Entertainment: Jun 20, 2010

Delivery_Failure: Your Dell_Package cannot be delivered by_FedEx until you participate! m m ^ a \s* b mmxUPPERCASE line-noise lowercase line-noise lowercase line-noise 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 $INPUT_RECORD_SEPARATOR), or an empty list if the command failed. # These evaluate exit before doing the print: 3: '', pos=8 # poor man's grep The auto-decrement operator is not magical. } []gsx; There are two ways to check for the existence of a key in a pseudo-hashThe first is to use exists()This checks to see if the given field has ever been setIt acts this way to match the behavior of a regular hashFor instance: As explained above, a closure is an anonymous function with access to the lexical variables visible when that function was compiledIt retains access to those variables even though it doesn't get run until later, such as in a signal handler or a Tk callback. Well, okay, not entirely like C's arrays, actuallyC doesn't know how to grow its arrays on demandPerl does. For constructs with three-part delimiters (s///, y///, and tr///), the search is repeated once more. c Do not reset search position on a failed match when /g is in effect. print ++($foo = 'a0'); # prints 'a1' has always meant to print "pop on over", even though push is a reserved wordThis has been generalized to work the same outside of quotes, so that use Term::Cap; 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 perlrun/"Command Switches". if (?^$?) { That should probably be written more simply as: s/^\s+//; Binding Operators To capture a command's STDOUT but discard its STDERR: . 'more '/eg; WARNING: This section describes an experimental featureDetails may change without notice in future versions. 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.) When searching for single-character non-pairing delimiters, such as /, combinations of \\ and \/ are skipped. However, when searching for single-character pairing delimiter like [, combinations of \\, \], and \[ are all skipped, and nested [, ] are skipped as well. When searching for multicharacter delimiters, nothing is skipped. } $phash->{pants} = undef; 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. Now inner() can only be called from within outer(), because of the temporary assignments of the closure (anonymous subroutine)But when it does, it has normal access to the lexical variable $x from the scope of outer(). The use warnings pragma or the -w switch will warn you if it interprets a reserved word as a stringBut it will no longer warn you about using lowercase words, because the string is effectively quoted. If you want to set @ARGV to your own list of files, go right ahead. This sets @ARGV to all plain text files if no @ARGV was given: 179231 chdir ($foo) || die; # (chdir $foo) || die 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: *foo{THING} returns undef if that particular THING hasn't been used yet, except in the case of scalars*foo{SCALAR} returns a reference to an anonymous scalar if $foo hasn't been used yetThis might change in a future release. Additionally, the quoting rules for the identifier are not related to Perl's quoting rules -- q(), qq(), and the like are not supported in place of '' and "", and the only interpolation is for backslashing the quoting character: Conditional Operator tr{}{} Transliteration no (but see below) As a scalar operator: This is one of the cases we mentioned earlier in which references could spring into existence when in an lvalue contextBefore this statement, $array[$x] may have been undefinedIf so, it's automatically defined with a hash reference so that we can look up {"foo"} in itLikewise $array[$x]->{"foo"} will automatically get defined with an array reference so that we can look up [0] in itThis process is called autovivification. qq/STRING/ 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.) print @ary; # prints 1324 $a *= 3; $ioref = *STDIN{IO}; $hashref = { return "\Q$str"; for ($variable) { # trim white space in $variable, cheap &{ $dispatch{$index} }(1,2,3); # call correct routine print "Final: '$1', pos=",pos,"\n" if /\G(.)/; EOL *foo{IO} is an alternative to the *HANDLE mechanism given in perldata/"Typeglobs and Filehandles" for passing filehandles into or out of subroutines, or storing into larger data structuresIts disadvantage is that it won't create a new filehandle for youIts advantage is that you have less risk of clobbering more than you want to with a typeglob assignment(It still conflates file and directory handles, though.) However, if you assign the incoming value to a scalar instead of a typeglob as we do in the examples below, there's no risk of that happening. s/\bgreen\b/mauve/g; # don't change wintergreen $path =~ s|/usr/bin|/usr/local/bin|; } foreach my $pat (@compiled) { $h = newprint("Howdy"); $output = `cmd 2>/dev/null`; @EXPORT = qw( foo bar baz ); # symbolic dereferencing Unary "!" performs logical negation, i.e., "not". See also not for a lower precedence version of this. # expand variables in $_, but dynamics only, using 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, it repeats the list. echo hi there if (/^-v/) { $verbose++ } Note, however, that this does not always work for quoting Perl code: `STRING` \Q, \U, \u, \L, \l (possibly paired with \E) are converted to corresponding Perl constructs. Thus, "$foo\Qbaz$bar" is converted to $foo . (quotemeta("baz" . $bar)) internally. The other combinations are replaced with appropriate expansions. If you want your here-docs to be indented with the rest of the code, you'll need to remove leading whitespace from each line manually: last if /^--$/; Function Templates $array{ shift } $coderef->(1,2,3); # Subroutine call 'EOF', m'', s''', tr///, y/// bar \a alarm (bell) (BEL) sub outer { Unary "not" returns the logical negation of the expression to its right. It's the equivalent of "!" except for the very low precedence. print ($foo & 255) + 1, "\n"; "a " . $b -> {c}; $arg = shift; If "/" is the delimiter then the initial m is optional. With the m you can use any pair of non-alphanumeric, non-whitespace characters as delimiters. This is particularly useful for matching path names that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is the delimiter, then the match-only-once rule of ?PATTERN? applies. If "'" is the delimiter, no interpolation is performed on the PATTERN. sub get_rec { named Unicode character $arrayref->[0] = "January"; # Array element is roughly equivalent to: Scalar or list context propagates downward into the 2nd or 3rd argument, whichever is selected. # Time passes. 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 TTY> =~ /^y/i && foo(); # do foo if desired $score[$x][$y][$z] += 42; 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 perlfunc/eof for how to reset line numbers on each file. sub match { print "3: '"; A string enclosed by backticks (grave accents) first undergoes double-quote interpolation. It is then interpreted as an external command, and the output of that command is the value of the backtick string, like in a shell. In scalar context, a single string consisting of all output is returned. In list context, a list of values is returned, one per line of output. (You can set $/ to use a different line terminator.) The command is executed each time the pseudo-literal is evaluated. The status value of the command is returned in $? (see perlvar for the interpretation of $?). Unlike in csh, no translation is done on the return data--newlines remain newlines. Unlike in any of the shells, single quotes do not hide variable names in the command from interpretation. To pass a literal dollar-sign through to the shell you need to hide it with a backslash. The generalized form of backticks is qx//. (Because backticks always undergo shell expansion as well, see perlsec for security con (*** The previous line contains the naughty word "$1".\n) chdir +($foo) || die; # (chdir $foo) || die } $a = $b or $c; # bug: this is wrong 3: 'pp', pos=7 } Address-of operator. (But see the "\" operator for taking a reference.) Examples: 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. print ". That's all!