the slashes. If single quotes are used, no interpretation is done on the replacement string (the `/e' modifier overrides this, however). Unlike Perl 4, Perl 5 treats backticks as normal delimiters; the replacement text is not evaluated as a command. If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own pair of quotes, which may or may not be bracketing quotes, e.g., `s(foo)(bar)' or `< s>. A `/e' will cause the replacement portion to be treated as a full-fledged Perl expression and evaluated right then and there. It is, however, syntax checked at compile-time. A second e modifier will cause the replacement portion to be evaled before being run as a Perl expression. Examples: s/\bgreen\b/mauve/g; # don't change wintergreen $path =~ s|/usr/bin|/usr/local/bin|; s/Login: $foo/Login: $bar/; # run-time pattern ($foo = $bar) =~ s/this/that/; # copy first, then change $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count $_ = 'abc123xyz'; s/\d+/$&*2/e; # yields 'abc246xyz' s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz' s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz' s/%(.)/$percent{$1}/g; # change percent escapes; no /e s/%(.)/$percent{$1} || $&/ge; # expr now, so /e s/^=(\w+)/&pod($1)/ge; # use function call # expand variables in $_, but dynamics only, using # symbolic dereferencing s/\$(\w+)/${$1}/g; # Add one to the value of any numbers in the string s/(\d+)/1 + $1/eg; # This will expand any embedded scalar variable # (including lexicals) in $_ : First $1 is interpolated # to the variable name, and then evaluated s/(\$\w+)/$1/eeg; # Delete (most) C comments. $program =~ s { /\* # Match the opening delimiter. .*? # Match a minimal number of characters. \*/ # Match the closing delimiter. } []gsx; s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively for ($variable) { # trim white space in $variable, cheap s/^\s+//; s/\s+$//; } s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields Note the use of $ instead of \ in the last example. Unlike *sed*, we use the \ form in only the left hand side. Anywhere else it's $. Occasionally, you can't use just a `/g' to get all the changes to occur that you might want. Here are two common cases: # put commas in the right places in an integer 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; # expand tabs to 8-column spacing 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e; tr/SEARCHLIST/REPLACEMENTLIST/cdsUC y/SEARCHLIST/REPLACEMENTLIST/cdsUC Transliterates all occurrences of the characters found in the search list with the corresponding character in the replacement list. It returns the number of characters replaced or deleted. If no string is specified via the =~ or !~ operator, the $_ string is transliterated. (The string specified with =~ must be a scalar variable, an array element, a hash element, or an assignment to one of those, i.e., an lvalue.) A character range may be specified with a hyphen, so `tr/A-J/0-9/' does the same replacement as `tr/ACEGIBDFHJ/0246813579/'. For *sed* devotees, y is provided as a synonym for tr. If the SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has its own pair of quotes, which may or may not be bracketing quotes, e.g., `tr[A-Z][a-z]' or `tr(+\-*/)/ABCD/'. Note also that the whole range idea is rather unportable between character sets-and even within character sets they may cause results you probably didn't expect. A sound principle is to use only ranges that begin from and end at either alphabets of equal case (a-e, A-E), or digits (0-4). Anything else is unsafe. If in doubt, spell out the character sets in full. Options: c Complement the SEARCHLIST. d Delete found but unreplaced characters. s Squash duplicate replaced characters. U Translate to/from UTF-8. C Translate to/from 8-bit char (octet). If the `/c' modifier is specified, the SEARCHLIST character set is complemented. If the `/d' modifier is specified, any characters specified by SEARCHLIST not found in REPLACEMENTLIST are deleted. (Note that this is slightly more flexible than the behavior of some tr programs, which delete anything they find in the SEARCHLIST, period.) If the `/s' modifier is specified, sequences of characters that were transliterated to the same character are squashed down to a single instance of the character. If the `/d' modifier is used, the REPLACEMENTLIST is always interpreted exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter than the SEARCHLIST, the final character is replicated till it is long enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated. This latter is useful for counting characters in a class