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diff --git a/contrib/perl5/pod/perlop.pod b/contrib/perl5/pod/perlop.pod deleted file mode 100644 index 9cae3a2..0000000 --- a/contrib/perl5/pod/perlop.pod +++ /dev/null @@ -1,1936 +0,0 @@ -=head1 NAME - -perlop - Perl operators and precedence - -=head1 SYNOPSIS - -Perl operators have the following associativity and precedence, -listed from highest precedence to lowest. Operators borrowed from -C keep the same precedence relationship with each other, even where -C's precedence is slightly screwy. (This makes learning Perl easier -for C folks.) With very few exceptions, these all operate on scalar -values only, not array values. - - left terms and list operators (leftward) - left -> - nonassoc ++ -- - right ** - right ! ~ \ and unary + and - - left =~ !~ - left * / % x - left + - . - left << >> - nonassoc named unary operators - nonassoc < > <= >= lt gt le ge - nonassoc == != <=> eq ne cmp - left & - left | ^ - left && - left || - nonassoc .. ... - right ?: - right = += -= *= etc. - left , => - nonassoc list operators (rightward) - right not - left and - left or xor - -In the following sections, these operators are covered in precedence order. - -Many operators can be overloaded for objects. See L<overload>. - -=head1 DESCRIPTION - -=head2 Terms and List Operators (Leftward) - -A TERM has the highest precedence in Perl. They include variables, -quote and quote-like operators, any expression in parentheses, -and any function whose arguments are parenthesized. Actually, there -aren't really functions in this sense, just list operators and unary -operators behaving as functions because you put parentheses around -the arguments. These are all documented in L<perlfunc>. - -If any list operator (print(), etc.) or any unary operator (chdir(), etc.) -is followed by a left parenthesis as the next token, the operator and -arguments within parentheses are taken to be of highest precedence, -just like a normal function call. - -In the absence of parentheses, the precedence of list operators such as -C<print>, C<sort>, or C<chmod> is either very high or very low depending on -whether you are looking at the left side or the right side of the operator. -For example, in - - @ary = (1, 3, sort 4, 2); - print @ary; # prints 1324 - -the commas on the right of the sort are evaluated before the sort, -but the commas on the left are evaluated after. In other words, -list operators tend to gobble up all arguments that follow, and -then act like a simple TERM with regard to the preceding expression. -Be careful with parentheses: - - # These evaluate exit before doing the print: - print($foo, exit); # Obviously not what you want. - print $foo, exit; # Nor is this. - - # These do the print before evaluating exit: - (print $foo), exit; # This is what you want. - print($foo), exit; # Or this. - print ($foo), exit; # Or even this. - -Also note that - - print ($foo & 255) + 1, "\n"; - -probably doesn't do what you expect at first glance. See -L<Named Unary Operators> for more discussion of this. - -Also parsed as terms are the C<do {}> and C<eval {}> constructs, as -well as subroutine and method calls, and the anonymous -constructors C<[]> and C<{}>. - -See also L<Quote and Quote-like Operators> toward the end of this section, -as well as L<"I/O Operators">. - -=head2 The Arrow Operator - -"C<< -> >>" is an infix dereference operator, just as it is in C -and C++. If the right side is either a C<[...]>, C<{...}>, or a -C<(...)> 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 L<perlreftut> and L<perlref>. - -Otherwise, the right side is a method name or a simple scalar -variable containing either the method name or a subroutine reference, -and the left side must be either an object (a blessed reference) -or a class name (that is, a package name). See L<perlobj>. - -=head2 Auto-increment and Auto-decrement - -"++" and "--" work as in C. That is, if placed before a variable, they -increment or decrement the variable before returning the value, and if -placed after, increment or decrement the variable after returning the value. - -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 -C</^[a-zA-Z]*[0-9]*\z/>, the increment is done as a string, preserving each -character within its range, with carry: - - print ++($foo = '99'); # prints '100' - print ++($foo = 'a0'); # prints 'a1' - print ++($foo = 'Az'); # prints 'Ba' - print ++($foo = 'zz'); # prints 'aaa' - -The auto-decrement operator is not magical. - -=head2 Exponentiation - -Binary "**" is the exponentiation operator. It binds even more -tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is -implemented using C's pow(3) function, which actually works on doubles -internally.) - -=head2 Symbolic Unary Operators - -Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower -precedence version of this. - -Unary "-" performs arithmetic negation if the operand is numeric. If -the operand is an identifier, a string consisting of a minus sign -concatenated with the identifier is returned. Otherwise, if the string -starts with a plus or minus, a string starting with the opposite sign -is returned. One effect of these rules is that C<-bareword> is equivalent -to C<"-bareword">. - -Unary "~" performs bitwise negation, i.e., 1's complement. For -example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and -L<Bitwise String Operators>.) Note that the width of the result is -platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64 -bits wide on a 64-bit platform, so if you are expecting a certain bit -width, remember use the & operator to mask off the excess bits. - -Unary "+" has no effect whatsoever, even on strings. It is useful -syntactically for separating a function name from a parenthesized expression -that would otherwise be interpreted as the complete list of function -arguments. (See examples above under L<Terms and List Operators (Leftward)>.) - -Unary "\" creates a reference to whatever follows it. See L<perlreftut> -and L<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. - -=head2 Binding Operators - -Binary "=~" binds a scalar expression to a pattern match. Certain operations -search or modify the string $_ by default. This operator makes that kind -of operation work on some other string. The right argument is a search -pattern, substitution, or transliteration. The left argument is what is -supposed to be searched, substituted, or transliterated instead of the default -$_. When used in scalar context, the return value generally indicates the -success of the operation. Behavior in list context depends on the particular -operator. See L</"Regexp Quote-Like Operators"> for details. - -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. - -Binary "!