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perlop (1)


perlop - Perl operators and precedence


Please see following description for synopsis


Perl Programmers Reference Guide                        PERLOP(1)

     perlop - Perl operators and precedence

  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
     multiplication has higher precedence so "4 * 5" is evaluated
     first yielding "2 + 20 == 22" and not "6 * 5 == 30".

     Operator associativity defines what happens if a sequence of
     the same operators is used one after another: whether the
     evaluator will evaluate the left operations first or the
     right.  For example, in "8 - 4 - 2", subtraction is left
     associative so Perl evaluates the expression left to right.
     "8 - 4" is evaluated first making the expression "4 - 2 ==
     2" and not "8 - 2 == 6".

     Perl operators have the following associativity and
     precedence, listed from highest 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

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         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 expression in
     parentheses, and any function whose arguments are
     parenthesized.  Actually, there aren't really functions in
     this sense, just list operators and unary operators behaving
     as functions because you put parentheses around the
     arguments.  These are all documented in 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,

         @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

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     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:

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

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

     The auto-increment operator has a little extra builtin magic
     to it.  If you increment a variable that is numeric, or that
     has ever been used in a numeric context, you get a normal
     increment.  If, however, the variable has been used in only
     string contexts since it was set, and has a value that is
     not the empty string and matches the pattern
     "/^[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.

     Binary "**" is the exponentiation operator.  It binds even
     more tightly than unary minus, so -2**4 is -(2**4), not
     (-2)**4. (This is implemented using C's pow(3) function,
     which actually works on doubles internally.)

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

     Unary "-" performs arithmetic negation if the operand is
     numeric.  If the operand is an identifier, a string
     consisting of a minus sign concatenated with the identifier
     is returned.  Otherwise, if the string starts with a plus or
     minus, a string starting with the opposite sign is returned.
     One effect of these rules is that -bareword is equivalent to
     the string "-bareword".  If, however, the string begins with
     a non-alphabetic character (excluding "+" or "-"), Perl will
     attempt to convert the string to a numeric and the
     arithmetic negation is performed. If the string cannot be
     cleanly converted to a numeric, Perl will give the warning
     Argument "the string" isn't numeric in negation (-) at ....

     Unary "~" performs bitwise negation, i.e., 1's complement.
     For example, "0666 & ~027" is 0640.  (See also "Integer
     Arithmetic" and "Bitwise String Operators".)  Note that the

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     width of the result is platform-dependent: ~0 is 32 bits
     wide on a 32-bit platform, but 64 bits wide on a 64-bit
     platform, so if you are expecting a certain bit width,
     remember to use the & operator to mask off the excess bits.

     Unary "+" has no effect whatsoever, even on strings.  It is
     useful syntactically for 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 behavior with
     the behavior of backslash within a string, although both
     forms do convey the notion of protecting the next thing from

  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. Note that this means that
     its contents will be interpolated twice, so

       '\\' =~ q'\\';

     is not ok, as the regex engine will end up trying to compile
     the pattern "\", which it will consider a syntax error.

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

  Multiplicative Operators
     Binary "*" multiplies two numbers.

     Binary "/" divides two numbers.

     Binary "%" is the modulo operator, which computes the
     division remainder of its first argument with respect to its
     second argument.  Given integer operands $a and $b: If $b is
     positive, then "$a % $b" is $a minus the largest multiple of

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     $b less than or equal to $a.  If $b is negative, then "$a %
     $b" is $a minus the smallest multiple of $b that is not less
     than $a (i.e. the result will be less than or equal to
     zero).  If the operands $a and $b are floating point values
     and the absolute value of $b (that is "abs($b)") is less
     than "(UV_MAX + 1)", only the integer portion of $a and $b
     will be used in the operation (Note: here "UV_MAX" means the
     maximum of the unsigned integer type).  If the absolute
     value of the right operand ("abs($b)") is greater than or
     equal to "(UV_MAX + 1)", "%" computes the floating-point
     remainder $r in the equation "($r = $a - $i*$b)" where $i is
     a certain integer that makes $r have the same sign as the
     right operand $b (not as the left operand $a like C function
     "fmod()") and the absolute value less than that of $b.  Note
     that when "use integer" is in scope, "%" gives you direct
     access to the modulo operator as 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 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

     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

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     Note that both "<<" and ">>" in Perl are implemented
     directly using "<<" and ">>" in C.  If "use integer" (see
     "Integer Arithmetic") is in force then signed C integers are
     used, else unsigned C integers are used.  Either way, the
     implementation isn't going to generate results larger than
     the size of the integer type Perl was built with (32 bits or
     64 bits).

