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author | Karl Williamson <khw@cpan.org> | 2015-04-18 20:29:34 -0600 |
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committer | Karl Williamson <khw@cpan.org> | 2015-04-18 20:38:39 -0600 |
commit | ba7f043cd8beaaa711b275c2a15eb10e1f9ab7f8 (patch) | |
tree | 681251c3a092f34905074ff6bdb687893594ce4e /pod/perlop.pod | |
parent | 15c526cb7d1cfe028631e7503100bb5c1ee2f822 (diff) | |
download | perl-ba7f043cd8beaaa711b275c2a15eb10e1f9ab7f8.tar.gz |
perlop: Nits and update for v5.22
Diffstat (limited to 'pod/perlop.pod')
-rw-r--r-- | pod/perlop.pod | 681 |
1 files changed, 351 insertions, 330 deletions
diff --git a/pod/perlop.pod b/pod/perlop.pod index e5d15dd8c5..3c364c1dfb 100644 --- a/pod/perlop.pod +++ b/pod/perlop.pod @@ -6,15 +6,15 @@ 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 C<$x + $y> +independent of the type of the operands. For example S<C<$x + $y>> is always a numeric addition, and if C<$x> or C<$y> do not contain numbers, an attempt is made to convert them to numbers first. This is in contrast to many other dynamic languages, where the operation is determined by the type of the first argument. It also means that Perl has two versions of some operators, one for numeric -and one for string comparison. For example C<$x == $y> compares -two numbers for equality, and C<$x eq $y> compares two strings. +and one for string comparison. For example S<C<$x == $y>> compares +two numbers for equality, and S<C<$x eq $y>> compares two strings. There are a few exceptions though: C<x> can be either string repetition or list repetition, depending on the type of the left @@ -28,16 +28,16 @@ Operator precedence and associativity work in Perl more or less like they do in mathematics. I<Operator precedence> means some operators are evaluated before -others. For example, in C<2 + 4 * 5>, the multiplication has higher -precedence so C<4 * 5> is evaluated first yielding C<2 + 20 == -22> and not C<6 * 5 == 30>. +others. For example, in S<C<2 + 4 * 5>>, the multiplication has higher +precedence so S<C<4 * 5>> is evaluated first yielding S<C<2 + 20 == +22>> and not S<C<6 * 5 == 30>>. I<Operator associativity> defines what happens if a sequence of the same operators is used one after another: whether the evaluator will -evaluate the left operations first or the right. For example, in C<8 -- 4 - 2>, subtraction is left associative so Perl evaluates the -expression left to right. C<8 - 4> is evaluated first making the -expression C<4 - 2 == 2> and not C<8 - 2 == 6>. +evaluate the left operations first, or the right first. For example, in +S<C<8 - 4 - 2>>, subtraction is left associative so Perl evaluates the +expression left to right. S<C<8 - 4>> is evaluated first making the +expression S<C<4 - 2 == 2>> and not S<C<8 - 2 == 6>>. Perl operators have the following associativity and precedence, listed from highest precedence to lowest. Operators borrowed from @@ -85,7 +85,7 @@ 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<perlfunc>. -If any list operator (print(), etc.) or any unary operator (chdir(), etc.) +If any list operator (C<print()>, etc.) or any unary operator (C<chdir()>, etc.) is followed by a left parenthesis as the next token, the operator and arguments within parentheses are taken to be of highest precedence, just like a normal function call. @@ -98,7 +98,7 @@ 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, +the commas on the right of the C<sort> are evaluated before the C<sort>, but the commas on the left are evaluated after. In other words, list operators tend to gobble up all arguments that follow, and then act like a simple TERM with regard to the preceding expression. @@ -119,7 +119,7 @@ Also note that probably doesn't do what you expect at first glance. The parentheses enclose the argument list for C<print> which is evaluated (printing -the result of C<$foo & 255>). Then one is added to the return value +the result of S<C<$foo & 255>>). Then one is added to the return value of C<print> (usually 1). The result is something like this: 1 + 1, "\n"; # Obviously not what you meant. @@ -130,7 +130,7 @@ To do what you meant properly, you must write: See L<Named Unary Operators> for more discussion of this. -Also parsed as terms are the C<do {}> and C<eval {}> constructs, as +Also parsed as terms are the S<C<do {}>> and S<C<eval {}>> constructs, as well as subroutine and method calls, and the anonymous constructors C<[]> and C<{}>. @@ -160,7 +160,7 @@ details of that feature, consult L<perlref/Postfix Dereference Syntax>. =head2 Auto-increment and Auto-decrement X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<--> -"++" and "--" work as in C. That is, if placed before a variable, +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. @@ -202,9 +202,10 @@ The auto-decrement operator is not magical. =head2 Exponentiation X<**> X<exponentiation> X<power> -Binary "**" is the exponentiation operator. It binds even more -tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is -implemented using C's pow(3) function, which actually works on doubles +Binary C<"**"> is the exponentiation operator. It binds even more +tightly than unary minus, so C<-2**4> is C<-(2**4)>, not C<(-2)**4>. +(This is +implemented using C's C<pow(3)> function, which actually works on doubles internally.) Note that certain exponentiation expressions are ill-defined: @@ -215,29 +216,30 @@ are platform-dependent. =head2 Symbolic Unary Operators X<unary operator> X<operator, unary> -Unary "!" performs logical negation, that is, "not". See also C<not> for a lower +Unary C<"!"> performs logical negation, that is, "not". See also C<not> for a lower precedence version of this. X<!> -Unary "-" performs arithmetic negation if the operand is numeric, +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 -bareword is equivalent -to the string "-bareword". If, however, the string begins with a -non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert -the string to a numeric and the arithmetic negation is performed. If the +returned. One effect of these rules is that C<-bareword> is equivalent +to the string C<"-bareword">. 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<Argument "the string" isn't numeric in negation (-) at ...>. X<-> X<negation, arithmetic> -Unary "~" performs bitwise negation, that is, 1's complement. For -example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and +Unary C<"~"> performs bitwise negation, that is, 1's complement. For +example, S<C<0666 & ~027>> is 0640. (See also L<Integer Arithmetic> and L<Bitwise String Operators>.) Note that the width of the result is -platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64 +platform-dependent: C<~0> is 32 bits wide on a 32-bit platform, but 64 bits wide on a 64-bit platform, so if you are expecting a certain bit -width, remember to use the "&" operator to mask off the excess bits. +width, remember to use the C<"&"> operator to mask off the excess bits. X<~> X<negation, binary> When complementing strings, if all characters have ordinal values under @@ -246,20 +248,20 @@ characters will be in either 32- or 64-bit complements, depending on your architecture. So for example, C<~"\x{3B1}"> is C<"\x{FFFF_FC4E}"> on 32-bit machines and C<"\x{FFFF_FFFF_FFFF_FC4E}"> on 64-bit machines. -If the experimental "bitwise" feature is enabled via C<use feature -'bitwise'>, then unary "~" always treats its argument as a number, and an -alternate form of the operator, "~.", always treats its argument as a +If the experimental "bitwise" feature is enabled via S<C<use feature +'bitwise'>>, 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<~0> and C<~"0"> will both give 2**32-1 on 32-bit platforms, whereas C<~.0> and C<~."0"> will both yield C<"\xff">. This feature -produces a warning unless you use C<no warnings 'experimental::bitwise'>. +produces a warning unless you use S<C<no warnings 'experimental::bitwise'>>. -Unary "+" has no effect whatsoever, even on strings. It is useful +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<Terms and List Operators (Leftward)>.) X<+> -Unary "\" creates a reference to whatever follows it. See L<perlreftut> +Unary C<"\"> creates a reference to whatever follows it. See L<perlreftut> and L<perlref>. Do not confuse this behavior with the behavior of backslash within a string, although both forms do convey the notion of protecting the next thing from interpolation. @@ -268,14 +270,14 @@ X<\> X<reference> X<backslash> =head2 Binding Operators X<binding> X<operator, binding> X<=~> X<!~> -Binary "=~" binds a scalar expression to a pattern match. Certain operations -search or modify the string $_ by default. This operator makes that kind +Binary C<"=~"> binds a scalar expression to a pattern match. Certain operations +search or modify the string C<$_> 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 -$_. When used in scalar context, the return value generally indicates the -success of the operation. The exceptions are substitution (s///) -and transliteration (y///) with the C</r> (non-destructive) option, +C<$_>. When used in scalar context, the return value generally indicates the +success of the operation. The exceptions are substitution (C<s///>) +and transliteration (C<y///>) with the C</r> (non-destructive) option, which cause the B<r>eturn 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<perlretut> for @@ -291,50 +293,50 @@ contents will be interpolated twice, so is not ok, as the regex engine will end up trying to compile the pattern C<\>, which it will consider a syntax error. -Binary "!~" is just like "=~" except the return value is negated in +Binary C<"!~"> is just like C<"=~"> except the return value is negated in the logical sense. -Binary "!~" with a non-destructive substitution (s///r) or transliteration -(y///r) is a syntax error. +Binary C<"!