=head1 NAME
+X<operator>
perlop - Perl operators and precedence
-=head1 SYNOPSIS
+=head1 DESCRIPTION
+
+=head2 Operator Precedence and Associativity
+X<operator, precedence> X<precedence> X<associativity>
+
+Operator precedence and associativity work in Perl more or less like
+they do in mathematics.
+
+I<Operator precedence> means some operators are evaluated before
+others. For example, in C<2 + 4 * 5>, the multiplication has higher
+precedence so C<4 * 5> is evaluated first yielding C<2 + 20 ==
+22> and not C<6 * 5 == 30>.
+
+I<Operator associativity> defines what happens if a sequence of the
+same operators is used one after another: whether the evaluator will
+evaluate the left operations first or the right. For example, in C<8
+- 4 - 2>, subtraction is left associative so Perl evaluates the
+expression left to right. C<8 - 4> is evaluated first making the
+expression C<4 - 2 == 2> and not C<8 - 2 == 6>.
Perl operators have the following associativity and precedence,
listed from highest precedence to lowest. Operators borrowed from
left << >>
nonassoc named unary operators
nonassoc < > <= >= lt gt le ge
- nonassoc == != <=> eq ne cmp
+ nonassoc == != <=> eq ne cmp ~~
left &
left | ^
left &&
- left ||
+ left || //
nonassoc .. ...
right ?:
right = += -= *= etc.
nonassoc list operators (rightward)
right not
left and
- left or xor
+ left or xor err
In the following sections, these operators are covered in precedence order.
Many operators can be overloaded for objects. See L<overload>.
-=head1 DESCRIPTION
-
=head2 Terms and List Operators (Leftward)
+X<list operator> X<operator, list> X<term>
A TERM has the highest precedence in Perl. They include variables,
quote and quote-like operators, any expression in parentheses,
print ($foo & 255) + 1, "\n";
-probably doesn't do what you expect at first glance. See
-L<Named Unary Operators> for more discussion of this.
+probably doesn't do what you expect at first glance. The parentheses
+enclose the argument list for C<print> which is evaluated (printing
+the result of C<$foo & 255>). Then one is added to the return value
+of C<print> (usually 1). The result is something like this:
+
+ 1 + 1, "\n"; # Obviously not what you meant.
+
+To do what you meant properly, you must write:
+
+ print(($foo & 255) + 1, "\n");
+
+See L<Named Unary Operators> for more discussion of this.
Also parsed as terms are the C<do {}> and 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,
-as well as L<"I/O Operators">.
+as well as L</"I/O Operators">.
=head2 The Arrow Operator
+X<arrow> X<dereference> X<< -> >>
"C<< -> >>" is an infix dereference operator, just as it is in C
and C++. If the right side is either a C<[...]>, C<{...}>, or a
or a class name (that is, a package name). See L<perlobj>.
=head2 Auto-increment and Auto-decrement
+X<increment> X<auto-increment> X<++> X<decrement> X<auto-decrement> X<-->
+
+"++" and "--" work as in C. That is, if placed before a variable,
+they increment or decrement the variable by one before returning the
+value, and if placed after, increment or decrement after returning the
+value.
+
+ $i = 0; $j = 0;
+ print $i++; # prints 0
+ print ++$j; # prints 1
+
+Note that just as in C, Perl doesn't define B<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.
+Avoid statements like:
+
+ $i = $i ++;
+ print ++ $i + $i ++;
-"++" and "--" work as in C. That is, if placed before a variable, they
-increment or decrement the variable before returning the value, and if
-placed after, increment or decrement the variable after returning the value.
+Perl will not guarantee what the result of the above statements is.
The auto-increment operator has a little extra builtin magic to it. If
you increment a variable that is numeric, or that has ever been used in
a numeric context, you get a normal increment. If, however, the
variable has been used in only string contexts since it was set, and
has a value that is not the empty string and matches the pattern
-C</^[a-zA-Z]*[0-9]*$/>, the increment is done as a string, preserving each
+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 = '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
+will return C<0> rather than C<undef>).
+
The auto-decrement operator is not magical.
=head2 Exponentiation
+X<**> X<exponentiation> X<power>
Binary "**" is the exponentiation operator. It binds even more
tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
internally.)
=head2 Symbolic Unary Operators
+X<unary operator> X<operator, unary>
Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
precedence version of this.
+X<!>
Unary "-" performs arithmetic negation if the operand is numeric. If
the operand is an identifier, a string consisting of a minus sign
concatenated with the identifier is returned. Otherwise, if the string
starts with a plus or minus, a string starting with the opposite sign
-is returned. One effect of these rules is that C<-bareword> is equivalent
-to C<"-bareword">.
+is returned. One effect of these rules is that -bareword is equivalent
+to the string "-bareword". If, however, the string begins with a
+non-alphabetic character (exluding "+" or "-"), Perl will attempt to convert
+the string to a numeric and the arithmetic negation is performed. If the
+string cannot be cleanly converted to a numeric, Perl will give the warning
+B<Argument "the string" isn't numeric in negation (-) at ...>.
+X<-> X<negation, arithmetic>
Unary "~" performs bitwise negation, i.e., 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 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>
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
arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
+X<+>
Unary "\" creates a reference to whatever follows it. See L<perlreftut>
and L<perlref>. Do not confuse this behavior with the behavior of
backslash within a string, although both forms do convey the notion
of protecting the next thing from interpolation.
+X<\> X<reference> X<backslash>
=head2 Binding Operators
+X<binding> X<operator, binding> X<=~> X<!~>
Binary "=~" binds a scalar expression to a pattern match. Certain operations
search or modify the string $_ by default. This operator makes that kind
supposed to be searched, substituted, or transliterated instead of the default
$_. When used in scalar context, the return value generally indicates the
success of the operation. Behavior in list context depends on the particular
-operator. See L</"Regexp Quote-Like Operators"> for details.
+operator. See L</"Regexp Quote-Like Operators"> for details and
+L<perlretut> for examples using these operators.
If the right argument is an expression rather than a search pattern,
substitution, or transliteration, it is interpreted as a search pattern at run
-time. This can be less efficient than an explicit search, because the
-pattern must be compiled every time the expression is evaluated.
