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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 mathematics.

       Operator precedence means some operators are evaluated before others.  For example, in "2 + 4 * 5", the multi-
       plication 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", sub-
       traction 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 precedence to lowest.
       Operators borrowed from C keep the same precedence relationship with each other, even where C's precedence is
       slightly screwy.  (This makes learning Perl easier for C folks.)  With very few exceptions, these all operate
       on scalar values only, not array values.

           left        terms and list operators (leftward)
           left        ->
           nonassoc    ++ --
           right       **
           right       ! ~ \ and unary + and -
           left        =~ !~
           left        * / % x
           left        + - .
           left        << >>
           nonassoc    named unary operators
           nonassoc    < > <= >= lt gt le ge
           nonassoc    == != <=> eq ne cmp
           left        &
           left        | ^
           left        &&
           left        ||
           nonassoc    ..  ...
           right       ?:
           right       = += -= *= etc.
           left        , =>
           nonassoc    list operators (rightward)
           right       not
           left        and
           left        or xor

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

       Many operators can be overloaded for objects.  See overload.

       Terms and List Operators (Leftward)

       A TERM has the highest precedence in Perl.  They include variables, quote and quote-like operators, any expres-
       sion in parentheses, and any function whose arguments are parenthesized.  Actually, there aren't really func-
       tions 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 technically 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 decremented. You just know it
       will be done sometime before or after the value is returned. This also means that modifying a variable twice in
       the same statement will lead to undefined behaviour.  Avoid statements like:

           $i = $i ++;
           print ++ $i + $i ++;

       Perl will not guarantee what the result of the above statements is.

       The auto-increment operator has a little extra builtin magic to it.  If you increment a variable that is
       numeric, or that has ever been used in a numeric context, you get a normal increment.  If, however, the vari-
       able 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 incrementing (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 char-
       acter (exluding "+" or "-"), Perl will attempt to convert the string to a numeric and the arithmetic negation
       is performed. If the string cannot be cleanly converted to a numeric, Perl will give the warning 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 cer-
       tain 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 separating 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 behav-
       ior 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 indicates
       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.

       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 "%" computes the modulus of two numbers.  Given integer operands $a and $b: If $b is positive, then "$a
       % $b" is $a minus the largest multiple of $b that is not greater than $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).  Note that when "use integer" is in scope, "%" gives you direct access to the modulus 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 parenthe-
       ses, 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 Arithmetic".)

       Binary ">>" returns the value of its left argument shifted right by the number of bits specified by the right
       argument.  Arguments should be integers.  (See also "Integer Arithmetic".)

       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($file).".bak"" is equiv-
       alent to "-f "$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 argument.

       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 argument.

       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 argument.

       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 argument.

       Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise less than, equal to, or
       greater than the right argument.

       "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 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 provides "and" and "or" opera-
       tors (see below).  The short-circuit behavior is identical.  The precedence of "and" and "or" is much lower,
       however, so that you can safely use them after a list operator without the need for parentheses:

           unlink "alpha", "beta", "gamma"
                   or gripe(), next LINE;

       With the C-style operators that would have been written like this:

           unlink("alpha", "beta", "gamma")
                   || (gripe(), next LINE);

       Using "or" for assignment is unlikely to do what you want; see below.

       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 operand 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 evaluation, 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 incre-
       ment would produce, the sequence goes until the next value would be longer than the final value specified.

       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 (meaning that you can
       assign to them):

           ($a_or_b ? $a : $b) = $c;

       Because this operator produces an assignable result, using assignments without parentheses will get you in
       trouble.  For example, this:

           $a % 2 ? $a += 10 : $a += 2

       Really means this:

           (($a % 2) ? ($a += 10) : $a) += 2

       Rather than this:

           ($a % 2) ? ($a += 10) : ($a += 2)

       That should probably be written more simply as:

           $a += ($a % 2) ? 10 : 2;

       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 recognized:

           **=    +=    *=    &=    <<=    &&=
                  -=    /=    |=    >>=    ||=
                  .=    %=    ^=
                        x=

       Although these are grouped by family, they all have the precedence of assignment.

       Unlike in C, the scalar assignment operator produces a valid lvalue.  Modifying an assignment is equivalent to
       doing the assignment and then modifying the variable that was assigned to.  This is useful for modifying a copy
       of something, like this:

           ($tmp = $global) =~ tr [A-Z] [a-z];

       Likewise,

           ($a += 2) *= 3;

       is equivalent to

           $a += 2;
           $a *= 3;

       Similarly, a list assignment in list context produces the list of lvalues assigned to, and a list assignment in
       scalar context returns the number of elements produced by the expression on the right hand side of the assign-
       ment.

