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operator is not equiva lent 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 reg ular 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 didnt expect. A sound principle is to use only ranges that begin from and end at either alphabets of equal case (a-e, A-E), or digits (0-4). Anything else is unsafe. If in doubt, spell out the character sets in full. Options: c Complement the SEARCHLIST. d Delete found but unreplaced characters. s Squash duplicate replaced characters. If the "/c" modifier is specified, the SEARCHLIST character set is complemented. If the "/d" modifier is specified, any char acters 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 charac ter. If the "/d" modifier is used, the REPLACEMENTLIST is always interpreted exactly as specified. Otherwise, if the REPLACE MENTLIST is shorter than the SEARCHLIST, the final character is replicated till it is long enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated. This latter is useful for counting characters in a class or for squashing character sequences in a class. Examples: $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case $cnt = tr/*/*/; # count the stars in $_ $cnt = $sky =~ tr/*/*/; # count the stars in $sky $cnt = tr/0-9//; # count the digits in $_ tr/a-zA-Z//s; # bookkeeper -> bokeper ($HOST = $host) =~ tr/a-z/A-Z/; tr/a-zA-Z/ /cs; # change non-alphas to single space tr [\200-\377] [\000-\177]; # delete 8th bit If multiple transliterations are given for a character, only the first one is used: tr/AAA/XYZ/ will transliterate any A to X. Because the transliteration table is built at compile time, neither the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote interpolation. That means that if you want to use variables, you must use an eval(): eval "tr/$oldlist/$newlist/"; die $@ if $@; eval "tr/$oldlist/$newlist/, 1" or die $@; <" which terminates a fileglob started with "<". When searching for single-character non-pairing delimiters, such as "/", combinations of "\\" and "\/" are skipped. However, when searching for single-character pairing delimiter like "[", combina tions of "\\", "\]", and "\[" are all skipped, and nested "[", "]" are skipped as well. When searching for multicharacter delimiters, nothing is skipped. For constructs with three-part delimiters ("s///", "y///", and "tr///"), the search is repeated once more. During this search no attention is paid to the semantics of the construct. Thus: "$hash{"$foo/$bar"}" or: m/ bar # NOT a comment, this slash / terminated m//! /x do not form legal quoted expressions. The quoted part ends on the first """ and "/", and the rest happens to be a syntax error. Because the slash that terminated "m//" was followed by a "SPACE", the example above is not "m//x", but rather "m//" with no "/x" mod ifier. So the embedded "#" is interpreted as a literal "#". Also no attention is paid to "\c\" during this search. Thus the second "\" in "qq/\c\/" is interpreted as a part of "\/", and the following "/" is not recognized as a delimiter. Instead, use "\034" or "\x1c" at the end of quoted constructs. Removal of backslashes before delimiters During the second pass, text between the starting and ending delim iters is copied to a safe location, and the "\" is removed from combinations consisting of "\" and delimiter--or delimiters, mean ing both starting and ending delimiters will should these differ. This removal does not happen for multi-character delimiters. Note that the combination "\\" is left intact, just as it was. Starting from this step no information about the delimiters is used in parsing. Interpolation The next step is interpolation in the text obtained, which is now delimiter-independent. There are four different cases. "<" "\Q", "\U", "\u", "\L", "\l" (possibly paired with "\E") are converted to corresponding Perl constructs. Thus, "$foo\Qbaz$bar" is converted to "$foo . (quotemeta("baz" . $bar))" internally. The other combinations 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, back slashes between "\Q" and "\E" may lead to counterintuitive results. So, "\Q\t\E" is converted to "quotemeta("\t")", which is the same as "\\\t" (since TAB is not alphanumeric). Note also that: $str = \t; return "\Q$str"; may be closer to the conjectural intention of the writer of "\Q\t\E". Interpolated scalars and arrays are converted internally to the "join" and "." catenation operations. Thus, "$foo XXX @arr" becomes: $foo . " XXX " . (join $", @arr) . ""; All operations above are performed simultaneously, left to right. Because the result of "\Q STRING \E" has all metacharacters quoted, there is no way to insert a literal "$" or "@" inside a "\Q\E" pair. If protected by "\", "$" will be quoted to became "\\\$"; if not, it is interpreted as the start of an interpo lated scalar. Note also that the interpolation code needs to make a decision on where the interpolated scalar ends. For instance, whether "a $b -> {c}" really means: "a " . $b . " -> {c}"; or: "a " . $b -> {c}; Most of the time, the longest possible text that does not include spaces between components and which contains matching braces or brackets. because the outcome may be determined by voting based on heuristic estimators, the result is not strictly predictable. Fortunately, its usually correct for ambiguous cases. "?RE?", "/RE/", "m/RE/", "s/RE/foo/", Processing of "\Q", "\U", "\u", "\L", "\l", and interpolation happens (almost) as with "qq//" constructs, but the substitu tion of "\" followed by RE-special chars (including "\") is not performed. Moreover, inside "(?{BLOCK})", "(?