+time. Note that this means that its contents will be interpolated twice, so
+
+ '\\' =~ q'\\';
+
+is not ok, as the regex engine will end up trying to compile the
+pattern C<\>, which it will consider a syntax error.
Binary "!~" is just like "=~" except the return value is negated in
the logical sense.
=head2 Multiplicative Operators
+X<operator, multiplicative>
Binary "*" multiplies two numbers.
+X<*>
Binary "/" divides two numbers.
+X</> X<slash>
Binary "%" computes the modulus of two numbers. Given integer
operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
C<$a> minus the largest multiple of C<$b> that is not greater than
C<$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
-result will be less than or equal to zero).
-Note than when C<use integer> is in scope, "%" give you direct access
+result will be less than or equal to zero). If the operands
+C<$a> and C<$b> are floting point values, only the integer portion
+of C<$a> and C<$b> will be used in the operation.
+Note that when C<use integer> is in scope, "%" gives you direct access
to the modulus operator as implemented by your C compiler. This
operator is not as well defined for negative operands, but it will
execute faster.
+X<%> X<remainder> X<modulus> X<mod>
Binary "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, it repeats the list.
+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
+or an empty list, depending on the context.
+X<x>
print '-' x 80; # print row of dashes
=head2 Additive Operators
+X<operator, additive>
Binary "+" returns the sum of two numbers.
+X<+>
Binary "-" returns the difference of two numbers.
+X<->
Binary "." concatenates two strings.
+X<string, concatenation> X<concatenation>
+X<cat> X<concat> X<concatenate> X<.>
=head2 Shift Operators
+X<shift operator> X<operator, shift> X<<< << >>>
+X<<< >> >>> X<right shift> X<left shift> X<bitwise shift>
+X<shl> X<shr> X<shift, right> X<shift, left>
Binary "<<" returns the value of its left argument shifted left by the
number of bits specified by the right argument. Arguments should be
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
+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
+or 64 bits).
+
+The result of overflowing the range of the integers is undefined
+because it is undefined also in C. In other words, using 32-bit
+integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
+of bits is also undefined.
+
=head2 Named Unary Operators
+X<operator, named unary>
The various named unary operators are treated as functions with one
-argument, with optional parentheses. These include the filetest
-operators, like C<-f>, C<-M>, etc. See L<perlfunc>.
+argument, with optional parentheses.
If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
is followed by a left parenthesis as the next token, the operator and
arguments within parentheses are taken to be of highest precedence,
-just like a normal function call. Examples:
+just like a normal function call. For example,
+because named unary operators are higher precedence than ||:
chdir $foo || die; # (chdir $foo) || die
chdir($foo) || die; # (chdir $foo) || die
chdir ($foo) || die; # (chdir $foo) || die
chdir +($foo) || die; # (chdir $foo) || die
-but, because * is higher precedence than ||:
+but, because * is higher precedence than named operators:
chdir $foo * 20; # chdir ($foo * 20)
chdir($foo) * 20; # (chdir $foo) * 20
rand (10) * 20; # (rand 10) * 20
rand +(10) * 20; # rand (10 * 20)
+Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
+treated like named unary operators, but they don't follow this functional
+parenthesis rule. That means, for example, that C<-f($file).".bak"> is
+equivalent to C<-f "$file.bak">.
+X<-X> X<filetest> X<operator, filetest>
+
See also L<"Terms and List Operators (Leftward)">.
=head2 Relational Operators
+X<relational operator> X<operator, relational>
Binary "<" returns true if the left argument is numerically less than
the right argument.
+X<< < >>
Binary ">" returns true if the left argument is numerically greater
than the right argument.
+X<< > >>
Binary "<=" returns true if the left argument is numerically less than
or equal to the right argument.
+X<< <= >>
Binary ">=" returns true if the left argument is numerically greater
than or equal to the right argument.
+X<< >= >>
Binary "lt" returns true if the left argument is stringwise less than
the right argument.
+X<< lt >>
Binary "gt" returns true if the left argument is stringwise greater
than the right argument.
+X<< gt >>
Binary "le" returns true if the left argument is stringwise less than
or equal to the right argument.
+X<< le >>
Binary "ge" returns true if the left argument is stringwise greater
than or equal to the right argument.
+X<< ge >>
=head2 Equality Operators
+X<equality> X<equal> X<equals> X<operator, equality>
Binary "==" returns true if the left argument is numerically equal to
the right argument.
+X<==>
Binary "!=" returns true if the left argument is numerically not equal
to the right argument.
+X<!=>
Binary "<=>" returns -1, 0, or 1 depending on whether the left
argument is numerically less than, equal to, or greater than the right
-argument.
+argument. If your platform supports NaNs (not-a-numbers) as numeric
+values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
+"<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
+returns true, as does NaN != anything else. If your platform doesn't
+support NaNs then NaN is just a string with numeric value 0.
+X<< <=> >> X<spaceship>
+
+ perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
+ perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
Binary "eq" returns true if the left argument is stringwise equal to
the right argument.
+X<eq>
Binary "ne" returns true if the left argument is stringwise not equal
to the right argument.
+X<ne>
-Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise
-less than, equal to, or greater than the right argument.
+Binary "cmp" returns -1, 0, or 1 depending on whether the left
+argument is stringwise less than, equal to, or greater than the right
+argument.
+X<cmp>
+
+Binary "~~" does a smart match between its arguments. Smart matching
+is described in L<perlsyn/"Smart Matching in Detail">.
+This operator is only available if you enable the "~~" feature:
+see L<feature> for more information.
+X<~~>
"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
by the current locale if C<use locale> is in effect. See L<perllocale>.
=head2 Bitwise And
+X<operator, bitwise, and> X<bitwise and> X<&>
-Binary "&" returns its operators ANDed together bit by bit.
+Binary "&" returns its operands ANDed together bit by bit.
(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
+Note that "&" has lower priority than relational operators, so for example
+the brackets are essential in a test like
+
+ print "Even\n" if ($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 operators ORed together bit by bit.
+Binary "|" returns its operands ORed together bit by bit.
(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
-Binary "^" returns its operators XORed together bit by bit.
+Binary "^" returns its operands XORed together bit by bit.