       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.

       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 equivalent 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 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 "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 behav-
       iors, 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 dis-
       tribution) 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      (ESC)
           \x1b        hex char        (ESC)
           \x{263a}    wide hex char   (SMILEY)
           \c[         control char    (ESC)
           \N{name}    named Unicode character

       NOTE: Unlike C and other languages, Perl has no \v escape sequence for the vertical tab (VT - ASCII 11).

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

           \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 characters 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 illusion that the operating system, device drivers, C
       libraries, and Perl all conspire to preserve.  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 interpolated.  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 equiva-
       lent to interpolating "join $", @array".    "Punctuation" arrays such as "@+" are only interpolated if the name
       is enclosed in braces "@{+}".

       You cannot include a literal "$" or "@" within a "\Q" sequence.  An unescaped "$" or "@" interpolates the cor-
       responding 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 sec-
       ond pass, after variables are interpolated, so that regular expressions may be incorporated into the pattern
       from the variables.  If this is not what you want, use "\Q" to interpolate a variable literally.

       Apart from the behavior described above, Perl does not expand multiple levels of interpolation.  In particular,
       contrary to the expectations of shell programmers, back-quotes do 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.

       ?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.

       m/PATTERN/cgimosx
       /PATTERN/cgimosx
               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 "!~" operator, 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 additional
               considerations that apply when "use locale" is in effect.

               Options are:

                   c   Do not reset search position on a failed match when /g is in effect.
                   g   Match globally, i.e., find all occurrences.
                   i   Do case-insensitive pattern matching.
                   m   Treat string as multiple lines.
                   o   Compile pattern only once.
                   s   Treat string as single line.
                   x   Use extended regular expressions.

               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 recompiled) 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 recompila-
               tions, 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/imosx".

               If the PATTERN evaluates to the empty string, the last successfully matched regular 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 genuine empty pattern (which will always match).

               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 sub-
               strings matched by any capturing parentheses in the regular expression.  If there are no parentheses,
               it returns a list of all the matched strings, as if there were parentheses around the whole pattern.

               In scalar context, each execution of "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.

               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" asser-
               tion 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, currently, "\G" is only properly sup-
               ported 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 = new URI::URL "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!

       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

       qr/STRING/imosx
               This operator quotes (and possibly compiles) its STRING as a regular expression.  STRING is interpo-
               lated 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.

               For example,

                   $rex = qr/my.STRING/is;
                   s/$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 opti-
               mizations, but none would be triggered in the above example if we did not use qr() operator.)

               Options are:

                   i   Do case-insensitive pattern matching.
                   m   Treat string as multiple lines.
                   o   Compile pattern only once.
                   s   Treat string as single line.
                   x   Use extended regular expressions.

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

       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 honored.  The collected standard output
               of the command is returned; standard error is unaffected.  In scalar context, it comes back as a single
               (potentially multi-line) string, or undef if the command failed.  In list context, returns a list of
               lines (however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR), or an empty list if the com-
               mand failed.

               Because backticks do not affect standard error, use shell file descriptor syntax (assuming the shell
               supports this) if you care to address this.  To capture a command's STDERR and STDOUT together:

                   $output = 'cmd 2>&1';

               To capture a command's STDOUT but discard its STDERR:

                   $output = 'cmd 2>/dev/null';

               To capture a command's STDERR but discard its STDOUT (ordering is important here):

                   $output = 'cmd 2>&1 1>/dev/null';

               To exchange a command's STDOUT and STDERR in order to capture the STDERR but leave its STDOUT to come
               out the old STDERR:

                   $output = 'cmd 3>&1 1>&2 2>&3 3>&-';

               To read both a command's STDOUT and its STDERR separately, it's easiest to redirect them separately to
               files, and then read from those files when the program is done:

                   system("program args 1>program.stdout 2>program.stderr");

               Using single-quote as a delimiter protects the command from Perl's double-quote interpolation, passing
               it on to the shell instead:

                   $perl_info  = qx(ps $$);            # that's Perl's $$
                   $shell_info = qx'ps $$';            # that's the new shell's $$

               How that string gets evaluated is entirely subject to the command interpreter on your system.  On most
               platforms, you will have to protect shell metacharacters if you want them treated literally.  This is
               in practice difficult to do, as it's unclear how to escape which characters.  See 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 com-
               mands, 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 perlport).  To be safe, you may need to
               set $| ($AUTOFLUSH in English) or call the "autoflush()" 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 interpolations.  See the platform-spe-
               cific release notes for more details about your particular environment.

