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+=head1 NAME
+X<operator>
+
+perlop - Perl operators and precedence
+
+=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
+C keep the same precedence relationship with each other, even where
+C's precedence is slightly screwy. (This makes learning Perl easier
+for C folks.) With very few exceptions, these all operate on scalar
+values only, not array values.
+
+ left terms and list operators (leftward)
+ left ->
+ nonassoc ++ --
+ right **
+ right ! ~ \ and unary + and -
+ left =~ !~
+ left * / % x
+ left + - .
+ left << >>
+ nonassoc named unary operators
+ nonassoc < > <= >= lt gt le ge
+ nonassoc == != <=> eq ne cmp ~~
+ left &
+ left | ^
+ left &&
+ left || //
+ nonassoc .. ...
+ right ?:
+ right = += -= *= etc.
+ left , =>
+ nonassoc list operators (rightward)
+ right not
+ left and
+ left or xor
+
+In the following sections, these operators are covered in precedence order.
+
+Many operators can be overloaded for objects. See L<overload>.
+
+=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,
+and any function whose arguments are parenthesized. Actually, there
+aren't really functions in this sense, just list operators and unary
+operators behaving as functions because you put parentheses around
+the arguments. These are all documented in L<perlfunc>.
+
+If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
+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.
+
+In the absence of parentheses, the precedence of list operators such as
+C<print>, C<sort>, or C<chmod> is either very high or very low depending on
+whether you are looking at the left side or the right side of the operator.
+For example, in
+
+ @ary = (1, 3, sort 4, 2);
+ print @ary; # prints 1324
+
+the commas on the right of the sort are evaluated before the sort,
+but the commas on the left are evaluated after. In other words,
+list operators tend to gobble up all arguments that follow, and
+then act like a simple TERM with regard to the preceding expression.
+Be careful with parentheses:
+
+ # These evaluate exit before doing the print:
+ print($foo, exit); # Obviously not what you want.
+ print $foo, exit; # Nor is this.
+
+ # These do the print before evaluating exit:
+ (print $foo), exit; # This is what you want.
+ print($foo), exit; # Or this.
+ print ($foo), exit; # Or even this.
+
+Also note that
+
+ print ($foo & 255) + 1, "\n";
+
+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">.
+
+=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
+C<(...)> subscript, then the left side must be either a hard or
+symbolic reference to an array, a hash, or a subroutine respectively.
+(Or technically speaking, a location capable of holding a hard
+reference, if it's an array or hash reference being used for
+assignment.) See L<perlreftut> and L<perlref>.
+
+Otherwise, the right side is a method name or a simple scalar
+variable containing either the method name or a subroutine reference,
+and the left side must be either an object (a blessed reference)
+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 + $i ++;
+
+Perl will not guarantee what the result of the above statements is.
+
+The auto-increment operator has a little extra builtin magic to it. If
+you increment a variable that is numeric, or that has ever been used in
+a numeric context, you get a normal increment. If, however, the
+variable has been used in only string contexts since it was set, and
+has a value that is not the empty string and matches the pattern
+C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each
+character within its range, with carry:
+
+ print ++($foo = '99'); # prints '100'
+ print ++($foo = 'a0'); # prints 'a1'
+ print ++($foo = 'Az'); # prints 'Ba'
+ print ++($foo = 'zz'); # prints 'aaa'
+
+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
+implemented using C's pow(3) function, which actually works on doubles
+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 -bareword is equivalent
+to the string "-bareword". If, however, the string begins with a
+non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert
+the string to a numeric and the arithmetic negation is performed. If the
+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 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 $_ by default. This operator makes that kind
+of operation work on some other string. The right argument is a search
+pattern, substitution, or transliteration. The left argument is what is
+supposed to be searched, substituted, or transliterated instead of the default
+$_. When used in scalar context, the return value generally indicates the
+success of the operation. Behavior in list context depends on the particular
+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. 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 "%" is the modulo operator, which computes the division
+remainder of its first argument with respect to its second argument.
+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<$b> less than or equal to
+C<$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
+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
+the integer portion of C<$a> and C<$b> will be used in the operation
+(Note: here C<UV_MAX> means the maximum of the unsigned integer type).
+If the absolute value of the right operand (C<abs($b)>) is greater than
+or equal to C<(UV_MAX + 1)>, "%" computes the floating-point remainder
+C<$r> in the equation C<($r = $a - $i*$b)> where C<$i> is a certain
+integer that makes C<$r> have the same sign as the right operand
+C<$b> (B<not> as the left operand C<$a> like C function C<fmod()>)
+and the absolute value less than that of C<$b>.
+Note that when C<use integer> is in scope, "%" gives you direct access
+to the modulo operator as implemented by your C compiler. This
+operator is not as well defined for negative operands, but it will
+execute faster.
+X<%> X<remainder> X<modulo> X<mod>
+
+Binary "x" is the repetition operator. In scalar context or if the left
+operand is not enclosed in parentheses, it returns a string consisting
+of the left operand repeated the number of times specified by the right
+operand. In list context, if the left operand is enclosed in
+parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
+If the right operand is zero or negative, it returns an empty string
+or an empty list, depending on the context.
+X<x>
+
+ print '-' x 80; # print row of dashes
+
+ print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
+
+ @ones = (1) x 80; # a list of 80 1's
+ @ones = (5) x @ones; # set all elements to 5
+
+
+=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
+integers. (See also L<Integer Arithmetic>.)
+
+Binary ">>" 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
+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.
+
+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. 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 named operators:
+
+ chdir $foo * 20; # chdir ($foo * 20)
+ chdir($foo) * 20; # (chdir $foo) * 20
+ chdir ($foo) * 20; # (chdir $foo) * 20
+ chdir +($foo) * 20; # chdir ($foo * 20)
+
+ rand 10 * 20; # rand (10 * 20)
+ rand(10) * 20; # (rand 10) * 20
+ rand (10) * 20; # (rand 10) * 20
+ rand +(10) * 20; # rand (10 * 20)
+
+Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
+treated like named unary operators, but they don't follow this functional
+parenthesis rule. That means, for example, that C<-f($file).".bak"> is
+equivalent to C<-f "$file.bak">.
+X<-X> X<filetest> X<operator, filetest>
+
+See also L<"Terms and List Operators (Leftward)">.
+
+=head2 Relational Operators
+X<relational operator> X<operator, relational>
+
+Binary "<" returns true if the left argument is numerically less than
+the right argument.
+X<< < >>
+
+Binary ">" returns true if the left argument is numerically greater
+than the right argument.
+X<< > >>
+
+Binary "<=" returns true if the left argument is numerically less than
+or equal to the right argument.
+X<< <= >>
+
+Binary ">=" returns true if the left argument is numerically greater
+than or equal to the right argument.
