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X11R7.4 - man page for perlop (x11r4 section 1)

PERLOP(1)			 Perl Programmers Reference Guide			PERLOP(1)

NAME
       perlop - Perl operators and precedence

DESCRIPTION
       Operator Precedence and Associativity

       Operator precedence and associativity work in Perl more or less like they do in mathemat-
       ics.

       Operator precedence means some operators are evaluated before others.  For example, in "2
       + 4 * 5", the multiplication has higher precedence so "4 * 5" is evaluated first yielding
       "2 + 20 == 22" and not "6 * 5 == 30".

       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 "8 - 4 - 2", subtraction is left associative so Perl evaluates the
       expression left to right.  "8 - 4" is evaluated first making the expression "4 - 2 == 2"
       and not "8 - 2 == 6".

       Perl operators have the following associativity and precedence, listed from highest prece-
       dence 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 eas-
       ier 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 err

       In the following sections, these operators are covered in precedence order.

       Many operators can be overloaded for objects.  See overload.

       Terms and List Operators (Leftward)

       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 parenthe-
       sized.  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 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 "print", "sort",
       or "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 "print" which is evaluated (printing the result of "$foo & 255").  Then one is
       added to the return value of "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 "Named Unary Operators" for more discussion of this.

       Also parsed as terms are the "do {}" and "eval {}" constructs, as well as subroutine and
       method calls, and the anonymous constructors "[]" and "{}".

       See also "Quote and Quote-like Operators" toward the end of this section, as well as "I/O
       Operators".

       The Arrow Operator

       ""->"" is an infix dereference operator, just as it is in C and C++.  If the right side is
       either a "[...]", "{...}", or a "(...)" subscript, then the left side must be either a
       hard or symbolic reference to an array, a hash, or a subroutine respectively.  (Or techni-
       cally speaking, a location capable of holding a hard reference, if it's an array or hash
       reference being used for assignment.)  See perlreftut and 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 perlobj.

       Auto-increment and Auto-decrement

       "++" 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 when the variable is incremented or decre-
       mented. 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 nor-
       mal 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
       "/^[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'

       "undef" is always treated as numeric, and in particular is changed to 0 before increment-
       ing (so that a post-increment of an undef value will return 0 rather than "undef").

       The auto-decrement operator is not magical.

       Exponentiation

       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.)

       Symbolic Unary Operators

       Unary "!" performs logical negation, i.e., "not".  See also "not" for a lower precedence
       version of this.

       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 Argument "the string" isn't numeric in negation (-) at ....

       Unary "~" performs bitwise negation, i.e., 1's complement.  For example, "0666 & ~027" is
       0640.  (See also "Integer Arithmetic" and "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.

       Unary "+" has no effect whatsoever, even on strings.  It is useful syntactically for sepa-
       rating a function name from a parenthesized expression that would otherwise be interpreted
       as the complete list of function arguments.  (See examples above under "Terms and List
       Operators (Leftward)".)

       Unary "\" creates a reference to whatever follows it.  See perlreftut and 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.

       Binding Operators

       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 indi-
       cates the success of the operation.  Behavior in list context depends on the particular
       operator.  See "Regexp Quote-Like Operators" for details and 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 "\", which it
       will consider a syntax error.

       Binary "!~" is just like "=~" except the return value is negated in the logical sense.

       Multiplicative Operators

       Binary "*" multiplies two numbers.

       Binary "/" divides two numbers.

       Binary "%" is the modulo operator, which computes the division remainder of its first
       argument with respect to its second argument.  Given integer operands $a and $b: If $b is
       positive, then "$a % $b" is $a minus the largest multiple of $b less than or equal to $a.
       If $b is negative, then "$a % $b" is $a minus the smallest multiple of $b that is not less
       than $a (i.e. the result will be less than or equal to zero).  If the operands $a and $b
       are floating point values and the absolute value of $b (that is "abs($b)") is less than
       "(UV_MAX + 1)", only the integer portion of $a and $b will be used in the operation (Note:
       here "UV_MAX" means the maximum of the unsigned integer type).  If the absolute value of
       the right operand ("abs($b)") is greater than or equal to "(UV_MAX + 1)", "%" computes the
       floating-point remainder $r in the equation "($r = $a - $i*$b)" where $i is a certain
       integer that makes $r have the same sign as the right operand $b (not as the left operand
       $a like C function "fmod()") and the absolute value less than that of $b.  Note that when
       "use integer" is in scope, "%" gives you direct access to the modulo operator as imple-
       mented by your C compiler.  This operator is not as well defined for negative operands,
       but it will execute faster.

