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 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 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>>.
+I<Operator precedence> means some operators group more tightly than others.
+For example, in C<2 + 4 * 5>, the multiplication has higher precedence, so C<4
+* 5> is grouped together as the right-hand operand of the addition, rather
+than C<2 + 4> being grouped together as the left-hand operand of the
+multiplication. It is as if the expression were written C<2 + (4 * 5)>, not
+C<(2 + 4) * 5>. So the expression yields C<2 + 20 == 22>, rather than
+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
+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
+operand of the first subtraction. It is as if the expression were written
+C<(9 - 3) - 2>, not C<9 - (3 - 2)>. So the expression yields C<6 - 2 == 4>,
+rather than C<9 - 1 == 8>.
+
+For simple operators that evaluate all their operands and then combine the
+values in some way, precedence and associativity (and parentheses) imply some
+ordering requirements on those combining operations. For example, in C<2 + 4 *
+5>, the grouping implied by precedence means that the multiplication of 4 and
+5 must be performed before the addition of 2 and 20, simply because the result
+of that multiplication is required as one of the operands of the addition. But
+the order of operations is not fully determined by this: in C<2 * 2 + 4 * 5>
+both multiplications must be performed before the addition, but the grouping
+does not say anything about the order in which the two multiplications are
+performed. In fact Perl has a general rule that the operands of an operator
+are evaluated in left-to-right order. A few operators such as C<&&=> have
+special evaluation rules that can result in an operand not being evaluated at
+all; in general, the top-level operator in an expression has control of
+operand evaluation.
Perl operators have the following associativity and precedence,
listed from highest precedence to lowest. Operators borrowed from
left and
left or xor
-In the following sections, these operators are covered in precedence order.
+In the following sections, these operators are covered in detail, in the
+same order in which they appear in the table above.
Many operators can be overloaded for objects. See L<overload>.
print(($foo & 255) + 1, "\n");
-See L<Named Unary Operators> for more discussion of this.
+See L</Named Unary Operators> for more discussion of this.
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<{}>.
-See also L<Quote and Quote-like Operators> toward the end of this section,
+See also L</Quote and Quote-like Operators> toward the end of this section,
as well as L</"I/O Operators">.
=head2 The Arrow Operator
=head2 Symbolic Unary Operators
X<unary operator> X<operator, unary>
-Unary C<"!"> performs logical negation, that is, "not". See also C<not> for a lower
-precedence version of this.
+Unary C<"!"> performs logical negation, that is, "not". See also
+L<C<not>|/Logical Not> for a lower precedence version of this.
X<!>
Unary C<"-"> performs arithmetic negation if the operand is numeric,
X<-> X<negation, arithmetic>
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
+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: 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 C<"&"> operator to mask off the excess bits.
X<~> X<negation, binary>
-When complementing strings, if all characters have ordinal values under
-256, then their complements will, also. But if they do not, all
-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.
+Starting in Perl 5.28, it is a fatal error to try to complement a string
+containing a character with an ordinal value above 255.
-If the experimental "bitwise" feature is enabled via S<C<use feature
-'bitwise'>>, then unary C<"~"> always treats its argument as a number, and an
+If the "bitwise" feature is enabled via S<C<use
+feature 'bitwise'>> or C<use v5.28>, 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 S<C<no warnings 'experimental::bitwise'>>.
+whereas C<~.0> and C<~."0"> will both yield C<"\xff">. Until Perl 5.28,
+this feature produced a warning in the C<"experimental::bitwise"> category.
Unary C<"+"> has no effect whatsoever, even on strings. It is useful
syntactically for separating a function name from a parenthesized expression
that would otherwise be interpreted as the complete list of function
-arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
+arguments. (See examples above under L</Terms and List Operators (Leftward)>.)
X<+>
-Unary C<"\"> creates a reference to whatever follows it. See L<perlreftut>
+Unary C<"\"> creates references. If its operand is a single sigilled
+thing, it creates a reference to that object. If its operand is a
+parenthesised list, then it creates references to the things mentioned
+in the list. Otherwise it puts its operand in list context, and creates
+a list of references to the scalars in the list provided by the operand.
