Operator precedence means some operators are evaluated beforeothers. For example, in 2 + 4 * 5, the multiplication has higherprecedence so 4 * 5 is evaluated first yielding 2 + 20 ==22 and not 6 * 5 == 30.

Operator associativity defines what happens if a sequence of thesame operators is used one after another: whether the evaluator willevaluate the left operations first or the right. For example, in 8- 4 - 2, subtraction is left associative so Perl evaluates theexpression left to right. 8 - 4 is evaluated first making theexpression 4 - 2 == 2 and not 8 - 2 == 6.

Perl operators have the following associativity and precedence,listed from highest precedence to lowest. Operators borrowed fromC keep the same precedence relationship with each other, even whereC's precedence is slightly screwy. (This makes learning Perl easierfor C folks.) With very few exceptions, these all operate on scalarvalues only, not array values.

 leftterms and list operators (leftward)
  left->
  nonassoc++ --
  right**
  right! ~ \ and unary + and -
  left=~ !~
  left* / % x
  left+ - .
  left<< >>
  nonassocnamed unary operators
  nonassoc< > <= >= lt gt le ge
  nonassoc== != <=> eq ne cmp ~~
  left&
  left| ^
  left&&
  left|| //
  nonassoc..  ...
  right?:
  right= += -= *= etc.
  left, =>
  nonassoclist operators (rightward)
  rightnot
  leftand
  leftor xor

In the following sections, these operators are covered in precedence order.

Many operators can be overloaded for objects. See overload.

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, therearen't really functions in this sense, just list operators and unaryoperators behaving as functions because you put parentheses aroundthe arguments. These are all documented in 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 andarguments 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 asprint, sort, or chmod is either very high or very low depending onwhether 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, andthen 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. The parenthesesenclose the argument list for print which is evaluated (printingthe result of $foo & 255). Then one is added to the return valueof print (usually 1). The result is something like this:

 1 + 1, "\n"; # Obviously not what you meant.

To do what you meant properly, you must write:

 print(($foo & 255) + 1, "\n");

See Named Unary Operators for more discussion of this.

Also parsed as terms are the do {} and eval {} constructs, aswell as subroutine and method calls, and the anonymousconstructors [] and {}.

See also Quote and Quote-like Operators toward the end of this section,as well as I/O Operators.

The Arrow Operator

"->" is an infix dereference operator, just as it is in Cand C++. If the right side is either a [...], {...}, or a(...) subscript, then the left side must be either a hard orsymbolic reference to an array, a hash, or a subroutine respectively.(Or technically speaking, a location capable of holding a hardreference, if it's an array or hash reference being used forassignment.) See perlreftut and perlref.

Otherwise, the right side is a method name or a simple scalarvariable 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 perlobj.

Auto-increment and Auto-decrement

"++" and "--" work as in C. That is, if placed before a variable,they increment or decrement the variable by one before returning thevalue, and if placed after, increment or decrement after returning thevalue.

 $i = 0;  $j = 0;
  print $i++;  # prints 0
  print ++$j;  # prints 1

Note that just as in C, Perl doesn't define when the variable isincremented or decremented. You just know it will be done sometimebefore or after the value is returned. This also means that modifyinga variable twice in the same statement will lead to undefined behavior.Avoid statements like:

 $i = $i ++;
  print ++ $i + $i ++;

Perl will not guarantee what the result of the above statements is.

The auto-increment operator has a little extra builtin magic to it. Ifyou increment a variable that is numeric, or that has ever been used ina numeric context, you get a normal increment. If, however, thevariable has been used in only string contexts since it was set, andhas a value that is not the empty string and matches the pattern/^[a-zA-Z]*[0-9]*\z/, the increment is done as a string, preserving eachcharacter 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"

undef is always treated as numeric, and in particular is changedto 0 before incrementing (so that a post-increment of an undef valuewill return 0 rather than undef).

The auto-decrement operator is not magical.

Exponentiation

Binary "**" is the exponentiation operator. It binds even moretightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This isimplemented using C's pow(3) function, which actually works on doublesinternally.)

Symbolic Unary Operators

Unary "!" performs logical negation, that is, "not". See also not for a lowerprecedence version of this.

Unary "-" performs arithmetic negation if the operand is numeric,including any string that looks like a number. If the operand isan identifier, a string consisting of a minus sign concatenatedwith the identifier is returned. Otherwise, if the string startswith a plus or minus, a string starting with the opposite sign isreturned. One effect of these rules is that -bareword is equivalentto the string "-bareword". If, however, the string begins with anon-alphabetic character (excluding "+" or "-"), Perl will attempt to convertthe string to a numeric and the arithmetic negation is performed. If thestring cannot be cleanly converted to a numeric, Perl will give the warningArgument "the string" isn't numeric in negation (-) at ....

Unary "~" performs bitwise negation, that is, 1's complement. Forexample, 0666 & ~027 is 0640. (See also Integer Arithmetic andBitwise String Operators.) Note that the width of the result isplatform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64bits wide on a 64-bit platform, so if you are expecting a certain bitwidth, remember to use the "&" operator to mask off the excess bits.

When complementing strings, if all characters have ordinal values under256, then their complements will, also. But if they do not, allcharacters will be in either 32- or 64-bit complements, depending on yourarchitecture. So for example, ~"\x{3B1}" is "\x{FFFF_FC4E}" on32-bit machines and "\x{FFFF_FFFF_FFFF_FC4E}" on 64-bit machines.

Unary "+" has no effect whatsoever, even on strings. It is usefulsyntactically for separating a function name from a parenthesized expressionthat would otherwise be interpreted as the complete list of functionarguments. (See examples above under Terms and List Operators (Leftward).)

Unary "\" creates a reference to whatever follows it. See perlreftutand perlref. Do not confuse this behavior with the behavior ofbackslash within a string, although both forms do convey the notionof protecting the next thing from interpolation.

Binding Operators

Binary "=~" binds a scalar expression to a pattern match. Certain operationssearch or modify the string $_ by default. This operator makes that kindof operation work on some other string. The right argument is a searchpattern, substitution, or transliteration. The left argument is what issupposed to be searched, substituted, or transliterated instead of the default$_. When used in scalar context, the return value generally indicates thesuccess of the operation. The exceptions are substitution (s///)and transliteration (y///) with the /r (non-destructive) option,which cause the return value to be the result of the substitution.Behavior in list context depends on the particular operator.See Regexp Quote-Like Operators for details and perlretut forexamples using these operators.

If the right argument is an expression rather than a search pattern,substitution, or transliteration, it is interpreted as a search pattern at runtime. Note that this means that its contents will be interpolated twice, so

 '\' =~ q'\';

is not ok, as the regex engine will end up trying to compile thepattern \, which it will consider a syntax error.

Binary "!~" is just like "=~" except the return value is negated inthe logical sense.

Binary "!~" with a non-destructive substitution (s///r) or transliteration(y///r) is a syntax error.

Multiplicative Operators

Binary "*" multiplies two numbers.

Binary "/" divides two numbers.

Binary "%" is the modulo operator, which computes the divisionremainder of its first argument with respect to its second argument.Given integeroperands $a and $b: If $b is positive, then $a % $b is$a minus the largest multiple of $b less than or equal to$a. If $b is negative, then $a % $b is $a minus thesmallest multiple of $b that is not less than $a (that is, theresult will be less than or equal to zero). If the operands$a and $b are floating point values and the absolute value of$b (that is abs($b)) is less than (UV_MAX + 1), onlythe integer portion of $a and $b will be used in the operation(Note: here UV_MAX means the maximum of the unsigned integer type).If the absolute value of the right operand (abs($b)) is greater thanor equal to (UV_MAX + 1), "%" computes the floating-point remainder$r in the equation ($r = $a - $i*$b) where $i is a certaininteger that makes $r have the same sign as the right operand$b (not as the left operand $a like C function fmod())and the absolute value less than that of $b.Note that when use integer is in scope, "%" gives you direct accessto the modulo operator as implemented by your C compiler. Thisoperator is not as well defined for negative operands, but it willexecute faster.

Binary "x" is the repetition operator. In scalar context or if the leftoperand is not enclosed in parentheses, it returns a string consistingof the left operand repeated the number of times specified by the rightoperand. In list context, if the left operand is enclosed inparentheses or is a list formed by qw/STRING/, it repeats the list.If the right operand is zero or negative, it returns an empty stringor an empty list, depending on the context.

 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 thenumber of bits specified by the right argument. Arguments should beintegers. (See also Integer Arithmetic.)

Binary >> returns the value of its left argument shifted right bythe number of bits specified by the right argument. Arguments shouldbe integers. (See also Integer Arithmetic.)

Note that both << and >> in Perl are implemented directly using<< and >> in C. If use integer (see Integer Arithmetic) isin force then signed C integers are used, else unsigned C integers areused. Either way, the implementation isn't going to generate resultslarger than the size of the integer type Perl was built with (32 bitsor 64 bits).

The result of overflowing the range of the integers is undefinedbecause it is undefined also in C. In other words, using 32-bitintegers, 1 << 32 is undefined. Shifting by a negative numberof bits is also undefined.

If you get tired of being subject to your platform's native integers,the use bigint pragma neatly sidesteps the issue altogether:

 print 20 << 20;  # 20971520
  print 20 << 40;  # 5120 on 32-bit machines, 
  # 21990232555520 on 64-bit machines
  use bigint;
  print 20 << 100; # 25353012004564588029934064107520

Named Unary Operators

The various named unary operators are treated as functions with oneargument, with optional parentheses.

 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 named operators:

 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)

Regarding precedence, the filetest operators, like -f, -M, etc. aretreated like named unary operators, but they don't follow this functionalparenthesis rule. That means, for example, that -f($file).".bak" isequivalent to -f "$file.bak".

Relational Operators

Perl operators that return true or false generally return values that can be safely used as numbers. For example, the relationaloperators in this section and the equality operators in the nextone return 1 for true and a special version of the defined emptystring, "", which counts as a zero but is exempt from warningsabout improper numeric conversions, just as "0 but true" is.

Binary "<" returns true if the left argument is numerically less thanthe right argument.

Binary ">" returns true if the left argument is numerically greaterthan the right argument.

Binary "<=" returns true if the left argument is numerically less thanor equal to the right argument.

Binary ">=" returns true if the left argument is numerically greaterthan or equal to the right argument.

Binary "lt" returns true if the left argument is stringwise less thanthe right argument.

Binary "gt" returns true if the left argument is stringwise greaterthan the right argument.

Binary "le" returns true if the left argument is stringwise less thanor equal to the right argument.

Binary "ge" returns true if the left argument is stringwise greaterthan or equal to the right argument.

Equality Operators

Binary "==" returns true if the left argument is numerically equal tothe right argument.