# comment )", and a "#"-comment in a "//x"-regular expression, no processing is performed whatsoever. This is the first step at which the presence of the "//x" modifier is relevant. Interpolation has several quirks: $|, $(, and $) are not inter polated, 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 vari able $arr followed by a digit, which would be the interpreta tion of "/$arr[0-9]/". Since voting among different estimators may occur, the result is not predictable. It is at this step that "\1" is begrudgingly converted to $1 in the replacement text of "s///" to correct the incorrigible sed hackers who havent picked up the saner idiom yet. A warning is emitted if the "use warnings" pragma or the -w command-line flag (that is, the $^W variable) was set. The lack of processing of "\\" creates specific restrictions on the post-processed text. If the 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 charac ter 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 illus tration, the delimiter is "m", the modifier is "mx", and after backslash-removal the RE is the same as for "m/ ^ a \s* b /mx". Theres more than one reason youre encouraged to restrict your delimiters to non-alphanumeric, non-whitespace choices. This step is the last one for all constructs except regular expres sions, which are processed further. Interpolation of regular expressions Previous steps were performed during the compilation of Perl code, but this one happens at run time--although it may be optimized to be calculated at compile time if appropriate. After preprocessing described above, and possibly after evaluation if catenation, join ing, casing translation, or metaquoting are involved, the resulting string is passed to the RE engine for compilation. Whatever happens in the RE engine might be better discussed in perlre, but for the sake of continuity, we shall do so here. This is another step where the presence of the "//x" modifier is relevant. The RE engine scans the string from left to right and converts it to a finite automaton. Backslashed characters are either replaced with corresponding lit eral 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 termi nator of a "{}"-delimited construct. It is possible to inspect both the string given to RE engine and the resulting finite automaton. See the arguments "debug"/"debug color" in the "use re" pragma, as well as Perls -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 different line termina tor.) The command is executed each time the pseudo-literal is evalu ated. 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 auto matically assigned to the global variable $_, destroying whatever was there previously. (This may seem like an odd thing to you, but youll use the construct in almost every Perl script you write.) The $_ vari able is not implicitly localized. Youll have to put a "local $_;" before the loop if you want that to happen. The following lines are equivalent: while (defined($_ = )) { print; } while ($_ = ) { print; } while () { print; } for (;;) { print; } print while defined($_ = ); print while ($_ = ); print while ; This also behaves similarly, but avoids $_ : while (my $line = ) { print $line } In these loop constructs, the assigned value (whether assignment is automatic or explicit) is then tested to see whether it is defined. The defined test avoids problems where line has a string value that would be treated as false by Perl, for example a "" or a "0" with no trailing newline. If you really mean for such values to terminate the loop, they should be tested for explicitly: while (($_ = ) ne 0) { ... } while () { last unless $_; ... } In other boolean contexts, "" without an explicit "defined" test or comparison elicit a warning if the "use warnings" pragma or the -w command-line switch (the $^W variable) is in effect. The filehandles STDIN, STDOUT, and STDERR are predefined. (The file handles "stdin", "stdout", and "stderr" will also work except in pack ages, where they would be interpreted as local identifiers rather than global.) Additional filehandles may be created with the open() func tion, amongst others. See perlopentut and "open" in perlfunc for details on this. If a is used in a context that is looking for a list, a list comprising all input lines is returned, one line per list element. Its easy to grow to a rather large data space this way, so use with care. may also be spelled "readline(*FILEHANDLE)". See "read line" in perlfunc. The null filehandle <> is special: it can be used to emulate the behav ior of sed and awk. Input from <> comes either from standard input, or from each file listed on the command line. Heres how it works: the first time <> is evaluated, the @ARGV array is checked, and if it is empty, $ARGV[0] is set to "-", which when opened gives you standard input. The @ARGV array is then processed as a list of filenames. The loop while (<>) { ... # code for each line } is equivalent to the following Perl-like pseudo code: unshift(@ARGV, -) unless @ARGV; while ($ARGV = shift) { open(ARGV, $ARGV); while () { ... # code for each line } } except that it isnt so cumbersome to say, and will actually work. It really does shift the @ARGV array and put the current filename into the $ARGV variable. It also uses filehandle ARGV internally--<> is just a synonym for , which is magical. (The pseudo code above doesnt work because it treats as non-magical.) You can modify @ARGV before the first <> as long as the array ends up containing the list of filenames you really want. Line numbers ($.) 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 automati cally filters compressed arguments through gzip: @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV; If you want to pass switches into your script, you can use one of the Getopts modules or put a loop on the front like this: while ($_ = $ARGV[0], /^-/) { shift; last if /^--$/; if (/^-D(.