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perlop



SYNOPSIS

       Perl operators have the following associativity and prece­
       dence, listed from highest precedence to lowest.  Opera­
       tors 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 val­
       ues 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 over­
       load.


DESCRIPTION

       Terms and List Operators (Leftward)

       A TERM has the highest precedence in Perl.  They include
       variables, quote and quote-like operators, any expression
       in parentheses, and any function whose arguments are
       parenthesized.  Actually, there aren't really functions in
       this sense, just list operators and unary operators behav­
       ing 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
       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 argu­
       ments 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.  See
       "Named Unary Operators" for more discussion of this.

       Also parsed as terms are the "do {}" and "eval {}" con­
       structs, 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 assign­
       ment.)  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 before
       returning the value, and if placed after, increment or
           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 con­
       sisting 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 "-bareword".

       Unary "~" performs bitwise negation, i.e., 1's complement.
       For example, "0666 & ~027" is 0640.  (See also "Integer
       Arithmetic" and "Bitwise String Operators".)  Note that
       the width of the result is platform-dependent: ~0 is 32
       bits wide on a 32-bit platform, but 64 bits wide on a
       64-bit platform, so if you are expecting a certain bit
       width, remember 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 (Left­
       ward)".)

       Unary "\" creates a reference to whatever follows it.  See
       perlreftut and perlref.  Do not confuse this behavior with
       the behavior of backslash within a string, although both
       forms do convey the notion of protecting the next thing
       from interpolation.

       Binding Operators

       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 implemented by
       your C compiler.  This operator is not as well defined for
       negative operands, but it will execute faster.

       Binary "x" is the repetition operator.  In scalar context
       or if the left operand is not enclosed in parentheses, it
       returns a string consisting of the left operand repeated
       the number of times specified by the right operand.  In
       list context, if the left operand is enclosed in parenthe­
       ses, it repeats the list.

           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 argu­
       ment.  Arguments should be integers.  (See also "Integer
       Arithmetic".)

       Binary ">>" returns the value of its left argument shifted
       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.  These
       include the filetest operators, like "-f", "-M", etc.  See
       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.  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)

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

       Relational Operators

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

       Binary ">" returns true if the left argument is numeri­
       cally greater than the right argument.

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

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

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

       Binary "!=" returns true if the left argument is numeri­
       cally 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 sup­
       ports 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 string­
       wise equal to the right argument.

       Binary "ne" returns true if the left argument is string­
       wise 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 Opera­
       tors".)

       Note that "&" has lower priority than relational opera­
       tors, 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 Opera­
       tors".)

       Binary "^" returns its operands XORed together bit by bit.
       (See also "Integer Arithmetic" and "Bitwise String Opera­
       tors".)

       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 differ from C's in that,
       rather than returning 0 or 1, they return the last value
       evaluated.  Thus, a reasonably portable way to find out
       the home directory (assuming it's not "0") might be:

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

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

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

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

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

       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 cre­
       tors.  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 evalu­
       ated 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 end­
       point.  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 float­
       ing 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 repre­
       sentation.

       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 ($. == 1 .. /^$/);
           s/^/> / if (/^$/ .. eof()); # quote body

                   # ...
               }
           } continue {
               close ARGV if eof;             # reset $. each file
           }

       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 mag­
       ical 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 spec­
       ified is not in the sequence that the magical increment
       would produce, the sequence goes until the next value
       would be longer than the final value specified.

       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

       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 deref­
       erencing 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;

       In list context, it's just the list argument separator,
       and inserts both its arguments into the list.

       The => digraph is mostly just a synonym for the comma
       operator.  It's useful for documenting arguments that come
       in pairs.  As of release 5.001, it also forces any word to
       the left of it to be interpreted as a string.

       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 opera­
       tors 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-cir­
       cuits: 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 expres­
       sion 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.

       ing expressions.  It cannot short circuit, of course.

       C Operators Missing From Perl

       Here is what C has that Perl doesn't:

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

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

       (TYPE)  Type-casting operator.

       Quote and Quote-like Operators

       While we usually think of quotes as literal values, in
       Perl they function as operators, providing various kinds
       of interpolating and pattern matching capabilities.  Perl
       provides customary quote characters for these behaviors,
       but also provides a way for you to choose your quote char­
       acter 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

       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

       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 Uni­
       code.  For documentation of "\N{name}", see charnames.