~" is just like "=~" except the return value is negated in -the logical sense. - -=head2 Multiplicative Operators - -Binary "*" multiplies two numbers. - -Binary "/" divides two numbers. - -Binary "%" computes the modulus of two numbers. Given integer -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). -Note than when C<use integer> is in scope, "%" gives you direct access -to the modulus operator as implemented by your C compiler. This -operator is not as well defined for negative operands, but it will -execute faster. - -Binary "x" is the repetition operator. In scalar context or if the left -operand is not enclosed in parentheses, it returns a string consisting -of the left operand repeated the number of times specified by the right -operand. In list context, if the left operand is enclosed in -parentheses, it repeats the list. - - print '-' x 80; # print row of dashes - - print "\t" x ($tab/8), ' ' x ($tab%8); # tab over - - @ones = (1) x 80; # a list of 80 1's - @ones = (5) x @ones; # set all elements to 5 - - -=head2 Additive Operators - -Binary "+" returns the sum of two numbers. - -Binary "-" returns the difference of two numbers. - -Binary "." concatenates two strings. - -=head2 Shift Operators - -Binary "<<" returns the value of its left argument shifted left by the -number of bits specified by the right argument. Arguments should be -integers. (See also L<Integer Arithmetic>.) - -Binary ">>" returns the value of its left argument shifted right by -the number of bits specified by the right argument. Arguments should -be integers. (See also L<Integer Arithmetic>.) - -=head2 Named Unary Operators - -The various named unary operators are treated as functions with one -argument, with optional parentheses. These include the filetest -operators, like C<-f>, C<-M>, etc. See L<perlfunc>. - -If any list operator (print(), etc.) or any unary operator (chdir(), etc.) -is followed by a left parenthesis as the next token, the operator and -arguments within parentheses are taken to be of highest precedence, -just like a normal function call. For example, -because named unary operators are higher precedence than ||: - - chdir $foo || die; # (chdir $foo) || die - chdir($foo) || die; # (chdir $foo) || die - chdir ($foo) || die; # (chdir $foo) || die - chdir +($foo) || die; # (chdir $foo) || die - -but, because * is higher precedence than named operators: - - chdir $foo * 20; # chdir ($foo * 20) - chdir($foo) * 20; # (chdir $foo) * 20 - chdir ($foo) * 20; # (chdir $foo) * 20 - chdir +($foo) * 20; # chdir ($foo * 20) - - rand 10 * 20; # rand (10 * 20) - rand(10) * 20; # (rand 10) * 20 - rand (10) * 20; # (rand 10) * 20 - rand +(10) * 20; # rand (10 * 20) - -See also L<"Terms and List Operators (Leftward)">. - -=head2 Relational Operators - -Binary "<" returns true if the left argument is numerically less than -the right argument. - -Binary ">" returns true if the left argument is numerically greater -than the right argument. - -Binary "<=" returns true if the left argument is numerically less than -or equal to the right argument. - -Binary ">=" returns true if the left argument is numerically greater -than or equal to the right argument. - -Binary "lt" returns true if the left argument is stringwise less than -the right argument. - -Binary "gt" returns true if the left argument is stringwise greater -than the right argument. - -Binary "le" returns true if the left argument is stringwise less than -or equal to the right argument. - -Binary "ge" returns true if the left argument is stringwise greater -than or equal to the right argument. - -=head2 Equality Operators - -Binary "==" returns true if the left argument is numerically equal to -the right argument. - -Binary "!=" returns true if the left argument is numerically not equal -to the right argument. - -Binary "<=>" returns -1, 0, or 1 depending on whether the left -argument is numerically less than, equal to, or greater than the right -argument. If your platform supports NaNs (not-a-numbers) as numeric -values, using them with "<=>" returns undef. NaN is not "<", "==", ">", -"<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN -returns true, as does NaN != anything else. If your platform doesn't -support NaNs then NaN is just a string with numeric value 0. - - perl -le '$a = NaN; print "No NaN support here" if $a == $a' - perl -le '$a = NaN; print "NaN support here" if $a != $a' - -Binary "eq" returns true if the left argument is stringwise equal to -the right argument. - -Binary "ne" returns true if the left argument is stringwise not equal -to the right argument. - -Binary "cmp" returns -1, 0, or 1 depending on whether the left -argument is stringwise less than, equal to, or greater than the right -argument. - -"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified -by the current locale if C<use locale> is in effect. See L<perllocale>. - -=head2 Bitwise And - -Binary "&" returns its operators ANDed together bit by bit. -(See also L<Integer Arithmetic> and L<Bitwise String Operators>.) - -=head2 Bitwise Or and Exclusive Or - -Binary "|" returns its operators ORed together bit by bit. -(See also L<Integer Arithmetic> and L<Bitwise String Operators>.) - -Binary "^" returns its operators XORed together bit by bit. -(See also L<Integer Arithmetic> and L<Bitwise String Operators>.) - -=head2 C-style Logical And - -Binary "&&" performs a short-circuit logical AND operation. That is, -if the left operand is false, the right operand is not even evaluated. -Scalar or list context propagates down to the right operand if it -is evaluated. - -=head2 C-style Logical Or - -Binary "||" performs a short-circuit logical OR operation. That is, -if the left operand is true, the right operand is not even evaluated. -Scalar or list context propagates down to the right operand if it -is evaluated. - -The C<||> and C<&&> operators differ from C's in that, rather than returning -0 or 1, they return the last value evaluated. Thus, a reasonably portable -way to find out the home directory (assuming it's not "0") might be: - - $home = $ENV{'HOME'} || $ENV{'LOGDIR'} || - (getpwuid($<))[7] || die "You're homeless!\n"; - -In particular, this means that you shouldn't use this -for selecting between two aggregates for assignment: - - @a = @b || @c; # this is wrong - @a = scalar(@b) || @c; # really meant this - @a = @b ? @b : @c; # this works fine, though - -As more readable alternatives to C<&&> and C<||> when used for -control flow, Perl provides C<and> and C<or> operators (see below). -The short-circuit behavior is identical. The precedence of "and" and -"or" is much lower, however, so that you can safely use them after a -list operator without the need for parentheses: - - unlink "alpha", "beta", "gamma" - or gripe(), next LINE; - -With the C-style operators that would have been written like this: - - unlink("alpha", "beta", "gamma") - || (gripe(), next LINE); - -Using "or" for assignment is unlikely to do what you want; see below. - -=head2 Range Operators - -Binary ".." is the range operator, which is really two different -operators depending on the context. In list context, it returns an -array 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 -C<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 C<foreach> loops, but older -versions of Perl might burn a lot of memory when you write something -like this: - - for (1 .. 1_000_000) { - # code - } - -In scalar context, ".." returns a boolean value. The operator is -bistable, like a flip-flop, and emulates the line-range (comma) operator -of B<sed>, B<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, I<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 B<awk>), but it still returns true once. -If you don't want it to test the right operand till the next -evaluation, as in B<sed>, just use three dots ("...") instead of -two. In all other regards, "..." behaves just like ".." does. - -The right operand is not evaluated while the operator is in the -"false" state, and the left operand is not evaluated while the -operator is in the "true" state. The precedence is a little lower -than || and &&. The value returned is either the empty string for -false, or a sequence number (beginning with 1) for true. The -sequence number is reset for each range encountered. The final -sequence number in a range has the string "E0" appended to it, which -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 C<$.