*)/) { $debug = $1 } if (/^-v/) { $verbose++ } # ... # other switches } while (<>) { # ... # code for each line } The <> symbol will return "undef" for end-of-file only once. If you call it again after this, it will assume you are processing another @ARGV list, and if you havent set @ARGV, will read input from STDIN. If what the angle brackets contain is a simple scalar variable (e.g., <$foo>), then that variable contains the name of the filehandle to input from, or its typeglob, or a reference to the same. For example: $fh = \*STDIN; $line = <$fh>; If whats within the angle brackets is neither a filehandle nor a sim ple 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 syn tactic grounds alone. That means "<$x>" is always a readline() from an indirect handle, but "<$hash{key}>" is always a glob(). Thats because $x is a simple scalar variable, but $hash{key} is not--its a hash ele ment. Even "<$x >" (note the extra space) is treated as "glob("$x ")", not "readline($x)". One level of double-quote interpretation is done first, but you cant say "<$foo>" because thats an indirect filehandle as explained in the previous paragraph. (In older versions of Perl, programmers would insert curly brackets to force interpretation as a filename glob: "<${foo}>". These days, its 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 () { chomp; chmod 0644, $_; } except that the globbing is actually done internally using the standard "File::Glob" extension. Of course, the shortest way to do the above is: chmod 0644, <*.c>; A (file)glob evaluates its (embedded) argument only when it is starting a new list. All values must be read before it will start over. In list context, this isnt important because you automatically get them all anyway. However, in scalar context the operator returns the next value each time its called, or "undef" when the list has run out. As with filehandle reads, an automatic "defined" is generated when the glob occurs in the test part of a "while", because legal glob returns (e.g. a file called 0) would otherwise terminate the loop. Again, "undef" is returned only once. So if youre expecting a single value from a glob, it is much better to say ($file) = ; than $file = ; because the latter will alternate between returning a filename and returning false. If youre trying to do variable interpolation, its definitely better to use the glob() function, because the older notation can cause people to become confused with the indirect filehandle notation. @files = glob("$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 dont do variable substi tution. 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 $file > 5 + 100 * 2**16) { } } the compiler will precompute the number which that expression repre sents so that the interpreter wont have to. No-ops Perl doesnt officially have a no-op operator, but the bare constants 0 and 1 are special-cased to not produce a warning in a void context, so you can for example safely do 1 while foo(); Bitwise String Operators Bitstrings of any size may be manipulated by the bitwise operators ("~ | & ^"). If the operands to a binary bitwise op are strings of different sizes, | and ^ ops act as though the shorter operand had additional zero bits on the right, while the & op acts as though the longer operand were truncated to the length of the shorter. The granularity for such extension or truncation is one or more bytes. # ASCII-based examples print "j p \n" ^ " a h"; # prints "JAPH\n" print "JA" | " ph\n"; # prints "japh\n" print "japh\nJunk" & _____; # prints "JAPH\n"; print p N$ ^ " E>") 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 twos-complement machines. Floating-point Arithmetic While "use integer" provides integer-only arithmetic, there is no anal ogous mechanism to provide automatic rounding or truncation to a cer tain number of decimal places. For rounding to a certain number of digits, sprintf() or printf() is usually the easiest route. See perl faq4. Floating-point numbers are only approximations to what a mathematician would call real numbers. There are infinitely more reals than floats, so some corners must be cut. For example: printf "%.20g\n", 123456789123456789; # produces 123456789123456784 Testing for exact equality of floating-point equality or inequality is not a good idea. Heres a (relatively expensive) work-around to com pare whether two floating-point numbers are equal to a particular num ber of decimal places. See Knuth, volume II, for a more robust treat ment 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 cant work with complex numbers. Rounding in financial applications can have serious implications, and the rounding method used should be specified precisely. In these cases, it probably pays not to trust whichever system rounding is being used by Perl, but to instead implement the rounding function you need yourself. Bigger Numbers The standard Math::BigInt and Math::BigFloat modules provide variable- precision arithmetic and overloaded operators, although theyre cur rently pretty slow. At the cost of some space and considerable speed, they avoid the normal pitfalls associated with limited-precision repre sentations. use Math::BigInt; $x = Math::BigInt->new(123456789123456789); print $x * $x; # prints +15241578780673678515622620750190521 There are several modules that let you calculate with (bound only by memory and cpu-time) unlimited or fixed precision. There are also some non-standard modules that provide faster implementations via external C libraries. Here is a short, but incomplete summary: Math::Fraction big, unlimited fractions like 9973 / 12967 Math::String treat string sequences like numbers Math::FixedPrecision calculate with a fixed precision Math::Currency for currency calculations Bit::Vector manipulate bit vectors fast (uses C) Math::BigIntFast Bit::Vector wrapper for big numbers Math::Pari provides access to the Pari C library Math::BigInteger uses an external C library Math::Cephes uses external Cephes C library (no big numbers) Math::Cephes::Fraction fractions via the Cephes library Math::GMP another one using an external C library Choose wisely. perl v5.8.8 2008-04-25 PERLOP(1)

