=head1 DESCRIPTION
+In Perl, the operator determines what operation is performed,
+independent of the type of the operands. For example C<$a + $b>
+is always a numeric addition, and if C<$a> or C<$b> do not contain
+numbers, an attempt is made to convert them to numbers first.
+
+This is in contrast to many other dynamic languages, where the
+operation is determined by the type of the first argument. It also
+means that Perl has two versions of some operators, one for numeric
+and one for string comparison. For example C<$a == $b> compares
+two numbers for equality, and C<$a eq $b> 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
+operand, and C<&>, C<|> and C<^> can be either string or numeric bit
+operations.
+
=head2 Operator Precedence and Associativity
X<operator, precedence> X<precedence> X<associativity>
left || //
nonassoc .. ...
right ?:
- right = += -= *= etc.
+ right = += -= *= etc. goto last next redo dump
left , =>
nonassoc list operators (rightward)
right not
and the left side must be either an object (a blessed reference)
or a class name (that is, a package name). See L<perlobj>.
+The dereferencing cases (as opposed to method-calling cases) are
+somewhat extended by the experimental C<postderef> feature. For the
+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<-->
Note that just as in C, Perl doesn't define B<when> the variable is
incremented or decremented. You just know it will be done sometime
before or after the value is returned. This also means that modifying
-a variable twice in the same statement will lead to undefined behaviour.
+a variable twice in the same statement will lead to undefined behavior.
Avoid statements like:
$i = $i ++;
C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
character within its range, with carry:
- print ++($foo = '99'); # prints '100'
- print ++($foo = 'a0'); # prints 'a1'
- print ++($foo = 'Az'); # prints 'Ba'
- print ++($foo = 'zz'); # prints 'aaa'
+ print ++($foo = "99"); # prints "100"
+ print ++($foo = "a0"); # prints "a1"
+ print ++($foo = "Az"); # prints "Ba"
+ print ++($foo = "zz"); # prints "aaa"
C<undef> is always treated as numeric, and in particular is changed
to C<0> before incrementing (so that a post-increment of an undef value
=head2 Symbolic Unary Operators
X<unary operator> X<operator, unary>
-Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
+Unary "!" performs logical negation, that is, "not". See also C<not> for a lower
precedence version of this.
X<!>
B<Argument "the string" isn't numeric in negation (-) at ...>.
X<-> X<negation, arithmetic>
-Unary "~" performs bitwise negation, i.e., 1's complement. For
+Unary "~" performs bitwise negation, that is, 1's complement. For
example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
L<Bitwise String Operators>.) Note that the width of the result is
platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
bits wide on a 64-bit platform, so if you are expecting a certain bit
-width, remember to use the & operator to mask off the excess bits.
+width, remember to use the "&" operator to mask off the excess bits.
X<~> X<negation, binary>
+When complementing strings, if all characters have ordinal values under
+256, then their complements will, also. But if they do not, all
+characters will be in either 32- or 64-bit complements, depending on your
+architecture. So for example, C<~"\x{3B1}"> is C<"\x{FFFF_FC4E}"> on
+32-bit machines and C<"\x{FFFF_FFFF_FFFF_FC4E}"> on 64-bit machines.
+
Unary "+" has no effect whatsoever, even on strings. It is useful
syntactically for separating a function name from a parenthesized expression
that would otherwise be interpreted as the complete list of function
substitution, or transliteration, it is interpreted as a search pattern at run
time. Note that this means that its contents will be interpolated twice, so
- '\\' =~ q'\\';
+ '\\' =~ q'\\';
is not ok, as the regex engine will end up trying to compile the
pattern C<\>, which it will consider a syntax error.
operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
C<$a> minus the largest multiple of C<$b> less than or equal to
C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
-smallest multiple of C<$b> that is not less than C<$a> (i.e. the
+smallest multiple of C<$b> that is not less than C<$a> (that is, the
result will be less than or equal to zero). If the operands
C<$a> and C<$b> are floating point values and the absolute value of
C<$b> (that is C<abs($b)>) is less than C<(UV_MAX + 1)>, only
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.
-If the right operand is zero or negative, it returns an empty string
+If the right operand is zero or negative (raising a warning on
+negative), it returns an empty string
or an empty list, depending on the context.
X<x>
=head2 Additive Operators
X<operator, additive>
-Binary "+" returns the sum of two numbers.
+Binary C<+> returns the sum of two numbers.
X<+>
-Binary "-" returns the difference of two numbers.
+Binary C<-> returns the difference of two numbers.
X<->
-Binary "." concatenates two strings.
+Binary C<.> concatenates two strings.
X<string, concatenation> X<concatenation>
X<cat> X<concat> X<concatenate> X<.>
X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
X<shl> X<shr> X<shift, right> X<shift, left>
-Binary "<<" 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 ">>" 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 "<<" and ">>" in Perl are implemented directly using
-"<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
+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
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
integers, 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:
+
+ print 20 << 20; # 20971520
+ print 20 << 40; # 5120 on 32-bit machines,
+ # 21990232555520 on 64-bit machines
+ use bigint;
+ print 20 << 100; # 25353012004564588029934064107520
+
=head2 Named Unary Operators
X<operator, named unary>
is followed by a left parenthesis as the next token, the operator and
arguments within parentheses are taken to be of highest precedence,
just like a normal function call. For example,
-because named unary operators are higher precedence than ||:
+because named unary operators are higher precedence than C<||>:
chdir $foo || die; # (chdir $foo) || die
chdir($foo) || die; # (chdir $foo) || die
=head2 Relational Operators
X<relational operator> X<operator, relational>
+Perl operators that return true or false generally return values
+that can be safely used as numbers. For example, the relational
+operators in this section and the equality operators in the next
+one return C<1> for true and a special version of the defined empty
+string, C<"">, which counts as a zero but is exempt from warnings
+about improper numeric conversions, just as C<"0 but true"> is.
+
Binary "<" returns true if the left argument is numerically less than
the right argument.
X<< < >>
support NaNs then NaN is just a string with numeric value 0.
X<< <=> >> X<spaceship>
- perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
- perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
+ $ perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
+ $ perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
+
+(Note that the L<bigint>, L<bigrat>, and L<bignum> pragmas all
+support "NaN".)
Binary "eq" returns true if the left argument is stringwise equal to
the right argument.
argument.
X<cmp>
-Binary "~~" does a smart match between its arguments. Smart matching
-is described in L<perlsyn/"Smart matching in detail">.
+Binary "~~" 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 C<use locale> is in effect. See L<perllocale>.
+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.
+
+=head2 Smartmatch Operator
+
+First available in Perl 5.10.1 (the 5.10.0 version behaved differently),
+binary C<~~> does a "smartmatch" between its arguments. This is mostly
+used implicitly in the C<when> construct described in L<perlsyn>, although
+not all C<when> clauses call the smartmatch operator. Unique among all of
+Perl's operators, the smartmatch operator can recurse. The smartmatch
+operator is L<experimental|perlpolicy/experimental> and its behavior is
+subject to change.
+
+It is also unique in that all other Perl operators impose a context
+(usually string or numeric context) on their operands, autoconverting
+those operands to those imposed contexts. In contrast, smartmatch
+I<infers> contexts from the actual types of its operands and uses that
+type information to select a suitable comparison mechanism.
+
+The C<~~> operator compares its operands "polymorphically", determining how
+to compare them according to their actual types (numeric, string, array,
+hash, etc.) Like the equality operators with which it shares the same
+precedence, C<~~> returns 1 for true and C<""> for false. It is often best
+read aloud as "in", "inside of", or "is contained in", because the left
+operand is often looked for I<inside> the right operand. That makes the
+order of the operands to the smartmatch operand often opposite that of
+the regular match operator. In other words, the "smaller" thing is usually
+placed in the left operand and the larger one in the right.
