X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlop.pod;h=cc657c1446e961364502e8e24eea053ebd6fb669;hb=5081475eefaf24307ce7eaf4c87aafd588b37e98;hp=4781b7fbbea26ea0d52ada404c650f4efa9ee11a;hpb=a3cb178b0bad32fa8be934503d051b96a3cb1fea;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlop.pod b/pod/perlop.pod index 4781b7f..cc657c1 100644 --- a/pod/perlop.pod +++ b/pod/perlop.pod @@ -38,6 +38,8 @@ operate on scalar values only, not array values. In the following sections, these operators are covered in precedence order. +Many operators can be overloaded for objects. See L. + =head1 DESCRIPTION =head2 Terms and List Operators (Leftward) @@ -114,7 +116,7 @@ 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 null and matches the pattern +has a value that is not the empty string and matches the pattern C, the increment is done as a string, preserving each character within its range, with carry: @@ -144,8 +146,9 @@ starts with a plus or minus, a string starting with the opposite sign is returned. One effect of these rules is that C<-bareword> is equivalent to C<"-bareword">. -Unary "~" performs bitwise negation, i.e., 1's complement. -(See also L and L.) +Unary "~" performs bitwise negation, i.e., 1's complement. For example, +C<0666 &~ 027> is 0640. (See also L and L.) Unary "+" has no effect whatsoever, even on strings. It is useful syntactically for separating a function name from a parenthesized expression @@ -184,11 +187,18 @@ operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is C<$a> minus the largest multiple of C<$b> that is not greater than C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the smallest multiple of C<$b> that is not less than C<$a> (i.e. the -result will be less than or equal to zero). +result will be less than or equal to zero). If C is +in effect, the native hardware will be used instead of this rule, +which may be construed a bug that will be fixed at some point. -Binary "x" is the repetition operator. In a scalar context, it +Note than when C is in scope, "%" give you direct access +to the modulus operator as implemented by your C compiler. This +operator is not as well defined for negative operands, but it will +execute faster. + +Binary "x" is the repetition operator. In scalar context, it returns a string consisting of the left operand repeated the number of -times specified by the right operand. In a list context, if the left +times specified by the right operand. In list context, if the left operand is a list in parentheses, it repeats the list. print '-' x 80; # print row of dashes @@ -331,11 +341,18 @@ way to find out the home directory (assuming it's not "0") might be: $home = $ENV{'HOME'} || $ENV{'LOGDIR'} || (getpwuid($<))[7] || die "You're homeless!\n"; -As more readable alternatives to C<&&> and C<||>, 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: +In particular, this means that you shouldn't use this +for selecting between two aggregates for assignment: + + @a = @b || @c; # this is wrong + @a = scalar(@b) || @c; # really meant this + @a = @b ? @b : @c; # this works fine, though + +As more readable alternatives to C<&&> and C<||> when used for +control flow, Perl provides C and C operators (see below). +The short-circuit behavior is identical. The precedence of "and" and +"or" is much lower, however, so that you can safely use them after a +list operator without the need for parentheses: unlink "alpha", "beta", "gamma" or gripe(), next LINE; @@ -345,21 +362,24 @@ With the C-style operators that would have been written like this: unlink("alpha", "beta", "gamma") || (gripe(), next LINE); -=head2 Range Operator +Use "or" for assignment is unlikely to do what you want; see below. + +=head2 Range Operators Binary ".." is the range operator, which is really two different -operators depending on the context. In a list context, it returns an +operators depending on the context. In list context, it returns an array of values counting (by ones) from the left value to the right -value. This is useful for writing C loops and for doing -slice operations on arrays. Be aware that under the current implementation, -a temporary array is created, so you'll burn a lot of memory if you -write something like this: +value. This is useful for writing C 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 C loops, but older versions of Perl might burn +a lot of memory when you write something like this: for (1 .. 