     The result of overflowing the range of the integers is
     undefined because it is undefined also in C.  In other
     words, using 32-bit integers, "1 << 32" is undefined.
     Shifting by a negative number of bits is also undefined.

  Named Unary Operators
     The various named unary operators are treated as functions
     with one argument, with optional parentheses.

     If any list operator (print(), etc.) or any unary operator
     (chdir(), etc.)  is followed by a left parenthesis as the
     next token, the operator and arguments within parentheses
     are taken to be of highest precedence, just like a normal
     function call.  For example, because named unary operators
     are higher precedence than ||:

         chdir $foo    || die;       # (chdir $foo) || die
         chdir($foo)   || die;       # (chdir $foo) || die
         chdir ($foo)  || die;       # (chdir $foo) || die
         chdir +($foo) || die;       # (chdir $foo) || die

     but, because * is higher precedence than named operators:

         chdir $foo * 20;    # chdir ($foo * 20)
         chdir($foo) * 20;   # (chdir $foo) * 20
         chdir ($foo) * 20;  # (chdir $foo) * 20
         chdir +($foo) * 20; # chdir ($foo * 20)

         rand 10 * 20;       # rand (10 * 20)
         rand(10) * 20;      # (rand 10) * 20
         rand (10) * 20;     # (rand 10) * 20
         rand +(10) * 20;    # rand (10 * 20)

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

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

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

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     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.

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     Binary "~~" does a smart match between its arguments. Smart
     matching is described in "Smart matching in detail" in

     "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

     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

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

     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.

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

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     is exactly equivalent to "defined($a) ? $a : $b".  This is
     very useful for providing default values for variables.  If
     you actually want to test if at least one of $a and $b is
     defined, use "defined($a // $b)".

     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 the "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

         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:

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         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,
     even across calls to a subroutine that contains it. It is
     false as long as its left operand is false.  Once the left
     operand is true, the range operator stays true until the
     right operand is true, 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 until the next
     evaluation, as in sed, just use three dots ("...") instead
     of two.  In all other regards, "..." behaves just like ".."

     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.

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     As a scalar operator:

         if (101 .. 200) { print; } # print 2nd hundred lines, short for
                                    #   if ($. == 101 .. $. == 200) { print; }

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

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

         # parse mail messages
         while (<>) {
             $in_header =   1  .. /^$/;
             $in_body   = /^$/ .. eof;
             if ($in_header) {
                 # do something
             } else { # in body
                 # do something else
         } continue {
             close ARGV if eof;             # reset $. each file

     Here's a simple example to illustrate the difference between
     the two range operators:

         @lines = ("   - Foo",
                   "01 - Bar",
                   "1  - Baz",
                   "   - Quux");

         foreach (@lines) {
             if (/0/ .. /1/) {
                 print "$_\n";

     This program will print only the line containing "Bar". If
     the range operator is changed to "...", 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

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     strings.  You can say

         @alphabet = ('A' .. 'Z');

     to get all normal letters of the English alphabet, or

         $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];

     to get a hexadecimal digit, or

         @z2 = ('01' .. '31');  print $z2[$mday];

     to get dates with leading zeros.

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

     If the initial value specified isn't part of a magical
     increment sequence (that is, a non-empty string matching
     "/^[a-zA-Z]*[0-9]*\z/"), only the initial value will be
     returned.  So the following will only return an alpha:

         use charnames 'greek';
         my @greek_small =  ("\N{alpha}" .. "\N{omega}");

     To get lower-case greek letters, use this instead:

         my @greek_small =  map { chr } ( ord("\N{alpha}") .. ord("\N{omega}") );

     Because each operand is evaluated in integer form, "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!

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     The operator may be assigned to if both the 2nd and 3rd
     arguments are legal lvalues (meaning that you can assign to

         ($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:

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

     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

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


         ($a += 2) *= 3;

     is equivalent to

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

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

  Comma Operator
     Binary "," is the comma operator.  In scalar context it
     evaluates its left argument, throws that value away, then
     evaluates its right argument and returns that value.  This
     is just like C's comma operator.

     In list context, it's just the list argument separator, and
     inserts both its arguments into the list.  These arguments
     are also evaluated from left to right.

     The "=>" operator is a synonym for the comma except that it
     causes its left operand to be interpreted as a string if it
     begins with a letter or underscore and is composed only of
     letters, digits and underscores.  This includes operands
     that might otherwise be interpreted as operators, constants,
     single number v-strings or function calls. If in doubt about
     this behaviour, the left operand can be quoted explicitly.