       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 "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.

	   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

       Additive Operators

       Binary "+" returns the sum of two numbers.

       Binary "-" returns the difference of two numbers.

       Binary "." concatenates two strings.

       Shift Operators

       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 "Integer Arith-
       metic".)

       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 "Integer Arith-
       metic".)

       Note that both "<<" and ">>" in Perl are implemented directly using "<<" and ">>" in C.
       If "use integer" (see "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, "1 << 32" is undefined.  Shifting by a
       negative number of bits is also undefined.

       Named Unary Operators

       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 "-f", "-M", etc. are treated like named
       unary operators, but they don't follow this functional parenthesis rule.  That means, for
       example, that "-f($file).".bak"" is equivalent to "-f "$file.bak"".

       See also "Terms and List Operators (Leftward)".

       Relational Operators

       Binary "<" returns true if the left argument is numerically less than the right argument.

       Binary ">" returns true if the left argument is numerically greater than the right argu-
       ment.

       Binary "<=" returns true if the left argument is numerically less than or equal to the
       right argument.

       Binary ">=" returns true if the left argument is numerically greater than or equal to the
       right argument.

       Binary "lt" returns true if the left argument is stringwise less than the right argument.

       Binary "gt" returns true if the left argument is stringwise greater than the right argu-
       ment.

       Binary "le" returns true if the left argument is stringwise less than or equal to the
       right argument.

       Binary "ge" returns true if the left argument is stringwise greater than or equal to the
       right argument.

       Equality Operators

       Binary "==" returns true if the left argument is numerically equal to the right argument.

       Binary "!=" returns true if the left argument is numerically not equal to the right argu-
       ment.

       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.

	   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.

       Binary "ne" returns true if the left argument is stringwise not equal to the right argu-
       ment.

       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.

       "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified by the current
       locale if "use locale" is in effect.  See perllocale.

       Bitwise And

       Binary "&" returns its operands ANDed together bit by bit.  (See also "Integer Arithmetic"
       and "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;

       Bitwise Or and Exclusive Or

       Binary "|" returns its operands ORed together bit by bit.  (See also "Integer Arithmetic"
       and "Bitwise String Operators".)

       Binary "^" returns its operands XORed together bit by bit.  (See also "Integer Arithmetic"
       and "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;

       C-style 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.

       C-style Logical Defined-Or

       Although it has no direct equivalent in C, Perl's "//" operator is related to its C-style
       or.  In fact, it's exactly the same as "||", except that it tests the left hand side's
       definedness instead of its truth.  Thus, "$a // $b" is similar to "defined($a) || $b"
       (except that it returns the value of $a rather than the value of "defined($a)") and is
       exactly equivalent to "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 $a and $b is
       defined, use "defined($a // $b)".

       C-style Logical Or

       Binary "||" performs a short-circuit logical OR operation.  That is, if the left operand
       is true, the right operand is not even evaluated.  Scalar or list context propagates down
       to the right operand if it is evaluated.

       The "||", "//" and "&&" operators return the last value evaluated (unlike C's "||" and
       "&&", 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 "&&", "//" and "||" when used for control flow, Perl pro-
       vides "and", "err" and "or" operators (see below).  The short-circuit behavior is identi-
       cal.  The precedence of "and", "err" and "or" is much lower, however, so that you can
       safely use them after a list operator without the need for parentheses:

	   unlink "alpha", "beta", "gamma"
		   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.

       Range Operators

       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 "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 "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 sed, awk, and various editors.
       Each ".." operator maintains its own boolean state.  It is false as long as its left oper-
       and is false.  Once the left operand is true, the range operator stays true until the
       right operand is true, 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 awk), but it still
       returns true once.  If you don't want it to test the right operand till the next evalua-
       tion, as in 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 ("==") to the current input line number (the $. variable).

       To be pedantic, the comparison is actually "int(EXPR) == int(EXPR)", but that is only an
       issue if you use a floating point expression; when implicitly using $. as described in the
       previous paragraph, the comparison is "int(EXPR) == int($.)" which is only an issue when
       $.  is set to a floating point value and you are not reading from a file.  Furthermore,
       "span" .. "spat" or "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) ...

	   next LINE if (1 .. /^$/);  # skip header lines, short for
				      #   ... if ($. == 1 .. /^$/);
				      # (typically in a loop labeled LINE)

	   s/^/> / if (/^$/ .. eof());	# quote body

	   # parse mail messages
	   while (<>) {
	       $in_header =   1  .. /^$/;
	       $in_body   = /^$/ .. eof;
	       if ($in_header) {
		   # ...
	       } else { # in body
		   # ...
	       }
	   } 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 "...", 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 pro-
       duce, the sequence goes until the next value would be longer than the final value speci-
       fied.