Binary "!=" returns true if the left argument is numerically not equalto the right argument.

Binary "<=>" returns -1, 0, or 1 depending on whether the leftargument is numerically less than, equal to, or greater than the rightargument. If your platform supports NaNs (not-a-numbers) as numericvalues, using them with "<=>" returns undef. NaN is not "<", "==", ">","<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaNreturns true, as does NaN != anything else. If your platform doesn'tsupport NaNs then NaN is just a string with numeric value 0.

 $ perl -le '$a = "NaN" print "No NaN support here" if $a == $a'
  $ perl -le '$a = "NaN" print "NaN support here" if $a != $a'

(Note that the bigint, bigrat, and bignum pragmas all support "NaN".)

Binary "eq" returns true if the left argument is stringwise equal tothe right argument.

Binary "ne" returns true if the left argument is stringwise not equalto the right argument.

Binary "cmp" returns -1, 0, or 1 depending on whether the leftargument is stringwise less than, equal to, or greater than the rightargument.

Binary "~~" does a smartmatch between its arguments. Smart matchingis described in the next section.

"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specifiedby the current locale if a legacy use locale (but notuse locale ':not_characters') is in effect. Seeperllocale. Do not mix these with Unicode, only with legacy binaryencodings. The standard Unicode::Collate andUnicode::Collate::Locale modules offer much more powerful solutions tocollation issues.

Smartmatch Operator

First available in Perl 5.10.1 (the 5.10.0 version behaved differently),binary ~~ does a "smartmatch" between its arguments. This is mostlyused implicitly in the when construct described in perlsyn, althoughnot all when clauses call the smartmatch operator. Unique among all ofPerl's operators, the smartmatch operator can recurse.

It is also unique in that all other Perl operators impose a context(usually string or numeric context) on their operands, autoconvertingthose operands to those imposed contexts. In contrast, smartmatchinfers contexts from the actual types of its operands and uses thattype information to select a suitable comparison mechanism.

The ~~ operator compares its operands "polymorphically", determining howto compare them according to their actual types (numeric, string, array,hash, etc.) Like the equality operators with which it shares the sameprecedence, ~~ returns 1 for true and "" for false. It is often bestread aloud as "in", "inside of", or "is contained in", because the leftoperand is often looked for inside the right operand. That makes theorder of the operands to the smartmatch operand often opposite that ofthe regular match operator. In other words, the "smaller" thing is usuallyplaced in the left operand and the larger one in the right.

The behavior of a smartmatch depends on what type of things its argumentsare, as determined by the following table. The first row of the tablewhose types apply determines the smartmatch behavior. Because whatactually happens is mostly determined by the type of the second operand,the table is sorted on the right operand instead of on the left.

 Left  Right  Description and pseudocode   
  ===============================================================
  Any   undef  check whether Any is undefined 
  like: !defined Any
 
  Any   Object invoke ~~ overloading on Object, or die
 
  Right operand is an ARRAY:
 
  Left  Right  Description and pseudocode   
  ===============================================================
  ARRAY1 ARRAY2 recurse on paired elements of ARRAY1 and ARRAY2[2]
  like: (ARRAY1[0] ~~ ARRAY2[0])
  && (ARRAY1[1] ~~ ARRAY2[1]) && ...
  HASH  ARRAY  any ARRAY elements exist as HASH keys 
  like: grep { exists HASH->{$_} } ARRAY
  Regexp ARRAY  any ARRAY elements pattern match Regexp
  like: grep { /Regexp/ } ARRAY
  undef ARRAY  undef in ARRAY 
  like: grep { !defined } ARRAY
  Any   ARRAY  smartmatch each ARRAY element[3]   
  like: grep { Any ~~ $_ } ARRAY
 
  Right operand is a HASH:
 
  Left  Right  Description and pseudocode   
  ===============================================================
  HASH1 HASH2  all same keys in both HASHes  
  like: keys HASH1 ==
  grep { exists HASH2->{$_} } keys HASH1
  ARRAY HASH   any ARRAY elements exist as HASH keys 
  like: grep { exists HASH->{$_} } ARRAY
  Regexp HASH   any HASH keys pattern match Regexp 
  like: grep { /Regexp/ } keys HASH
  undef HASH   always false (undef can't be a key)   
  like: 0 == 1
  Any   HASH   HASH key existence 
  like: exists HASH->{Any}
 
  Right operand is CODE:
    
  Left  Right  Description and pseudocode   
  ===============================================================
  ARRAY CODE   sub returns true on all ARRAY elements[1]
  like: !grep { !CODE->($_) } ARRAY
  HASH  CODE   sub returns true on all HASH keys[1]
  like: !grep { !CODE->($_) } keys HASH
  Any   CODE   sub passed Any returns true  
  like: CODE->(Any)

Right operand is a Regexp:

 Left  Right  Description and pseudocode   
  ===============================================================
  ARRAY Regexp any ARRAY elements match Regexp   
  like: grep { /Regexp/ } ARRAY
  HASH  Regexp any HASH keys match Regexp 
  like: grep { /Regexp/ } keys HASH
  Any   Regexp pattern match 
  like: Any =~ /Regexp/
 
  Other:
 
  Left  Right  Description and pseudocode   
  ===============================================================
  Object Any invoke ~~ overloading on Object,
   or fall back to...
 
  Any   Num numeric equality  
  like: Any == Num
  Num   nummy[4] numeric equality
  like: Num == nummy
  undef Any check whether undefined
  like: !defined(Any)
  Any   Any string equality   
  like: Any eq Any

Notes:

1.: Empty hashes or arrays match.
2.: That is, each element smartmatches the element of the same index in the other array.[3]
3.: If a circular reference is found, fall back to referential equality.
4.: Either an actual number, or a string that looks like one.

The smartmatch implicitly dereferences any non-blessed hash or arrayreference, so the HASH and ARRAY entries apply in those cases.For blessed references, the Object entries apply. Smartmatchesinvolving hashes only consider hash keys, never hash values.

The "like" code entry is not always an exact rendition. For example, thesmartmatch operator short-circuits whenever possible, but grep doesnot. Also, grep in scalar context returns the number of matches, but~~ returns only true or false.

Unlike most operators, the smartmatch operator knows to treat undefspecially:

 use v5.10.1;
  @array = (1, 2, 3, undef, 4, 5);
  say "some elements undefined" if undef ~~ @array;

Each operand is considered in a modified scalar context, the modificationbeing that array and hash variables are passed by reference to theoperator, which implicitly dereferences them. Both elementsof each pair are the same:

 use v5.10.1;
 
  my %hash = (red => 1, blue   => 2, green  => 3,
  orange => 4, yellow => 5, purple => 6,
  black  => 7, grey   => 8, white  => 9);
 
  my @array = qw(red blue green);
 
  say "some array elements in hash keys" if  @array ~~  %hash;
  say "some array elements in hash keys" if \@array ~~ \%hash;
 
  say "red in array" if "red" ~~  @array;
  say "red in array" if "red" ~~ \@array;
 
  say "some keys end in e" if /e$/ ~~  %hash;
  say "some keys end in e" if /e$/ ~~ \%hash;

Two arrays smartmatch if each element in the first array smartmatches(that is, is "in") the corresponding element in the second array,recursively.

 use v5.10.1;
  my @little = qw(red blue green);
  my @bigger = ("red", "blue", [ "orange", "green" ] );
  if (@little ~~ @bigger) {  # true!
  say "little is contained in bigger";
  }

Because the smartmatch operator recurses on nested arrays, thiswill still report that "red" is in the array.

 use v5.10.1;
  my @array = qw(red blue green);
  my $nested_array = [[[[[[[ @array ]]]]]]];
  say "red in array" if "red" ~~ $nested_array;

If two arrays smartmatch each other, then they are deepcopies of each others' values, as this example reports:

 use v5.12.0;
  my @a = (0, 1, 2, [3, [4, 5], 6], 7); 
  my @b = (0, 1, 2, [3, [4, 5], 6], 7); 
 
  if (@a ~~ @b && @b ~~ @a) {
  say "a and b are deep copies of each other";
  } 
  elsif (@a ~~ @b) {
  say "a smartmatches in b";
  } 
  elsif (@b ~~ @a) {
  say "b smartmatches in a";
  } 
  else {
  say "a and b don't smartmatch each other at all";
  }

If you were to set $b[3] = 4, then instead of reporting that "a and bare deep copies of each other", it now reports that "b smartmatches in a".That because the corresponding position in @a contains an array that(eventually) has a 4 in it.

Smartmatching one hash against another reports whether both contain thesame keys, no more and no less. This could be used to see whether tworecords have the same field names, without caring what values those fieldsmight have. For example:

 use v5.10.1;
  sub make_dogtag {
  state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
 
  my ($class, $init_fields) = @_;
 
  die "Must supply (only) name, rank, and serial number"
  unless $init_fields ~~ $REQUIRED_FIELDS;
 
  ...
  }

or, if other non-required fields are allowed, use ARRAY ~~ HASH:

 use v5.10.1;
  sub make_dogtag {
  state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
 
  my ($class, $init_fields) = @_;
 
  die "Must supply (at least) name, rank, and serial number"
  unless [keys %{$init_fields}] ~~ $REQUIRED_FIELDS;
 
  ...
  }

The smartmatch operator is most often used as the implicit operator of awhen clause. See the section on "Switch Statements" in perlsyn.

Smartmatching of Objects

To avoid relying on an object's underlying representation, if thesmartmatch's right operand is an object that doesn't overload ~~,it raises the exception "Smartmatching a non-overloaded objectbreaks encapsulation". That's because one has no business diggingaround to see whether something is "in" an object. These are allillegal on objects without a ~~ overload:

 %hash ~~ $object
    42 ~~ $object
    "fred" ~~ $object

However, you can change the way an object is smartmatched by overloadingthe ~~ operator. This is allowed to extend the usual smartmatch semantics.For objects that do have an ~~ overload, see overload.

Using an object as the left operand is allowed, although not very useful.Smartmatching rules take precedence over overloading, so even if theobject in the left operand has smartmatch overloading, this will beignored. A left operand that is a non-overloaded object falls back on astring or numeric comparison of whatever the ref operator returns. Thatmeans that

 $object ~~ X

does not invoke the overload method with X as an argument.Instead the above table is consulted as normal, and based on the type ofX, overloading may or may not be invoked. For simple strings ornumbers, in becomes equivalent to this:

 $object ~~ $number  ref($object) == $number
  $object ~~ $string  ref($object) eq $string

For example, this reports that the handle smells IOish(but please don't really do this!):

 use IO::Handle;
  my $fh = IO::Handle->new();
  if ($fh ~~ /\bIO\b/) {
  say "handle smells IOish";
  }

That's because it treats $fh as a string like"IO::Handle=GLOB(0x8039e0)", then pattern matches against that.