(See also L<Integer Arithmetic> and 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;
+
=head2 C-style Logical And
+X<&&> X<logical and> X<operator, logical, and>
Binary "&&" performs a short-circuit logical AND operation. That is,
if the left operand is false, the right operand is not even evaluated.
is evaluated.
=head2 C-style Logical Or
+X<||> X<operator, logical, or>
Binary "||" performs a short-circuit logical OR operation. That is,
if the left operand is true, the right operand is not even evaluated.
Scalar or list context propagates down to the right operand if it
is evaluated.
-The C<||> and C<&&> operators differ from C's in that, rather than returning
-0 or 1, they return the last value evaluated. Thus, a reasonably portable
-way to find out the home directory (assuming it's not "0") might be:
+=head2 C-style 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($a)>) and is exactly equivalent to
+C<defined($a) ? $a : $b>. This is very useful for providing default values
+for variables. If you actually want to test if at least one of C<$a> and
+C<$b> is defined, use C<defined($a // $b)>.
- $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
- (getpwuid($<))[7] || die "You're homeless!\n";
+The C<||>, C<//> and C<&&> operators return the last value evaluated
+(unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
+portable way to find out the home directory might be:
+
+ $home = $ENV{'HOME'} // $ENV{'LOGDIR'} //
+ (getpwuid($<))[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
-control flow, Perl provides 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
+As more readable alternatives to C<&&>, C<//> and C<||> when used for
+control flow, Perl provides C<and>, C<err> and C<or> operators (see below).
+The short-circuit behavior is identical. The precedence of "and", "err"
+and "or" is much lower, however, so that you can safely use them after a
list operator without the need for parentheses:
unlink "alpha", "beta", "gamma"
Using "or" for assignment is unlikely to do what you want; see below.
=head2 Range Operators
+X<operator, range> X<range> X<..> X<...>
Binary ".." is the range operator, which is really two different
-operators depending on the context. In list context, it returns an
-array of values counting (up by ones) from the left value to the right
+operators depending on the context. In list context, it returns a
+list of values counting (up by ones) from the left value to the right
value. If the left value is greater than the right value then it
-returns the empty array. The range operator is useful for writing
-C<foreach (1..10)> loops and for doing slice operations on arrays. In
+returns the empty list. The range operator is useful for writing
+C<foreach (1..10)> loops and for doing slice operations on arrays. In
the current implementation, no temporary array is created when the
range operator is used as the expression in C<foreach> loops, but older
versions of Perl might burn a lot of memory when you write something
# code
}
+The range operator also works on strings, using the magical auto-increment,
+see below.
+
In scalar context, ".." returns a boolean value. The operator is
bistable, like a flip-flop, and emulates the line-range (comma) operator
of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
doesn't affect its numeric value, but gives you something to search
for if you want to exclude the endpoint. You can exclude the
beginning point by waiting for the sequence number to be greater
-than 1. If either operand of scalar ".." is a constant expression,
-that operand is implicitly compared to the C<$.> variable, the
-current line number. Examples:
+than 1.
+
+If either operand of scalar ".." is a constant expression,
+that operand is considered true if it is equal (C<==>) to the current
+input line number (the C<$.> variable).
+
+To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
+but that is only an issue if you use a floating point expression; when
+implicitly using C<$.> as described in the previous paragraph, the
+comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
+is set to a floating point value and you are not reading from a file.
+Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
+you want in scalar context because each of the operands are evaluated
+using their integer representation.
+
+Examples:
As a scalar operator:
- if (101 .. 200) { print; } # print 2nd hundred lines
- next line if (1 .. /^$/); # skip header lines
- s/^/> / if (/^$/ .. eof()); # quote body
+ if (101 .. 200) { print; } # print 2nd hundred lines, short for
+ # if ($. == 101 .. $. == 200) ...
+
+ next LINE if (1 .. /^$/); # skip header lines, short for
+ # ... if ($. == 1 .. /^$/);
+ # (typically in a loop labeled LINE)
+
+ s/^/> / if (/^$/ .. eof()); # quote body
# parse mail messages
while (<>) {
$in_header = 1 .. /^$/;
- $in_body = /^$/ .. eof();
- # do something based on those
+ $in_body = /^$/ .. eof;
+ if ($in_header) {
+ # ...
+ } else { # in body
+ # ...
+ }
} continue {
- close ARGV if eof; # reset $. each file
+ close ARGV if eof; # reset $. each file
}
-As a list operator:
+Here's a simple example to illustrate the difference between
+the two range operators:
+
+ @lines = (" - Foo",
+ "01 - Bar",
+ "1 - Baz",
+ " - Quux");
+
+ foreach (@lines) {
+ if (/0/ .. /1/) {
+ print "$_\n";
+ }
+ }
+
+This program will print only the line containing "Bar". If
+the range operator is changed to C<...>, it will also print the
+"Baz" line.
+
+And now some examples as a list operator:
for (101 .. 200) { print; } # print $_ 100 times
@foo = @foo[0 .. $#foo]; # an expensive no-op
@alphabet = ('A' .. 'Z');
-to get all normal letters of the alphabet, or
+to get all normal letters of the English alphabet, or
$hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
@z2 = ('01' .. '31'); print $z2[$mday];
-to get dates with leading zeros. If the final value specified is not
-in the sequence that the magical increment would produce, the sequence
-goes until the next value would be longer than the final value
-specified.
+to get dates with leading zeros.
+
+If the final value specified is not in the sequence that the magical
+increment would produce, the sequence goes until the next value would
+be longer than the final value specified.
+
+If the initial value specified isn't part of a magical increment
+sequence (that is, a non-empty string matching "/^[a-zA-Z]*[0-9]*\z/"),
+only the initial value will be returned. So the following will only
+return an alpha:
+
+ use charnames 'greek';
+ my @greek_small = ("\N{alpha}" .. "\N{omega}");
+
+To get lower-case greek letters, use this instead:
+
+ my @greek_small = map { chr } ( ord("\N{alpha}") .. ord("\N{omega}") );
+
+Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
+return two elements in list context.