or for squashing character sequences in a class. The first `/U' or `/C' modifier applies to the left side of the translation. The second one applies to the right side. If present, these modifiers override the current utf8 state. Examples: $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case $cnt = tr/*/*/; # count the stars in $_ $cnt = $sky =~ tr/*/*/; # count the stars in $sky $cnt = tr/0-9//; # count the digits in $_ tr/a-zA-Z//s; # bookkeeper -> bokeper ($HOST = $host) =~ tr/a-z/A-Z/; tr/a-zA-Z/ /cs; # change non-alphas to single space tr [\200-\377] [\000-\177]; # delete 8th bit tr/\0-\xFF//CU; # change Latin-1 to Unicode tr/\0-\x{FF}//UC; # change Unicode to Latin-1 If multiple transliterations are given for a character, only the first one is used: tr/AAA/XYZ/ will transliterate any A to X. Because the transliteration table is built at compile time, neither the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote interpolation. That means that if you want to use variables, you must use an eval(): eval "tr/$oldlist/$newlist/"; die $@ if $@; eval "tr/$oldlist/$newlist/, 1" or die $@; Gory details of parsing quoted constructs ----------------------------------------- When presented with something that might have several different interpretations, Perl uses the *DWIM* (that's "Do What I Mean") principle to pick the most probable interpretation. This strategy is so successful that Perl programmers often do not suspect the ambivalence of what they write. But from time to time, Perl's notions differ substantially from what the author honestly meant. This section hopes to clarify how Perl handles quoted constructs. Although the most common reason to learn this is to unravel labyrinthine regular expressions, because the initial steps of parsing are the same for all quoting operators, they are all discussed together. The most important Perl parsing rule is the first one discussed below: when processing a quoted construct, Perl first finds the end of that construct, then interprets its contents. If you understand this rule, you may skip the rest of this section on the first reading. The other rules are likely to contradict the user's expectations much less frequently than this first one. Some passes discussed below are performed concurrently, but because their results are the same, we consider them individually. For different quoting constructs, Perl performs different numbers of passes, from one to five, but these passes are always performed in the same order. Finding the end The first pass is finding the end of the quoted construct, whether it be a multicharacter delimiter `"\nEOF\n"' in the `<> which terminates a fileglob started with `< < '>. When searching for single-character non-pairing delimiters, such as /, combinations of \\ and `\/' are skipped. However, when searching for single-character pairing delimiter like [, combinations of \\, `\]', and `\[' are all skipped, and nested [, ] are skipped as well. When searching for multicharacter delimiters, nothing is skipped. For constructs with three-part delimiters (s///, y///, and tr///), the search is repeated once more. During this search no attention is paid to the semantics of the construct. Thus: "$hash{"$foo/$bar"}" or: m/ bar # NOT a comment, this slash / terminated m//! /x do not form legal quoted expressions. The quoted part ends on the first `"' and /, and the rest happens to be a syntax error. Because the slash that terminated m// was followed by a `SPACE', the example above is not `m//x', but rather m// with no `/x' modifier. So the embedded `#' is interpreted as a literal `#'. Removal of backslashes before delimiters During the second pass, text between the starting and ending delimiters is copied to a safe location, and the \ is removed from combinations consisting of \ and delimiter-or delimiters, meaning both starting and ending delimiters will should these differ. This removal does not happen for multi-character delimiters. Note that the combination \\ is left intact, just as it was. Starting from this step no information about the delimiters is used in parsing. Interpolation The next step is interpolation in the text obtained, which is now delimiter-independent. There are four different cases. `<<'EOF'', `m''', `s'''', tr///, y/// No interpolation is performed. ", q// The only interpolation is removal of \ from pairs \\. "", ```', `qq//', qx//, `< > `\Q', `\U', `\u', `\L', `\l' (possibly paired with `\E') are converted to corresponding Perl constructs. Thus, `"$foo\Qbaz$bar"' is converted to `$foo . (quotemeta("baz" . $bar))' internally. The