       All systems use the virtual "\n" to represent a line ter­
       minator, 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
       string "\$" to be inserted.  You'll need to write some­
       thing like "m/\Quser\E\@\Qhost/".

       Patterns are subject to an additional level of interpreta­
       tion as a regular expression.  This is done as a second
       pass, after variables are interpolated, so that regular
       expressions may be incorporated into the pattern from the
       variables.  If this is not what you want, use "\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 cur­
               rent 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

               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 "?PAT­
               TERN?" applies.  If "'" is the delimiter, no
               interpolation is performed 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 expen­
               sive run-time recompilations, and is useful when
               the value you are interpolating won't change over
               the life of the script.  However, mentioning "/o"
               constitutes a promise that you won't change the
               variables in the pattern.  If you change them,
               Perl won't even notice.  See also
               "qr/STRING/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 con­
               text returns a list consisting of the subexpres­
               sions 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; }

               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 match­
               ing--that is, matching as many times as possible
               within the string.  How it behaves depends on the
               context.  In list context, it returns a list of
               the substrings matched by any capturing parenthe­
               ses 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").  Modify­
               ing the target string also resets the search posi­
               tion.

               You can intermix "m//g" matches with "m/\G.../g",
               where "\G" is a zero-width assertion that matches
               the exact position where the previous "m//g", if
               any, left off.  Without the "/g" modifier, the
               "\G" assertion still anchors at pos(), but the
               match is of course only attempted once.  Using
               "\G" without "/g" on a target string that has not
               previously had a "/g" match applied to it is the
               same as using the "\A" assertion to match the
               beginning of the string.  Note also that, cur­
               rently, "\G" is only properly supported when
               anchored at the very beginning of the pattern.

               Examples:

                   # list context
                   ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);

                   # scalar context
                   $/ = "";
                   while (defined($paragraph = <>)) {
                       while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
                           $sentences++;
                       }
                       print "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 dif­
               ferent 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!


                   $_ .= 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 inter­
               polated 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
               syntax for STRING, and for a detailed look at the
               semantics 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 redirec­
               tions will be honored.  The collected standard
               output of the command is returned; standard error
               is unaffected.  In scalar context, it comes back
               as a single (potentially multi-line) string, or
               undef if the command failed.  In list context,
               returns a list of lines (however you've defined
               lines with $/ or $INPUT_RECORD_SEPARATOR), or an
               empty list if the command failed.

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

                   $output = `cmd 2>&1`;

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

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

               To capture a command's STDERR but discard its STD­
               OUT (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 and safest to redirect
               them separately to files, and then read from those
               files when the program is done:

                   system("program args 1>/tmp/program.stdout 2>/tmp/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 $$);            # that's Perl's $$

               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 charac­
               ter, 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 restric­
               tions on the length of the command line.  You must
               ensure your strings don't exceed this limit after
               any necessary interpolations.  See the platform-
               specific release notes for more details about your
               particular environment.

               Using this operator can lead to programs that are
               difficult to port, because the shell commands
               called vary between systems, and may in fact not
               be present at all.  As one example, the "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 back­
               ticks 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
               delimiters.  It can be understood as being roughly
               equivalent to:

                   split(' ', q/STRING/);

               the differences being that it generates a real
               list at compile time, and in scalar context it
               returns the last element in the list.  So this
               expression:

                   qw(foo bar baz)

               is semantically equivalent to the list:

       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 modi­
               fied.  (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 addi­
               tional 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, how­
               ever).  Unlike Perl 4, Perl 5 treats backticks as
               normal delimiters; the replacement text is not
               evaluated as a command.  If the PATTERN is delim­
               ited by bracketing quotes, the REPLACEMENT has its
               own pair of quotes, which may or may not be brack­
               eting 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

                   $count = ($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 white space in $_, expensively

                   for ($variable) {           # trim white space in $variable, cheap
                       s/^\s+//;
                       s/\s+$//;
                   }

                   s/([^ ]*) *([^ ]*)/$2 $1/;  # reverse 1st two fields

               Note the use of $ instead of \ in the last exam­
               ple.  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