PERLOP(1)	       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 ==

       Operator associativity defines what happens if a sequence of the same
       operators is used one after another: whether the evaluator will evalu
       ate 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 Cs precedence
       is slightly screwy.  (This makes learning Perl easier for C folks.)
       With very few exceptions, these all operate on scalar values only, not
       array values.

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

       In the following sections, these operators are covered in precedence

       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 arent
       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 opera
       tor and arguments within parentheses are taken to be of highest prece
       dence, just like a normal function call.

       In the absence of parentheses, the precedence of list operators such as
       "print", "sort", or "chmod" is either very high or very low depending
       on whether you are looking at the left side or the right side of the
       operator.  For example, in

	   @ary = (1, 3, sort 4, 2);
	   print @ary;	       # prints 1324

       the commas on the right of the sort are evaluated before the sort, but
       the commas on the left are evaluated after.  In other words, list oper
       ators tend to gobble up all arguments that follow, and then act like a
       simple TERM with regard to the preceding expression.  Be careful with

	   # 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 doesnt do what you expect at first glance.  The parentheses
       enclose the argument list for "print" which is evaluated (printing the
       result of "$foo & 255").  Then one is added to the return value of
       "print" (usually 1).  The result is something like this:

	   1 + 1, "\n";    # Obviously not what you meant.

       To do what you meant properly, you must write:

	   print(($foo & 255) + 1, "\n");

       See "Named Unary Operators" for more discussion of this.

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

       See also "Quote and Quote-like Operators" toward the end of this sec
       tion, 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 its 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 doesnt define when the variable is incre
       mented or decremented. You just know it will be done sometime before or
       after the value is returned. This also means that modifying a variable
       twice in the same statement will lead to undefined behaviour.  Avoid
       statements like:

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

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

       The auto-increment operator has a little extra builtin magic to it.  If
       you increment a variable that is numeric, or that has ever been used in
       a numeric context, you get a normal increment.  If, however, the vari
       able has been used in only string contexts since it was set, and has a
       value that is not the empty string and matches the pattern
       "/^[a-zA-Z]*[0-9]*\z/", the increment is done as a string, preserving
       each character within its range, with carry:

	   print ++($foo = 99);      # prints 100
	   print ++($foo = a0);      # prints a1
	   print ++($foo = Az);      # prints Ba
	   print ++($foo = zz);      # prints aaa

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

       The auto-decrement operator is not magical.