Bitwise And

Binary "&" returns its operands ANDed together bit by bit.(See also Integer Arithmetic and Bitwise String Operators.)

Note that "&" has lower priority than relational operators, so for examplethe parentheses are essential in a test like

 print "Even\n" if ($x & 1) == 0;

Bitwise Or and Exclusive Or

Binary "|" returns its operands ORed together bit by bit.(See also Integer Arithmetic and Bitwise String Operators.)

Binary "^" returns its operands XORed together bit by bit.(See also Integer Arithmetic and Bitwise String Operators.)

Note that "|" and "^" have lower priority than relational operators, sofor example the brackets are essential in a test like

 print "false\n" if (8 | 2) != 10;

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 itis 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 itis evaluated.

Logical Defined-Or

Although it has no direct equivalent in C, Perl's // operator is relatedto its C-style or. In fact, it's exactly the same as ||, except that ittests the left hand side's definedness instead of its truth. Thus,EXPR1 // EXPR2 returns the value of EXPR1 if it's defined,otherwise, the value of EXPR2 is returned. (EXPR1 is evaluatedin scalar context, EXPR2 in the context of // itself). Usually,this is the same result as defined(EXPR1) ? EXPR1 : EXPR2 (except thatthe ternary-operator form can be used as a lvalue, while EXPR1 // EXPR2cannot). This is very useful forproviding default values for variables. If you actually want to test ifat least one of $a and $b is defined, use defined($a // $b).

The ||, // and && operators return the last value evaluated(unlike C's || and &&, which return 0 or 1). Thus, a reasonablyportable way to find out the home directory might be:

 $home =  $ENV{HOME}
   // $ENV{LOGDIR}
   // (getpwuid($<))[7]
   // die "You're homeless!\n";

In particular, this means that you shouldn't use thisfor 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 alternatives to && and || when used forcontrol flow, Perl provides the 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 alist 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);

It would be even more readable to write that this way:

 unless(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 differentoperators depending on the context. In list context, it returns alist of values counting (up by ones) from the left value to the rightvalue. If the left value is greater than the right value then itreturns the empty list. The range operator is useful for writingforeach (1..10) loops and for doing slice operations on arrays. Inthe current implementation, no temporary array is created when therange operator is used as the expression in foreach loops, but olderversions of Perl might burn a lot of memory when you write somethinglike this:

 for (1 .. 1_000_000) {
 # code
  }

The range operator also works on strings, using the magicalauto-increment, see below.

In scalar context, ".." returns a boolean value. The operator isbistable, like a flip-flop, and emulates the line-range (comma)operator of sed, awk, and various editors. Each ".." operatormaintains its own boolean state, even across calls to a subroutinethat contains it. It is false as long as its left operand is false.Once the left operand is true, the range operator stays true until theright operand is true, AFTER which the range operator becomes falseagain. It doesn't become false till the next time the range operatoris evaluated. It can test the right operand and become false on thesame evaluation it became true (as in awk), but it still returnstrue once. If you don't want it to test the right operand until thenext evaluation, as in sed, just use three dots ("...") instead oftwo. 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 theoperator is in the "true" state. The precedence is a little lowerthan || and &&. The value returned is either the empty string forfalse, or a sequence number (beginning with 1) for true. The sequencenumber is reset for each range encountered. The final sequence numberin a range has the string "E0" appended to it, which doesn't affectits numeric value, but gives you something to search for if you wantto exclude the endpoint. You can exclude the beginning point bywaiting for the sequence number to be greater than 1.

If either operand of scalar ".." is a constant expression,that operand is considered true if it is equal (==) to the currentinput line number (the $. variable).

To be pedantic, the comparison is actually int(EXPR) == int(EXPR),but that is only an issue if you use a floating point expression; whenimplicitly using $. as described in the previous paragraph, thecomparison is int(EXPR) == int($.) which is only an issue when $.is set to a floating point value and you are not reading from a file.Furthermore, "span" .. "spat" or 2.18 .. 3.14 will not do whatyou want in scalar context because each of the operands are evaluatedusing their integer representation.

Examples:

As a scalar operator:

 if (101 .. 200) { print; } # print 2nd hundred lines, short for
    #  if ($. == 101 .. $. == 200) { print; }
 
  next LINE if (1 .. /^$/);  # skip header lines, short for
    #   next LINE if ($. == 1 .. /^$/);
    # (typically in a loop labeled LINE)
 
  s/^/> / if (/^$/ .. eof());  # quote body
 
  # parse mail messages
  while (<>) {
  $in_header =   1  .. /^$/;
  $in_body   = /^$/ .. eof;
  if ($in_header) {
  # do something
  } else { # in body
  # do something else
  }
  } continue {
  close ARGV if eof; # reset $. each file
  }

Here's a simple example to illustrate the difference betweenthe two range operators:

 @lines = ("   - Foo",
   "01 - Bar",
   "1  - Baz",
   "   - Quux");
 
  foreach (@lines) {
  if (/0/ .. /1/) {
  print "$_\n";
  }
  }

This program will print only the line containing "Bar". Ifthe range operator is changed to ..., it will also print the"Baz" line.

And now some examples 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 magicalauto-increment algorithm if the operands are strings. Youcan say

 @alphabet = ("A" .. "Z");

to get all normal letters of the English 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 magicalincrement would produce, the sequence goes until the next value wouldbe longer than the final value specified.

If the initial value specified isn't part of a magical incrementsequence (that is, a non-empty string matching /^[a-zA-Z]*[0-9]*\z/),only the initial value will be returned. So the following will onlyreturn an alpha:

 use charnames "greek";
  my @greek_small =  ("\N{alpha}" .. "\N{omega}");

To get the 25 traditional lowercase Greek letters, including both sigmas,you could use this instead:

 use charnames "greek";
  my @greek_small =  map { chr } ( ord("\N{alpha}") 
  ..
  ord("\N{omega}") 
    );

However, because there are many other lowercase Greek characters thanjust those, to match lowercase Greek characters in a regular expression,you would use the pattern /(?:(?=\p{Greek})\p{Lower})+/.

Because each operand is evaluated in integer form, 2.18 .. 3.14 willreturn two elements in list context.

 @list = (2.18 .. 3.14); # same as @list = (2 .. 3);

Conditional Operator

Ternary "?:" is the conditional operator, just as in C. It works muchlike an if-then-else. If the argument before the ? is true, theargument 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 2ndor 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 arelegal lvalues (meaning that you can assign to them):

 ($a_or_b ? $a : $b) = $c;

Because this operator produces an assignable result, using assignmentswithout 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 lvaluemight 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 precedenceof assignment.

Unlike in C, the scalar assignment operator produces a valid lvalue.Modifying an assignment is equivalent to doing the assignment andthen modifying the variable that was assigned to. This is usefulfor modifying a copy of something, like this:

 ($tmp = $global) =~ tr/13579/24680/;

Although as of 5.14, that can be also be accomplished this way:

 use v5.14;
  $tmp = ($global =~  tr/13579/24680/r);

Likewise,

 ($a += 2) *= 3;

is equivalent to

 $a += 2;
  $a *= 3;

Similarly, a list assignment in list context produces the list oflvalues assigned to, and a list assignment in scalar context returnsthe number of elements produced by the expression on the right handside of the assignment.

Comma Operator

Binary "," is the comma operator. In scalar context it evaluatesits left argument, throws that value away, then evaluates its rightargument and returns that value. This is just like C's comma operator.

In list context, it's just the list argument separator, and insertsboth its arguments into the list. These arguments are also evaluatedfrom left to right.

The => operator is a synonym for the comma except that it causes aword on its left to be interpreted as a string if it begins with a letteror underscore and is composed only of letters, digits and underscores.This includes operands that might otherwise be interpreted as operators,constants, single number v-strings or function calls. If in doubt aboutthis behavior, the left operand can be quoted explicitly.

Otherwise, the => operator behaves exactly as the comma operatoror list argument separator, according to context.

For example:

 use constant FOO => "something";
 
  my %h = ( FOO => 23 );

is equivalent to:

 my %h = ("FOO", 23);

It is NOT:

 my %h = ("something", 23);

The => operator is helpful in documenting the correspondencebetween keys and values in hashes, and other paired elements in lists.

 %hash = ( $key => $value );
  login( $username => $password );

The special quoting behavior ignores precedence, and hence may apply topart of the left operand:

 print time.shift => "bbb";

That example prints something like "1314363215shiftbbb", because the=> implicitly quotes the shift immediately on its left, ignoringthe fact that time.shift is the entire left operand.

List Operators (Rightward)

On the right side of a list operator, the comma 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 listoperators without the need for parentheses:

 open HANDLE, "< :utf8", "filename" or die "Can't open: $!\n";

However, some people find that code harder to read than writingit with parentheses:

 open(HANDLE, "< :utf8", "filename") or die "Can't open: $!\n";

in which case you might as well just use the more customary "||" operator:

 open(HANDLE, "< :utf8", "filename") || die "Can't open: $!\n";

See also discussion of list operators in Terms and List Operators (Leftward).

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 surroundingexpressions. It's equivalent to && except for the very lowprecedence. This means that it short-circuits: the rightexpression is evaluated only if the left expression is true.

Logical or and Exclusive Or

Binary "or" returns the logical disjunction of the two surroundingexpressions. It's equivalent to || except for the very low precedence.This makes it useful for control flow:

 print FH $dataor die "Can't write to FH: $!";

This means that it short-circuits: the right expression is evaluatedonly if the left expression is false. Due to its precedence, you mustbe careful to avoid using it as replacement for the || operator.It usually works out better for flow control than in assignments:

 $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 assignmenttakes 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).

There is no low precedence operator for defined-OR.

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 dereferencingoperators are typed: $, @, %, and &.)
(TYPE)
Type-casting operator.

Quote and Quote-like Operators

While we usually think of quotes as literal values, in Perl theyfunction as operators, providing various kinds of interpolating andpattern matching capabilities. Perl provides customary quote charactersfor these behaviors, but also provides a way for you to choose yourquote character for any of them. In the following table, a {} representsany pair of delimiters you choose.

 Customary  Generic Meaning Interpolates
 '' q{}  Literal  no
 ""qq{}  Literal  yes
 ``qx{}  Command  yes*
 qw{} Word list  no
 // m{}   Pattern match  yes*
 qr{}  Pattern  yes*
  s{}{} Substitution  yes*
 tr{}{}  Transliteration  no (but see below)
  y{}{}  Transliteration  no (but see below)
  <<EOF here-doc yes*
 
 * unless the delimiter is ''.

Non-bracketing delimiters use the same character fore and aft, but the foursorts of ASCII brackets (round, angle, square, curly) all nest, which meansthat

 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 (standard as of v5.8,and from CPAN before then) is able to do this properly.