+X<< >= >>
+
+Binary "lt" returns true if the left argument is stringwise less than
+the right argument.
+X<< lt >>
+
+Binary "gt" returns true if the left argument is stringwise greater
+than the right argument.
+X<< gt >>
+
+Binary "le" returns true if the left argument is stringwise less than
+or equal to the right argument.
+X<< le >>
+
+Binary "ge" returns true if the left argument is stringwise greater
+than or equal to the right argument.
+X<< ge >>
+
+=head2 Equality Operators
+X<equality> X<equal> X<equals> X<operator, equality>
+
+Binary "==" returns true if the left argument is numerically equal to
+the right argument.
+X<==>
+
+Binary "!=" returns true if the left argument is numerically not equal
+to the right argument.
+X<!=>
+
+Binary "<=>" returns -1, 0, or 1 depending on whether the left
+argument is numerically less than, equal to, or greater than the right
+argument. If your platform supports NaNs (not-a-numbers) as numeric
+values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
+"<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
+returns true, as does NaN != anything else. If your platform doesn't
+support NaNs then NaN is just a string with numeric value 0.
+X<< <=> >> X<spaceship>
+
+ perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
+ perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
+
+Binary "eq" returns true if the left argument is stringwise equal to
+the right argument.
+X<eq>
+
+Binary "ne" returns true if the left argument is stringwise not equal
+to the right argument.
+X<ne>
+
+Binary "cmp" returns -1, 0, or 1 depending on whether the left
+argument is stringwise less than, equal to, or greater than the right
+argument.
+X<cmp>
+
+Binary "~~" does a smart match between its arguments. Smart matching
+is described in L<perlsyn/"Smart matching in detail">.
+X<~~>
+
+"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
+by the current locale if C<use locale> is in effect. See L<perllocale>.
+
+=head2 Bitwise And
+X<operator, bitwise, and> X<bitwise and> X<&>
+
+Binary "&" returns its operands ANDed together bit by bit.
+(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
+
+Note that "&" has lower priority than relational operators, so for example
+the brackets are essential in a test like
+
+ print "Even\n" if ($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 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>.)
+
+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.
+Scalar or list context propagates down to the right operand if it
+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.
+
+=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)>.
+
+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 = @b || @c; # this is wrong
+ @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 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
+list operator without the need for parentheses:
+
+ unlink "alpha", "beta", "gamma"
+ or gripe(), next LINE;
+
+With the C-style operators that would have been written like this:
+
+ unlink("alpha", "beta", "gamma")
+ || (gripe(), next LINE);
+
+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 a
+list of values counting (up by ones) from the left value to the right
+value. If the left value is greater than the right value then it
+returns the empty list. The range operator is useful for writing
+C<foreach (1..10)> loops and for doing slice operations on arrays. In
+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
+like this:
+
+ for (1 .. 1_000_000) {
+ # 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 own boolean state, even across calls to a subroutine
+that contains it. It is false as long as its left operand is false.
+Once the left operand is true, the range operator stays true until the
+right operand is true, I<AFTER> which the range operator becomes false
+again. It doesn't become false till the next time the range operator
+is evaluated. It can test the right operand and become false on the
+same evaluation it became true (as in B<awk>), but it still returns
+true once. If you don't want it to test the right operand until the
+next evaluation, as in B<sed>, just use three dots ("...") instead of
+two. In all other regards, "..." behaves just like ".." does.
+
+The right operand is not evaluated while the operator is in the
+"false" state, and the left operand is not evaluated while the
+operator is in the "true" state. The precedence is a little lower
+than || and &&. The value returned is either the empty string for
+false, or a sequence number (beginning with 1) for true. The sequence
+number is reset for each range encountered. The final sequence number
+in a range has the string "E0" appended to it, which doesn't affect
+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 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, short for
+ # if ($. == 101 .. $. == 200) { print; }
+
+ next LINE if (1 .. /^$/); # skip header lines, short for
+ # next LINE if ($. == 1 .. /^$/);
+ # (typically in a loop labeled LINE)
+
+ s/^/> / if (/^$/ .. eof()); # quote body
+
+ # parse mail messages
+ while (<>) {
+ $in_header = 1 .. /^$/;
+ $in_body = /^$/ .. eof;
+ if ($in_header) {
+ # do something
+ } else { # in body
+ # do something else
+ }
+ } continue {
+ close ARGV if eof; # reset $. each file
+ }
+
+Here's a simple example to illustrate the difference between
+the two range operators:
+
+ @lines = (" - Foo",
+ "01 - Bar",
+ "1 - Baz",
+ " - Quux");
+
+ foreach (@lines) {
+ if (/0/ .. /1/) {
+ print "$_\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 .. $#foo]; # an expensive no-op
+ @foo = @foo[$#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');
+
+to get all normal letters of the English alphabet, or
+
+ $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
+
+to get a hexadecimal digit, or
+
+ @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.
+
+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
+argument before the : is returned, otherwise the argument after the :
+is returned. For example:
+
+ printf "I have %d dog%s.\n", $n,
+ ($n == 1) ? '' : "s";
+
+Scalar or list context propagates downward into the 2nd
+or 3rd argument, whichever is selected.
+
+ $a = $ok ? $b : $c; # get a scalar
+ @a = $ok ? @b : @c; # get an array
+ $a = $ok ? @b : @c; # oops, that's just a count!
+
+The operator may be assigned to if both the 2nd and 3rd arguments are
+legal lvalues (meaning that you can assign to them):
+
+ ($a_or_b ? $a : $b) = $c;
+
+Because this operator produces an assignable result, using assignments
+without parentheses will get you in trouble. For example, this:
+
+ $a % 2 ? $a += 10 : $a += 2
+
+Really means this:
+
+ (($a % 2) ? ($a += 10) : $a) += 2
+
+Rather than this:
+
+ ($a % 2) ? ($a += 10) : ($a += 2)
+
+That should probably be written more simply as:
+
+ $a += ($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.
+
+Assignment operators work as in C. That is,
+
+ $a += 2;
+
+is equivalent to
+
+ $a = $a + 2;
+
+although without duplicating any side effects that dereferencing the lvalue
+might trigger, such as from tie(). Other assignment operators work similarly.
+The following are recognized:
+
+ **= += *= &= <<= &&=
+ -= /= |= >>= ||=
+ .= %= ^= //=
+ x=
+
+Although these are grouped by family, they all have the precedence
+of assignment.
+
+Unlike in C, the scalar assignment operator produces a valid lvalue.
+Modifying an assignment is equivalent to doing the assignment and
+then modifying the variable that was assigned to. This is useful
+for modifying a copy of something, like this:
+
+ ($tmp = $global) =~ tr [A-Z] [a-z];
+
+Likewise,
+
+ ($a += 2) *= 3;
+
+is equivalent to
+
+ $a += 2;
+ $a *= 3;
+
+Similarly, a list assignment in list context produces the list of
+lvalues assigned to, and a list assignment in scalar context returns
+the number of elements produced by the expression on the right hand
+side of the assignment.