> variable, the -current line number. Examples: - -As a scalar operator: - - if (101 .. 200) { print; } # print 2nd hundred lines - next line if (1 .. /^$/); # 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 list context) makes use of the magical -auto-increment algorithm if the operands are strings. You -can say - - @alphabet = ('A' .. 'Z'); - -to get all normal letters of the alphabet, or - - $hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15]; - -to get a hexadecimal digit, or - - @z2 = ('01' .. '31'); print $z2[$mday]; - -to get dates with leading zeros. If the final value specified is not -in the sequence that the magical increment would produce, the sequence -goes until the next value would be longer than the final value -specified. - -=head2 Conditional Operator - -Ternary "?:" is the conditional operator, just as in C. It works much -like an if-then-else. If the argument before the ? is true, the -argument before the : is returned, otherwise the argument after the : -is returned. For example: - - printf "I have %d dog%s.\n", $n, - ($n == 1) ? '' : "s"; - -Scalar or list context propagates downward into the 2nd -or 3rd argument, whichever is selected. - - $a = $ok ? $b : $c; # get a scalar - @a = $ok ? @b : @c; # get an array - $a = $ok ? @b : @c; # oops, that's just a count! - -The operator may be assigned to if both the 2nd and 3rd arguments are -legal lvalues (meaning that you can assign to them): - - ($a_or_b ? $a : $b) = $c; - -Because this operator produces an assignable result, using assignments -without parentheses will get you in trouble. For example, this: - - $a % 2 ? $a += 10 : $a += 2 - -Really means this: - - (($a % 2) ? ($a += 10) : $a) += 2 - -Rather than this: - - ($a % 2) ? ($a += 10) : ($a += 2) - -That should probably be written more simply as: - - $a += ($a % 2) ? 10 : 2; - -=head2 Assignment Operators - -"=" is the ordinary assignment operator. - -Assignment operators work as in C. That is, - - $a += 2; - -is equivalent to - - $a = $a + 2; - -although without duplicating any side effects that dereferencing the lvalue -might trigger, such as from tie(). Other assignment operators work similarly. -The following are recognized: - - **= += *= &= <<= &&= - -= /= |= >>= ||= - .= %= ^= - x= - -Although these are grouped by family, they all have the precedence -of assignment. - -Unlike in C, the scalar assignment operator produces a valid lvalue. -Modifying an assignment is equivalent to doing the assignment and -then modifying the variable that was assigned to. This is useful -for modifying a copy of something, like this: - - ($tmp = $global) =~ tr [A-Z] [a-z]; - -Likewise, - - ($a += 2) *= 3; - -is equivalent to - - $a += 2; - $a *= 3; - -Similarly, a list assignment in list context produces the list of -lvalues assigned to, and a list assignment in scalar context returns -the number of elements produced by the expression on the right hand -side of the assignment. - -=head2 Comma Operator - -Binary "," is the comma operator. In scalar context it evaluates -its left argument, throws that value away, then evaluates its right -argument and returns that value. This is just like C's comma operator. - -In list context, it's just the list argument separator, and inserts -both its arguments into the list. - -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. - -=head2 List Operators (Rightward) - -On the right side of a list operator, it has very low precedence, -such that it controls all comma-separated expressions found there. -The only operators with lower precedence are the logical operators -"and", "or", and "not", which may be used to evaluate calls to list -operators without the need for extra parentheses: - - open HANDLE, "filename" - or die "Can't open: $!\n"; - -See also discussion of list operators in L<Terms and List Operators (Leftward)>. - -=head2 Logical Not - -Unary "not" returns the logical negation of the expression to its right. -It's the equivalent of "!" except for the very low precedence. - -=head2 Logical And - -Binary "and" returns the logical conjunction of the two surrounding -expressions. It's equivalent to && except for the very low -precedence. This means that it short-circuits: i.e., the right -expression is evaluated only if the left expression is true. - -=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 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. - -=head2 C Operators Missing From Perl - -Here is what C has that Perl doesn't: - -=over 8 - -=item unary & - -Address-of operator. (But see the "\" operator for taking a reference.) - -=item unary * - -Dereference-address operator. (Perl's prefix dereferencing -operators are typed: $, @, %, and &.) - -=item (TYPE) - -Type-casting operator. - -=back - -=head2 Quote and Quote-like Operators - -While we usually think of quotes as literal values, in Perl they -function as operators, providing various kinds of interpolating and -pattern matching capabilities. Perl provides customary quote characters -for these behaviors, but also provides a way for you to choose your -quote character for any of them. In the following table, a C<{}> represents -any pair of delimiters you choose. - - Customary Generic Meaning Interpolates - '' q{} Literal no - "" qq{} Literal yes - `` qx{} Command yes (unless '' is delimiter) - qw{} Word list no - // m{} Pattern match yes (unless '' is delimiter) - qr{} Pattern yes (unless '' is delimiter) - s{}{} Substitution yes (unless '' is delimiter) - tr{}{} Transliteration no (but see below) - -Non-bracketing delimiters use the same character fore and aft, but the four -sorts of brackets (round, angle, square, curly) will all nest, which means -that - - q{foo{bar}baz} - -is the same as - - 'foo{bar}baz' - -Note, however, that this does not always work for quoting Perl code: - - $s = q{ if($a eq "}") ... }; # WRONG - -is a syntax error. The C<Text::Balanced> module on CPAN is able to do this -properly. - -There can be whitespace between the operator and the quoting -characters, except when C<#> is being used as the quoting character. -C<q#foo#> is parsed as the string C<foo>, while C<q #foo#> is the -operator C<q> followed by a comment. Its argument will be taken -from the next line. This allows you to write: - - s {foo} # Replace foo - {bar} # with bar. - -For constructs that do interpolate, variables beginning with "C<$>" -or "C<@>" are interpolated, as are the following escape sequences. Within -a transliteration, the first eleven of these sequences may be used. - - \t tab (HT, TAB) - \n newline (NL) - \r return (CR) - \f form feed (FF) - \b backspace (BS) - \a alarm (bell) (BEL) - \e escape (ESC) - \033 octal char (ESC) - \x1b hex char (ESC) - \x{263a} wide hex char (SMILEY) - \c[ control char (ESC) - \N{name} named char - - \l lowercase next char - \u uppercase next char - \L lowercase till \E - \U uppercase till \E - \E end case modification - \Q quote non-word characters till \E - -If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u> -and C<\U> is taken from the current locale. See L<perllocale>. For -documentation of C<\N{name}>, see L<charnames>. - -All systems use the virtual C<"\n"> to represent a line terminator, -called a "newline". There is