~"> with a non-destructive substitution (C<s///r>) or transliteration +(C<y///r>) is a syntax error. =head2 Multiplicative Operators X<operator, multiplicative> -Binary "*" multiplies two numbers. +Binary C<"*"> multiplies two numbers. X<*> -Binary "/" divides two numbers. +Binary C<"/"> divides two numbers. X</> X<slash> -Binary "%" is the modulo operator, which computes the division +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<$m> and C<$n>: If C<$n> is positive, then C<$m % $n> is +operands C<$m> and C<$n>: If C<$n> is positive, then S<C<$m % $n>> is C<$m> minus the largest multiple of C<$n> less than or equal to -C<$m>. If C<$n> is negative, then C<$m % $n> is C<$m> minus the +C<$m>. If C<$n> is negative, then S<C<$m % $n>> is C<$m> minus the smallest multiple of C<$n> that is not less than C<$m> (that is, the result will be less than or equal to zero). If the operands C<$m> and C<$n> are floating point values and the absolute value of -C<$n> (that is C<abs($n)>) is less than C<(UV_MAX + 1)>, only +C<$n> (that is C<abs($n)>) is less than S<C<(UV_MAX + 1)>>, only the integer portion of C<$m> and C<$n> will be used in the operation (Note: here C<UV_MAX> means the maximum of the unsigned integer type). If the absolute value of the right operand (C<abs($n)>) is greater than -or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder -C<$r> in the equation C<($r = $m - $i*$n)> where C<$i> is a certain +or equal to S<C<(UV_MAX + 1)>>, C<"%"> computes the floating-point remainder +C<$r> in the equation S<C<($r = $m - $i*$n)>> where C<$i> is a certain integer that makes C<$r> have the same sign as the right operand C<$n> (B<not> as the left operand C<$m> like C function C<fmod()>) and the absolute value less than that of C<$n>. -Note that when C<use integer> is in scope, "%" gives you direct access +Note that when S<C<use integer>> 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<remainder> X<modulo> X<mod> -Binary "x" is the repetition operator. In scalar context or if the left +Binary C<"x"> is the repetition operator. In scalar context or if the left operand is not enclosed in parentheses, it returns a string consisting of the left operand repeated the number of times specified by the right operand. In list context, if the left operand is enclosed in -parentheses or is a list formed by C<qw/STRING/>, it repeats the list. +parentheses or is a list formed by C<qw/I<STRING>/>, it repeats the list. 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. @@ -351,13 +353,13 @@ X<x> =head2 Additive Operators X<operator, additive> -Binary C<+> returns the sum of two numbers. +Binary C<"+"> returns the sum of two numbers. X<+> -Binary C<-> returns the difference of two numbers. +Binary C<"-"> returns the difference of two numbers. X<-> -Binary C<.> concatenates two strings. +Binary C<"."> concatenates two strings. X<string, concatenation> X<concatenation> X<cat> X<concat> X<concatenate> X<.> @@ -366,16 +368,16 @@ X<shift operator> X<operator, shift> X<<< << >>> X<<< >> >>> X<right shift> X<left shift> X<bitwise shift> X<shl> X<shr> X<shift, right> X<shift, left> -Binary C<<< << >>> returns the value of its left argument shifted left by the +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<Integer Arithmetic>.) -Binary C<<< >> >>> returns the value of its left argument shifted right by +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<Integer Arithmetic>.) Note that both C<<< << >>> and C<<< >> >>> in Perl are implemented directly using -C<<< << >>> and C<<< >> >>> in C. If C<use integer> (see L<Integer Arithmetic>) is +C<<< << >>> and C<<< >> >>> in C. If S<C<use integer>> (see L<Integer Arithmetic>) is in force then signed C integers are used, else unsigned C integers are used. Either way, the implementation isn't going to generate results larger than the size of the integer type Perl was built with (32 bits @@ -383,11 +385,11 @@ or 64 bits). The result of overflowing the range of the integers is undefined because it is undefined also in C. In other words, using 32-bit -integers, C<< 1 << 32 >> is undefined. Shifting by a negative number +integers, S<C<< 1 << 32 >>> is undefined. Shifting by a negative number of bits is also undefined. If you get tired of being subject to your platform's native integers, -the C<use bigint> pragma neatly sidesteps the issue altogether: +the S<C<use bigint>> pragma neatly sidesteps the issue altogether: print 20 << 20; # 20971520 print 20 << 40; # 5120 on 32-bit machines, @@ -401,7 +403,7 @@ X<operator, named unary> The various named unary operators are treated as functions with one argument, with optional parentheses. -If any list operator (print(), etc.) or any unary operator (chdir(), etc.) +If any list operator (C<print()>, etc.) or any unary operator (C<chdir()>, etc.) is followed by a left parenthesis as the next token, the operator and arguments within parentheses are taken to be of highest precedence, just like a normal function call. For example, @@ -412,7 +414,7 @@ because named unary operators are higher precedence than C<||>: chdir ($foo) || die; # (chdir $foo) || die chdir +($foo) || die; # (chdir $foo) || die -but, because * is higher precedence than named operators: +but, because C<"*"> is higher precedence than named operators: chdir $foo * 20; # chdir ($foo * 20) chdir($foo) * 20; # (chdir $foo) * 20 @@ -427,7 +429,7 @@ but, because * is higher precedence than named operators: Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are treated like named unary operators, but they don't follow this functional parenthesis rule. That means, for example, that C<-f($file).".bak"> is -equivalent to C<-f "$file.bak">. +equivalent to S<C<-f "$file.bak">>. X<-X> X<filetest> X<operator, filetest> See also L<"Terms and List Operators (Leftward)">. @@ -440,90 +442,93 @@ 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<1> 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 C<"0 but true"> is. +about improper numeric conversions, just as S<C<"0 but true">> is. -Binary "<" returns true if the left argument is numerically less than +Binary C<< "<" >> returns true if the left argument is numerically less than the right argument. X<< < >> -Binary ">" returns true if the left argument is numerically greater +Binary C<< ">" >> returns true if the left argument is numerically greater than the right argument. X<< > >> -Binary "<=" returns true if the left argument is numerically less than +Binary C<< "<=" >> returns true if the left argument is numerically less than or equal to the right argument. X<< <= >> -Binary ">=" returns true if the left argument is numerically greater +Binary C<< ">=" >> returns true if the left argument is numerically greater than or equal to the right argument. X<< >= >> -Binary "lt" returns true if the left argument is stringwise less than +Binary C<"lt"> returns true if the left argument is stringwise less than the right argument. X<< lt >> -Binary "gt" returns true if the left argument is stringwise greater +Binary C<"gt"> returns true if the left argument is stringwise greater than the right argument. X<< gt >> -Binary "le" returns true if the left argument is stringwise less than +Binary C<"le"> returns true if the left argument is stringwise less than or equal to the right argument. X<< le >> -Binary "ge" returns true if the left argument is stringwise greater +Binary C<"ge"> returns true if the left argument is stringwise greater than or equal to the right argument. X<< ge >> =head2 Equality Operators X<equality> X<equal> X<equals> X<operator, equality> -Binary "==" returns true if the left argument is numerically equal to +Binary C<< "==" >> returns true if the left argument is numerically equal to the right argument. X<==> -Binary "!=" returns true if the left argument is numerically not equal +Binary C<< "!=" >> returns true if the left argument is numerically not equal to the right argument. X<!=> -Binary "<=>" returns -1, 0, or 1 depending on whether the left +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 NaNs (not-a-numbers) as numeric -values, using them with "<=>" returns undef. NaN is not "<", "==", ">", -"<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN -returns true, as does NaN != anything else. If your platform doesn't -support NaNs then NaN is just a string with numeric value 0. -X<< <=> >> X<spaceship> +argument. If your platform supports C<NaN>'s (not-a-numbers) as numeric +values, using them with C<< "<=>" >> returns undef. C<NaN> is not +C<< "<" >>, C<< "==" >>, C<< ">" >>, C<< "<=" >> or C<< ">=" >> anything +(even C<NaN>), so those 5 return false. S<C<< NaN != NaN >>> returns +true, as does S<C<NaN !