=head1 NAME X perlop - Perl operators and precedence =head1 DESCRIPTION In Perl, the operator determines what operation is performed, independent of the type of the operands. For example S> is always a numeric addition, and if C or C do not contain numbers, an attempt is made to convert them to numbers first. This is in contrast to many other dynamic languages, where the operation is determined by the type of the first argument. It also means that Perl has two versions of some operators, one for numeric and one for string comparison. For example S> compares two numbers for equality, and S> compares two strings. There are a few exceptions though: C can be either string repetition or list repetition, depending on the type of the left operand, and C, C, C and C can be either string or numeric bit operations. =head2 Operator Precedence and Associativity X X X Operator precedence and associativity work in Perl more or less like they do in mathematics. I means some operators group more tightly than others. For example, in C, the multiplication has higher precedence, so C is grouped together as the right-hand operand of the addition, rather than C being grouped together as the left-hand operand of the multiplication. It is as if the expression were written C, not C. So the expression yields C, rather than C. I defines what happens if a sequence of the same operators is used one after another: usually that they will be grouped at the left or the right. For example, in C, subtraction is left associative, so C is grouped together as the left-hand operand of the second subtraction, rather than C being grouped together as the right-hand operand of the first subtraction. It is as if the expression were written C, not C. So the expression yields C, rather than C. For simple operators that evaluate all their operands and then combine the values in some way, precedence and associativity (and parentheses) imply some ordering requirements on those combining operations. For example, in C, the grouping implied by precedence means that the multiplication of 4 and 5 must be performed before the addition of 2 and 20, simply because the result of that multiplication is required as one of the operands of the addition. But the order of operations is not fully determined by this: in C both multiplications must be performed before the addition, but the grouping does not say anything about the order in which the two multiplications are performed. In fact Perl has a general rule that the operands of an operator are evaluated in left-to-right order. A few operators such as C have special evaluation rules that can result in an operand not being evaluated at all; in general, the top-level operator in an expression has control of operand evaluation. Some comparison operators, as their associativity, I with some operators of the same precedence (but never with operators of different precedence). This chaining means that each comparison is performed on the two arguments surrounding it, with each interior argument taking part in two comparisons, and the comparison results are implicitly ANDed. Thus S $y E= $z">> behaves exactly like S $y && $y E= $z">>, assuming that C is as simple a scalar as it looks. The ANDing short-circuits just like C does, stopping the sequence of comparisons as soon as one yields false. In a chained comparison, each argument expression is evaluated at most once, even if it takes part in two comparisons, but the result of the evaluation is fetched for each comparison. (It is not evaluated at all if the short-circuiting means that it's not required for any comparisons.) This matters if the computation of an interior argument is expensive or non-deterministic. For example, if($x > is an error. Perl operators have the following associativity and precedence, listed from highest precedence to lowest. Operators borrowed from C keep the same precedence relationship with each other, even where C's precedence is slightly screwy. (This makes learning Perl easier for C folks.) With very few exceptions, these all operate on scalar values only, not array values. left terms and list operators (leftward) left -> nonassoc ++ -- right ** right ! ~ ~. \ and unary + and - left =~ !~ left * / % x left + - . left > nonassoc named unary operators nonassoc isa chained = lt gt le ge chain/na == != eq ne cmp ~~ left & &. left | |. ^ ^. left && left || // nonassoc .. ... right ?: right = += -= *= etc. goto last next redo dump left , => nonassoc list operators (rightward) right not left and left or xor In the following sections, these operators are covered in detail, in the same order in which they appear in the table above. Many operators can be overloaded for objects. See L. =head2 Terms and List Operators (Leftward) X X X A TERM has the highest precedence in Perl. They include variables, quote and quote-like operators, any expression in parentheses, and any function whose arguments are parenthesized. Actually, there aren't really functions in this sense, just list operators and unary operators behaving as functions because you put parentheses around the arguments. These are all documented in L. If any list operator (C, etc.) or any unary operator (C, etc.) is followed by a left parenthesis as the next token, the operator and arguments within parentheses are taken to be of highest precedence, just like a normal function call. In the absence of parentheses, the precedence of list operators such as C, C, or C is either very high or very low depending on whether you are looking at the left side or the right side of the operator. For example, in @ary = (1, 3, sort 4, 2); print @ary; # prints 1324 the commas on the right of the C are evaluated before the C, but the commas on the left are evaluated after. In other words, list operators tend to gobble up all arguments that follow, and then act like a simple TERM with regard to the preceding expression. Be careful with parentheses: # These evaluate exit before doing the print: print($foo, exit); # Obviously not what you want. print $foo, exit; # Nor is this. # These do the print before evaluating exit: (print $foo), exit; # This is what you want. print($foo), exit; # Or this. print ($foo), exit; # Or even this. Also note that print ($foo & 255) + 1, "\n"; probably doesn't do what you expect at first glance. The parentheses enclose the argument list for C which is evaluated (printing the result of S>). Then one is added to the return value of C (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 L for more discussion of this. Also parsed as terms are the S> and S> constructs, as well as subroutine and method calls, and the anonymous constructors C and C. See also L toward the end of this section, as well as L"I/O Operators">. =head2 The Arrow Operator X X X >> "C >>" is an infix dereference operator, just as it is in C and C++. If the right side is either a C, C, or a C subscript, then the left side must be either a hard or symbolic reference to an array, a hash, or a subroutine respectively. (Or technically speaking, a location capable of holding a hard reference, if it's an array or hash reference being used for assignment.) See L and L. Otherwise, the right side is a method name or a simple scalar variable containing either the method name or a subroutine reference, and the left side must be either an object (a blessed reference) or a class name (that is, a package name). See L. The dereferencing cases (as opposed to method-calling cases) are somewhat extended by the C feature. For the details of that feature, consult L. =head2 Auto-increment and Auto-decrement X X X X X X C and C work as in C. That is, if placed before a variable, they increment or decrement the variable by one before returning the value, and if placed after, increment or decrement after returning the value. $i = 0; $j = 0; print $i++; # prints 0 print ++$j; # prints 1 Note that just as in C, Perl doesn't define B the variable