               A character range may be specified with a hyphen,
               so "tr/A-J/0-9/" does the same replacement as
               "tr/ACEGIBDFHJ/0246813579/".  For sed devotees,
               "y" is provided as a synonym for "tr".  If the
               SEARCHLIST is delimited by bracketing quotes, the
               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
               character sets they may cause results you probably
               didn't expect.  A sound principle is to use only
               ranges that begin from and end at either alphabets
               of equal case (a-e, A-E), or digits (0-4).  Any­
               thing 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" modi­
               fier is specified, any characters specified by
               SEARCHLIST not found in REPLACEMENTLIST are
               deleted.  (Note that this is slightly more flexi­
               ble 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 sin­
               gle instance of the character.

               If the "/d" modifier is used, the REPLACEMENTLIST
               is always interpreted exactly as specified.  Oth­
               erwise, 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.

                   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 char­
               acter, only the first one is used:

                   tr/AAA/XYZ/

               will transliterate any A to X.

               Because the transliteration table is built at com­
               pile time, neither the SEARCHLIST nor the REPLACE­
               MENTLIST are subjected to double quote interpola­
               tion.  That means that if you want to use vari­
               ables, 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 mate­
               rial, 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 iden­
               tifier, which is valid, and matches the first
               empty line.)  The terminating string must appear
               by itself (unquoted and with no surrounding
               whitespace) on the terminating line.

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

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

                      print << `EOC`; # execute commands
                   echo hi there
                   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 con­
               struct, such as in "s///eg", the quoted material
               must come on the lines following the final delim­
               iter.  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 new­
               line after it; otherwise, Perl will give the warn­
               ing 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:

       What I Mean") principle to pick the most probable inter­
       pretation.  This strategy is so successful that Perl pro­
       grammers often do not suspect the ambivalence of what they
       write.  But from time to time, Perl's notions differ sub­
       stantially from what the author honestly meant.

       This section hopes to clarify how Perl handles quoted con­
       structs.  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 dis­
       cussed below: when processing a quoted construct, Perl
       first finds the end of that construct, then interprets its
       contents.  If you understand this rule, you may skip the
       rest of this section on the first reading.  The other
       rules are likely to contradict the user's expectations
       much less frequently than this first one.

       Some passes discussed below are performed concurrently,
       but because their results are the same, we consider them
       individually.  For different quoting constructs, Perl per­
       forms 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 con­
           struct, whether it be a multicharacter delimiter
           "\nEOF\n" in the "<<EOF" construct, a "/" that termi­
           nates a "qq//" construct, a "]" which terminates
           "qq[]" construct, or a ">" which terminates a fileglob
           started with "<".

           When searching for single-character non-pairing delim­
           iters, 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 multicharac­
           ter 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 seman­
           tics of the construct.  Thus:

               "$hash{"$foo/$bar"}"

           or:
           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 delim­
           iters.  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 appropri­
               ate expansions.

               Let it be stressed that whatever falls between
               "\Q" and "\E" is interpolated in the usual way.
               Something like "\Q\\E" has no "\E" inside.
               instead, it has "\Q", "\\", and "E", so the result
               is the same as for "\\\\E".  As a general rule,
               backslashes between "\Q" and "\E" may lead to
               counterintuitive results.  So, "\Q\t\E" is con­
               verted 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 opera­
               tions.  Thus, "$foo XXX '@arr'" becomes:
               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 heuristic estimators, the result is not
               strictly predictable.  Fortunately, it's usually
               correct for ambiguous 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[SOME­
               THING] 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 regu­
               lar expression from the variable $arr followed by
               a digit, which would be the interpretation of
               "/$arr[0-9]/".  Since voting among different esti­
               mators may occur, the result is not predictable.

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

               The lack of processing of "\\" creates specific
               restrictions on the post-processed text.  If the
               delimiter is "/", one cannot get the combination
               fier 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-whites­
               pace 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 metaquot­
           ing are involved, the resulting string is passed to
           the RE engine for compilation.

           Whatever happens in the RE engine might be better dis­
           cussed 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 automa­
           ton.