=> I<anything else>>. If your platform doesn't +support C<NaN>'s then C<NaN> is just a string with numeric value 0. +X<< <=> >> +X<spaceship> $ perl -le '$x = "NaN"; print "No NaN support here" if $x == $x' $ perl -le '$x = "NaN"; print "NaN support here" if $x != $x' (Note that the L<bigint>, L<bigrat>, and L<bignum> pragmas all -support "NaN".) +support C<"NaN">.) -Binary "eq" returns true if the left argument is stringwise equal to +Binary C<"eq"> returns true if the left argument is stringwise equal to the right argument. X<eq> -Binary "ne" returns true if the left argument is stringwise not equal +Binary C<"ne"> returns true if the left argument is stringwise not equal to the right argument. X<ne> -Binary "cmp" returns -1, 0, or 1 depending on whether the left +Binary C<"cmp"> returns -1, 0, or 1 depending on whether the left argument is stringwise less than, equal to, or greater than the right argument. X<cmp> -Binary "~~" does a smartmatch between its arguments. Smart matching +Binary C<"~~"> does a smartmatch between its arguments. Smart matching is described in the next section. X<~~> -"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified -by the current locale if a legacy C<use locale> (but not -C<use locale ':not_characters'>) is in effect. See -L<perllocale>. Do not mix these with Unicode, only with legacy binary -encodings. The standard L<Unicode::Collate> and -L<Unicode::Collate::Locale> modules offer much more powerful solutions to -collation issues. +C<"lt">, C<"le">, C<"ge">, C<"gt"> and C<"cmp"> use the collation (sort) +order specified by the current C<LC_COLLATE> locale if a S<C<use +locale>> form that includes collation is in effect. See L<perllocale>. +Do not mix these with Unicode, +only use them with legacy 8-bit locale encodings. +The standard C<L<Unicode::Collate>> and +C<L<Unicode::Collate::Locale>> modules offer much more powerful +solutions to collation issues. For case-insensitive comparisions, look at the L<perlfunc/fc> case-folding function, available in Perl v5.16 or later: @@ -737,9 +742,9 @@ copies of each others' values, as this example reports: } -If you were to set C<$b[3] = 4>, then instead of reporting that "a and b -are deep copies of each other", it now reports that "b smartmatches in a". -That because the corresponding position in C<@a> contains an array that +If you were to set S<C<$b[3] = 4>>, then instead of reporting that "a and b +are deep copies of each other", it now reports that C<"b smartmatches in a">. +That's because the corresponding position in C<@a> contains an array that (eventually) has a 4 in it. Smartmatching one hash against another reports whether both contain the @@ -806,7 +811,7 @@ means that does I<not> invoke the overload method with C<I<X>> as an argument. Instead the above table is consulted as normal, and based on the type of C<I<X>>, overloading may or may not be invoked. For simple strings or -numbers, in becomes equivalent to this: +numbers, "in" becomes equivalent to this: $object ~~ $number ref($object) == $number $object ~~ $string ref($object) eq $string @@ -826,48 +831,48 @@ C<"IO::Handle=GLOB(0x8039e0)">, then pattern matches against that. =head2 Bitwise And X<operator, bitwise, and> X<bitwise and> X<&> -Binary "&" returns its operands ANDed together bit by bit. Although no +Binary C<"&"> returns its operands ANDed together bit by bit. Although no warning is currently raised, the result is not well defined when this operation is performed on operands that aren't either numbers (see -L<Integer Arithmetic>) or bitstrings (see L<Bitwise String Operators>). +L<Integer Arithmetic>) nor bitstrings (see L<Bitwise String Operators>). -Note that "&" has lower priority than relational operators, so for example +Note that C<"&"> has lower priority than relational operators, so for example the parentheses are essential in a test like print "Even\n" if ($x & 1) == 0; -If the experimental "bitwise" feature is enabled via C<use feature -'bitwise'>, then this operator always treats its operand as numbers. This -feature produces a warning unless you use C<no warnings -'experimental::bitwise'>. +If the experimental "bitwise" feature is enabled via S<C<use feature +'bitwise'>>, then this operator always treats its operand as numbers. This +feature produces a warning unless you also use C<S<no warnings +'experimental::bitwise'>>. =head2 Bitwise Or and Exclusive Or X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor> X<bitwise xor> X<^> -Binary "|" returns its operands ORed together bit by bit. +Binary C<"|"> returns its operands ORed together bit by bit. -Binary "^" returns its operands XORed 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<Integer Arithmetic>) or bitstrings (see L<Bitwise String +numbers (see L<Integer Arithmetic>) nor bitstrings (see L<Bitwise String Operators>). -Note that "|" and "^" have lower priority than relational operators, so -for example the brackets are essential in a test like +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 experimental "bitwise" feature is enabled via C<use feature -'bitwise'>, then this operator always treats its operand as numbers. This -feature produces a warning unless you use C<no warnings -'experimental::bitwise'>. +If the experimental "bitwise" feature is enabled via S<C<use feature +'bitwise'>>, then this operator always treats its operand as numbers. This +feature produces a warning unless you also use S<C<no warnings +'experimental::bitwise'>>. =head2 C-style Logical And X<&&> X<logical and> X<operator, logical, and> -Binary "&&" performs a short-circuit logical AND operation. That is, +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. @@ -875,7 +880,7 @@ is evaluated. =head2 C-style Logical Or X<||> X<operator, logical, or> -Binary "||" performs a short-circuit logical OR operation. That is, +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. @@ -884,17 +889,17 @@ is evaluated. X<//> X<operator, logical, defined-or> 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 +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, -C<< EXPR1 // EXPR2 >> returns the value of C<< EXPR1 >> if it's defined, +S<C<< EXPR1 // EXPR2 >>> returns the value of C<< EXPR1 >> if it's defined, otherwise, the value of C<< EXPR2 >> is returned. (C<< EXPR1 >> is evaluated in scalar context, C<< EXPR2 >> in the context of C<< // >> itself). Usually, -this is the same result as C<< defined(EXPR1) ? EXPR1 : EXPR2 >> (except that -the ternary-operator form can be used as a lvalue, while C<< EXPR1 // EXPR2 >> +this is the same result as S<C<< defined(EXPR1) ? EXPR1 : EXPR2 >>> (except that +the ternary-operator form can be used as a lvalue, while S<C<< EXPR1 // EXPR2 >>> cannot). This is very useful for providing default values for variables. If you actually want to test if -at least one of C<$x> and C<$y> is defined, use C<defined($x // $y)>. +at least one of C<$x> and C<$y> is defined, use S<C<defined($x // $y)>>. 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 @@ -914,8 +919,8 @@ for selecting between two aggregates for assignment: As alternatives to C<&&> and C<||> when used for control flow, Perl provides the C<and> and C<or> operators (see below). -The short-circuit behavior is identical. The precedence of "and" -and "or" is much lower, however, so that you can safely use them after a +The short-circuit behavior is identical. The precedence of C<"and"> +and C<"or"> is much lower, however, so that you can safely use them after a list operator without the need for parentheses: unlink "alpha", "beta", "gamma" @@ -933,17 +938,17 @@ It would be even more readable to write that this way: next LINE; } -Using "or" for assignment is unlikely to do what you want; see below. +Using C<"or"> for assignment is unlikely to do what you want; see below. =head2 Range Operators X<operator, range> X<range> X<..> X<...> -Binary ".." is the range operator, which is really two different +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 -C<foreach (1..10)> loops and for doing slice operations on arrays. In +S<C<foreach (1..10)>> loops and for doing slice operations on arrays. In the current implementation, no temporary array is created when the range operator is used as the expression in C<foreach> loops, but older versions of Perl might burn a lot of memory when you write something @@ -956,9 +961,9 @@ like this: The range operator also works on strings, using the magical auto-increment, see below. -In scalar context, ".." returns a boolean value. The operator is +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<sed>, B<awk>, and various editors. Each ".." operator +operator of B<sed>, B<awk>, 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 @@ -967,8 +972,8 @@ 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<awk>), but it still returns true once. If you don't want it to test the right operand until the -next evaluation, as in B<sed>, just use three dots ("...") instead of -two. In all other regards, "..." behaves just like ".." does. +next evaluation, as in B<sed>, 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 @@ -976,21 +981,21 @@ operator is in the "true" state. The precedence is a little lower than || and &&. The value returned is either the empty string for false, or a sequence number (beginning with 1) for true. The sequence number is reset for each range encountered. The final sequence number -in a range has the string "E0" appended to it, which doesn't affect +in a range has the string C<"E0"> appended to it, which doesn't affect its numeric value, but gives you something to search for if you want to exclude the endpoint. You can exclude the beginning point by waiting for the sequence number to be greater than 1. -If either operand of scalar ".." is a constant expression, +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 C<int(EXPR) == int(EXPR)>, +To be pedantic, the comparison is actually S<C<int(EXPR) == int(EXPR)>>, but that is only an issue if you use a floating point expression; when implicitly using C<$.> as described in the previous paragraph, the -comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.> +comparison is S<C<int(EXPR) == int($.)>> which is only an issue when C<$.> is set to a floating point value and you are not reading from a file. -Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what +Furthermore, S<C<"span" .. "spat">> or S<C<2.18 .. 3.14>> will not do what you want in scalar context because each of the operands are evaluated using their integer representation. @@ -1088,7 +1093,7 @@ you could use the pattern C</(?:(?=\p{Greek})\p{Lower})+/> (or the L<experimental feature|perlrecharclass/Extended Bracketed Character Classes> C<S</(?[ \p{Greek} & \p{Lower} ])+/>>). -Because each operand is evaluated in integer form, C<2.18 .. 3.14> will +Because each operand is evaluated in integer form, S<C<2.18 .. 3.14>> will return two elements in list context. @list = (2.18 .. 3.14); # same as @list = (2 .. 