+
+ @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
=head2 Conditional Operator
+X<operator, conditional> X<operator, ternary> X<ternary> X<?:>
Ternary "?:" is the conditional operator, just as in C. It works much
like an if-then-else. If the argument before the ? is true, the
$a += ($a % 2) ? 10 : 2;
=head2 Assignment Operators
+X<assignment> X<operator, assignment> X<=> X<**=> X<+=> X<*=> X<&=>
+X<<< <<= >>> X<&&=> X<-=> X</=> X<|=> X<<< >>= >>> X<||=> X<//=> X<.=>
+X<%=> X<^=> X<x=>
"=" is the ordinary assignment operator.
**= += *= &= <<= &&=
-= /= |= >>= ||=
- .= %= ^=
- x=
+ .= %= ^= //=
+ x=
Although these are grouped by family, they all have the precedence
of assignment.
side of the assignment.
=head2 Comma Operator
+X<comma> X<operator, comma> X<,>
Binary "," is the comma operator. In scalar context it evaluates
its left argument, throws that value away, then evaluates its right
argument and returns that value. This is just like C's comma operator.
In list context, it's just the list argument separator, and inserts
-both its arguments into the list.
+both its arguments into the list. These arguments are also evaluated
+from left to right.
+
+The C<< => >> operator is a synonym for the comma, but forces any word
+(consisting entirely of word characters) to its left to be interpreted
+as a string (as of 5.001). This includes words that might otherwise be
+considered a constant or function call.
+
+ use constant FOO => "something";
+
+ my %h = ( FOO => 23 );
+
+is equivalent to:
+
+ my %h = ("FOO", 23);
+
+It is I<NOT>:
+
+ my %h = ("something", 23);
+
+If the argument on the left is not a word, it is first interpreted as
+an expression, and then the string value of that is used.
+
+The C<< => >> operator is helpful in documenting the correspondence
+between keys and values in hashes, and other paired elements in lists.
-The => digraph is mostly just a synonym for the comma operator. It's useful for
-documenting arguments that come in pairs. As of release 5.001, it also forces
-any word to the left of it to be interpreted as a string.
+ %hash = ( $key => $value );
+ login( $username => $password );
=head2 List Operators (Rightward)
+X<operator, list, rightward> X<list operator>
On the right side of a list operator, it has very low precedence,
such that it controls all comma-separated expressions found there.
See also discussion of list operators in L<Terms and List Operators (Leftward)>.
=head2 Logical Not
+X<operator, logical, not> X<not>
Unary "not" returns the logical negation of the expression to its right.
It's the equivalent of "!" except for the very low precedence.
=head2 Logical And
+X<operator, logical, and> X<and>
Binary "and" returns the logical conjunction of the two surrounding
expressions. It's equivalent to && except for the very low
precedence. This means that it short-circuits: i.e., the right
expression is evaluated only if the left expression is true.
-=head2 Logical or and Exclusive Or
+=head2 Logical or, Defined or, and Exclusive Or
+X<operator, logical, or> X<operator, logical, xor> X<operator, logical, err>
+X<operator, logical, defined or> X<operator, logical, exclusive or>
+X<or> X<xor> X<err>
Binary "or" returns the logical disjunction of the two surrounding
expressions. It's equivalent to || except for the very low precedence.
@info = stat($file) || die; # oops, scalar sense of stat!
@info = stat($file) or die; # better, now @info gets its due
-Then again, you could always use parentheses.
+Then again, you could always use parentheses.
+
+Binary "err" is equivalent to C<//>--it's just like binary "or", except it
+tests its left argument's definedness instead of its truth. There are two
+ways to remember "err": either because many functions return C<undef> on
+an B<err>or, or as a sort of correction: C<$a = ($b err 'default')>. This
+keyword is only available when the 'err' feature is enabled: see
+L<feature> for more information.
Binary "xor" returns the exclusive-OR of the two surrounding expressions.
It cannot short circuit, of course.
=head2 C Operators Missing From Perl
+X<operator, missing from perl> X<&> X<*>
+X<typecasting> X<(TYPE)>
Here is what C has that Perl doesn't:
=back
=head2 Quote and Quote-like Operators
+X<operator, quote> X<operator, quote-like> X<q> X<qq> X<qx> X<qw> X<m>
+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
for these behaviors, but also provides a way for you to choose your
quote character for any of them. In the following table, a C<{}> represents
-any pair of delimiters you choose.
+any pair of delimiters you choose.
Customary Generic Meaning Interpolates
'' q{} Literal no
"" qq{} Literal yes
- `` qx{} Command yes (unless '' is delimiter)
+ `` qx{} Command yes*
qw{} Word list no
- // m{} Pattern match yes (unless '' is delimiter)
- qr{} Pattern yes (unless '' is delimiter)
- s{}{} Substitution yes (unless '' is delimiter)
+ // m{} Pattern match yes*
+ qr{} Pattern yes*
+ s{}{} Substitution yes*
tr{}{} 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
-that
+that
- q{foo{bar}baz}
+ q{foo{bar}baz}
-is the same as
+is the same as
'foo{bar}baz'
$s = q{ if($a eq "}") ... }; # WRONG
-is a syntax error. The C<Text::Balanced> module on CPAN is able to do this
-properly.
+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.
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.
-For constructs that do interpolate, variables beginning with "C<$>"
-or "C<@>" are interpolated, as are the following escape sequences. Within
-a transliteration, the first eleven of these sequences may be used.
+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>
\t tab (HT, TAB)
\n newline (NL)
\x1b hex char (ESC)
\x{263a} wide hex char (SMILEY)
\c[ control char (ESC)
- \N{name} named char
+ \N{name} named Unicode character
+
+B<NOTE>: Unlike C and other languages, Perl has no \v escape sequence for
+the vertical tab (VT - ASCII 11).
+
+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
\E end case modification
\Q quote non-word characters till \E
-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>. For
-documentation of C<\N{name}>, see L<charnames>.
+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>.
+If Unicode (for example, C<\N{}> or wide hex characters 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. For documentation of C<\N{name}>,
+see L<charnames>.
All systems use the virtual C<"\n"> to represent a line terminator,
called a "newline". There is no such thing as an unvarying, physical
printing C<"\n"> may 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<"\012\015"> or C<"\cJ\cM">) for line terminators,
+and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators,
and although they often accept just C<"\012">, they seldom tolerate just
C<"\015">. If you get in the habit of using C<"\n"> for networking,
you may be burned some day.