           Backslashed characters are either replaced with corre­
           sponding 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 com­
           ments 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 con­
           struct 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 "{}"-delim­
           ited 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,

       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 com­
       mand is the value of the backtick string, like in a shell.
       In scalar context, a single string consisting of all out­
       put 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 con­
       cerns.)

       In scalar context, evaluating a filehandle in angle brack­
       ets 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 vari­
       able, 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" state­
       ment (even if disguised as a "for(;;)" loop), the value is
       automatically assigned to the global variable $_, destroy­
       ing whatever was there previously.  (This may seem like an
       odd thing to you, but you'll use the construct in almost
       every Perl script you write.)  The $_ variable is not
       implicitly localized.  You'll have to put a "local $_;"
       before the loop if you want that to happen.

       The following lines are equivalent:

           while (defined($_ = <STDIN>)) { print; }
           while ($_ = <STDIN>) { print; }
           while (<STDIN>) { print; }
           for (;<STDIN>;) { print; }
           print while defined($_ = <STDIN>);
           print while ($_ = <STDIN>);
           print while <STDIN>;


       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 emu­
       late 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 actu­
       ally 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 synonym for
       <ARGV>, which is magical.  (The pseudo code above doesn't
       work because it treats <ARGV> as non-magical.)


           @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 filehan­
       dle nor a simple scalar variable containing a filehandle
       name, typeglob, or typeglob reference, it is interpreted
       as a filename pattern to be globbed, and either a list of
       filenames or the next filename in the list is returned,
       depending on context.  This distinction is determined on
       syntactic grounds alone.  That means "<$x>" is always a
       readline() from an indirect handle, but "<$hash{key}>" is
       always a glob().  That's because $x is a simple scalar
       variable, but $hash{key} is not--it's a hash element.

       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:
       the standard "File::Glob" extension.  Of course, the
       shortest way to do the above is:

           chmod 0644, <*.c>;

       A (file)glob evaluates its (embedded) argument only when
       it is starting a new list.  All values must be read before
       it will start over.  In list context, this isn't important
       because you automatically get them all anyway.  However,
       in scalar context the operator returns the next value each
       time it's called, or "undef" when the list has run out.
       As with filehandle reads, an automatic "defined" is gener­
       ated 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 defi­
       nitely 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 evalua­
       tion at compile time whenever it determines that all argu­
       ments 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) {

       ferent 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 num­
       ber, 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
       sprintf() or printf() is usually the easiest route.  See
       perlfaq4.

       Floating-point numbers are only approximations to what a
       mathematician would call real numbers.  There are
       infinitely more reals than floats, so some corners must be
       cut.  For example:

           printf "%.20g\n", 123456789123456789;
           #        produces 123456789123456784

       Testing for exact equality of floating-point equality or
       inequality is not a good idea.  Here's a (relatively
       expensive) work-around to compare whether two floating-
       point numbers are equal to a particular number of decimal
       places.  See Knuth, volume II, for a more robust treatment
       of this topic.

           sub fp_equal {
               my ($X, $Y, $POINTS) = @_;
               my ($tX, $tY);
               $tX = sprintf("%.${POINTS}g", $X);
               $tY = sprintf("%.${POINTS}g", $Y);
               return $tX eq $tY;
           }

       The POSIX module (part of the standard perl distribution)
       implements ceil(), floor(), and other mathematical and
       trigonometric functions.  The Math::Complex module (part
       of the standard perl distribution) defines mathematical
       functions that work on both the reals and the imaginary
       numbers.  Math::Complex not as efficient as POSIX, but
       POSIX can't work with complex numbers.

       Rounding in financial applications can have serious impli­
       cations, and the rounding method used should be specified
       precisely.  In these cases, it probably pays not to trust
       whichever system rounding is being used by Perl, but to
       instead implement the rounding function you need yourself.

       Bigger Numbers

       The standard Math::BigInt and Math::BigFloat modules pro­
       vide variable-precision arithmetic and overloaded opera­
       tors, although they're currently pretty slow. At the cost
       of some space and considerable speed, they avoid the nor­
       mal pitfalls associated with limited-precision representa­
       tions.

           use Math::BigInt;
           $x = Math::BigInt->new('123456789123456789');
           print $x * $x;
               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 v5.8.1                 2003-09-02                  PERLOP(1)
  
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