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

       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 con
       catenated 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 (exluding "+" or "-"), Perl will attempt to con
       vert 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" isnt numeric in negation (-) at ....

       Unary "~" performs bitwise negation, i.e., 1s complement.  For exam
       ple, "0666 & ~027" is 0640.  (See also "Integer Arithmetic" and "Bit
       wise String Operators".)  Note that the width of the result is plat
       form-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 syn
       tactically for separating a function name from a parenthesized expres
       sion that would otherwise be interpreted as the complete list of func
       tion arguments.	(See examples above under "Terms and List Operators

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

       Binding Operators

       Binary "=~" binds a scalar expression to a pattern match.  Certain
       operations search or modify the string $_ by default.  This operator
       makes that kind of operation work on some other string.	The right
       argument is a search pattern, substitution, or transliteration.	The
       left argument is what is supposed to be searched, substituted, or
       transliterated instead of the default $_.  When used in scalar context,
       the return value generally indicates the success of the operation.
       Behavior in list context depends on the particular operator.  See "Reg
       exp Quote-Like Operators" for details and perlretut for examples using
       these operators.

       If the right argument is an expression rather than a search pattern,
       substitution, or transliteration, it is interpreted as a search pattern
       at run time.

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

       Multiplicative Operators

       Binary "*" multiplies two numbers.

       Binary "/" divides two numbers.

       Binary "%" computes the modulus of two numbers.	Given integer operands
       $a and $b: If $b is positive, then "$a % $b" is $a minus the largest
       multiple of $b that is not greater than $a.  If $b is negative, then
       "$a % $b" is $a minus the smallest multiple of $b that is not less than
       $a (i.e. the result will be less than or equal to zero).  Note that
       when "use integer" is in scope, "%" gives you direct access to the mod
       ulus 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 con
       sisting 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 1s
	   @ones = (5) x @ones;        # set all elements to 5

       Additive Operators

       Binary "+" returns the sum of two numbers.

       Binary "-" returns the difference of two numbers.

       Binary "." concatenates two strings.

       Shift Operators

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

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

       Note that both "<<" and ">>" in Perl are implemented directly using
       "<<" and ">>" in C.  If "use integer" (see "Integer Arithmetic") is in
       force then signed C integers are used, else unsigned C integers are
       used.  Either way, the implementation isnt 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 inte
       gers, "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 opera
       tor and arguments within parentheses are taken to be of highest prece
       dence, 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 dont follow this func
       tional parenthesis rule.  That means, for example, that
       "-f($file).".bak"" is equivalent to "-f "$file.bak"".

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

       Relational Operators

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

       Binary ">" returns true if the left argument is numerically greater
       than the right 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 doesnt support NaNs
       then NaN is just a string with numeric value 0.

	   perl -le $a = "NaN"; print "No NaN support here" if $a == $a
	   perl -le $a = "NaN"; print "NaN support here" if $a != $a

       Binary "eq" returns true if the left argument is stringwise equal to
       the right argument.

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

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

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

       Bitwise And

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

       Note that "&" has lower priority than relational operators, so for
       example the brackets are essential in a test like

	       print "Even\n" if ($x & 1) == 0;

       Bitwise Or and Exclusive Or

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

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

       Note that "|" and "^" have lower priority than relational operators, so
       for example the brackets are essential in a test like

	       print "false\n" if (8 | 2) != 10;

       C-style Logical And

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

       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

       The "||" and "&&" operators return the last value evaluated (unlike Cs
       "||" 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 "Youre homeless!\n";

       In particular, this means that you shouldnt use this for selecting
       between two aggregates for assignment:

	   @a = @b || @c;	       # this is wrong
	   @a = scalar(@b) || @c;      # really meant this
	   @a = @b ? @b : @c;	       # this works fine, though