There can be whitespace between the operator and the quotingcharacters, except when # is being used as the quoting character.q#foo# is parsed as the string foo, while q #foo# is theoperator q followed by a comment. Its argument will be takenfrom the next line. This allows you to write:

 s {foo}  # Replace foo
     {bar}  # with bar.

The following escape sequences are available in constructs that interpolate,and in transliterations:

 Sequence Note  Description
  \t  tab   (HT, TAB)
  \n  newline   (NL)
  \r  return (CR)
  \f  form feed (FF)
  \b  backspace (BS)
  \a  alarm (bell)  (BEL)
  \e  escape (ESC)
  \x{263A} [1,8]  hex char  (example: SMILEY)
  \x1b [2,8]  restricted range hex char (example: ESC)
  \N{name} [3] named Unicode character or character sequence
  \N{U+263D}   [4,8]  Unicode character (example: FIRST QUARTER MOON)
  \c[  [5] control char  (example: chr(27))
  \o{23072} [6,8]  octal char (example: SMILEY)
  \033 [7,8]  restricted range octal char  (example: ESC)

[1]
The result is the character specified by the hexadecimal number betweenthe braces. See [8] below for details on which character.
Only hexadecimal digits are valid between the braces. If an invalidcharacter is encountered, a warning will be issued and the invalidcharacter and all subsequent characters (valid or invalid) within thebraces will be discarded.
If there are no valid digits between the braces, the generated character isthe NULL character (\x{00}). However, an explicit empty brace (\x{})will not cause a warning (currently).
[2]
The result is the character specified by the hexadecimal number in the range0x00 to 0xFF. See [8] below for details on which character.
Only hexadecimal digits are valid following \x. When \x is followedby fewer than two valid digits, any valid digits will be zero-padded. Thismeans that \x7 will be interpreted as \x07, and a lone <\x> will beinterpreted as \x00. Except at the end of a string, having fewer thantwo valid digits will result in a warning. Note that although the warningsays the illegal character is ignored, it is only ignored as part of theescape and will still be used as the subsequent character in the string.For example:
```
  Original Result Warns?
   "\x7"   "\x07" no
   "\x" "\x00" no
   "\x7q"  "\x07q"   yes
   "\xq"   "\x00q"   yes
 
```
[3]
The result is the Unicode character or character sequence given by name.See charnames.
[4]
\N{U+hexadecimal number} means the Unicode character whose Unicode codepoint is hexadecimal number.
[5]
The character following \c is mapped to some other character as shown in thetable:
```
 Sequence   Value
    \c@  chr(0)
    \cA  chr(1)
    \ca  chr(1)
    \cB  chr(2)
    \cb  chr(2)
    ...
    \cZ  chr(26)
    \cz  chr(26)
    \c[  chr(27)
    \c]  chr(29)
    \c^  chr(30)
    \c?  chr(127)
 
```
In other words, it's the character whose code point has had 64 xor'd withits uppercase. \c? is DELETE because ord("@") ^ 64 is 127, and\c@ is NULL because the ord of "@" is 64, so xor'ing 64 itself produces 0.
Also, \c\X yields chr(28) . "X" for any X, but cannot come at theend of a string, because the backslash would be parsed as escaping the endquote.
On ASCII platforms, the resulting characters from the list above are thecomplete set of ASCII controls. This isn't the case on EBCDIC platforms; seeOPERATOR DIFFERENCES in perlebcdic for the complete list of what thesesequences mean on both ASCII and EBCDIC platforms.
Use of any other character following the "c" besides those listed above isdiscouraged, and some are deprecated with the intention of removingthose in a later Perl version. What happens for any of theseother characters currently though, is that the value is derived by xor'ingwith the seventh bit, which is 64.
To get platform independent controls, you can use \N{...}.
[6]
The result is the character specified by the octal number between the braces.See [8] below for details on which character.
If a character that isn't an octal digit is encountered, a warning is raised,and the value is based on the octal digits before it, discarding it and allfollowing characters up to the closing brace. It is a fatal error if there areno octal digits at all.
[7]
The result is the character specified by the three-digit octal number in therange 000 to 777 (but best to not use above 077, see next paragraph). See[8] below for details on which character.
Some contexts allow 2 or even 1 digit, but any usage without exactlythree digits, the first being a zero, may give unintended results. (Forexample, in a regular expression it may be confused with a backreference;see Octal escapes in perlrebackslash.) Starting in Perl 5.14, you mayuse \o{} instead, which avoids all these problems. Otherwise, it is best touse this construct only for ordinals \077 and below, remembering to pad tothe left with zeros to make three digits. For larger ordinals, either use\o{}, or convert to something else, such as to hex and use \x{}instead.
Having fewer than 3 digits may lead to a misleading warning message that saysthat what follows is ignored. For example, "\128" in the ASCII character setis equivalent to the two characters "\n8", but the warning Illegal octaldigit '8' ignored will be thrown. If "\n8" is what you want, you canavoid this warning by padding your octal number with 0's: "\0128".
[8]
Several constructs above specify a character by a number. That numbergives the character's position in the character set encoding (indexed from 0).This is called synonymously its ordinal, code position, or code point. Perlworks on platforms that have a native encoding currently of either ASCII/Latin1or EBCDIC, each of which allow specification of 256 characters. In general, ifthe number is 255 (0xFF, 0377) or below, Perl interprets this in the platform'snative encoding. If the number is 256 (0x100, 0400) or above, Perl interpretsit as a Unicode code point and the result is the corresponding Unicodecharacter. For example \x{50} and \o{120} both are the number 80 indecimal, which is less than 256, so the number is interpreted in the nativecharacter set encoding. In ASCII the character in the 80th position (indexedfrom 0) is the letter "P", and in EBCDIC it is the ampersand symbol "&".\x{100} and \o{400} are both 256 in decimal, so the number is interpretedas a Unicode code point no matter what the native encoding is. The name of thecharacter in the 256th position (indexed by 0) in Unicode isLATIN CAPITAL LETTER A WITH MACRON.
There are a couple of exceptions to the above rule. \N{U+hex number} isalways interpreted as a Unicode code point, so that \N{U+0050} is "P" evenon EBCDIC platforms. And if use encoding is in effect, thenumber is considered to be in that encoding, and is translated from that intothe platform's native encoding if there is a corresponding native character;otherwise to Unicode.

NOTE: Unlike C and other languages, Perl has no \v escape sequence forthe vertical tab (VT - ASCII 11), but you may use \ck or \x0b. (\vdoes have meaning in regular expression patterns in Perl, see perlre.)

The following escape sequences are available in constructs that interpolate,but not in transliterations.

 \llowercase next character only
  \utitlecase (not uppercase!) next character only
  \Llowercase all characters till \E or end of string
  \Uuppercase all characters till \E or end of string
  \Ffoldcase all characters till \E or end of string
  \Q  quote (disable) pattern metacharacters till \E or
  end of string
  \Eend either case modification or quoted section
 (whichever was last seen)

See quotemeta for the exact definition of characters thatare quoted by \Q.

\L, \U, \F, and \Q can stack, in which case you need one\E for each. For example:

 say"This \Qquoting \ubusiness \Uhere isn't quite\E done yet,\E is it?";
  This quoting\ Business\ HERE\ ISN\'T\ QUITE\ done\ yet\, is it?

If use locale is in effect (but not use locale ':not_characters'),the case map used by \l, \L,\u, and \U is taken from the current locale. See perllocale.If Unicode (for example, \N{} or code points of 0x100 orbeyond) is being used, the case map used by \l, \L, \u, and\U is as defined by Unicode. That means that case-mappinga single character can sometimes produce several characters.Under use locale, \F produces the same results as \L.

All systems use the virtual "\n" to represent a line terminator,called a "newline". There is no such thing as an unvarying, physicalnewline character. It is only an illusion that the operating system,device drivers, C libraries, and Perl all conspire to preserve. Not allsystems read "\r" as ASCII CR and "\n" as ASCII LF. For example,on the ancient Macs (pre-MacOS X) of yesteryear, these used to be reversed,and on systems without line terminator,printing "\n" might emit no actual data. In general, use "\n" whenyou mean a "newline" for your system, but use the literal ASCII when youneed an exact character. For example, most networking protocols expectand prefer a CR+LF ("\015\012" or "\cM\cJ") 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.

For constructs that do interpolate, variables beginning with "$"or "@" are interpolated. Subscripted variables such as $a[3] or$href->{key}[0] are also interpolated, as are array and hash slices.But method calls such as $obj->meth are not.

Interpolating an array or slice interpolates the elements in order,separated by the value of $", so is equivalent to interpolatingjoin $", @array. "Punctuation" arrays such as @* are usuallyinterpolated only if the name is enclosed in braces @{*}, but thearrays @_, @+, and @- are interpolated even without braces.

For double-quoted strings, the quoting from \Q is applied afterinterpolation and escapes are processed.

 "abc\Qfoo\tbar$s\Exyz"

is equivalent to

 "abc" . quotemeta("foo\tbar$s") . "xyz"

For the pattern of regex operators (qr//, m// and s///),the quoting from \Q is applied after interpolation is processed,but before escapes are processed. This allows the pattern to matchliterally (except for $ and @). For example, the following matches:

 '\s\t' =~ /\Q\s\t/

Because $ or @ trigger interpolation, you'll need to use somethinglike /\Quser\E\@\Qhost/ to match them literally.

Patterns are subject to an additional level of interpretation as aregular expression. This is done as a second pass, after variables areinterpolated, so that regular expressions may be incorporated into thepattern from the variables. If this is not what you want, use \Q tointerpolate a variable literally.

Apart from the behavior described above, Perl does not expandmultiple levels of interpolation. In particular, contrary to theexpectations of shell programmers, back-quotes do NOT interpolatewithin double quotes, nor do single quotes impede evaluation ofvariables when used within double quotes.