+X<newline> X<line terminator> X<eol> X<end of line>
+X<\n> X<\r> X<\r\n>
+
+For constructs that do interpolate, variables beginning with "C<$>"
+or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
+C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
+But method calls such as C<< $obj->meth >> are not.
-You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
-An unescaped C<$> or C<@> interpolates the corresponding variable,
+Interpolating an array or slice interpolates the elements in order,
+separated by the value of C<$">, so is equivalent to interpolating
+C<join $", @array>. "Punctuation" arrays such as C<@+> are only
+interpolated if the name is enclosed in braces C<@{+}>.
+
+You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
+An unescaped C<$> or C<@> interpolates the corresponding variable,
while escaping will cause the literal string C<\$> to be inserted.
-You'll need to write something like C<m/\Quser\E\@\Qhost/>.
+You'll need to write something like C<m/\Quser\E\@\Qhost/>.
Patterns are subject to an additional level of interpretation as a
regular expression. This is done as a second pass, after variables are
variables when used within double quotes.
=head2 Regexp Quote-Like Operators
+X<operator, regexp>
Here are the quote-like operators that apply to pattern
matching and related activities.
=over 8
=item ?PATTERN?
+X<?>
This is just like the C</pattern/> search, except that it matches only
once between calls to the reset() operator. This is a useful
reset if eof; # clear ?? status for next file
}
-This usage is vaguely depreciated, which means it just might possibly
+This usage is vaguely deprecated, which means it just might possibly
be removed in some distant future version of Perl, perhaps somewhere
around the year 2168.
=item m/PATTERN/cgimosx
+X<m> X<operator, match>
+X<regexp, options> X<regexp> X<regex, options> X<regex>
+X</c> X</i> X</m> X</o> X</s> X</x>
=item /PATTERN/cgimosx
x Use extended regular expressions.
If "/" is the delimiter then the initial C<m> is optional. With the C<m>
-you can use any pair of non-alphanumeric, non-whitespace characters
+you can use any pair of non-alphanumeric, non-whitespace characters
as delimiters. This is particularly useful for matching path names
that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
PATTERN may contain variables, which will be interpolated (and the
pattern recompiled) every time the pattern search is evaluated, except
-for when the delimiter is a single quote. (Note that C<$)> and C<$|>
-might not be interpolated because they look like end-of-string tests.)
+for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and
+C<$|> are not interpolated because they look like end-of-string tests.)
If you want such a pattern to be compiled only once, add a C</o> after
the trailing delimiter. This avoids expensive run-time recompilations,
and is useful when the value you are interpolating won't change over
the life of the script. However, mentioning C</o> constitutes a promise
that you won't change the variables in the pattern. If you change them,
-Perl won't even notice. See also L<"qr//">.
+Perl won't even notice. See also L<"qr/STRING/imosx">.
If the PATTERN evaluates to the empty string, the last
-I<successfully> matched regular expression is used instead.
+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 -
+the other flags are taken from the original pattern. If no match has
+previously succeeded, this will (silently) act instead as a genuine
+empty pattern (which will always match).
+
+Note that it's possible to confuse Perl into thinking C<//> (the empty
+regex) is really C<//> (the defined-or operator). Perl is usually pretty
+good about this, but some pathological cases might trigger this, such as
+C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
+(C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
+will assume you meant defined-or. If you meant the empty regex, just
+use parentheses or spaces to disambiguate, or even prefix the empty
+regex with an C<m> (so C<//> becomes C<m//>).
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
You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
zero-width assertion that matches the exact position where the previous
-C<m//g>, if any, left off. The C<\G> assertion is not supported without
-the C</g> modifier. (Currently, without C</g>, C<\G> behaves just like
-C<\A>, but that's accidental and may change in the future.)
+C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion
+still anchors at pos(), but the match is of course only attempted once.
+Using C<\G> without C</g> on a target string that has not previously had a
+C</g> match applied to it is the same as using the C<\A> assertion to match
+the beginning of the string. Note also that, currently, C<\G> is only
+properly supported when anchored at the very beginning of the pattern.
Examples:
($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
# scalar context
- $/ = ""; $* = 1; # $* deprecated in modern perls
+ $/ = "";
while (defined($paragraph = <>)) {
while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
$sentences++;
print "3: '";
print $1 while /(p)/gc; print "', pos=", pos, "\n";
}
+ print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
The last example should print:
1: '', pos=7
2: 'q', pos=8
3: '', pos=8
+ Final: 'q', pos=8
+
+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.
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,
MiXeD line-noise. That's all!
=item q/STRING/
+X<q> X<quote, single> X<'> X<''>
-=item C<'STRING'>
+=item 'STRING'
A single-quoted, literal string. A backslash represents a backslash
unless followed by the delimiter or another backslash, in which case
$baz = '\n'; # a two-character string
=item qq/STRING/
+X<qq> X<quote, double> X<"> X<"">
=item "STRING"
$baz = "\n"; # a one-character string
=item qr/STRING/imosx
+X<qr> X</i> X</m> X</o> X</s> X</x>
-This operators quotes--and compiles--its I<STRING> as a regular
+This operator quotes (and possibly compiles) its I<STRING> as a regular
expression. I<STRING> is interpolated the same way as I<PATTERN>
in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
is done. Returns a Perl value which may be used instead of the
for a detailed look at the semantics of regular expressions.
=item qx/STRING/
+X<qx> X<`> X<``> X<backtick>
=item `STRING`
-A string which is (possibly) interpolated and then executed as a system
-command with C</bin/sh> or its equivalent. Shell wildcards, pipes,
-and redirections will be honored. The collected standard output of the
-command is returned; standard error is unaffected. In scalar context,
-it comes back as a single (potentially multi-line) string. In list
-context, returns a list of lines (however you've defined lines with $/
-or $INPUT_RECORD_SEPARATOR).
+A string which is (possibly) interpolated and then executed as a
+system command with C</bin/sh> or its equivalent. Shell wildcards,
+pipes, and redirections will be honored. The collected standard
+output of the command is returned; standard error is unaffected. In
+scalar context, it comes back as a single (potentially multi-line)
+string, or undef if the command failed. In list context, returns a
+list of lines (however you've defined lines with $/ or
+$INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
Because backticks do not affect standard error, use shell file descriptor
syntax (assuming the shell supports this) if you care to address this.