       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, "2.18 .. 3.14" will return two elements
       in list context.

	   @list = (2.18 .. 3.14); # same as @list = (2 .. 3);

       Conditional Operator

       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 (mean-
       ing 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;

       Assignment Operators

       "=" 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 recog-
       nized:

	   **=	  +=	*=    &=    <<=    &&=
		  -=	/=    |=    >>=    ||=
		  .=	%=    ^=	   //=
			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 assign-
       ment 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 expres-
       sion on the right hand side of the assignment.

       Comma Operator

       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 "=>" operator is a synonym for the comma, but forces any word (consisting entirely of
       word characters) to its left to be interpreted as a string (as of 5.001).  This includes
       words that might otherwise be considered a constant or function call.

	   use constant FOO => "something";

	   my %h = ( FOO => 23 );

       is equivalent to:

	   my %h = ("FOO", 23);

       It is NOT:

	   my %h = ("something", 23);

       If the argument on the left is not a word, it is first interpreted as an expression, and
       then the string value of that is used.

       The "=>" 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 );

       List Operators (Rightward)

       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 "Terms and List Operators (Leftward)".

       Logical Not

       Unary "not" returns the logical negation of the expression to its right.  It's the equiva-
       lent of "!" except for the very low precedence.

       Logical 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.

       Logical or, Defined or, and Exclusive Or

       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 "err" is equivalent to "//"--it's just like binary "or", except it tests its left
       argument's definedness instead of its truth.  There are two ways to remember "err":
       either because many functions return "undef" on an error, or as a sort of correction:
       "$a=($b err 'default')"

       Binary "xor" returns the exclusive-OR of the two surrounding expressions.  It cannot short
       circuit, of course.

       C Operators Missing From Perl

       Here is what C has that Perl doesn't:

       unary & Address-of operator.  (But see the "\" operator for taking a reference.)

       unary * Dereference-address operator. (Perl's prefix dereferencing operators are typed: $,
	       @, %, and &.)

       (TYPE)  Type-casting operator.

       Quote and Quote-like Operators

       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 "{}" 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 "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 "#"
       is being used as the quoting character.	"q#foo#" is parsed as the string "foo", while "q
       #foo#" is the operator "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.

	   \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

       The character following "\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 interest-
       ing 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 let-
       ters, '[', '\', ']', '^', '_' and '?' will work, resulting in 0x00 through 0x1F and 0x7F.

       NOTE: Unlike C and other languages, Perl has no \v escape sequence for the vertical tab
       (VT - ASCII 11), but you may use "\ck" or "\x0b".

       The following escape sequences are available in constructs that interpolate but not in
       transliterations.

	   \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 "use locale" is in effect, the case map used by "\l", "\L", "\u" and "\U" is taken from
       the current locale.  See perllocale.  If Unicode (for example, "\N{}" or wide hex charac-
       ters of 0x100 or beyond) is being used, the case map used by "\l", "\L", "\u" and "\U" is
       as defined by Unicode.  For documentation of "\N{name}", see charnames.

       All systems use the virtual "\n" to represent a line terminator, called a "newline".
       There is no such thing as an unvarying, physical newline character.  It is only an illu-
       sion that the operating system, device drivers, C libraries, and Perl all conspire to pre-
       serve.  Not all systems read "\r" as ASCII CR and "\n" as ASCII LF.  For example, on a
       Mac, these are reversed, and on systems without line terminator, printing "\n" may emit no
       actual data.  In general, use "\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 ("\015\012" or "\cM\cJ") for line terminators, and although they
       often accept just "\012", they seldom tolerate just "\015".  If you get in the habit of
       using "\n" for networking, you may be burned some day.

       For constructs that do interpolate, variables beginning with ""$"" or ""@"" are interpo-
       lated.  Subscripted variables such as $a[3] or "$href->{key}[0]" are also interpolated, as
       are array and hash slices.  But method calls such as "$obj->meth" are not.

       Interpolating an array or slice interpolates the elements in order, separated by the value
       of $", so is equivalent to interpolating "join $", @array".    "Punctuation" arrays such
       as "@*" are only interpolated if the name is enclosed in braces "@{*}", but special arrays
       @_, "@+", and "@-" are interpolated, even without braces.