is incremented or decremented. You just know it will be done sometime before or after the value is returned. This also means that modifying a variable twice in the same statement will lead to undefined behavior. Avoid statements like: $i = $i ++; print ++ $i + $i ++; Perl will not guarantee what the result of the above statements is. The auto-increment operator has a little extra builtin magic to it. If you increment a variable that is numeric, or that has ever been used in a numeric context, you get a normal increment. If, however, the variable has been used in only string contexts since it was set, and has a value that is not the empty string and matches the pattern C^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each character within its range, with carry: print ++($foo = "99"); # prints "100" print ++($foo = "a0"); # prints "a1" print ++($foo = "Az"); # prints "Ba" print ++($foo = "zz"); # prints "aaa" C is always treated as numeric, and in particular is changed to C before incrementing (so that a post-increment of an undef value will return C rather than C). The auto-decrement operator is not magical. =head2 Exponentiation X X X Binary C is the exponentiation operator. It binds even more tightly than unary minus, so C is C, not C. (This is implemented using C's C function, which actually works on doubles internally.) Note that certain exponentiation expressions are ill-defined: these include C, C, and C. Do not expect any particular results from these special cases, the results are platform-dependent. =head2 Symbolic Unary Operators X X Unary C performs logical negation, that is, "not". See also L|/Logical Not> for a lower precedence version of this. X Unary C performs arithmetic negation if the operand is numeric, including any string that looks like a number. If the operand is an identifier, a string consisting of a minus sign concatenated with the identifier is returned. Otherwise, if the string starts with a plus or minus, a string starting with the opposite sign is returned. One effect of these rules is that C is equivalent to the string C. If, however, the string begins with a non-alphabetic character (excluding C or C), Perl will attempt to convert the string to a numeric, and the arithmetic negation is performed. If the string cannot be cleanly converted to a numeric, Perl will give the warning B. X X Unary C performs bitwise negation, that is, 1's complement. For example, S> is 0640. (See also L and L.) Note that the width of the result is platform-dependent: C is 32 bits wide on a 32-bit platform, but 64 bits wide on a 64-bit platform, so if you are expecting a certain bit width, remember to use the C operator to mask off the excess bits. X X Starting in Perl 5.28, it is a fatal error to try to complement a string containing a character with an ordinal value above 255. If the "bitwise" feature is enabled via S> or C, then unary C always treats its argument as a number, and an alternate form of the operator, C, always treats its argument as a string. So C and C will both give 2**32-1 on 32-bit platforms, whereas C and C will both yield C. Until Perl 5.28, this feature produced a warning in the C category. Unary C has no effect whatsoever, even on strings. It is useful syntactically for separating a function name from a parenthesized expression that would otherwise be interpreted as the complete list of function arguments. (See examples above under L.) X Unary C creates references. If its operand is a single sigilled thing, it creates a reference to that object. If its operand is a parenthesised list, then it creates references to the things mentioned in the list. Otherwise it puts its operand in list context, and creates a list of references to the scalars in the list provided by the operand. See L and L. Do not confuse this behavior with the behavior of backslash within a string, although both forms do convey the notion of protecting the next thing from interpolation. X X X =head2 Binding Operators X X X X Binary C binds a scalar expression to a pattern match. Certain operations search or modify the string C by default. This operator makes that kind of operation work on some other string. The right argument is a search pattern, substitution, or transliteration. The left argument is what is supposed to be searched, substituted, or transliterated instead of the default C. When used in scalar context, the return value generally indicates the success of the operation. The exceptions are substitution (C) and transliteration (C) with the C (non-destructive) option, which cause the Beturn value to be the result of the substitution. Behavior in list context depends on the particular operator. See L"Regexp Quote-Like Operators"> for details and L for examples 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 run time. 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 the pattern C, which it will consider a syntax error. Binary C is just like C except the return value is negated in the logical sense. Binary C with a non-destructive substitution (C) or transliteration (C) is a syntax error. =head2 Multiplicative Operators X Binary C multiplies two numbers. X Binary C divides two numbers. X> X Binary C is the modulo operator, which computes the division remainder of its first argument with respect to its second argument. Given integer operands C and C: If C is positive, then S> is C minus the largest multiple of C less than or equal to C. If C is negative, then S> is C minus the smallest multiple of C that is not less than C (that is, the result will be less than or equal to zero). If the operands C and C are floating point values and the absolute value of C (that is C) is less than S>, only the integer portion of C and C will be used in the operation (Note: here C means the maximum of the unsigned integer type). If the absolute value of the right operand (C) is greater than or equal to S>, C computes the floating-point remainder C in the equation S> where C is a certain integer that makes C have the same sign as the right operand C (B as the left operand C like C function C) and the absolute value less than that of C. Note that when S> is in scope, C gives you direct access to the modulo operator as implemented by your C compiler. This operator is not as well defined for negative operands, but it will execute faster. X X X X Binary C is the repetition operator. In scalar context, or if the left operand is neither enclosed in parentheses nor a C list, it performs a string repetition. In that case it supplies scalar context to the left operand, and returns a string consisting of the left operand string repeated the number of times specified by the right operand. If the C is in list context, and the left operand is either enclosed in parentheses or a C list, it performs a list repetition. In that case it supplies list context to the left operand, and returns a list consisting of the left operand list repeated the number of times specified by the right operand. If the right operand is zero or negative (raising a warning on negative), it returns an empty string or an empty list, depending on the context. X 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 =head2 Additive Operators X Binary C returns the sum of two numbers. X Binary C returns the difference of two numbers. X Binary C concatenates two strings. X X X X X X<.