+
+=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. 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
+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.
+
+Otherwise, the C<< => >> operator behaves exactly as the comma operator
+or list argument separator, according to context.
+
+For example:
+
+ use constant FOO => "something";
+
+ my %h = ( FOO => 23 );
+
+is equivalent to:
+
+ my %h = ("FOO", 23);
+
+It is I<NOT>:
+
+ my %h = ("something", 23);
+
+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
+
+=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.
+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:
+
+ open HANDLE, "filename"
+ or die "Can't open: $!\n";
+
+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, Defined or, and Exclusive Or
+X<operator, logical, or> X<operator, logical, xor>
+X<operator, logical, defined 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
+
+ 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.
+
+ $a = $b or $c; # bug: this is wrong
+ ($a = $b) or $c; # really means this
+ $a = $b || $c; # better written this way
+
+However, when it's a list-context assignment and you're trying to use
+"||" for control flow, you probably need "or" so that the assignment
+takes higher precedence.
+
+ @info = stat($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.
+
+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:
+
+=over 8
+
+=item unary &
+
+Address-of operator. (But see the "\" operator for taking a reference.)
+
+=item unary *
+
+Dereference-address operator. (Perl's prefix dereferencing
+operators are typed: $, @, %, and &.)
+
+=item (TYPE)
+
+Type-casting operator.
+
+=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.
+
+ Customary Generic Meaning Interpolates
+ '' q{} Literal no
+ "" qq{} Literal yes
+ `` qx{} Command yes*
+ qw{} Word list no
+ // 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
+
+ q{foo{bar}baz}
+
+is the same as
+
+ 'foo{bar}baz'
+
+Note, however, that this does not always work for quoting Perl code:
+
+ $s = q{ if($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.
+
+There can be whitespace between the operator and the quoting
+characters, except when C<#> is being used as the quoting character.
+C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the
+operator C<q> followed by a comment. Its argument will be taken
+from the next line. This allows you to write:
+
+ s {foo} # Replace foo
+ {bar} # with bar.
+
+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)
+ \r return (CR)
+ \f form feed (FF)
+ \b backspace (BS)
+ \a alarm (bell) (BEL)
+ \e escape (ESC)
+ \033 octal char (example: ESC)
+ \x1b hex char (example: ESC)
+ \x{263a} wide hex char (example: SMILEY)
+ \c[ control char (example: ESC)
+ \N{name} named Unicode character
+ \N{U+263D} Unicode character (example: FIRST QUARTER MOON)
+
+The character following C<\c> is mapped to some other character by
+converting letters to upper case and then (on ASCII systems) by inverting
+the 7th bit (0x40). The most interesting range is from '@' to '_'
+(0x40 through 0x5F), resulting in a control character from 0x00
+through 0x1F. A '?' maps to the DEL character. On EBCDIC systems only
+'@', the letters, '[', '\', ']', '^', '_' and '?' will work, resulting
+in 0x00 through 0x1F and 0x7F.
+
+C<\N{U+I<wide hex char>}> means the Unicode character whose Unicode ordinal
+number is I<wide hex char>.
+For documentation of C<\N{name}>, see L<charnames>.
+
+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>
+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
+ \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>.
+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.
+
+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
+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,
+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<< $obj->meth >> are not.
+
+Interpolating an array or slice interpolates the elements in order,
+separated by the value of C<$">, so is equivalent to interpolating
+C<join $", @array>. "Punctuation" arrays such as C<@*> are only
+interpolated if the name is enclosed in braces C<@{*}>, but special
+arrays C<@_>, C<@+>, and C<@-> are interpolated, even without braces.
+
+You cannot include a literal C<$> 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/>.
+
+Patterns are subject to an additional level of interpretation as a
+regular expression. This is done as a second pass, after variables are
+interpolated, so that regular expressions may be incorporated into the
+pattern from the variables. If this is not what you want, use C<\Q> to
+interpolate a variable literally.
+
+Apart from the behavior described above, Perl does not expand
+multiple levels of interpolation. In particular, contrary to the
+expectations of shell programmers, back-quotes do I<NOT> interpolate
+within double quotes, nor do single quotes impede evaluation of
+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 qr/STRING/msixpo
+X<qr> X</i> X</m> X</o> X</s> X</x> X</p>
+
+This operator quotes (and possibly compiles) its I<STRING> as a regular
+expression. I<STRING> is interpolated the same way as I<PATTERN>
+in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation
+is done. Returns a Perl value which may be used instead of the
+corresponding C</STRING/msixpo> 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.
+
+For example,
+
+ $rex = qr/my.STRING/is;
+ print $rex; # prints (?si-xm:my.STRING)
+ s/$rex/foo/;
+
+is equivalent to
+
+ s/my.STRING/foo/is;
+
+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 =~ $re; # or used standalone
+ $string =~ /$re/; # or this way
+
+Since Perl may compile the pattern at the moment of execution of qr()
+operator, using qr() may have speed advantages in some situations,
+notably if the result of qr() is used standalone:
+
+ sub match {
+ my $patterns = shift;
+ my @compiled = map qr/$_/i, @$patterns;
+ grep {
+ my $success = 0;
+ foreach my $pat (@compiled) {
+ $success = 1, last if /$pat/;
+ }
+ $success;
+ } @_;
+ }
+
+Precompilation of the pattern into an internal representation at
+the moment of qr() avoids a need to recompile the pattern every
+time a match C</$pat/> is attempted. (Perl has many other internal
+optimizations, but none would be triggered in the above example if
+we did not use qr() operator.)
+
+Options are:
+
+ m Treat string as multiple lines.
+ s Treat string as single line. (Make . match a newline)
+ i Do case-insensitive pattern matching.
+ x Use extended regular expressions.
+ p When matching preserve a copy of the matched string so
+ that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
+ o Compile pattern only once.
+
+If a precompiled pattern is embedded in a larger pattern then the effect
+of 'msixp' will be propagated appropriately. The effect of the 'o'
+modifier has is not propagated, being restricted to those patterns
+explicitly using it.
+
+See L<perlre> for additional information on valid syntax for STRING, and
+for a detailed look at the semantics of regular expressions.
+
+=item m/PATTERN/msixpogc
+X<m> X<operator, match>
+X<regexp, options> X<regexp> X<regex, options> X<regex>
+X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c>
+
+=item /PATTERN/msixpogc
+
+Searches a string for a pattern match, and in scalar context returns
+true if it succeeds, false if it fails. If no string is specified
+via the C<=~> or C<!~> operator, the $_ string is searched. (The
+string specified with C<=~> need not be an lvalue--it may be the
+result of an expression evaluation, but remember the C<=~> binds
+rather tightly.) See also L<perlre>. See L<perllocale> for
+discussion of additional considerations that apply when C<use locale>
+is in effect.