     Otherwise, the "=>" operator behaves exactly as the comma
     operator or list argument separator, according to context.

     For example:

         use constant FOO => "something";

         my %h = ( FOO => 23 );

     is equivalent to:

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

     It is NOT:

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

     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 );

  Yada Yada Operator
     The yada yada operator (noted "...") is a placeholder for
     code. Perl parses it without error, but when you try to
     execute a yada yada, it throws an exception with the text

             sub unimplemented { ... }

             eval { unimplemented() };
             if( $@ eq 'Unimplemented' ) {
               print "I found the yada yada!\n";

     You can only use the yada yada to stand in for a complete
     statement.  These examples of the yada yada work:

             { ... }

             sub foo { ... }


             eval { ... };

             sub foo {
                             my( $self ) = shift;


             do { my $n; ...; print 'Hurrah!' };

     The yada yada cannot stand in for an expression that is part
     of a larger statement since the "..." is also the three-dot
     version of the range operator (see "Range Operators"). These
     examples of the yada yada are still syntax errors:

             print ...;

             open my($fh), '>', '/dev/passwd' or ...;

             if( $condition && ... ) { print "Hello\n" };

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     There are some cases where Perl can't immediately tell the
     difference between an expression and a statement. For
     instance, the syntax for a block and an anonymous hash
     reference constructor look the same unless there's something
     in the braces that give Perl a hint. The yada yada is a
     syntax error if Perl doesn't guess that the "{ ... }" is a
     block. In that case, it doesn't think the "..." is the yada
     yada because it's expecting an expression instead of a

             my @transformed = map { ... } @input;  # syntax error

     You can use a ";" inside your block to denote that the "{
     ... }" is a block and not a hash reference constructor. Now
     the yada yada works:

             my @transformed = map {; ... } @input; # ; disambiguates

             my @transformed = map { ...; } @input; # ; disambiguates

  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, Defined or, and Exclusive Or
     Binary "or" returns the logical disjunction of the two
     surrounding expressions.  It's equivalent to || except for
     the very low precedence.  This makes it useful for control

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         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 behaviors, but
     also provides a way for you to choose your quote character
     for any of them.  In the following table, a "{}" represents
     any pair of delimiters you choose.

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         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


     is the same as


     Note, however, that this does not always work for quoting
     Perl code:

             $s = q{ if($a eq "}") ... }; # WRONG

     is a syntax error. The "Text::Balanced" module (from CPAN,
     and starting from Perl 5.8 part of the standard
     distribution) is able to do this properly.

     There can be whitespace between the operator and the quoting
     characters, except when "#" is being used as the quoting
     character.  "q#foo#" is parsed as the string "foo", while "q
     #foo#" is the operator "q" followed by a comment.  Its
     argument will be taken from the next line.  This allows you
     to write:

         s {foo}  # Replace foo
           {bar}  # with bar.

     The following escape sequences are available in constructs
     that interpolate and in transliterations.

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         \t          tab             (HT, TAB)
         \n          newline         (NL)
         \r          return          (CR)
         \f          form feed       (FF)
         \b          backspace       (BS)
         \a          alarm (bell)    (BEL)
         \e          escape          (ESC)
         \033        octal char      (example: ESC)
         \x1b        hex char        (example: ESC)
         \x{263a}    wide hex char   (example: SMILEY)
         \c[         control char    (example: ESC)
         \N{name}    named Unicode character
         \N{U+263D}  Unicode character (example: FIRST QUARTER MOON)

     The character following "\c" is mapped to some other
     character by converting letters to upper case and then (on
     ASCII systems) by inverting the 7th bit (0x40). The most
     interesting range is from '@' to '_' (0x40 through 0x5F),
     resulting in a control character from 0x00 through 0x1F. A
     '?' maps to the DEL character. On EBCDIC systems only '@',
     the letters, '[', '\', ']', '^', '_' and '?' will work,
     resulting in 0x00 through 0x1F and 0x7F.

     "\N{U+wide hex char}" means the Unicode character whose
     Unicode ordinal number is wide hex char.  For documentation
     of "\N{name}", see charnames.

     NOTE: Unlike C and other languages, Perl has no "\v" escape
     sequence for the vertical tab (VT - ASCII 11), but you may
     use "\ck" or "\x0b".  ("\v" does have meaning in regular
     expression patterns in Perl, see perlre.)

     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.