> =head2 Shift Operators X X X>> X> >>> X X X X X X X Binary C>> returns the value of its left argument shifted left by the number of bits specified by the right argument. Arguments should be integers. (See also L.) Binary C>" >>> returns the value of its left argument shifted right by the number of bits specified by the right argument. Arguments should be integers. (See also L.) If S> (see L) is in force then signed C integers are used (I), otherwise unsigned C integers are used (I), even for negative shiftees. In arithmetic right shift the sign bit is replicated on the left, in logical shift zero bits come in from the left. Either way, the implementation isn't going to generate results larger than the size of the integer type Perl was built with (32 bits or 64 bits). Shifting by negative number of bits means the reverse shift: left shift becomes right shift, right shift becomes left shift. This is unlike in C, where negative shift is undefined. Shifting by more bits than the size of the integers means most of the time zero (all bits fall off), except that under S> right overshifting a negative shiftee results in -1. This is unlike in C, where shifting by too many bits is undefined. A common C behavior is "shift by modulo wordbits", so that for example 1 >> 64 == 1 >> (64 % 64) == 1 >> 0 == 1 # Common C behavior. but that is completely accidental. If you get tired of being subject to your platform's native integers, the S> pragma neatly sidesteps the issue altogether: print 20 The various named unary operators are treated as functions with one argument, with optional parentheses. If any list operator (C, etc.) or any unary operator (C, etc.) is followed by a left parenthesis as the next token, the operator and arguments within parentheses are taken to be of highest precedence, just like a normal function call. For example, because named unary operators are higher precedence than C: 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 C 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 C, C, etc. are treated like named unary operators, but they don't follow this functional parenthesis rule. That means, for example, that C is equivalent to S>. X X X See also L"Terms and List Operators (Leftward)">. =head2 Relational Operators X X Perl operators that return true or false generally return values that can be safely used as numbers. For example, the relational operators in this section and the equality operators in the next one return C for true and a special version of the defined empty string, C, which counts as a zero but is exempt from warnings about improper numeric conversions, just as S> is. Binary C> returns true if the left argument is numerically less than the right argument. X> Binary C" >> returns true if the left argument is numerically greater than the right argument. X >> Binary C> returns true if the left argument is numerically less than or equal to the right argument. X> Binary C=" >> returns true if the left argument is numerically greater than or equal to the right argument. X= >> Binary C returns true if the left argument is stringwise less than the right argument. X> Binary C returns true if the left argument is stringwise greater than the right argument. X> Binary C returns true if the left argument is stringwise less than or equal to the right argument. X> Binary C returns true if the left argument is stringwise greater than or equal to the right argument. X> A sequence of relational operators, such as S $y E= $z">>, performs chained comparisons, in the manner described above in the section L"Operator Precedence and Associativity">. Beware that they do not chain with equality operators, which have lower precedence. =head2 Equality Operators X X X X Binary C> returns true if the left argument is numerically equal to the right argument. X Binary C> returns true if the left argument is numerically not equal to the right argument. X Binary C returns true if the left argument is stringwise equal to the right argument. X Binary C returns true if the left argument is stringwise not equal to the right argument. X A sequence of the above equality operators, such as S>, performs chained comparisons, in the manner described above in the section L"Operator Precedence and Associativity">. Beware that they do not chain with relational operators, which have higher precedence. Binary C" >> returns -1, 0, or 1 depending on whether the left argument is numerically less than, equal to, or greater than the right argument. If your platform supports C's (not-a-numbers) as numeric values, using them with C" >> returns undef. C is not C>, C>, C" >>, C> or C=" >> anything (even C), so those 5 return false. S>> returns true, as does S I>. If your platform doesn't support C's then C is just a string with numeric value 0. X >> X $ perl -le '$x = "NaN"; print "No NaN support here" if $x == $x' $ perl -le '$x = "NaN"; print "NaN support here" if $x != $x' (Note that the L, L, and L pragmas all support C.) Binary C returns -1, 0, or 1 depending on whether the left argument is stringwise less than, equal to, or greater than the right argument. X Binary C does a smartmatch between its arguments. Smart matching is described in the next section. X The two-sided ordering operators C=E"> and C, and the smartmatch operator C, are non-associative with respect to each other and with respect to the equality operators of the same precedence. C, C, C, C and C use the collation (sort) order specified by the current C locale if a S> form that includes collation is in effect. See L. Do not mix these with Unicode, only use them with legacy 8-bit locale encodings. The standard C> and C> modules offer much more powerful solutions to collation issues. For case-insensitive comparisons, look at the L case-folding function, available in Perl v5.16 or later: if ( fc($x) eq fc($y) ) { ... } =head2 Class Instance Operator X Binary C evaluates to true when the left argument is an object instance of the class (or a subclass derived from that class) given by the right argument. If the left argument is not defined, not a blessed object instance, nor does not derive from the class given by the right argument, the operator evaluates as false. The right argument may give the class either as a bareword or a scalar expression that yields a string class name: if( $obj isa Some::Class ) { ... } if( $obj isa "Different::Class" ) { ... } if( $obj isa $name_of_class ) { ... } This is an experimental feature and is available from Perl 5.31.6 when enabled by C. It emits a warning in the C<:isa> category. =head2 Smartmatch Operator First available in Perl 5.10.1 (the 5.10.0 version behaved differently), binary C does a "smartmatch" between its arguments. This is mostly used implicitly in the C construct described in L, although not all C clauses call the smartmatch operator. Unique among all of Perl's operators, the smartmatch operator can recurse. The smartmatch operator is L and its behavior is subject to change. It is also unique in that all other Perl operators impose a context (usually string or numeric context) on their operands, autoconverting those operands to those imposed contexts. In contrast, smartmatch I contexts from the actual types of its operands and uses that type information to select a suitable comparison mechanism. The C operator compares its operands "polymorphically", determining how to compare them according to their actual types (numeric, string, array, hash, etc.). Like the equality operators with which it shares the same precedence, C returns 1 for true and C for false. It is often best read aloud as "in", "inside of", or "is contained in", because the left operand is often looked for I the right operand. That makes the order of the operands to the smartmatch operand often opposite that of the regular match operator. In other words, the "smaller" thing is usually placed in the left operand and the larger one in the right. The behavior of a smartmatch depends on what type of things its arguments are, as determined by the following table. The first row of the table whose types apply determines the smartmatch behavior. Because what actually 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: =over =item 1. Empty hashes or arrays match. =item 2. That is, each element smartmatches the element of the same index in the other array.