+See L<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.
execute faster.
X<%> X<remainder> X<modulo> X<mod>
-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/I<STRING>/>, it repeats the list.
+Binary C<x> is the repetition operator. In scalar context, or if the
+left operand is neither enclosed in parentheses nor a C<qw//> list,
+it performs a string repetition. In that case it supplies scalar
+context to the left operand, and returns a string consisting of the
+left operand string repeated the number of times specified by the right
+operand. If the C<x> is in list context, and the left operand is either
+enclosed in parentheses or a C<qw//> list, it performs a list repetition.
+In that case it supplies list context to the left operand, and returns
+a list consisting of the left operand list repeated the number of times
+specified by the right operand.
If the right operand is zero or negative (raising a warning on
negative), it returns an empty string
or an empty list, depending on the context.
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>.)
+integers. (See also L</Integer Arithmetic>.)
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>.)
+be integers. (See also L</Integer Arithmetic>.)
-If S<C<use integer>> (see L<Integer Arithmetic>) is in force then
+If S<C<use integer>> (see L</Integer Arithmetic>) is in force then
signed C integers are used (I<arithmetic shift>), otherwise unsigned C
integers are used (I<logical shift>), even for negative shiftees.
In arithmetic right shift the sign bit is replicated on the left,
the S<C<use bigint>> pragma neatly sidesteps the issue altogether:
print 20 << 20; # 20971520
- print 20 << 40; # 5120 on 32-bit machines,
+ print 20 << 40; # 5120 on 32-bit machines,
# 21990232555520 on 64-bit machines
use bigint;
print 20 << 100; # 25353012004564588029934064107520
equivalent to S<C<-f "$file.bak">>.
X<-X> X<filetest> X<operator, filetest>
-See also L<"Terms and List Operators (Leftward)">.
+See also L</"Terms and List Operators (Leftward)">.
=head2 Relational Operators
X<relational operator> X<operator, relational>
-Perl operators that return true or false generally return values
+Perl operators that return true or false generally return values
that can be safely used as numbers. For example, the relational
operators in this section and the equality operators in the next
one return C<1> for true and a special version of the defined empty
C<L<Unicode::Collate::Locale>> modules offer much more powerful
solutions to collation issues.
-For case-insensitive comparis
ions, look at the L<perlfunc/fc> case-folding
+For case-insensitive comparisons, look at the L<perlfunc/fc> case-folding
function, available in Perl v5.16 or later:
if ( fc($x) eq fc($y) ) { ... }
actually happens is mostly determined by the type of the second operand,
the table is sorted on the right operand instead of on the left.
- Left Right Description and pseudocode
+ Left Right Description and pseudocode
===============================================================
- Any undef check whether Any is undefined
+ Any undef check whether Any is undefined
like: !defined Any
Any Object invoke ~~ overloading on Object, or die
Right operand is an ARRAY:
- Left Right Description and pseudocode
+ Left Right Description and pseudocode
===============================================================
ARRAY1 ARRAY2 recurse on paired elements of ARRAY1 and ARRAY2[2]
like: (ARRAY1[0] ~~ ARRAY2[0])
&& (ARRAY1[1] ~~ ARRAY2[1]) && ...