     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

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     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 equivalent to
     interpolating "join $", @array".    "Punctuation" arrays
     such as "@*" are only interpolated if the name is enclosed
     in braces "@{*}", but special arrays @_, "@+", and "@-" are
     interpolated, even without braces.

     You cannot include a literal "$" or "@" within a "\Q"
     sequence.  An unescaped "$" or "@" interpolates the
     corresponding variable, while escaping will cause the
     literal string "\$" to be inserted.  You'll need to write
     something like "m/\Quser\E\@\Qhost/".

     Patterns are subject to an additional level of
     interpretation as a regular expression.  This is done as a
     second pass, after variables are interpolated, so that
     regular 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.

             This operator quotes (and possibly compiles) its
             STRING as a regular expression.  STRING is

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             interpolated the same way as PATTERN in
             "m/PATTERN/".  If "'" is used as the delimiter, no
             interpolation is done.  Returns a Perl value which
             may be used instead of the corresponding
             "/STRING/msixpo" expression. The returned value is a
             normalized version of the original pattern. It
             magically differs from a string containing the same
             characters: "ref(qr/x/)" returns "Regexp", even
             though dereferencing the result returns undef.

             For example,

                 $rex = qr/my.STRING/is;
                 print $rex;                 # prints (?si-xm:my.STRING)

             is equivalent to


             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/;
                     } @_;

             Precompilation of the pattern into an internal
             representation at the moment of qr() avoids a need
             to recompile the pattern every time a match "/$pat/"
             is attempted.  (Perl has many other internal
             optimizations, but none would be triggered in the
             above example if we did not use qr() operator.)

             Options are:

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                 m   Treat string as multiple lines.
                 s   Treat string as single line. (Make . match a newline)
                 i   Do case-insensitive pattern matching.
                 x   Use extended regular expressions.
                 p   When matching preserve a copy of the matched string so
                     that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
                 o   Compile pattern only once.

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

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

             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

             Options are as described in "qr//"; in addition, the
             following match process modifiers are available:

                 g   Match globally, i.e., find all occurrences.
                 c   Do not reset search position on a failed match when /g is in effect.

             If "/" is the delimiter then the initial "m" is
             optional.  With the "m" you can use any pair of non-
             whitespace characters as delimiters.  This is
             particularly useful for matching path names that
             contain "/", to avoid LTS (leaning toothpick
             syndrome).  If "?" is the delimiter, then the match-
             only-once rule of "?PATTERN?" applies.  If "'" is
             the delimiter, no interpolation is performed on the
             PATTERN.  When using a character valid in an
             identifier, whitespace is required after the "m".

             PATTERN may contain variables, which will be
             interpolated (and the pattern recompiled) every time
             the pattern search is evaluated, except for when the
             delimiter is a single quote.  (Note that $(, $), and

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             $| are not interpolated because they look like end-
             of-string tests.)  If you want such a pattern to be
             compiled only once, add a "/o" after the trailing
             delimiter.  This avoids expensive run-time
             recompilations, and is useful when the value you are
             interpolating won't change over the life of the
             script.  However, mentioning "/o" constitutes a
             promise that you won't change the variables in the
             pattern.  If you change them, Perl won't even
             notice.  See also "STRING/msixpo"" in "qr.

     The empty pattern //
             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).

             Note that it's possible to confuse Perl into
             thinking "//" (the empty regex) is really "//" (the
             defined-or operator).  Perl is usually pretty good
             about this, but some pathological cases might
             trigger this, such as "$a///" (is that "($a) / (//)"
             or "$a // /"?) and "print $fh //" ("print $fh(//" or
             "print($fh //"?).  In all of these examples, Perl
             will assume you meant defined-or.  If you meant the
             empty regex, just use parentheses or spaces to
             disambiguate, or even prefix the empty regex with an
             "m" (so "//" becomes "m//").

     Matching in list context
             If the "/g" option is not used, "m//" in list
             context returns a list consisting of the
             subexpressions matched by the parentheses in the
             pattern, i.e., ($1, $2, $3...).  (Note that here $1
             etc. are also set, and that this differs from Perl
             4's behavior.)  When there are no parentheses in the
             pattern, the return value is the list "(1)" for
             success.  With or without parentheses, an empty list
             is returned upon failure.


                 open(TTY, '/dev/tty');
                 <TTY> =~ /^y/i && foo();    # do foo if desired

                 if (/Version: *([0-9.]*)/) { $version = $1; }

                 next if m#^/usr/spool/uucp#;

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                 # poor man's grep
                 $arg = shift;
                 while (<>) {
                     print if /$arg/o;       # compile only once

                 if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))

             This last example splits $foo into the first two
             words and the remainder of the line, and assigns
             those three fields to $F1, $F2, and $Etc.  The
             conditional is true if any variables were assigned,
             i.e., if the pattern matched.