no such thing as an unvarying, physical -newline character. It is only an illusion that the operating system, -device drivers, C libraries, and Perl all conspire to preserve. Not all -systems read C<"\r"> as ASCII CR and C<"\n"> as ASCII LF. For example, -on a Mac, these are reversed, and on systems without line terminator, -printing C<"\n"> may emit no actual data. In general, use C<"\n"> when -you mean a "newline" for your system, but use the literal ASCII when you -need an exact character. For example, most networking protocols expect -and prefer a CR+LF (C<"\015\012"> or C<"\cM\cJ">) for line terminators, -and although they often accept just C<"\012">, they seldom tolerate just -C<"\015">. If you get in the habit of using C<"\n"> for networking, -you may be burned some day. - -You cannot include a literal C<$> or C<@> within a C<\Q> sequence. -An unescaped C<$> or C<@> interpolates the corresponding variable, -while escaping will cause the literal string C<\$> to be inserted. -You'll need to write something like C<m/\Quser\E\@\Qhost/>. - -Patterns are subject to an additional level of interpretation as a -regular expression. This is done as a second pass, after variables are -interpolated, so that regular expressions may be incorporated into the -pattern from the variables. If this is not what you want, use C<\Q> to -interpolate a variable literally. - -Apart from the behavior described above, Perl does not expand -multiple levels of interpolation. In particular, contrary to the -expectations of shell programmers, back-quotes do I<NOT> interpolate -within double quotes, nor do single quotes impede evaluation of -variables when used within double quotes. - -=head2 Regexp Quote-Like Operators - -Here are the quote-like operators that apply to pattern -matching and related activities. - -=over 8 - -=item ?PATTERN? - -This is just like the C</pattern/> search, except that it matches only -once between calls to the reset() operator. This is a useful -optimization when you want to see only the first occurrence of -something in each file of a set of files, for instance. Only C<??> -patterns local to the current package are reset. - - while (<>) { - if (?^$?) { - # blank line between header and body - } - } continue { - reset if eof; # clear ?? status for next file - } - -This usage is vaguely deprecated, which means it just might possibly -be removed in some distant future version of Perl, perhaps somewhere -around the year 2168. - -=item m/PATTERN/cgimosx - -=item /PATTERN/cgimosx - -Searches a string for a pattern match, and in scalar context returns -true if it succeeds, false if it fails. If no string is specified -via the C<=~> or C<!~> operator, the $_ string is searched. (The -string specified with C<=~> need not be an lvalue--it may be the -result of an expression evaluation, but remember the C<=~> binds -rather tightly.) See also L<perlre>. See L<perllocale> for -discussion of additional considerations that apply when C<use locale> -is in effect. - -Options are: - - c Do not reset search position on a failed match when /g is in effect. - g Match globally, i.e., find all occurrences. - i Do case-insensitive pattern matching. - m Treat string as multiple lines. - o Compile pattern only once. - s Treat string as single line. - x Use extended regular expressions. - -If "/" is the delimiter then the initial C<m> is optional. With the C<m> -you can use any pair of non-alphanumeric, non-whitespace characters -as delimiters. This is particularly useful for matching path names -that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is -the delimiter, then the match-only-once rule of C<?PATTERN?> applies. -If "'" is the delimiter, no interpolation is performed on the PATTERN. - -PATTERN may contain variables, which will be interpolated (and the -pattern recompiled) every time the pattern search is evaluated, except -for when the delimiter is a single quote. (Note that C<$(>, C<$)>, and -C<$|> are not interpolated because they look like end-of-string tests.) -If you want such a pattern to be compiled only once, add a C</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 C</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 L<"qr/STRING/imosx">. - -If the PATTERN evaluates to the empty string, the last -I<successfully> matched regular expression is used instead. - -If the C</g> option is not used, C<m//> in list context returns a -list consisting of the subexpressions matched by the parentheses in the -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.) When there are -no parentheses in the pattern, the return value is the list C<(1)> for -success. With or without parentheses, an empty list is returned upon -failure. - -Examples: - - open(TTY, '/dev/tty'); - <TTY> =~ /^y/i && foo(); # do foo if desired - - if (/Version: *([0-9.]*)/) { $version = $1; } - - next if m#^/usr/spool/uucp#; - - # poor man's grep - $arg = shift; - while (<>) { - print if /$arg/o; # compile only once - } - - if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/)) - -This last example splits $foo into the first two words and the -remainder of the line, and assigns those three fields to $F1, $F2, and -$Etc. The conditional is true if any variables were assigned, i.e., if -the pattern matched. - -The C</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 C<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 L<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</c> modifier (e.g. C<m//gc>). Modifying the target -string also resets the search position. - -You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a -zero-width assertion that matches the exact position where the previous -C<m//g>, if any, left off. Without the C</g> modifier, the C<\G> assertion -still anchors at pos(), but the match is of course only attempted once. -Using C<\G> without C</g> on a target string that has not previously had a -C</g> match applied to it is the same as using the C<\A> assertion to match -the beginning of the string. - -Examples: - - # list context - ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g); - - # scalar context - $/ = ""; - while (defined($paragraph = <>)) { - while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) { - $sentences++; - } - } - print "$sentences\n"; - - # using m//gc with \G - $_ = "ppooqppqq"; - while ($i++ < 2) { - print "1: '"; - print $1 while /(o)/gc; print "', pos=", pos, "\n"; - print "2: '"; - print $1 if /\G(q)/gc; print "', pos=", pos, "\n"; - print "3: '"; - print $1 while /(p)/gc; print "', pos=", pos, "\n"; - } - print "Final: '$1', pos=",pos,"\n" if /\G(.)/; - -The last example should print: - - 1: 'oo', pos=4 - 2: 'q', pos=5 - 3: 'pp', pos=7 - 1: '', pos=7 - 2: 'q', pos=8 - 3: '', pos=8 - Final: 'q', pos=8 - -Notice that the final match matched C<q> instead of C<p>, which a match -without the C<\G> anchor would have done. Also note that the final match -did not update C<pos> -- C<pos> is only updated on a C</g> match. If the -final match did indeed match C<p>, it's a good bet that you're running an -older (pre-5.6.0) Perl. - -A useful idiom for C<lex>-like scanners is C</\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. - - $_ = <<'EOL'; - $url = new URI::URL "http://www/"; die if $url eq "xXx"; - EOL - LOOP: - { - print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc; - print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc; - print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc; - print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc; - print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc; - print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc; - print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc; - print ". That's all!