+
+The behavior of a smartmatch depends on what type of things its arguments
+are, as determined by the following table. The first row of the table
+whose types apply determines the smartmatch behavior. Because what
+actually happens is mostly determined by the type of the second operand,
+the table is sorted on the right operand instead of on the left.
+
+ Left Right Description and pseudocode
+ ===============================================================
+ Any undef check whether Any is undefined
+ like: !defined Any
+
+ Any Object invoke ~~ overloading on Object, or die
+
+ Right operand is an ARRAY:
+
+ Left Right Description and pseudocode
+ ===============================================================
+ ARRAY1 ARRAY2 recurse on paired elements of ARRAY1 and ARRAY2[2]
+ like: (ARRAY1[0] ~~ ARRAY2[0])
+ && (ARRAY1[1] ~~ ARRAY2[1]) && ...
+ HASH ARRAY any ARRAY elements exist as HASH keys
+ like: grep { exists HASH->{$_} } ARRAY
+ Regexp ARRAY any ARRAY elements pattern match Regexp
+ like: grep { /Regexp/ } ARRAY
+ undef ARRAY undef in ARRAY
+ like: grep { !defined } ARRAY
+ Any ARRAY smartmatch each ARRAY element[3]
+ like: grep { Any ~~ $_ } ARRAY
+
+ Right operand is a HASH:
+
+ Left Right Description and pseudocode
+ ===============================================================
+ HASH1 HASH2 all same keys in both HASHes
+ like: keys HASH1 ==
+ grep { exists HASH2->{$_} } keys HASH1
+ ARRAY HASH any ARRAY elements exist as HASH keys
+ like: grep { exists HASH->{$_} } ARRAY
+ Regexp HASH any HASH keys pattern match Regexp
+ like: grep { /Regexp/ } keys HASH
+ undef HASH always false (undef can't be a key)
+ like: 0 == 1
+ Any HASH HASH key existence
+ like: exists HASH->{Any}
+
+ Right operand is CODE:
+
+ Left Right Description and pseudocode
+ ===============================================================
+ ARRAY CODE sub returns true on all ARRAY elements[1]
+ like: !grep { !CODE->($_) } ARRAY
+ HASH CODE sub returns true on all HASH keys[1]
+ like: !grep { !CODE->($_) } keys HASH
+ Any CODE sub passed Any returns true
+ like: CODE->(Any)
+
+Right operand is a Regexp:
+
+ Left Right Description and pseudocode
+ ===============================================================
+ ARRAY Regexp any ARRAY elements match Regexp
+ like: grep { /Regexp/ } ARRAY
+ HASH Regexp any HASH keys match Regexp
+ like: grep { /Regexp/ } keys HASH
+ Any Regexp pattern match
+ like: Any =~ /Regexp/
+
+ Other:
+
+ Left Right Description and pseudocode
+ ===============================================================
+ Object Any invoke ~~ overloading on Object,
+ or fall back to...
+
+ Any Num numeric equality
+ like: Any == Num
+ Num nummy[4] numeric equality
+ like: Num == nummy
+ undef Any check whether undefined
+ like: !defined(Any)
+ Any Any string equality
+ like: Any eq Any
+
+
+Notes:
+
+=over
+
+=item 1.
+Empty hashes or arrays match.
+
+=item 2.
+That is, each element smartmatches the element of the same index in the other array.[3]
+
+=item 3.
+If a circular reference is found, fall back to referential equality.
+
+=item 4.
+Either an actual number, or a string that looks like one.
+
+=back
+
+The smartmatch implicitly dereferences any non-blessed hash or array
+reference, so the C<I<HASH>> and C<I<ARRAY>> entries apply in those cases.
+For blessed references, the C<I<Object>> entries apply. Smartmatches
+involving hashes only consider hash keys, never hash values.
+
+The "like" code entry is not always an exact rendition. For example, the
+smartmatch operator short-circuits whenever possible, but C<grep> does
+not. Also, C<grep> in scalar context returns the number of matches, but
+C<~~> returns only true or false.
+
+Unlike most operators, the smartmatch operator knows to treat C<undef>
+specially:
+
+ use v5.10.1;
+ @array = (1, 2, 3, undef, 4, 5);
+ say "some elements undefined" if undef ~~ @array;
+
+Each operand is considered in a modified scalar context, the modification
+being that array and hash variables are passed by reference to the
+operator, which implicitly dereferences them. Both elements
+of each pair are the same:
+
+ use v5.10.1;
+
+ my %hash = (red => 1, blue => 2, green => 3,
+ orange => 4, yellow => 5, purple => 6,
+ black => 7, grey => 8, white => 9);
+
+ my @array = qw(red blue green);
+
+ say "some array elements in hash keys" if @array ~~ %hash;
+ say "some array elements in hash keys" if \@array ~~ \%hash;
+
+ say "red in array" if "red" ~~ @array;
+ say "red in array" if "red" ~~ \@array;
+
+ say "some keys end in e" if /e$/ ~~ %hash;
+ say "some keys end in e" if /e$/ ~~ \%hash;
+
+Two arrays smartmatch if each element in the first array smartmatches
+(that is, is "in") the corresponding element in the second array,
+recursively.
+
+ use v5.10.1;
+ my @little = qw(red blue green);
+ my @bigger = ("red", "blue", [ "orange", "green" ] );
+ if (@little ~~ @bigger) { # true!
+ say "little is contained in bigger";
+ }
+
+Because the smartmatch operator recurses on nested arrays, this
+will still report that "red" is in the array.
+
+ use v5.10.1;
+ my @array = qw(red blue green);
+ my $nested_array = [[[[[[[ @array ]]]]]]];
+ say "red in array" if "red" ~~ $nested_array;
+
+If two arrays smartmatch each other, then they are deep
+copies of each others' values, as this example reports:
+
+ use v5.12.0;
+ my @a = (0, 1, 2, [3, [4, 5], 6], 7);
+ my @b = (0, 1, 2, [3, [4, 5], 6], 7);
+
+ if (@a ~~ @b && @b ~~ @a) {
+ say "a and b are deep copies of each other";
+ }
+ elsif (@a ~~ @b) {
+ say "a smartmatches in b";
+ }
+ elsif (@b ~~ @a) {
+ say "b smartmatches in a";
+ }
+ else {
+ say "a and b don't smartmatch each other at all";
+ }
+
+
+If you were to set C<$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
+(eventually) has a 4 in it.
+
+Smartmatching one hash against another reports whether both contain the
+same keys, no more and no less. This could be used to see whether two
+records have the same field names, without caring what values those fields
+might have. For example:
+
+ use v5.10.1;
+ sub make_dogtag {
+ state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
+
+ my ($class, $init_fields) = @_;
+
+ die "Must supply (only) name, rank, and serial number"
+ unless $init_fields ~~ $REQUIRED_FIELDS;
+
+ ...
+ }
+
+or, if other non-required fields are allowed, use ARRAY ~~ HASH:
+
+ use v5.10.1;
+ sub make_dogtag {
+ state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
+
+ my ($class, $init_fields) = @_;
+
+ die "Must supply (at least) name, rank, and serial number"
+ unless [keys %{$init_fields}] ~~ $REQUIRED_FIELDS;
+
+ ...
+ }
+
+The smartmatch operator is most often used as the implicit operator of a
+C<when> clause. See the section on "Switch Statements" in L<perlsyn>.