+
+Options are as described in C<qr//>; in addition, the following match
+process modifiers are available:
+
+ g Match globally, i.e., find all occurrences.
+ c Do not reset search position on a failed match when /g is in effect.
+
+If "/" is the delimiter then the initial C<m> is optional. With the C<m>
+you can use any pair of non-whitespace characters
+as delimiters. This is particularly useful for matching path names
+that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
+the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
+If "'" is the delimiter, no interpolation is performed on the PATTERN.
+When using a character valid in an identifier, whitespace is required
+after the C<m>.
+
+PATTERN may contain variables, which will be interpolated (and the
+pattern recompiled) every time the pattern search is evaluated, except
+for when the delimiter is a single quote. (Note that C<$(>, 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/STRING/msixpo">.
+
+=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 -
+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//>).
+
+=item Matching in list context
+
+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.
+
+Examples:
+
+ open(TTY, '/dev/tty');
+ <TTY> =~ /^y/i && foo(); # do foo if desired
+
+ if (/Version: *([0-9.]*)/) { $version = $1; }
+
+ next if m#^/usr/spool/uucp#;
+
+ # poor man's grep
+ $arg = shift;
+ while (<>) {
+ print if /$arg/o; # compile only once
+ }
+
+ 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.
+
+The C</g> modifier specifies global pattern matching--that is,
+matching as many times as possible within the string. How it behaves
+depends on the context. In list context, it returns a list of the
+substrings matched by any capturing parentheses in the regular
+expression. If there are no parentheses, it returns a list of all
+the matched strings, as if there were parentheses around the whole
+pattern.
+
+In scalar context, each execution of C<m//g> finds the next match,
+returning true if it matches, and false if there is no further match.
+The position after the last match can be read or set using the 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
+string also resets the search position.
+
+=item \G assertion
+
+You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
+zero-width assertion that matches the exact position where the previous
+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:
+
+ # list context
+ ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
+
+ # scalar context
+ $/ = "";
+ while (defined($paragraph = <>)) {
+ while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
+ $sentences++;
+ }
+ }
+ print "$sentences\n";
+
+ # using m//gc with \G
+ $_ = "ppooqppqq";
+ while ($i++ < 2) {
+ print "1: '";
+ print $1 while /(o)/gc; print "', pos=", pos, "\n";
+ print "2: '";
+ print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
+ 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: 'oo', pos=4
+ 2: 'q', pos=5
+ 3: 'pp', pos=7
+ 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,
+doing different actions depending on which regexp matched. Each
+regexp tries to match where the previous one leaves off.
+
+ $_ = <<'EOL';
+ $url = URI::URL->new( "http://example.com/" ); die if $url eq "xXx";
+ EOL
+ LOOP:
+ {
+ print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
+ print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
+ print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
+ print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
+ print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
+ print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
+ print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
+ print ". That's all!\n";
+ }
+
+Here is the output (split into several lines):
+
+ line-noise lowercase line-noise lowercase UPPERCASE line-noise
+ UPPERCASE line-noise lowercase line-noise lowercase line-noise
+ lowercase lowercase line-noise lowercase lowercase line-noise
+ MiXeD line-noise. That's all!
+
+=item ?PATTERN?
+X<?>
+
+This is just like the C</pattern/> search, except that it matches only
+once between calls to the reset() operator. This is a useful
+optimization when you want to see only the first occurrence of
+something in each file of a set of files, for instance. Only C<??>
+patterns local to the current package are reset.
+
+ while (<>) {
+ if (?^$?) {
+ # blank line between header and body
+ }
+ } continue {
+ reset if eof; # clear ?? status for next file
+ }
+
+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 s/PATTERN/REPLACEMENT/msixpogce
+X<substitute> X<substitution> X<replace> X<regexp, replace>
+X<regexp, substitute> X</m> X</s> X</i> X</x> X</p> X</o> X</g> X</c> X</e>
+
+Searches a string for a pattern, and if found, replaces that pattern
+with the replacement text and returns the number of substitutions
+made. Otherwise it returns false (specifically, the empty string).
+
+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.)
+
+If the delimiter chosen is a single quote, no interpolation is
+done on either the PATTERN or the REPLACEMENT. Otherwise, if the
+PATTERN contains a $ that looks like a variable rather than an
+end-of-string test, the variable will be interpolated into the pattern
+at run-time. If you want the pattern compiled only once the first time
+the variable is interpolated, use the C</o> option. If the pattern
+evaluates to the empty string, the last successfully executed regular
+expression is used instead. See L<perlre> for further explanation on these.
+See L<perllocale> for discussion of additional considerations that apply
+when C<use locale> is in effect.
+
+Options are as with m// with the addition of the following replacement
+specific options:
+
+ e Evaluate the right side as an expression.
+ ee Evaluate the right side as a string then eval the result
+
+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
+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.,
+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
+compile-time. A second C<e> modifier will cause the replacement portion
+to be C<eval>ed before being run as a Perl expression.
+
+Examples:
+
+ 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
+
+ $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
+
+ $_ = 'abc123xyz';
+ s/\d+/$&*2/e; # yields 'abc246xyz'
+ s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
+ s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
+
+ s/%(.)/$percent{$1}/g; # change percent escapes; no /e
+ s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
+ s/^=(\w+)/pod($1)/ge; # use function call
+
+ # expand variables in $_, but dynamics only, using
+ # symbolic dereferencing
+ s/\$(\w+)/${$1}/g;
+
+ # Add one to the value of any numbers in the string
+ s/(\d+)/1 + $1/eg;
+
+ # This will expand any embedded scalar variable
+ # (including lexicals) in $_ : First $1 is interpolated
+ # to the variable name, and then evaluated
+ s/(\$\w+)/$1/eeg;
+
+ # Delete (most) C comments.
+ $program =~ s {
+ /\* # Match the opening delimiter.
+ .*? # Match a minimal number of characters.
+ \*/ # Match the closing delimiter.
+ } []gsx;
+
+ s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
+
+ for ($variable) { # trim whitespace in $variable, cheap
+ s/^\s+//;
+ s/\s+$//;
+ }
+
+ s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
+
+Note the use of $ instead of \ in the last example. Unlike
+B<sed>, we use the \<I<digit>> form in only the left hand side.
+Anywhere else it's $<I<digit>>.