             The "/g" modifier specifies global pattern
             matching--that is, matching as many times as
             possible within the string.  How it behaves depends
             on the context.  In list context, it returns a list
             of the substrings matched by any capturing
             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.

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


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

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

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              $_ = <<'EOL';
                   $url = URI::URL->new( "" ); die if $url eq "xXx";
                   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!

             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.

             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.

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             (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

             Options are as with m// with the addition of the
             following replacement specific options:

                 e   Evaluate the right side as an expression.
                 ee  Evaluate the right side as a string then eval the result

             Any non-whitespace delimiter may replace the
             slashes.  Add space after the "s" when using a
             character allowed in identifiers.  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 command.  If the PATTERN is
             delimited by bracketing quotes, the REPLACEMENT has
             its own pair of quotes, which may or may not be
             bracketing quotes, e.g., "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
             portion to be "eval"ed before being run as a Perl


                 s/\bgreen\b/mauve/g;                # don't change wintergreen

                 $path =~ s|/usr/bin|/usr/local/bin|;

                 s/Login: $foo/Login: $bar/; # run-time pattern

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                 ($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

                 # 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

                 # 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/([^ ]*) *([^ ]*)/$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;

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                 # expand tabs to 8-column spacing
                 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;

  Quote-Like Operators
         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

             $foo = q!I said, "You said, 'She said it.'"!;
             $bar = q('This is it.');
             $baz = '\n';                # a two-character 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

         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 command failed.

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

             $output = `cmd 2>&1`;

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

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

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

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

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

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

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

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

         The STDIN filehandle used by the command is inherited
         from Perl's STDIN.  For example:

             open BLAM, "blam" || die "Can't open: $!";
             open STDIN, "<&BLAM";
             print `sort`;

         will print the sorted contents of the file "blam".

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

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

         How that string gets evaluated is entirely subject to
         the command interpreter on your system.  On most
         platforms, you will have to protect shell metacharacters
         if you want them treated literally.  This is in practice
         difficult to do, as it's unclear how to escape which
         characters.  See perlsec for a clean and safe example of
         a manual fork() and exec() to emulate backticks safely.

         On some platforms (notably DOS-like ones), the shell may
         not be capable of dealing with multiline commands, so
         putting newlines in the string may not get you what you
         want.  You may be able to evaluate multiple commands in
         a single line by separating them with the command
         separator character, if your shell supports that (e.g.
         ";" on many Unix shells; "&" on the Windows NT "cmd"

         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.

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         Beware that some command shells may place restrictions
         on the length of the command line.  You must ensure your
         strings don't exceed this limit after any necessary
         interpolations.  See the platform-specific release notes
         for more details about your particular environment.

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

         Evaluates to a list of the words extracted out of
         STRING, using embedded whitespace as the word
         delimiters.  It can be understood as being roughly
         equivalent to:

             split(' ', q/STRING/);

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

             qw(foo bar baz)

         is semantically equivalent to the list:

             'foo', 'bar', 'baz'

         Some frequently seen examples:

             use POSIX qw( setlocale localeconv )
             @EXPORT = qw( foo bar baz );

         A common mistake is to try to separate the words with
         comma or to put comments into a multi-line "qw"-string.
         For this reason, the "use warnings" pragma and the -w
         switch (that is, the $^W variable) produces warnings if
         the STRING contains the "," or the "#" character.

         Transliterates all occurrences of the characters found
         in the search list with the corresponding character in

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         the replacement list.  It returns the number of
         characters replaced or deleted.  If no string is
         specified via the =~ or !~ operator, the $_ string is
         transliterated.  (The string specified with =~ must be a
         scalar variable, an array element, a hash element, or an
         assignment to one of those, i.e., an lvalue.)

         A character range may be specified with a hyphen, so
         "tr/A-J/0-9/" does the same replacement as
         "tr/ACEGIBDFHJ/0246813579/".  For sed devotees, "y" is
         provided as a synonym for "tr".  If the SEARCHLIST is
         delimited by bracketing quotes, the REPLACEMENTLIST has
         its own pair of quotes, which may or may not be
         bracketing quotes, e.g., "tr[A-Z][a-z]" or

         Note that "tr" does not do regular expression character
         classes such as "\d" or "[:lower:]".  The "tr" operator
         is not equivalent to the tr(1) utility.  If you want to
         map strings between lower/upper cases, see "lc" in
         perlfunc and "uc" in perlfunc, and in general consider
         using the "s" operator if you need regular expressions.