- HASH ARRAY any ARRAY elements exist as HASH keys
+ HASH ARRAY any ARRAY elements exist as HASH keys
like: grep { exists HASH->{$_} } ARRAY
Regexp ARRAY any ARRAY elements pattern match Regexp
like: grep { /Regexp/ } ARRAY
- undef ARRAY undef in ARRAY
+ undef ARRAY undef in ARRAY
like: grep { !defined } ARRAY
- Any ARRAY smartmatch each ARRAY element[3]
+ Any ARRAY smartmatch each ARRAY element[3]
like: grep { Any ~~ $_ } ARRAY
Right operand is a HASH:
- Left Right Description and pseudocode
+ Left Right Description and pseudocode
===============================================================
- HASH1 HASH2 all same keys in both HASHes
+ HASH1 HASH2 all same keys in both HASHes
like: keys HASH1 ==
grep { exists HASH2->{$_} } keys HASH1
- ARRAY HASH any ARRAY elements exist as HASH keys
+ ARRAY HASH any ARRAY elements exist as HASH keys
like: grep { exists HASH->{$_} } ARRAY
- Regexp HASH any HASH keys pattern match Regexp
+ Regexp HASH any HASH keys pattern match Regexp
like: grep { /Regexp/ } keys HASH
- undef HASH always false (undef can't be a key)
+ undef HASH always false (undef can't be a key)
like: 0 == 1
- Any HASH HASH key existence
+ Any HASH HASH key existence
like: exists HASH->{Any}
Right operand is CODE:
- Left Right Description and pseudocode
+ Left Right Description and pseudocode
===============================================================
ARRAY CODE sub returns true on all ARRAY elements[1]
like: !grep { !CODE->($_) } ARRAY
HASH CODE sub returns true on all HASH keys[1]
like: !grep { !CODE->($_) } keys HASH
- Any CODE sub passed Any returns true
+ Any CODE sub passed Any returns true
like: CODE->(Any)
Right operand is a Regexp:
- Left Right Description and pseudocode
+ Left Right Description and pseudocode
===============================================================
- ARRAY Regexp any ARRAY elements match Regexp
+ ARRAY Regexp any ARRAY elements match Regexp
like: grep { /Regexp/ } ARRAY
- HASH Regexp any HASH keys match Regexp
+ HASH Regexp any HASH keys match Regexp
like: grep { /Regexp/ } keys HASH
- Any Regexp pattern match
+ Any Regexp pattern match
like: Any =~ /Regexp/
Other:
- Left Right Description and pseudocode
+ Left Right Description and pseudocode
===============================================================
Object Any invoke ~~ overloading on Object,
or fall back to...
- Any Num numeric equality
+ Any Num numeric equality
like: Any == Num
Num nummy[4] numeric equality
like: Num == nummy
undef Any check whether undefined
like: !defined(Any)
- Any Any string equality
+ Any Any string equality
like: Any eq Any
=over
=item 1.
-Empty hashes or arrays match.
+Empty hashes or arrays match.
=item 2.
That is, each element smartmatches the element of the same index in the other array.[3]
=item 3.
-If a circular reference is found, fall back to referential equality.
+If a circular reference is found, fall back to referential equality.
=item 4.
Either an actual number, or a string that looks like one.
my @bigger = ("red", "blue", [ "orange", "green" ] );
if (@little ~~ @bigger) { # true!
say "little is contained in bigger";
- }
+ }
Because the smartmatch operator recurses on nested arrays, this
will still report that "red" is in the array.
copies of each others' values, as this example reports:
use v5.12.0;
- my @a = (0, 1, 2, [3, [4, 5], 6], 7);
- my @b = (0, 1, 2, [3, [4, 5], 6], 7);
+ my @a = (0, 1, 2, [3, [4, 5], 6], 7);
+ my @b = (0, 1, 2, [3, [4, 5], 6], 7);
if (@a ~~ @b && @b ~~ @a) {
say "a and b are deep copies of each other";
- }
+ }
elsif (@a ~~ @b) {
say "a smartmatches in b";
- }
+ }
elsif (@b ~~ @a) {
say "b smartmatches in a";
- }
+ }
else {
say "a and b don't smartmatch each other at all";
- }
+ }
If you were to set S<C<$b[3] = 4>>, then instead of reporting that "a and b
...
}
-or, if other non-required fields are allowed, use ARRAY ~~ HASH:
-
- use v5.10.1;
- sub make_dogtag {
- state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
-
- my ($class, $init_fields) = @_;
-
- die "Must supply (at least) name, rank, and serial number"
- unless [keys %{$init_fields}] ~~ $REQUIRED_FIELDS;
-
- ...
- }
+However, this only does what you mean if C<$init_fields> is indeed a hash
+reference. The condition C<$init_fields ~~ $REQUIRED_FIELDS> also allows the
+strings C<"name">, C<"rank">, C<"serial_num"> as well as any array reference
+that contains C<"name"> or C<"rank"> or C<"serial_num"> anywhere to pass
+through.