+
+=head3 Smartmatching of Objects
+
+To avoid relying on an object's underlying representation, if the
+smartmatch's right operand is an object that doesn't overload C<~~>,
+it raises the exception "C<Smartmatching a non-overloaded object
+breaks encapsulation>". That's because one has no business digging
+around to see whether something is "in" an object. These are all
+illegal on objects without a C<~~> overload:
+
+ %hash ~~ $object
+ 42 ~~ $object
+ "fred" ~~ $object
+
+However, you can change the way an object is smartmatched by overloading
+the C<~~> operator. This is allowed to extend the usual smartmatch semantics.
+For objects that do have an C<~~> overload, see L<overload>.
+
+Using an object as the left operand is allowed, although not very useful.
+Smartmatching rules take precedence over overloading, so even if the
+object in the left operand has smartmatch overloading, this will be
+ignored. A left operand that is a non-overloaded object falls back on a
+string or numeric comparison of whatever the C<ref> operator returns. That
+means that
+
+ $object ~~ X
+
+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:
+
+ $object ~~ $number ref($object) == $number
+ $object ~~ $string ref($object) eq $string
+
+For example, this reports that the handle smells IOish
+(but please don't really do this!):
+
+ use IO::Handle;
+ my $fh = IO::Handle->new();
+ if ($fh ~~ /\bIO\b/) {
+ say "handle smells IOish";
+ }
+
+That's because it treats C<$fh> as a string like
+C<"IO::Handle=GLOB(0x8039e0)">, then pattern matches against that.
=head2 Bitwise And
X<operator, bitwise, and> X<bitwise and> X<&>
-Binary "&" returns its operands ANDed together bit by bit.
-(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
+Binary "&" returns its operands ANDed together bit by bit. Although no
+warning is currently raised, the result is not well defined when this operation
+is performed on operands that aren't either numbers (see
+L<Integer Arithmetic>) or bitstrings (see L<Bitwise String Operators>).
Note that "&" has lower priority than relational operators, so for example
-the brackets are essential in a test like
+the parentheses are essential in a test like
- print "Even\n" if ($x & 1) == 0;
+ print "Even\n" if ($x & 1) == 0;
=head2 Bitwise Or and Exclusive Or
X<operator, bitwise, or> X<bitwise or> X<|> X<operator, bitwise, xor>
X<bitwise xor> X<^>
Binary "|" returns its operands ORed together bit by bit.
-(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
Binary "^" returns its operands XORed together bit by bit.
-(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
+
+Although no warning is currently raised, the results are not well
+defined when these operations are performed on operands that aren't either
+numbers (see L<Integer Arithmetic>) or bitstrings (see L<Bitwise String
+Operators>).
Note that "|" and "^" have lower priority than relational operators, so
for example the brackets are essential in a test like
- print "false\n" if (8 | 2) != 10;
+ print "false\n" if (8 | 2) != 10;
=head2 C-style Logical And
X<&&> X<logical and> X<operator, logical, and>
Scalar or list context propagates down to the right operand if it
is evaluated.
-=head2 C-style Logical Defined-Or
+=head2 Logical Defined-Or
X<//> X<operator, logical, defined-or>
Although it has no direct equivalent in C, Perl's C<//> operator is related
to its C-style or. In fact, it's exactly the same as C<||>, except that it
-tests the left hand side's definedness instead of its truth. Thus, C<$a // $b>
-is similar to C<defined($a) || $b> (except that it returns the value of C<$a>
-rather than the value of C<defined($a)>) and yields the same result as
-C<defined($a) ? $a : $b> (except that the ternary-operator form can be
-used as a lvalue, while C<$a // $b> cannot). This is very useful for
+tests the left hand side's definedness instead of its truth. Thus,
+C<< EXPR1 // EXPR2 >> returns the value of C<< EXPR1 >> if it's defined,
+otherwise, the value of C<< EXPR2 >> is returned. (C<< EXPR1 >> is evaluated
+in scalar context, C<< EXPR2 >> in the context of C<< // >> itself). Usually,
+this is the same result as C<< defined(EXPR1) ? EXPR1 : EXPR2 >> (except that
+the ternary-operator form can be used as a lvalue, while C<< EXPR1 // EXPR2 >>
+cannot). This is very useful for
providing default values for variables. If you actually want to test if
at least one of C<$a> and C<$b> is defined, use C<defined($a // $b)>.
(unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
portable way to find out the home directory might be:
- $home = $ENV{'HOME'} // $ENV{'LOGDIR'} //
- (getpwuid($<))[7] // die "You're homeless!\n";
+ $home = $ENV{HOME}
+ // $ENV{LOGDIR}
+ // (getpwuid($<))[7]
+ // die "You're homeless!\n";
In particular, this means that you shouldn't use this
for selecting between two aggregates for assignment:
@a = scalar(@b) || @c; # really meant this
@a = @b ? @b : @c; # this works fine, though
-As more readable alternatives to C<&&> and C<||> when used for
+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
unlink("alpha", "beta", "gamma")
|| (gripe(), next LINE);
+It would be even more readable to write that this way:
+
+ unless(unlink("alpha", "beta", "gamma")) {
+ gripe();
+ next LINE;
+ }
+
Using "or" for assignment is unlikely to do what you want; see below.
=head2 Range Operators
And now some examples as a list operator:
- for (101 .. 200) { print; } # print $_ 100 times
- @foo = @foo[0 .. $#foo]; # an expensive no-op
- @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
+ for (101 .. 200) { print } # print $_ 100 times
+ @foo = @foo[0 .. $#foo]; # an expensive no-op
+ @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
The range operator (in list context) makes use of the magical
auto-increment algorithm if the operands are strings. You
can say
- @alphabet = ('A' .. 'Z');
+ @alphabet = ("A" .. "Z");
to get all normal letters of the English alphabet, or
- $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
+ $hexdigit = (0 .. 9, "a" .. "f")[$num & 15];
to get a hexadecimal digit, or
- @z2 = ('01' .. '31'); print $z2[$mday];
+ @z2 = ("01" .. "31");
+ print $z2[$mday];
to get dates with leading zeros.
be longer than the final value specified.
If the initial value specified isn't part of a magical increment
-sequence (that is, a non-empty string matching "/^[a-zA-Z]*[0-9]*\z/"),
+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
return an alpha:
- use charnames 'greek';
+ use charnames "greek";
my @greek_small = ("\N{alpha}" .. "\N{omega}");
-To get lower-case greek letters, use this instead:
+To get the 25 traditional lowercase Greek letters, including both sigmas,
+you could use this instead:
- my @greek_small = map { chr } ( ord("\N{alpha}") ..
- ord("\N{omega}") );
+ use charnames "greek";
+ my @greek_small = map { chr } ( ord("\N{alpha}")
+ ..
+ ord("\N{omega}")
+ );
+
+However, because there are I<many> other lowercase Greek characters than
+just those, to match lowercase Greek characters in a regular expression,
+you could use the pattern C</(?:(?=\p{Greek})\p{Lower})+/> (or the
+L<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
return two elements in list context.
is returned. For example:
printf "I have %d dog%s.\n", $n,
- ($n == 1) ? '' : "s";
+ ($n == 1) ? "" : "s";
Scalar or list context propagates downward into the 2nd
or 3rd argument, whichever is selected.
then modifying the variable that was assigned to. This is useful
for modifying a copy of something, like this:
- ($tmp = $global) =~ tr [A-Z] [a-z];
+ ($tmp = $global) =~ tr/13579/24680/;
+
+Although as of 5.14, that can be also be accomplished this way:
+
+ use v5.14;
+ $tmp = ($global =~ tr/13579/24680/r);
Likewise,
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
-its left operand to be interpreted as a string if it begins with a letter
+The C<< => >> operator is a synonym for the comma except that it causes a
+word on its left to be interpreted as a string if it begins with a letter
or underscore and is composed only of letters, digits and underscores.