1_000_000) { # code } -In a scalar context, ".." returns a boolean value. The operator is +In scalar context, ".." returns a boolean value. The operator is bistable, like a flip-flop, and emulates the line-range (comma) operator of B, B, and various editors. Each ".." operator maintains its own boolean state. It is false as long as its left operand is false. @@ -373,13 +393,13 @@ If you don't want it to test the right operand till the next evaluation 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 null string for false, or a sequence number +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 numeric literal, +greater than 1. If either operand of scalar ".." is a constant expression, that operand is implicitly compared to the C<$.> variable, the current line number. Examples: @@ -389,13 +409,22 @@ As a scalar operator: next line if (1 .. /^$/); # skip header lines s/^/> / if (/^$/ .. eof()); # quote body + # parse mail messages + while (<>) { + $in_header = 1 .. /^$/; + $in_body = /^$/ .. eof(); + # do something based on those + } continue { + close ARGV if eof; # reset $. each file + } + As a list operator: for (101 .. 200) { print; } # print $_ 100 times @foo = @foo[0 .. $#foo]; # an expensive no-op @foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items -The range operator (in a list context) makes use of the magical +The range operator (in list context) makes use of the magical auto-increment algorithm if the operands are strings. You can say @@ -438,6 +467,19 @@ legal lvalues (meaning that you can assign to them): This is not necessarily guaranteed to contribute to the readability of your program. +Because this operator produces an assignable result, using assignments +without parentheses will get you in trouble. For example, this: + + $a % 2 ? $a += 10 : $a += 2 + +Really means this: + + (($a % 2) ? ($a += 10) : $a) += 2 + +Rather than this: + + ($a % 2) ? ($a += 10) : ($a += 2) + =head2 Assignment Operators "=" is the ordinary assignment operator. @@ -480,11 +522,11 @@ is equivalent to =head2 Comma Operator -Binary "," is the comma operator. In a scalar context it evaluates +Binary "," is the comma operator. In scalar context it evaluates its left argument, throws that value away, then evaluates its right argument and returns that value. This is just like C's comma operator. -In a list context, it's just the list argument separator, and inserts +In list context, it's just the list argument separator, and inserts both its arguments into the list. The =E digraph is mostly just a synonym for the comma operator. It's useful for @@ -519,9 +561,27 @@ expression is evaluated only if the left expression is true. =head2 Logical or and Exclusive Or Binary "or" returns the logical disjunction of the two surrounding -expressions. It's equivalent to || except for the very low -precedence. This means that it short-circuits: i.e., the right -expression is evaluated only if the left expression is false. +expressions. It's equivalent to || except for the very low precedence. +This makes it useful for control flow + + print FH $data or die "Can't write to FH: $!"; + +This means that it short-circuits: i.e., the right expression is evaluated +only if the left expression is false. Due to its precedence, you should +probably avoid using this for assignment, only for control flow. + + $a = $b or $c; # bug: this is wrong + ($a = $b) or $c; # really means this + $a = $b || $c; # better written this way + +However, when it's a list context assignment and you're trying to use +"||" for control flow, you probably need "or" so that the assignment +takes higher precedence. + + @info = stat($file) || die; # oops, scalar sense of stat! + @info = stat($file) or die; # better, now @info gets its due + +Then again, you could always use parentheses. Binary "xor" returns the exclusive-OR of the two surrounding expressions. It cannot short circuit, of course. @@ -561,7 +621,7 @@ the same character fore and aft, but the 4 sorts of brackets Customary Generic Meaning Interpolates '' q{} Literal no "" qq{} Literal yes - `` qx{} Command yes + `` qx{} Command yes (unless '' is delimiter) qw{} Word list no // m{} Pattern match yes s{}{} Substitution yes @@ -581,7 +641,7 @@ or "C<@>" are interpolated, as are the following sequences. Within a transliteration, the first ten of these sequences may be used. \t tab (HT, TAB) - \n newline (LF, NL) + \n newline (NL) \r return (CR) \f form feed (FF) \b backspace (BS) @@ -596,10 +656,29 @@ a transliteration, the first ten of these sequences may be used. \L lowercase till \E \U uppercase till \E \E end case modification - \Q quote regexp metacharacters till \E + \Q quote non-word characters till \E If C is in effect, the case map used by C<\l>, C<\L>, C<\u> -and <\U> is taken from the current locale. See L. +and C<\U> is taken from the current locale. See L. + +All systems use the virtual C<"\n"> to represent a line terminator, +called a "newline". There is no such thing as an unvarying, physical +newline character. It is an illusion that the operating system, +device drivers, C libraries, and Perl all conspire to preserve. Not all +systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example, +on a Mac, these are reversed, and on systems without line terminator, +printing C<"\n"> may emit no actual data. In general, use C<"\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 (C<"\012\015"> or C<"\cJ\cM">) for line terminators, +and although they often accept just C<"\012">, they seldom tolerate just +C<"\015">. If you get in the habit of using C<"\n"> for networking, +you may be burned some day. + +You cannot include a literal C<$> or C<@> within a C<\Q> sequence. +An unescaped C<$> or C<@> interpolates the corresponding variable, +while escaping will cause the literal string C<\$> to be inserted. +You'll need to write something like C. Patterns are subject to an additional level of interpretation as a regular expression. This is done as a second pass, after variables are @@ -617,6 +696,18 @@ evaluation of variables when used within double quotes. Here are the quote-like operators that apply to pattern matching and related activities. +Most of this section is related to use of regular expressions from Perl. +Such a use may be considered from two points of view: Perl handles a +a string and a "pattern" to RE (regular expression) engine to match, +RE engine finds (or does not find) the match, and Perl uses the findings +of RE engine for its operation, possibly asking the engine for other matches. + +RE engine has no idea what Perl is going to do with what it finds, +similarly, the rest of Perl has no idea what a particular regular expression +means to RE engine. This creates a clean separation, and in this section +we discuss matching from Perl point of view only. The other point of +view may be found in L. + =over 8 =item ?PATTERN? @@ -627,6 +718,14 @@ optimization when you want to see only the first occurrence of something in each file of a set of files, for instance. Only C patterns local to the current package are reset. + while (<>) { + if (?^$?) { + # blank line between header and body + } + } continue { + reset if eof; # clear ?? status for next file + } + This usage is vaguely deprecated, and may be removed in some future version of Perl. @@ -634,13 +733,13 @@ version of Perl. =item /PATTERN/cgimosx -Searches a string for a pattern match, and in a scalar context returns +Searches a string for a pattern match, and in scalar context returns true (1) or false (''). If no string is specified via the C<=~> or C operator, the $_ string is searched. (The string specified with C<=~> need not be an lvalue--it may be the result of an expression evaluation, but remember the C<=~> binds rather tightly.) See also L. -See L for discussion of additional considerations which apply +See L for discussion of additional considerations that apply when C is in effect. Options are: @@ -654,8 +753,11 @@ Options are: x Use extended regular expressions. If "/" is the delimiter then the initial C is optional. With the C -you can use any pair of non-alphanumeric, non-whitespace characters as -delimiters. This is particularly useful for matching Unix path names +you can use any pair of non-alphanumeric, non-whitespace characters +as delimiters (if single quotes are used, no interpretation is done +on the replacement string. Unlike Perl 4, Perl 5 treats backticks as normal +delimiters; the replacement text is not evaluated as a command). +This is particularly useful for matching Unix path names that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is the delimiter, then the match-only-once rule of C applies. @@ -669,15 +771,16 @@ interpolating won't change over the life of the script. However, mentioning C constitutes a promise that you won't change the variables in the pattern. If you change them, Perl won't even notice. -If the PATTERN evaluates to a null string, the last -successfully matched regular expression is used instead. +If the PATTERN evaluates to the empty string, the last +I matched regular expression is used instead. -If used in a context that requires a list value, a pattern match returns a +If the C option is not used, C in a list context returns a list consisting of the subexpressions matched by the parentheses in the -pattern, i.e., (C<$1>, $2, $3...). (Note that here $1 etc. are also set, and -that this differs from Perl 4's behavior.) If the match fails, a null -array is returned. If the match succeeds, but there were no parentheses, -a list value of (1) is returned. +pattern, i.e., (C<$1>, C<$2>, C<$3>...). (Note that here +C<$1> etc. are also set, and +that this differs from Perl 4's behavior.) If there are no