$output = `cmd 3>&1 1>&2 2>&3 3>&-`;
To read both a command's STDOUT and its STDERR separately, it's easiest
-and safest to redirect them separately to files, and then read from those
-files when the program is done:
+to redirect them separately to files, and then read from those files
+when the program is done:
+
+ system("program args 1>program.stdout 2>program.stderr");
+
+The STDIN filehandle used by the command is inherited from Perl's STDIN.
+For example:
- system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
+ open BLAM, "blam" || die "Can't open: $!";
+ open STDIN, "<&BLAM";
+ print `sort`;
+
+will print the sorted contents of the file "blam".
Using single-quote as a delimiter protects the command from Perl's
double-quote interpolation, passing it on to the shell instead:
a glue language, and one of the things it glues together is commands.
Just understand what you're getting yourself into.
-See L<"I/O Operators"> for more discussion.
+See L</"I/O Operators"> for more discussion.
=item qw/STRING/
+X<qw> X<quote, list> X<quote, words>
Evaluates to a list of the words extracted out of STRING, using embedded
whitespace as the word delimiters. It can be understood as being roughly
split(' ', q/STRING/);
-the difference being that it generates a real list at compile time. So
+the differences being that it generates a real list at compile time, and
+in scalar context it returns the last element in the list. So
this expression:
qw(foo bar baz)
A common mistake is to try to separate the words with comma or to
put comments into a multi-line C<qw>-string. For this reason, the
-C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
+C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
produces warnings if the STRING contains the "," or the "#" character.
=item s/PATTERN/REPLACEMENT/egimosx
+X<substitute> X<substitution> X<replace> X<regexp, replace>
+X<regexp, substitute> X</e> X</g> X</i> X</m> X</o> X</s> X</x>
Searches a string for a pattern, and if found, replaces that pattern
with the replacement text and returns the number of substitutions
\*/ # Match the closing delimiter.
} []gsx;
- s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively
+ s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
- for ($variable) { # trim white space in $variable, cheap
+ for ($variable) { # trim whitespace in $variable, cheap
s/^\s+//;
s/\s+$//;
}
to occur that you might want. Here are two common cases:
# put commas in the right places in an integer
- 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
+ 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
# expand tabs to 8-column spacing
1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
-=item tr/SEARCHLIST/REPLACEMENTLIST/cdsUC
+=item tr/SEARCHLIST/REPLACEMENTLIST/cds
+X<tr> X<y> X<transliterate> X</c> X</d> X</s>
-=item y/SEARCHLIST/REPLACEMENTLIST/cdsUC
+=item y/SEARCHLIST/REPLACEMENTLIST/cds
Transliterates all occurrences of the characters found in the search list
with the corresponding character in the replacement list. It returns
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.)
-A character range may be specified with a hyphen, so C<tr/A-J/0-9/>
+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 <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.
+
Note also that the whole range idea is rather unportable between
character sets--and even within character sets they may cause results
you probably didn't expect. A sound principle is to use only ranges
c Complement the SEARCHLIST.
d Delete found but unreplaced characters.
s Squash duplicate replaced characters.
- U Translate to/from UTF-8.
- C Translate to/from 8-bit char (octet).
If the C</c> modifier is specified, the SEARCHLIST character set
is complemented. If the C</d> modifier is specified, any characters
This latter is useful for counting characters in a class or for
squashing character sequences in a class.
-The first C</U> or C</C> modifier applies to the left side of the translation.
-The second one applies to the right side. If present, these modifiers override
-the current utf8 state.
-
Examples:
$ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
tr [\200-\377]
[\000-\177]; # delete 8th bit
- tr/\0-\xFF//CU; # change Latin-1 to Unicode
- tr/\0-\x{FF}//UC; # change Unicode to Latin-1
-
If multiple transliterations are given for a character, only the
first one is used:
eval "tr/$oldlist/$newlist/, 1" or die $@;
+=item <<EOF
+X<here-doc> X<heredoc> X<here-document> X<<< << >>>
+
+A line-oriented form of quoting is based on the shell "here-document"
+syntax. Following a C<< << >> you specify a string to terminate
+the quoted material, and all lines following the current line down to
+the terminating string are the value of the item.
+
+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.
+
+If the terminating string is quoted, the type of quotes used determine
+the treatment of the text.
+
+=over 4
+
+=item Double Quotes
+
+Double quotes indicate that the text will be interpolated using exactly
+the same rules as normal double quoted strings.
+
+ print <<EOF;
+ The price is $Price.
+ EOF
+
+ print << "EOF"; # same as above
+ The price is $Price.
+ EOF
+
+
+=item Single Quotes
+
+Single quotes indicate the text is to be treated literally with no
+interpolation of its content. This is similar to single quoted
+strings except that backslashes have no special meaning, with C<\\>
+being treated as two backslashes and not one as they would in every
+other quoting construct.
+
+This is the only form of quoting in perl where there is no need
+to worry about escaping content, something that code generators
+can and do make good use of.
+
+=item Backticks
+
+The content of the here doc is treated just as it would be if the
+string were embedded in backticks. Thus the content is interpolated
+as though it were double quoted and then executed via the shell, with
+the results of the execution returned.
+
+ print << `EOC`; # execute command and get results
+ echo hi there
+ EOC
+
+=back
+
+It is possible to stack multiple here-docs in a row:
+
+ print <<"foo", <<"bar"; # you can stack them
+ I said foo.
+ foo
+ I said bar.
+ bar
+
+ myfunc(<< "THIS", 23, <<'THAT');
+ Here's a line
+ or two.
+ THIS
+ and here's another.
+ THAT
+
+Just don't forget that you have to put a semicolon on the end
+to finish the statement, as Perl doesn't know you're not going to
+try to do this:
+
+ print <<ABC
+ 179231
+ ABC
+ + 20;
+
+If you want to remove the line terminator from your here-docs,
+use C<chomp()>.
+
+ chomp($string = <<'END');
+ This is a string.
+ 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:
+
+ ($quote = <<'FINIS') =~ s/^\s+//gm;
+ The Road goes ever on and on,
+ down from the door where it began.
+ 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
+
+ s/this/<<E . 'that'
+ the other
+ E
+ . 'more '/eg;
+
+you 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...>.