This includes operands that might otherwise be interpreted as operators,
constants, single number v-strings or function calls. If in doubt about
-this behaviour, the left operand can be quoted explicitly.
+this behavior, the left operand can be quoted explicitly.
Otherwise, the C<< => >> operator behaves exactly as the comma operator
or list argument separator, according to context.
The C<< => >> operator is helpful in documenting the correspondence
between keys and values in hashes, and other paired elements in lists.
- %hash = ( $key => $value );
- login( $username => $password );
-
-=head2 Yada Yada Operator
-X<...> X<... operator> X<yada yada operator>
-
-The yada yada operator (noted C<...>) is a placeholder for code. Perl
-parses it without error, but when you try to execute a yada yada, it
-throws an exception with the text C<Unimplemented>:
-
- sub unimplemented { ... }
-
- eval { unimplemented() };
- if( $@ eq 'Unimplemented' ) {
- print "I found the yada yada!\n";
- }
-
-You can only use the yada yada to stand in for a complete statement.
-These examples of the yada yada work:
-
- { ... }
-
- sub foo { ... }
-
- ...;
-
- eval { ... };
-
- sub foo {
- my( $self ) = shift;
-
- ...;
- }
-
- do { my $n; ...; print 'Hurrah!' };
-
-The yada yada cannot stand in for an expression that is part of a
-larger statement since the C<...> is also the three-dot version of the
-range operator (see L<Range Operators>). These examples of the yada
-yada are still syntax errors:
-
- print ...;
-
- open my($fh), '>', '/dev/passwd' or ...;
-
- if( $condition && ... ) { print "Hello\n" };
-
-There are some cases where Perl can't immediately tell the difference
-between an expression and a statement. For instance, the syntax for a
-block and an anonymous hash reference constructor look the same unless
-there's something in the braces that give Perl a hint. The yada yada
-is a syntax error if Perl doesn't guess that the C<{ ... }> is a
-block. In that case, it doesn't think the C<...> is the yada yada
-because it's expecting an expression instead of a statement:
-
- my @transformed = map { ... } @input; # syntax error
-
-You can use a C<;> inside your block to denote that the C<{ ... }> is
-a block and not a hash reference constructor. Now the yada yada works:
-
- my @transformed = map {; ... } @input; # ; disambiguates
-
- my @transformed = map { ...; } @input; # ; disambiguates
+ %hash = ( $key => $value );
+ login( $username => $password );
+
+The special quoting behavior ignores precedence, and hence may apply to
+I<part> of the left operand:
+
+ print time.shift => "bbb";
+
+That example prints something like "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.
=head2 List Operators (Rightward)
X<operator, list, rightward> X<list operator>
-On the right side of a list operator, it has very low precedence,
+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
-operators without the need for extra parentheses:
+operators without the need for parentheses:
+
+ open HANDLE, "< :utf8", "filename" or die "Can't open: $!\n";
- open HANDLE, "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";
+
+in which case you might as well just use the more customary "||" operator:
+
+ open(HANDLE, "< :utf8", "filename") || die "Can't open: $!\n";
See also discussion of list operators in L<Terms and List Operators (Leftward)>.
X<operator, logical, and> X<and>
Binary "and" returns the logical conjunction of the two surrounding
-expressions. It's equivalent to && except for the very low
-precedence. This means that it short-circuits: i.e., the right
+expressions. It's equivalent to C<&&> except for the very low
+precedence. This means that it short-circuits: the right
expression is evaluated only if the left expression is true.
-=head2 Logical or, Defined or, and Exclusive Or
+=head2 Logical or and Exclusive Or
X<operator, logical, or> X<operator, logical, xor>
-X<operator, logical, defined or> X<operator, logical, exclusive or>
+X<operator, logical, exclusive or>
X<or> X<xor>
Binary "or" returns the logical disjunction of the two surrounding
-expressions. It's equivalent to || except for the very low precedence.
-This makes it useful for control flow
+expressions. It's equivalent to C<||> except for the very low precedence.
+This makes it useful for control flow:
print FH $data or die "Can't write to FH: $!";
-This means that it short-circuits: i.e., the right expression is evaluated
-only if the left expression is false. Due to its precedence, you should
-probably avoid using this for assignment, only for control flow.
+This means that it short-circuits: the right expression is evaluated
+only if the left expression is false. Due to its precedence, you must
+be careful to avoid using it as replacement for the C<||> operator.
+It usually works out better for flow control than in assignments:
$a = $b or $c; # bug: this is wrong
($a = $b) or $c; # really means this
$a = $b || $c; # better written this way
However, when it's a list-context assignment and you're trying to use
-"||" for control flow, you probably need "or" so that the assignment
+C<||> for control flow, you probably need "or" so that the assignment
takes higher precedence.
@info = stat($file) || die; # oops, scalar sense of stat!
Then again, you could always use parentheses.
-Binary "xor" returns the exclusive-OR of the two surrounding expressions.
-It cannot short circuit, of course.
+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.
=head2 C Operators Missing From Perl
X<operator, missing from perl> X<&> X<*>
X<qr> X<s> X<tr> X<'> X<''> X<"> X<""> X<//> X<`> X<``> X<<< << >>>
X<escape sequence> X<escape>
-
While we usually think of quotes as literal values, in Perl they
function as operators, providing various kinds of interpolating and
pattern matching capabilities. Perl provides customary quote characters
qr{} Pattern yes*
s{}{} Substitution yes*
tr{}{} Transliteration no (but see below)
+ y{}{} Transliteration no (but see below)
<<EOF here-doc yes*
* unless the delimiter is ''.
Non-bracketing delimiters use the same character fore and aft, but the four
-sorts of brackets (round, angle, square, curly) will all nest, which means
+sorts of ASCII brackets (round, angle, square, curly) all nest, which means
that
- q{foo{bar}baz}
+ q{foo{bar}baz}
is the same as
- 'foo{bar}baz'
+ 'foo{bar}baz'
Note, however, that this does not always work for quoting Perl code:
- $s = q{ if($a eq "}") ... }; # WRONG
+ $s = q{ if($a eq "}") ... }; # WRONG
-is a syntax error. The C<Text::Balanced> module (from CPAN, and
-starting from Perl 5.8 part of the standard distribution) is able
-to do this properly.
+is a syntax error. The C<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.
s {foo} # Replace foo
{bar} # with bar.
-The following escape sequences are available in constructs that interpolate
-and in transliterations.
+The following escape sequences are available in constructs that interpolate,
+and in transliterations:
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{}>
\b backspace (BS)
\a alarm (bell) (BEL)
\e escape (ESC)
- \x{263a} [1,8] hex char (example: SMILEY)
+ \x{263A} [1,8] hex char (example: SMILEY)
\x1b [2,8] restricted range hex char (example: ESC)
\N{name} [3] named Unicode character or character sequence
\N{U+263D} [4,8] Unicode character (example: FIRST QUARTER MOON)
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{}>)
-will not cause a warning.
+will not cause a warning (currently).
=item [2]
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 C<\x> alone will be
+means that C<\x7> will be interpreted as C<\x07>, and a lone <\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 while the warning
+two valid digits will result in a warning. Note that although the warning
says the illegal character is ignored, it is only ignored as part of the
escape and will still be used as the subsequent character in the string.
For example:
\c[ chr(27)
\c] chr(29)
\c^ chr(30)
- \c? chr(127)
+ \c_ chr(31)
+ \c? chr(127) # (on ASCII platforms)
+
+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.
Also, C<\c\I<X>> yields 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
On ASCII platforms, the resulting characters from the list above are the
complete set of ASCII controls. This isn't the case on EBCDIC platforms; see
-L<perlebcdic/OPERATOR DIFFERENCES> for the complete list of what these
-sequences mean on both ASCII and EBCDIC platforms.