The smartmatch operator is most often used as the implicit operator of a
C<when> clause. See the section on "Switch Statements" in L<perlsyn>.
numbers, "in" becomes equivalent to this:
$object ~~ $number ref($object) == $number
- $object ~~ $string ref($object) eq $string
+ $object ~~ $string ref($object) eq $string
For example, this reports that the handle smells IOish
(but please don't really do this!):
my $fh = IO::Handle->new();
if ($fh ~~ /\bIO\b/) {
say "handle smells IOish";
- }
+ }
That's because it treats C<$fh> as a string like
C<"IO::Handle=GLOB(0x8039e0)">, then pattern matches against that.
Binary C<"&"> returns its operands ANDed together bit by bit. Although no
warning is currently raised, the result is not well defined when this operation
is performed on operands that aren't either numbers (see
-L<Integer Arithmetic>) nor bitstrings (see L<Bitwise String Operators>).
+L</Integer Arithmetic>) nor bitstrings (see L</Bitwise String Operators>).
Note that C<"&"> has lower priority than relational operators, so for example
the parentheses are essential in a test like
print "Even\n" if ($x & 1) == 0;
-If the experimental "bitwise" feature is enabled via 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'>>.
+If the "bitwise" feature is enabled via S<C<use feature 'bitwise'>> or
+C<use v5.28>, then this operator always treats its operands as numbers.
+Before Perl 5.28 this feature produced a warning in the
+C<"experimental::bitwise"> category.
=head2 Bitwise Or and Exclusive Or
X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
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>) nor bitstrings (see L<Bitwise String
+numbers (see L</Integer Arithmetic>) nor bitstrings (see L</Bitwise String
Operators>).
Note that C<"|"> and C<"^"> have lower priority than relational operators, so
print "false\n" if (8 | 2) != 10;
-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'>>.
+If the "bitwise" feature is enabled via S<C<use feature 'bitwise'>> or
+C<use v5.28>, then this operator always treats its operands as numbers.
+Before Perl 5.28. this feature produced a warning in the
+C<"experimental::bitwise"> category.
=head2 C-style Logical And
X<&&> X<logical and> X<operator, logical, and>
unless(unlink("alpha", "beta", "gamma")) {
gripe();
next LINE;
- }
+ }
Using C<"or"> for assignment is unlikely to do what you want; see below.
increment would produce, the sequence goes until the next value would
be longer than the final value specified.
+As of Perl 5.26, the list-context range operator on strings works as expected
+in the scope of L<< S<C<"use feature 'unicode_strings">>|feature/The
+'unicode_strings' feature >>. In previous versions, and outside the scope of
+that feature, it exhibits L<perlunicode/The "Unicode Bug">: its behavior
+depends on the internal encoding of the range endpoint.
+
If the initial value specified isn't part of a magical increment
sequence (that is, a non-empty string matching C</^[a-zA-Z]*[0-9]*\z/>),
only the initial value will be returned. So the following will only
you could use this instead:
use charnames "greek";
- my @greek_small = map { chr } ( ord("\N{alpha}")
+ my @greek_small = map { chr } ( ord("\N{alpha}")
..
- ord("\N{omega}")
+ ord("\N{omega}")
);
However, because there are I<many> other lowercase Greek characters than
side of the assignment.
The three dotted bitwise assignment operators (C<&.=> C<|.=> C<^.=>) are new in
-Perl 5.22 and experimental. See L</Bitwise String Operators>.
+Perl 5.22. See L</Bitwise String Operators>.
=head2 Comma Operator
X<comma> X<operator, comma> X<,>
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";
+ open HANDLE, "< :encoding(UTF-8)", "filename"
+ or die "Can't open: $!\n";
However, some people find that code harder to read than writing
it with parentheses:
- open(HANDLE, "< :utf8", "filename") or die "Can't open: $!\n";
+ open(HANDLE, "< :encoding(UTF-8)", "filename")
+ or die "Can't open: $!\n";
in which case you might as well just use the more customary C<"||"> operator:
- open(HANDLE, "< :utf8", "filename") || die "Can't open: $!\n";
+ open(HANDLE, "< :encoding(UTF-8)", "filename")
+ || die "Can't open: $!\n";
-See also discussion of list operators in L<Terms and List Operators (Leftward)>.