parentheses, +the return value is the list C<(1)> for success or C<('')> upon failure. +With parentheses, C<()> is returned upon failure. Examples: @@ -703,12 +806,12 @@ the pattern matched. The C modifier specifies global pattern matching--that is, matching as many times as possible within the string. How it behaves depends on -the context. In a list context, it returns a list of all the +the context. In list context, it returns a list of all the substrings matched by all the 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 a scalar context, C iterates through the string, returning TRUE +In scalar context, C iterates through the string, returning TRUE each time it matches, and FALSE when it eventually runs out of matches. (In other words, it remembers where it left off last time and restarts the search at that point. You can actually find the current match @@ -812,17 +915,53 @@ A double-quoted, interpolated string. =item `STRING` -A string which is interpolated and then executed as a system command. -The collected standard output of the command is returned. In scalar -context, it comes back as a single (potentially multi-line) string. -In list context, returns a list of lines (however you've defined lines -with $/ or $INPUT_RECORD_SEPARATOR). +A string which is (possibly) interpolated and then executed as a system +command with C 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. In list +context, returns a list of lines (however you've defined lines with $/ +or $INPUT_RECORD_SEPARATOR). + +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: - $today = qx{ date }; + $output = `cmd 2>&1`; + +To capture a command's STDOUT but discard its STDERR: + + $output = `cmd 2>/dev/null`; + +To capture a command's STDERR but discard its STDOUT (ordering is +important here): + + $output = `cmd 2>&1 1>/dev/null`; + +To exchange a command's STDOUT and STDERR in order to capture the STDERR +but leave its STDOUT to come out the old STDERR: + + $output = `cmd 3>&1 1>&2 2>&3 3>&-`; + +To read both a command's STDOUT and its STDERR separately, it's easiest +and safest to redirect them separately to files, and then read from those +files when the program is done: + + system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr"); + +Using single-quote as a delimiter protects the command from Perl's +double-quote interpolation, passing it on to the shell instead: + + $perl_info = qx(ps $$); # that's Perl's $$ + $shell_info = qx'ps $$'; # that's the new shell's $$ + +Note that how the 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 L for a clean and safe example of a manual fork() and exec() +to emulate backticks safely. -Note that how the 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. 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 @@ -835,8 +974,14 @@ 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. -Also realize that using this operator frequently leads to unportable -programs. +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 C command under +the POSIX shell is very different from the C command under DOS. +That doesn't mean you should go out of your way to avoid backticks +when they're the right way to get something done. Perl was made to be +a glue language, and one of the things it glues together is commands. +Just understand what you're getting yourself into. See L<"I/O Operators"> for more discussion. @@ -847,13 +992,16 @@ whitespace as the word delimiters. It is exactly equivalent to split(' ', q/STRING/); +This equivalency means that if used in scalar context, you'll get split's +(unfortunate) scalar context behavior, complete with mysterious warnings. + Some frequently seen examples: use POSIX qw( setlocale localeconv ) @EXPORT = qw( foo bar baz ); A common mistake is to try to separate the words with comma or to put -comments into a multi-line qw-string. For this reason the C<-w> +comments into a multi-line C-string. For this reason the C<-w> switch produce warnings if the STRING contains the "," or the "#" character. @@ -865,7 +1013,7 @@ made. Otherwise it returns false (specifically, the empty string). If no string is specified via the C<=~> or C operator, the C<$_> variable is searched and modified. (The string specified with C<=~> must -be a scalar variable, an array element, a hash element, or an assignment +be scalar variable, an array element, a hash element, or an assignment to one of those, i.e., an lvalue.) If the delimiter chosen is single quote, no variable interpolation is @@ -874,9 +1022,9 @@ 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 C option. If the pattern -evaluates to a null string, the last successfully executed regular +evaluates to the empty string, the last successfully executed