+
+Occasionally, you can't use just a C</g> to get all the changes
+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,$2/g;
+
+ # expand tabs to 8-column spacing
+ 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
+
+=back
+
+=head2 Quote-Like Operators
+X<operator, quote-like>
+
+=over 4
+
+=item q/STRING/
+X<q> X<quote, single> X<'> X<''>
+
+=item 'STRING'
+
+A single-quoted, literal string. A backslash represents a backslash
+unless followed by the delimiter or another backslash, in which case
+the delimiter or backslash is interpolated.
+
+ $foo = q!I said, "You said, 'She said it.'"!;
+ $bar = q('This is it.');
+ $baz = '\n'; # a two-character string
+
+=item qq/STRING/
+X<qq> X<quote, double> X<"> X<"">
+
+=item "STRING"
+
+A double-quoted, interpolated string.
+
+ $_ .= qq
+ (*** The previous line contains the naughty word "$1".\n)
+ if /\b(tcl|java|python)\b/i; # :-)
+ $baz = "\n"; # a one-character string
+
+=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, 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.
+To capture a command's STDERR and STDOUT together:
+
+ $output = `cmd 2>&1`;
+
+To capture a command's STDOUT but discard its STDERR:
+
+ $output = `cmd 2>/dev/null`;
+
+To capture a command's STDERR but discard its STDOUT (ordering is
+important here):
+
+ $output = `cmd 2>&1 1>/dev/null`;
+
+To exchange a command's STDOUT and STDERR in order to capture the STDERR
+but leave its STDOUT to come out the old STDERR:
+
+ $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
+
+To read both a command's STDOUT and its STDERR separately, it's easiest
+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:
+
+ 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:
+
+ $perl_info = qx(ps $$); # that's Perl's $$
+ $shell_info = qx'ps $$'; # that's the new shell's $$
+
+How that string gets evaluated is entirely subject to the command
+interpreter on your system. On most platforms, you will have to protect
+shell metacharacters if you want them treated literally. This is in
+practice difficult to do, as it's unclear how to escape which characters.
+See L<perlsec> for a clean and safe example of a manual fork() and exec()
+to emulate backticks safely.
+
+On some platforms (notably DOS-like ones), the shell may not be
+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).
+
+Beginning with v5.6.0, Perl will attempt to flush all files opened for
+output before starting the child process, but this may not be supported
+on some platforms (see L<perlport>). To be safe, you may need to set
+C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
+C<IO::Handle> on any open handles.
+
+Beware that some command shells may place restrictions on the length
+of the command line. You must ensure your strings don't exceed this
+limit after any necessary interpolations. See the platform-specific
+release notes for more details about your particular environment.
+
+Using this operator can lead to programs that are difficult to port,
+because the shell commands called vary between systems, and may in
+fact not be present at all. As one example, the C<type> command under
+the POSIX shell is very different from the C<type> command under DOS.
+That doesn't mean you should go out of your way to avoid backticks
+when they're the right way to get something done. Perl was made to be
+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.
+
+=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
+equivalent to:
+
+ 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
+this expression:
+
+ qw(foo bar baz)
+
+is semantically equivalent to the list:
+
+ 'foo', 'bar', 'baz'
+
+Some frequently seen examples:
+
+ use POSIX qw( setlocale localeconv )
+ @EXPORT = qw( foo bar baz );
+
+A common mistake is to try to separate the words with comma or to
+put comments into a multi-line C<qw>-string. For this reason, the
+C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable)
+produces warnings if the STRING contains the "," or the "#" character.
+
+
+=item tr/SEARCHLIST/REPLACEMENTLIST/cds
+X<tr> X<y> X<transliterate> X</c> X</d> X</s>
+
+=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
+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.)
+
+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.
+
+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
+that begin from and end at either alphabets of equal case (a-e, A-E),
+or digits (0-4). Anything else is unsafe. If in doubt, spell out the
+character sets in full.
+
+Options:
+
+ c Complement the SEARCHLIST.
+ d Delete found but unreplaced characters.
+ s Squash duplicate replaced characters.
+
+If the C</c> modifier is specified, the SEARCHLIST character set
+is complemented. If the C</d> modifier is specified, any characters
+specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
+(Note that this is slightly more flexible than the behavior of some
+B<tr> programs, which delete anything they find in the SEARCHLIST,
+period.) If the C</s> modifier is specified, sequences of characters
+that were transliterated to the same character are squashed down
+to a single instance of the character.
+
+If the C</d> modifier is used, the REPLACEMENTLIST is always interpreted
+exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter
+than the SEARCHLIST, the final character is replicated till it is long
+enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated.
+This latter is useful for counting characters in a class or for
+squashing character sequences in a class.
+
+Examples:
+
+ $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
+
+ $cnt = tr/*/*/; # count the stars in $_
+
+ $cnt = $sky =~ tr/*/*/; # count the stars in $sky
+
+ $cnt = tr/0-9//; # count the digits in $_
+
+ tr/a-zA-Z//s; # bookkeeper -> bokeper
+
+ ($HOST = $host) =~ tr/a-z/A-Z/;
+
+ tr/a-zA-Z/ /cs; # change non-alphas to single space
+
+ tr [\200-\377]
+ [\000-\177]; # delete 8th bit
+
+If multiple transliterations are given for a character, only the
+first one is used:
+
+ tr/AAA/XYZ/
+
+will transliterate any A to X.
+
+Because the transliteration table is built at compile time, neither
+the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
+interpolation. That means that if you want to use variables, you
+must use an eval():
+
+ eval "tr/$oldlist/$newlist/";
+ die $@ if $@;
+
+ eval "tr/$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")
+principle to pick the most probable interpretation. This strategy
+is so successful that Perl programmers often do not suspect the
+ambivalence of what they write. But from time to time, Perl's
+notions differ substantially from what the author honestly meant.
+
+This section hopes to clarify how Perl handles quoted constructs.
+Although the most common reason to learn this is to unravel labyrinthine
+regular expressions, because the initial steps of parsing are the
+same for all quoting operators, they are all discussed together.
+
+The most important Perl parsing rule is the first one discussed
+below: when processing a quoted construct, Perl first finds the end
+of that construct, then interprets its contents. If you understand
+this rule, you may skip the rest of this section on the first
+reading. The other rules are likely to contradict the user's
+expectations much less frequently than this first one.
+
+Some passes discussed below are performed concurrently, but because
+their results are the same, we consider them individually. For different
+quoting constructs, Perl performs different numbers of passes, from
+one to four, but these passes are always performed in the same order.
+
+=over 4
+
+=item Finding the end
+
+The first pass is finding the end of the quoted construct, where
+the information about the delimiters is used in parsing.
+During this search, text between the starting and ending delimiters
+is copied to a safe location. The text copied gets delimiter-independent.
+
+If the construct is a here-doc, the ending delimiter is a line
+that has a terminating string as the content. Therefore C<<<EOF> is
+terminated by C<EOF> immediately followed by C<"\n"> and starting
+from the first column of the terminating line.