\n"; $objref = new Doggie (Tail => 'short', Ears => 'long'); We said that references spring into existence as necessary if they are undefined, but we didn't say what happens if a value used as a reference is already defined, but isn't a hard referenceIf you use it as a reference, it'll be treated as a symbolic referenceThat is, the value of the scalar is taken to be the name of a variable, rather than a direct link to a (possibly) anonymous value. On some platforms (notably DOS-like ones), the shell may not be capable of dealing with multiline commands, so putting newlines in the string may not get you what you want. You may be able to evaluate multiple commands in a single line by separating them with the command separator character, if your shell supports that (e.g. ; on many Unix shells; & on the Windows NT cmd shell). x Use extended regular expressions. Interpolation /x if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) return $x + inner(); Regexp Quote-Like Operators chdir +($foo) * 20; # chdir ($foo * 20) 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: Access to lexicals that change over type--like those in the for loop above--only works with closures, not general subroutinesIn the general case, then, named subroutines do not nest properly, although anonymous ones doIf you are accustomed to using nested subroutines in other programming languages with their own private variables, you'll have to work at it a bit in PerlThe intuitive coding of this type of thing incurs mysterious warnings about ``will not stay shared''For example, this won't work: use POSIX qw( setlocale localeconv ) use Tk; unary * Logical And "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified by the current locale if use locale is in effect. See perllocale. The price is $Price. print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc; push(@$arrayref, $filename); .= %= ^= Binary "ne" returns true if the left argument is stringwise not equal to the right argument. 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.) o Compile pattern only once. $hashref->{"KEY"} = "VALUE"; # Hash element Because curly brackets (braces) are used for several other things including BLOCKs, you may occasionally have to disambiguate braces at the beginning of a statement by putting a + or a return in front so that Perl realizes the opening brace isn't starting a BLOCKThe economy and mnemonic value of using curlies is deemed worth this occasional extra hassle. . # code for each line The > symbol will return undef for end-of-file only once. If you call it again after this, it will assume you are processing another @ARGV list, and if you haven't set @ARGV, will read input from STDIN. EOF s Squash duplicate replaced characters. } There are several I/O operators you should know about. (($a % 2) ? ($a += 10) : $a) += 2 s Treat string as single line. } As a special case, \(@foo) returns a list of references to the contents of @foo, not a reference to @foo itselfLikewise for %foo, except that the key references are to copies (since the keys are just strings rather than full-fledged scalars). Symbolic Unary Operators # Delete (most) C comments. print "1: '"; Perl will raise an exception if you try to access nonexistent fieldsTo avoid inconsistencies, always use the fields::phash() function provided by the fields pragma. Occasionally, you can't use just a /g to get all the changes to occur that you might want. Here are two common cases: In human terms, it's a funny way of passing arguments to a subroutine when you define it as well as when you call itIt's useful for setting up little bits of code to run later, such as callbacksYou can even do object-oriented stuff with it, though Perl already provides a different mechanism to do that--see perlobj. $string =~ /foo${re}bar/; # can be interpolated in other patterns while ($_ = STDIN>) { print; } ($a % 2) ? ($a += 10) : ($a += 2) x Use extended regular expressions. qr{} Pattern yes* cerns.) C-style Logical Or 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. # to the variable name, and then evaluated 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: print "2: '"; 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. while ( >) { # list context myfunc( "THIS", 23, 'THAT'); 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 pattern. Using "or" for assignment is unlikely to do what you want; see below. sub hashem { return { @_ } } # ok I said foo. abc"def while (my($k,$v) = each %$struct) { 4. FINIS $pack = "THAT"; Binary "." concatenates two strings. Note the semicolonExcept for the code inside not being immediately executed, a sub {} is not so much a declaration as it is an operator, like do{} or eval{}(However, no matter how many times you execute that particular line (unless you're in an eval(".")), $coderef will still have a reference to the same anonymous subroutine.) A TERM has the highest precedence in Perl. They include variables, quote and quote-like operators, any expression in parentheses, and any function whose arguments are parenthesized. Actually, there aren't really functions in this sense, just list operators and unary operators behaving as functions because you put parentheses around the arguments. These are all documented in perlfunc. i Do case-insensitive pattern matching. Here is the output (split into several lines): In particular, this means that you shouldn't use this for selecting between two aggregates for assignment: eval "tr/$oldlist/$newlist/, 1" or die $@; } 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. All systems use the virtual "\n" to represent a line terminator, called a "newline". There is no such thing as an unvarying, physical newline character. It is only an illusion that the operating system, device drivers, C libraries, and Perl all conspire to preserve. Not all systems read "\r" as ASCII CR and "\n" as ASCII LF. For example, on a Mac, these are reversed, and on systems without line terminator, printing "\n" may emit no actual data. In general, use "\n" when you mean a "newline" for your system, but use the literal ASCII when you need an exact character. For example, most networking protocols expect and prefer a CR+LF ("\015\012" or "\cM\cJ") for line terminators, and although they often accept just "\012", they seldom tolerate just "\015". If you get in the habit of using "\n" for networking, you may be burned some day. $a += 2; To capture a command's STDERR but discard its STDOUT (ordering is important here): chmod 0644, $_; $a = $ok ? $b : $c; # get a scalar } 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: } print $fh "her um well a hmmm\n"; @list = \($a, @b, %c); # same thing! Admittedly, it's a little silly to use the curlies in this case, but the BLOCK can contain any arbitrary expression, in particular, subscripted expressions: splutter(*STDOUT); # pass the whole glob o Compile pattern only once. The various named unary operators are treated as functions with one argument, with optional parentheses. These include the filetest operators, like -f, -M, etc. See perlfunc. 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. Binary "le" returns true if the left argument is stringwise less than or equal to the right argument. 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 perlfunc/pos. 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. Interpolating an array or slice interpolates the elements in order, separated by the value of $", so is equivalent to interpolating join $", @array. "Punctuation" arrays such as @+ are only interpolated if the name is enclosed in braces @{+}. [\000-\177]; # delete 8th bit Here's a trick for interpolating a subroutine call into a string: This is powerful, and slightly dangerous, in that it's possible to intend (with the utmost sincerity) to use a hard reference, and accidentally use a symbolic reference insteadTo protect against that, you can say open(TTY, '/dev/tty'); 4. { eval "tr/$oldlist/$newlist/"; The red() and green() functions would be similarTo create these, we'll assign a closure to a typeglob of the name of the function we're trying to build. s/(\d+)/1 + $1/eg; print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc; while (defined($paragraph = >)) { It's important to understand that we are specifically not dereferencing $arrayref[0] or $hashref{"KEY"} thereThe dereference of the scalar variable happens before it does any key lookupsAnything more complicated than a simple scalar variable must use methods 2 or 3 belowHowever, a "simple scalar" includes an identifier that itself uses method 1 recursivelyTherefore, the following prints "howdy". s Treat string as single line. q{foo{bar}baz} except that the globbing is actually done internally using the standard File::Glob extension. Of course, the shortest way to do the above is: print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc; A reference to an anonymous subroutine can be created by using sub without a subname: @files = glob($files[$i]); The => digraph is mostly just a synonym for the comma operator. It's useful for documenting arguments that come in pairs. As of release 5.001, it also forces any word to the left of it to be interpreted as a string. # (including lexicals) in $_ : First $1 is interpolated \c[ control char (ESC) sub showem { {; @_ } } # ok Some frequently seen examples: } Whatever happens in the RE engine might be better discussed in perlre, but for the sake of continuity, we shall do so here. sub newprint { # put commas in the right places in an integer $baz = '\n'; # a two-character string $line = $fh>; This prints @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc $_ |" : $_ } @ARGV; 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. 