other combinations are replaced with appropriate expansions. Let it be stressed that *whatever falls between `\Q' and `\E'* is interpolated in the usual way. Something like `"\Q\\E"' has no `\E' inside. instead, it has `\Q', \\, and E, so the result is the same as for `"\\\\E"'. As a general rule, backslashes between `\Q' and `\E' may lead to counterintuitive results. So, `"\Q\t\E"' is converted to `quotemeta("\t")', which is the same as `"\\\t"' (since TAB is not alphanumeric). Note also that: $str = '\t'; return "\Q$str"; may be closer to the conjectural *intention* of the writer of `"\Q\t\E"'. Interpolated scalars and arrays are converted internally to the join and . catentation operations. Thus, `"$foo XXX '@arr'"' becomes: $foo . " XXX '" . (join $", @arr) . "'"; All operations above are performed simultaneously, left to right. Because the result of `"\Q STRING \E"' has all metacharacters quoted, there is no way to insert a literal `$' or `@' inside a `\Q\E' pair. If protected by \, `$' will be quoted to became `"\\\$"'; if not, it is interpreted as the start of an interpolated scalar. Note also that the interpolation code needs to make a decision on where the interpolated scalar ends. For instance, whether `< "a $b -' {c}" >> really means: "a " . $b . " -> {c}"; or: "a " . $b -> {c}; Most of the time, the longest possible text that does not include spaces between components and which contains matching braces or brackets. because the outcome may be determined by voting based on heuristic estimators, the result is not strictly predictable. Fortunately, it's usually correct for ambiguous cases. `?RE?', `/RE/', `m/RE/', `s/RE/foo/', Processing of `\Q', `\U', `\u', `\L', `\l', and interpolation happens (almost) as with `qq//' constructs, but the substitution of \ followed by RE-special chars (including \) is not performed. Moreover, inside `(?{BLOCK})', `(?# comment )', and a `#'-comment in a `//x'-regular expression, no processing is performed whatsoever. This is the first step at which the presence of the `//x' modifier is relevant. Interpolation has several quirks: $|, $(, and $) are not interpolated, and constructs `$var[SOMETHING]' are voted (by several different estimators) to be either an array element or `$var' followed by an RE alternative. This is where the notation `${arr[$bar]}' comes handy: `/${arr[0-9]}/' is interpreted as array element `-9', not as a regular expression from the variable `$arr' followed by a digit, which would be the interpretation of `/$arr[0-9]/'. Since voting among different estimators may occur, the result is not predictable. It is at this step that \1 is begrudgingly converted to $1 in the replacement text of s/// to correct the incorrigible *sed* hackers who haven't picked up the saner idiom yet. A warning is emitted if the `use warnings' pragma or the -w command-line flag (that is, the $^W variable) was set. The lack of processing of \\ creates specific restrictions on the post-processed text. If the delimiter is /, one cannot get the combination `\/' into the result of this step. / will finish the regular expression, `\/' will be stripped to / on the previous step, and `\\/' will be left as is. Because / is equivalent to `\/' inside a regular expression, this does not matter unless the delimiter happens to be character special to the RE engine, such as in `s*foo*bar*', `m[foo]', or `?foo?'; or an alphanumeric char, as in: m m ^ a \s* b mmx; In the RE above, which is intentionally obfuscated for illustration, the delimiter is m, the modifier is mx, and after backslash-removal the RE is the same as for `m/ ^ a s* b /mx'). There's more than one reason you're encouraged to restrict your delimiters to non-alphanumeric, non-whitespace choices. This step is the last one for all constructs except regular expressions, which are processed further. Interpolation of regular expressions Previous steps were performed during the compilation of Perl code, but this one happens at run time-although it may be optimized to be calculated at compile time if appropriate. After preprocessing described above, and possibly after evaluation if catenation, joining, casing translation, or metaquoting are involved, the resulting string is passed to the RE engine for compilation. Whatever happens in the RE engine might be better discussed in *Note Perlre: perlre,, but for the sake of continuity, we shall do so here. This is another step where the presence of the `//x' modifier is relevant. The RE engine scans the string from left to right and converts it to a finite automaton. Backslashed characters are either replaced with corresponding literal strings (as with `\{'), or else they generate special nodes in the finite automaton (as with `\b'). Characters special to the RE engine (such as |) generate corresponding nodes or groups of nodes. `(?