+See also discussion of list operators in L</Terms and List Operators (Leftward)>.
=head2 Logical Not
X<operator, logical, not> X<not>
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
+There can (and in some cases, must) be whitespace between the operator
+and the quoting
characters, except when C<#> is being used as the quoting character.
C<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
s {foo} # Replace foo
{bar} # with bar.
+The cases where whitespace must be used are when the quoting character
+is a word character (meaning it matches C</\w/>):
+
+ q XfooX # Works: means the string 'foo'
+ qXfooX # WRONG!
+
The following escape sequences are available in constructs that interpolate,
-and in transliterations:
+and in transliterations whose delimiters aren't single quotes (C<"'">).
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{}>
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
+An exception to the above rule is that S<C<\N{U+I<hex number>}>> is
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.
+on EBCDIC platforms.
=back
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/I<PATTERN>/>. If C<"'"> is used as the delimiter, no interpolation
-is done. Returns a Perl value which may be used instead of the
+in C<m/I<PATTERN>/>. If C<"'"> is used as the delimiter, no variable
+interpolation is done. Returns a Perl value which may be used instead of the
corresponding C</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
+however, dereferencing it is not well defined (you currently get the
normalized version of the original pattern, but this may change).
m Treat string as multiple lines.
s Treat string as single line. (Make . match a newline)
i Do case-insensitive pattern matching.
- x Use extended regular expressions.
+ x Use extended regular expressions; specifying two
+ x's means \t and the SPACE character are ignored within
+ square-bracketed character classes
p When matching preserve a copy of the matched string so
that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be
defined (ignored starting in v5.20) as these are always
- defined starting in that relese
+ defined starting in that release
o Compile pattern only once.
- a ASCII-restrict: Use ASCII for \d, \s, \w; specifying two
- a's further restricts things to that that no ASCII
- character will match a non-ASCII one under /i.
+ a ASCII-restrict: Use ASCII for \d, \s, \w and [[:posix:]]
+ character classes; specifying two a's adds the further
+ restriction that no ASCII character will match a
+ non-ASCII one under /i.
l Use the current run-time locale's rules.
u Use Unicode rules.
d Use Unicode or native charset, as in 5.12 and earlier.
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,
+The C</a>, C</d>, C</l>, and C</u> modifiers (added in Perl 5.14)
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.
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?I<PATTERN>?> below. If C<"'"> (single quote) is the delimiter,
-no interpolation is performed on the I<PATTERN>.
+no
variable interpolation is performed on the I<PATTERN>.
When using a delimiter character valid in an identifier, whitespace is required
after the C<m>.
list consisting of the subexpressions matched by the parentheses in the
pattern, that is, (C<$1>, C<$2>, C<$3>...) (Note that here C<$1> etc. are
also set). When there are no parentheses in the pattern, the return
-value is the list C<(1)> for success.
+value is the list C<(1)> for success.
With or without parentheses, an empty list is returned upon failure.
Examples:
Notice that the final match matched C<q> instead of C<p>, which a match
without the C<\G> anchor would have done. Also note that the final match
did not update C<pos>. C<pos> is only updated on a C</g> match. If the
-final match did indeed match C<p>, it's a good bet that you're running a
-very old (pre-5.6.0) version of Perl.
+final match did indeed match C<p>, it's a good bet that you're running an
+ancient (pre-5.6.0) version of Perl.
A useful idiom for C<lex>-like scanners is C</\G.../gc>. You can
combine several regexps like this to process a string part-by-part,
=item C<m?I<PATTERN>?msixpodualngc>
X<?> X<operator, match-once>
-=item C<?I<PATTERN>?msixpodualngc>
-
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
C<m>.