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. $_ .= qq \f form feed (FF) Final: 'q', pos=8 Only package variables (globals, even if localized) are visible to symbolic referencesLexical variables (declared with my()) aren't in a symbol table, and thus are invisible to this mechanismFor example: while ( *.c>) { Then again, you could always use parentheses. When presented with something that might have several different interpretations, Perl uses the DWIM (that's "Do What I Mean") principle to pick the most probable interpretation. This strategy is so successful that Perl programmers often do not suspect the ambivalence of what they write. But from time to time, Perl's notions differ substantially from what the author honestly meant. s/%(.)/$percent{$1} || $&/ge; # expr now, so /e down from the door where it began. Dereference-address operator. (Perl's prefix dereferencing operators are typed: $, @, %, and &.) print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc; the commas on the right of the sort are evaluated before the sort, but the commas on the left are evaluated after. In other words, list operators tend to gobble up all arguments that follow, and then act like a simple TERM with regard to the preceding expression. Be careful with parentheses: print "refs 1 and 2 refer to the same thing\n"; sub outer { print "$sentences\n"; print FH $data or die "Can't write to FH: $!"; sub hashem { +{ @_ } } # ok || (gripe(), next LINE); 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 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 evaled before being run as a Perl expression. THAT No interpolation is performed. $in_header = 1 /^$/; Binary " " is the range operator, which is really two different operators depending on the context. In list context, it returns an 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 array. 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: $array{ shift @_ } chomp; q/STRING/ The left side of the arrow can be any expression returning a reference, including a previous dereferenceNote that $array[$x] is not the same thing as $array->[$x] here: $arrayref = \@ARGV; "STRING" For this to work, the array must contain extra informationThe first element of the array has to be a hash reference that maps field names to array indicesHere is an example: LOOP: In the RE above, which is intentionally obfuscated for illustration, the delimiter is m, the modifier is mx, and after backslash-removal the RE is the same as for m/ ^ a s* b /mx). There's more than one reason you're encouraged to restrict your delimiters to non-alphanumeric, non-whitespace choices. 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. WARNING Removal of backslashes before delimiters you have to write no strict 'refs'; # allow symbol table manipulation 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. Binary "^" returns its operators XORed together bit by bit. (See also Integer Arithmetic and Bitwise String Operators.) return scalar $fh>; ($file) = blurch*>; Binary "eq" returns true if the left argument is stringwise equal to the right argument. print exists $phash->[0]{shoes};# false, 'shoes' can't be used 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. testing . x Use extended regular expressions. or die "Can't open: $!\n"; for (1 1_000_000) { s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively Precompilation of the pattern into an internal representation at the moment of qr() avoids a need to recompile the pattern every time a match /$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.) will have the same effect(This would have been a syntax error in Perl 5.000, though Perl 4 allowed it in the spaceless form.) This construct is not considered to be a symbolic reference when you're using strict refs: ${ "bareword" }; # Error, symbolic reference `` qx{} Command yes* $globref->print("output\n"); # iff IO::Handle is loaded See "I/O Operators" for more discussion. } @z2 = ('01' '31'); print $z2[$mday]; $name = "foo"; } $a % 2 ? $a += 10 : $a += 2 perl -le '$a = NaN; print "No NaN support here" if $a == $a' Finding the end ${"${pack}::$name"} = 5; # Sets $THAT::foo without eval Logical Not $fh = \*STDIN; Because this operator produces an assignable result, using assignments without parentheses will get you in trouble. For example, this: my $fh = shift; People frequently expect it to work like thisSo it does. my $success = 0; As a list operator: 7. $array{ shift() } $_ = 'EOL'; use strict 'refs'; foo It isn't possible to create a true reference to an IO handle (filehandle or dirhandle) using the backslash operatorThe most you can get is a reference to a typeglob, which is actually a complete symbol table entryBut see the explanation of the *foo{THING} syntax belowHowever, you can still use type globs and globrefs as though they were IO handles. print ${push} "over"; THIS $rec = get_rec(*STDIN); # pass the whole glob Auto-increment and Auto-decrement Anywhere you'd put an identifier (or chain of identifiers) as part of a variable or subroutine name, you can replace the identifier with a BLOCK returning a reference of the correct typeIn other words, the previous examples could be written like this: A reference to an anonymous hash can be created using curly brackets: because the latter will alternate between returning a filename and returning false. $$hashref{"KEY"} = "VALUE"; # CASE 0 &$g("earthlings"); $bar = q('This is it.'); s/Login: $foo/Login: $bar/; # run-time pattern print $1 while /(o)/gc; print "', pos=", pos, "\n"; bar # NOT a comment, this slash / terminated m//! /PATTERN/cgimosx 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. $file = blurch*>; # . # other switches e Evaluate the right side as an expression. die $@ if $@; A typeglob may be dereferenced the same way a reference can, because the dereference syntax always indicates the type of reference desiredSo ${*foo} and ${\$foo} both indicate the same scalar variable. E $arrayref = [1, 2, ['a', 'b', 'c']]; But don't have to be: @info = stat($file) or die; # better, now @info gets its due print $1 if /\G(q)/gc; print "', pos=", pos, "\n"; The standard Tie::RefHash module provides a convenient workaround to this. Making References the other This has the interesting effect of creating a function local to another function, something not normally supported in Perl. ($HOST = $host) =~ tr/a-z/A-Z/; # Add one to the value of any numbers in the string ($quote = 'FINIS') =~ s/^\s+//gm; and here's another. i Do case-insensitive pattern matching. This operator quotes (and possibly compiles) its STRING as a regular expression. STRING is interpolated the same way as PATTERN in m/PATTERN/. If "'" is used as the delimiter, no interpolation is done. Returns a Perl value which may be used instead of the corresponding /STRING/imosx expression. ${$name x 2} = 3; # Sets $foofoo Shift Operators 6. C Operators Missing From Perl The way it works is that when the @{.} is seen in the double-quoted string, it's evaluated as a blockThe block creates a reference to an anonymous array containing the results of the call to mysub(1,2,3)So the whole block returns a reference to an array, which is then dereferenced by @{.