#...)' comments are ignored. All the rest is either converted to literal strings to match, or else is ignored (as is whitespace and `#'-style comments if `//x' is present). Parsing of the bracketed character class construct, `[...]', is rather different than the rule used for the rest of the pattern. The terminator of this construct is found using the same rules as for finding the terminator of a `{}'-delimited construct, the only exception being that ] immediately following [ is treated as though preceded by a backslash. Similarly, the terminator of `(?{...})' is found using the same rules as for finding the terminator of a `{}'-delimited construct. It is possible to inspect both the string given to RE engine and the resulting finite automaton. See the arguments debug/`debugcolor' in the `use *Note Re: (pm.info)re,' pragma, as well as Perl's *-Dr* command-line switch documented in `"Command Switches"', *Note Perlrun: perlrun,. Optimization of regular expressions This step is listed for completeness only. Since it does not change semantics, details of this step are not documented and are subject to change without notice. This step is performed over the finite automaton that was generated during the previous pass. It is at this stage that `split()' silently optimizes `/^/' to mean `/^/m'. I/O Operators ------------- There are several I/O operators you should know about. A string enclosed by backticks (grave accents) first undergoes double-quote interpolation. It is then interpreted as an external command, and the output of that command is the value of the pseudo-literal, j string consisting of all output is returned. In list context, a list of values is returned, one per line of output. (You can set $/ to use a different line terminator.) The command is executed each time the pseudo-literal is evaluated. The status value of the command is returned in $? (see *Note Perlvar: perlvar, for the interpretation of $?). Unlike in *csh*, no translation is done on the return data-newlines remain newlines. Unlike in any of the shells, single quotes do not hide variable names in the command from interpretation. To pass a literal dollar-sign through to the shell you need to hide it with a backslash. The generalized form of backticks is qx//. (Because backticks always undergo shell expansion as well, see *Note Perlsec: perlsec, for security concerns.) In scalar context, evaluating a filehandle in angle brackets yields the next line from that file (the newline, if any, included), or undef at end-of-file or on error. When $/ is set to undef (sometimes known as file-slurp mode) and the file is empty, it returns " the first time, followed by undef subsequently. Ordinarily you must assign the returned value to a variable, but there is one situation where an automatic assignment happens. If and only if the input symbol is the only thing inside the conditional of a while statement (even if disguised as a `for(;;)' loop), the value is automatically assigned to the global variable $_, destroying whatever was there previously. (This may seem like an odd thing to you, but you'll use the construct in almost every Perl script you write.) The $_ variables is not implicitly localized. You'll have to put a `local $_;' before the loop if you want that to happen. The following lines are equivalent: while (defined($_ = )) { print; } while ($_ = ) { print; } while () { print; } for (;;) { print; } print while defined($_ = ); print while ($_ = ); print while ; This also behaves similarly, but avoids $_ : while (my $line = ) { print $line } In these loop constructs, the assigned value (whether assignment is automatic or explicit) is then tested to see whether it is defined. The defined test avoids problems where line has a string value that would be treated as false by Perl, for example a "" or a "0" with no trailing newline. If you really mean for such values to terminate the loop, they should be tested for explicitly: while (($_ = ) ne '0') { ... } while () { last unless $_; ... } In other boolean contexts, `< <*filehandle*' >> without an explicit defined test or comparison elicit a warning if the `use warnings' pragma or the -w command-line switch (the $^W variable) is in effect. The filehandles STDIN, STDOUT, and STDERR are predefined. (The filehandles stdin, stdout, and stderr will also work except in packages, where they would be