       As more readable alternatives to "&&" and "||" when used for control
       flow, Perl provides "and" and "or" operators (see below).  The short-
       circuit behavior is identical.  The precedence of "and" and "or" is
       much lower, however, so that you can safely use them after a list oper
       ator without the need for parentheses:

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

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

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

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

       Range Operators

       Binary ".." is the range operator, which is really two different opera
       tors 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 imple
       mentation, no temporary array is created when the range operator is
       used as the expression in "foreach" loops, but older versions of Perl
       might burn a lot of memory when you write something like this:

	   for (1 .. 1_000_000) {
	       # code

       The range operator also works on strings, using the magical auto-incre
       ment, see below.

       In scalar context, ".." returns a boolean value.  The operator is
       bistable, like a flip-flop, and emulates the line-range (comma) opera
       tor of sed, awk, and various editors.  Each ".." operator maintains its
       own boolean state.  It is false as long as its left operand is false.
       Once the left operand is true, the range operator stays true until the
       right operand is true, AFTER which the range operator becomes false
       again.  It doesnt 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 dont want it to test the right operand till the next
       evaluation, as in sed, just use three dots ("...") instead of two.  In
       all other regards, "..." behaves just like ".." does.

       The right operand is not evaluated while the operator is in the "false"
       state, and the left operand is not evaluated while the operator is in
       the "true" state.  The precedence is a little lower than || and &&.
       The value returned is either the empty string for false, or a sequence
       number (beginning with 1) for true.  The sequence number is reset for
       each range encountered.	The final sequence number in a range has the
       string "E0" appended to it, which doesnt 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 num
       ber (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 $. as described in the previous paragraph, the compar
       ison is "int(EXPR) == int($.)" which is only an issue when $.  is set
       to a floating point value and you are not reading from a file.  Fur
       thermore, "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.


       As a scalar operator:

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

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

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

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

       Heres a simple example to illustrate the difference between the two
       range operators:

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

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

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

       And now some examples as a list operator:

	   for (101 .. 200) { print; } # print $_ 100 times
	   @foo = @foo[0 .. $#foo];    # an expensive no-op
	   @foo = @foo[$#foo-4 .. $#foo];      # slice last 5 items

       The range operator (in list context) makes use of the magical auto-
       increment algorithm if the operands are strings.  You can say

	   @alphabet = (A .. Z);

       to get all normal letters of the English alphabet, or

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

       to get a hexadecimal digit, or

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

       to get dates with leading zeros.  If the final value specified is not
       in the sequence that the magical increment would produce, the sequence
       goes until the next value would be longer than the final value speci

       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 argu
       ment 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 argu
       ment, whichever is selected.

	   $a = $ok ? $b : $c;	# get a scalar
	   @a = $ok ? @b : @c;	# get an array
	   $a = $ok ? @b : @c;	# oops, thats just a count!

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

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

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

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

       Really means this:

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

       Rather than this:

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

       That should probably be written more simply as:

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

       Assignment Operators

       "=" is the ordinary assignment operator.

       Assignment operators work as in C.  That is,

	   $a += 2;

       is equivalent to

	   $a = $a + 2;

       although without duplicating any side effects that dereferencing the
       lvalue might trigger, such as from tie().  Other assignment operators
       work similarly.	The following are recognized:

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

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

       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 modi
       fying a copy of something, like this:

	   ($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 lval
       ues 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 argu
       ment and returns that value.  This is just like Cs comma operator.

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

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

	   use constant FOO => "something";

	   my %h = ( FOO => 23 );

       is equivalent to:

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

       It is NOT:

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

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

       The "=>" operator is helpful in documenting the correspondence between
       keys and values in hashes, and other paired elements in lists.

	       %hash = ( $key => $value );
	       login( $username => $password );

       List Operators (Rightward)

       On the right side of a list operator, it has very low precedence, such
       that it controls all comma-separated expressions found there.  The only
       operators with lower precedence are the logical operators "and", "or",
       and "not", which may be used to evaluate calls to list operators with
       out the need for extra parentheses:

	   open HANDLE, "filename"
	       or die "Cant open: $!\n";

       See also discussion of list operators in "Terms and List Operators

       Logical Not

       Unary "not" returns the logical negation of the expression to its
       right.  Its the equivalent of "!" except for the very low precedence.