regular expression is used instead. See L for further explanation on these. -See L for discussion of additional considerations which apply +See L for discussion of additional considerations that apply when C is in effect. Options are: @@ -897,7 +1045,7 @@ 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., C or CfooE/bar/>. A C will cause the -replacement portion to be interpreter as a full-fledged Perl expression +replacement portion to be interpreted as a full-fledged Perl expression and eval()ed right then and there. It is, however, syntax checked at compile-time. @@ -909,9 +1057,9 @@ Examples: s/Login: $foo/Login: $bar/; # run-time pattern - ($foo = $bar) =~ s/this/that/; + ($foo = $bar) =~ s/this/that/; # copy first, then change - $count = ($paragraph =~ s/Mister\b/Mr./g); + $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count $_ = 'abc123xyz'; s/\d+/$&*2/e; # yields 'abc246xyz' @@ -922,18 +1070,27 @@ Examples: s/%(.)/$percent{$1} || $&/ge; # expr now, so /e s/^=(\w+)/&pod($1)/ge; # use function call + # expand variables in $_, but dynamics only, using + # symbolic dereferencing + s/\$(\w+)/${$1}/g; + # /e's can even nest; this will expand - # simple embedded variables in $_ + # any embedded scalar variable (including lexicals) in $_ s/(\$\w+)/$1/eeg; - # Delete C comments. + # Delete (most) C comments. $program =~ s { /\* # Match the opening delimiter. .*? # Match a minimal number of characters. \*/ # Match the closing delimiter. } []gsx; - s/^\s*(.*?)\s*$/$1/; # trim white space + s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively + + for ($variable) { # trim white space in $variable, cheap + s/^\s+//; + s/\s+$//; + } s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields @@ -987,7 +1144,7 @@ character. If the C 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 null, the SEARCHLIST is replicated. +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. @@ -1028,14 +1185,190 @@ an eval(): =back +=head2 Gory details of parsing quoted constructs + +When presented with something which may have several different +interpretations, Perl uses the principle B (expanded to Do What I Mean +- not what I wrote) to pick up the most probable interpretation of the +source. This strategy is so successful that Perl users usually do not +suspect ambivalence of what they write. However, time to time Perl's ideas +differ from what the author meant. + +The target of this section is to clarify the Perl's way of interpreting +quoted constructs. The most frequent reason one may have to want to know the +details discussed in this section is hairy regular expressions. However, the +first steps of parsing are the same for all Perl quoting operators, so here +they are discussed together. + +Some of the passes discussed below are performed concurrently, but as +far as results are the same, we consider them one-by-one. For different +quoting constructs Perl performs different number of passes, from +one to five, but they are always performed in the same order. + +=over + +=item Finding the end + +First pass is finding the end of the quoted construct, be it multichar ender +C<"\nEOF\n"> of C<< construct, C which terminates C construct, +C> which terminates C construct, or C> which terminates a +fileglob started with C<<>. + +When searching for multichar construct no skipping is performed. When +searching for one-char non-matching delimiter, such as C, combinations +C<\\> and C<\/> are skipped. When searching for one-char matching delimiter, +such as C<]>, combinations C<\\>, C<\]> and C<\[> are skipped, and +nested C<[>, C<]> are skipped as well. + +For 3-parts constructs C etc. the search is repeated once more. + +During this search no attension is paid to the semantic of the construct, thus + + "$hash{"$foo/$bar"}" + +or + + m/ + bar # This is not a comment, this slash / terminated m//! + /x + +do not form legal quoted expressions. Note that since the slash which +terminated C was followed by a C, this is not C, +thus C<#> was interpreted as a literal C<#>. + +=item Removal of backslashes before delimiters + +During the second pass the text between the starting delimiter and +the ending delimiter is copied to a safe location, and the C<\> is +removed from combinations consisting of C<\> and delimiter(s) (both starting +and ending delimiter if they differ). + +The removal does not happen for multi-char delimiters. + +Note that the combination C<\\> is left as it was! + +Starting from this step no information about the delimiter(s) is used in the +parsing. + +=item Interpolation + +Next step is interpolation in the obtained delimiter-independent text. +There are many different cases. + +=over + +=item C<<<'EOF'>, C, C, C, C + +No interpolation is performed. + +=item C<''>, C + +The only interpolation is removal of C<\> from pairs C<\\>. + +=item C<"">, C<``>, C, C, C<> + +C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are converted +to corresponding Perl constructs, thus C<"$foo\Qbaz$bar"> is converted to + + $foo . (quotemeta("baz" . $bar)); + +Other combinations of C<\> with following chars are substituted with +appropriate expansions. + +Interpolated scalars and arrays are converted to C and C<.