\n"; - } - -Here is the output (split into several lines): - - line-noise lowercase line-noise lowercase UPPERCASE line-noise - UPPERCASE line-noise lowercase line-noise lowercase line-noise - lowercase lowercase line-noise lowercase lowercase line-noise - MiXeD line-noise. That's all! - -=item q/STRING/ - -=item C<'STRING'> - -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. - - $foo = q!I said, "You said, 'She said it.'"!; - $bar = q('This is it.'); - $baz = '\n'; # a two-character string - -=item qq/STRING/ - -=item "STRING" - -A double-quoted, interpolated string. - - $_ .= qq - (*** The previous line contains the naughty word "$1".\n) - if /\b(tcl|java|python)\b/i; # :-) - $baz = "\n"; # a one-character string - -=item qr/STRING/imosx - -This operator quotes (and possibly compiles) its I<STRING> as a regular -expression. I<STRING> is interpolated the same way as I<PATTERN> -in C<m/PATTERN/>. If "'" is used as the delimiter, no interpolation -is done. Returns a Perl value which may be used instead of the -corresponding C</STRING/imosx> expression. - -For example, - - $rex = qr/my.STRING/is; - s/$rex/foo/; - -is equivalent to - - s/my.STRING/foo/is; - -The result may be used as a subpattern in a match: - - $re = qr/$pattern/; - $string =~ /foo${re}bar/; # can be interpolated in other patterns - $string =~ $re; # or used standalone - $string =~ /$re/; # or this way - -Since Perl may compile the pattern at the moment of execution of qr() -operator, using qr() may have speed advantages in some situations, -notably if the result of qr() is used standalone: - - sub match { - my $patterns = shift; - my @compiled = map qr/$_/i, @$patterns; - grep { - my $success = 0; - foreach my $pat (@compiled) { - $success = 1, last if /$pat/; - } - $success; - } @_; - } - -Precompilation of the pattern into an internal representation at -the moment of qr() avoids a need to recompile the pattern every -time a match C</$pat/> is attempted. (Perl has many other internal -optimizations, but none would be triggered in the above example if -we did not use qr() operator.) - -Options are: - - i Do case-insensitive pattern matching. - m Treat string as multiple lines. - o Compile pattern only once. - s Treat string as single line. - x Use extended regular expressions. - -See L<perlre> for additional information on valid syntax for STRING, and -for a detailed look at the semantics of regular expressions. - -=item qx/STRING/ - -=item `STRING` - -A string which is (possibly) interpolated and then executed as a -system command with C</bin/sh> or its equivalent. Shell wildcards, -pipes, and redirections will be honored. The collected standard -output of the command is returned; standard error is unaffected. In -scalar context, it comes back as a single (potentially multi-line) -string, or undef if the command failed. In list context, returns a -list of lines (however you've defined lines with $/ or -$INPUT_RECORD_SEPARATOR), or an empty list if the command failed. - -Because backticks do not affect standard error, use shell file descriptor -syntax (assuming the shell supports this) if you care to address this. -To capture a command's STDERR and STDOUT together: - - $output = `cmd 2>&1`; - -To capture a command's STDOUT but discard its STDERR: - - $output = `cmd 2>/dev/null`; - -To capture a command's STDERR but discard its STDOUT (ordering is -important here): - - $output = `cmd 2>&1 1>/dev/null`; - -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 $$'; # that's the new shell's $$ - -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 L<perlsec> for a clean and safe example of a manual fork() and exec() -to emulate backticks safely. - -On some platforms (notably DOS-like ones), the shell may not be -capable of dealing with multiline commands, so putting newlines in -the string may not get you what you want. You may be able to evaluate -multiple commands in a single line by separating them with the command -separator character, if your shell supports that (e.g. C<;> on many Unix -shells; C<&> on the Windows NT C<cmd> shell). - -Beginning with v5.6.0, Perl will attempt to flush all files opened for -output before starting the child process, but this may not be supported -on some platforms (see L<perlport>). To be safe, you may need to set -C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of -C<IO::Handle> on any open handles. - -Beware that some command shells may place restrictions on the length -of the command line. You must ensure your strings don't exceed this -limit after any necessary interpolations. See the platform-specific -release notes for more details about your particular environment. - -Using this operator can lead to programs that are difficult to port, -because the shell commands called vary between systems, and may in -fact not be present at all. As one example, the C<type> command under -the POSIX shell is very different from the C<type> command under DOS. -That doesn't mean you should go out of your way to avoid backticks -when they're the right way to get something done. Perl was made to be -a glue language, and one of the things it glues together is commands. -Just understand what you're getting yourself into. - -See L<"I/O Operators"> for more discussion. - -=item qw/STRING/ - -Evaluates to a list of the words extracted out of STRING, using embedded -whitespace as the word delimiters. It can be understood as being roughly -equivalent to: - - split(' ', q/STRING/); - -the difference being that it generates a real list at compile time. So -this expression: - - qw(foo bar baz) - -is semantically equivalent to the list: - - 'foo', 'bar', 'baz' - -Some frequently seen examples: - - use POSIX qw( setlocale localeconv ) - @EXPORT = qw( foo bar baz ); - -A common mistake is to try to separate the words with comma or to -put comments into a multi-line C<qw>-string. For this reason, the -C<use warnings> pragma and the B<-w> switch (that is, the C<$^W> variable) -produces warnings if the STRING contains the "," or the "#" character. - -=item s/PATTERN/REPLACEMENT/egimosx - -Searches a string for a pattern, and if found, replaces that pattern -with the replacement text and returns the number of substitutions -made. Otherwise it returns false (specifically, the empty string). - -If no string is specified via the C<=~> or C<!