+L<perlebcdic/OPERATOR DIFFERENCES> for a full discussion of the
+differences between these for ASCII versus EBCDIC platforms.
-Use of any other character following the "c" besides those listed above is
-discouraged, and some are deprecated with the intention of removing
-those in Perl 5.16. What happens for any of these
-other characters currently though, is that the value is derived by inverting
-the 7th bit (0x40).
+Use of any other character following the C<"c"> besides those listed above is
+discouraged, and as of Perl v5.20, the only characters actually allowed
+are the printable ASCII ones, minus the left brace C<"{">. What happens
+for any of the allowed other characters is that the value is derived by
+xor'ing with the seventh bit, which is 64, and a warning raised if
+enabled. Using the non-allowed characters generates a fatal error.
To get platform independent controls, you can use C<\N{...}>.
=item [7]
-The result is the character specified by the three digit octal number in the
+The result is the character specified by the three-digit octal number in the
range 000 to 777 (but best to not use above 077, see next paragraph). See
L</[8]> below for details on which character.
Some contexts allow 2 or even 1 digit, but any usage without exactly
three digits, the first being a zero, may give unintended results. (For
-example, 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
+example, in a regular expression it may be confused with a backreference;
+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{}>
+C<\o{}>, or convert to something else, such as to hex and use C<\x{}>
instead.
-Having fewer than 3 digits may lead to a misleading warning message that says
-that what follows is ignored. For example, C<"\128"> in the ASCII character set
-is equivalent to the two characters C<"\n8">, but the warning C<Illegal octal
-digit '8' ignored> will be thrown. To avoid this warning, make sure to pad
-your octal number with C<0>'s: C<"\0128">.
-
=item [8]
-Several of the constructs above specify a character by a number. That number
+Several constructs above specify a character by a number. That number
gives the character's position in the character set encoding (indexed from 0).
-This is called synonymously its ordinal, code position, or code point). Perl
+This is called synonymously its ordinal, code position, or code point. Perl
works on platforms that have a native encoding currently of either ASCII/Latin1
or EBCDIC, each of which allow specification of 256 characters. In general, if
the number is 255 (0xFF, 0377) or below, Perl interprets this in the platform's
native encoding. If the number is 256 (0x100, 0400) or above, Perl interprets
-it as as a Unicode code point and the result is the corresponding Unicode
+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 "&".
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 100th position (indexed by 0) in Unicode is
+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. C<\N{U+I<hex number>}> is
+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
number is considered to be in that encoding, and is translated from that into
=back
B<NOTE>: Unlike C and other languages, Perl has no C<\v> escape sequence for
-the vertical tab (VT - ASCII 11), but you may use C<\ck> or C<\x0b>. (C<\v>
+the vertical tab (VT, which is 11 in both ASCII and EBCDIC), but you may
+use C<\ck> 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,
but not in transliterations.
-X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q>
-
- \l lowercase next char
- \u uppercase next char
- \L lowercase till \E
- \U uppercase till \E
- \Q quote non-word characters till \E
+X<\l> X<\u> X<\L> X<\U> X<\E> X<\Q> X<\F>
+
+ \l lowercase next character only
+ \u titlecase (not uppercase!) next character only
+ \L lowercase all characters till \E or end of string
+ \U uppercase all characters till \E or end of string
+ \F foldcase all characters till \E or end of string
+ \Q quote (disable) pattern metacharacters till \E or
+ end of string
\E end either case modification or quoted section
+ (whichever was last seen)
+
+See L<perlfunc/quotemeta> for the exact definition of characters that
+are quoted by C<\Q>.
-If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
-C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
+C<\L>, C<\U>, C<\F>, and C<\Q> can stack, in which case you need one
+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.
+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>
+for all locales but a UTF-8 one, where it instead uses the Unicode
+definition.
All systems use the virtual C<"\n"> to represent a line terminator,
called a "newline". There is no such thing as an unvarying, physical
newline character. It is only an illusion that the operating system,
device drivers, C libraries, and Perl all conspire to preserve. Not all
systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example,
-on a Mac, these are reversed, and on systems without line terminator,
-printing C<"\n"> may emit no actual data. In general, use C<"\n"> when
+on the ancient Macs (pre-MacOS X) of yesteryear, these used to be reversed,
+and on systems without 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
and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
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 only
-interpolated if the name is enclosed in braces C<@{*}>, but special
-arrays C<@_>, C<@+>, and C<@-> are interpolated, even without braces.
+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.
For double-quoted strings, the quoting from C<\Q> is applied after
interpolation and escapes are processed.
is done. Returns a Perl value which may be used instead of the
corresponding C</STRING/msixpodual> 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",
-even though dereferencing the result returns undef.
+a string containing the same characters: C<ref(qr/x/)> returns "Regexp";
+however, dereferencing it is not well defined (you currently get the
+normalized version of the original pattern, but this may change).
+
For example,
The result may be used as a subpattern in a match:
$re = qr/$pattern/;
- $string =~ /foo${re}bar/; # can be interpolated in other patterns
+ $string =~ /foo${re}bar/; # can be interpolated in other
+ # patterns
$string =~ $re; # or used standalone
$string =~ /$re/; # or this way
i Do case-insensitive pattern matching.
x Use extended regular expressions.
p When matching preserve a copy of the matched string so
- that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
+ that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be
+ defined.
o Compile pattern only once.
- l Use the locale
- u Use Unicode rules
- a 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
- d Use Unicode or native charset, as in 5.12 and earlier
+ a ASCII-restrict: Use ASCII for \d, \s, \w; specifying two
+ a's further restricts /i matching so that no ASCII
+ character will match a non-ASCII one.
+ l Use the locale.
+ u Use Unicode rules.
+ d Use Unicode or native charset, as in 5.12 and earlier.
If a precompiled pattern is embedded in a larger pattern then the effect
-of 'msixpluad' will be propagated appropriately. The effect the 'o'
+of "msixpluad" will be propagated appropriately. The effect the "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,
-control the character set semantics.
+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
for a detailed look at the semantics of regular expressions. In
-particular, all the modifiers execpt C</o> are further explained in
-L<perlre/Modifiers>. C</o> is described in the next section.
+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/msixpodualgc
X<m> X<operator, match>
process modifiers are available:
g Match globally, i.e., find all occurrences.
- c Do not reset search position on a failed match when /g is in effect.
+ 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>
-you can use any pair of non-whitespace characters
+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
the delimiter, then a match-only-once rule applies,
-described in C<m?PATTERN?> below.
-If "'" is the delimiter, no interpolation is performed on the PATTERN.
+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
after the C<m>.
after the trailing delimiter.
Once upon a time, Perl would recompile regular expressions
unnecessarily, and this modifier was useful to tell it not to do so, in the
-interests of speed. But now, the only reasons to use C</o> are either:
+interests of speed. But now, the only reasons to use C</o> are one of:
=over
don't change, and you need to wring out the last little bit of speed by
having Perl skip testing for that. (There is a maintenance penalty for
doing this, as mentioning C</o> constitutes a promise that you won't
-change the variables in the pattern. If you change them, Perl won't
+change the variables in the pattern. If you do change them, Perl won't
even notice.)
=item 2
regardless of whether they change or not. (But there are saner ways
of accomplishing this than using C</o>.)
+=item 3
+
+If the pattern contains embedded code, such as
+
+ use re 'eval';
+ $code = 'foo(?{ $x })';
+ /$code/
+
+then perl will recompile each time, even though the pattern string hasn't
+changed, to ensure that the current value of C<$x> is seen each time.