} and stuck into the double-quoted stringThis chicanery is also useful for arbitrary expressions: 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. Interpolation has several quirks: $|, $(, and $) are not interpolated, and constructs $var[SOMETHING] are voted (by several different estimators) to be either an array element or $var followed by an RE alternative. This is where the notation ${arr[$bar]} comes handy: /${arr[0-9]}/ is interpreted as array element -9, not as a regular expression from the variable $arr followed by a digit, which would be the interpretation of /$arr[0-9]/. Since voting among different estimators may occur, the result is not predictable. Starting from this step no information about the delimiters is used in parsing. # These do the print before evaluating exit: $cnt = tr/*/*/; # count the stars in $_ Bitwise Or and Exclusive Or local $value = 10; Also parsed as terms are the do {} and eval {} constructs, as well as subroutine and method calls, and the anonymous constructors [] and {}. print while ($_ = STDIN>); Binary "*" multiplies two numbers. or: FILEHANDLE> may also be spelled readline(*FILEHANDLE). See perlfunc/readline. Equality Operators if (/^-D(.*)/) { $debug = $1 } 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. print "\t" x ($tab/8), ' ' x ($tab%8); # tab over $a = $ok ? @b : @c; # oops, that's just a count! print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc; This also behaves similarly, but avoids $_ : \*/ # Match the closing delimiter. 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. I/O Operators s/my.STRING/foo/is; Taking a reference to an enumerated list is not the same as using square brackets--instead it's the same as creating a list of references! { rand(10) * 20; # (rand 10) * 20 if ($ref1 == $ref2) { # cheap numeric compare of references 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: &{$coderef}(1,2,3); 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: \x{263a} wide hex char (SMILEY) $rec = get_rec(*STDIN{IO}); # pass both file and dir handles and even 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 In contrast, hard references are more like hard links in a Unix file system: They are used to access an underlying object without concern for what its (other) name isWhen the word "reference" is used without an adjective, as in the following paragraph, it is usually talking about a hard reference. Greetings, earthlings! 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. while ( ARGV>) { 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). 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. One more thing hereThe arrow is optional between brackets subscripts, so you can shrink the above down to \u uppercase next char 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). s {foo} # Replace foo \E end case modification ${ bareword }; # Okay, means $bareword. And then at least you can use the values(), which will be real refs, instead of the keys(), which won't. Gory details of parsing quoted constructs $bar = ${$scalarref}; lowercase lowercase line-noise lowercase lowercase line-noise Not-so-symbolic references Parsing of the bracketed character class construct, [ .], is rather different than the rule used for the rest of the pattern. The terminator of this construct is found using the same rules as for finding the terminator of a {}-delimited construct, the only exception being that ] immediately following [ is treated as though preceded by a backslash. Similarly, the terminator of (?{ .}) is found using the same rules as for finding the terminator of a {}-delimited construct. print "${push}over"; $program =~ s { 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 #. while ( >) { 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. This can be less efficient than an explicit search, because the pattern must be compiled every time the expression is evaluated. ($tmp = $global) =~ tr [A-Z] [a-z]; $re = qr/$pattern/; 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. 1. $r = \@a; while ($ARGV = shift) { $objref = Doggie->new(Tail => 'short', Ears => 'long'); 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. is semantically equivalent to the list: 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. The operator may be assigned to if both the 2nd and 3rd arguments are legal lvalues (meaning that you can assign to them): s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz' for my $name (@colors) { my $fh = shift; 5. As more readable alternatives to && and || when used for control flow, Perl provides and and or operators (see below). The short-circuit behavior is identical. The precedence of "and" and "or" is much lower, however, so that you can safely use them after a list operator without the need for parentheses: use fields; for (101 200) { print; } # print $_ 100 times Quote and Quote-like Operators @ary = (1, 3, sort 4, 2); Similarly, because of all the subscripting that is done using single words, we've applied the same rule to any bareword that is used for subscripting a hashSo now, instead of writing 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(): . 'more '/eg; @foo = @foo[0 $#foo]; # an expensive no-op is equivalent to while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) { ($a = $b) or $c; # really means this ($a += 2) *= 3; \x1b hex char (ESC) # expand tabs to 8-column spacing \U uppercase till \E Binary "/" divides two numbers. or two. /\* # Match the opening delimiter. shift; 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 Binary "!=" returns true if the left argument is numerically not equal to the right argument. } tr/AAA/XYZ/ For better performance, Perl can also do the translation from field names to array indices at compile time for typed object referencesSee fields. $in_body = /^$/ eof(); This step is the last one for all constructs except regular expressions, which are processed further. reset if eof; # clear ?? status for next file 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>. my $x = shift; my $x = $_[0] + 35; Exponentiation print "My sub returned @{[mysub(1,2,3)]} that time.\n"; Binary ">" returns true if the left argument is numerically greater than the right argument. Using a reference as a string produces both its referent's type, including any package blessing as described in perlobj, as well as the numeric address expressed in hexThe ref() operator returns just the type of thing the reference is pointing to, without the addressSee perlfunc/ref for details and examples of its use. If the /d modifier is used, the REPLACEMENTLIST is always interpreted exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter than the SEARCHLIST, the final character is replicated till it is long enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated. This latter is useful for counting characters in a class or for squashing character sequences in a class. m Treat string as multiple lines. $coderef = *handler{CODE}; A work-around is the following: To exchange a command's STDOUT and STDERR in order to capture the STDERR but leave its STDOUT to come out the old STDERR: @a = $ok ? @b : @c; # get an array # using m//gc with \G 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 perlfunc/open for details on this. unary & 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. \e escape (ESC) print ${ push } "over"; $shell_info = qx'ps $$'; # that's the new shell's $$ Which, in the degenerate case of using only ordinary arrays, gives you multidimensional arrays just like C's: $$arrayref[0] = "January"; (print $foo), exit; # This is what you want. Note that tr does not do regular expression character classes such as \d or [:lower:]. The tr> operator is not equivalent to the tr(1) utility. If you want to map strings between lower/upper cases, see perlfunc/lc and perlfunc/uc, and in general consider using the s operator if you need regular expressions. d Delete found but unreplaced characters. Bitwise And is the same as Binary "+" returns the sum of two numbers. For constructs that do interpolate, variables beginning with "$" 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. chdir $foo * 20; # chdir ($foo * 20) print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc; keys %$struct; # will return ("foo", "bar") in some order Note particularly that $x continues to refer to the value passed into newprint() despite "my $x" having gone out of scope by the time the anonymous subroutine runsThat's what a closure is all about. my $value = 20; tr/SEARCHLIST/REPLACEMENTLIST/cds s/PATTERN/REPLACEMENT/egimosx The following lines are equivalent: Here are the quote-like operators that apply to pattern matching and related activities. @ARGV = grep { -f && -T } glob('*') unless @ARGV; On the other hand, if you want the other meaning, you can do this: Examples: delete() on a pseudo-hash element only deletes the value corresponding to the key, not the key itselfTo delete the key, you'll have to explicitly delete it from the first hash element. @foo = @foo[$#foo-4 $#foo]; # slice last 5 items unshift(@ARGV, '-') unless @ARGV; If a reference happens to be a reference to an object, then there are probably methods to access the things referred to, and you should probably stick to those methods unless you're in the class package that defines the object's methodsIn other words, be nice, and don't violate the object's encapsulation without a very good reasonPerl does not enforce encapsulationWe are not totalitarians hereWe do expect some basic civility though. The bless() operator may be used to associate the object a reference points to with a package functioning as an object classSee perlobj. 