~> operator, the C<$_> -variable is searched and modified. (The string specified with C<=~> must -be scalar variable, an array element, a hash element, or an assignment -to one of those, i.e., an lvalue.) - -If the delimiter chosen is a single quote, no interpolation is -done on either the PATTERN or the REPLACEMENT. Otherwise, if the -PATTERN contains a $ that looks like a variable rather than an -end-of-string test, the variable will be interpolated into the pattern -at run-time. If you want the pattern compiled only once the first time -the variable is interpolated, use the C</o> option. If the pattern -evaluates to the empty string, the last successfully executed regular -expression is used instead. See L<perlre> for further explanation on these. -See L<perllocale> for discussion of additional considerations that apply -when C<use locale> is in effect. - -Options are: - - e Evaluate the right side as an expression. - g Replace globally, i.e., all occurrences. - i Do case-insensitive pattern matching. - m Treat string as multiple lines. - o Compile pattern only once. - s Treat string as single line. - x Use extended regular expressions. - -Any non-alphanumeric, non-whitespace delimiter may replace the -slashes. If single quotes are used, no interpretation is done on the -replacement string (the C</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., -C<s(foo)(bar)> or C<< s<foo>/bar/ >>. A C</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 C<e> modifier will cause the replacement portion -to be C<eval>ed before being run as a Perl expression. - -Examples: - - s/\bgreen\b/mauve/g; # don't change wintergreen - - $path =~ s|/usr/bin|/usr/local/bin|; - - s/Login: $foo/Login: $bar/; # run-time pattern - - ($foo = $bar) =~ s/this/that/; # copy first, then change - - $count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count - - $_ = 'abc123xyz'; - s/\d+/$&*2/e; # yields 'abc246xyz' - s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz' - s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz' - - s/%(.)/$percent{$1}/g; # change percent escapes; no /e - s/%(.)/$percent{$1} || $&/ge; # expr now, so /e - s/^=(\w+)/&pod($1)/ge; # use function call - - # expand variables in $_, but dynamics only, using - # symbolic dereferencing - s/\$(\w+)/${$1}/g; - - # Add one to the value of any numbers in the string - s/(\d+)/1 + $1/eg; - - # This will expand any embedded scalar variable - # (including lexicals) in $_ : First $1 is interpolated - # to the variable name, and then evaluated - s/(\$\w+)/$1/eeg; - - # Delete (most) C comments. - $program =~ s { - /\* # Match the opening delimiter. - .*? # Match a minimal number of characters. - \*/ # Match the closing delimiter. - } []gsx; - - s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively - - for ($variable) { # trim white space in $variable, cheap - s/^\s+//; - s/\s+$//; - } - - s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields - -Note the use of $ instead of \ in the last example. Unlike -B<sed>, we use the \<I<digit>> form in only the left hand side. -Anywhere else it's $<I<digit>>. - -Occasionally, you can't use just a C</g> to get all the changes -to occur that you might want. Here are two common cases: - - # put commas in the right places in an integer - 1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; - - # expand tabs to 8-column spacing - 1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e; - -=item tr/SEARCHLIST/REPLACEMENTLIST/cds - -=item y/SEARCHLIST/REPLACEMENTLIST/cds - -Transliterates all occurrences of the characters found in the search list -with the corresponding character in the replacement list. It returns -the number of characters replaced or deleted. If no string is -specified via the =~ or !~ operator, the $_ string is transliterated. (The -string specified with =~ must be a scalar variable, an array element, a -hash element, or an assignment to one of those, i.e., an lvalue.) - -A character range may be specified with a hyphen, so C<tr/A-J/0-9/> -does the same replacement as C<tr/ACEGIBDFHJ/0246813579/>. -For B<sed> devotees, C<y> is provided as a synonym for C<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., C<tr[A-Z][a-z]> or C<tr(+\-*/)/ABCD/>. - -Note that C<tr> does B<not> do regular expression character classes -such as C<\d> or C<[:lower:]>. The <tr> operator is not equivalent to -the tr(1) utility. If you want to map strings between lower/upper -cases, see L<perlfunc/lc> and L<perlfunc/uc>, and in general consider -using the C<s> operator if you need regular expressions. - -Note also that the whole range idea is rather unportable between -character sets--and even within character sets they may cause results -you probably didn't expect. A sound principle is to use only ranges -that begin from and end at either alphabets of equal case (a-e, A-E), -or digits (0-4). Anything else is unsafe. If in doubt, spell out the -character sets in full. - -Options: - - c Complement the SEARCHLIST. - d Delete found but unreplaced characters. - s Squash duplicate replaced characters. - -If the C</c> modifier is specified, the SEARCHLIST character set -is complemented. If the C</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 -B<tr> programs, which delete anything they find in the SEARCHLIST, -period.) If the C</s> modifier is specified, sequences of characters -that were transliterated to the same character are squashed down -to a single instance of the character. - -If the C</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. - -Examples: - - $ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case - - $cnt = tr/*/*/; # count the stars in $_ - - $cnt = $sky =~ tr/*/*/; # count the stars in $sky - - $cnt = tr/0-9//; # count the digits in $_ - - tr/a-zA-Z//s; # bookkeeper -> bokeper - - ($HOST = $host) =~ tr/a-z/A-Z/; - - tr/a-zA-Z/ /cs; # change non-alphas to single space - - tr [\200-\377] - [\000-\177]; # delete 8th bit - -If multiple transliterations are given for a character, only the -first one is used: - - tr/AAA/XYZ/ - -will transliterate any A to X. - -Because the transliteration table is built at compile time, neither -the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote -interpolation. That means that if you want to use variables, you -must use an eval(): - - eval "tr/$oldlist/$newlist/"; - die $@ if $@; - - eval "tr/$oldlist/$newlist/, 1" or die $@; - -=back - -=head2 Gory details of parsing quoted constructs - -When presented with something that might have several different -interpretations, Perl uses the B<DWIM> (that's "Do What I Mean") -principle to pick the most probable interpretation. This strategy -is so successful that Perl programmers often do not suspect the -ambivalence of what they write. But from time to time, Perl's -notions differ substantially from what the author honestly meant. - -This section hopes to clarify how Perl handles quoted constructs. -Although the most common reason to learn this is to unravel labyrinthine -regular expressions, because the initial steps of parsing are the -same for all quoting operators, they are all discussed together. - -The most important Perl parsing rule is the first one discussed -below: when processing a quoted construct, Perl first finds the end -of that construct, then interprets its contents. If you understand -this rule, you may skip the rest of this section on the first -reading. The other rules are likely to contradict the user's -expectations much less frequently than this first one. - -Some passes discussed below are performed concurrently, but because -their results are the same, we consider them individually. For different -quoting constructs, Perl performs different numbers of passes, from -one to five, but these passes are always performed in the same order. - -=over 4 - -=item Finding the end - -The first pass is finding the end of the quoted construct, whether -it be a multicharacter delimiter C<"\nEOF\n"> in the C<<<EOF> -construct, a C</> that terminates a C<qq//> construct, a C<]> which -terminates C<qq[]> construct, or a C<< > >> which terminates a -fileglob started with C<< < >>. - -When searching for single-character non-pairing delimiters, such -as C</>, combinations of C<\\> and C<\/> are skipped. However, -when searching for single-character pairing delimiter like C<[>, -combinations of