Regexp Quote-Like Operators

Here are the quote-like operators that apply to patternmatching and related activities.

qr/STRING/msixpodual
This operator quotes (and possibly compiles) its STRING as a regularexpression. STRING is interpolated the same way as PATTERNin m/PATTERN/. If "'" is used as the delimiter, no interpolationis done. Returns a Perl value which may be used instead of thecorresponding /STRING/msixpodual expression. The returned value is anormalized version of the original pattern. It magically differs froma string containing the same characters: ref(qr/x/) returns "Regexp";however, dereferencing it is not well defined (you currently get the normalized version of the original pattern, but this may change).
For example,
```
 $rex = qr/my.STRING/is;
  print $rex; # prints (?si-xm:my.STRING)
  s/$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 the 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 atthe moment of qr() avoids a need to recompile the pattern everytime a match /$pat/ is attempted. (Perl has many other internaloptimizations, but none would be triggered in the above example ifwe did not use qr() operator.)
Options (specified by the following modifiers) are:
```
 mTreat string as multiple lines.
   sTreat string as single line. (Make . match a newline)
  iDo case-insensitive pattern matching.
  xUse extended regular expressions.
  pWhen matching preserve a copy of the matched string so
  that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
  oCompile pattern only once.
  a   ASCII-restrict: Use ASCII for \d, \s, \w; specifying two a's
   further restricts /i matching so that no ASCII character will
   match a non-ASCII one
   l   Use the locale
   u   Use Unicode rules
   d   Use Unicode or native charset, as in 5.12 and earlier
 
```
If a precompiled pattern is embedded in a larger pattern then the effectof "msixpluad" will be propagated appropriately. The effect the "o"modifier has is not propagated, being restricted to those patternsexplicitly using it.
The last four modifiers listed above, added in Perl 5.14,control the character set semantics, but /a is the only one you are likelyto want to specify explicitly; the other three are selectedautomatically by various pragmas.
See perlre for additional information on valid syntax for STRING, andfor a detailed look at the semantics of regular expressions. Inparticular, all modifiers except the largely obsolete /o are furtherexplained in Modifiers in perlre. /o is described in the next section.
m/PATTERN/msixpodualgc
/PATTERN/msixpodualgc
Searches a string for a pattern match, and in scalar context returnstrue if it succeeds, false if it fails. If no string is specifiedvia the =~ or !~ operator, the $_ string is searched. (Thestring specified with =~ need not be an lvalue--it may be theresult of an expression evaluation, but remember the =~ bindsrather tightly.) See also perlre.
Options are as described in qr// above; in addition, the following matchprocess modifiers are available:
```
 g  Match globally, i.e., find all occurrences.
  c  Do not reset search position on a failed match when /g is in effect.
 
```
If "/" is the delimiter then the initial m is optional. With the myou can use any pair of non-whitespace (ASCII) charactersas delimiters. This is particularly useful for matching path namesthat contain "/", to avoid LTS (leaning toothpick syndrome). If "?" isthe delimiter, then a match-only-once rule applies,described in m?PATTERN? below.If "'" is the delimiter, no interpolation is performed on the PATTERN.When using a character valid in an identifier, whitespace is requiredafter the m.
PATTERN may contain variables, which will be interpolatedevery time the pattern search is evaluated, exceptfor when the delimiter is a single quote. (Note that $(, $), and$| are not interpolated because they look like end-of-string tests.)Perl will not recompile the pattern unless an interpolatedvariable that it contains changes. You can force Perl to skip thetest and never recompile by adding a /o (which stands for "once")after the trailing delimiter.Once upon a time, Perl would recompile regular expressionsunnecessarily, and this modifier was useful to tell it not to do so, in theinterests of speed. But now, the only reasons to use /o are either:
1
The variables are thousands of characters long and you know that theydon't change, and you need to wring out the last little bit of speed byhaving Perl skip testing for that. (There is a maintenance penalty fordoing this, as mentioning /o constitutes a promise that you won'tchange the variables in the pattern. If you do change them, Perl won'teven notice.)
2
you want the pattern to use the initial values of the variablesregardless of whether they change or not. (But there are saner waysof accomplishing this than using /o.)
The bottom line is that using /o is almost never a good idea.
The empty pattern //
If the PATTERN evaluates to the empty string, the lastsuccessfully matched regular expression is used instead. In thiscase, only the g and c flags on the empty pattern are honored;the other flags are taken from the original pattern. If no match haspreviously succeeded, this will (silently) act instead as a genuineempty pattern (which will always match).
Note that it's possible to confuse Perl into thinking // (the emptyregex) is really // (the defined-or operator). Perl is usually prettygood about this, but some pathological cases might trigger this, such as$a/// (is that ($a) / (//) or $a // /?) and print $fh //(print $fh(// or print($fh //?). In all of these examples, Perlwill assume you meant defined-or. If you meant the empty regex, justuse parentheses or spaces to disambiguate, or even prefix the emptyregex with an m (so // becomes m//).
Matching in list context
If the /g option is not used, m// in list context returns alist consisting of the subexpressions matched by the parentheses in thepattern, that is, ($1, $2, $3...). (Note that here $1 etc. arealso set, and that this differs from Perl 4's behavior.) When there areno parentheses in the pattern, the return value is the list (1) forsuccess. With or without parentheses, an empty list is returned uponfailure.
Examples:
```
 open(TTY, "+</dev/tty")
 || die "can't access /dev/tty: $!";
 
  <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 (no longer needed!)
  }
 
  if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
 
```
This last example splits $foo into the first two words and theremainder of the line, and assigns those three fields to $F1, $F2, and$Etc. The conditional is true if any variables were assigned; that is,if the pattern matched.
The /g modifier specifies global pattern matching--that is,matching as many times as possible within the string. How it behavesdepends on the context. In list context, it returns a list of thesubstrings matched by any capturing parentheses in the regularexpression. If there are no parentheses, it returns a list of allthe matched strings, as if there were parentheses around the wholepattern.
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. A failed match normally resets thesearch position to the beginning of the string, but you can avoid thatby adding the /c modifier (for example, m//gc). Modifying the targetstring also resets the search position.

\G assertion

You can intermix m//g matches with m/\G.../g, where \G is azero-width assertion that matches the exact position where theprevious m//g, if any, left off. Without the /g modifier, the\G assertion still anchors at pos() as it was at the start ofthe operation (see pos), but the match is of course onlyattempted once. Using \G without /g on a target string that hasnot previously had a /g match applied to it is the same as usingthe \A assertion to match the beginning of the string. Note alsothat, currently, \G is only properly supported when anchored at thevery beginning of the pattern.

Examples:

 # list context
  ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
 
  # scalar context
  local $/ = "";
  while ($paragraph = <>) {
 while ($paragraph =~ /\p{Ll}['")]*[.!?]+['")]*\s/g) {
  $sentences++;
 }
  }
  say $sentences;

Here's another way to check for sentences in a paragraph:

 my $sentence_rx = qr{
 (?: (?<= ^ ) | (?<= \s ) )  # after start-of-string or whitespace
 \p{Lu}  # capital letter
 .*? # a bunch of anything
 (?<= \S )   # that ends in non-whitespace
 (?<! \b [DMS]r  )   # but isn't a common abbreviation
 (?<! \b Mrs )
 (?<! \b Sra )
 (?<! \b St  )
 [.?!]   # followed by a sentence ender
 (?= $ | \s ) # in front of end-of-string or whitespace
   }sx;
  local $/ = "";
  while (my $paragraph = <>) {
 say "NEW PARAGRAPH";
 my $count = 0;
 while ($paragraph =~ /($sentence_rx)/g) {
  printf "\tgot sentence %d: <%s>\n", ++$count, $1;
 }
  }

Here's how to use 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";
  }
  print "Final: '$1', pos=",pos,"\n" if /\G(.)/;

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
  Final: 'q', pos=8

Notice that the final match matched q instead of p, which a matchwithout the \G anchor would have done. Also note that the final matchdid not update pos. pos is only updated on a /g match. If thefinal match did indeed match p, it's a good bet that you're running avery old (pre-5.6.0) version of Perl.

A useful idiom for lex-like scanners is /\G.../gc. You cancombine several regexps like this to process a string part-by-part,doing different actions depending on which regexp matched. Eachregexp tries to match where the previous one leaves off.

 $_ = <<'EOL';
  $url = URI::URL->new( "http://example.com/" ); die if $url eq "xXx"
  EOL
 
  LOOP: {
  print(" digits"),   redo LOOP if /\G\d+\b[,.;]?\s*/gc;
  print(" lowercase"), redo LOOP if /\G\p{Ll}+\b[,.;]?\s*/gc;
  print(" UPPERCASE"), redo LOOP if /\G\p{Lu}+\b[,.;]?\s*/gc;
  print(" Capitalized"),  redo LOOP if /\G\p{Lu}\p{Ll}+\b[,.;]?\s*/gc;
  print(" MiXeD"), redo LOOP if /\G\pL+\b[,.;]?\s*/gc;
  print(" alphanumeric"), redo LOOP if /\G[\p{Alpha}\pN]+\b[,.;]?\s*/gc;
  print(" line-noise"),   redo LOOP if /\G\W+/gc;
  print ". That's all!\n"
  }

Here is the output (split into several lines):

 line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE
  line-noise lowercase line-noise lowercase line-noise lowercase
  lowercase line-noise lowercase lowercase line-noise lowercase
  lowercase line-noise MiXeD line-noise. That's all!

m?PATTERN?msixpodualgc
?PATTERN?msixpodualgc
This is just like the m/PATTERN/ search, except that it matchesonly once between calls to the reset() operator. This is a usefuloptimization when you want to see only the first occurrence ofsomething in each file of a set of files, for instance. Only m??patterns local to the current package are reset.
```
 while (<>) {
 if (m?^$?) {
  # blank line between header and body
 }
  } continue {
 reset if eof; # clear m?? status for next file
  }
 
```
Another example switched the first "latin1" encoding it findsto "utf8" in a pod file:
```
 s//utf8/ if m? ^ =encoding \h+ \K latin1 ?x;
 
```
The match-once behavior is controlled by the match delimiter being?; with any other delimiter this is the normal m// operator.
For historical reasons, the leading m in m?PATTERN? is optional,but the resulting ?PATTERN? syntax is deprecated, will warn onusage and might be removed from a future stable release of Perl (withoutfurther notice!).
s/PATTERN/REPLACEMENT/msixpodualgcer
Searches a string for a pattern, and if found, replaces that patternwith the replacement text and returns the number of substitutionsmade. Otherwise it returns false (specifically, the empty string).
If the /r (non-destructive) option is used then it runs thesubstitution on a copy of the string and instead of returning thenumber of substitutions, it returns the copy whether or not asubstitution occurred. The original string is never changed when/r is used. The copy will always be a plain string, even if theinput is an object or a tied variable.
If no string is specified via the =~ or !~ operator, the $_variable is searched and modified. Unless the /r option is used,the string specified must be a scalar variable, an array element, ahash element, or an assignment to one of those; that is, some sort ofscalar lvalue.
If the delimiter chosen is a single quote, no interpolation isdone on either the PATTERN or the REPLACEMENT. Otherwise, if thePATTERN contains a $ that looks like a variable rather than anend-of-string test, the variable will be interpolated into the patternat run-time. If you want the pattern compiled only once the first timethe variable is interpolated, use the /o option. If the patternevaluates to the empty string, the last successfully executed regularexpression is used instead. See perlre for further explanation on these.
Options are as with m// with the addition of the following replacementspecific options:
```
 eEvaluate the right side as an expression.
  ee  Evaluate the right side as a string then eval the result.
  r   Return substitution and leave the original string untouched.
 
```
Any non-whitespace delimiter may replace the slashes. Add space afterthe s when using a character allowed in identifiers. If single quotesare used, no interpretation is done on the replacement string (the /emodifier overrides this, however). Unlike Perl 4, Perl 5 treats backticksas normal delimiters; the replacement text is not evaluated as a command.If the PATTERN is delimited by bracketing quotes, the REPLACEMENT hasits own pair of quotes, which may or may not be bracketing quotes, for example,s(foo)(bar) or s/bar/. A /e will cause thereplacement portion to be treated as a full-fledged Perl expressionand evaluated right then and there. It is, however, syntax checked atcompile-time. A second e modifier will cause the replacement portionto 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 = $bar) =~ s/this/that/;# copy first, then change
  ($foo = "$bar") =~ s/this/that/;# convert to string, copy, then change
  $foo = $bar =~ s/this/that/r;# Same as above using /r
  $foo = $bar =~ s/this/that/r
  =~ s/that/the other/r;# Chained substitutes using /r
  @foo = map { s/this/that/r } @bar# /r is very useful in maps
 
  $count = ($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
 
  $_ = 'abc123xyz';
  $a = s/abc/def/r;   # $a is 'def123xyz' and
  # $_ remains 'abc123xyz'.
 
  # 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;
 
  # Titlecase words in the last 30 characters only
  substr($str, -30) =~ s/\b(\p{Alpha}+)\b/\u\L$1/g;
 
  # 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 whitespace in $_, expensively
 
  for ($variable) {# trim whitespace in $variable, cheap
 s/^\s+//;
 s/\s+$//;
  }
 
  s/([^ ]*) *([^ ]*)/$2 $1/;# reverse 1st two fields
 