3); @@ -1096,10 +1101,10 @@ return two elements in list context. =head2 Conditional Operator X<operator, conditional> X<operator, ternary> X<ternary> X<?:> -Ternary "?:" is the conditional operator, just as in C. It works much -like an if-then-else. If the argument before the ? is true, the -argument before the : is returned, otherwise the argument after the : -is returned. For example: +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"; @@ -1138,7 +1143,7 @@ X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=> X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=> X<%=> X<^=> X<x=> X<&.=> X<|.=> X<^.=> -"=" is the ordinary assignment operator. +C<"="> is the ordinary assignment operator. Assignment operators work as in C. That is, @@ -1149,7 +1154,7 @@ is equivalent to $x = $x + 2; although without duplicating any side effects that dereferencing the lvalue -might trigger, such as from tie(). Other assignment operators work similarly. +might trigger, such as from C<tie()>. Other assignment operators work similarly. The following are recognized: **= += *= &= &.= <<= &&= @@ -1190,13 +1195,13 @@ 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<&.= |.= ^.=>) are new in +The three dotted bitwise assignment operators (C<&.=> C<|.=> C<^.=>) are new in Perl 5.22 and experimental. See L</Bitwise String Operators>. =head2 Comma Operator X<comma> X<operator, comma> X<,> -Binary "," is the comma operator. In scalar context it evaluates +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. @@ -1204,7 +1209,8 @@ 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 is a synonym for the comma except that it causes a +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, @@ -1239,7 +1245,7 @@ I<part> of the left operand: print time.shift => "bbb"; -That example prints something like "1314363215shiftbbb", because the +That example prints something like C<"1314363215shiftbbb">, because the C<< => >> implicitly quotes the C<shift> immediately on its left, ignoring the fact that C<time.shift> is the entire left operand. @@ -1249,7 +1255,7 @@ X<operator, list, rightward> X<list operator> 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 list +C<"and">, C<"or">, and C<"not">, which may be used to evaluate calls to list operators without the need for parentheses: open HANDLE, "< :utf8", "filename" or die "Can't open: $!\n"; @@ -1259,7 +1265,7 @@ it 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: +in which case you might as well just use the more customary C<"||"> operator: open(HANDLE, "< :utf8", "filename") || die "Can't open: $!\n"; @@ -1268,13 +1274,13 @@ See also discussion of list operators in L<Terms and List Operators (Leftward)>. =head2 Logical Not X<operator, logical, not> X<not> -Unary "not" returns the logical negation of the expression to its right. -It's the equivalent of "!" except for the very low precedence. +Unary C<"not"> 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<operator, logical, and> X<and> -Binary "and" returns the logical conjunction of the two surrounding +Binary C<"and"> 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. @@ -1284,7 +1290,7 @@ X<operator, logical, or> X<operator, logical, xor> X<operator, logical, exclusive or> X<or> X<xor> -Binary "or" returns the logical disjunction of the two surrounding +Binary C<"or"> 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: @@ -1300,7 +1306,7 @@ It usually works out better for flow control than in assignments: $x = $y || $z; # better written this way However, when it's a list-context assignment and you're trying to use -C<||> for control flow, you probably need "or" so that the assignment +C<||> for control flow, you probably need C<"or"> so that the assignment takes higher precedence. @info = stat($file) || die; # oops, scalar sense of stat! @@ -1308,7 +1314,7 @@ takes higher precedence. Then again, you could always use parentheses. -Binary C<xor> returns the exclusive-OR of the two surrounding expressions. +Binary C<"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. @@ -1323,12 +1329,12 @@ Here is what C has that Perl doesn't: =item unary & -Address-of operator. (But see the "\" operator for taking a reference.) +Address-of operator. (But see the C<"\"> operator for taking a reference.) =item unary * Dereference-address operator. (Perl's prefix dereferencing -operators are typed: $, @, %, and &.) +operators are typed: C<$>, C<@>, C<%>, and C<&>.) =item (TYPE) @@ -1376,12 +1382,12 @@ Note, however, that this does not always work for quoting Perl code: $s = q{ if($x eq "}") ... }; # WRONG -is a syntax error. The C<Text::Balanced> module (standard as of v5.8, +is a syntax error. The C<L<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 quoting characters, except when C<#> is being used as the quoting character. -C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the +C<q#foo#> is parsed as the string C<foo>, while S<C<q #foo#>> is the operator C<q> followed by a comment. Its argument will be taken from the next line. This allows you to write: @@ -1432,7 +1438,7 @@ The result is the character specified by the hexadecimal number in the range Only hexadecimal digits are valid following C<\x>. When C<\x> is followed by fewer than two valid digits, any valid digits will be zero-padded. This -means that C<\x7> will be interpreted as C<\x07>, and a lone <\x> will be +means that C<\x7> will be interpreted as C<\x07>, and a lone C<"\x"> will be interpreted as C<\x00>. 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 @@ -1452,7 +1458,7 @@ See L<charnames>. =item [4] -C<\N{U+I<hexadecimal number>}> means the Unicode character whose Unicode code +S<C<\N{U+I<hexadecimal number>}>> means the Unicode character whose Unicode code point is I<hexadecimal number>. =item [5] @@ -1470,17 +1476,19 @@ table: \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) + \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<\c?> is DELETE on ASCII platforms because S<C<ord("?") ^ 64>> is 127, and -C<\c@> is NULL because the ord of "@" is 64, so xor'ing 64 itself produces 0. +C<\c@> is NULL because the ord of C<"@"> is 64, so xor'ing 64 itself produces 0. -Also, C<\c\I<X>> yields C< chr(28) . "I<X>"> for any I<X>, but cannot come at the +Also, C<\c\I<X>> yields S<C< chr(28) . "I<X>">> for any I<X>, but cannot come at the end of a string, because the backslash would be parsed as escaping the end quote. @@ -1521,8 +1529,9 @@ see L<perlrebackslash/Octal escapes>.) Starting in Perl 5.14, you may use C<\o{}> instead, which avoids all these problems. Otherwise, it is best to use this construct only for ordinals C<\077> and below, remembering to pad to the left with zeros to make three digits. For larger ordinals, either use -C<\o{}>, or convert to something else, such as to hex and use C<\x{}> -instead. +C<\o{}>, or convert to something else, such as to hex and use C<\N{U+}> +(which is portable between platforms with different character sets) or +C<\x{}> instead. =item [8] @@ -1537,15 +1546,15 @@ it as a Unicode code point and the result is the corresponding Unicode character. For example C<\x{50}> and C<\o{120}> 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 "P", and in EBCDIC it is the ampersand symbol "&". +from 0) is the letter C<"P">, and in EBCDIC it is the ampersand symbol C<"&">. C<\x{100}> and C<\o{400}> 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<LATIN CAPITAL LETTER A WITH MACRON>. There are a couple of exceptions to the above rule. S<C<\N{U+I<hex number>}>> is -always interpreted as a Unicode code point, so that C<\N{U+0050}> is "P" even -on EBCDIC platforms. And if L<C<S<use encoding>>|encoding> is in effect, the +always interpreted as a Unicode code point, so that C<\N{U+0050}> is C<"P"> even +on EBCDIC platforms. And if C<S<L<use encoding|encoding>>> is in effect, the number is considered to be in that encoding, and is translated from that into the platform's native encoding if there is a corresponding native character; otherwise to Unicode. @@ -1554,8 +1563,7 @@ otherwise to Unicode. B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for the vertical tab (VT, which is 11 in both ASCII and EBCDIC), but you may -use C<\ck> or -C<\x0b>. (C<\v> +use C<\N{VT}>, C<\ck>, C<\N{U+0b}>, or C<\x0b>. (C<\v> does have meaning in regular expression patterns in Perl, see L<perlre>.) The following escape sequences are available in constructs that interpolate, @@ -1581,14 +1589,14 @@ C<\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 C<use locale> is in effect (but not C<use locale ':not_characters'>), -the case map used by C<\l>, C<\L>, -C<\u>, and C<\U> is taken from the current locale. See L<perllocale>. -If Unicode (for example, C<\N{}> or code points of 0x100 or -beyond) is being used, the case map used by C<\l>, C<\L>, C<\u>, and -C<\U> is as defined by Unicode. That means that case-mapping -a single character can sometimes produce several characters. -Under C<use locale>, C<\F> produces the same results as C<\L> +If a S<C<use locale>> form that includes C<LC_CTYPE> is in effect (see +L<perllocale>), the case map used by C<\l>, C<\L>, C<\u>, and C<\U> is +taken from the current locale. If Unicode (for example, C<\N{}> or code +points of 0x100 or beyond) is being used, the case map used by C<\l>, +C<\L>, C<\u>, and C<\U> is as defined by Unicode. That means that +case-mapping a single character can sometimes produce a sequence of +several characters. +Under S<C<use locale>>, C<\F> produces the same results as C<\L> for all locales but a UTF-8 one, where it instead uses the Unicode definition. @@ -1598,7 +1606,7 @@ 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<"\r"> as ASCII CR and C<"\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, +and on systems without a line terminator, printing C<"\n"> might emit no actual data. In general, use C<"\n"> when you mean a "newline" for your system, but use the literal ASCII when you need an exact character. For example, most networking protocols expect @@ -1616,7 +1624,7 @@ But method calls such as C<< $obj->meth >> are not. Interpolating an array or slice interpolates the elements in order, separated by the value of C<$">, so is equivalent to interpolating -C<join $", @array>. "Punctuation" arrays such as C<@*> are usually +S<C<join $", @array>>. "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. @@ -1659,14 +1667,14 @@ matching and related activities. =over 8 -=item qr/STRING/msixpodualn +=item C<qr/I<STRING>/msixpodualn> X<qr> X</i> X</m> X</o> X</s> X</x> X</p> This operator quotes (and possibly compiles) its I<STRING> as a regular expression. I<STRING> is interpolated the same way as I<PATTERN> -in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation +in C<m/I<PATTERN>/>. If C<"'"> is used as the delimiter, no interpolation is done. Returns a Perl value which may be used instead of the -corresponding C</STRING/msixpodualn> expression. The returned value is a +corresponding C</I<STRING>/msixpodualn> expression. The returned value is a normalized version of the original pattern. It magically differs from a string containing the same characters: C<ref(qr/x/)> returns "Regexp"; however, dereferencing it is not well defined (you currently get the @@ -1691,9 +1699,9 @@ The result may be used as a subpattern in a match: $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: +Since Perl may compile the pattern at the moment of execution of the C<qr()> +operator, using C<qr()> may have speed advantages in some situations, +notably if the result of C<qr()> is used standalone: sub match { my $patterns = shift; @@ -1708,10 +1716,10 @@ notably if the result of qr() is used standalone: } Precompilation of the pattern into an internal representation at -the moment of qr() avoids a need to recompile the pattern every +the moment of C<qr()> avoids the need to recompile the pattern every time a match C</$pat/> is attempted. (Perl has many other internal optimizations, but none would be triggered in the above example if -we did not use qr() operator.) +we did not use C<qr()> operator.) Options (specified by the following modifiers) are: @@ -1721,19 +1729,20 @@ Options (specified by the following modifiers) are: x Use extended regular expressions. p When matching preserve a copy of the matched string so that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be - defined. + defined (ignored starting in v5.20) as these are always + defined starting in that relese o Compile 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. + a's further restricts things to that 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 "msixpluadn" will be propagated appropriately. The effect the "o" -modifier has is not propagated, being restricted to those patterns +of C<"msixpluadn"> will be propagated appropriately. The effect that the +C</o> modifier has is not propagated, being restricted to those patterns explicitly using it. The last four modifiers listed above, added in Perl 5.14, @@ -1741,21 +1750,21 @@ control the character set rules, but C</a> is the only one you are likely to want to specify explicitly; the other three are selected automatically by various pragmas. -See L<perlre> for additional information on valid syntax for STRING, and +See L<perlre> for additional information on valid syntax for I<STRING>, and for a detailed look at the semantics of regular expressions. In particular, all modifiers except the largely obsolete C</o> are further explained in L<perlre/Modifiers>. C</o> is described in the next section. -=item m/PATTERN/msixpodualngc +=item C<m/I<PATTERN>/msixpodualngc> X<m> X<operator, match> X<regexp, options> X<regexp> X<regex, options> X<regex> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> -=item /PATTERN/msixpodualngc +=item C</I<PATTERN>/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 $_ string is searched. (The +via the C<=~> or C<!~> operator, the C<$_> 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<perlre>. @@ -1767,17 +1776,17 @@ process modifiers are available: c Do not reset search position on a failed match when /g is in effect. -If "/" is the delimiter then the initial C<m> is optional. With the C<m> +If C<"/"> is the delimiter then the initial C<m> is optional. With the C<m> you can use any pair of non-whitespace (ASCII) characters as delimiters. This is particularly useful for matching path names -that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is +that contain C<"/">, to avoid LTS (leaning toothpick syndrome). If C<"?"> is the delimiter, then a match-only-once rule applies, -described in C<m?PATTERN?> below. If "'" (single quote) is the delimiter, -no interpolation is performed on the PATTERN. -When using a character valid in an identifier, whitespace is required +described in C<m?I<PATTERN>?> below. If C<"'"> (single quote) is the delimiter, +no interpolation is performed on the I<PATTERN>. +When using a delimiter character valid in an identifier, whitespace is required after the C<m>. -PATTERN may contain variables, which will be interpolated +I<PATTERN> 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<$|> are not interpolated because they look like end-of-string tests.) @@ -1822,9 +1831,9 @@ Use C</o> if you want to avoid this. The bottom line is that using C</o> is almost never a good idea. -=item The empty pattern // +=item The empty pattern C<//> -If the PATTERN evaluates to the empty string, the last +If the I<PATTERN> evaluates to the empty string, the last I<successfully> matched regular expression is used instead. In this case, only the C<g> and C<c> flags on the empty pattern are honored; the other flags are taken from the original pattern. If no match has @@ -1834,8 +1843,8 @@ empty pattern (which will always match). Note that it's possible to confuse Perl into thinking C<//> (the empty regex) is really C<//> (the defined-or operator). Perl is usually pretty good about this, but some pathological cases might trigger this, such as -C<$x///> (is that C<($x) / (//)> or C<$x // />?) and C<print $fh //> -(C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl +C<$x///> (is that S<C<($x) / (//)>> or S<C<$x // />>?) and S<C<print $fh //>> +(S<C<print $fh(//>> or S<C<print($fh //>>?). 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<m> (so C<//> becomes C<m//>). @@ -1868,9 +1877,9 @@ Examples: if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) -This last example splits $foo into the first two words and the -remainder of the line, and assigns those three fields to $F1, $F2, and -$Etc. The conditional is true if any variables were assigned; that is, +This last example splits C<$foo> into the first two words and the +remainder of the line, and assigns those three fields to C<$F1>, C<$F2>, and +C<$Etc>. The conditional is true if any variables were assigned; that is, if the pattern matched. The C</g> modifier specifies global pattern matching--that is, @@ -1889,7 +1898,7 @@ search position to the beginning of the string, but you can avoid that by adding the C</c> modifier (for example, C<m//gc>). Modifying the target string also resets the search position. -=item \G assertion +=item C<\G I<assertion>> You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a zero-width assertion that matches the exact position where the @@ -2004,13 +2013,13 @@ Here is the output (split into several lines): lowercase line-noise lowercase lowercase line-noise lowercase lowercase line-noise MiXeD line-noise. That's all! -=item m?PATTERN?msixpodualngc +=item C<m?I<PATTERN>?msixpodualngc> X<?> X<operator, match-once> -=item ?PATTERN?msixpodualngc +=item C<?I<PATTERN>?msixpodualngc> -This is just like the C<m/PATTERN/> search, except that it matches -only once between calls to the reset() operator. This is a useful +This is just like the C<m/I<PATTERN>/> search, except that it matches +only once between calls to the C<reset()> operator. This is a useful optimization when you want to see only the first occurrence of something in each file of a set of files, for instance. Only C<m??> patterns local to the current package are reset. @@ -2031,12 +2040,12 @@ to "utf8" in a pod file: The match-once behavior is controlled by the match delimiter being C<?>; with any other delimiter this is the normal C<m//> operator. -In the past, the leading C<m> in C<m?PATTERN?> was optional, but omitting it +In the past, the leading C<m> in C<m?I<PATTERN>?> 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<m>. -=item s/PATTERN/REPLACEMENT/msixpodualngcer +=item C<s/I<PATTERN>/I<REPLACEMENT>/msixpodualngcer> X<substitute> X<substitution> X<replace> X<regexp, replace> X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e> X</r> @@ -2058,15 +2067,15 @@ hash element, or an assignment to one of those; that is, some sort of scalar lvalue. If the delimiter chosen is a single quote, no interpolation is -done on either the PATTERN or the REPLACEMENT. Otherwise, if the -PATTERN contains a $ that looks like a variable rather than an +done on either the I<PATTERN> or the I<REPLACEMENT>. Otherwise, if the +I<PATTERN> contains a C<$> that looks like a variable rather than an end-of-string test, the variable will be interpolated into the pattern at run-time. If you want the pattern compiled only once the first time the variable is interpolated, use the C</o> option. If the pattern evaluates to the empty string, the last successfully executed regular expression is used instead. See L<perlre> for further explanation on these. -Options are as with m// with the addition of the following replacement +Options are as with C<m//> with the addition of the following replacement specific options: e Evaluate the right side as an expression. @@ -2080,7 +2089,7 @@ the C<s> when using a character allowed in identifiers. If single quotes are used, no interpretation is done on the replacement string (the C</e> modifier overrides this, however). Note that Perl treats backticks as normal delimiters; the replacement text is not evaluated as a command. -If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has +If the I<PATTERN> is delimited by bracketing quotes, the I<REPLACEMENT> has its own pair of quotes, which may or may not be