C<\\>, C<\]>, and C<\[> are all skipped, and nested -C<[>, C<]> are skipped as well. When searching for multicharacter -delimiters, nothing is skipped. - -For constructs with three-part delimiters (C<s///>, C<y///>, and -C<tr///>), the search is repeated once more. - -During this search no attention is paid to the semantics of the construct. -Thus: - - "$hash{"$foo/$bar"}" - -or: - - m/ - bar # NOT a comment, this slash / terminated m//! - /x - -do not form legal quoted expressions. The quoted part ends on the -first C<"> and C</>, and the rest happens to be a syntax error. -Because the slash that terminated C<m//> was followed by a C<SPACE>, -the example above is not C<m//x>, but rather C<m//> with no C</x> -modifier. So the embedded C<#> is interpreted as a literal C<#>. - -=item Removal of backslashes before delimiters - -During the second pass, text between the starting and ending -delimiters is copied to a safe location, and the C<\> is removed -from combinations consisting of C<\> and delimiter--or delimiters, -meaning both starting and ending delimiters will should these differ. -This removal does not happen for multi-character delimiters. -Note that the combination C<\\> is left intact, just as it was. - -Starting from this step no information about the delimiters is -used in parsing. - -=item Interpolation - -The next step is interpolation in the text obtained, which is now -delimiter-independent. There are four different cases. - -=over 4 - -=item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///> - -No interpolation is performed. - -=item C<''>, C<q//> - -The only interpolation is removal of C<\> from pairs C<\\>. - -=item C<"">, C<``>, C<qq//>, C<qx//>, C<< <file*glob> >> - -C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are -converted to corresponding Perl constructs. Thus, C<"$foo\Qbaz$bar"> -is converted to C<$foo . (quotemeta("baz" . $bar))> internally. -The other combinations are replaced with appropriate expansions. - -Let it be stressed that I<whatever falls between C<\Q> and C<\E>> -is interpolated in the usual way. Something like C<"\Q\\E"> has -no C<\E> inside. instead, it has C<\Q>, C<\\>, and C<E>, so the -result is the same as for C<"\\\\E">. As a general rule, backslashes -between C<\Q> and C<\E> may lead to counterintuitive results. So, -C<"\Q\t\E"> is converted to C<quotemeta("\t")>, which is the same -as C<"\\\t"> (since TAB is not alphanumeric). Note also that: - - $str = '\t'; - return "\Q$str"; - -may be closer to the conjectural I<intention> of the writer of C<"\Q\t\E">. - -Interpolated scalars and arrays are converted internally to the C<join> and -C<.> catenation operations. Thus, C<"$foo XXX '@arr'"> becomes: - - $foo . " XXX '" . (join $", @arr) . "'"; - -All operations above are performed simultaneously, left to right. - -Because the result of C<"\Q STRING \E"> has all metacharacters -quoted, there is no way to insert a literal C<$> or C<@> inside a -C<\Q\E> pair. If protected by C<\>, C<$> will be quoted to became -C<"\\\$">; if not, it is interpreted as the start of an interpolated -scalar. - -Note also that the interpolation code needs to make a decision on -where the interpolated scalar ends. For instance, whether -C<< "a $b -> {c}" >> really means: - - "a " . $b . " -> {c}"; - -or: - - "a " . $b -> {c}; - -Most of the time, the longest possible text that does not include -spaces between components and which contains matching braces or -brackets. because the outcome may be determined by voting based -on heuristic estimators, the result is not strictly predictable. -Fortunately, it's usually correct for ambiguous cases. - -=item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>, - -Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l>, and interpolation -happens (almost) as with C<qq//> constructs, but the substitution -of C<\> followed by RE-special chars (including C<\>) is not -performed. Moreover, inside C<(?{BLOCK})>, C<(?# comment )>, and -a C<#>-comment in a C<//x>-regular expression, no processing is -performed whatsoever. This is the first step at which the presence -of the C<//x> modifier is relevant. - -Interpolation has several quirks: C<$|>, C<$(>, and C<$)> are not -interpolated, and constructs C<$var[SOMETHING]> are voted (by several -different estimators) to be either an array element or C<$var> -followed by an RE alternative. This is where the notation -C<${arr[$bar]}> comes handy: C</${arr[0-9]}/> is interpreted as -array element C<-9>, not as a regular expression from the variable -C<$arr> followed by a digit, which would be the interpretation of -C</$arr[0-9]/>. Since voting among different estimators may occur, -the result is not predictable. - -It is at this step that C<\1> is begrudgingly converted to C<$1> in -the replacement text of C<s///> to correct the incorrigible -I<sed> hackers who haven't picked up the saner idiom yet. A warning -is emitted if the C<use warnings> pragma or the B<-w> command-line flag -(that is, the C<$^W> variable) was set. - -The lack of processing of C<\\> creates specific restrictions on -the post-processed text. If the delimiter 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. Because C</> is -equivalent to C<\/> inside a regular expression, this does not -matter unless the delimiter happens to be character special to the -RE engine, such as in C<s*foo*bar*>, C<m[foo]>, or C<?foo?>; or an -alphanumeric char, as in: - - m m ^ a \s* b mmx; - -In the RE above, which is intentionally obfuscated for illustration, the -delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the -RE is the same as for C<m/ ^ a s* b /mx>). There's more than one -reason you're encouraged to restrict your delimiters to non-alphanumeric, -non-whitespace choices. - -=back - -This step is the last one for all constructs except regular expressions, -which are processed further. - -=item Interpolation of regular expressions - -Previous steps were performed during the compilation of Perl code, -but this one happens at run time--although it may be optimized to -be calculated at compile time if appropriate. After preprocessing -described above, and possibly after evaluation if catenation, -joining, casing translation, or metaquoting are involved, the -resulting I<string> is passed to the RE engine for compilation. - -Whatever happens in the RE engine might be better discussed in L<perlre>, -but for the sake of continuity, we shall do so here. - -This is another step where the presence of the C<//x> modifier is -relevant. The RE engine scans the string from left to right and -converts it to a finite automaton. - -Backslashed characters are either replaced with corresponding -literal strings (as with C<\{>), or else they generate special nodes -in the finite automaton (as with C<\b>). Characters special to the -RE engine (such as C<|>) generate corresponding nodes or groups of -nodes. C<(?#...)> comments are ignored. All the rest is either -converted to literal strings to match, or else is ignored (as is -whitespace and C<#>-style comments if C<//x> is present). - -Parsing of the bracketed character class construct, C<[...]>, 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 C<{}>-delimited construct, the only -exception being that C<]> immediately following C<[> is treated as -though preceded by a backslash. Similarly, the terminator of -C<(?{...