[3] =item 3. If a circular reference is found, fall back to referential equality. =item 4. Either an actual number, or a string that looks like one. =back The smartmatch implicitly dereferences any non-blessed hash or array reference, so the C> and C> entries apply in those cases. For blessed references, the C> entries apply. Smartmatches involving hashes only consider hash keys, never hash values. The "like" code entry is not always an exact rendition. For example, the smartmatch operator short-circuits whenever possible, but C does not. Also, C in scalar context returns the number of matches, but C returns only true or false. Unlike most operators, the smartmatch operator knows to treat C specially: 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 modification being that array and hash variables are passed by reference to the operator, which implicitly dereferences them. Both elements of 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, this will 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 deep copies 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 S>, then instead of reporting that "a and b are deep copies of each other", it now reports that C. That's because the corresponding position in C contains an array that (eventually) has a 4 in it. Smartmatching one hash against another reports whether both contain the same keys, no more and no less. This could be used to see whether two records have the same field names, without caring what values those fields might 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; ... } However, this only does what you mean if C is indeed a hash reference. The condition C also allows the strings C, C, C as well as any array reference that contains C or C or C anywhere to pass through. The smartmatch operator is most often used as the implicit operator of a C clause. See the section on "Switch Statements" in L. =head3 Smartmatching of Objects To avoid relying on an object's underlying representation, if the smartmatch's right operand is an object that doesn't overload C, it raises the exception "C". That's because one has no business digging around to see whether something is "in" an object. These are all illegal on objects without a C overload: %hash ~~ $object 42 ~~ $object "fred" ~~ $object However, you can change the way an object is smartmatched by overloading the C operator. This is allowed to extend the usual smartmatch semantics. For objects that do have an C overload, see L. Using an object as the left operand is allowed, although not very useful. Smartmatching rules take precedence over overloading, so even if the object in the left operand has smartmatch overloading, this will be ignored. A left operand that is a non-overloaded object falls back on a string or numeric comparison of whatever the C operator returns. That means that $object ~~ X does I invoke the overload method with C> as an argument. Instead the above table is consulted as normal, and based on the type of C>, overloading may or may not be invoked. For simple strings or numbers, "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 C as a string like C, then pattern matches against that. =head2 Bitwise And X X X Binary C returns its operands ANDed together bit by bit. Although no warning is currently raised, the result is not well defined when this operation is performed on operands that aren't either numbers (see L) nor bitstrings (see L). Note that C has lower priority than relational operators, so for example the parentheses are essential in a test like print "Even\n" if ($x & 1) == 0; If the "bitwise" feature is enabled via S> or C, then this operator always treats its operands as numbers. Before Perl 5.28 this feature produced a warning in the C category. =head2 Bitwise Or and Exclusive Or X X X X X X Binary C returns its operands ORed together bit by bit. Binary C returns its operands XORed together bit by bit. Although no warning is currently raised, the results are not well defined when these operations are performed on operands that aren't either numbers (see L) nor bitstrings (see L). Note that C and C have lower priority than relational operators, so for example the parentheses are essential in a test like print "false\n" if (8 | 2) != 10; If the "bitwise" feature is enabled via S> or C, then this operator always treats its operands as numbers. Before Perl 5.28. this feature produced a warning in the C category. =head2 C-style Logical And X X X Binary C performs a short-circuit logical AND operation. That is, if the left operand is false, the right operand is not even evaluated. Scalar or list context propagates down to the right operand if it is evaluated. =head2 C-style Logical Or X X Binary C performs a short-circuit logical OR operation. That is, if the left operand is true, the right operand is not even evaluated. Scalar or list context propagates down to the right operand if it is evaluated. =head2 Logical Defined-Or X/> X Although it has no direct equivalent in C, Perl's C/> operator is related to its C-style "or". In fact, it's exactly the same as C, except that it tests the left hand side's definedness instead of its truth. Thus, S>> returns the value of C> if it's defined, otherwise, the value of C> is returned. (C> is evaluated in scalar context, C> in the context of C> itself). Usually, this is the same result as S>> (except that the ternary-operator form can be used as a lvalue, while S>> cannot). This is very useful for providing default values for variables. If you actually want to test if at least one of C and C is defined, use S>. The C, C/> and C operators return the last value evaluated (unlike C's C and C, which return 0 or 1). Thus, a reasonably portable way to find out the home directory might be: $home = $ENV{HOME} // $ENV{LOGDIR} // (getpwuid($ and C when used for control flow, Perl provides the C and C operators (see below). The short-circuit behavior is identical. The precedence of C and C is much lower, however, so that you can safely use them after a list operator without the need for parentheses: unlink "alpha", "beta", "gamma" or gripe(), next LINE; With the C-style operators that would have been written like this: unlink("alpha", "beta", "gamma") || (gripe(), next LINE); It would be even more readable to write that this way: unless(unlink("alpha", "beta", "gamma")) { gripe(); next LINE; } Using C for assignment is unlikely to do what you want; see below. =head2 Range Operators X X X<..> X<...> Binary C is the range operator, which is really two different operators depending on the context. In list context, it returns a list of values counting (up by ones) from the left value to the right value. If the left value is greater than the right value then it returns the empty list. The range operator is useful for writing S> loops and for doing slice operations on arrays. In the current implementation, no temporary array is created when the range operator is used as the expression in C loops, but older versions of Perl might burn a lot of memory when you write something like this: for (1 .. 