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: ${$hashref->{"KEY"}} = "VALUE"; # CASE 3 &$coderef(1,2,3); See also "Terms and List Operators (Leftward)". sub inner { return $x * 19 } # WRONG 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 $Etc. The conditional is true if any variables were assigned, i.e., if the pattern matched. @files = glob("$dir/*.[ch]"); This will still print 10, not 20Remember that local() affects package variables, which are all "global" to the package. Unlike in C, the scalar assignment operator produces a valid lvalue. Modifying an assignment is equivalent to doing the assignment and then modifying the variable that was assigned to. This is useful for modifying a copy of something, like this: return $x + inner(); } @ones = (5) x @ones; # set all elements to 5 @a = @b || @c; # this is wrong 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. Unary "~" performs bitwise negation, i.e., 1's complement. For example, 0666 & ~027 is 0640. (See also Integer Arithmetic and Bitwise String Operators.) Note that the width of the result is platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64 bits wide on a 64-bit platform, so if you are expecting a certain bit width, remember use the & operator to mask off the excess bits. qr/STRING/imosx Binary ">=" returns true if the left argument is numerically greater than or equal to the right argument. than Symbolic references are names of variables or other objects, just as a symbolic link in a Unix filesystem contains merely the name of a fileThe *glob notation is something of a symbolic reference(Symbolic references are sometimes called "soft references", but please don't call them that; references are confusing enough without useless synonyms.) Interpolation of regular expressions $baz = "\n"; # a one-character string 2. to get a hexadecimal digit, or $success; Multiplicative Operators print "That yields @{[$n + 5]} widgets\n"; 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. 3. During this search no attention is paid to the semantics of the construct. Thus: The range operator also works on strings, using the magical auto-increment, see below. You can even set them to pipe commands. For example, this automatically filters compressed arguments through gzip: $output = `cmd 2>&1`; print while STDIN>; In scalar context, " " returns a boolean value. The operator is bistable, like a flip-flop, and emulates the line-range (comma) operator of sed, awk, and various editors. Each " " operator maintains its own boolean state. It is false as long as its left operand is false. Once the left operand is true, the range operator stays true until the right operand is true, AFTER which the range operator becomes false again. It doesn't become false till the next time the range operator is evaluated. It can test the right operand and become false on the same evaluation it became true (as in awk), but it still returns true once. If you don't want it to test the right operand till the next evaluation, as in sed, just use three dots (" .") instead of two. In all other regards, " ." behaves just like " " does. Binary "-" returns the difference of two numbers. while ($i++ 2) { // m{} Pattern match yes* } push(@{$arrayref}, $filename); qw/STRING/ qx/STRING/ ${$hashref}{"KEY"} = "VALUE"; . # code for each line split(' ', q/STRING/); o Compile pattern only once. $$name = 1; # Sets $foo A (file)glob evaluates its (embedded) argument only when it is starting a new list. All values must be read before it will start over. In list context, this isn't important because you automatically get them all anyway. However, in scalar context the operator returns the next value each time it's called, or undef when the list has run out. As with filehandle reads, an automatic defined is generated when the glob occurs in the test part of a while, because legal glob returns (e.g. a file called 0) would otherwise terminate the loop. Again, undef is returned only once. So if you're expecting a single value from a glob, it is much better to say If use locale is in effect, the case map used by \l, \L, \u and \U is taken from the current locale. See perllocale. If Unicode (for example, \N{} or wide hex characters of 0x100 or beyond) is being used, the case map used by \l, \L, \u and \U is as defined by Unicode. For documentation of \N{name}, see charnames. close ARGV if eof; # reset $. each file 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 ||: 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. } while (defined($_ = STDIN>)) { print; } print ++($foo = 'zz'); # prints 'aaa' 2: 'q', pos=8 NOTE: The current user-visible implementation of pseudo-hashes (the weird use of the first array element) is deprecated starting from Perl 5.8.0 and will be removed in Perl 5.10.0, and the feature will be implemented differentlyNot only is the current interface rather ugly, but the current implementation slows down normal array and hash use quite noticeablyThe 'fields' pragma interface will remain available. {bar} # with bar. For example, if you wanted a function to make a new hash and return a reference to it, you have these options: "" qq{} Literal yes for (; STDIN>;) { print; } s/\s+$//; values %$struct; # will return ("FOO", "BAR") in same some order A double-quoted, interpolated string. unlink("alpha", "beta", "gamma") "++" and "--" work as in C. That is, if placed before a variable, they increment or decrement the variable before returning the value, and if placed after, increment or decrement the variable after returning the value. if (/Version: *([0-9.]*)/) { $version = $1; } open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); Some passes discussed below are performed concurrently, but because their results are the same, we consider them individually. For different quoting constructs, Perl performs different numbers of passes, from one to five, but these passes are always performed in the same order. 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. A new feature contributing to readability in perl version 5.001 is that the brackets around a symbolic reference behave more like quotes, just as they always have within a stringThat is, 'foo', 'bar', 'baz' Relational Operators $rex = qr/my.STRING/is; ?PATTERN? \L lowercase till \E grep { chdir($foo) * 20; # (chdir $foo) * 20 print exists $phash->{bar}; # false, 'bar' has not been used. Howdy, world! $coderef = \&handler; References are often returned by special subroutines called constructorsPerl objects are just references to a special type of object that happens to know which package it's associated withConstructors are just special subroutines that know how to create that associationThey do so by starting with an ordinary reference, and it remains an ordinary reference even while it's also being an objectConstructors are often named new() and called indirectly: # This will expand any embedded scalar variable $menubar = $main->Frame(-relief => "raised", $home = $ENV{'HOME'} || $ENV{'LOGDIR'} || With the C-style operators that would have been written like this: EOF here-doc yes* $struct->{bar}; # same as $struct->[2], i.e"BAR" while ( >) { 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. See perlre for additional information on valid syntax for STRING, and for a detailed look at the semantics of regular expressions. m/ Options are: Using a string or number as a reference produces a symbolic reference, as explained aboveUsing a reference as a number produces an integer representing its storage location in memoryThe only useful thing to be done with this is to compare two references numerically to see whether they refer to the same location. $output = `cmd 2>&1 1>/dev/null`; system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr"); release 5 of Perl it was difficult to represent complex data structures, because