+
+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
+supported in place of C<''> and C<"">, and the only interpolation is for
+backslashing the quoting character:
+
+ print << "abc\"def";
+ testing...
+ abc"def
+
+Finally, quoted strings cannot span multiple lines. The general rule is
+that the identifier must be a string literal. Stick with that, and you
+should be safe.
+
=back
=head2 Gory details of parsing quoted constructs
+X<quote, gory details>
When presented with something that might have several different
interpretations, Perl uses the B<DWIM> (that's "Do What I Mean")
quoting constructs, Perl performs different numbers of passes, from
one to five, but these passes are always performed in the same order.
-=over
+=over 4
=item Finding the end
The first pass is finding the end of the quoted construct, whether
-it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF>
+it be a multicharacter delimiter C<"EOF\n"> in the C<<<EOF>
construct, a C</> that terminates a C<qq//> construct, a C<]> which
terminates C<qq[]> construct, or a C<< > >> which terminates a
fileglob started with C<< < >>.
when searching for single-character pairing delimiter like C<[>,
combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested
C<[>, C<]> are skipped as well. When searching for multicharacter
-delimiters, nothing is skipped.
+delimiters like C<"EOF\n">, nothing is skipped, though the delimiter
+must start from the first column of the terminating line.
For constructs with three-part delimiters (C<s///>, C<y///>, and
C<tr///>), the search is repeated once more.
or:
- m/
+ m/
bar # NOT a comment, this slash / terminated m//!
/x
the example above is not C<m//x>, but rather C<m//> with no C</x>
modifier. So the embedded C<#> is interpreted as a literal C<#>.
+Also no attention is paid to C<\c\> (multichar control char syntax) during
+this search. Thus the second C<\> in C<qq/\c\/> is interpreted as a part
+of C<\/>, and the following C</> is not recognized as a delimiter.
+Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
+
=item Removal of backslashes before delimiters
During the second pass, text between the starting and ending
delimiters is copied to a safe location, and the C<\> is removed
from combinations consisting of C<\> and delimiter--or delimiters,
-meaning both starting and ending delimiters will should these differ.
-This removal does not happen for multi-character delimiters.
-Note that the combination C<\\> is left intact, just as it was.
+meaning both starting and ending delimiters will be handled,
+should these differ. This removal does not happen for multi-character
+delimiters. Note that the combination C<\\> is left intact.
Starting from this step no information about the delimiters is
used in parsing.
=item Interpolation
+X<interpolation>
The next step is interpolation in the text obtained, which is now
-delimiter-independent. There are four different cases.
+delimiter-independent. There are multiple cases.
-=over
+=over 4
-=item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
+=item C<<<'EOF'>
No interpolation is performed.
+=item C<m''>, C<s'''>
+
+No interpolation is performed at this stage, see
+L</"Interpolation of regular expressions"> for comments on later
+processing of their contents.
+
=item C<''>, C<q//>
-The only interpolation is removal of C<\> from pairs C<\\>.
+The only interpolation is removal of C<\> from pairs of C<\\>.
+
+=item C<tr///>, C<y///>
+
+No variable interpolation occurs. Escape sequences such as \200
+and the common escapes such as \t for tab are converted to literals.
+The character C<-> is treated specially and therefore C<\-> is treated
+as a literal C<->.
-=item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>
+=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
converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
Interpolated scalars and arrays are converted internally to the C<join> and
-C<.> catentation operations. Thus, C<"$foo XXX '@arr'"> becomes:
+C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
$foo . " XXX '" . (join $", @arr) . "'";
scalar.
Note also that the interpolation code needs to make a decision on
-where the interpolated scalar ends. For instance, whether
+where the interpolated scalar ends. For instance, whether
C<< "a $b -> {c}" >> really means:
"a " . $b . " -> {c}";
on heuristic estimators, the result is not strictly predictable.
Fortunately, it's usually correct for ambiguous cases.
-=item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
+=item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
happens (almost) as with C<qq//> constructs, but the substitution
In the RE above, which is intentionally obfuscated for illustration, the
delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the
-RE is the same as for C<m/ ^ a s* b /mx>). There's more than one
+RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
reason you're encouraged to restrict your delimiters to non-alphanumeric,
non-whitespace choices.
which are processed further.
=item Interpolation of regular expressions
+X<regexp, interpolation>
Previous steps were performed during the compilation of Perl code,
but this one happens at run time--although it may be optimized to
switch documented in L<perlrun/"Command Switches">.
=item Optimization of regular expressions
+X<regexp, optimization>
This step is listed for completeness only. Since it does not change
semantics, details of this step are not documented and are subject
=back
=head2 I/O Operators
+X<operator, i/o> X<operator, io> X<io> X<while> X<filehandle>
+X<< <> >> X<@ARGV>
There are several I/O operators you should know about.
A string enclosed by backticks (grave accents) first undergoes
double-quote interpolation. It is then interpreted as an external
command, and the output of that command is the value of the
-pseudo-literal, j
-string consisting of all output is returned. In list context, a
-list of values is returned, one per line of output. (You can set
-C<$/> to use a different line terminator.) The command is executed
-each time the pseudo-literal is evaluated. The status value of the
-command is returned in C<$?> (see L<perlvar> for the interpretation
-of C<$?>). Unlike in B<csh>, no translation is done on the return
-data--newlines remain newlines. Unlike in any of the shells, single
-quotes do not hide variable names in the command from interpretation.
-To pass a literal dollar-sign through to the shell you need to hide
-it with a backslash. The generalized form of backticks is C<qx//>.
-(Because backticks always undergo shell expansion as well, see
-L<perlsec> for security concerns.)
+backtick string, like in a shell. In scalar context, a single string
+consisting of all output is returned. In list context, a list of
+values is returned, one per line of output. (You can set C<$/> to use
+a different line terminator.) The command is executed each time the
+pseudo-literal is evaluated. The status value of the command is
+returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
+Unlike in B<csh>, no translation is done on the return data--newlines
+remain newlines. Unlike in any of the shells, single quotes do not
+hide variable names in the command from interpretation. To pass a
+literal dollar-sign through to the shell you need to hide it with a
+backslash. The generalized form of backticks is C<qx//>. (Because
+backticks always undergo shell expansion as well, see L<perlsec> for
+security concerns.)