         Note also that the whole range idea is rather unportable
         between character sets--and even within character sets
         they may cause results you probably didn't expect.  A
         sound principle is to use only ranges that begin from
         and end at either alphabets of equal case (a-e, A-E), or
         digits (0-4).  Anything else is unsafe.  If in doubt,
         spell out the character sets in full.


             c   Complement the SEARCHLIST.
             d   Delete found but unreplaced characters.
             s   Squash duplicate replaced characters.

         If the "/c" modifier is specified, the SEARCHLIST
         character set is complemented.  If the "/d" modifier is
         specified, any characters specified by SEARCHLIST not
         found in REPLACEMENTLIST are deleted.  (Note that this
         is slightly more flexible than the behavior of some tr
         programs, which delete anything they find in the
         SEARCHLIST, period.) If the "/s" modifier is specified,
         sequences of characters that were transliterated to the
         same character are squashed down to a single instance of
         the character.

         If the "/d" modifier is used, the REPLACEMENTLIST is
         always interpreted exactly as specified.  Otherwise, if
         the REPLACEMENTLIST is shorter than the SEARCHLIST, the
         final character is replicated till it is long enough.

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         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.


             $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:


         will transliterate any A to X.

         Because the transliteration table is built at compile
         time, neither the SEARCHLIST nor the REPLACEMENTLIST are
         subjected to double quote interpolation.  That means
         that if you want to use variables, you must use an

             eval "tr/$oldlist/$newlist/";
             die $@ if $@;

             eval "tr/$oldlist/$newlist/, 1" or die $@;

         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.  An unquoted identifier
         works like double quotes.  There may not be a space

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         between the "<<" and the identifier, unless the
         identifier is explicitly quoted.  (If you put a space it
         will be treated as a null identifier, which is valid,
         and matches the first empty line.)  The terminating
         string must appear by itself (unquoted and with no
         surrounding whitespace) on the terminating line.

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

         Double Quotes
             Double quotes indicate that the text will be
             interpolated using exactly the same rules as normal
             double quoted strings.

                    print <<EOF;
                 The price is $Price.

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

         Single Quotes
             Single quotes indicate the text is to be treated
             literally with no interpolation of its content. This
             is similar to single quoted strings except that
             backslashes have no special meaning, with "\\" being
             treated as two backslashes and not one as they would
             in every other quoting construct.

             This is the only form of quoting in perl where there
             is no need to worry about escaping content,
             something that code generators can and do make good
             use of.

             The content of the here doc is treated just as it
             would be if the string were embedded in backticks.
             Thus the content is interpolated as though it were
             double quoted and then executed via the shell, with
             the results of the execution returned.

                    print << `EOC`; # execute command and get results
                 echo hi there

         It is possible to stack multiple here-docs in a row:

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                print <<"foo", <<"bar"; # you can stack them
             I said foo.
             I said bar.

                myfunc(<< "THIS", 23, <<'THAT');
             Here's a line
             or two.
             and here's another.

         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
                + 20;

         If you want to remove the line terminator from your
         here-docs, use "chomp()".

             chomp($string = <<'END');
             This is a string.

         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.

         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
              . 'more '/eg;

         you have to write

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             s/this/<<E . 'that'
              . 'more '/eg;
             the other

         If the terminating identifier is on the last line of the
         program, you must be sure there is a newline after it;
         otherwise, Perl will give the warning Can't find string
         terminator "END" anywhere before EOF....

         Additionally, the quoting rules for the end of string
         identifier are not related to Perl's quoting rules.
         "q()", "qq()", and the like are not supported in place
         of '' and "", and the only interpolation is for
         backslashing the quoting character:

             print << "abc\"def";

         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 expectations much less
     frequently than this first one.

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

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     Finding the end
         The first pass is finding the end of the quoted
         construct, where the information about the delimiters is
         used in parsing.  During this search, text between the
         starting and ending delimiters is copied to a safe
         location. The text copied gets delimiter-independent.

         If the construct is a here-doc, the ending delimiter is
         a line that has a terminating string as the content.
         Therefore "<<EOF" is terminated by "EOF" immediately
         followed by "\n" and starting from the first column of
         the terminating line.  When searching for the
         terminating line of a here-doc, nothing is skipped. In
         other words, lines after the here-doc syntax are
         compared with the terminating string line by line.