all references had to be symbolic--and even then it was difficult to refer to a variable instead of a symbol table entryPerl now not only makes it easier to use symbolic references to variables, but also lets you have "hard" references to any piece of data or codeAny scalar may hold a hard referenceBecause arrays and hashes contain scalars, you can now easily build arrays of arrays, arrays of hashes, hashes of arrays, arrays of hashes of functions, and so on. A line-oriented form of quoting is based on the shell "here-document" syntax. Following a you specify a string to terminate the quoted material, and all lines following the current line down to the terminating string are the value of the item. The terminating string may be either an identifier (a word), or some quoted text. If quoted, the type of quotes you use determines the treatment of the text, just as in regular quoting. An unquoted identifier works like double quotes. There must be no space between the and the identifier, unless the identifier is quoted. (If you put a space it will be treated as a null identifier, which is valid, and matches the first empty line.) The terminating string must appear by itself (unquoted and with no surrounding whitespace) on the terminating line. } "", ``, qq//, qx//, file*glob> Binary "ge" returns true if the left argument is stringwise greater than or equal to the right argument. Previous steps were performed during the compilation of Perl code, but this one happens at run time--although it may be optimized to be calculated at compile time if appropriate. After preprocessing described above, and possibly after evaluation if catenation, joining, casing translation, or metaquoting are involved, the resulting string is passed to the RE engine for compilation. Interpolated scalars and arrays are converted internally to the join and . catenation operations. Thus, "$foo XXX '@arr'" becomes: m Treat string as multiple lines. Although these are grouped by family, they all have the precedence of assignment. 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. $cnt = $sky =~ tr/*/*/; # count the stars in $sky Comma Operator $scalarref = \$foo; 'Adam' => 'Eve', 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e; probably doesn't do what you expect at first glance. See Named Unary Operators for more discussion of this. ${$name} = 2; # Sets $foo \Q quote non-word characters till \E $string =~ /$re/; # or this way } continue { s/%(.)/$percent{$1}/g; # change percent escapes; no /e print delete $phash->{foo}; # prints $phash->[1], "FOO" $str = '\t'; # . # code for each line no strict 'refs'; Each year,thousands of people just like you receive_grant_money Named Unary Operators while ( STDIN>) { print; } $cnt = tr/0-9//; # count the digits in $_ However, since prototype checking happens at compile time, the assignment above happens too late to be of much useYou could address this by putting the whole loop of assignments within a BEGIN block, forcing it to occur during compilation. References are easy to use in PerlThere is just one overriding principle: Perl does no implicit referencing or dereferencingWhen a scalar is holding a reference, it always behaves as a simple scalarIt doesn't magically start being an array or hash or subroutine; you have to tell it explicitly to do so, by dereferencing it. qw(foo bar baz) print '-' x 80; # print row of dashes print $$ref; Pseudo-hashes: Using an array as a hash ($n == 1) ? '' : "s"; $_ = "ppooqppqq"; Type-casting operator. 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 Binary " =" returns true if the left argument is numerically less than or equal to the right argument. 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: s/(\$\w+)/$1/eeg; will transliterate any A to X. 2: 'q', pos=5 } Binary " " returns true if the left argument is numerically less than the right argument. s/^/> / if (/^$/ eof()); # quote body 'STRING' Binary "&" returns its operators ANDed together bit by bit. (See also Integer Arithmetic and Bitwise String Operators.) * unless the delimiter is ''. print($foo), exit; # Or this. s/this/ E . 'that' EOC $perl_info = qx(ps $$); # that's Perl's $$ The price is $Price. 1: 'oo', pos=4 A reference can be created by using a special syntax, lovingly known as the *foo{THING} syntax*foo{THING} returns a reference to the THING slot in *foo (which is the symbol table entry which holds everything known as foo). $main = MainWindow->new(); You might also think of closure as a way to write a subroutine template without using eval()Here's a small example of how closures work: 'Clyde' => 'Bonnie', Also note that In list context, it's just the list argument separator, and inserts both its arguments into the list. This applies only to lexical variables, by the wayDynamic variables continue to work as they have always workedClosure is not something that most Perl programmers need trouble themselves about to begin with. Binary "gt" returns true if the left argument is stringwise greater than the right argument. \033 octal char (ESC) print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc; i Do case-insensitive pattern matching. 1. sub showem { { return @_ } } # ok } sub splutter { chdir ($foo) * 20; # (chdir $foo) * 20 @$name = (); # Clears @foo 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: Rather than this: print $foo, exit; # Nor is this. 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 $a (i.e. the result will be less than or equal to zero). Note than when use integer is in scope, "%" gives you direct access to the modulus operator as implemented by your C compiler. This operator is not as well defined for negative operands, but it will execute faster. next if m#^/usr/spool/uucp#; s/$rex/foo/; The next step is interpolation in the text obtained, which is now delimiter-independent. There are four different cases. E print while defined($_ = STDIN>); $hashref = *ENV{HASH}; 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: 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. Case 2 is also deceptive in that you're accessing a variable called %hashref, not dereferencing through $hashref to the hash it's presumably referencingThat would be case 3. the other g Match globally, i.e., find all occurrences. Binary "lt" returns true if the left argument is stringwise less than the right argument. chdir($foo) || die; # (chdir $foo) || die Processing of \Q, \U, \u, \L, \l, and interpolation happens (almost) as with qq// constructs, but the substitution of \ followed by RE-special chars (including \) is not performed. Moreover, inside (?{BLOCK}), (?# comment ), and a #-comment in a //x-regular expression, no processing is performed whatsoever. This is the first step at which the presence of the //x modifier is relevant. 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. $output = `cmd 3>&1 1>&2 2>&3 3>&-`; print $globref "output\n"; Subroutine calls and lookups of individual array elements arise often enough that it gets cumbersome to use method 2As a form of syntactic sugar, the examples for method 2 may be written: 'foo{bar}baz' next line if (1 /^$/); # skip header lines See also discussion of list operators in Terms and List Operators (Leftward). $a = $a + 2; $name->[0] = 4; # Sets $foo[0] 3. 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: "->" 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. tr [\200-\377] Anywhere you'd put an identifier (or chain of identifiers) as part of a variable or subroutine name, you can replace the identifier with a simple scalar variable containing a reference of the correct type: $a = $b || $c; # better written this way my $x = $_[0] + 35; $foo = q!I said, "You said, 'She said it.'"!; m Treat string as multiple lines. The first pass is finding the end of the quoted construct, whether it be a multicharacter delimiter "\nEOF\n" in the EOF construct, a / that terminates a qq// construct, a ] which terminates qq[] construct, or a > which terminates a fileglob started with . 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. rand 10 * 20; # rand (10 * 20) $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case while (my $line = STDIN>) { print $line } local *inner = sub { return $x * 19 }; Anonymous subroutines act as closures with respect to my() variables, that is, variables lexically visible within the current scopeClosure is a notion out of the Lisp world that says if you define an anonymous function in a particular lexical context, it pretends to run in that context even when it's called outside the context. printf "I have %d dog%s.