1_000_000) { # code } The range operator also works on strings, using the magical auto-increment, see below. In scalar context, C returns a boolean value. The operator is bistable, like a flip-flop, and emulates the line-range (comma) operator of B, B, and various editors. Each C operator maintains its own boolean state, even across calls to a subroutine that 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 the right operand is true, I which the range operator becomes false again. It doesn't become false till the next time the range operator is evaluated. It can test the right operand and become false on the same evaluation it became true (as in B), but it still returns true once. If you don't want it to test the right operand until the next evaluation, as in B, just use three dots (C) instead of two. In all other regards, C behaves just like C does. The right operand is not evaluated while the operator is in the "false" state, and the left operand is not evaluated while the operator is in the "true" state. The precedence is a little lower than || and &&. The value returned is either the empty string for false, or a sequence number (beginning with 1) for true. The sequence number is reset for each range encountered. The final sequence number in a range has the string C appended to it, which doesn't affect its numeric value, but gives you something to search for if you want to exclude the endpoint. You can exclude the beginning point by waiting for the sequence number to be greater than 1. If either operand of scalar C is a constant expression, that operand is considered true if it is equal (C) to the current input line number (the C variable). To be pedantic, the comparison is actually S>, but that is only an issue if you use a floating point expression; when implicitly using C as described in the previous paragraph, the comparison is S> which is only an issue when C is set to a floating point value and you are not reading from a file. Furthermore, S> or S> will not do what you want in scalar context because each of the operands are evaluated using 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 $. each file } Here's a simple example to illustrate the difference between the two range operators: @lines = (" - Foo", "01 - Bar", "1 - Baz", " - Quux"); foreach (@lines) { if (/0/ .. /1/) { print "$_\n"; } } This program will print only the line containing "Bar". If the range operator is changed to C<...>, 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 Because each operand is evaluated in integer form, S> will return two elements in list context. @list = (2.18 .. 3.14); # same as @list = (2 .. 3); The range operator in list context can make use of the magical auto-increment algorithm if both operands are strings, subject to the following rules: =over =item * With one exception (below), if both strings look like numbers to Perl, the magic increment will not be applied, and the strings will be treated as numbers (more specifically, integers) instead. For example, C is the same as C, and C produces C. =item * The exception to the above rule is when the left-hand string begins with C and is longer than one character, in this case the magic increment I be applied, even though strings like C would normally look like a number to Perl. For example, C produces C, and C produces C through C - this may seem surprising, but see the following rules for why it works this way. To get dates with leading zeros, you can say: @z2 = ("01" .. "31"); print $z2[$mday]; If you want to force strings to be interpreted as numbers, you could say @numbers = ( 0+$first .. 0+$last ); B In Perl versions 5.30 and below, I string on the left-hand side beginning with C, including the string C itself, would cause the magic string increment behavior. This means that on these Perl versions, C would produce C through C, which was inconsistent with C, which produces the empty list. This also means that C now produces a list of integers instead of a list of strings. =item * If the initial value specified isn't part of a magical increment sequence (that is, a non-empty string matching C^[a-zA-Z]*[0-9]*\z/>), only the initial value will be returned. For example, C produces C, but C produces only C. =item * For other initial values that are strings that do follow the rules of the magical increment, the corresponding sequence will be returned. For example, you can 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. =item * If the final value specified is not in the sequence that the magical increment would produce, the sequence goes until the next value would be longer than the final value specified. If the length of the final string is shorter than the first, the empty list is returned. For example, C is the same as C, C produces C through C, and C returns the empty list. =back As of Perl 5.26, the list-context range operator on strings works as expected in the scope of L>|feature/The 'unicode_strings' feature >>. In previous versions, and outside the scope of that feature, it exhibits L: its behavior depends on the internal encoding of the range endpoint. Because the magical increment only works on non-empty strings matching C^[a-zA-Z]*[0-9]*\z/>, the following will only return 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 I other lowercase Greek characters than just those, to match lowercase Greek characters in a regular expression, you could use the pattern C(?:(?=\p{Greek})\p{Lower})+/> (or the L C>). =head2 Conditional Operator X X X X Ternary C is the conditional operator, just as in C. It works much like an if-then-else. If the argument before the C> is true, the argument before the C<:> is returned, otherwise the argument after the C<:> is returned. For example: printf "I have %d dog%s.\n", $n, ($n == 1) ? "" : "s"; Scalar or list context propagates downward into the 2nd or 3rd argument, whichever is selected. $x = $ok ? $y : $z; # get a scalar @x = $ok ? @y : @z; # get an array $x = $ok ? @y : @z; # oops, that's just a count! The operator may be assigned to if both the 2nd and 3rd arguments are legal lvalues (meaning that you can assign to them): ($x_or_y ? $x : $y) = $z; Because this operator produces an assignable result, using assignments without parentheses will get you in trouble. For example, this: $x % 2 ? $x += 10 : $x += 2 Really means this: (($x % 2) ? ($x += 10) : $x) += 2 Rather than this: ($x % 2) ? ($x += 10) : ($x += 2) That should probably be written more simply as: $x += ($x % 2) ? 10 : 2; =head2 Assignment Operators X X X X X X X X>> X X X=> X X>= >>> X X/=> X<.> X X X X X X C is the ordinary assignment operator. Assignment operators work as in C. That is, $x += 2; is equivalent to $x = $x + 2; although without duplicating any side effects that dereferencing the lvalue might trigger, such as from C. Other assignment operators work similarly. The following are recognized: **= += *= &= &.= >= ||= .= %= ^= ^.= //= x= Although these are grouped by family, they all have the precedence of assignment. These combined assignment operators can only operate on scalars, whereas the ordinary assignment operator can assign to arrays, hashes, lists and even references. (See L and L, and L.) Unlike in C, the scalar assignment operator produces a valid lvalue. Modifying an assignment is equivalent to doing the assignment and then modifying the variable that was assigned to. This is useful for modifying a copy of something, like this: ($tmp = $global) =~ tr/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, ($x += 2) *= 3; is equivalent to $x += 2; $x *= 3; Similarly, a list assignment in list context produces the list of lvalues assigned to, and a list assignment in scalar context returns the number of elements produced by the expression on the right hand side of the assignment. The three dotted bitwise assignment operators (C C C) are new in Perl 5.22. See L. =head2 Comma Operator X X X Binary C is the comma operator. In scalar context it evaluates its left argument, throws that value away, then evaluates its right argument and returns that value. This is just like C's comma operator. In list context, it's just the list argument separator, and inserts both its arguments into the list. These arguments are also evaluated from left to right. The C >> operator (sometimes pronounced "fat comma") is a synonym for the comma except that it causes a word on its left to be interpreted as a string if it begins with a letter or 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 about this behavior, the left operand can be quoted explicitly. Otherwise, the C >> operator behaves exactly as the comma operator or 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 I: my %h = ("something", 23); The C >> operator is helpful in documenting the correspondence between 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 to I of the left operand: print time.shift => "bbb"; That example prints something like C, because