interpreted as local identifiers rather than global.) Additional filehandles may be created with the open() function, amongst others. See *Note Perlopentut: perlopentut, and `open', *Note Perlfunc: perlfunc, for details on this. If a is used in a context that is looking for a list, a list comprising all input lines is returned, one line per list element. It's easy to grow to a rather large data space this way, so use with care. may also be spelled `readline(*FILEHANDLE)'. See `readline', *Note Perlfunc: perlfunc,. The null filehandle <> is special: it can be used to emulate the behavior of *sed* and *awk*. Input from <> comes either from standard input, or from each file listed on the command line. Here's how it works: the first time <> is evaluated, the @ARGV array is checked, and if it is empty, `$ARGV[0]' is set to "-", which when opened gives you standard input. The @ARGV array is then processed as a list of filenames. The loop while (<>) { ... # code for each line } is equivalent to the following Perl-like pseudo code: unshift(@ARGV, '-') unless @ARGV; while ($ARGV = shift) { open(ARGV, $ARGV); while () { ... # code for each line } } except that it isn't so cumbersome to say, and will actually work. It really does shift the @ARGV array and put the current filename into the $ARGV variable. It also uses filehandle *ARGV* internally-<> is just a synonym for , which is magical. (The pseudo code above doesn't work because it treats as non-magical.) You can modify @ARGV before the first <> as long as the array ends up containing the list of filenames you really want. Line numbers ($.) continue as though the input were one big happy file. See the example in `eof', *Note Perlfunc: perlfunc, for how to reset line numbers on each file. If you want to set @ARGV to your own list of files, go right ahead. This sets @ARGV to all plain text files if no @ARGV was given: @ARGV = grep { -f && -T } glob('*') unless @ARGV; You can even set them to pipe commands. For example, this automatically filters compressed arguments through *gzip*: @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV; If you want to pass switches into your script, you can use one of the Getopts modules or put a loop on the front like this: while ($_ = $ARGV[0], /^-/) { shift; last if /^--$/; if (/^-D(.*)/) { $debug = $1 } if (/^-v/) { $verbose++ } # ... # other switches } while (<>) { # ... # code for each line } The <> symbol will return undef for end-of-file only once. If you call it again after this, it will assume you are processing another @ARGV list, and if you haven't set @ARGV, will read input from STDIN. If angle brackets contain is a simple scalar variable (e.g., <$foo>), then that variable contains the name of the filehandle to input from, or its typeglob, or a reference to the same. For example: $fh = \*STDIN; $line = <$fh>; If what's within the angle brackets is neither a filehandle nor a simple scalar variable containing a filehandle name, typeglob, or typeglob reference, it is interpreted as a filename pattern to be globbed, and either a list of filenames or the next filename in the list is returned, depending on context. This distinction is determined on syntactic grounds alone. That means `< <$x' >> is always a readline() from an indirect handle, but `< <$hash{key}' >> is always a glob(). That's because $x is a simple scalar variable, but `$hash{key}' is not-it's a hash element. One level of double-quote interpretation is done first, but you can't say `< <$foo' >> because that's an indirect filehandle as explained in the previous paragraph. (In older versions of Perl, programmers would insert curly brackets to force interpretation as a filename glob: `< <${foo}' >>. These days, it's considered cleaner to call the internal function directly as `glob($foo)', which is probably the right way to have done it in the first place.) For example: while (<*.c>) { chmod 0644, $_; } is roughly equivalent to: open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); while () { chop; chmod 0644, $_; } except that the globbing is actually done internally using the standard File::Glob extension. Of course, the shortest way to do the above is: chmod 0644, <*.c>; A (file)glob evaluates its (embedded) argument only when it is starting a new list. All values must be read before it will start over. In list context, this isn't important because you automatically get them all anyway. However, in scalar context the operator returns the next value each time it's called, or C run out. As with filehandle reads, an automatic defined is generated when the glob occurs in the test part of a while, because legal glob returns (e.g. a file called 0) would otherwise terminate the loop. Again, undef is returned only once. So if you're expecting a single value from a glob, it is much better to say ($file) = ; than $file = ; because the latter will alternate between returning a filename and returning false. It you're trying to do variable interpolation, it's definitely better to use the glob() function, because the older notation can cause people to become confused with the indirect filehandle notation. @files = glob("$dir/*.