               Using this operator can lead to programs that are difficult to port, because the shell commands called
               vary between systems, and may in fact not be present at all.  As one example, the "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 language, 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 delim-
               iters.  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.

       s/PATTERN/REPLACEMENT/egimosx
               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 interpolated 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:

                   e   Evaluate the right side as an expression.
                   g   Replace globally, i.e., all occurrences.
                   i   Do case-insensitive pattern matching.
                   m   Treat string as multiple lines.
                   o   Compile pattern only once.
                   s   Treat string as single line.
                   x   Use extended regular expressions.

               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 normal delimiters; the replacement text is not evaluated as a com-
               mand.  If the PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own pair of quotes,
               which may or may not be bracketing quotes, e.g., "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, syntax 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;

       tr/SEARCHLIST/REPLACEMENTLIST/cds
       y/SEARCHLIST/REPLACEMENTLIST/cds
               Transliterates all occurrences of the characters found in the search list with the corresponding char-
               acter 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 SEARCH-
               LIST is delimited by bracketing quotes, the REPLACEMENTLIST 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 char-
               acter 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.  Other-
               wise, 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 REPLACE-
               MENTLIST 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.  Following a "<<" you
               specify a string to terminate the quoted material, and all lines following the current line down to the
               terminating string are the value of the item.  The terminating string may be either an identifier (a
               word), or some quoted text.  If quoted, the type of quotes you use determines the treatment of the
               text, just as in regular quoting.  An unquoted identifier works like double quotes.  There must be no
               space between the "<<" and the identifier, unless the identifier is quoted.  (If you put a space it
               will be treated as a null identifier, which is valid, and matches the first empty line.)  The terminat-
               ing string must appear by itself (unquoted and with no surrounding whitespace) on the terminating line.

                      print <<EOF;
                   The price is $Price.
                   EOF

                      print << "EOF"; # same as above
                   The price is $Price.
                   EOF

                      print << 'EOC'; # execute commands
                   echo hi there
                   echo lo there
                   EOC

                      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 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 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 back-
               slashing 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 common reason to learn
       this is to unravel labyrinthine regular expressions, because the initial steps of parsing are the same for all
       quoting operators, they are all discussed together.

       The most important Perl parsing rule is the first one discussed below: when processing a quoted construct, Perl
       first finds the end of that construct, then interprets its contents.  If you understand this rule, you may skip
       the rest of this section on the first reading.  The other rules are likely to contradict the user's expecta-
       tions much less frequently than this first one.

       Some passes discussed below are performed concurrently, but because their results are the same, we consider
       them individually.  For different quoting constructs, Perl performs different numbers of passes, from one to
       five, but these passes are always performed in the same order.

       Finding the end
           The first pass is finding the end of the quoted construct, whether it be a multicharacter delimiter
           "\nEOF\n" in the "<<EOF" construct, a "/" that terminates a "qq//" construct, a "]" which terminates "qq[]"
           construct, or a ">" which terminates a fileglob started with "<".

           When searching for single-character non-pairing delimiters, such as "/", combinations of "\\" and "\/" are
           skipped.  However, when searching for single-character pairing delimiter like "[", combinations of "\\",
           "\]", and "\[" are all skipped, and nested "[", "]" are skipped as well.  When searching for multicharacter
           delimiters, nothing is skipped.

           For constructs with three-part delimiters ("s///", "y///", and "tr///"), the search is repeated once more.

           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\" 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.

       Removal of backslashes before delimiters
           During the second pass, text between the starting and ending delimiters is copied to a safe location, and
           the "\" is removed from combinations consisting of "\" and delimiter--or delimiters, meaning both starting
           and ending delimiters will should these differ.  This removal does not happen for multi-character
           delimiters.  Note that the combination "\\" is left intact, just as it was.

           Starting from this step no information about the delimiters is used in parsing.

       Interpolation
           The next step is interpolation in the text obtained, which is now delimiter-independent.  There are four
           different cases.

           "<<'EOF'", "m''", "s'''", "tr///", "y///"
               No interpolation is performed.

           '', "q//"
               The only interpolation is removal of "\" from pairs "\\".

           "", '', "qq//", "qx//", "<file*glob>"
               "\Q", "\U", "\u", "\L", "\l" (possibly paired with "\E") are converted to corresponding Perl con-
               structs.  Thus, "$foo\Qbaz$bar" is converted to "$foo . (quotemeta("baz" . $bar))" internally.  The
               other combinations are replaced with appropriate expansions.

               Let it be stressed that whatever falls between "\Q" and "\E" is interpolated in the usual way.  Some-
               thing 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, backslashes 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 interpolated 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 heuris-
               tic estimators, the result is not strictly predictable.  Fortunately, it's usually correct for ambigu-
               ous cases.