```
Note the use of $ instead of \ in the last example. Unlikesed, we use the \<digit> form in only the left hand side.Anywhere else it's $<digit>.
Occasionally, you can't use just a /g to get all the changesto 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;
 
```
s///le is treated as a substitution followed by the le operator, notthe /le flags. This may change in a future version of Perl. Itproduces a warning if warnings are enabled. To disambiguate, use a spaceor change the order of the flags:
```
 s/foo/bar/ le 5;  # "le" infix operator
  s/foo/bar/el; # "e" and "l" flags
 
```

Quote-Like Operators

q/STRING/
'STRING'
A single-quoted, literal string. A backslash represents a backslashunless followed by the delimiter or another backslash, in which casethe 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

qx/STRING/
`STRING`
A string which is (possibly) interpolated and then executed as asystem command with /bin/sh or its equivalent. Shell wildcards,pipes, and redirections will be honored. The collected standardoutput of the command is returned; standard error is unaffected. Inscalar context, it comes back as a single (potentially multi-line)string, or undef if the command failed. In list context, returns alist of lines (however you've defined lines with $/ or$INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
Because backticks do not affect standard error, use shell file descriptorsyntax (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 isimportant here):
```
 $output = `cmd 2>&1 1>/dev/null`;
 
```
To exchange a command's STDOUT and STDERR in order to capture the STDERRbut 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 easiestto redirect them separately to files, and then read from those fileswhen the program is done:
```
 system("program args 1>program.stdout 2>program.stderr");
 
```
The STDIN filehandle used by the command is inherited from Perl's STDIN.For example:
```
 open(SPLAT, "stuff")   || die "can't open stuff: $!";
  open(STDIN, "<&SPLAT") || die "can't dupe SPLAT: $!";
  print STDOUT `sort`;
 
```
will print the sorted contents of the file named "stuff".
Using single-quote as a delimiter protects the command from Perl'sdouble-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 commandinterpreter on your system. On most platforms, you will have to protectshell metacharacters if you want them treated literally. This is inpractice difficult to do, as it's unclear how to escape which characters.See 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 becapable of dealing with multiline commands, so putting newlines inthe string may not get you what you want. You may be able to evaluatemultiple commands in a single line by separating them with the commandseparator character, if your shell supports that (for example, ; on many Unix shells and & on the Windows NT cmd shell).
Beginning with v5.6.0, Perl will attempt to flush all files opened foroutput before starting the child process, but this may not be supportedon some platforms (see perlport). To be safe, you may need to set$| ($AUTOFLUSH in English) or call the autoflush() method ofIO::Handle on any open handles.
Beware that some command shells may place restrictions on the lengthof the command line. You must ensure your strings don't exceed thislimit after any necessary interpolations. See the platform-specificrelease 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 infact not be present at all. As one example, the type command underthe POSIX shell is very different from the type command under DOS.That doesn't mean you should go out of your way to avoid backtickswhen they're the right way to get something done. Perl was made to bea glue language, and one of the things it glues together is commands.Just understand what you're getting yourself into.
See I/O Operators for more discussion.
qw/STRING/
Evaluates to a list of the words extracted out of STRING, using embeddedwhitespace as the word delimiters. It can be understood as being roughlyequivalent to:
```
 split(" ", q/STRING/);
 
```
the differences being that it generates a real list at compile time, andin scalar context it returns the last element in the list. Sothis 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 toput comments into a multi-line qw-string. For this reason, theuse warnings pragma and the -w switch (that is, the $^W variable)produces warnings if the STRING contains the "," or the "#" character.
tr/SEARCHLIST/REPLACEMENTLIST/cdsr
y/SEARCHLIST/REPLACEMENTLIST/cdsr
Transliterates all occurrences of the characters found in the search listwith the corresponding character in the replacement list. It returnsthe number of characters replaced or deleted. If no string isspecified via the =~ or !~ operator, the $_ string is transliterated.
If the /r (non-destructive) option is present, a new copy of the stringis made and its characters transliterated, and this copy is returned nomatter whether it was modified or not: the original string is alwaysleft unchanged. The new copy is always a plain string, even if the inputstring is an object or a tied variable.
Unless the /r option is used, the string specified with =~ must be ascalar variable, an array element, a hash element, or an assignment to oneof those; in other words, 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 theSEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST hasits own pair of quotes, which may or may not be bracketing quotes;for example, tr[aeiouy][yuoiea] or tr(+\-*/)/ABCD/.
Note that tr does not do regular expression character classes such as\d or \pL. The tr operator is not equivalent to the tr(1)utility. If you want to map strings between lower/upper cases, seelc and uc, and in general consider using the soperator if you need regular expressions. The \U, \u, \L, and\l string-interpolation escapes on the right side of a substitutionoperator will perform correct case-mappings, but tr[a-z][A-Z] will not(except sometimes on legacy 7-bit data).
Note also that the whole range idea is rather unportable betweencharacter sets--and even within character sets they may cause resultsyou probably didn't expect. A sound principle is to use only rangesthat 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 thecharacter sets in full.
Options:
```
 cComplement the SEARCHLIST.
  dDelete found but unreplaced characters.
  sSquash duplicate replaced characters.
   rReturn the modified string and leave the original string
 untouched.
 
```
If the /c modifier is specified, the SEARCHLIST character setis complemented. If the /d modifier is specified, any charactersspecified by SEARCHLIST not found in REPLACEMENTLIST are deleted.(Note that this is slightly more flexible than the behavior of sometr programs, which delete anything they find in the SEARCHLIST,period.) If the /s modifier is specified, sequences of charactersthat were transliterated to the same character are squashed downto a single instance of the character.
If the /d modifier is used, the REPLACEMENTLIST is always interpretedexactly as specified. Otherwise, if the REPLACEMENTLIST is shorterthan the SEARCHLIST, the final character is replicated till it is longenough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.This latter is useful for counting characters in a class or forsquashing character sequences in a class.
Examples:
```
 $ARGV[1] =~ tr/A-Z/a-z/;# canonicalize to lower case ASCII
 
  $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/;
  $HOST = $host  =~ tr/a-z/A-Z/r;   # same thing
 
  $HOST = $host =~ tr/a-z/A-Z/r # chained with s///r
   =~ s/:/ -p/r;
 
  tr/a-zA-Z/ /cs;# change non-alphas to single space
 
  @stripped = map tr/a-zA-Z/ /csr, @original;
 # /r with map
 
  tr [\200-\377]
       [\000-\177];# wickedly delete 8th bit
 
```
If multiple transliterations are given for a character, only thefirst one is used:
```
 tr/AAA/XYZ/
 
```
will transliterate any A to X.
Because the transliteration table is built at compile time, neitherthe SEARCHLIST nor the REPLACEMENTLIST are subjected to double quoteinterpolation. That means that if you want to use variables, youmust use an eval():
```
 eval "tr/$oldlist/$newlist/";
  die $@ if $@;
 
  eval "tr/$oldlist/$newlist/, 1" or die $@;
 
```
<<EOF
A line-oriented form of quoting is based on the shell "here-document"syntax. Following a << you specify a string to terminatethe quoted material, and all lines following the current line down tothe terminating string are the value of the item.
The terminating string may be either an identifier (a word), or somequoted text. An unquoted identifier works like double quotes.There may not be a space between the << and the identifier,unless the identifier is explicitly quoted. (If you put a space itwill be treated as a null identifier, which is valid, and matches thefirst empty line.) The terminating string must appear by itself(unquoted and with no surrounding whitespace) on the terminating line.
If the terminating string is quoted, the type of quotes used determinethe treatment of the text.
- Double Quotes
  Double quotes indicate that the text will be interpolated using exactlythe same rules as normal double quoted strings.
```
   print <<EOF;
  The price is $Price.
  EOF
 
    print << "EOF"; # same as above
  The price is $Price.
  EOF
 
```
- Single Quotes
  Single quotes indicate the text is to be treated literally with nointerpolation of its content. This is similar to single quotedstrings except that backslashes have no special meaning, with \being treated as two backslashes and not one as they would in everyother quoting construct.
  Just as in the shell, a backslashed bareword following the <<means the same thing as a single-quoted string does:
```
$cost = <<'VISTA';  # hasta la ...
  That'll be $10 please, ma'am.
  VISTA
 