       Logical And

       Binary "and" returns the logical conjunction of the two surrounding
       expressions.  Its equivalent to && except for the very low precedence.
       This means that it short-circuits: i.e., the right expression is evalu
       ated only if the left expression is true.

       Logical or and Exclusive Or

       Binary "or" returns the logical disjunction of the two surrounding
       expressions.  Its equivalent to || except for the very low precedence.
       This makes it useful for control flow

	   print FH $data	       or die "Cant write to FH: $!";

       This means that it short-circuits: i.e., the right expression is evalu
       ated 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 its a list-context assignment and youre 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 expres
       sions.  It cannot short circuit, of course.

       C Operators Missing From Perl

       Here is what C has that Perl doesnt:

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

       unary * Dereference-address operator. (Perls 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 func
       tion as operators, providing various kinds of interpolating and pattern
       matching capabilities.  Perl provides customary quote characters for
       these behaviors, but also provides a way for you to choose your quote
       character for any of them.  In the following table, a "{}" represents
       any pair of delimiters you choose.

	   Customary  Generic	     Meaning	    Interpolates
		      q{}	   Literal	       no
	       ""      qq{}	     Literal		 yes
		     qx{}	   Command	       yes*
		       qw{}	    Word list		 no
	       //	m{}	  Pattern match 	 yes*
		       qr{}	     Pattern		 yes*
			s{}{}	   Substitution 	 yes*
		       tr{}{}	 Transliteration	 no (but see below)
	       <{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 "@{+}".

       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.  Youll
       need to write something like "m/\Quser\E\@\Qhost/".

       Patterns are subject to an additional level of interpretation as a reg
       ular 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 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

		   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 match, and in scalar context
	       returns true if it succeeds, false if it fails.	If no string
	       is specified via the "=~" or "!~" operator, the $_ string is
	       searched.  (The string specified with "=~" need not be an
	       lvalue--it may be the result of an expression evaluation, but
	       remember the "=~" binds rather tightly.)  See also perlre.  See
	       perllocale for discussion of additional considerations that
	       apply when "use locale" is in effect.

	       Options are:

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

	       If "/" is the delimiter then the initial "m" is optional.  With
	       the "m" you can use any pair of non-alphanumeric, non-whites
	       pace 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 delim
	       iter, no interpolation is performed on the PATTERN.

	       PATTERN may contain variables, which will be interpolated (and
	       the pattern recompiled) every time the pattern search is evalu
	       ated, except for when the delimiter is a single quote.  (Note
	       that $(, $), and $| are not interpolated because they look like
	       end-of-string tests.)  If you want such a pattern to be com
	       piled only once, add a "/o" after the trailing delimiter.  This
	       avoids expensive run-time recompilations, and is useful when
	       the value you are interpolating wont change over the life of
	       the script.  However, mentioning "/o" constitutes a promise
	       that you wont change the variables in the pattern.  If you
	       change them, Perl wont even notice.  See also

	       If the PATTERN evaluates to the empty string, the last success
	       fully matched regular expression is used instead. In this case,
	       only the "g" and "c" flags on the empty pattern is honoured -
	       the other flags are taken from the original pattern. If no
	       match has previously succeeded, this will (silently) act
	       instead as a genuine empty pattern (which will always match).

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


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

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

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

		   # poor mans 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 begin
	       ning of the string, but you can avoid that by adding the "/c"
	       modifier (e.g. "m//gc").  Modifying the target string also
	       resets the search position.

	       You can intermix "m//g" matches with "m/\G.../g", where "\G" is
	       a zero-width assertion that matches the exact position where
	       the previous "m//g", if any, left off.  Without the "/g" modi
	       fier, 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 pattern.


		   # 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", its
	       a good bet that youre 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.