> Perl +constructs, thus C<"'@arr'"> becomes + + "'" . (join $", @arr) . "'"; + +Since all three above steps are performed simultaneously left-to-right, +the is no way to insert a literal C<$> or C<@> inside C<\Q\E> pair: it +cannot be protected by C<\>, since any C<\> (except in C<\E>) is +interpreted as a literal inside C<\Q\E>, and any $ is +interpreted as starting an interpolated scalar. + +Note also that the interpolating code needs to make decision where the +interpolated scalar ends, say, whether C<"a $b -> {c}"> means + + "a " . $b . " -> {c}"; + +or + + "a " . $b -> {c}; + +Most the time the decision is to take the longest possible text which does +not include spaces between components and contains matching braces/brackets. + +=item C, C, C, C, + +Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> and interpolation happens +(almost) as with qq// constructs, but I followed by +other chars is not performed>! Moreover, inside C<(?{BLOCK})> no processing +is performed at all. + +Interpolation has several quirks: $|, $( and $) are not interpolated, and +constructs C<$var[SOMETHING]> are I (by several different estimators) +to be an array element or $var followed by a RE alternative. This is +the place where the notation C<${arr[$bar]}> comes handy: C +is interpreted as an array element -9, not as a regular expression from +variable $arr followed by a digit, which is the interpretation of +C. + +Note that absense of processing of C<\\> creates specific restrictions on the +post-processed text: if the delimeter is C, one cannot get the combination +C<\/> into the result of this step: C will finish the regular expression, +C<\/> will be stripped to C on the previous step, and C<\\/> will be left +as is. Since C is equivalent to C<\/> inside a regular expression, this +does not matter unless the delimiter is special character for RE engine, as +in C, C, or C. + +=back + +This step is the last one for all the constructs except regular expressions, +which are processed further. + +=item Interpolation of regular expressions + +All the previous steps were performed during the compilation of Perl code, +this one happens in run time (though it may be optimized to be calculated +at compile time if appropriate). After all the preprocessing performed +above (and possibly after evaluation if catenation, joining, up/down-casing +and quotemeta()ing are involved) the resulting I is passed to RE +engine for compilation. + +Whatever happens in the RE engine is better be discussed in L, +but for the sake of continuity let us do it here. + +This is the first step where presense of the C switch is relevant. +RE engine scans the string left-to-right, and converts it to a finite +automaton. + +Backslashed chars are either substituted by corresponding literal +strings, or generate special nodes of the finite automaton. Characters +which are special to RE engine generate corresponding nodes. C<(?#...)> +comments are ignored. All the rest is either converted to literal strings +to match, or is ignored (as is whitespace and C<#>-style comments if +C is present). + +Note that the parsing of the construct C<[...]> is performed using +absolutely different rules than the rest of the regular expression. +Similarly, the C<(?{...})> is only checked for matching braces. + +=item Optimization of regular expressions + +This step is listed for compeleteness only. Since it does not change +semantics, details of this step are not documented and are subject +to change. + +=back + =head2 I/O Operators There are several I/O operators you should know about. -A string is enclosed by backticks (grave accents) first undergoes +A string enclosed by backticks (grave accents) first undergoes variable substitution just like a double quoted string. It is then interpreted as a command, and the output of that command is the value -of the pseudo-literal, like in a shell. In a scalar context, a single -string consisting of all the output is returned. In a list context, +of the pseudo-literal, like in a shell. In scalar context, a single +string consisting of all the output is returned. In list context, a list of values is returned, one for each line of output. (You can set C<$/> to use a different line