+When searching for the terminating line of a here-doc, nothing
+is skipped. In other words, lines after the here-doc syntax
+are compared with the terminating string line by line.
+
+For the constructs except here-docs, single characters are used as starting
+and ending delimiters. If the starting delimiter is an opening punctuation
+(that is C<(>, C<[>, C<{>, or C<< < >>), the ending delimiter is the
+corresponding closing punctuation (that is C<)>, C<]>, C<}>, or C<< > >>).
+If the starting delimiter is an unpaired character like C</> or a closing
+punctuation, the ending delimiter is same as the starting delimiter.
+Therefore a C</> terminates a C<qq//> construct, while a C<]> terminates
+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</>,
+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).
+
+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
+terminates the left part and starts the right part at once.
+If the left part is delimited by bracketing punctuations (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, whitespaces
+and comments are allowed between both parts, though the comment must follow
+at least one whitespace; otherwise a character expected as the start of
+the comment may be regarded as the starting delimiter of the right part.
+
+During this search no attention is paid to the semantics of the construct.
+Thus:
+
+ "$hash{"$foo/$bar"}"
+
+or:
+
+ m/
+ bar # NOT a comment, this slash / terminated m//!
+ /x
+
+do not form legal quoted expressions. The quoted part ends on the
+first C<"> and C</>, and the rest happens to be a syntax error.
+Because the slash that terminated C<m//> was followed by a C<SPACE>,
+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 Interpolation
+X<interpolation>
+
+The next step is interpolation in the text obtained, which is now
+delimiter-independent. There are multiple cases.
+
+=over 4
+
+=item C<<<'EOF'>
+
+No interpolation is performed.
+Note that the combination C<\\> is left intact, since escaped delimiters
+are not available for here-docs.
+
+=item C<m''>, the pattern of C<s'''>
+
+No interpolation is performed at this stage.
+Any backslashed sequences including C<\\> are treated at the stage
+to L</"parsing regular expressions">.
+
+=item C<''>, C<q//>, C<tr'''>, C<y'''>, the replacement of C<s'''>
+
+The only interpolation is removal of C<\> from pairs of C<\\>.
+Therefore C<-> in C<tr'''> and C<y'''> is treated literally
+as a hyphen and no character range is available.
+C<\1> in the replacement of C<s'''> does not work as C<$1>.
+
+=item C<tr///>, C<y///>
+
+No variable interpolation occurs. String modifying combinations for
+case and quoting such as C<\Q>, C<\U>, and C<\E> are not recognized.
+The other escape sequences such as C<\200> and C<\t> and backslashed
+characters such as C<\\> and C<\-> are converted to appropriate literals.
+The character C<-> is treated specially and therefore C<\-> is treated
+as a literal C<->.
+
+=item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >>, C<<<"EOF">
+
+C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are
+converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar">
+is converted to C<$foo . (quotemeta("baz" . $bar))> internally.
+The other escape sequences such as C<\200> and C<\t> and backslashed
+characters such as C<\\> and C<\-> are replaced with appropriate
+expansions.
+
+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
+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
+as C<"\\\t"> (since TAB is not alphanumeric). Note also that:
+
+ $str = '\t';
+ return "\Q$str";
+
+may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">.
+
+Interpolated scalars and arrays are converted internally to the C<join> and
+C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes:
+
+ $foo . " XXX '" . (join $", @arr) . "'";
+
+All operations above are performed simultaneously, left to right.
+
+Because the result of C<"\Q STRING \E"> has all metacharacters
+quoted, there is no way to insert a literal C<$> or C<@> inside a
+C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became
+C<"\\\$">; if not, it is interpreted as the start of an interpolated
+scalar.
+
+Note also that the interpolation code needs to make a decision on
+where the interpolated scalar ends. For instance, whether
+C<< "a $b -> {c}" >> really means:
+
+ "a " . $b . " -> {c}";
+
+or:
+
+ "a " . $b -> {c};
+
+Most of the time, the longest possible text that does not include
+spaces between components and which contains matching braces or
+brackets. because the outcome may be determined by voting based
+on heuristic estimators, the result is not strictly predictable.
+Fortunately, it's usually correct for ambiguous cases.
+
+=item the replacement of C<s///>
+
+Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation
+happens as with C<qq//> constructs.
+
+It is at this step that C<\1> is begrudgingly converted to C<$1> in
+the replacement text of C<s///>, in order to correct the incorrigible
+I<sed> hackers who haven't picked up the saner idiom yet. A warning
+is emitted if the C<use warnings> pragma or the B<-w> command-line flag
+(that is, the C<$^W> variable) was set.
+
+=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>,
+and interpolation happens (almost) as with C<qq//> constructs.
+
+Processing of C<\N{...}> is also done here, and compiled into an intermediate
+form for the regex compiler. (This is because, as mentioned below, the regex
+compilation may be done at execution time, and C<\N{...}> is a compile-time
+construct.)
+
+However any other combinations of C<\> followed by a character
+are not substituted but only skipped, in order to parse them
+as regular expressions at the following step.
+As C<\c> is skipped at this step, C<@> of C<\c@> in RE is possibly
+treated as an array symbol (for example C<@foo>),
+even though the same text in C<qq//> gives interpolation of C<\c@>.
+
+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
+of the C<//x> modifier is relevant.
+
+Interpolation in patterns has several quirks: C<$|>, C<$(>, C<$)>, C<@+>
+and C<@-> are not interpolated, and constructs C<$var[SOMETHING]> are
+voted (by several different estimators) to be either an array element
+or C<$var> followed by an RE alternative. This is where the notation
+C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as
+array element C<-9>, not as a regular expression from the variable
+C<$arr> followed by a digit, which would be the interpretation of
+C</$arr[0-9]/>. Since voting among different estimators may occur,
+the result is not predictable.
+
+The lack of processing of C<\\> creates specific restrictions on
+the post-processed text. If the delimiter is C</>, one cannot get
+the combination C<\/> into the result of this step. C</> will
+finish the regular expression, C<\/> will be stripped to C</> on
+the previous step, and C<\\/> will be left as is. Because C</> is
+equivalent to C<\/> inside a regular expression, this does not
+matter unless the delimiter happens to be character special to the
+RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an
+alphanumeric char, as in:
+
+ m m ^ a \s* b mmx;
+
+In the RE above, which is intentionally obfuscated for illustration, the
+delimiter is C<m>, the modifier is C<mx>, and after delimiter-removal the
+RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
+reason you're encouraged to restrict your delimiters to non-alphanumeric,
+non-whitespace choices.
+
+=back
+
+This step is the last one for all constructs except regular expressions,
+which are processed further.