       You cannot include a literal "$" or "@" within a "\Q" sequence.	An unescaped "$" or "@"
       interpolates the corresponding variable, while escaping will cause the literal string "\$"
       to be inserted.	You'll need to write something like "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 expres-
       sions may be incorporated into the pattern from the variables.  If this is not what you
       want, use "\Q" to interpolate a variable literally.

       Apart from the behavior described above, Perl does not expand multiple levels of interpo-
       lation.	In particular, contrary to the expectations of shell programmers, back-quotes do
       NOT interpolate within double quotes, nor do single quotes impede evaluation of variables
       when used within double quotes.

       Regexp Quote-Like Operators

       Here are the quote-like operators that apply to pattern matching and related activities.

       qr/STRING/msixo
	       This operator quotes (and possibly compiles) its STRING as a regular expression.
	       STRING is interpolated the same way as PATTERN in "m/PATTERN/".	If "'" is used as
	       the delimiter, no interpolation is done.  Returns a Perl value which may be used
	       instead of the corresponding "/STRING/imosx" expression. The returned value is a
	       normalized version of the original pattern. It magically differs from a string
	       containing the same characters: ref(qr/x/) returns "Regexp", even though derefer-
	       encing 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 "/$pat/" is
	       attempted.  (Perl has many other internal optimizations, but none would be trig-
	       gered 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.
		   o   Compile pattern only once.

	       If a precompiled pattern is embedded in a larger pattern then the effect of 'msix'
	       will be propagated appropriately.  The effect of the 'o' modifier has is not prop-
	       agated, being restricted to those patterns explicitly using it.

	       See perlre for additional information on valid syntax for STRING, and for a
	       detailed look at the semantics of regular expressions.

       m/PATTERN/msixogc
       /PATTERN/msixogc
	       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 "=~" or "!~" oper-
	       ator, the $_ string is searched.  (The string specified with "=~" need not be an
	       lvalue--it may be the result of an expression evaluation, but remember the "=~"
	       binds rather tightly.)  See also perlre.  See perllocale for discussion of addi-
	       tional considerations that apply when "use locale" is in effect.

	       Options are as described in "qr//"; in addition, the following match process modi-
	       fiers 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 "m" is optional.  With the "m" you can
	       use any pair of non-alphanumeric, non-whitespace characters as delimiters.  This
	       is particularly useful for matching path names that contain "/", to avoid LTS
	       (leaning toothpick syndrome).  If "?" is the delimiter, then the match-only-once
	       rule of "?PATTERN?" applies.  If "'" is the delimiter, no interpolation is per-
	       formed on the PATTERN.

	       PATTERN may contain variables, which will be interpolated (and the pattern recom-
	       piled) every time the pattern search is evaluated, except for when the delimiter
	       is a single quote.  (Note that $(, $), and $| are not interpolated because they
	       look like end-of-string tests.)	If you want such a pattern to be compiled only
	       once, add a "/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 "/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 "qr/STRING/msixo".

       The empty pattern //
	       If the PATTERN evaluates to the empty string, the last successfully matched regu-
	       lar expression is used instead. In this case, only the "g" and "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 gen-
	       uine empty pattern (which will always match).

	       Note that it's possible to confuse Perl into thinking "//" (the empty regex) is
	       really "//" (the defined-or operator).  Perl is usually pretty good about this,
	       but some pathological cases might trigger this, such as "$a///" (is that "($a) /
	       (//)" or "$a // /"?) and "print $fh //" ("print $fh(//" or "print($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 "m" (so "//" becomes "m//").

       Matching in list context
	       If the "/g" option is not used, "m//" in list context returns a list consisting of
	       the subexpressions matched by the parentheses in the pattern, i.e., ($1, $2,
	       $3...).	(Note that here $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 "(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 "/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 paren-
	       theses 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 "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 "pos" in perlfunc.	A
	       failed match normally resets the search position to the beginning of the string,
	       but you can avoid that by adding the "/c" modifier (e.g. "m//gc").  Modifying the
	       target string also resets the search position.

       \G assertion
	       You can intermix "m//g" matches with "m/\G.../g", where "\G" is a zero-width
	       assertion that matches the exact position where the previous "m//g", if any, left
	       off.  Without the "/g" modifier, the "\G" assertion still anchors at pos(), but
	       the match is of course only attempted once.  Using "\G" without "/g" on a target
	       string that has not previously had a "/g" match applied to it is the same as using
	       the "\A" assertion to match the beginning of the string.  Note also that, cur-
	       rently, "\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 "q" instead of "p", which a match without the
	       "\G" anchor would have done. Also note that the final match did not update "pos"
	       -- "pos" is only updated on a "/g" match. If the final match did indeed match "p",
	       it's a good bet that you're running an older (pre-5.6.0) Perl.