bracketing quotes, for example, C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</e> will cause the replacement portion to be treated as a full-fledged Perl expression @@ -2155,8 +2164,8 @@ Examples: s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields -Note the use of $ instead of \ in the last example. Unlike -B<sed>, we use the \<I<digit>> form in only the left hand side. +Note the use of C<$> instead of C<\> in the last example. Unlike +B<sed>, we use the \<I<digit>> form only in the left hand side. Anywhere else it's $<I<digit>>. Occasionally, you can't use just a C</g> to get all the changes @@ -2175,10 +2184,10 @@ X<operator, quote-like> =over 4 -=item q/STRING/ +=item C<q/I<STRING>/> X<q> X<quote, single> X<'> X<''> -=item 'STRING' +=item C<'I<STRING>'> A single-quoted, literal string. A backslash represents a backslash unless followed by the delimiter or another backslash, in which case @@ -2188,10 +2197,10 @@ the delimiter or backslash is interpolated. $bar = q('This is it.'); $baz = '\n'; # a two-character string -=item qq/STRING/ +=item C<qq/I<STRING>/> X<qq> X<quote, double> X<"> X<""> -=item "STRING" +=item "I<STRING>" A double-quoted, interpolated string. @@ -2200,19 +2209,19 @@ A double-quoted, interpolated string. if /\b(tcl|java|python)\b/i; # :-) $baz = "\n"; # a one-character string -=item qx/STRING/ +=item C<qx/I<STRING>/> X<qx> X<`> X<``> X<backtick> -=item `STRING` +=item C<`I<STRING>`> A string which is (possibly) interpolated and then executed as a system command with F</bin/sh> or its equivalent. Shell wildcards, pipes, and redirections will be honored. The collected standard output of the command is returned; standard error is unaffected. In scalar context, it comes back as a single (potentially multi-line) -string, or undef if the command failed. In list context, returns a -list of lines (however you've defined lines with $/ or -$INPUT_RECORD_SEPARATOR), or an empty list if the command failed. +string, or C<undef> if the command failed. In list context, returns a +list of lines (however you've defined lines with C<$/> or +C<$INPUT_RECORD_SEPARATOR>), or an empty list if the command failed. Because backticks do not affect standard error, use shell file descriptor syntax (assuming the shell supports this) if you care to address this. @@ -2259,7 +2268,7 @@ How that string gets evaluated is entirely subject to the command interpreter on your system. On most platforms, you will have to protect shell metacharacters if you want them treated literally. This is in practice difficult to do, as it's unclear how to escape which characters. -See L<perlsec> for a clean and safe example of a manual fork() and exec() +See L<perlsec> for a clean and safe example of a manual C<fork()> and C<exec()> to emulate backticks safely. On some platforms (notably DOS-like ones), the shell may not be @@ -2272,8 +2281,8 @@ many Unix shells and C<&> on the Windows NT C<cmd> shell). Perl will attempt to flush all files opened for output before starting the child process, but this may not be supported on some platforms (see L<perlport>). To be safe, you may need to set -C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of -C<IO::Handle> on any open handles. +C<$|> (C<$AUTOFLUSH> in C<L<English>>) or call the C<autoflush()> method of +C<L<IO::Handle>> on any open handles. Beware that some command shells may place restrictions on the length of the command line. You must ensure your strings don't exceed this @@ -2291,10 +2300,10 @@ Just understand what you're getting yourself into. See L</"I/O Operators"> for more discussion. -=item qw/STRING/ +=item C<qw/I<STRING>/> X<qw> X<quote, list> X<quote, words> -Evaluates to a list of the words extracted out of STRING, using embedded +Evaluates to a list of the words extracted out of I<STRING>, using embedded whitespace as the word delimiters. It can be understood as being roughly equivalent to: @@ -2315,20 +2324,20 @@ Some frequently seen examples: use POSIX qw( setlocale localeconv ) @EXPORT = qw( foo bar baz ); -A common mistake is to try to separate the words with comma or to +A common mistake is to try to separate the words with commas or to put comments into a multi-line C<qw>-string. For this reason, the -C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable) -produces warnings if the STRING contains the "," or the "#" character. +S<C<use warnings>> pragma and the B<-w> switch (that is, the C<$^W> variable) +produces warnings if the I<STRING> contains the C<","> or the C<"#"> character. -=item tr/SEARCHLIST/REPLACEMENTLIST/cdsr +=item C<tr/I<SEARCHLIST>/I<REPLACEMENTLIST>/cdsr> X<tr> X<y> X<transliterate> X</c> X</d> X</s> -=item y/SEARCHLIST/REPLACEMENTLIST/cdsr +=item C<y/I<SEARCHLIST>/I<REPLACEMENTLIST>/cdsr> Transliterates all occurrences of the characters found in the search list with the corresponding character in the replacement list. It returns the number of characters replaced or deleted. If no string is -specified via the C<=~> or C<!~> operator, the $_ string is transliterated. +specified via the C<=~> or C<!~> operator, the C<$_> string is transliterated. If the C</r> (non-destructive) option is present, a new copy of the string is made and its characters transliterated, and this copy is returned no @@ -2343,12 +2352,23 @@ of those; in other words, an lvalue. A character range may be specified with a hyphen, so C<tr/A-J/0-9/> does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>. For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the -SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has +I<SEARCHLIST> is delimited by bracketing quotes, the I<REPLACEMENTLIST> has its own pair of quotes, which may or may not be bracketing quotes; for example, C<tr[aeiouy][yuoiea]> or C<tr(+\-*/)/ABCD/>. +Characters may be literals or any of the escape sequences accepted in +double-quoted strings. But there is no interpolation, so C<"$"> and +C<"@"> are treated as literals. A hyphen at the beginning or end, or +preceded by a backslash is considered a literal. Escape sequence +details are in L<the table near the beginning of this section|/Quote and +Quote-like Operators>. It is a bug in Perl v5.22 that something like + + tr/\N{U+20}-\N{U+7E}foobar// + +does not treat that range as fully Unicode. + Note that C<tr> does B<not> do regular expression character classes such as -C<\d> or C<\pL>. The C<tr> operator is not equivalent to the tr(1) +C<\d> or C<\pL>. The C<tr> operator is not equivalent to the C<L<tr(1)>> utility. If you want to map strings between lower/upper cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider using the C<s> operator if you need regular expressions. The C<\U>, C<\u>, C<\L>, and @@ -2371,19 +2391,19 @@ Options: r Return the modified string and leave the original string untouched. -If the C</c> modifier is specified, the SEARCHLIST character set +If the C</c> modifier is specified, the I<SEARCHLIST> character set is complemented. If the C</d> modifier is specified, any characters -specified by SEARCHLIST not found in REPLACEMENTLIST are deleted. +specified by I<SEARCHLIST> not found in I<REPLACEMENTLIST> are deleted. (Note that this is slightly more flexible than the behavior of some -B<tr> programs, which delete anything they find in the SEARCHLIST, +B<tr> programs, which delete anything they find in the I<SEARCHLIST>, period.) If the C</s> modifier is specified, sequences of characters that were transliterated to the same character are squashed down to a single instance of the character. -If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted -exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter -than the SEARCHLIST, the final character is replicated till it is long -enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated. +If the C</d> modifier is used, the I<REPLACEMENTLIST> is always interpreted +exactly as specified. Otherwise, if the I<REPLACEMENTLIST> is shorter +than the I<SEARCHLIST>, the final character is replicated till it is long +enough. If the I<REPLACEMENTLIST> is empty, the I<SEARCHLIST> is replicated. This latter is useful for counting characters in a class or for squashing character sequences in a class. @@ -2421,16 +2441,16 @@ first one is used: will transliterate any A to X. Because the transliteration table is built at compile time, neither -the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote +the I<SEARCHLIST> nor the I<REPLACEMENTLIST> are subjected to double quote interpolation. That means that if you want to use variables, you -must use an eval(): +must use an C<eval()>: eval "tr/$oldlist/$newlist/"; die $@ if $@; eval "tr/$oldlist/$newlist/, 1" or die $@; -=item <<EOF +=item C<< <<I<EOF> >> X<here-doc> X<heredoc> X<here-document> X<<< << >>> A line-oriented form of quoting is based on the shell "here-document" @@ -2606,10 +2626,10 @@ one to four, but these passes are always performed in the same order. =item Finding the end -The first pass is finding the end of the quoted construct, where -the information about the delimiters is used in parsing. -During this search, text between the starting and ending delimiters -is copied to a safe location. The text copied gets delimiter-independent. +The first pass is finding the end of the quoted construct. This results +in saving to a safe location a copy of the text (between the starting +and ending delimiters), normalized as necessary to avoid needing to know +what the original delimiters were. If the construct is a here-doc, the ending delimiter is a line that has a terminating string as the content. Therefore C<<<EOF> is @@ -2624,7 +2644,7 @@ and ending delimiters. If the starting delimiter is an opening punctuation (that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>). If the starting delimiter is an unpaired character like C</> or a closing -punctuation, the ending delimiter is same as the starting delimiter. +punctuation, the ending delimiter is the same as the starting delimiter. Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates both C<qq[]> and C<qq]]> constructs. @@ -2632,7 +2652,7 @@ When searching for single-character delimiters, escaped delimiters and C<\\> are skipped. For example, while searching for terminating C</>, combinations of C<\\> and C<\/> are skipped. If the delimiters are bracketing, nested pairs are also skipped. For example, while searching -for closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>, +for a closing C<]> paired with the opening C<[>, combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested C<[> and C<]> are skipped as well. However, when backslashes are used as the delimiters (like C<qq\\> and C<tr\\\>), nothing is skipped. @@ -2649,7 +2669,7 @@ terminates the left part and starts the right part at once. If the left part is delimited by bracketing punctuation (that is C<()>, C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespace -and comments are allowed between the two parts, though the comment must follow +and comments are allowed between the two parts, although the comment must follow at 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. @@ -2698,7 +2718,7 @@ to L</"parsing regular expressions">. =item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''> The only interpolation is removal of C<\> from pairs of C<\\>. -Therefore C<-> in C<tr'''> and C<y'''> is treated literally +Therefore C<"-"> in C<tr'''> and C<y'''> is treated literally as a hyphen and no character range is available. C<\1> in the replacement of C<s'''> does not work as C<$1>. @@ -2708,14 +2728,14 @@ No variable interpolation occurs. String modifying combinations for case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized. The other escape sequences such as C<\200> and C<\t> and backslashed characters such as C<\\> and C<\-> are converted to appropriate literals. -The character C<-> is treated specially and therefore C<\-> is treated -as a literal C<->. +The character C<"-"> is treated specially and therefore C<\-> is treated +as a literal C<"-">. =item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF"> C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> (possibly paired with C<\E>) are converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar"> -is converted to C<$foo . (quotemeta("baz" . $bar))> internally. +is converted to S<C<$foo . (quotemeta("baz" . $bar))>> internally. The other escape sequences such as C<\200> and C<\t> and backslashed characters such as C<\\> and C<\-> are replaced with appropriate expansions. @@ -2734,21 +2754,21 @@ as C<"\\\t"> (since TAB is not alphanumeric). Note also that: may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">. Interpolated scalars and arrays are converted internally to the C<join> and -C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes: +C<"."> catenation operations. Thus, S<C<"$foo XXX '@arr'">> becomes: $foo . " XXX '" . (join $", @arr) . "'"; All operations above are performed simultaneously, left to right. -Because the result of C<"\Q STRING \E"> has all metacharacters +Because the result of S<C<"\Q I<STRING> \E">> has all metacharacters quoted, there is no way to insert a literal C<$> or C<@> inside a -C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became +C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to become C<"\\\$">; if not, it is interpreted as the start of an interpolated scalar. Note also that the interpolation code needs to make a decision on where the interpolated scalar ends. For instance, whether -C<< "a $x -> {c}" >> really means: +S<C<< "a $x -> {c}" >>> really means: "a " . $x . " -> {c}"; @@ -2770,7 +2790,7 @@ happens as with C<qq//> constructs. It is at this step that C<\1> is begrudgingly converted to C<$1> in the replacement text of C<s///>, in order to correct the incorrigible I<sed> hackers who haven't picked up the saner idiom yet. A warning -is emitted if the C<use warnings> pragma or the B<-w> command-line flag +is emitted if the S<C<use warnings>> pragma or the B<-w> command-line flag (that is, the C<$^W> variable) was set. =item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>, @@ -2794,10 +2814,10 @@ Code blocks such as C<(?{BLOCK})> are handled by temporarily passing control back to the perl parser, in a similar way that an interpolated array subscript expression such as C<"foo$array[1+f("[xyz")]bar"> would be. -Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and -a C<#>-comment in a C<//x>-regular expression, no processing is +Moreover, inside C<(?{BLOCK})>, S<C<(?# comment )>>, and +a C<#>-comment in a C</x>-regular expression, no processing is performed whatsoever. This is the first step at which the presence -of the C<//x> modifier is relevant. +of the C</x> modifier is relevant. Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+> and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are @@ -2823,7 +2843,7 @@ alphanumeric char, as in: In the RE above, which is intentionally obfuscated for illustration, the delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the -RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one +RE is the same as for S<C<m/ ^ a \s* b /mx>>. There's more than one reason you're encouraged to restrict your delimiters to non-alphanumeric, non-whitespace choices. @@ -2845,9 +2865,9 @@ resulting I<string> is passed to the RE engine for compilation. Whatever happens in the RE engine might be better discussed in L<perlre>, but for the sake of continuity, we shall do so here. -This is another step where the presence of the C<//x> modifier is +This is another step where the presence of the C</x> modifier is relevant. The RE engine scans the string from left to right and -converts it to a finite automaton. +converts it into a finite automaton. Backslashed characters are either replaced with corresponding literal strings (as with C<\{>), or else they generate special nodes @@ -2855,7 +2875,7 @@ in the finite automaton (as with C<\b>). Characters special to the RE engine (such as C<|>) generate corresponding nodes or groups of nodes. C<(?#...)> comments are ignored. All the rest is either converted to literal strings to match, or else is ignored (as is -whitespace and C<#>-style comments if C<//x> is present). +whitespace and C<#>-style comments if C</x> is present). Parsing of the bracketed character class construct, C<[...]>, is rather different than the rule used for the rest of the pattern. @@ -2870,7 +2890,7 @@ logically balancing terminating C<}> is found. It is possible to inspect both the string given to RE engine and the resulting finite automaton. See the arguments C<debug>/C<debugcolor> -in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line +in the S<C<use L<re>>> pragma, as well as Perl's B<-Dr> command-line switch documented in L<perlrun/"Command Switches">. =item Optimization of regular expressions @@ -2920,11 +2940,11 @@ Ordinarily you must assign the returned value to a variable, but there is one situation where an automatic assignment happens. If and only if the input symbol is the only thing inside the conditional of a C<while> statement (even if disguised as a C<for(;;)> loop), -the value is automatically assigned to the global variable $_, +the value is automatically assigned to the global variable C<$_>, destroying whatever was there previously. (This may seem like an odd thing to you, but you'll use the construct in almost every Perl -script you write.) The $_ variable is not implicitly localized. -You'll have to put a C<local $_;> before the loop if you want that +script you write.) The C<$_> variable is not implicitly localized. +You'll have to put a S<C<local $_;>> before the loop if you want that to happen. The following lines are equivalent: @@ -2946,41 +2966,41 @@ In these loop constructs, the assigned value (whether assignment is automatic or explicit) is then tested to see whether it is defined. The defined test avoids problems where the line has a string value that would be treated as false by Perl; for example a "" or -a "0" with no trailing newline. If you really mean for such values +a C<"0"> with no trailing newline. If you really mean for such values to terminate the loop, they should be tested for explicitly: while (($_ = <STDIN>) ne '0') { ... } while (<STDIN>) { last unless $_; ... } -In other boolean contexts, C<< <FILEHANDLE> >> without an +In other boolean contexts, C<< <I<FILEHANDLE>> >> without an explicit C<defined> test or comparison elicits a warning if the -C<use warnings> pragma or the B<-w> +S<C<use warnings>> pragma or the B<-w> command-line switch (the C<$^W> variable) is in effect. The filehandles STDIN, STDOUT, and STDERR are predefined. (The filehandles C<stdin>, C<stdout>, and C<stderr> will also work except in packages, where they would be interpreted as local identifiers rather than global.) Additional filehandles may be created with -the open() function, amongst others. See L<perlopentut> and +the C<open()> function, amongst others. See L<perlopentut> and L<perlfunc/open> for details on this. X<stdin> X<stdout> X<sterr> -If a <FILEHANDLE> is used in a context that is looking for +If a C<< <I<FILEHANDLE>> >> is used in a context that is looking for a list, a list comprising all input lines is returned, one line per list element. It's easy to grow to a rather large data space this way, so use with care. -<FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>. +C<< <I<FILEHANDLE>> >> may also be spelled C<readline(*I<FILEHANDLE>)>. See L<perlfunc/readline>. -The null filehandle <> is special: it can be used to emulate the +The null filehandle C<< <> >> is special: it can be used to emulate the behavior of B<sed> and B<awk>, and any other Unix filter program that takes a list of filenames, doing the same to each line -of input from all of them. Input from <> comes either from +of input from all of them. Input from C<< <> >> comes either from standard