+
+=item parsing regular expressions
+X<regexp, parse>
+
+Previous steps were performed during the compilation of Perl code,
+but this one happens at run time, although it may be optimized to
+be calculated at compile time if appropriate. After preprocessing
+described above, and possibly after evaluation if concatenation,
+joining, casing translation, or metaquoting are involved, the
+resulting I<string> is passed to the RE engine for compilation.
+
+Whatever happens in the RE engine might be better discussed in L<perlre>,
+but for the sake of continuity, we shall do so here.
+
+This is another step where the presence of the C<//x> modifier is
+relevant. The RE engine scans the string from left to right and
+converts it to a finite automaton.
+
+Backslashed characters are either replaced with corresponding
+literal strings (as with C<\{>), or else they generate special nodes
+in the finite automaton (as with C<\b>). Characters special to the
+RE engine (such as C<|>) generate corresponding nodes or groups of
+nodes. C<(?#...)> comments are ignored. All the rest is either
+converted to literal strings to match, or else is ignored (as is
+whitespace and C<#>-style comments if C<//x> is present).
+
+Parsing of the bracketed character class construct, C<[...]>, is
+rather different than the rule used for the rest of the pattern.
+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.
+
+It is possible to inspect both the string given to RE engine and the
+resulting finite automaton. See the arguments C<debug>/C<debugcolor>
+in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line
+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
+to change without notice. This step is performed over the finite
+automaton that was generated during the previous pass.
+
+It is at this stage that C<split()> silently optimizes C</^/> to
+mean C</^/m>.
+
+=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
+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<$?>).
+Unlike in B<csh>, no translation is done on the return data--newlines
+remain newlines. Unlike in any of the shells, single quotes do not
+hide variable names in the command from interpretation. To pass a
+literal dollar-sign through to the shell you need to hide it with a
+backslash. The generalized form of backticks is C<qx//>. (Because
+backticks always undergo shell expansion as well, see L<perlsec> for
+security concerns.)
+X<qx> X<`> X<``> X<backtick> X<glob>
+
+In scalar context, evaluating a filehandle in angle brackets yields
+the next line from that file (the newline, if any, included), or
+C<undef> at end-of-file or on error. When C<$/> is set to C<undef>
+(sometimes known as file-slurp mode) and the file is empty, it
+returns C<''> the first time, followed by C<undef> subsequently.
+
+Ordinarily you must assign the returned value to a variable, but
+there is one situation where an automatic assignment happens. If
+and only if the input symbol is the only thing inside the conditional
+of a C<while> statement (even if disguised as a C<for(;;)> loop),
+the value is automatically assigned to the global variable $_,
+destroying whatever was there previously. (This may seem like an
+odd thing to you, but you'll use the construct in almost every Perl
+script you write.) The $_ variable is not implicitly localized.
+You'll have to put a C<local $_;> before the loop if you want that
+to happen.
+
+The following lines are equivalent:
+
+ while (defined($_ = <STDIN>)) { print; }
+ while ($_ = <STDIN>) { print; }
+ while (<STDIN>) { print; }
+ for (;<STDIN>;) { print; }
+ print while defined($_ = <STDIN>);
+ print while ($_ = <STDIN>);
+ print while <STDIN>;
+
+This also behaves similarly, but avoids $_ :
+
+ while (my $line = <STDIN>) { print $line }
+
+In these loop constructs, the assigned value (whether assignment
+is automatic or explicit) is then tested to see whether it is
+defined. The defined test avoids problems where line has a string
+value that would be treated as false by Perl, for example a "" or
+a "0" with no trailing newline. If you really mean for such values
+to terminate the loop, they should be tested for explicitly:
+
+ while (($_ = <STDIN>) ne '0') { ... }
+ while (<STDIN>) { last unless $_; ... }
+
+In other boolean contexts, C<< <filehandle> >> without an
+explicit C<defined> test or comparison elicits a warning if the
+C<use warnings> pragma or the B<-w>
+command-line switch (the C<$^W> variable) is in effect.
+
+The filehandles STDIN, STDOUT, and STDERR are predefined. (The
+filehandles C<stdin>, C<stdout>, and C<stderr> will also work except
+in packages, where they would be interpreted as local identifiers
+rather than global.) Additional filehandles may be created with
+the open() function, amongst others. See L<perlopentut> and
+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
+list element. It's easy to grow to a rather large data space this
+way, so use with care.
+
+<FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>.
+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
+standard input, or from each file listed on the command line. Here's
+how it works: the first time <> is evaluated, the @ARGV array is
+checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened
+gives you standard input. The @ARGV array is then processed as a list
+of filenames. The loop
+
+ while (<>) {
+ ... # code for each line
+ }
+
+is equivalent to the following Perl-like pseudo code:
+
+ unshift(@ARGV, '-') unless @ARGV;
+ while ($ARGV = shift) {
+ open(ARGV, $ARGV);
+ while (<ARGV>) {
+ ... # code for each line
+ }
+ }
+
+except that it isn't so cumbersome to say, and will actually work.
+It really does shift the @ARGV array and put the current filename
+into the $ARGV variable. It also uses filehandle I<ARGV>
+internally. <> is just a synonym for <ARGV>, which
+is magical. (The pseudo code above doesn't work because it treats
+<ARGV> as non-magical.)
+
+Since the null filehandle uses the two argument form of L<perlfunc/open>
+it interprets special characters, so if you have a script like this:
+
+ while (<>) {
+ print;
+ }
+
+and call it with C<perl dangerous.pl 'rm -rfv *|'>, it actually opens a
+pipe, executes the C<rm> command and reads C<rm>'s output from that pipe.
+If you want all items in C<@ARGV> to be interpreted as file names, you
+can use the module C<ARGV::readonly> from CPAN.
+
+You can modify @ARGV before the first <> as long as the array ends up
+containing the list of filenames you really want. Line numbers (C<$.>)
+continue as though the input were one big happy file. See the example
+in L<perlfunc/eof> for how to reset line numbers on each file.
+
+If you want to set @ARGV to your own list of files, go right ahead.
+This sets @ARGV to all plain text files if no @ARGV was given:
+
+ @ARGV = grep { -f && -T } glob('*') unless @ARGV;
+
+You can even set them to pipe commands. For example, this automatically
+filters compressed arguments through B<gzip>:
+
+ @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
+
+If you want to pass switches into your script, you can use one of the
+Getopts modules or put a loop on the front like this:
+
+ while ($_ = $ARGV[0], /^-/) {
+ shift;
+ last if /^--$/;
+ if (/^-D(.*)/) { $debug = $1 }
+ if (/^-v/) { $verbose++ }
+ # ... # other switches
+ }
+
+ while (<>) {
+ # ... # code for each line
+ }
+
+The <> symbol will return C<undef> for end-of-file only once.