	       A useful idiom for "lex"-like scanners is "/\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://www/" );   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!

       ?PATTERN?
	       This is just like the "/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 "??"  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.

       s/PATTERN/REPLACEMENT/msixogce
	       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 "=~" or "!~" operator, the $_ variable is
	       searched and modified.  (The string specified with "=~" 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 interpo-
	       lated into the pattern at run-time.  If you want the pattern compiled only once
	       the first time the variable is interpolated, use the "/o" option.  If the pattern
	       evaluates to the empty string, the last successfully executed regular expression
	       is used instead.  See perlre for further explanation on these.  See perllocale for
	       discussion of additional considerations that apply when "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-alphanumeric, non-whitespace delimiter may replace the slashes.	If single
	       quotes are used, no interpretation is done on the replacement string (the "/e"
	       modifier overrides this, however).  Unlike Perl 4, Perl 5 treats backticks as nor-
	       mal delimiters; the replacement text is not evaluated as a command.  If the PAT-
	       TERN is delimited by bracketing quotes, the REPLACEMENT has its own pair of
	       quotes, which may or may not be bracketing quotes, e.g., "s(foo)(bar)" or
	       "s<foo>/bar/".  A "/e" will cause the replacement portion to be treated as a full-
	       fledged Perl expression and evaluated right then and there.  It is, however, syn-
	       tax checked at compile-time. A second "e" modifier will cause the replacement por-
	       tion to be "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 sed, we use the
	       \<digit> form in only the left hand side.  Anywhere else it's $<digit>.

	       Occasionally, you can't use just a "/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;

       Quote-Like Operators

       q/STRING/
       '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

       qq/STRING/
       "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

       qx/STRING/
       `STRING`
	   A string which is (possibly) interpolated and then executed as a system command with
	   "/bin/sh" or its equivalent.  Shell wildcards, pipes, and redirections will be hon-
	   ored.  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 com-
	   mand'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 inter-
	   polation, 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 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. ";" on
	   many Unix shells; "&" on the Windows NT "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 perl-
	   port).  To be safe, you may need to set $| ($AUTOFLUSH in English) or call the "aut-
	   oflush()" method of "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 inter-
	   polations.  See the platform-specific release notes for more details about your par-
	   ticular 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 "type" command under the POSIX shell is very different from the "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 lan-
	   guage, and one of the things it glues together is commands.	Just understand what
	   you're getting yourself into.

	   See "I/O Operators" for more discussion.

       qw/STRING/
	   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 "qw"-string.  For this reason, the "use warnings" pragma and the -w switch
	   (that is, the $^W variable) produces warnings if the STRING contains the "," or the
	   "#" character.

       tr/SEARCHLIST/REPLACEMENTLIST/cds
       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 "tr/A-J/0-9/" does the same
	   replacement as "tr/ACEGIBDFHJ/0246813579/".	For sed devotees, "y" is provided as a
	   synonym for "tr".  If the SEARCHLIST is delimited by bracketing quotes, the REPLACE-
	   MENTLIST has its own pair of quotes, which may or may not be bracketing quotes, e.g.,
	   "tr[A-Z][a-z]" or "tr(+\-*/)/ABCD/".

	   Note that "tr" does not do regular expression character classes such as "\d" or
	   "[:lower:]".  The "tr" operator is not equivalent to the tr(1) utility.  If you want
	   to map strings between lower/upper cases, see "lc" in perlfunc and "uc" in perlfunc,
	   and in general consider using the "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" modifier is specified, the SEARCHLIST character set is complemented.  If
	   the "/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 tr programs, which delete anything they find in the SEARCHLIST,
	   period.) If the "/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 "/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 $@;

       <<EOF
	   A line-oriented form of quoting is based on the shell "here-document" syntax.  Follow-
	   ing a "<<" you specify a string to terminate the quoted material, and all lines fol-
	   lowing 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
	   "<<" 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 sur-
	   rounding whitespace) on the terminating line.

	   If the terminating string is quoted, the type of quotes used determine the treatment
	   of the text.

	   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

	   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 "\\" 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.

	   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

	   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 "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 "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 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 -- "q()", "qq()", and the like are not supported in place of ''
	   and "", 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.

       Gory details of parsing quoted constructs

       When presented with something that might have several different interpretations, Perl uses
       the 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 com-
       mon reason to learn this is to unravel labyrinthine regular expressions, because the ini-
       tial 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 con-
       tents.  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 fre-
       quently 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 dif-
       ferent numbers of passes, from one to four, but these passes are always performed in the
       same order.