input, or from each file listed on the command line. Here's -how it works: the first time <> is evaluated, the @ARGV array is -checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened -gives you standard input. The @ARGV array is then processed as a list +how it works: the first time C<< <> >> is evaluated, the C<@ARGV> array is +checked, and if it is empty, C<$ARGV[0]> is set to C<"-">, which when opened +gives you standard input. The C<@ARGV> array is then processed as a list of filenames. The loop while (<>) { @@ -2998,11 +3018,11 @@ is equivalent to the following Perl-like pseudo code: } except that it isn't so cumbersome to say, and will actually work. -It really does shift the @ARGV array and put the current filename -into the $ARGV variable. It also uses filehandle I<ARGV> -internally. <> is just a synonym for <ARGV>, which +It really does shift the C<@ARGV> array and put the current filename +into the C<$ARGV> variable. It also uses filehandle I<ARGV> +internally. C<< <> >> is just a synonym for C<< <ARGV> >>, which is magical. (The pseudo code above doesn't work because it treats -<ARGV> as non-magical.) +C<< <ARGV> >> as non-magical.) Since the null filehandle uses the two argument form of L<perlfunc/open> it interprets special characters, so if you have a script like this: @@ -3011,7 +3031,7 @@ it interprets special characters, so if you have a script like this: print; } -and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a +and call it with S<C<perl dangerous.pl 'rm -rfv *|'>>, it actually opens a pipe, executes the C<rm> command and reads C<rm>'s output from that pipe. If you want all items in C<@ARGV> to be interpreted as file names, you can use the module C<ARGV::readonly> from CPAN, or use the double bracket: @@ -3022,17 +3042,17 @@ can use the module C<ARGV::readonly> from CPAN, or use the double bracket: Using double angle brackets inside of a while causes the open to use the three argument form (with the second argument being C<< < >>), so all -arguments in ARGV are treated as literal filenames (including "-"). -(Note that for convenience, if you use C<< <<>> >> and if @ARGV is +arguments in C<ARGV> are treated as literal filenames (including C<"-">). +(Note that for convenience, if you use C<< <<>> >> and if C<@ARGV> is empty, it will still read from the standard input.) -You can modify @ARGV before the first <> as long as the array ends up +You can modify C<@ARGV> before the first C<< <> >> as long as the array ends up containing the list of filenames you really want. Line numbers (C<$.>) continue as though the input were one big happy file. See the example in L<perlfunc/eof> for how to reset line numbers on each file. -If you want to set @ARGV to your own list of files, go right ahead. -This sets @ARGV to all plain text files if no @ARGV was given: +If you want to set C<@ARGV> to your own list of files, go right ahead. +This sets C<@ARGV> to all plain text files if no C<@ARGV> was given: @ARGV = grep { -f && -T } glob('*') unless @ARGV; @@ -3042,7 +3062,7 @@ filters compressed arguments through B<gzip>: @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV; If you want to pass switches into your script, you can use one of the -Getopts modules or put a loop on the front like this: +C<Getopts> modules or put a loop on the front like this: while ($_ = $ARGV[0], /^-/) { shift; @@ -3056,12 +3076,12 @@ Getopts modules or put a loop on the front like this: # ... # code for each line } -The <> symbol will return C<undef> for end-of-file only once. +The C<< <> >> symbol will return C<undef> for end-of-file only once. If you call it again after this, it will assume you are processing another -@ARGV list, and if you haven't set @ARGV, will read input from STDIN. +C<@ARGV> list, and if you haven't set C<@ARGV>, will read input from STDIN. If what the angle brackets contain is a simple scalar variable (for example, -<$foo>), then that variable contains the name of the +C<$foo>), then that variable contains the name of the filehandle to input from, or its typeglob, or a reference to the same. For example: @@ -3073,9 +3093,9 @@ scalar variable containing a filehandle name, typeglob, or typeglob reference, it is interpreted as a filename pattern to be globbed, and either a list of filenames or the next filename in the list is returned, depending on context. This distinction is determined on syntactic -grounds alone. That means C<< <$x> >> is always a readline() from -an indirect handle, but C<< <$hash{key}> >> is always a glob(). -That's because $x is a simple scalar variable, but C<$hash{key}> is +grounds alone. That means C<< <$x> >> is always a C<readline()> from +an indirect handle, but C<< <$hash{key}> >> is always a C<glob()>. +That's because C<$x> is a simple scalar variable, but C<$hash{key}> is not--it's a hash element. Even C<< <$x > >> (note the extra space) is treated as C<glob("$x ")>, not C<readline($x)>. @@ -3100,7 +3120,7 @@ is roughly equivalent to: } except that the globbing is actually done internally using the standard -C<File::Glob> extension. Of course, the shortest way to do the above is: +C<L<File::Glob>> extension. Of course, the shortest way to do the above is: chmod 0644, <*.c>; @@ -3127,7 +3147,7 @@ because the latter will alternate between returning a filename and returning false. If you're trying to do variable interpolation, it's definitely better -to use the glob() function, because the older notation can cause people +to use the C<glob()> function, because the older notation can cause people to become confused with the indirect filehandle notation. @files = glob("$dir/*.[ch]"); @@ -3199,9 +3219,9 @@ operation you intend by using C<""> or C<0+>, as in the examples below. $biz = "$foo" ^ "$bar"; # both ops explicitly stringy This somewhat unpredictable behavior can be avoided with the experimental -"bitwise" feature, new in Perl 5.22. You can enable it via C<use feature -'bitwise'>. By default, it will warn unless the "experimental::bitwise" -warnings category has been disabled. (C<use experimental 'bitwise'> will +"bitwise" feature, new in Perl 5.22. You can enable it via S<C<use feature +'bitwise'>>. By default, it will warn unless the C<"experimental::bitwise"> +warnings category has been disabled. (S<C<use experimental 'bitwise'>> will enable the feature and disable the warning.) Under this feature, the four standard bitwise operators (C<~ | & ^>) are always numeric. Adding a dot after each operator (C<~. |. &. ^.>) forces it to treat its operands as @@ -3243,26 +3263,26 @@ the enclosing BLOCK. An inner BLOCK may countermand this by saying which lasts until the end of that BLOCK. Note that this doesn't mean everything is an integer, merely that Perl will use integer operations for arithmetic, comparison, and bitwise operators. For -example, even under C<use integer>, if you take the C<sqrt(2)>, you'll +example, even under S<C<use integer>>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731> or so. -Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<", -and ">>") always produce integral results. (But see also -L<Bitwise String Operators>.) However, C<use integer> still has meaning for +Used on numbers, the bitwise operators (C<&> C<|> C<^> C<~> C<< << >> +C<< >> >>) always produce integral results. (But see also +L<Bitwise String Operators>.) However, S<C<use integer>> still has meaning for them. By default, their results are interpreted as unsigned integers, but -if C<use integer> is in effect, their results are interpreted +if S<C<use integer>> is in effect, their results are interpreted as signed integers. For example, C<~0> usually evaluates to a large -integral value. However, C<use integer; ~0> is C<-1> on two's-complement +integral value. However, S<C<use integer; ~0>> is C<-1> on two's-complement machines. =head2 Floating-point Arithmetic X<floating-point> X<floating point> X<float> X<real> -While C<use integer> provides integer-only arithmetic, there is no +While S<C<use integer>> provides integer-only arithmetic, there is no analogous mechanism to provide automatic rounding or truncation to a certain number of decimal places. For rounding to a certain number -of digits, sprintf() or printf() is usually the easiest route. +of digits, C<sprintf()> or C<printf()> is usually the easiest route. See L<perlfaq4>. Floating-point numbers are only approximations to what a mathematician @@ -3287,10 +3307,10 @@ this topic. } The POSIX module (part of the standard perl distribution) implements -ceil(), floor(), and other mathematical and trigonometric functions. -The Math::Complex module (part of the standard perl distribution) +C<ceil()>, C<floor()>, and other mathematical and trigonometric functions. +The C<L<Math::Complex>> module (part of the standard perl distribution) defines mathematical functions that work on both the reals and the -imaginary numbers. Math::Complex not as efficient as POSIX, but +imaginary numbers. C<Math::Complex> is not as efficient as POSIX, but POSIX can't work with complex numbers. Rounding in financial applications can have serious implications, and @@ -3302,7 +3322,8 @@ need yourself. =head2 Bigger Numbers X<number, arbitrary precision> -The standard C<Math::BigInt>, C<Math::BigRat>, and C<Math::BigFloat> modules, +The standard C<L<Math::BigInt>>, C<L<Math::BigRat>>, and +C<L<Math::BigFloat>> modules, along with the C<bignum>, C<bigint>, and C<bigrat> pragmas, provide variable-precision arithmetic and overloaded operators, although they're currently pretty slow. At the cost of some space and @@ -3326,8 +3347,8 @@ Or with rationals: x/y is 9/44 x*y is 1/11 -Several modules let you calculate with (bound only by memory and CPU time) -unlimited or fixed precision. There +Several modules let you calculate with unlimited or fixed precision +(bound only by memory and CPU time). There are also some non-standard modules that provide faster implementations via external C libraries. |