+If you call it again after this, it will assume you are processing another
+@ARGV list, and if you haven't set @ARGV, will read input from STDIN.
+
+If what the angle brackets contain is a simple scalar variable (e.g.,
+<$foo>), then that variable contains the name of the
+filehandle to input from, or its typeglob, or a reference to the
+same. For example:
+
+ $fh = \*STDIN;
+ $line = <$fh>;
+
+If what's within the angle brackets is neither a filehandle nor a simple
+scalar variable containing a filehandle name, typeglob, or typeglob
+reference, it is interpreted as a filename pattern to be globbed, and
+either a list of filenames or the next filename in the list is returned,
+depending on context. This distinction is determined on syntactic
+grounds alone. That means C<< <$x> >> is always a readline() from
+an indirect handle, but C<< <$hash{key}> >> is always a glob().
+That's because $x is a simple scalar variable, but C<$hash{key}> is
+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
+in the previous paragraph. (In older versions of Perl, programmers
+would insert curly brackets to force interpretation as a filename glob:
+C<< <${foo}> >>. These days, it's considered cleaner to call the
+internal function directly as C<glob($foo)>, which is probably the right
+way to have done it in the first place.) For example:
+
+ while (<*.c>) {
+ chmod 0644, $_;
+ }
+
+is roughly equivalent to:
+
+ open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
+ while (<FOO>) {
+ chomp;
+ chmod 0644, $_;
+ }
+
+except that the globbing is actually done internally using the standard
+C<File::Glob> extension. Of course, the shortest way to do the above is:
+
+ chmod 0644, <*.c>;
+
+A (file)glob evaluates its (embedded) argument only when it is
+starting a new list. All values must be read before it will start
+over. In list context, this isn't important because you automatically
+get them all anyway. However, in scalar context the operator returns
+the next value each time it's called, or C<undef> when the list has
+run out. As with filehandle reads, an automatic C<defined> is
+generated when the glob occurs in the test part of a C<while>,
+because legal glob returns (e.g. a file called F<0>) would otherwise
+terminate the loop. Again, C<undef> is returned only once. So if
+you're expecting a single value from a glob, it is much better to
+say
+
+ ($file) = <blurch*>;
+
+than
+
+ $file = <blurch*>;
+
+because the latter will alternate between returning a filename and
+returning false.
+
+If you're trying to do variable interpolation, it's definitely better
+to use the glob() function, because the older notation can cause people
+to become confused with the indirect filehandle notation.
+
+ @files = glob("$dir/*.[ch]");
+ @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
+operator are static and have no side effects. In particular, string
+concatenation happens at compile time between literals that don't do
+variable substitution. Backslash interpolation also happens at
+compile time. You can say
+
+ 'Now is the time for all' . "\n" .
+ 'good men to come to.'
+
+and this all reduces to one string internally. Likewise, if
+you say
+
+ foreach $file (@filenames) {
+ if (-s $file > 5 + 100 * 2**16) { }
+ }
+
+the compiler will precompute the number which that expression
+represents so that the interpreter won't have to.
+
+=head2 No-ops
+X<no-op> X<nop>
+
+Perl doesn't officially have a no-op operator, but the bare constants
+C<0> and C<1> are special-cased to not produce a warning in a void
+context, so you can for example safely do
+
+ 1 while foo();
+
+=head2 Bitwise String Operators
+X<operator, bitwise, string>
+
+Bitstrings of any size may be manipulated by the bitwise operators
+(C<~ | & ^>).
+
+If the operands to a binary bitwise op are strings of different
+sizes, B<|> and B<^> ops act as though the shorter operand had
+additional zero bits on the right, while the B<&> op acts as though
+the longer operand were truncated to the length of the shorter.
+The granularity for such extension or truncation is one or more
+bytes.
+
+ # ASCII-based examples
+ print "j p \n" ^ " a h"; # prints "JAPH\n"
+ print "JA" | " ph\n"; # prints "japh\n"
+ print "japh\nJunk" & '_____'; # prints "JAPH\n";
+ print 'p N$' ^ " E<H\n"; # prints "Perl\n";
+
+If you are intending to manipulate bitstrings, be certain that
+you're supplying bitstrings: If an operand is a number, that will imply
+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
+ $foo = '150' | '105'; # yields string '155' (under ASCII)
+
+ $baz = 0+$foo & 0+$bar; # both ops explicitly numeric
+ $biz = "$foo" ^ "$bar"; # both ops explicitly stringy
+
+See L<perlfunc/vec> for information on how to manipulate individual bits
+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
+
+ use integer;
+
+you may tell the compiler that it's okay to use integer operations
+(if it feels like it) from here to the end of the enclosing BLOCK.
+An inner BLOCK may countermand this by saying
+
+ no integer;
+
+which lasts until the end of that BLOCK. Note that this doesn't
+mean everything is only an integer, merely that Perl may use integer
+operations if it is so inclined. For example, even under C<use
+integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731>
+or so.
+
+Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
+and ">>") always produce integral results. (But see also
+L<Bitwise String Operators>.) However, C<use integer> still has meaning for
+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 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
+certain number of decimal places. For rounding to a certain number
+of digits, sprintf() or printf() is usually the easiest route.
+See L<perlfaq4>.
+
+Floating-point numbers are only approximations to what a mathematician
+would call real numbers. There are infinitely more reals than floats,
+so some corners must be cut. For example:
+
+ printf "%.20g\n", 123456789123456789;
+ # produces 123456789123456784
+
+Testing for exact floating-point equality or inequality is not a
+good idea. Here's a (relatively expensive) work-around to compare
+whether two floating-point numbers are equal to a particular number of
+decimal places. See Knuth, volume II, for a more robust treatment of
+this topic.
+
+ sub fp_equal {
+ my ($X, $Y, $POINTS) = @_;
+ my ($tX, $tY);
+ $tX = sprintf("%.${POINTS}g", $X);
+ $tY = sprintf("%.${POINTS}g", $Y);
+ return $tX eq $tY;
+ }
+
+The POSIX module (part of the standard perl distribution) implements
+ceil(), floor(), and other mathematical and trigonometric functions.
+The Math::Complex module (part of the standard perl distribution)
+defines mathematical functions that work on both the reals and the
+imaginary numbers. Math::Complex not as efficient as POSIX, but
+POSIX can't work with complex numbers.
+
+Rounding in financial applications can have serious implications, and
+the rounding method used should be specified precisely. In these
+cases, it probably pays not to trust whichever system rounding is
+being used by Perl, but to instead implement the rounding function you
+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
+considerable speed, they avoid the normal pitfalls associated with
+limited-precision representations.
+
+ use Math::BigInt;
+ $x = Math::BigInt->new('123456789123456789');
+ print $x * $x;
+
+ # prints +15241578780673678515622620750190521
+
+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