           "?RE?", "/RE/", "m/RE/", "s/RE/foo/",
               Processing of "\Q", "\U", "\u", "\L", "\l", and interpolation happens (almost) as with "qq//" con-
               structs, but the substitution of "\" followed by RE-special chars (including "\") is not performed.
               Moreover, inside "(?{BLOCK})", "(?# comment )", and a "#"-comment in a "//x"-regular expression, no
               processing is performed whatsoever.  This is the first step at which the presence of the "//x" modifier
               is relevant.

               Interpolation has several quirks: $|, $(, and $) are not interpolated, and constructs $var[SOMETHING]
               are voted (by several different estimators) 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.

               It is at this step that "\1" is begrudgingly converted to $1 in the replacement text of "s///" to cor-
               rect 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.

               The lack of processing of "\\" creates specific restrictions on the post-processed text.  If the delim-
               iter 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 delim-
               iter 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 backslash-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.

       Interpolation of regular expressions
           Previous steps were performed during the compilation of Perl code, but this one happens at run
           time--although it may be optimized to be calculated at compile time if appropriate.  After preprocessing
           described above, and possibly after evaluation if catenation, 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 previous 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 inter-
       preted 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 backticks 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 con-
       struct 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 val-
       ues 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 variable) is in effect.

       The filehandles STDIN, STDOUT, and STDERR are predefined.  (The filehandles "stdin", "stdout", 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 filehandle ARGV internally--<> is just a syn-
       onym for <ARGV>, which is magical.  (The pseudo code above doesn't work because it treats <ARGV> as non-magi-
       cal.)

       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 compressed 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 reference to the same.  For example:

           $fh = \*STDIN;
           $line = <$fh>;

       If what's within the angle brackets is neither a filehandle nor a simple scalar variable containing a filehan-
       dle 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 any-
       way.  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 single value from a glob, it is much better to
       say

           ($file) = <blurch*>;

       than

           $file = <blurch*>;

       because the latter will alternate between returning a filename and returning false.

       If you're trying to do variable interpolation, it's definitely better to use the glob() function, because the
       older notation can cause people to become confused with the indirect filehandle notation.

           @files = glob("$dir/*.[ch]");
           @files = glob($files[$i]);

       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 particular, string concatenation happens at
       compile time between literals that don't do variable substitution.  Backslash interpolation also happens at
       compile time.  You can say

           'Now is the time for all' . "\n" .
               'good men to come to.'

       and this all reduces to one string internally.  Likewise, if you say

           foreach $file (@filenames) {
               if (-s $file > 5 + 100 * 2**16) {  }
           }

       the compiler will precompute the number which that expression represents so that the interpreter won't have to.

       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 bitstrings: 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 vector.

       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 saying

           no integer;

       which lasts until the end of that BLOCK.  Note that this doesn't mean everything is only an integer, merely
       that Perl may use integer operations if it is so inclined.  For example, even under "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 integers, but if "use integer" is in effect, their results are inter-
       preted as signed integers.  For example, "~0" usually evaluates to a large integral value.  However, "use inte-
       ger; ~0" is "-1" on twos-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 deci-
       mal 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 mathe-
       matical 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 speci-
       fied precisely.  In these cases, it probably pays not to trust whichever system rounding is being used by Perl,
       but to instead implement the rounding function you need yourself.

       Bigger Numbers

       The standard Math::BigInt and Math::BigFloat modules provide variable-precision arithmetic and overloaded oper-
       ators, although they're currently pretty slow. At the cost of some space and considerable speed, they avoid the
       normal pitfalls associated with limited-precision representations.

           use Math::BigInt;
           $x = Math::BigInt->new('123456789123456789');
           print $x * $x;

           # prints +15241578780673678515622620750190521

       There are several modules that let you calculate with (bound only by memory and cpu-time) unlimited or fixed
       precision. There are also some non-standard modules that provide faster implementations via external C
       libraries.

       Here is a short, but incomplete summary:

               Math::Fraction          big, unlimited fractions like 9973 / 12967
               Math::String            treat string sequences like numbers
               Math::FixedPrecision    calculate with a fixed precision
               Math::Currency          for currency calculations
               Bit::Vector             manipulate bit vectors fast (uses C)
               Math::BigIntFast        Bit::Vector wrapper for big numbers
               Math::Pari              provides access to the Pari C library
               Math::BigInteger        uses an external C library
               Math::Cephes            uses external Cephes C library (no big numbers)
               Math::Cephes::Fraction  fractions via the Cephes library
               Math::GMP               another one using an external C library

       Choose wisely.



perl v5.8.8                       2006-01-07                         PERLOP(1)