+Use C</o> if you want to avoid this.
+
=back
+The bottom line is that using C</o> is almost never a good idea.
+
=item The empty pattern //
If the 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 is honoured -
+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
previously succeeded, this will (silently) act instead as a genuine
empty pattern (which will always match).
If the C</g> option is not used, C<m//> in list context returns a
list consisting of the subexpressions matched by the parentheses in the
-pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here C<$1> etc. are
-also set, and that this differs from Perl 4's behavior.) When there are
-no parentheses in the pattern, the return value is the list C<(1)> for
-success. With or without parentheses, an empty list is returned upon
-failure.
+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.
+With or without parentheses, an empty list is returned upon failure.
Examples:
- open(TTY, '/dev/tty');
- <TTY> =~ /^y/i && foo(); # do foo if desired
+ open(TTY, "+</dev/tty")
+ || die "can't access /dev/tty: $!";
- if (/Version: *([0-9.]*)/) { $version = $1; }
+ <TTY> =~ /^y/i && foo(); # do foo if desired
- next if m#^/usr/spool/uucp#;
+ if (/Version: *([0-9.]*)/) { $version = $1; }
- # poor man's grep
- $arg = shift;
- while (<>) {
- print if /$arg/o; # compile only once
- }
+ next if m#^/usr/spool/uucp#;
- if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
+ # poor man's grep
+ $arg = shift;
+ while (<>) {
+ print if /$arg/o; # compile only once (no longer needed!)
+ }
+
+ if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
This last example splits $foo into the first two words and the
remainder of the line, and assigns those three fields to $F1, $F2, and
-$Etc. The conditional is true if any variables were assigned, i.e., if
-the pattern matched.
+$Etc. The conditional is true if any variables were assigned; that is,
+if the pattern matched.
The C</g> modifier specifies global pattern matching--that is,
matching as many times as possible within the string. How it behaves
The position after the last match can be read or set using the C<pos()>
function; see L<perlfunc/pos>. A failed match normally resets the
search position to the beginning of the string, but you can avoid that
-by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
+by adding the C</c> modifier (for example, C<m//gc>). Modifying the target
string also resets the search position.
=item \G assertion
($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
# scalar context
- $/ = "";
- while (defined($paragraph = <>)) {
- while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
+ local $/ = "";
+ while ($paragraph = <>) {
+ while ($paragraph =~ /\p{Ll}['")]*[.!?]+['")]*\s/g) {
$sentences++;
}
}
- print "$sentences\n";
+ say $sentences;
+
+Here's another way to check for sentences in a paragraph:
+
+ my $sentence_rx = qr{
+ (?: (?<= ^ ) | (?<= \s ) ) # after start-of-string or
+ # whitespace
+ \p{Lu} # capital letter
+ .*? # a bunch of anything
+ (?<= \S ) # that ends in non-
+ # whitespace
+ (?<! \b [DMS]r ) # but isn't a common abbr.
+ (?<! \b Mrs )
+ (?<! \b Sra )
+ (?<! \b St )
+ [.?!] # followed by a sentence
+ # ender
+ (?= $ | \s ) # in front of end-of-string
+ # or whitespace
+ }sx;
+ local $/ = "";
+ while (my $paragraph = <>) {
+ say "NEW PARAGRAPH";
+ my $count = 0;
+ while ($paragraph =~ /($sentence_rx)/g) {
+ printf "\tgot sentence %d: <%s>\n", ++$count, $1;
+ }
+ }
+
+Here's how to use C<m//gc> with C<\G>:
- # using m//gc with \G
$_ = "ppooqppqq";
while ($i++ < 2) {
print "1: '";
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 an
-older (pre-5.6.0) Perl.
+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.
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,
regexp tries to match where the previous one leaves off.
$_ = <<'EOL';
- $url = URI::URL->new( "http://example.com/" ); die if $url eq "xXx";
+ $url = URI::URL->new( "http://example.com/" );
+ die if $url eq "xXx";
EOL
- LOOP:
- {
+
+ LOOP: {
print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
- print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
- print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
- print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
- print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
- print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
- print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
+ print(" lowercase"), redo LOOP
+ if /\G\p{Ll}+\b[,.;]?\s*/gc;
+ print(" UPPERCASE"), redo LOOP
+ if /\G\p{Lu}+\b[,.;]?\s*/gc;
+ print(" Capitalized"), redo LOOP
+ if /\G\p{Lu}\p{Ll}+\b[,.;]?\s*/gc;
+ print(" MiXeD"), redo LOOP if /\G\pL+\b[,.;]?\s*/gc;
+ print(" alphanumeric"), redo LOOP
+ if /\G[\p{Alpha}\pN]+\b[,.;]?\s*/gc;
+ print(" line-noise"), redo LOOP if /\G\W+/gc;
print ". That's all!\n";
- }
+ }
Here is the output (split into several lines):
- line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE
- line-noise lowercase line-noise lowercase line-noise lowercase
- lowercase line-noise lowercase lowercase line-noise lowercase
- lowercase line-noise MiXeD line-noise. That's all!
+ line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE
+ line-noise lowercase line-noise lowercase line-noise lowercase
+ lowercase line-noise lowercase lowercase line-noise lowercase
+ lowercase line-noise MiXeD line-noise. That's all!
-=item m?PATTERN?
+=item m?PATTERN?msixpodualgc
X<?> X<operator, match-once>
-=item ?PATTERN?
+=item ?PATTERN?msixpodualgc
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
reset if eof; # clear m?? status for next file
}
-The match-once behaviour is controlled by the match delimiter being
+Another example switched the first "latin1" encoding it finds
+to "utf8" in a pod file:
+
+ s//utf8/ if m? ^ =encoding \h+ \K latin1 ?x;
+
+The match-once behavior is controlled by the match delimiter being
C<?>; with any other delimiter this is the normal C<m//> operator.
For historical reasons, the leading C<m> in C<m?PATTERN?> is optional,
but the resulting C<?PATTERN?> syntax is deprecated, will warn on
-usage and may be removed from a future stable release of Perl without
-further notice.
+usage and might be removed from a future stable release of Perl (without
+further notice!).
=item s/PATTERN/REPLACEMENT/msixpodualgcer
X<substitute> X<substitution> X<replace> X<regexp, replace>
with the replacement text and returns the number of substitutions
made. Otherwise it returns false (specifically, the empty string).
-If the C</r> (non-destructive) option is used then it will perform the
+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
number of substitutions, it returns the copy whether or not a
-substitution occurred. The original string will always remain unchanged in
-this case. The copy will always be a plain string, even if the input is an
-object or a tied variable.
+substitution occurred. The original string is never changed when
+C</r> is used. The copy will always be a plain string, even if the
+input is an object or a tied variable.
If no string is specified via the C<=~> or C<!~> operator, the C<$_>
-variable is searched and modified. (The string specified with C<=~> must
-be scalar variable, an array element, a hash element, or an assignment
-to one of those, i.e., an lvalue.)
+variable is searched and modified. Unless the C</r> option is used,
+the string specified must be a scalar variable, an array element, a
+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
specific options:
e Evaluate the right side as an expression.
- ee Evaluate the right side as a string then eval the result.
- r Return substitution and leave the original string untouched.
+ ee Evaluate the right side as a string then eval the
+ result.
+ r Return substitution and leave the original string
+ untouched.