[ch]"); @files = glob($files[$i]); Constant Folding ---------------- Like C, Perl does a certain amount of expression evaluation at compile time whenever it determines that all arguments to an operator are static and have no side effects. In particular, string concatenation happens at compile time between literals that don't do variable substitution. Backslash interpolation also happens at compile time. You can say 'Now is the time for all' . "\n" . 'good men to come to.' and this all reduces to one string internally. Likewise, if you say foreach $file (@filenames) { if (-s $file > 5 + 100 * 2**16) { } } the compiler will precompute the number which that expression represents so that the interpreter won't have to. Bitwise String Operators ------------------------ Bitstrings of any size may be manipulated by the bitwise operators (`~ | & ^'). If the operands to a binary bitwise op are strings of different sizes, | and ^ ops act as though the shorter operand had additional zero bits on the right, while the & op acts as though the longer operand were truncated to the length of the shorter. The granularity for such extension or truncation is one or more bytes. # ASCII-based examples print "j p \n" ^ " a h"; # prints "JAPH\n" print "JA" | " ph\n"; # prints "japh\n" print "japh\nJunk" & '_____'; # prints "JAPH\n"; print 'p N$' ^ " E>") always produce integral results. (But see also `Bitwise String Operators' in this node.) However, `use integer' still has meaning for them. By default, their results are interpreted as unsigned integers, but if `use integer' is in effect, their results are interpreted as signed integers. For example, `~0' usually evaluates to a large integral value. However, `use integer; ~0' is `-1' on twos-complement machines. Floating-point Arithmetic ------------------------- While `use integer' provides integer-only arithmetic, there is no analogous mechanism to provide automatic rounding or truncation to a certain number of decimal places. For rounding to a certain number of digits, sprintf() or printf() is usually the easiest route. See *Note Perlfaq4: perlfaq4,. Floating-point numbers are only approximations to what a mathematician would call real numbers. There are infinitely more reals than floats, so some corners must be cut. For example: printf "%.20g\n", 123456789123456789; # produces 123456789123456784 Testing for exact equality of floating-point equality or inequality is not a good idea. Here's a (relatively expensive) work-around to compare whether two floating-point numbers are equal to a particular number of decimal places. See Knuth, volume II, for a more robust treatment of this topic. sub fp_equal { my ($X, $Y, $POINTS) = @_; my ($tX, $tY); $tX = sprintf("%.${POINTS}g", $X); $tY = sprintf("%.${POINTS}g", $Y); return $tX eq $tY; } The POSIX module (part of the standard perl distribution) implements ceil(), floor(), and other mathematical and trigonometric functions. The Math::Complex module (part of the standard perl distribution) defines mathematical functions that work on both the reals and the imaginary numbers. Math::Complex not as efficient as POSIX, but POSIX can't work with complex numbers. Rounding in financial applications can have serious implications, and the rounding method used should be specified precisely. In these cases, it probably pays not to trust whichever system rounding is being used by Perl, but to instead implement the rounding function you need yourself. Bigger Numbers -------------- The standard Math::BigInt and Math::BigFloat modules provide variable-precision arithmetic and overloaded operators, although they're currently pretty slow. At the cost of some space and considerable speed, they avoid the normal pitfalls associated with limited-precision representations. use Math::BigInt; $x = Math::BigInt->new('123456789123456789'); print $x * $x; # prints +15241578780673678515622620750190521 The non-standard modules SSLeay::BN and Math::Pari provide equivalent functionality (and much more) with a substantial performance savings.