=item C<s/I<PATTERN>/I<REPLACEMENT>/msixpodualngcer>
-X<substitute> X<substitution> X<replace> X<regexp, replace>
+X<s> 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>
Searches a string for a pattern, and if found, replaces that pattern
with the replacement text and returns the number of substitutions
-made. Otherwise it returns false (specifically, the empty string).
+made. Otherwise it returns false (a value that is both an empty string (C<"">)
+and numeric zero (C<0>) as described in L</Relational Operators>).
If the C</r> (non-destructive) option is used then it runs the
substitution on a copy of the string and instead of returning the
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
+If the delimiter chosen is a single quote, no variable interpolation is
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
capable of dealing with multiline commands, so putting newlines in
the string may not get you what you want. You may be able to evaluate
multiple commands in a single line by separating them with the command
-separator character, if your shell supports that (for example, C<;> on
+separator character, if your shell supports that (for example, C<;> on
many Unix shells and C<&> on the Windows NT C<cmd> shell).
Perl will attempt to flush all files opened for
a glue language, and one of the things it glues together is commands.
Just understand what you're getting yourself into.
+Like C<system>, backticks put the child process exit code in C<$?>.
+If you'd like to manually inspect failure, you can check all possible
+failure modes by inspecting C<$?> like this:
+
+ if ($? == -1) {
+ print "failed to execute: $!\n";
+ }
+ elsif ($? & 127) {
+ printf "child died with signal %d, %s coredump\n",
+ ($? & 127), ($? & 128) ? 'with' : 'without';
+ }
+ else {
+ printf "child exited with value %d\n", $? >> 8;
+ }
+
+Use the L<open> pragma to control the I/O layers used when reading the
+output of the command, for example:
+
+ use open IN => ":encoding(UTF-8)";
+ my $x = `cmd-producing-utf-8`;
+
See L</"I/O Operators"> for more discussion.
=item C<qw/I<STRING>/>
split(" ", q/STRING/);
-the differences being that it generates a real list at compile time, and
+the differences being that it only splits on ASCII whitespace,
+generates a real list at compile time, and
in scalar context it returns the last element in the list. So
this expression:
scalar variable, an array element, a hash element, or an assignment to one
of those; in other words, an lvalue.
-A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
-does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>.
-For B<sed> devotees, C<y> is provided as a synonym for C<tr>. If the
-I<SEARCHLIST> is delimited by bracketing quotes, the I<REPLACEMENTLIST> has
-its own pair of quotes, which may or may not be bracketing quotes;
-for example, C<tr[aeiouy][yuoiea]> or C<tr(+\-*/)/ABCD/>.
+If the characters delimiting I<SEARCHLIST> and I<REPLACEMENTLIST>
+are single quotes (C<tr'I<SEARCHLIST>'I<REPLACEMENTLIST>'>), the only
+interpolation is removal of C<\> from pairs of C<\\>.
-Characters may be literals or any of the escape sequences accepted in
-double-quoted strings. But there is no interpolation, so C<"$"> 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>.
+Otherwise, 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 I<SEARCHLIST> is delimited by bracketing quotes, the
+I<REPLACEMENTLIST> must have 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 (if the delimiters aren't single quotes)
+any of the escape sequences accepted in double-quoted strings. But
+there is never any variable 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>.
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 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
-C<\l> string-interpolation escapes on the right side of a substitution
-operator will perform correct case-mappings, but C<tr[a-z][A-Z]> will not
-(except sometimes on legacy 7-bit data).
+utility. C<tr[a-z][A-Z]> will uppercase the 26 letters "a" through "z",
+but for case changing not confined to ASCII, use
+L<C<lc>|perlfunc/lc>, L<C<uc>|perlfunc/uc>,
+L<C<lcfirst>|perlfunc/lcfirst>, L<C<ucfirst>|perlfunc/ucfirst>
+(all documented in L<perlfunc>), or the
+L<substitution operator C<sE<sol>I<PATTERN>E<sol>I<REPLACEMENT>E<sol>>|/sE<sol>PATTERNE<sol>REPLACEMENTE<sol>msixpodualngcer>
+(with C<\U>, C<\u>, C<\L>, and C<\l> string-interpolation escapes in the
+I<REPLACEMENT> portion).