		$_ = <&1;

	       To capture a commands STDOUT but discard its STDERR:

		   $output = cmd 2>/dev/null;

	       To capture a commands STDERR but discard its STDOUT (ordering
	       is important here):

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

	       To exchange a commands 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 commands STDOUT and its STDERR separately, its
	       easiest to redirect them separately to files, and then read
	       from those files when the program is done:

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

	       Using single-quote as a delimiter protects the command from
	       Perls double-quote interpolation, passing it on to the shell

		   $perl_info  = qx(ps $$);	       # thats Perls $$
		   $shell_info = qxps $$;	     # thats the new shells $$

	       How that string gets evaluated is entirely subject to the com
	       mand 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 its
	       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 sup
	       ports that (e.g. ";" on many Unix shells; "&" on the Windows NT
	       "cmd" shell).

	       Beginning with v5.6.0, Perl will attempt to flush all files
	       opened for output before starting the child process, but this
	       may not be supported on some platforms (see perlport).  To be
	       safe, you may need to set $| ($AUTOFLUSH in English) or call
	       the "autoflush()" method of "IO::Handle" on any open handles.

	       Beware that some command shells may place restrictions on the
	       length of the command line.  You must ensure your strings dont
	       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 doesnt mean you should go out
	       of your way to avoid backticks when theyre 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 under
	       stand what youre 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 under
	       stood 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 rea
	       son, the "use warnings" pragma and the -w switch (that is, the
	       $^W variable) produces warnings if the STRING contains the ","
	       or the "#" character.

	       Searches a string for a pattern, and if found, replaces that
	       pattern with the replacement text and returns the number of
	       substitutions made.  Otherwise it returns false (specifically,
	       the empty string).

	       If no string is specified via the "=~" or "!~" operator, the $_
	       variable is searched and modified.  (The string specified with
	       "=~" must be scalar variable, an array element, a hash element,
	       or an assignment to one of those, i.e., an lvalue.)

	       If the delimiter chosen is a single quote, no interpolation is
	       done on either the PATTERN or the REPLACEMENT.  Otherwise, if
	       the PATTERN contains a $ that looks like a variable rather than
	       an end-of-string test, the variable will be interpolated into
	       the pattern at run-time.  If you want the pattern compiled only
	       once the first time the variable is interpolated, use the "/o"
	       option.	If the pattern evaluates to the empty string, the last
	       successfully executed regular expression is used instead.  See
	       perlre for further explanation on these.  See perllocale for
	       discussion of additional considerations that apply when "use
	       locale" is in effect.

	       Options are:

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

	       Any non-alphanumeric, non-whitespace delimiter may replace the
	       slashes.  If single quotes are used, no interpretation is done
	       on the replacement string (the "/e" modifier overrides this,
	       however).  Unlike Perl 4, Perl 5 treats backticks as normal
	       delimiters; the replacement text is not evaluated as a command.
	       If the PATTERN is delimited by bracketing quotes, the REPLACE
	       MENT has its own pair of quotes, which may or may not be brack
	       eting quotes, e.g., "s(foo)(bar)" or "s/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" modi
	       fier will cause the replacement portion to be "eval"ed before
	       being run as a Perl expression.


		   s/\bgreen\b/mauve/g; 	       # dont change wintergreen

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

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

		   ($foo = $bar) =~ s/this/that/;      # copy first, then change

		   $count = ($paragraph =~ s/Mister\b/Mr./g);  # get change-count

		   $_ = abc123xyz;
		   s/\d+/$&*2/e;	       # yields abc246xyz
		   s/\d+/sprintf("%5d",$&)/e;  # yields abc  246xyz
		   s/\w/$& x 2/eg;	       # yields aabbcc	224466xxyyzz

		   s/%(.)/$percent{$1}/g;      # change percent escapes; no /e
		   s/%(.)/$percent{$1} || $&/ge;       # expr now, so /e
		   s/^=(\w+)/&pod($1)/ge;      # use function call

		   # expand variables in $_, but dynamics only, using
		   # symbolic dereferencing

		   # 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 \ form in only the left hand side.  Any
	       where else its $.

	       Occasionally, you cant use just a "/g" to get all the changes
	       to occur that you might want.  Here are two common cases:

		   # put commas in the right places in an integer
		   1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;

		   # expand tabs to 8-column spacing
		   1 while s/\t+/  x (length($&)*8 - length($)%8)/e;

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

	       A character range may be specified with a hyphen, so
	       "tr/A-J/0-9/" does the same replacement as "tr/ACEG
	       IBDFHJ/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 

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