the C >> implicitly quotes the C immediately on its left, ignoring the fact that C is the entire left operand. =head2 List Operators (Rightward) X X 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 C, C, and C, which may be used to evaluate calls to list operators without the need for parentheses: open HANDLE, " operator: open(HANDLE, ". =head2 Logical Not X X Unary C returns the logical negation of the expression to its right. It's the equivalent of C except for the very low precedence. =head2 Logical And X X Binary C returns the logical conjunction of the two surrounding expressions. It's equivalent to C except for the very low precedence. This means that it short-circuits: the right expression is evaluated only if the left expression is true. =head2 Logical or and Exclusive Or X X X X X Binary C returns the logical disjunction of the two surrounding expressions. It's equivalent to C except for the very low precedence. This makes it useful for control flow: print FH $data or die "Can't write to FH: $!"; This means that it short-circuits: the right expression is evaluated only if the left expression is false. Due to its precedence, you must be careful to avoid using it as replacement for the C operator. It usually works out better for flow control than in assignments: $x = $y or $z; # bug: this is wrong ($x = $y) or $z; # really means this $x = $y || $z; # better written this way However, when it's a list-context assignment and you're trying to use C for control flow, you probably need C so that the assignment takes higher precedence. @info = stat($file) || die; # oops, scalar sense of stat! @info = stat($file) or die; # better, now @info gets its due Then again, you could always use parentheses. Binary C returns the exclusive-OR of the two surrounding expressions. It cannot short-circuit (of course). There is no low precedence operator for defined-OR. =head2 C Operators Missing From Perl X X X X X Here is what C has that Perl doesn't: =over 8 =item unary & Address-of operator. (But see the C operator for taking a reference.) =item unary * Dereference-address operator. (Perl's prefix dereferencing operators are typed: C, C, C, and C.) =item (TYPE) Type-casting operator. =back =head2 Quote and Quote-like Operators X X X X X X X X X X X X X X X/> X X X>> X X While we usually think of quotes as literal values, in Perl they function as operators, providing various kinds of interpolating and pattern matching capabilities. Perl provides customary quote characters for these behaviors, but also provides a way for you to choose your quote character for any of them. In the following table, a C represents any 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) > module (standard as of v5.8, and from CPAN before then) is able to do this properly. There can (and in some cases, must) be whitespace between the operator and the quoting characters, except when C is being used as the quoting character. C is parsed as the string C, while S> is the operator C followed by a comment. Its argument will be taken from the next line. This allows you to write: s {foo} # Replace foo {bar} # with bar. The cases where whitespace must be used are when the quoting character is a word character (meaning it matches C\w/>): q XfooX # Works: means the string 'foo' qXfooX # WRONG! The following escape sequences are available in constructs that interpolate, and in transliterations whose delimiters aren't single quotes (C). In all the ones with braces, any number of blanks and/or tabs adjoining and within the braces are allowed (and ignored). X X X X X X X X X X X X X 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 shown: SMILEY) \x{ 263A } Same, but shows optional blanks inside and adjoining the braces \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) Note that any escape sequence using braces inside interpolated constructs may have optional blanks (tab or space characters) adjoining with and inside of the braces, as illustrated above by the second S> example. =over 4 =item [1] The result is the character specified by the hexadecimal number between the braces. See L[8]> below for details on which character. Blanks (tab or space characters) may separate the number from either or both of the braces. Otherwise, only hexadecimal digits are valid between the braces. If an invalid character is encountered, a warning will be issued and the invalid character and all subsequent characters (valid or invalid) within the braces will be discarded. If there are no valid digits between the braces, the generated character is the NULL character (C). However, an explicit empty brace (C) will not cause a warning (currently). =item [2] The result is the character specified by the hexadecimal number in the range 0x00 to 0xFF. See L[8]> below for details on which character. Only hexadecimal digits are valid following C. When C is followed by fewer than two valid digits, any valid digits will be zero-padded. This means that C will be interpreted as C, and a lone C will be interpreted as C. Except at the end of a string, having fewer than two valid digits will result in a warning. Note that although the warning says the illegal character is ignored, it is only ignored as part of the escape 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 =item [3] The result is the Unicode character or character sequence given by I. See L. =item [4] S}>> means the Unicode character whose Unicode code point is I. =item [5] The character following C is mapped to some other character as shown in the table: 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) # See below for chr(28) \c] chr(29) \c^ chr(30) \c_ chr(31) \c? chr(127) # (on ASCII platforms; see below for link to # EBCDIC discussion) In other words, it's the character whose code point has had 64 xor'd with its uppercase. C is DELETE on ASCII platforms because S> is 127, and C is NULL because the ord of C is 64, so xor'ing 64 itself produces 0. Also, C> yields S">> for any I, but cannot come at the end of a string, because the backslash would be parsed as escaping the end quote. On ASCII platforms, the resulting characters from the list above are the complete set of ASCII controls. This isn't the case on EBCDIC platforms; see L for a full discussion of the differences between these for ASCII versus EBCDIC platforms. Use of any other character following the C besides those listed above is discouraged, and as of Perl v5.20, the only characters actually allowed are the printable ASCII ones, minus the left brace C. What happens for any of the allowed other characters is that the value is derived by xor'ing with the seventh bit, which is 64, and a warning raised if enabled. Using the non-allowed characters generates a fatal error. To get platform independent controls, you can use C. =item [6] The result is the character specified by the octal number between the braces. See L[8]> below for details on which character. Blanks (tab or space characters) may separate the number from either or both of the braces. Otherwise, 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 all following characters up to the closing brace. It is a fatal error if there are no octal digits at all. =item [7] The result is the character specified by the three-digit octal number in the range 000 to 777 (but best to not use above 077, see next paragraph). See L[8]> below for details on which character. Some contexts allow 2 or even 1 digit, but any usage without exactly three digits, the first being a zero, may give unintended results. (For example, in a regular expression it may be confused with a backreference; see L.) Starting in Perl 5.14, you may use C instead, which avoids all these problems. Otherwise, it is best to use this construct only for ordinals C and below, remembering to pad to the left with zeros to make three digits. For larger ordinals, either use C, or convert to something else, such as to hex and use C (which is portable between platforms with different character sets) or C instead. =item [8] Several constructs above specify a character by a number. That number gives the character's position in the character set encoding (indexed from 0). This is called synonymously its ordinal, code position, or code point. Perl works on platforms that have a native encoding currently of either ASCII/Latin1 or EBCDIC, each of which allow specification of 256 characters. In general, if the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's native encoding. If the number is 256 (0x100, 0400) or above, Perl interprets it as a Unicode code point and the result is the corresponding Unicode character. For example C and C both are the number 80 in decimal, which is less than 256, so the number is interpreted in the native character set encoding. In ASCII the character in the 80th position (indexed from 0) is the letter C, and in EBCDIC it is the ampersand symbol C. C and C are both 256 in decimal, so the number is interpreted as a Unicode code point no matter what the native encoding is. The name of the character in the 256th position (indexed by 0) in Unicode is C. An exception to the above rule is that S}>> is always interpreted as a Unicode code point, so that C is C even on EBCDIC platforms. =back B: Unlike C and other languages, Perl has no C escape sequence for the vertical tab (VT, which is 11 in both ASCII and EBCDIC), but you may use C, C, C, or C. (C does have meaning in regular expression patterns in Perl, see L.) The following escape sequences are available in constructs that interpolate, but not in transliterations. X X X X X X X \l lowercase next character only \u titlecase (not uppercase!) next character only \L lowercase all characters till \E or end of string \U uppercase all characters till \E or end of string \F foldcase all characters till \E or end of string \Q quote (disable) pattern metacharacters till \E or end of string \E end either case modification or quoted section (whichever was last seen) See L for the exact definition of characters that are quoted by C. C, C, C, and C can stack, in which case you need one C 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 a S> form that includes C is in effect (see L), the case map used by C, C, C, and C is taken from the current locale. If Unicode (for example, C or code points of 0x100 or beyond) is being used, the case map used by C, C, C, and C is as defined by Unicode. That means that case-mapping a single character can sometimes produce a sequence of several characters. Under S>, C produces the same results as C for all locales but a UTF-8 one, where it instead uses the Unicode definition. All systems use the virtual C to represent a line terminator, called a "newline". There is no such thing as an unvarying, physical newline character. It is only an illusion that the operating system, device drivers, C libraries, and Perl all conspire to preserve. Not all systems read C as ASCII CR and C as ASCII LF. For example, on the ancient Macs (pre-MacOS X) of yesteryear, these used to be reversed, and on systems without a line terminator, printing C might emit no actual data. In general, use C when you mean a "newline" for your system, but use the literal ASCII when you need an exact character. For example, most networking protocols expect and prefer a CR+LF (C or C) for line terminators, and although they often accept just C, they seldom tolerate just C. If you get in the habit of using C for networking, you may be burned some day. X X X X X X X For constructs that do interpolate, variables beginning with "C" or "C" are interpolated. Subscripted variables such as C or C{key}[0] >> are also interpolated, as are array and hash slices. But method calls such as Cmeth >> are not. Interpolating an array or slice interpolates the elements in order, separated by the value of C, so is equivalent to interpolating S>. "Punctuation" arrays such as C are usually interpolated only if the name is enclosed in braces C, but the arrays C, C, and C are interpolated even without braces. For double-quoted strings, the quoting from C is applied after interpolation and escapes are processed. "abc\Qfoo\tbar$s\Exyz" is equivalent to "abc" . quotemeta("foo\tbar$s") . "xyz" For the pattern of regex operators (C, C and C), the quoting from C is applied after interpolation is processed, but before escapes are processed. This allows the pattern to match literally (except for C and C). For example, the following matches: '\s\t' =~ /\Q\s\t/ Because C or C trigger interpolation, you'll need to use something like C\Quser\E\@\Qhost/> to match them literally. Patterns are subject to an additional level of interpretation as a regular expression. This is done as a second pass, after variables are interpolated, so that regular expressions may be incorporated into the pattern from the variables. If this is not what you want, use C to interpolate a variable literally. Apart from the behavior described above, Perl does not expand multiple levels of interpolation. In particular, contrary to the expectations of shell programmers, back-quotes do I interpolate within double quotes, nor do single quotes impede evaluation of variables when used within double quotes. =head2 Regexp Quote-Like Operators X Here are the quote-like operators that apply to pattern matching and related activities. =over 8 =item C/msixpodualn> X X X X X X X This operator quotes (and possibly compiles) its I as a regular expression. I is interpolated the same way as I in C/>. If C is used as the delimiter, no variable interpolation is done. Returns a Perl value which may be used instead of the corresponding C/msixpodualn> expression. The returned value is a normalized version of the original pattern. It magically differs from a string containing the same characters: C 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 C operator, using C may have speed advantages in some situations, notably if the result of C is used standalone: sub match { my $patterns = shift; my @compiled = map qr/$_/i, @$patterns; grep { my $success = 0; foreach my $pat (@compiled) { $success = 1, last if /$pat/; } $success; } @_; } Precompilation of the pattern into an internal representation at the moment of C avoids the need to recompile the pattern every time a match C$pat/> is attempted. (Perl has many other internal optimizations, but none would be triggered in the above example if we did not use C operator.) Options (specified by the following modifiers) are: m Treat string as multiple lines. s Treat string as single line. (Make . match a newline) i Do case-insensitive pattern matching. x Use extended regular expressions; specifying two x's means \t and the SPACE character are ignored within square-bracketed character classes p When matching preserve a copy of the matched string so that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined (ignored starting in v5.20) as these are always defined starting in that release o Compile pattern only once. a ASCII-restrict: Use ASCII for \d, \s, \w and [[:posix:]] character classes; specifying two a's adds the further restriction that no ASCII character will match a non-ASCII one under /i. l Use the current run-time locale's rules. u Use Unicode rules. d Use Unicode or native charset, as in 5.12 and earlier. n Non-capture mode. Don't let () fill in $1, $2, etc... If a precompiled pattern is embedded in a larger pattern then the effect of C will be propagated appropriately. The effect that the C modifier has is not propagated, being restricted to those patterns explicitly using it. The C, C, C, and C modifiers (added in Perl 5.14) control the character set rules, but C is the only one you are likely to want to specify explicitly; the other three are selected automatically by various pragmas. See L for additional information on valid syntax for I, and for a detailed look at the semantics of regular expressions. In particular, all modifiers except the largely obsolete C are further explained in L. C is described in the next section. =item C/msixpodualngc> X X X X X X X X X X X X X X =item C/msixpodualngc> Searches a string for a pattern match, and in scalar context returns true if it succeeds, false if it fails. If no string is specified via the C or C operator, the C string is searched. (The string specified with C need not be an lvalue--it may be the result of an expression evaluation, but remember the C binds rather tightly.) See also L. Options are as described in C above; in addition, the following match process 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 C is the delimiter then the initial C is optional. With the C you can use any pair of non-whitespace (ASCII) characters as delimiters. This is particularly useful for matching path names that contain C, to avoid LTS (leaning toothpick syndrome). If C is the delimiter, then a match-only-once rule applies, described in C?