Any non-whitespace delimiter may replace the slashes. Add space after
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). Unlike Perl 4, Perl 5 treats backticks
+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
-its own pair of quotes, which may or may not be bracketing quotes, e.g.,
+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
and evaluated right then and there. It is, however, syntax checked at
Examples:
- s/\bgreen\b/mauve/g; # don't change wintergreen
+ s/\bgreen\b/mauve/g; # don't change wintergreen
$path =~ s|/usr/bin|/usr/local/bin|;
s/Login: $foo/Login: $bar/; # run-time pattern
- ($foo = $bar) =~ s/this/that/; # copy first, then change
- ($foo = "$bar") =~ s/this/that/; # convert to string, copy, then change
+ ($foo = $bar) =~ s/this/that/; # copy first, then
+ # change
+ ($foo = "$bar") =~ s/this/that/; # convert to string,
+ # copy, then change
$foo = $bar =~ s/this/that/r; # Same as above using /r
$foo = $bar =~ s/this/that/r
- =~ s/that/the other/r; # Chained substitutes using /r
- @foo = map { s/this/that/r } @bar # /r is very useful in maps
+ =~ s/that/the other/r; # Chained substitutes
+ # using /r
+ @foo = map { s/this/that/r } @bar # /r is very useful in
+ # maps
- $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
+ $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-cnt
$_ = 'abc123xyz';
s/\d+/$&*2/e; # yields 'abc246xyz'
# Add one to the value of any numbers in the string
s/(\d+)/1 + $1/eg;
+ # Titlecase words in the last 30 characters only
+ substr($str, -30) =~ s/\b(\p{Alpha}+)\b/\u\L$1/g;
+
# This will expand any embedded scalar variable
# (including lexicals) in $_ : First $1 is interpolated
# to the variable name, and then evaluated
\*/ # Match the closing delimiter.
} []gsx;
- s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
+ s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_,
+ # expensively
- for ($variable) { # trim whitespace in $variable, cheap
+ for ($variable) { # trim whitespace in $variable,
+ # cheap
s/^\s+//;
s/\s+$//;
}
# expand tabs to 8-column spacing
1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
-C<s///le> is treated as a substitution followed by the C<le> operator, not
-the C</le> flags. This may change in a future version of Perl. It
-produces a warning if warnings are enabled. To disambiguate, use a space
-or change the order of the flags:
-
- s/foo/bar/ le 5; # "le" infix operator
- s/foo/bar/el; # "e" and "l" flags
-
=back
=head2 Quote-Like Operators
=item `STRING`
A string which is (possibly) interpolated and then executed as a
-system command with C</bin/sh> or its equivalent. Shell wildcards,
+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)
The STDIN filehandle used by the command is inherited from Perl's STDIN.
For example:
- open SPLAT, "stuff" or die "can't open stuff: $!";
- open STDIN, "<&SPLAT" or die "can't dupe SPLAT: $!";
+ open(SPLAT, "stuff") || die "can't open stuff: $!";
+ open(STDIN, "<&SPLAT") || die "can't dupe SPLAT: $!";
print STDOUT `sort`;
will print the sorted contents of the file named F<"stuff">.
capable of dealing with multiline commands, so putting newlines in
the string may not get you what you want. You may be able to evaluate
multiple commands in a single line by separating them with the command
-separator character, if your shell supports that (e.g. C<;> on many Unix
-shells; C<&> on the Windows NT C<cmd> shell).
+separator character, if your shell supports that (for example, C<;> on
+many Unix shells and C<&> on the Windows NT C<cmd> shell).
-Beginning with v5.6.0, Perl will attempt to flush all files opened for
+Perl will attempt to flush all files opened for
output before starting the child process, but this may not be supported
on some platforms (see L<perlport>). To be safe, you may need to set
C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
whitespace as the word delimiters. It can be understood as being roughly
equivalent to:
- split(' ', q/STRING/);
+ split(" ", q/STRING/);
the differences being that it generates a real list at compile time, and
in scalar context it returns the last element in the list. So
is semantically equivalent to the list:
- 'foo', 'bar', 'baz'
+ "foo", "bar", "baz"
Some frequently seen examples:
C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
produces warnings if the STRING contains the "," or the "#" character.
-
=item tr/SEARCHLIST/REPLACEMENTLIST/cdsr
X<tr> X<y> X<transliterate> X</c> X</d> X</s>
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 =~ or !~ operator, the $_ string is transliterated. (The
-string specified with =~ must be a scalar variable, an array element, a
-hash element, or an assignment to one of those, i.e., an lvalue.)
+specified via the C<=~> or C<!~> operator, the $_ string is transliterated.
-If the C</r> (non-destructive) option is used then it will perform the
-replacement on a copy of the string and return the copy whether or not it
-was modified. The original string will always remain unchanged in
-this case. The copy will always be a plain string, even if the input is an
-object or a tied variable.
+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
+matter whether it was modified or not: the original string is always
+left unchanged. The new copy is always a plain string, even if the input
+string is an object or a tied variable.
+
+Unless the C</r> option is used, the string specified with C<=~> must be a
+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
SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has
-its own pair of quotes, which may or may not be bracketing quotes,
-e.g., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>.
-
-Note that C<tr> does B<not> do regular expression character classes
-such as C<\d> or C<[:lower:]>. The C<tr> operator is not equivalent to
-the 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.
+its own pair of quotes, which may or may not be bracketing quotes;
+for example, C<tr[aeiouy][yuoiea]> or C<tr(+\-*/)/ABCD/>.
+
+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)
+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).
Note also that the whole range idea is rather unportable between
character sets--and even within character sets they may cause results
Examples:
- $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
+ $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case ASCII
$cnt = tr/*/*/; # count the stars in $_
tr/a-zA-Z//s; # bookkeeper -> bokeper
($HOST = $host) =~ tr/a-z/A-Z/;
- $HOST = $host =~ tr/a-z/A-Z/r; # same thing
+ $HOST = $host =~ tr/a-z/A-Z/r; # same thing
- $HOST = $host =~ tr/a-z/A-Z/r # chained with s///
+ $HOST = $host =~ tr/a-z/A-Z/r # chained with s///r
=~ s/:/ -p/r;
tr/a-zA-Z/ /cs; # change non-alphas to single space
# /r with map
tr [\200-\377]
- [\000-\177]; # delete 8th bit
+ [\000-\177]; # wickedly delete 8th bit
If multiple transliterations are given for a character, only the
first one is used:
being treated as two backslashes and not one as they would in every
other quoting construct.
+Just as in the shell, a backslashed bareword following the C<<< << >>>
+means the same thing as a single-quoted string does:
+
+ $cost = <<'VISTA'; # hasta la ...
+ That'll be $10 please, ma'am.
+ VISTA
+
+ $cost = <<\VISTA; # Same thing!
+ That'll be $10 please, ma'am.
+ VISTA
+
This is the only form of quoting in perl where there is no need
to worry about escaping content, something that code generators
can and do make good use of.
FINIS
If you use a here-doc within a delimited construct, such as in C<s///eg>,
-the quoted material must come on the lines following the final delimiter.
-So instead of
+the quoted material must still come on the line following the
+C<<< <<FOO >>> marker, which means it may be inside the delimited
+construct:
s/this/<<E . 'that'
the other
E
. 'more '/eg;
-you have to write
+It works this way as of Perl 5.18. Historically, it was inconsistent, and
+you would have to write
s/this/<<E . 'that'
. 'more '/eg;
the other
E
-If the terminating identifier is on the last line of the program, you
-must be sure there is a newline after it; otherwise, Perl will give the
-warning B<Can't find string terminator "END" anywhere before EOF...>.
+outside of string evals.
-Additionally, the quoting rules for the end of string identifier are not
-related to Perl's quoting rules. C<q()>, C<qq()>, and the like are not
+Additionally, quoting rules for the end-of-string identifier are
+unrelated to Perl's quoting rules. C<q()>, C<qq()>, and the like are not
supported in place of C<''> and C<"">, and the only interpolation is for
backslashing the quoting character:
If the starting delimiter is an unpaired character like C</> or a closing
punctuation, the ending delimiter is same as the starting delimiter.
Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
-C<qq[]> and C<qq]]> constructs.
+both C<qq[]> and C<qq]]> constructs.
When searching for single-character delimiters, escaped delimiters
-and C<\\> are skipped. For example, while searching for terminating C</>,
+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<\]>,
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.
-During the search for the end, backslashes that escape delimiters
-are removed (exactly speaking, they are not copied to the safe location).
+During the search for the end, backslashes that escape delimiters or
+other backslashes are removed (exactly speaking, they are not copied to the
+safe location).
For constructs with three-part delimiters (C<s///>, C<y///>, and
C<tr///>), the search is repeated once more.
-If the first delimiter is not an opening punctuation, three delimiters must
-be same such as C<s!!!> and C<tr)))>, in which case the second delimiter
+If the first delimiter is not an opening punctuation, the three delimiters must
+be the same, such as C<s!!!> and C<tr)))>,
+in which case the second delimiter
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 both parts, though the comment must follow
+and comments are allowed between the two parts, though 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.
=item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
-C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
+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.
The other escape sequences such as C<\200> and C<\t> and backslashed
Let it be stressed that I<whatever falls between C<\Q> and C<\E>>
is interpolated in the usual way. Something like C<"\Q\\E"> has
-no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the
+no C<\E> inside. Instead, it has C<\Q>, C<\\>, and C<E>, so the
result is the same as for C<"\\\\E">. As a general rule, backslashes
between C<\Q> and C<\E> may lead to counterintuitive results. So,
C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same
=item the replacement of C<s///>
-Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
+Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F> and interpolation
happens as with C<qq//> constructs.
It is at this step that C<\1> is begrudgingly converted to C<$1> in
=item C<RE> in C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
-Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\E>,
+Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, C<\F>, C<\E>,
and interpolation happens (almost) as with C<qq//> constructs.
Processing of C<\N{...}> is also done here, and compiled into an intermediate
treated as an array symbol (for example C<@foo>),
even though the same text in C<qq//> gives interpolation of C<\c@>.
+Code blocks such as C<(?{BLOCK})> are handled by temporarily passing control
+back to the perl parser, in a similar way that an interpolated array
+subscript expression such as C<"foo$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
performed whatsoever. This is the first step at which the presence
The terminator of this construct is found using the same rules as
for finding the terminator of a C<{}>-delimited construct, the only
exception being that C<]> immediately following C<[> is treated as
-though preceded by a backslash. Similarly, the terminator of
-C<(?{...})> is found using the same rules as for finding the
-terminator of a C<{}>-delimited construct.
+though preceded by a backslash.
+
+The terminator of runtime C<(?{...})> is found by temporarily switching
+control to the perl parser, which should stop at the point where the
+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>
print while ($_ = <STDIN>);
print while <STDIN>;
-This also behaves similarly, but avoids $_ :
+This also behaves similarly, but assigns to a lexical variable
+instead of to C<$_>:
while (my $line = <STDIN>) { print $line }
In these loop constructs, the assigned value (whether assignment
is automatic or explicit) is then tested to see whether it is
-defined. The defined test avoids problems where line has a string
-value that would be treated as false by Perl, for example a "" or
+defined. The defined test avoids problems where the line has a string
+value that would be treated as false by Perl; for example a "" or
a "0" with no trailing newline. If you really mean for such values
to terminate the loop, they should be tested for explicitly:
while (($_ = <STDIN>) ne '0') { ... }
while (<STDIN>) { last unless $_; ... }
-In other boolean contexts, C<< <filehandle> >> without an
+In other boolean contexts, C<< <FILEHANDLE> >> without an
explicit C<defined> test or comparison elicits a warning if the
C<use warnings> pragma or the B<-w>
command-line switch (the C<$^W> variable) is in effect.
See L<perlfunc/readline>.
The null filehandle <> is special: it can be used to emulate the
-behavior of B<sed> and B<awk>. Input from <> comes either from
+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
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<$ARGV[0]> is set to "-", which when opened
If you call it again after this, it will assume you are processing another
@ARGV list, and if you haven't set @ARGV, will read input from STDIN.
-If what the angle brackets contain is a simple scalar variable (e.g.,
+If what the angle brackets contain is a simple scalar variable (for example,
<$foo>), then that variable contains the name of the
filehandle to input from, or its typeglob, or a reference to the
same. For example:
the next value each time it's called, or C<undef> when the list has
run out. As with filehandle reads, an automatic C<defined> is
generated when the glob occurs in the test part of a C<while>,
-because legal glob returns (e.g. a file called F<0>) would otherwise
+because legal glob returns (for example,
+a file called F<0>) would otherwise
terminate the loop. Again, C<undef> is returned only once. So if
you're expecting a single value from a glob, it is much better to
say
variable substitution. Backslash interpolation also happens at
compile time. You can say
- 'Now is the time for all' . "\n" .
- 'good men to come to.'
+ 'Now is the time for all'
+ . "\n"
+ . 'good men to come to.'
and this all reduces to one string internally. Likewise, if
you say
if (-s $file > 5 + 100 * 2**16) { }
}
-the compiler will precompute the number which that expression
+the compiler precomputes the number which that expression
represents so that the interpreter won't have to.
=head2 No-ops
X<no-op> X<nop>
Perl doesn't officially have a no-op operator, but the bare constants
-C<0> and C<1> are special-cased to not produce a warning in a void
+C<0> and C<1> are special-cased not to produce a warning in void
context, so you can for example safely do
1 while foo();
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
=head2 Bigger Numbers
X<number, arbitrary precision>
-The standard Math::BigInt and Math::BigFloat modules provide
+The standard C<Math::BigInt>, C<Math::BigRat>, and C<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
considerable speed, they avoid the normal pitfalls associated with
limited-precision representations.
- use Math::BigInt;
- $x = Math::BigInt->new('123456789123456789');
- print $x * $x;
+ use 5.010;
+ use bigint; # easy interface to Math::BigInt
+ $x = 123456789123456789;
+ say $x * $x;
+ +15241578780673678515622620750190521
+
+Or with rationals:
- # prints +15241578780673678515622620750190521
+ use 5.010;
+ use bigrat;
+ $a = 3/22;
+ $b = 4/6;
+ say "a/b is ", $a/$b;
+ say "a*b is ", $a*$b;
+ a/b is 9/44
+ a*b is 1/11
-There are several modules that let you calculate with (bound only by
-memory and cpu-time) unlimited or fixed precision. There are also
-some non-standard modules that provide faster implementations via
-external C libraries.
+Several modules let you calculate with (bound only by memory and CPU time)
+unlimited or fixed precision. There are also some non-standard modules that
+provide faster implementations via external C libraries.
Here is a short, but incomplete summary:
- Math::Fraction big, unlimited fractions like 9973 / 12967
Math::String treat string sequences like numbers
Math::FixedPrecision calculate with a fixed precision
Math::Currency for currency calculations
Bit::Vector manipulate bit vectors fast (uses C)
Math::BigIntFast Bit::Vector wrapper for big numbers
Math::Pari provides access to the Pari C library
- Math::BigInteger uses an external C library
- Math::Cephes uses external Cephes C library (no big numbers)
+ Math::Cephes uses the external Cephes C library (no
+ big numbers)
Math::Cephes::Fraction fractions via the Cephes library
Math::GMP another one using an external C library
+ Math::GMPz an alternative interface to libgmp's big ints
+ Math::GMPq an interface to libgmp's fraction numbers
+ Math::GMPf an interface to libgmp's floating point numbers
Choose wisely.
https://perl5.git.perl.org / perl5.git / blobdiff