\n", $n, is equivalent to the following Perl-like pseudo code: and not worry about whether the subscripts are reserved wordsIn the rare event that you do wish to do something like In the absence of parentheses, the precedence of list operators such as print, sort, or chmod is either very high or very low depending on whether you are looking at the left side or the right side of the operator. For example, in perl -le '$a = NaN; print "NaN support here" if $a != $a' A single-quoted, literal string. A backslash represents a backslash unless followed by the delimiter or another backslash, in which case the delimiter or backslash is interpolated. 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/. print "abc\"def"; Likewise, print "$k => $v\n"; if (101 200) { print; } # print 2nd hundred lines @ones = (1) x 80; # a list of 80 1's echo lo there $globref = *foo{GLOB}; \n newline (NL) 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 implicitly compared to the $. variable, the current line number. Examples: rand +(10) * 20; # rand (10 * 20) Examples: Symbolic references Unary "\" creates a reference to whatever follows it. See perlreftut and perlref. Do not confuse this behavior with the behavior of backslash within a string, although both forms do convey the notion of protecting the next thing from interpolation. sub showem { { @_ } } # ambiguous (currently ok, but may change) Binary "or" returns the logical disjunction of the two surrounding expressions. It's equivalent to || except for the very low precedence. This makes it useful for control flow print if /$arg/o; # compile only once ${$arrayref}[0] = "January"; print exists $phash->{foo}; # true, 'foo' was set in the declaration print ++($foo = 'Az'); # prints 'Ba' } All operations above are performed simultaneously, left to right. s Treat string as single line. The leading +{ and {; always serve to disambiguate the expression to mean either the HASH reference, or the BLOCK. $pseudohash = fields::phash(foo => "FOO", bar => "BAR"); tr/a-zA-Z//s; # bookkeeper -> bokeper } The following escape sequences are available in constructs that interpolate and in transliterations. s/\d+/$&*2/e; # yields 'abc246xyz' 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. Here we've created a reference to an anonymous array of three elements whose final element is itself a reference to another anonymous array of three elements(The multidimensional syntax described later can be used to access thisFor example, after the above, $arrayref->[2][1] would have the value "b".) $array{ aaa }{ bbb }{ ccc } rand (10) * 20; # (rand 10) * 20 unlink "alpha", "beta", "gamma" print($foo, exit); # Obviously not what you want. $array[$x]->{"foo"}->[0] = "January"; is equivalent to 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; y/SEARCHLIST/REPLACEMENTLIST/cds The result may be used as a subpattern in a match: List Operators (Rightward) Anonymous hash and array composers like these can be intermixed freely to produce as complicated a structure as you wantThe multidimensional syntax described below works for these tooThe values above are literals, but variables and expressions would work just as well, because assignment operators in Perl (even within local() or my()) are executable statements, not compile-time declarations. \l lowercase next char During the second pass, text between the starting and ending delimiters is copied to a safe location, and the \ is removed from combinations consisting of \ and delimiter--or delimiters, meaning both starting and ending delimiters will should these differ. This removal does not happen for multi-character delimiters. Note that the combination \\ is left intact, just as it was. Range Operators If you try to dereference the key, it won't do a hard dereference, and you won't accomplish what you're attemptingYou might want to do something more like use strict 'refs'; } continue { to get all normal letters of the English alphabet, or The only interpolation is removal of \ from pairs \\. 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. Now all those different functions appear to exist independentlyYou can call red(), RED(), blue(), BLUE(), green(), etcThis technique saves on both compile time and memory use, and is less error-prone as well, since syntax checks happen at compile timeIt's critical that any variables in the anonymous subroutine be lexicals in order to create a proper closureThat's the reasons for the my on the loop iteration variable. s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz' 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. open(ARGV, $ARGV); m/PATTERN/cgimosx $foo . " XXX '" . (join $", @arr) . "'"; print exists $phash->{foo}; # false $$hashref{"KEY"} = "VALUE"; Additive Operators print EOF; print ABC To read both a command's STDOUT and its STDERR separately, it's easiest and safest to redirect them separately to files, and then read from those files when the program is done: ${$hashref}{"KEY"} = "VALUE"; # CASE 1 &$name(); # Calls &foo() (as in Perl 4) print exists $phash->[0]{bar}; # true, 'bar' is a valid field @a = @b ? @b : @c; # this works fine, though &$h("world"); (TYPE) $x{ $r } = $r; A character range may be specified with a hyphen, so tr/A-J/0-9/ does the same replacement as tr/ACEGIBDFHJ/0246813579/. For sed devotees, y is provided as a synonym for tr. If the SEARCHLIST is delimited by bracketing quotes, the REPLACEMENTLIST has its own pair of quotes, which may or may not be bracketing quotes, e.g., tr[A-Z][a-z] or tr(+\-*/)/ABCD/. print ++($foo = '99'); # prints '100' In other boolean contexts, filehandle> without an explicit defined test or comparison elicit a warning if the use warnings pragma or the -w command-line switch (the $^W variable) is in effect. 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. @info = stat($file) || die; # oops, scalar sense of stat! $array[$x]{"foo"}[0] = "January"; # scalar context Really means this: For example, sub hashem { { @_ } } # silently wrong References of the appropriate type can spring into existence if you dereference them in a context that assumes they existBecause we haven't talked about dereferencing yet, we can't show you any examples yet Binary "==" returns true if the left argument is numerically equal to the right argument. Binary "|" returns its operators ORed together bit by bit. (See also Integer Arithmetic and Bitwise String Operators.) the difference being that it generates a real list at compile time. So this expression: This is one of the only places where giving a prototype to a closure makes much senseIf you wanted to impose scalar context on the arguments of these functions (probably not a wise idea for this particular example), you could have written it this way instead: } Binary "!~" is just like "=~" except the return value is negated in the logical sense. }; } Options are: $string =~ $re; # or used standalone $sentences++; 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. and then only hard references will be allowed for the rest of the enclosing blockAn inner block may countermand that with 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. 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.) print $$$$refrefref; 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. Examples: (getpwuid($ ))[7] || die "You're homeless!\n"; s{}{} Substitution yes* ($foo = $bar) =~ s/this/that/; # copy first, then change 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. print exists $phash->[0]{foo}; # true, key still exists chmod 0644, *.c>; ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g); @a = scalar(@b) || @c; # really meant this 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. chdir $foo || die; # (chdir $foo) || die if /\b(tcl|java|python)\b/i; # :-) ($a_or_b ? $a : $b) = $c; 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. $push = "pop on "; print delete $phash->[0]{foo}; # now key is gone 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: } That's it for creating referencesBy now you're probably dying to know how to use references to get back to your long-lost dataThere are several basic methods. All of these are self-explanatory except for *foo{IO}It returns the IO handle, used for file handles (perlfunc/open), sockets (perlfunc/socket and perlfunc/socketpair), and directory handles (perlfunc/opendir)For compatibility with previous versions of Perl, *foo{FILEHANDLE} is a synonym for *foo{IO}, though it is deprecated as of 5.8.0If deprecation warnings are in effect, it will warn of its use. qw{} Word list no MiXeD line-noise. That's all! while ( FOO>) { 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. Using References print "foo", "bar"; # you can stack them $a += ($a % 2) ? 