 $cost = <<\VISTA;   # Same thing!
  That'll be $10 please, ma'am.
  VISTA
 
```
  This is the only form of quoting in perl where there is no needto worry about escaping content, something that code generatorscan and do make good use of.
- Backticks
  The content of the here doc is treated just as it would be if thestring were embedded in backticks. Thus the content is interpolatedas though it were double quoted and then executed via the shell, withthe results of the execution returned.
```
   print << `EOC`; # execute command and get results
  echo hi there
  EOC
 
```
It is possible to stack multiple here-docs in a row:
```
   print <<"foo", <<"bar"; # you can stack them
  I said foo.
  foo
  I said bar.
  bar
 
    myfunc(<< "THIS", 23, <<'THAT');
  Here's a line
  or two.
  THIS
  and here's another.
  THAT
 
```
Just don't forget that you have to put a semicolon on the endto finish the statement, as Perl doesn't know you're not going totry to do this:
```
   print <<ABC
  179231
  ABC
    + 20;
 
```
If you want to remove the line terminator from your here-docs,use chomp().
```
 chomp($string = <<'END');
  This is a string.
  END
 
```
If you want your here-docs to be indented with the rest of the code,you'll need to remove leading whitespace from each line manually:
```
 ($quote = <<'FINIS') =~ s/^\s+//gm;
    The Road goes ever on and on,
    down from the door where it began.
  FINIS
 
```
If you use a here-doc within a delimited construct, such as in s///eg,the quoted material must come on the lines following the final delimiter.So instead of
```
 s/this/<<E . 'that'
  the other
  E
  . 'more '/eg;
 
```
you have to write
```
 s/this/<<E . 'that'
  . 'more '/eg;
  the other
  E
 
```
If the terminating identifier is on the last line of the program, youmust be sure there is a newline after it; otherwise, Perl will give thewarning Can't find string terminator "END" anywhere before EOF....
Additionally, quoting rules for the end-of-string identifier areunrelated to Perl's quoting rules. q(), qq(), and the like are notsupported in place of '' and "", and the only interpolation is forbackslashing the quoting character:
```
 print << "abc\"def";
  testing...
  abc"def
 
```
Finally, quoted strings cannot span multiple lines. The general rule isthat the identifier must be a string literal. Stick with that, and youshould be safe.

Gory details of parsing quoted constructs

When presented with something that might have several differentinterpretations, Perl uses the DWIM (that's "Do What I Mean")principle to pick the most probable interpretation. This strategyis so successful that Perl programmers often do not suspect theambivalence of what they write. But from time to time, Perl'snotions 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 labyrinthineregular expressions, because the initial steps of parsing are thesame for all quoting operators, they are all discussed together.

The most important Perl parsing rule is the first one discussedbelow: when processing a quoted construct, Perl first finds the endof that construct, then interprets its contents. If you understandthis rule, you may skip the rest of this section on the firstreading. The other rules are likely to contradict the user'sexpectations much less frequently than this first one.

Some passes discussed below are performed concurrently, but becausetheir results are the same, we consider them individually. For differentquoting constructs, Perl performs different numbers of passes, fromone to four, but these passes are always performed in the same order.

Finding the end
The first pass is finding the end of the quoted construct, wherethe information about the delimiters is used in parsing.During this search, text between the starting and ending delimitersis copied to a safe location. The text copied gets delimiter-independent.
If the construct is a here-doc, the ending delimiter is a linethat has a terminating string as the content. Therefore <<EOF isterminated by EOF immediately followed by "\n" and startingfrom the first column of the terminating line.When searching for the terminating line of a here-doc, nothingis skipped. In other words, lines after the here-doc syntaxare compared with the terminating string line by line.
For the constructs except here-docs, single characters are used as startingand ending delimiters. If the starting delimiter is an opening punctuation(that is (, [, {, or <), the ending delimiter is thecorresponding closing punctuation (that is ), ], }, or >).If the starting delimiter is an unpaired character like / or a closingpunctuation, the ending delimiter is same as the starting delimiter.Therefore a / terminates a qq// construct, while a ] terminatesqq[] and qq]] constructs.
When searching for single-character delimiters, escaped delimitersand \ are skipped. For example, while searching for terminating /,combinations of \ and \/ are skipped. If the delimiters arebracketing, nested pairs are also skipped. For example, while searchingfor closing ] paired with the opening [, combinations of \, \],and \[ are all skipped, and nested [ and ] are skipped as well.However, when backslashes are used as the delimiters (like qq\ andtr\), nothing is skipped.During the search for the end, backslashes that escape delimiters orbackslashes are removed (exactly speaking, they are not copied to thesafe location).
For constructs with three-part delimiters (s///, y///, andtr///), the search is repeated once more.If the first delimiter is not an opening punctuation, three delimiters mustbe same such as s!!! and tr))), in which case the second delimiterterminates the left part and starts the right part at once.If the left part is delimited by bracketing punctuation (that is (),[], {}, or <>), the right part needs another pair ofdelimiters such as s(){} and tr[]//. In these cases, whitespaceand comments are allowed between both parts, though the comment must followat least one whitespace character; otherwise a character expected as the start of the comment may be regarded as the starting delimiter of the right part.
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 thefirst " 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 /xmodifier. So the embedded # is interpreted as a literal #.
Also no attention is paid to \c\ (multichar control char syntax) duringthis search. Thus the second \ in qq/\c\/ is interpreted as a partof \/, and the following / is not recognized as a delimiter.Instead, use \034 or \x1c at the end of quoted constructs.
Interpolation
The next step is interpolation in the text obtained, which is nowdelimiter-independent. There are multiple cases.
- <<'EOF'
  No interpolation is performed.Note that the combination \ is left intact, since escaped delimitersare not available for here-docs.
- m'', the pattern of s'''
  No interpolation is performed at this stage.Any backslashed sequences including \ are treated at the stageto parsing regular expressions.
- '', q//, tr''', y''', the replacement of s'''
  The only interpolation is removal of \ from pairs of \.Therefore - in tr''' and y''' is treated literallyas a hyphen and no character range is available.\1 in the replacement of s''' does not work as $1.
- tr///, y///
  No variable interpolation occurs. String modifying combinations forcase and quoting such as \Q, \U, and \E are not recognized.The other escape sequences such as \200 and \t and backslashedcharacters such as \ and \- are converted to appropriate literals.The character - is treated specially and therefore \- is treatedas a literal -.
- "", ``, qq//, qx//, <file*glob>, <<"EOF"
  \Q, \U, \u, \L, \l, \F (possibly paired with \E) areconverted to corresponding Perl constructs. Thus, "$foo\Qbaz$bar"is converted to $foo . (quotemeta("baz" . $bar)) internally.The other escape sequences such as \200 and \t and backslashedcharacters such as \ and \- are replaced with appropriateexpansions.
  Let it be stressed that whatever falls between \Q and \Eis interpolated in the usual way. Something like "\Q\E" hasno \E inside. instead, it has \Q, \, and E, so theresult is the same as for "\E". As a general rule, backslashesbetween \Q and \E may lead to counterintuitive results. So,"\Q\t\E" is converted to quotemeta("\t"), which is the sameas "\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. catenation 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 metacharactersquoted, 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 interpolatedscalar.
  Note also that the interpolation code needs to make a decision onwhere 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 includespaces between components and which contains matching braces orbrackets. because the outcome may be determined by voting basedon heuristic estimators, the result is not strictly predictable.Fortunately, it's usually correct for ambiguous cases.
- the replacement of s///
  Processing of \Q, \U, \u, \L, \l, \F and interpolationhappens as with qq// constructs.
  It is at this step that \1 is begrudgingly converted to $1 inthe replacement text of s///, in order to correct the incorrigiblesed hackers who haven't picked up the saner idiom yet. A warningis emitted if the use warnings pragma or the -w command-line flag(that is, the $^W variable) was set.
- RE in ?RE?, /RE/, m/RE/, s/RE/foo/,
  Processing of \Q, \U, \u, \L, \l, \F, \E,and interpolation happens (almost) as with qq// constructs.
  Processing of \N{...} is also done here, and compiled into an intermediateform for the regex compiler. (This is because, as mentioned below, the regexcompilation may be done at execution time, and \N{...} is a compile-timeconstruct.)
  However any other combinations of \ followed by a characterare not substituted but only skipped, in order to parse themas regular expressions at the following step.As \c is skipped at this step, @ of \c@ in RE is possiblytreated as an array symbol (for example @foo),even though the same text in qq// gives interpolation of \c@.
  Moreover, inside (?{BLOCK}), (?# comment ), anda #-comment in a //x-regular expression, no processing isperformed whatsoever. This is the first step at which the presenceof the //x modifier is relevant.
  Interpolation in patterns has several quirks: $|, $(, $), @+and @- are not interpolated, and constructs $var[SOMETHING] arevoted (by several different estimators) to be either an array elementor $var followed by an RE alternative. This is where the notation${arr[$bar]} comes handy: /${arr[0-9]}/ is interpreted asarray 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.
  The lack of processing of \ creates specific restrictions onthe post-processed text. If the delimiter is /, one cannot getthe combination \/ into the result of this step. / willfinish the regular expression, \/ will be stripped to / onthe previous step, and \/ will be left as is. Because / isequivalent to \/ inside a regular expression, this does notmatter unless the delimiter happens to be character special to theRE engine, such as in s*foo*bar*, m[foo], or ?foo?; or analphanumeric char, as in:
```
  m m ^ a \s* b mmx;
 
```
  In the RE above, which is intentionally obfuscated for illustration, thedelimiter is m, the modifier is mx, and after delimiter-removal theRE is the same as for m/ ^ a \s* b /mx. There's more than onereason 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.
parsing regular expressions
Previous steps were performed during the compilation of Perl code,but this one happens at run time, although it may be optimized tobe calculated at compile time if appropriate. After preprocessingdescribed above, and possibly after evaluation if concatenation,joining, casing translation, or metaquoting are involved, theresulting string is passed to the RE engine for compilation.
Whatever happens in the RE engine might be better discussed in perlre,but for the sake of continuity, we shall do so here.
This is another step where the presence of the //x modifier isrelevant. The RE engine scans the string from left to right andconverts it to a finite automaton.
Backslashed characters are either replaced with correspondingliteral strings (as with \{), or else they generate special nodesin the finite automaton (as with \b). Characters special to theRE engine (such as |) generate corresponding nodes or groups ofnodes. (?#...) comments are ignored. All the rest is eitherconverted to literal strings to match, or else is ignored (as iswhitespace and #-style comments if //x is present).
Parsing of the bracketed character class construct, [...], israther different than the rule used for the rest of the pattern.The terminator of this construct is found using the same rules asfor finding the terminator of a {}-delimited construct, the onlyexception being that ] immediately following [ is treated asthough preceded by a backslash. Similarly, the terminator of(?{...}) is found using the same rules as for finding theterminator of a {}-delimited construct.
It is possible to inspect both the string given to RE engine and theresulting finite automaton. See the arguments debug/debugcolorin the use re pragma, as well as Perl's -Dr command-lineswitch documented in Command Switches in perlrun.
Optimization of regular expressions
This step is listed for completeness only. Since it does not changesemantics, details of this step are not documented and are subjectto change without notice. This step is performed over the finiteautomaton that was generated during the previous pass.