> below. If C (single quote) is the delimiter, no variable interpolation is performed on the I. When using a delimiter character valid in an identifier, whitespace is required after the C. I may contain variables, which will be interpolated every time the pattern search is evaluated, except for when the delimiter is a single quote. (Note that C, C, and C are not interpolated because they look like end-of-string tests.) Perl will not recompile the pattern unless an interpolated variable that it contains changes. You can force Perl to skip the test and never recompile by adding a C (which stands for "once") after the trailing delimiter. Once upon a time, Perl would recompile regular expressions unnecessarily, and this modifier was useful to tell it not to do so, in the interests of speed. But now, the only reasons to use C are one of: =over =item 1 The variables are thousands of characters long and you know that they don't change, and you need to wring out the last little bit of speed by having Perl skip testing for that. (There is a maintenance penalty for doing this, as mentioning C constitutes a promise that you won't change the variables in the pattern. If you do change them, Perl won't even notice.) =item 2 you want the pattern to use the initial values of the variables regardless of whether they change or not. (But there are saner ways of accomplishing this than using C.) =item 3 If the pattern contains embedded code, such as use re 'eval'; $code = 'foo(?{ $x })'; /$code/ then perl will recompile each time, even though the pattern string hasn't changed, to ensure that the current value of C is seen each time. Use C if you want to avoid this. =back The bottom line is that using C is almost never a good idea. =item The empty pattern C/> If the I evaluates to the empty string, the last I matched regular expression is used instead. In this case, only the C and C flags on the empty pattern are honored; the other flags are taken from the original pattern. If no match has previously succeeded, this will (silently) act instead as a genuine empty pattern (which will always match). Note that it's possible to confuse Perl into thinking C/> (the empty regex) is really C/> (the defined-or operator). Perl is usually pretty good about this, but some pathological cases might trigger this, such as C (is that S> or S>?) and S> (S> or S>?). In all of these examples, Perl will assume you meant defined-or. If you meant the empty regex, just use parentheses or spaces to disambiguate, or even prefix the empty regex with an C (so C/> becomes C). =item Matching in list context If the C option is not used, C in list context returns a list consisting of the subexpressions matched by the parentheses in the pattern, that is, (C, C, C...) (Note that here C etc. are also set). When there are no parentheses in the pattern, the return value is the list C for success. With or without parentheses, an empty list is returned upon failure. Examples: open(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 C into the first two words and the remainder of the line, and assigns those three fields to C, C, and C. The conditional is true if any variables were assigned; that is, if the pattern matched. The C modifier specifies global pattern matching--that is, matching as many times as possible within the string. How it behaves depends on the context. In list context, it returns a list of the substrings matched by any capturing parentheses in the regular expression. If there are no parentheses, it returns a list of all the matched strings, as if there were parentheses around the whole pattern. In scalar context, each execution of C 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 C function; see L. A failed match normally resets the search position to the beginning of the string, but you can avoid that by adding the C modifier (for example, C). Modifying the target string also resets the search position. =item C> You can intermix C matches with C, where C is a zero-width assertion that matches the exact position where the previous C, if any, left off. Without the C modifier, the C assertion still anchors at C as it was at the start of the operation (see L), but the match is of course only attempted once. Using C without C on a target string that has not previously had a C match applied to it is the same as using the C assertion to match the beginning of the string. Note also that, currently, C is only properly supported when anchored at the very 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{ (?: (?) { say "NEW PARAGRAPH"; my $count = 0; while ($paragraph =~ /($sentence_rx)/g) { printf "\tgot sentence %d: \n", ++$count, $1; } } Here's how to use C with C: $_ = "ppooqppqq"; while ($i++ instead of C

, which a match without the C anchor would have done. Also note that the final match did not update C. C is only updated on a C match. If the final match did indeed match C

, it's a good bet that you're running an ancient (pre-5.6.0) version of Perl. A useful idiom for C-like scanners is C\G.../gc>. You can combine several regexps like this to process a string part-by-part, doing different actions depending on which regexp matched. Each regexp tries to match where the previous one leaves off. $_ = new( "http://example.com/" ); die if $url eq "xXx"; EOL LOOP: { print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc; print(" lowercase"), redo LOOP if /\G\p{Ll}+\b[,.;]?\s*/gc; print(" UPPERCASE"), redo LOOP if /\G\p{Lu}+\b[,.;]?\s*/gc; print(" Capitalized"), redo LOOP if /\G\p{Lu}\p{Ll}+\b[,.;]?\s*/gc; print(" MiXeD"), redo LOOP if /\G\pL+\b[,.;]?\s*/gc; print(" alphanumeric"), redo LOOP if /\G[\p{Alpha}\pN]+\b[,.;]?\s*/gc; print(" line-noise"), redo LOOP if /\G\W+/gc; print ". That's all!\n"; } Here is the output (split into several lines): line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE line-noise lowercase line-noise lowercase line-noise lowercase lowercase line-noise lowercase lowercase line-noise lowercase lowercase line-noise MiXeD line-noise. That's all! =item C?msixpodualngc> X> X This is just like the C/> search, except that it matches only once between calls to the C operator. This is a useful optimization when you want to see only the first occurrence of something in each file of a set of files, for instance. Only C patterns local to the current package are reset. while () { if (m?^$?) { # blank line between header and body } } continue { reset if eof; # clear m?? status for next file } Another example switched the first "latin1" encoding it finds to "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 C>; with any other delimiter this is the normal C operator. In the past, the leading C in C?> was optional, but omitting it would produce a deprecation warning. As of v5.22.0, omitting it produces a syntax error. If you encounter this construct in older code, you can just add C. =item C/I/msixpodualngcer> X X X X X X X X X X X