         For the constructs except here-docs, single characters
         are used as starting and ending delimiters. If the
         starting delimiter is an opening punctuation (that is
         "(", "[", "{", or "<"), the ending delimiter is the
         corresponding closing punctuation (that is ")", "]",
         "}", or ">").  If the starting delimiter is an unpaired
         character like "/" or a closing punctuation, the ending
         delimiter is same as the starting delimiter.  Therefore
         a "/" terminates a "qq//" construct, while a "]"
         terminates "qq[]" and "qq]]" constructs.

         When searching for single-character delimiters, escaped
         delimiters and "\\" are skipped. For example, while
         searching for terminating "/", combinations of "\\" and
         "\/" are skipped.  If the delimiters are bracketing,
         nested pairs are also skipped.  For example, while
         searching for closing "]" paired with the opening "[",
         combinations of "\\", "\]", and "\[" are all skipped,
         and nested "[" and "]" are skipped as well.  However,
         when backslashes are used as the delimiters (like "qq\\"
         and "tr\\\"), nothing is skipped.  During the search for
         the end, backslashes that escape delimiters are removed
         (exactly speaking, they are not copied to the safe

         For constructs with three-part delimiters ("s///",
         "y///", and "tr///"), the search is repeated once more.
         If the first delimiter is not an opening punctuation,
         three delimiters must be same such as "s!!!" and
         "tr)))", in which case the second delimiter terminates
         the left part and starts the right part at once.  If the
         left part is delimited by bracketing punctuations (that
         is "()", "[]", "{}", or "<>"), the right part needs
         another pair of delimiters such as "s(){}" and "tr[]//".
         In these cases, whitespaces and comments are allowed
         between both parts, though the comment must follow at

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         least one whitespace; otherwise a character expected as
         the start of the comment may be regarded as the starting
         delimiter of the right part.

         During this search no attention is paid to the semantics
         of the construct.  Thus:



               bar       # NOT a comment, this slash / terminated m//!

         do not form legal quoted expressions.   The quoted part
         ends on the first """ and "/", and the rest happens to
         be a syntax error.  Because the slash that terminated
         "m//" was followed by a "SPACE", the example above is
         not "m//x", but rather "m//" with no "/x" modifier.  So
         the embedded "#" is interpreted as a literal "#".

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

         The next step is interpolation in the text obtained,
         which is now delimiter-independent.  There are multiple

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

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

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

         "tr///", "y///"

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

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

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             For instance, whether "a $b -> {c}" really means:

               "a " . $b . " -> {c}";


               "a " . $b -> {c};

             Most of the time, the longest possible text that
             does not include spaces between components and which
             contains matching braces or brackets.  because the
             outcome may be determined by voting based on
             heuristic estimators, the result is not strictly
             predictable.  Fortunately, it's usually correct for
             ambiguous cases.

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

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

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

             Processing of "\N{...}" is also done here, and
             compiled into an intermediate form for the regex
             compiler.  (This is because, as mentioned below, the
             regex compilation may be done at execution time, and
             "\N{...}" is a compile-time construct.)

             However any other combinations of "\" followed by a
             character are not substituted but only skipped, in
             order to parse them as regular expressions at the
             following step.  As "\c" is skipped at this step,
             "@" of "\c@" in RE is possibly treated as an array
             symbol (for example @foo), even though the same text
             in "qq//" gives interpolation of "\c@".

             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.

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             Interpolation in patterns 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

             The lack of processing of "\\" creates specific
             restrictions on the post-processed text.  If the
             delimiter is "/", one cannot get the combination
             "\/" into the result of this step.  "/" will finish
             the regular expression, "\/" will be stripped to "/"
             on the previous step, and "\\/" will be left as is.
             Because "/" is equivalent to "\/" inside a regular
             expression, this does not matter unless the
             delimiter happens to be character special to the RE
             engine, such as in "s*foo*bar*", "m[foo]", or
             "?foo?"; or an alphanumeric char, as in:

               m m ^ a \s* b mmx;

             In the RE above, which is intentionally obfuscated
             for illustration, the delimiter is "m", the modifier
             is "mx", and after delimiter-removal the RE is the
             same as for "m/ ^ a \s* b /mx".  There's more than
             one reason you're encouraged to restrict your
             delimiters to non-alphanumeric, non-whitespace