It is at this stage that split() silently optimizes /^/ tomean /^/m.

I/O Operators

There are several I/O operators you should know about.

A string enclosed by backticks (grave accents) first undergoesdouble-quote interpolation. It is then interpreted as an externalcommand, and the output of that command is the value of thebacktick string, like in a shell. In scalar context, a single stringconsisting of all output is returned. In list context, a list ofvalues is returned, one per line of output. (You can set $/ to usea different line terminator.) The command is executed each time thepseudo-literal is evaluated. The status value of the command isreturned in $? (see perlvar for the interpretation of $?).Unlike in csh, no translation is done on the return data--newlinesremain newlines. Unlike in any of the shells, single quotes do nothide variable names in the command from interpretation. To pass aliteral dollar-sign through to the shell you need to hide it with abackslash. The generalized form of backticks is qx//. (Becausebackticks always undergo shell expansion as well, see perlsec forsecurity concerns.)

In scalar context, evaluating a filehandle in angle brackets yieldsthe next line from that file (the newline, if any, included), orundef at end-of-file or on error. When $/ is set to undef(sometimes known as file-slurp mode) and the file is empty, itreturns '' the first time, followed by undef subsequently.

Ordinarily you must assign the returned value to a variable, butthere is one situation where an automatic assignment happens. Ifand only if the input symbol is the only thing inside the conditionalof 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 anodd thing to you, but you'll use the construct in almost every Perlscript you write.) The $_ variable is not implicitly localized.You'll have to put a local $_; before the loop if you want thatto happen.

The following lines are equivalent:

 while (defined($_ = <STDIN>)) { print; }
  while ($_ = <STDIN>) { print; }
  while (<STDIN>) { print; }
  for (;<STDIN>;) { print; }
  print while defined($_ = <STDIN>);
  print while ($_ = <STDIN>);
  print while <STDIN>;

This also behaves similarly, but assigns to a lexical variable instead of to $_:

 while (my $line = <STDIN>) { print $line }

In these loop constructs, the assigned value (whether assignmentis automatic or explicit) is then tested to see whether it isdefined. The defined test avoids problems where the line has a stringvalue that would be treated as false by Perl; for example a "" ora "0" with no trailing newline. If you really mean for such valuesto terminate the loop, they should be tested for explicitly:

 while (($_ = <STDIN>) ne '0') { ... }
  while (<STDIN>) { last unless $_; ... }

In other boolean contexts, <FILEHANDLE> without anexplicit defined test or comparison elicits a warning if theuse warnings pragma or the -wcommand-line switch (the $^W variable) is in effect.

The filehandles STDIN, STDOUT, and STDERR are predefined. (Thefilehandles stdin, stdout, and stderr will also work exceptin packages, where they would be interpreted as local identifiersrather than global.) Additional filehandles may be created withthe open() function, amongst others. See perlopentut andopen for details on this.

If a <FILEHANDLE> is used in a context that is looking fora list, a list comprising all input lines is returned, one line perlist element. It's easy to grow to a rather large data space thisway, so use with care.

<FILEHANDLE> may also be spelled readline(*FILEHANDLE).See readline.

The null filehandle <> is special: it can be used to emulate thebehavior of sed and awk, and any other Unix filter programthat takes a list of filenames, doing the same to each lineof input from all of them. Input from <> comes either fromstandard input, or from each file listed on the command line. Here'show it works: the first time <> is evaluated, the @ARGV array ischecked, and if it is empty, $ARGV[0] is set to "-", which when openedgives you standard input. The @ARGV array is then processed as a listof 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 (<ARGV>) {
  ...# 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 filenameinto the $ARGV variable. It also uses filehandle ARGVinternally. <> is just a synonym for <ARGV>, whichis magical. (The pseudo code above doesn't work because it treats<ARGV> as non-magical.)

Since the null filehandle uses the two argument form of openit interprets special characters, so if you have a script like this:

 while (<>) {
  print;
  }

and call it with perl dangerous.pl 'rm -rfv *|', it actually opens apipe, executes the rm command and reads rm's output from that pipe.If you want all items in @ARGV to be interpreted as file names, youcan use the module ARGV::readonly from CPAN.

You can modify @ARGV before the first <> as long as the array ends upcontaining the list of filenames you really want. Line numbers ($.)continue as though the input were one big happy file. See the examplein eof 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 automaticallyfilters 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 theGetopts 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 what the angle brackets contain is a simple scalar variable (for example,<$foo>), then that variable contains the name of thefilehandle to input from, or its typeglob, or a reference to thesame. For example:

 $fh = \*STDIN;
  $line = <$fh>;

If what's within the angle brackets is neither a filehandle nor a simplescalar variable containing a filehandle name, typeglob, or typeglobreference, it is interpreted as a filename pattern to be globbed, andeither a list of filenames or the next filename in the list is returned,depending on context. This distinction is determined on syntacticgrounds alone. That means <$x> is always a readline() froman indirect handle, but <$hash{key}> is always a glob().That's because $x is a simple scalar variable, but $hash{key} isnot--it's a hash element. Even <$x > (note the extra space)is treated as glob("$x "), not readline($x).

One level of double-quote interpretation is done first, but you can'tsay <$foo> because that's an indirect filehandle as explainedin the previous paragraph. (In older versions of Perl, programmerswould insert curly brackets to force interpretation as a filename glob:<${foo}>. These days, it's considered cleaner to call theinternal function directly as glob($foo), which is probably the rightway 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 (<FOO>) {
 chomp;
 chmod 0644, $_;
  }

except that the globbing is actually done internally using the standardFile::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 isstarting a new list. All values must be read before it will startover. In list context, this isn't important because you automaticallyget them all anyway. However, in scalar context the operator returnsthe next value each time it's called, or undef when the list hasrun out. As with filehandle reads, an automatic defined isgenerated when the glob occurs in the test part of a while,because legal glob returns (for example,a file called 0) would otherwiseterminate the loop. Again, undef is returned only once. So ifyou're expecting a single value from a glob, it is much better tosay

 ($file) = <blurch*>;

than

 $file = <blurch*>;

because the latter will alternate between returning a filename andreturning false.

If you're trying to do variable interpolation, it's definitely betterto use the glob() function, because the older notation can cause peopleto 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 atcompile time whenever it determines that all arguments to anoperator are static and have no side effects. In particular, stringconcatenation happens at compile time between literals that don't dovariable substitution. Backslash interpolation also happens atcompile 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, ifyou say

 foreach $file (@filenames) {
 if (-s $file > 5 + 100 * 2**16) {  }
  }

the compiler precomputes the number which that expressionrepresents so that the interpreter won't have to.

No-ops

Perl doesn't officially have a no-op operator, but the bare constants0 and 1 are special-cased not to produce a warning in voidcontext, so you can for example safely do

 1 while foo();

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 differentsizes, | and ^ ops act as though the shorter operand hadadditional zero bits on the right, while the & op acts as thoughthe longer operand were truncated to the length of the shorter.The granularity for such extension or truncation is one or morebytes.

 # 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<H\n";# prints "Perl\n"

If you are intending to manipulate bitstrings, be certain thatyou're supplying bitstrings: If an operand is a number, that will implya numeric bitwise operation. You may explicitly show which type ofoperation you intend by using "" or 0+, as in the examples below.

 $foo =  150  |  105;# yields 255  (0x96 | 0x69 is 0xFF)
  $foo = '150' |  105;# yields 255
  $foo =  150  | '105';# yields 255
  $foo = '150' | '105';# yields string '155' (under ASCII)
 
  $baz = 0+$foo & 0+$bar;# both ops explicitly numeric
  $biz = "$foo" ^ "$bar";# both ops explicitly stringy

See vec for information on how to manipulate individual bitsin a bit vector.

Integer Arithmetic

By default, Perl assumes that it must do most of its arithmetic infloating point. But by saying

 use integer;

you may tell the compiler to use integer operations(see integer for a detailed explanation) from here to the end ofthe enclosing BLOCK. An inner BLOCK may countermand this by saying

 no integer;

which lasts until the end of that BLOCK. Note that this doesn'tmean everything is an integer, merely that Perl will use integeroperations for arithmetic, comparison, and bitwise operators. Forexample, even under use integer, if you take the sqrt(2), you'llstill get 1.4142135623731 or so.

Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",and ">>") always produce integral results. (But see alsoBitwise String Operators.) However, use integer still has meaning forthem. By default, their results are interpreted as unsigned integers, butif use integer is in effect, their results are interpretedas signed integers. For example, ~0 usually evaluates to a largeintegral value. However, use integer; ~0 is -1 on two's-complementmachines.

Floating-point Arithmetic

While use integer provides integer-only arithmetic, there is noanalogous mechanism to provide automatic rounding or truncation to acertain number of decimal places. For rounding to a certain numberof digits, sprintf() or printf() is usually the easiest route.See perlfaq4.

Floating-point numbers are only approximations to what a mathematicianwould 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 floating-point equality or inequality is not agood idea. Here's a (relatively expensive) work-around to comparewhether two floating-point numbers are equal to a particular number ofdecimal places. See Knuth, volume II, for a more robust treatment ofthis 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) implementsceil(), 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 theimaginary numbers. Math::Complex not as efficient as POSIX, butPOSIX can't work with complex numbers.

Rounding in financial applications can have serious implications, andthe rounding method used should be specified precisely. In thesecases, it probably pays not to trust whichever system rounding isbeing used by Perl, but to instead implement the rounding function youneed yourself.

Bigger Numbers

The standard Math::BigInt, Math::BigRat, and Math::BigFloat modules,along with the bigint, bigrat, and bitfloat pragmas, providevariable-precision arithmetic and overloaded operators, althoughthey're currently pretty slow. At the cost of some space andconsiderable speed, they avoid the normal pitfalls associated withlimited-precision representations.

use 5.010;
 use bigint;  # easy interface to Math::BigInt
 $x = 123456789123456789;
 say $x * $x;
  +15241578780673678515622620750190521

Or with rationals:

use 5.010;
 use bigrat;
 $a = 3/22;
 $b = 4/6;
 say "a/b is ", $a/$b;
 say "a*b is ", $a*$b;
  a/b is 9/44
  a*b is 1/11

Several modules let you calculate with (bound only by memory and CPU time)unlimited or fixed precision. There are also some non-standard modules thatprovide faster implementations via external C libraries.

Here is a short, but incomplete summary:

  Math::Fraction big, unlimited fractions like 9973 / 12967
   Math::String   treat string sequences like numbers
   Math::FixedPrecision   calculate with a fixed precision
   Math::Currency for currency calculations
   Bit::Vector manipulate bit vectors fast (uses C)
   Math::BigIntFast   Bit::Vector wrapper for big numbers
   Math::Pari provides access to the Pari C library
   Math::BigInteger   uses an external C library
   Math::Cephes   uses external Cephes C library (no big numbers)
   Math::Cephes::Fraction fractions via the Cephes library
   Math::GMP  another one using an external C library

Choose wisely.

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NAME
DESCRIPTION

Source : perldoc.perl.org - Official documentation for the Perl programming language
Site maintained by Jon Allen (JJ) See the project page for more details
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