C<6 * 5 == 30>.
I<Operator associativity> defines what happens if a sequence of the same
-operators is used one after another: whether they will be grouped at the left
+operators is used one after another:
+usually that they will be grouped at the left
or the right. For example, in C<9 - 3 - 2>, subtraction is left associative,
so C<9 - 3> is grouped together as the left-hand operand of the second
subtraction, rather than C<3 - 2> being grouped together as the right-hand
all; in general, the top-level operator in an expression has control of
operand evaluation.
+Some comparison operators, as their associativity, I<chain> 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<C<"$x E<lt> $y E<lt>= $z">> behaves exactly like S<C<"$x E<lt>
+$y && $y E<lt>= $z">>, assuming that C<"$y"> 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 < expensive_sub() <= $z) { ...
+
+is not entirely like
+
+ if($x < expensive_sub() && expensive_sub() <= $z) { ...
+
+but instead closer to
+
+ my $tmp = expensive_sub();
+ if($x < $tmp && $tmp <= $z) { ...
+
+in that the subroutine is only called once. However, it's not exactly
+like this latter code either, because the chained comparison doesn't
+actually involve any temporary variable (named or otherwise): there is
+no assignment. This doesn't make much difference where the expression
+is a call to an ordinary subroutine, but matters more with an lvalue
+subroutine, or if the argument expression yields some unusual kind of
+scalar by other means. For example, if the argument expression yields
+a tied scalar, then the expression is evaluated to produce that scalar
+at most once, but the value of that scalar may be fetched up to twice,
+once for each comparison in which it is actually used.
+
+In this example, the expression is evaluated only once, and the tied
+scalar (the result of the expression) is fetched for each comparison that
+uses it.
+
+ if ($x < $tied_scalar < $z) { ...
+
+In the next example, the expression is evaluated only once, and the tied
+scalar is fetched once as part of the operation within the expression.
+The result of that operation is fetched for each comparison, which
+normally doesn't matter unless that expression result is also magical due
+to operator overloading.
+
+ if ($x < $tied_scalar + 42 < $z) { ...
+
+Some operators are instead non-associative, meaning that it is a syntax
+error to use a sequence of those operators of the same precedence.
+For example, S<C<"$x .. $y .. $z">> is an error.
+
Perl operators have the following associativity and precedence,
listed from highest precedence to lowest. Operators borrowed from
C keep the same precedence relationship with each other, even where
left ->
nonassoc ++ --
right **
- right ! ~ \ and unary + and -
+ right ! ~ ~. \ and unary + and -
left =~ !~
left * / % x
left + - .
left << >>
nonassoc named unary operators
- nonassoc < > <= >= lt gt le ge
- nonassoc == != <=> eq ne cmp ~~
- left &
- left | ^
+ chained < > <= >= lt gt le ge
+ chain/na == != eq ne <=> cmp ~~
+ nonassoc isa
+ left & &.
+ left | |. ^ ^.
left &&
left || //
nonassoc .. ...
than or equal to the right argument.
X<< ge >>
+A sequence of relational operators, such as S<C<"$x E<lt> $y E<lt>=
+$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<equality> X<equal> X<equals> X<operator, equality>
to the right argument.
X<!=>
+Binary C<"eq"> returns true if the left argument is stringwise equal to
+the right argument.
+X<eq>
+
+Binary C<"ne"> returns true if the left argument is stringwise not equal
+to the right argument.
+X<ne>
+
+A sequence of the above equality operators, such as S<C<"$x == $y ==
+$z">>, 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<NaN>'s (not-a-numbers) as numeric
(Note that the L<bigint>, L<bigrat>, and L<bignum> pragmas all
support C<"NaN">.)
-Binary C<"eq"> returns true if the left argument is stringwise equal to
-the right argument.
-X<eq>
-
-Binary C<"ne"> returns true if the left argument is stringwise not equal
-to the right argument.
-X<ne>
-
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.
is described in the next section.