Most ranges are unportable between character sets, but certain ones
signal Perl to do special handling to make them portable. There are two
But, even for portable ranges, it is not generally obvious what is
included without having to look things up. A sound principle is to use
-only ranges that begin from and end at either ASCII alphabetics of equal
-case (C<b-e>, C<b-E>), or digits (C<1-4>). Anything else is unclear
-(and unportable unless C<\N{...}> is used). If in doubt, spell out the
-character sets in full.
+only ranges that both begin from and end at either ASCII alphabetics of
+equal case (C<b-e>, C<B-E>), or digits (C<1-4>). Anything else is
+unclear (and unportable unless C<\N{...}> is used). If in doubt, spell
+out the character sets in full.
Options:
untouched.
If the C</c> modifier is specified, the I<SEARCHLIST> character set
-is complemented. If the C</d> modifier is specified, any characters
+is complemented. So for example these two are equivalent (the exact
+maximum number will depend on your platform):
+
+ tr/\x00-\xfd/ABCD/c
+ tr/\xfe-\x{7fffffff}/ABCD/
+
+If the C</d> modifier is specified, any characters
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 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.
+period.)
+
+If the C</s> modifier is specified, runs of the same character in the
+result, where each those characters were substituted by the
+transliteration, are squashed down to a single instance of the character.
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.
+squashing character sequences in a class. For example, each of these pairs
+are equivalent:
-Examples:
+ tr/abcd// tr/abcd/abcd/
+ tr/abcd/AB/ tr/abcd/ABBB/
+ tr/abcd//d s/[abcd]//g
+ tr/abcd/AB/d (tr/ab/AB/ + s/[cd]//g) - but run together
+
+Some examples:
$ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case ASCII
the quoted material, and all lines following the current line down to
the terminating string are the value of the item.
+Prefixing the terminating string with a C<~> specifies that you
+want to use L</Indented Here-docs> (see below).
+
The terminating string may be either an identifier (a word), or some
quoted text. An unquoted identifier works like double quotes.
There may not be a space between the C<< << >> and the identifier,
-unless the identifier is explicitly quoted. (If you put a space it
-will be treated as a null identifier, which is valid, and matches the
-first empty line.) The terminating string must appear by itself
-(unquoted and with no surrounding whitespace) on the terminating line.
+unless the identifier is explicitly quoted. The terminating string
+must appear by itself (unquoted and with no surrounding whitespace)
+on the terminating line.
If the terminating string is quoted, the type of quotes used determine
the treatment of the text.
=back
+=over 4
+
+=item Indented Here-docs
+
+The here-doc modifier C<~> allows you to indent your here-docs to make
+the code more readable:
+
+ if ($some_var) {
+ print <<~EOF;
+ This is a here-doc
+ EOF
+ }
+
+This will print...
+
+ This is a here-doc
+
+...with no leading whitespace.
+
+The delimiter is used to determine the B<exact> whitespace to
+remove from the beginning of each line. All lines B<must> have
+at least the same starting whitespace (except lines only
+containing a newline) or perl will croak. Tabs and spaces can
+be mixed, but are matched exactly. One tab will not be equal to
+8 spaces!
+
+Additional beginning whitespace (beyond what preceded the
+delimiter) will be preserved:
+
+ print <<~EOF;
+ This text is not indented
+ This text is indented with two spaces
+ This text is indented with two tabs
+ EOF
+
+Finally, the modifier may be used with all of the forms
+mentioned above:
+
+ <<~\EOF;
+ <<~'EOF'
+ <<~"EOF"
+ <<~`EOF`
+
+And whitespace may be used between the C<~> and quoted delimiters:
+
+ <<~ 'EOF'; # ... "EOF", `EOF`
+
+=back
+
It is possible to stack multiple here-docs in a row:
print <<"foo", <<"bar"; # you can stack them
END
If you want your here-docs to be indented with the rest of the code,
-you'll need to remove leading whitespace from each line manually:
+use the C<<< <<~FOO >>> construct described under L</Indented Here-docs>:
- ($quote = <<'FINIS') =~ s/^\s+//gm;
+ $quote = <<~'FINIS';
The Road goes ever on and on,
down from the door where it began.