terminator.) The command is executed each time the pseudo-literal is evaluated. The status value of the @@ -1054,17 +1387,35 @@ Ordinarily you must assign that value to a variable, but there is one situation where an automatic assignment happens. I the input symbol is the only thing inside the conditional of a C or C loop, the value is automatically assigned to the variable -C<$_>. The assigned value is then tested to see if it is defined. -(This may seem like an odd thing to you, but you'll use the construct -in almost every Perl script you write.) Anyway, the following lines -are equivalent to each other: +C<$_>. In these loop constructs, the assigned value (whether assignment +is automatic or explicit) is then tested to see if it is defined. +The defined test avoids problems where line has a string value +that would be treated as false by perl e.g. "" or "0" with no trailing +newline. (This may seem like an odd thing to you, but you'll use the +construct in almost every Perl script you write.) Anyway, the following +lines are equivalent to each other: while (defined($_ = )) { print; } + while ($_ = ) { print; } while () { print; } for (;;) { print; } print while defined($_ = ); + print while ($_ = ); print while ; +and this also behaves similarly, but avoids the use of $_ : + + while (my $line = ) { print $line } + +If you really mean such values to terminate the loop they should be +tested for explicitly: + + while (($_ = ) ne '0') { ... } + while () { last unless $_; ... } + +In other boolean contexts, CIE> without explicit C +test or comparison will solicit a warning if C<-w> is in effect. + The filehandles STDIN, STDOUT, and STDERR are predefined. (The filehandles C, C, and C will also work except in packages, where they would be interpreted as local identifiers rather @@ -1080,7 +1431,7 @@ The null filehandle EE is special and can be used to emulate the behavior of B and B. Input from EE comes either from standard input, or from each file listed on the command line. Here's how it works: the first time EE is evaluated, the @ARGV array is -checked, and if it is null, C<$ARGV[0]> is set to "-", which when opened +checked, and if it is empty, C<$ARGV[0]> is set to "-", which when opened gives you standard input. The @ARGV array is then processed as a list of filenames. The loop @@ -1107,10 +1458,19 @@ doesn't work because it treats EARGVE as non-magical.) You can modify @ARGV before the first EE as long as the array ends up containing the list of filenames you really want. Line numbers (C<$.>) continue as if the input were one big happy file. (But see example -under eof() for how to reset line numbers on each file.) +under C for how to reset line numbers on each file.) + +If you want to set @ARGV to your own list of files, go right ahead. +This sets @ARGV to all plain text files if no @ARGV was given: + + @ARGV = grep { -f && -T } glob('*') unless @ARGV; + +You can even set them to pipe commands. For example, this automatically +filters compressed arguments through B: + + @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV; -If you want to set @ARGV to your own list of files, go right ahead. If -you want to pass switches into your script, you can use one of the +If you want to pass switches into your script, you can use one of the Getopts modules or put a loop on the front like this: while ($_ = $ARGV[0], /^-/) { @@ -1118,34 +1478,41 @@ Getopts modules or put a loop on the front like this: last if /^--$/; if (/^-D(.*)/) { $debug = $1 } if (/^-v/) { $verbose++ } - ... # other switches + # ... # other switches } + while (<>) { - ... # code for each line + # ... # code for each line } -The EE symbol will return FALSE only once. If you call it again after -this it will assume you are processing another @ARGV list, and if you -haven't set @ARGV, will input from STDIN. +The EE symbol will return C for end-of-file only once. +If you call it again after this it will assume you are processing another +@ARGV list, and if you haven't set @ARGV, will input from STDIN. If the string inside the angle brackets is a reference to a scalar variable (e.g., E$fooE), then that variable contains the name of the -filehandle to input from, or a reference to the same. For example: +filehandle to input from, or its typeglob, or a reference to the same. For example: $fh = \*STDIN; $line = <$fh>; -If the string inside angle brackets is not a filehandle or a scalar -variable containing a filehandle name or reference, then 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. One level of -$ interpretation is done first, but you can't say C$fooE> -because that's an indirect filehandle as explained in the previous -paragraph. (In older versions of