10 : 2; while ( >) { $globref = \*foo; # do something based on those Using a closure as a function template allows us to generate many functions that act similarlySuppose you wanted functions named after the colors that generated HTML font changes for the various colors: tr/a-zA-Z/ /cs; # change non-alphas to single space may be closer to the conjectural intention of the writer of "\Q\t\E". open HANDLE, "filename" @colors = qw(red blue green yellow orange purple violet); $g = newprint("Greetings"); $url = new URI::URL "http://www/"; die if $url eq "xXx"; $arrayref = *ARGV{ARRAY}; Options: $hexdigit = (0 9, 'a' 'f')[$num & 15]; while (($_ = STDIN>) ne '0') { . } print $phash->{foo}; # runtime exception or: x= $success = 1, last if /$pat/; 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 haven't picked up the saner idiom yet. A warning is emitted if the use warnings pragma or the -w command-line flag (that is, the $^W variable) was set. The Road goes ever on and on, g Replace globally, i.e., all occurrences. my $patterns = shift; "$hash{"$foo/$bar"}" 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 because it treats ARGV> as non-magical.) 1: '', pos=7 The following escape sequences are available in constructs that interpolate but not in transliterations. .*? # Match a minimal number of characters. $ref = "value"; Customary Generic Meaning Interpolates It is at this stage that split() silently optimizes /^/ to mean /^/m. + 20; s/this/ E . 'that' \r return (CR) $a += 2; Assignment operators work as in C. That is, $/ = ""; *$name = sub ($) { "FONT COLOR='$name'>$_[0]/FONT>" }; 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: $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count c Complement the SEARCHLIST. Options are: 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. $struct->{foo}; # same as $struct->[1], i.e"FOO" 2. you can force interpretation as a reserved word by adding anything that makes it more than a bareword: EOF use fields; # code Beginning with release 5.005 of Perl, you may use an array reference in some contexts that would normally require a hash referenceThis allows you to access array elements using symbolic names, as if they were fields in a structure. } } @_; \t tab (HT, TAB) $array{ "aaa" }{ "bbb" }{ "ccc" } 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: my @compiled = map qr/$_/i, @$patterns; 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). but, because * is higher precedence than named operators: Here is what C has that Perl doesn't: splutter(*STDOUT{IO}); # pass both file and dir handles **= += *= &= = &&= s/\$(\w+)/${$1}/g; If multiple transliterations are given for a character, only the first one is used: line-noise lowercase line-noise lowercase UPPERCASE line-noise } $_ = 'abc123xyz'; PATTERN may contain variables, which will be interpolated (and the pattern recompiled) every time the pattern search is evaluated, except for when the delimiter is a single quote. (Note that $(, $), and $| are not interpolated because they look like end-of-string tests.) If you want such a pattern to be compiled only once, add a /o after the trailing delimiter. This avoids expensive run-time recompilations, and is useful when the value you are interpolating won't change over the life of the script. However, mentioning /o constitutes a promise that you won't change the variables in the pattern. If you change them, Perl won't even notice. See also "qr/STRING/imosx". "=" is the ordinary assignment operator. or gripe(), next LINE; Binary "xor" returns the exclusive-OR of the two surrounding expressions. It cannot short circuit, of course. You may not (usefully) use a reference as the key to a hashIt will be converted into a string: $terminal = Term::Cap->Tgetent( { OSPEED => 9600 }); $s = q{ if($a eq "}") . }; # WRONG print ($foo), exit; # Or even this. print "Be ", red("careful"), "with that ", green("light"); References can be created in several ways. $struct = [{foo => 1, bar => 2}, "FOO", "BAR"]; print "EOF"; # same as above $x{ \$a } = $a; } Constant Folding 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. 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). The last example should print: $coderef = sub { print "Boink!\n" }; print exists $phash->{pants}; # true, your 'pants' have been touched Here's a line s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields -borderwidth => 2) # parse mail messages 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.) By using the backslash operator on a variable, subroutine, or value(This works much like the & (address-of) operator in C.) This typically creates another reference to a variable, because there's already a reference to the variable in the symbol tableBut the symbol table reference might go away, and you'll still have the reference that the backslash returnedHere are some examples: The null filehandle > is special: it can be used to emulate the behavior of sed and awk. Input from > comes either from standard input, or from each file listed on the command line. Here's how it works: the first time > is evaluated, the @ARGV array is checked, and if it is empty, $ARGV[0] is set to "-", which when opened gives you standard input. The @ARGV array is then processed as a list of filenames. The loop See also Quote and Quote-like Operators toward the end of this section, as well as "I/O Operators". return sub { my $y = shift; print "$x, $y!\n"; }; The Arrow Operator print $1 while /(p)/gc; print "', pos=", pos, "\n"; you can write just 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).) 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. $bar = $$scalarref; $refrefref = \\\"howdy"; $array{ +shift } Because of being able to omit the curlies for the simple case of $$x, people often make the mistake of viewing the dereferencing symbols as proper operators, and wonder about their precedenceIf they were, though, you could use parentheses instead of bracesThat's not the caseConsider the difference below; case 0 is a short-hand version of case 1, not case 2: Logical or and Exclusive Or # blank line between header and body *$name = *{uc $name} = sub { "FONT COLOR='$name'>@_/FONT>" }; -= /= |= >>= ||= EOF The range operator (in list context) makes use of the magical auto-increment algorithm if the operands are strings. You can say $phash = fields::phash([qw(foo bar pants)], ['FOO']); $scalarref = *foo{SCALAR}; I said bar. while ( STDIN>) { last unless $_; . } A reference to an anonymous array can be created using square brackets: while ($_ = $ARGV[0], /^-/) { ${$hashref{"KEY"}} = "VALUE"; # CASE 2 while ( >) { 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: C-style Logical And The || and && operators differ from C's in that, rather than returning 0 or 1, they return the last value evaluated. Thus, a reasonably portable way to find out the home directory (assuming it's not "0") might be: chmod 0644, $_; is equivalent to Assignment Operators 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 "-bareword". ABC print `EOC`; # execute commands Hard references are smart--they keep track of reference counts for you, automatically freeing the thing referred to when its reference count goes to zero(Reference counts for values in self-referential or cyclic data structures may not go to zero without a little help; see perlobj/"Two-Phased Garbage Collection" for a detailed explanation.) If that thing happens to be an object, the object is destructedSee perlobj for more about objects(In a sense, everything in Perl is an object, but we usually reserve the word for references to objects that have been officially "blessed" into a class package.) The second is to use exists() on the hash reference sitting in the first array elementThis checks to see if the given key is a valid field in the pseudo-hash. Optimization of regular expressions @list = (\$a, \@b, \%c); Using single-quote as a delimiter protects the command from Perl's double-quote interpolation, passing it on to the shell instead: $hashref = \%ENV; '' q{} Literal no s/^=(\w+)/&pod($1)/ge; # use function call \b backspace (BS) @alphabet = ('A' 'Z'); "a " . $b . " -> {c}"; 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: }

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