X<~~>
+The two-sided ordering operators C<"E<lt>=E<gt>"> and C<"cmp">, 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<"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>.
if ( fc($x) eq fc($y) ) { ... }
+=head2 Class Instance Operator
+X<isa operator>
+
+Binary C<isa> 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 $name_of_class ) { ... }
+
+This is an experimental feature and is available from Perl 5.31.6 when enabled
+by C<use feature 'isa'>. It emits a warning in the C<experimental::isa>
+category.
+
=head2 Smartmatch Operator
First available in Perl 5.10.1 (the 5.10.0 version behaved differently),
@numbers = ( 0+$first .. 0+$last );
+B<Note:> In Perl versions 5.30 and below, I<any> string on the left-hand
+side beginning with C<"0">, including the string C<"0"> itself, would
+cause the magic string increment behavior. This means that on these Perl
+versions, C<"0".."-1"> would produce C<"0"> through C<"99">, which was
+inconsistent with C<0..-1>, which produces the empty list. This also means
+that C<"0".."9"> 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
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<\t> X<\n> X<\r> X<\f> X<\b> X<\a> X<\e> X<\x> X<\0> X<\c> X<\N> X<\N{}>
X<\o{}>
\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)
\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<C<\x{ }>> 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.
-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.
+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<\x{00}>). However, an explicit empty brace (C<\x{}>)
The result is the character specified by the octal number between the braces.
See L</[8]> below for details on which character.
-If a character that isn't an octal digit is encountered, a warning is raised,
-and the value is based on the octal digits before it, discarding it and all
-following characters up to the closing brace. It is a fatal error if there are
-no octal digits at all.
+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]
# expand tabs to 8-column spacing
1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
+X</c>While C<s///> accepts the C</c> flag, it has no effect beyond
+producing a warning if warnings are enabled.
+
=back
=head2 Quote-Like Operators
=item C<`I<STRING>`>
A string which is (possibly) interpolated and then executed as a
-system command with F</bin/sh> or its equivalent. Shell wildcards,
-pipes, and redirections will be honored. The collected standard
-output of the command is returned; standard error is unaffected. In
-scalar context, it comes back as a single (potentially multi-line)
-string, or C<undef> if the command failed. In list context, returns a
+system command, via F</bin/sh> or its equivalent if required. Shell
+wildcards, pipes, and redirections will be honored. Similarly to
+C<system>, if the string contains no shell metacharacters then it will
+executed directly. The collected standard output of the command is
+returned; standard error is unaffected. In scalar context, it comes
+back as a single (potentially multi-line) string, or C<undef> if the
+shell (or command) could not be started. In list context, returns a
list of lines (however you've defined lines with C<$/> or
-C<$INPUT_RECORD_SEPARATOR>), or an empty list if the command failed.
+C<$INPUT_RECORD_SEPARATOR>), or an empty list if the shell (or command)
+could not be started.
Because backticks do not affect standard error, use shell file descriptor
syntax (assuming the shell supports this) if you care to address this.
scalar variable, an array element, a hash element, or an assignment to one
of those; in other words, an lvalue.
-If the characters delimiting I<SEARCHLIST> and I<REPLACEMENTLIST>
+The characters delimitting I<SEARCHLIST> and I<REPLACEMENTLIST>
+can be any printable character, not just forward slashes. If they
are single quotes (C<tr'I<SEARCHLIST>'I<REPLACEMENTLIST>'>), the only
interpolation is removal of C<\> from pairs of C<\\>.
row, that were transliterated to the same character are squashed down to
a single instance of that character.
- my $a = "aaaba"
- $a =~ tr/a/a/s # $a now is "aba"
+ my $a = "aaabbbca";
+ $a =~ tr/ab/dd/s; # $a now is "dcd"
If the C</d> modifier is used, the I<REPLACEMENTLIST> is always interpreted
exactly as specified. Otherwise, if the I<REPLACEMENTLIST> is shorter
C<< <I<FILEHANDLE>> >> may also be spelled C<readline(*I<FILEHANDLE>)>.
See L<perlfunc/readline>.
-The null filehandle C<< <> >> is special: it can be used to emulate the
+The null filehandle C<< <> >> (sometimes called the diamond operator) 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 C<< <> >> comes either from
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:
+can use the module C<ARGV::readonly> from CPAN, or use the double
+diamond bracket:
while (<<>>) {
print;
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