- FINIS
+ FINIS
If you use a here-doc within a delimited construct, such as in C<s///eg>,
the quoted material must still come on the line following the
C<[]>, C<{}>, or C<< <> >>), the right part needs another pair of
delimiters such as C<s(){}> and C<tr[]//>. In these cases, whitespace
and comments are allowed between the two parts, although the comment must follow
-at least one whitespace character; otherwise a character expected as the
+at least one whitespace character; otherwise a character expected as the
start of the comment may be regarded as the starting delimiter of the right part.
During this search no attention is paid to the semantics of the construct.
is emitted if the S<C<use warnings>> pragma or the B<-w> command-line flag
(that is, the C<$^W> variable) was set.
-=item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
+=item C<RE> in C<m?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F>, C<\E>,
and interpolation happens (almost) as with C<qq//> constructs.
the previous step, and C<\\/> will be left as is. Because C</> is
equivalent to C<\/> inside a regular expression, this does not
matter unless the delimiter happens to be character special to the
-RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
+RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<m?foo?>; or an
alphanumeric char, as in:
m m ^ a \s* b mmx;
odd thing to you, but you'll use the construct in almost every Perl
script you write.) The C<$_> variable is not implicitly localized.
You'll have to put a S<C<local $_;>> before the loop if you want that
-to happen.
+to happen. Furthermore, if the input symbol or an explicit assignment
+of the input symbol to a scalar is used as a C<while>/C<for> condition,
+then the condition actually tests for definedness of the expression's
+value, not for its regular truth value.
-The following lines are equivalent:
+Thus the following lines are equivalent:
while (defined($_ = <STDIN>)) { print; }
while ($_ = <STDIN>) { print; }
print while ($_ = <STDIN>);
print while <STDIN>;
-This also behaves similarly, but assigns to a lexical variable
+This also behaves similarly, but assigns to a lexical variable
instead of to C<$_>:
while (my $line = <STDIN>) { print $line }
@files = glob("$dir/*.[ch]");
@files = glob($files[$i]);
+If an angle-bracket-based globbing expression is used as the condition of
+a C<while> or C<for> loop, then it will be implicitly assigned to C<$_>.
+If either a globbing expression or an explicit assignment of a globbing
+expression to a scalar is used as a C<while>/C<for> condition, then
+the condition actually tests for definedness of the expression's value,
+not for its regular truth value.
+
=head2 Constant Folding
X<constant folding> X<folding>
compile time. You can say
'Now is the time for all'
- . "\n"
+ . "\n"
. 'good men to come to.'
and this all reduces to one string internally. Likewise, if
$baz = 0+$foo & 0+$bar; # both ops explicitly numeric
$biz = "$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 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
+This somewhat unpredictable behavior can be avoided with the "bitwise"
+feature, new in Perl 5.22. You can enable it via S<C<use feature
+'bitwise'>> or C<use v5.28>. Before Perl 5.28, it used to emit a warning
+in the C<"experimental::bitwise"> category. 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
strings:
- use experimental "bitwise";
+ use feature "bitwise";
$foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
$foo = '150' | 105; # yields 255
$foo = 150 | '105'; # yields 255
The assignment variants of these operators (C<&= |= ^= &.= |.= ^.=>)
behave likewise under the feature.
-The behavior of these operators is problematic (and subject to change)
-if either or both of the strings are encoded in UTF-8 (see
-L<perlunicode/Byte and Character Semantics>.
+It is a fatal error if an operand contains a character whose ordinal
+value is above 0xFF, and hence not expressible except in UTF-8. The
+operation is performed on a non-UTF-8 copy for other operands encoded in
+UTF-8. See L<perlunicode/Byte and Character Semantics>.
See L<perlfunc/vec> for information on how to manipulate individual bits
in a bit vector.
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
+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 S<C<use integer>> is in effect, their results are interpreted
as signed integers. For example, C<~0> usually evaluates to a large
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