       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-inde-
	   pendent.

	   If the construct is a here-doc, the ending delimiter is a line that has a terminating
	   string as the content. Therefore "<<EOF" is terminated by "EOF" immediately followed
	   by "\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 "(", "[",
	   "{", or "<"), the ending delimiter is the corresponding closing punctuation (that is
	   ")", "]", "}", or ">").  If the starting delimiter is an unpaired character like "/"
	   or a closing punctuation, the ending delimiter is same as the starting delimiter.
	   Therefore a "/" terminates a "qq//" construct, while a "]" terminates "qq[]" and
	   "qq]]" constructs.

	   When searching for single-character delimiters, escaped delimiters and "\\" are
	   skipped. For example, while searching for terminating "/", combinations of "\\" and
	   "\/" are skipped.  If the delimiters are bracketing, nested pairs are also skipped.
	   For example, while searching for closing "]" paired with the opening "[", combinations
	   of "\\", "\]", and "\[" are all skipped, and nested "[" and "]" are skipped as well.
	   However, when backslashes are used as the delimiters (like "qq\\" and "tr\\\"), noth-
	   ing 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 ("s///", "y///", and "tr///"), the search is
	   repeated once more.	If the first delimiter is not an opening punctuation, three
	   delimiters must be same such as "s!!!" and "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 "()", "[]", "{}", or "<>"), the right
	   part needs another pair of delimiters such as "s(){}" and "tr[]//".	In these cases,
	   whitespaces and comments are allowed between both parts, though the comment must fol-
	   low at least one whitespace; otherwise a character expected as the start of the com-
	   ment 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 """ and "/",
	   and the rest happens to be a syntax error.  Because the slash that terminated "m//"
	   was followed by a "SPACE", the example above is not "m//x", but rather "m//" with no
	   "/x" modifier.  So the embedded "#" is interpreted as a literal "#".

	   Also no attention is paid to "\c\" (multichar control char syntax) during this search.
	   Thus the second "\" in "qq/\c\/" is interpreted as a part of "\/", and the following
	   "/" is not recognized as a delimiter.  Instead, use "\034" or "\x1c" at the end of
	   quoted constructs.

       Interpolation
	   The next step is interpolation in the text obtained, which is now delimiter-indepen-
	   dent.  There are multiple cases.

	   "<<'EOF'"
	       No interpolation is performed.  Note that the combination "\\" is left intact,
	       since escaped delimiters are not available for here-docs.

	   "m''", the pattern of "s'''"
	       No interpolation is performed at this stage.  Any backslashed sequences including
	       "\\" are treated at the stage to "parsing regular expressions".

	   '', "q//", "tr'''", "y'''", the replacement of "s'''"
	       The only interpolation is removal of "\" from pairs of "\\".  Therefore "-" in
	       "tr'''" and "y'''" is treated literally as a hyphen and no character range is
	       available.  "\1" in the replacement of "s'''" does not work as $1.

	   "tr///", "y///"
	       No variable interpolation occurs.  String modifying combinations for case and
	       quoting such as "\Q", "\U", and "\E" are not recognized.  The other escape
	       sequences such as "\200" and "\t" and backslashed characters such as "\\" and "\-"
	       are converted to appropriate literals.  The character "-" is treated specially and
	       therefore "\-" is treated as a literal "-".

	   "", ``, "qq//", "qx//", "<file*glob>", "<<"EOF""
	       "\Q", "\U", "\u", "\L", "\l" (possibly paired with "\E") are converted to corre-
	       sponding Perl constructs.  Thus, "$foo\Qbaz$bar" is converted to "$foo .
	       (quotemeta("baz" . $bar))" internally.  The other escape sequences such as "\200"
	       and "\t" and backslashed characters such as "\\" and "\-" are replaced with appro-
	       priate expansions.

	       Let it be stressed that whatever falls between "\Q" and "\E" is interpolated in
	       the usual way.  Something like "\Q\\E" has no "\E" inside.  instead, it has "\Q",
	       "\\", and "E", so the result is the same as for "\\\\E".  As a general rule, back-
	       slashes between "\Q" and "\E" may lead to counterintuitive results.  So, "\Q\t\E"
	       is converted to "quotemeta("\t")", which is the same as "\\\t" (since TAB is not
	       alphanumeric).  Note also that:

		 $str = '\t';
		 return "\Q$str";

	       may be closer to the conjectural intention of the writer of "\Q\t\E".