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

     parsing regular expressions
         Previous steps were performed during the compilation of
         Perl code, but this one happens at run time, although it
         may be optimized to be calculated at compile time if
         appropriate.  After preprocessing described above, and
         possibly after evaluation if concatenation, joining,
         casing translation, or metaquoting are involved, the
         resulting string is passed to the RE engine for

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

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         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

         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

         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
     interpreted as an external command, and the output of that
     command is the value of the backtick string, like in a
     shell.  In scalar context, a single string consisting of all
     output is returned.  In list context, a list of values is
     returned, one per line of output.  (You can set $/ to use a

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     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 construct in almost every Perl script you
     write.)  The $_ variable is not implicitly localized.
     You'll have to put a "local $_;" before the loop if you want
     that to happen.

     The following lines are equivalent:

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

     This also behaves similarly, but avoids $_ :

         while (my $line = <STDIN>) { print $line }

     In these loop constructs, the assigned value (whether
     assignment is automatic or explicit) is then tested to see
     whether it is defined.  The defined test avoids problems
     where line has a string value that would be treated as false
     by Perl, for example a "" or a "0" with no trailing newline.
     If you really mean for such values to terminate the loop,
     they should be tested for explicitly:

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         while (($_ = <STDIN>) ne '0') { ... }
         while (<STDIN>) { last unless $_; ... }

     In other boolean contexts, "<filehandle>" without an
     explicit "defined" test or comparison elicits 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

         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 synonym for <ARGV>,
     which is magical.  (The pseudo code above doesn't work

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     because it treats <ARGV> as non-magical.)

     Since the null filehandle uses the two argument form of
     "open" in perlfunc it interprets special characters, so if
     you have a script like this:

         while (<>) {

     and call it with "perl 'rm -rfv *|'", it
     actually opens a pipe, executes the "rm" command and reads
     "rm"'s output from that pipe.  If you want all items in
     @ARGV to be interpreted as file names, you can use the
     module "ARGV::readonly" from CPAN.

     You can modify @ARGV before the first <> as long as the
     array ends up containing the list of filenames you really
     want.  Line numbers ($.)  continue as though the input were
     one big happy file.  See the example in "eof" in perlfunc
     for how to reset line numbers on each file.

     If you want to set @ARGV to your own list of files, go right
     ahead.  This sets @ARGV to all plain text files if no @ARGV
     was given:

         @ARGV = grep { -f && -T } glob('*') unless @ARGV;

     You can even set them to pipe commands.  For example, this
     automatically filters 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

         while ($_ = $ARGV[0], /^-/) {
             last if /^--$/;
             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,

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     will read input from STDIN.

     If what the angle brackets contain is a simple scalar
     variable (e.g., <$foo>), then that variable contains the
     name of the filehandle to input from, or its typeglob, or a
     reference to the same.  For example:

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

     If what's within the angle brackets is neither a filehandle
     nor a simple scalar variable containing a filehandle name,
     typeglob, or typeglob reference, it is interpreted as a
     filename pattern to be globbed, and either a list of
     filenames or the next filename in the list is returned,
     depending on context.  This distinction is determined on
     syntactic grounds alone.  That means "<$x>" is always a
     readline() from an indirect handle, but "<$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>) {
             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

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     will start over.  In list context, this isn't important
     because you automatically get them all anyway.  However, in
     scalar context the operator returns the next value each time
     it's called, or "undef" when the list has run out.  As with
     filehandle reads, an automatic "defined" is generated when
     the glob occurs in the test part of a "while", because legal
     glob returns (e.g. a file called 0) would otherwise
     terminate the loop.  Again, "undef" is returned only once.
     So if you're expecting a single value from a glob, it is
     much better to say

         ($file) = <blurch*>;


         $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.

     Perl doesn't officially have a no-op operator, but the bare

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     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

         no integer;

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     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 interpreted as signed integers.
     For example, "~0" usually evaluates to a large integral
     value.  However, "use integer; ~0" is "-1" on two's-
     complement machines.

  Floating-point Arithmetic
     While "use integer" provides integer-only arithmetic, there
     is no analogous mechanism to provide automatic rounding or
     truncation to a certain number of decimal places.  For
     rounding to a certain number of digits, sprintf() or
     printf() is usually the easiest route.  See perlfaq4.

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

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

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

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

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

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

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

     See attributes(5) for descriptions of the following

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     |Availability   | runtime/perl-512 |
     |Stability      | Uncommitted      |
     This software was built from source available at  The original
     community source was downloaded from

     Further information about this software can be found on the
     open source community website at

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