+X<qx> X<`> X<``> X<backtick> X<glob>
In scalar context, evaluating a filehandle in angle brackets yields
the next line from that file (the newline, if any, included), or
the value is automatically assigned to the global variable $_,
destroying whatever was there previously. (This may seem like an
odd thing to you, but you'll use the construct in almost every Perl
-script you write.) The $_ variables is not implicitly localized.
+script you write.) The $_ variable is not implicitly localized.
You'll have to put a C<local $_;> before the loop if you want that
to happen.
This also behaves similarly, but avoids $_ :
- while (my $line = <STDIN>) { print $line }
+ 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
while (<STDIN>) { last unless $_; ... }
In other boolean contexts, C<< <I<filehandle>> >> without an
-explicit C<defined> test or comparison elicit a warning if the
+explicit C<defined> test or comparison elicit a warning if the
C<use warnings> pragma or the B<-w>
command-line switch (the C<$^W> variable) is in effect.
rather than global.) Additional filehandles may be created with
the open() function, amongst others. See L<perlopentut> and
L<perlfunc/open> for details on this.
+X<stdin> X<stdout> X<sterr>
If a <FILEHANDLE> is used in a context that is looking for
a list, a list comprising all input lines is returned, one line per
continue as though the input were one big happy file. See the example
in L<perlfunc/eof> for how to reset line numbers on each file.
-If you want to set @ARGV to your own list of files, go right ahead.
+If you want to set @ARGV to your own list of files, go right ahead.
This sets @ARGV to all plain text files if no @ARGV was given:
@ARGV = grep { -f && -T } glob('*') unless @ARGV;
# ... # code for each line
}
-The <> symbol will return C<undef> for end-of-file only once.
-If you call it again after this, it will assume you are processing another
+The <> symbol will return C<undef> for end-of-file only once.
+If you call it again after this, it will assume you are processing another
@ARGV list, and if you haven't set @ARGV, will read input from STDIN.
-If angle brackets contain is a simple scalar variable (e.g.,
+If what the angle brackets contain is a simple scalar variable (e.g.,
<$foo>), then that variable contains the name of the
filehandle to input from, or its typeglob, or a reference to the
same. For example:
grounds alone. That means C<< <$x> >> is always a readline() from
an indirect handle, but C<< <$hash{key}> >> is always a glob().
That's because $x is a simple scalar variable, but C<$hash{key}> is
-not--it's a hash element.
+not--it's a hash element. Even C<< <$x > >> (note the extra space)
+is treated as C<glob("$x ")>, not C<readline($x)>.
One level of double-quote interpretation is done first, but you can't
say C<< <$foo> >> because that's an indirect filehandle as explained
open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
while (<FOO>) {
- chop;
+ chomp;
chmod 0644, $_;
}
starting a new list. All values must be read before it will start
over. In list context, this isn't important because you automatically
get them all anyway. However, in scalar context the operator returns
-the next value each time it's called, or C
+the next value each time it's called, or C<undef> when the list has
run out. As with filehandle reads, an automatic C<defined> is
generated when the glob occurs in the test part of a C<while>,
because legal glob returns (e.g. a file called F<0>) would otherwise
because the latter will alternate between returning a filename and
returning false.
-It you're trying to do variable interpolation, it's definitely better
+If you're trying to do variable interpolation, it's definitely better
to use the glob() function, because the older notation can cause people
to become confused with the indirect filehandle notation.
@files = glob($files[$i]);
=head2 Constant Folding
+X<constant folding> X<folding>
Like C, Perl does a certain amount of expression evaluation at
compile time whenever it determines that all arguments to an
the compiler will precompute 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
+context, so you can for example safely do
+
+ 1 while foo();
+
=head2 Bitwise String Operators
+X<operator, bitwise, string>
Bitstrings of any size may be manipulated by the bitwise operators
(C<~ | & ^>).
The granularity for such extension or truncation is one or more
bytes.
- # ASCII-based examples
+ # ASCII-based examples
print "j p \n" ^ " a h"; # prints "JAPH\n"
print "JA" | " ph\n"; # prints "japh\n"
print "japh\nJunk" & '_____'; # prints "JAPH\n";
a B<numeric> bitwise operation. You may explicitly show which type of
operation you intend by using C<""> or C<0+>, as in the examples below.
- $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF)
- $foo = '150' | 105 ; # yields 255
+ $foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
+ $foo = '150' | 105; # yields 255
$foo = 150 | '105'; # yields 255
$foo = '150' | '105'; # yields string '155' (under ASCII)
in a bit vector.
=head2 Integer Arithmetic
+X<integer>
By default, Perl assumes that it must do most of its arithmetic in
floating point. But by saying
or so.
Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
-and ">>") always produce integral results. (But see also L<Bitwise
-String Operators>.) However, C<use integer> still has meaning for
+and ">>") always produce integral results. (But see also
+L<Bitwise String Operators>.) However, C<use integer> still has meaning for
them. By default, their results are interpreted as unsigned integers, but
if C<use integer> is in effect, their results are interpreted
as signed integers. For example, C<~0> usually evaluates to a large
-integral value. However, C<use integer; ~0> is C<-1> on twos-complement
+integral value. However, C<use integer; ~0> is C<-1> on two's-complement
machines.
=head2 Floating-point Arithmetic
+X<floating-point> X<floating point> X<float> X<real>
While C<use integer> provides integer-only arithmetic, there is no
analogous mechanism to provide automatic rounding or truncation to a
need yourself.
=head2 Bigger Numbers
+X<number, arbitrary precision>
The standard Math::BigInt and Math::BigFloat modules provide
variable-precision arithmetic and overloaded operators, although
-they're currently pretty slow. At the cost of some space and
+they're currently pretty slow. At the cost of some space and
considerable speed, they avoid the normal pitfalls associated with
limited-precision representations.
# prints +15241578780673678515622620750190521
-The non-standard modules SSLeay::BN and Math::Pari provide
-equivalent functionality (and much more) with a substantial
-performance savings.
+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.
+
+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::Fraction fractions via the Cephes library
+ Math::GMP another one using an external C library
+
+Choose wisely.
=cut
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