	       Interpolated scalars and arrays are converted internally to the "join" and "."
	       catenation operations.  Thus, "$foo XXX '@arr'" becomes:

		 $foo . " XXX '" . (join $", @arr) . "'";

	       All operations above are performed simultaneously, left to right.

	       Because the result of "\Q STRING \E" has all metacharacters quoted, there is no
	       way to insert a literal "$" or "@" inside a "\Q\E" pair.  If protected by "\", "$"
	       will be quoted to became "\\\$"; 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 inter-
	       polated scalar ends.  For instance, whether "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.

	   the replacement of "s///"
	       Processing of "\Q", "\U", "\u", "\L", "\l", and interpolation happens as with
	       "qq//" constructs.

	       It is at this step that "\1" is begrudgingly converted to $1 in the replacement
	       text of "s///", in order to correct the incorrigible sed hackers who haven't
	       picked up the saner idiom yet.  A warning is emitted if the "use warnings" pragma
	       or the -w command-line flag (that is, the $^W variable) was set.

	   "RE" in "?RE?", "/RE/", "m/RE/", "s/RE/foo/",
	       Processing of "\Q", "\U", "\u", "\L", "\l", and interpolation happens (almost) as
	       with "qq//" constructs.

	       However combinations of "\" followed by RE-special chars are not substituted but
	       only skipped. The full list of RE-special chars is "\", ".", "^", "$", "@", "A",
	       "G", "Z", "d", "D", "w", "W", "s", "S", "b", "B", "p", "P", "X", "C", "+", "*",
	       "?", "|", "(", ")", "-", "N", "n", "r", "t", "f", "e", "a", "x", "c", "z", digits
	       (0 to 9), "[", "{", "]", "}", whitespaces (SPACE, TAB, LF, CR, FF, and VT in addi-
	       tion), and "#".	As "\c" is skipped at this step, "@" of "\c@" in RE is possibly
	       treated as an array symbol (for example one of @foo or "@-"), even though the same
	       text in "qq//" gives interpolation of "\c@".

	       Moreover, inside "(?{BLOCK})", "(?# comment )", and a "#"-comment in a "//x"-regu-
	       lar expression, no processing is performed whatsoever.  This is the first step at
	       which the presence of the "//x" modifier is relevant.

	       Interpolation in patterns has several quirks: $|, $(, $), "@+" and "@-" are not
	       interpolated, and constructs $var[SOMETHING] are voted (by several different esti-
	       mators) to be either an array element or $var followed by an RE alternative.  This
	       is where the notation "${arr[$bar]}" comes handy: "/${arr[0-9]}/" is interpreted
	       as array element "-9", not as a regular expression from the variable $arr followed
	       by a digit, which would be the interpretation of "/$arr[0-9]/".	Since voting
	       among different estimators may occur, the result is not predictable.

	       The lack of processing of "\\" creates specific restrictions on the post-processed
	       text.  If the delimiter is "/", one cannot get the combination "\/" into the
	       result of this step.  "/" will finish the regular expression, "\/" will be
	       stripped to "/" on the previous step, and "\\/" will be left as is.  Because "/"
	       is equivalent to "\/" inside a regular expression, this does not matter unless the
	       delimiter happens to be character special to the RE engine, such as in
	       "s*foo*bar*", "m[foo]", or "?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 "m", the modifier is "mx", and after delimiter-removal the RE is the same as
	       for "m/ ^ a \s* b /mx".	There's more than one reason you're encouraged to
	       restrict your delimiters to non-alphanumeric, non-whitespace choices.

	   This step is the last one for all constructs except regular expressions, which are
	   processed further.

       parsing regular expressions
	   Previous steps were performed during the compilation of Perl code, but this one hap-
	   pens 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
	   string is passed to the RE engine for compilation.

	   Whatever happens in the RE engine might be better discussed in perlre, but for the
	   sake of continuity, we shall do so here.

	   This is another step where the presence of the "//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
	   "\{"), or else they generate special nodes in the finite automaton (as with "\b").
	   Characters special to the RE engine (such as "|") generate corresponding nodes or
	   groups of nodes.  "(?#...)" comments are ignored.  All the rest is either converted to
	   literal strings to match, or else is ignored (as is whitespace and "#"-style comments
	   if "//x" is present).

	   Parsing of the bracketed character class construct, "[...]", 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 "{}"-delimited construct, the
	   only exception being that "]" immediately following "[" is treated as though preceded
	   by a backslash.  Similarly, the terminator of "(?{...})" is found using the same rules
	   as for finding the terminator of a "{}"-delimited construct.