Perl, programmers would insert curly -brackets to force interpretation as a filename glob: C${foo}E>. -These days, it's considered cleaner to call the internal function directly -as C, which is probably the right way to have done it in the -first place.) Example: +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 C$xE> is always a readline from +an indirect handle, but C$hash{key}E> is always a glob. +That's because $x is a simple scalar variable, but C<$hash{key}> is +not--it's a hash element. + +One level of double-quote interpretation is done first, but you can't +say C$fooE> because that's an indirect filehandle as explained +in the previous paragraph. (In older versions of Perl, programmers +would insert curly brackets to force interpretation as a filename glob: +C${foo}E>. These days, it's considered cleaner to call the +internal function directly as C, which is probably the right +way to have done it in the first place.) Example: while (<*.c>) { chmod 0644, $_; @@ -1173,10 +1540,13 @@ long" errors (unless you've installed tcsh(1L) as F). A glob evaluates its (embedded) argument only when it is starting a new list. All values must be read before it will start over. In a list context this isn't important, because you automatically get them all -anyway. In a scalar context, however, the operator returns the next value -each time it is called, or a FALSE value if you've just run out. Again, -FALSE is returned only once. So if you're expecting a single value from -a glob, it is much better to say +anyway. In scalar context, however, the operator returns the next value +each time it is called, or a C value if you've just run out. As +for filehandles an automatic C is generated when the glob +occurs in the test part of a C or C - because legal glob returns +(e.g. a file called F<0>) would otherwise terminate the loop. +Again, C is returned only once. So if you're expecting a single value +from a glob, it is much better to say ($file) = ; @@ -1197,7 +1567,7 @@ to become confused with the indirect filehandle notation. =head2 Constant Folding Like C, Perl does a certain amount of expression evaluation at -compile time, whenever it determines that all of the arguments to an +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 interpretation also happens at @@ -1210,7 +1580,7 @@ and this all reduces to one string internally. Likewise, if you say foreach $file (@filenames) { - if (-s $file > 5 + 100 * 2**16) { ... } + if (-s $file > 5 + 100 * 2**16) { } } the compiler will precompute the number that @@ -1267,7 +1637,7 @@ However, C still has meaning for them. By default, their results are interpreted as unsigned integers. However, if C is in effect, their results are interpreted as signed integers. For example, C<~0> usually evaluates -to a large integral value. However, C is -1. +to a large integral value. However, C is -1 on twos-complement machines. =head2 Floating-point Arithmetic @@ -1276,6 +1646,27 @@ similar ways to provide rounding or truncation at a certain number of decimal places. For rounding to a certain number of digits, sprintf() or printf() is usually the easiest route. +Floating-point numbers are only approximations to what a mathematician +would call real numbers. There are infinitely more reals than floats, +so some corners must be cut. For example: + + printf "%.20g\n", 123456789123456789; + # produces 123456789123456784 + +Testing for exact equality of floating-point equality or inequality is +not a good idea. Here's a (relatively expensive) work-around to compare +whether two floating-point numbers are equal to a particular number of +decimal places. See Knuth, volume II, for a more robust treatment of +this topic. + + sub fp_equal { + my ($X, $Y, $POINTS) = @_; + my ($tX, $tY); + $tX = sprintf("%.${POINTS}g", $X); + $tY = sprintf("%.${POINTS}g", $Y); + return $tX eq $tY; + } + The POSIX module (part of the standard perl distribution) implements ceil(), floor(), and a number of other mathematical and trigonometric functions. The Math::Complex module (part of the standard perl @@ -1288,3 +1679,17 @@ the rounding method used should be specified precisely. In these cases, it probably pays not to trust whichever system rounding is being used by Perl, but to instead implement the rounding function you need yourself. + +=head2 Bigger Numbers + +The standard Math::BigInt and Math::BigFloat modules provide +variable precision arithmetic and overloaded operators. +At the cost of some space and considerable speed, they +avoid the normal pitfalls associated with limited-precision +representations. + + use Math::BigInt; + $x = Math::BigInt->new('123456789123456789'); + print $x * $x; + + # prints +15241578780673678515622620750190521