	   It is possible to inspect both the string given to RE engine and the resulting finite
	   automaton.  See the arguments "debug"/"debugcolor" in the "use re" pragma, as well as
	   Perl's -Dr command-line switch documented in "Command Switches" in perlrun.

       Optimization of regular expressions
	   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 previ-
	   ous pass.

	   It is at this stage that "split()" silently optimizes "/^/" to mean "/^/m".

       I/O Operators

       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 $/ 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
       $? (see perlvar for the interpretation of $?).  Unlike in 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 back-
       ticks is "qx//".  (Because backticks always undergo shell expansion as well, see perlsec
       for security concerns.)

       In scalar context, evaluating a filehandle in angle brackets yields the next line from
       that file (the newline, if any, included), or "undef" at end-of-file or on error.  When $/
       is set to "undef" (sometimes known as file-slurp mode) and the file is empty, it returns
       '' the first time, followed by "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 "while" statement (even if disguised as a "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 "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, "<filehandle>" without an explicit "defined" test or comparison
       elicit a warning if the "use warnings" pragma or the -w command-line switch (the $^W vari-
       able) is in effect.

       The filehandles STDIN, STDOUT, and STDERR are predefined.  (The filehandles "stdin", "std-
       out", and "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 perlopentut and "open" in perlfunc for details on
       this.

       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 "readline(*FILEHANDLE)".  See "readline" in perlfunc.

       The null filehandle <> is special: it can be used to emulate the behavior of sed and 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, $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 file-
       handle 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 "open" in perlfunc it interprets
       special characters, so if you have a script like this:

	   while (<>) {
	       print;
	   }

       and call it with "perl dangerous.pl 'rm -rfv *|'", it actually opens a pipe, executes the
       "rm" command and reads "rm"'s output from that pipe.  If you want all items in @ARGV to be
       interpreted as file names, you can use the module "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 ($.)  continue as though the input were one
       big happy file.	See the example in "eof" in perlfunc 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 com-
       pressed arguments through 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 "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 refer-
       ence 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 "<$x>" is always a readline() from an indirect handle, but
       "<$hash{key}>" is always a glob().  That's because $x is a simple scalar variable, but
       $hash{key} is not--it's a hash element.	Even "<$x >" (note the extra space) is treated as
       "glob("$x ")", not "readline($x)".

       One level of double-quote interpretation is done first, but you can't say "<$foo>" because
       that's an indirect filehandle as explained in the previous paragraph.  (In older versions
       of Perl, programmers would insert curly brackets to force interpretation as a filename
       glob: "<${foo}>".  These days, it's considered cleaner to call the internal function
       directly as "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 "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 "undef" when the list has run out.  As
       with filehandle reads, an automatic "defined" is generated when the glob occurs in the
       test part of a "while", because legal glob returns (e.g. a file called 0) would otherwise
       terminate the loop.  Again, "undef" is returned only once.  So if you're expecting a sin-
       gle 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]);

       Constant 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 par-
       ticular, string concatenation happens at compile time between literals that don't do vari-
       able 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.

       No-ops

       Perl doesn't officially have a no-op operator, but the bare constants 0 and 1 are special-
       cased to not produce a warning in a void context, so you can for example safely do

	   1 while foo();

       Bitwise String Operators

       Bitstrings of any size may be manipulated by the bitwise operators ("~ | & ^").

       If the operands to a binary bitwise op are strings of different sizes, | and ^ ops act as
       though the shorter operand had additional zero bits on the right, while the & 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 bit-
       strings: If an operand is a number, that will imply a numeric bitwise operation.  You may
       explicitly show which type of operation you intend by using "" or "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 "vec" in perlfunc for information on how to manipulate individual bits in a bit vec-
       tor.

       Integer Arithmetic

       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 say-
       ing

	   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 exam-
       ple, even under "use integer", if you take the sqrt(2), you'll still get 1.4142135623731
       or so.

       Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<", and ">>") always produce
       integral results.  (But see also "Bitwise String Operators".)  However, "use integer"
       still has meaning for them.  By default, their results are interpreted as unsigned inte-
       gers, but if "use integer" is in effect, their results are interpreted as signed integers.
       For example, "~0" usually evaluates to a large integral value.  However, "use integer; ~0"
       is "-1" on two's-complement machines.

       Floating-point Arithmetic

       While "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 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 equality of 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 which-
       ever system rounding is being used by Perl, but to instead implement the rounding function
       you need yourself.

       Bigger Numbers

       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-pre-
       cision 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.

perl v5.8.9				    2007-11-17					PERLOP(1)


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