})> is found using the same rules as for finding the -terminator of a C<{}>-delimited construct. - -It is possible to inspect both the string given to RE engine and the -resulting finite automaton. See the arguments C<debug>/C<debugcolor> -in the C<use L<re>> pragma, as well as Perl's B<-Dr> command-line -switch documented in L<perlrun/"Command Switches">. - -=item Optimization of regular expressions - -This step is listed for completeness only. Since it does not change -semantics, details of this step are not documented and are subject -to change without notice. This step is performed over the finite -automaton that was generated during the previous pass. - -It is at this stage that C<split()> silently optimizes C</^/> to -mean C</^/m>. - -=back - -=head2 I/O Operators - -There are several I/O operators you should know about. - -A string enclosed by backticks (grave accents) first undergoes -double-quote interpolation. It is then interpreted as an external -command, and the output of that command is the value of the -backtick string, like in a shell. In scalar context, a single string -consisting of all output is returned. In list context, a list of -values is returned, one per line of output. (You can set C<$/> to use -a different line terminator.) The command is executed each time the -pseudo-literal is evaluated. The status value of the command is -returned in C<$?> (see L<perlvar> for the interpretation of C<$?>). -Unlike in B<csh>, no translation is done on the return data--newlines -remain newlines. Unlike in any of the shells, single quotes do not -hide variable names in the command from interpretation. To pass a -literal dollar-sign through to the shell you need to hide it with a -backslash. The generalized form of backticks is C<qx//>. (Because -backticks always undergo shell expansion as well, see L<perlsec> for -security concerns.) - -In scalar context, evaluating a filehandle in angle brackets yields -the next line from that file (the newline, if any, included), or -C<undef> at end-of-file or on error. When C<$/> is set to C<undef> -(sometimes known as file-slurp mode) and the file is empty, it -returns C<''> the first time, followed by C<undef> subsequently. - -Ordinarily you must assign the returned value to a variable, but -there is one situation where an automatic assignment happens. If -and only if the input symbol is the only thing inside the conditional -of a C<while> statement (even if disguised as a C<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 C<local $_;> before the loop if you want that -to happen. - -The following lines are equivalent: - - while (defined($_ = <STDIN>)) { print; } - while ($_ = <STDIN>) { print; } - while (<STDIN>) { print; } - for (;<STDIN>;) { print; } - print while defined($_ = <STDIN>); - print while ($_ = <STDIN>); - print while <STDIN>; - -This also behaves similarly, but avoids $_ : - - while (my $line = <STDIN>) { print $line } - -In these loop constructs, the assigned value (whether assignment -is automatic or explicit) is then tested to see whether it is -defined. The defined test avoids problems where line has a string -value that would be treated as false by Perl, for example a "" or -a "0" with no trailing newline. If you really mean for such values -to terminate the loop, they should be tested for explicitly: - - while (($_ = <STDIN>) ne '0') { ... } - while (<STDIN>) { last unless $_; ... } - -In other boolean contexts, C<< <I<filehandle>> >> without an -explicit C<defined> test or comparison elicit a warning if the -C<use warnings> pragma or the B<-w> -command-line switch (the C<$^W> variable) is in effect. - -The filehandles STDIN, STDOUT, and STDERR are predefined. (The -filehandles C<stdin>, C<stdout>, and C<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 L<perlopentut> and -L<perlfunc/open> for details on this. - -If a <FILEHANDLE> is used in a context that is looking for -a list, a list comprising all input lines is returned, one line per -list element. It's easy to grow to a rather large data space this -way, so use with care. - -<FILEHANDLE> may also be spelled C<readline(*FILEHANDLE)>. -See L<perlfunc/readline>. - -The null filehandle <> is special: it can be used to emulate the -behavior of B<sed> and B<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, C<$ARGV[0]> is set to "-", which when opened -gives you standard input. The @ARGV array is then processed as a list -of filenames. The loop - - while (<>) { - ... # code for each line - } - -is equivalent to the following Perl-like pseudo code: - - unshift(@ARGV, '-') unless @ARGV; - while ($ARGV = shift) { - open(ARGV, $ARGV); - while (<ARGV>) { - ... # code for each line - } - } - -except that it isn't so cumbersome to say, and will actually work. -It really does shift the @ARGV array and put the current filename -into the $ARGV variable. It also uses filehandle I<ARGV> -internally--<> is just a synonym for <ARGV>, which -is magical. (The pseudo code above doesn't work because it treats -<ARGV> as non-magical.) - -You can modify @ARGV before the first <> as long as the array ends up -containing the list of filenames you really want. Line numbers (C<$.>) -continue as though the input were one big happy file. See the example -in L<perlfunc/eof> 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<gzip>: - - @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV; - -If you want to pass switches into your script, you can use one of the -Getopts modules or put a loop on the front like this: - - while ($_ = $ARGV[0], /^-/) { - shift; - last if /^--$/; - if (/^-D(.*)/) { $debug = $1 } - if (/^-v/) { $verbose++ } - # ... # other switches - } - - while (<>) { - # ... # code for each line - } - -The <> symbol will return C<undef> for end-of-file only once. -If you call it again after this, it will assume you are processing another -@ARGV list, and if you haven't set @ARGV, will read input from STDIN. - -If angle brackets contain is a simple scalar variable (e.g., -<$foo>), then that variable contains the name of the -filehandle to input from, or its typeglob, or a reference to the -same. For example: - - $fh = \*STDIN; - $line = <$fh>; - -If what's within the angle brackets is neither a filehandle nor a simple -scalar variable containing a filehandle name, typeglob, or typeglob -reference, it is interpreted as a filename pattern to be globbed, and -either a list of filenames or the next filename in the list is returned, -depending on context. This distinction is determined on syntactic -grounds alone. That means C<< <$x> >> is always a readline() from -an indirect handle, but C<< <$hash{key}> >> 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<< <$foo> >> because that's an indirect filehandle as explained -in the previous paragraph. (In older versions of Perl, programmers -would insert curly brackets to force interpretation as a filename glob: -C<< <${foo}> >>. These days, it's considered cleaner to call the -internal function directly as C<glob($foo)>, which is probably the right -way to have done it in the first place.) For example: - - while (<*.c>) { - chmod 0644, $_; - } - -is roughly equivalent to: - - open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|"); - while (<FOO>) { - chomp; - chmod 0644, $_; - } - -except that the globbing is actually done internally using the standard -C<File::Glob> extension. Of course, the shortest way to do the above is: - - chmod 0644, <*.c>; - -A (file)glob evaluates its (embedded) argument only when it is -starting a new list. All values must be read before it will start -over. In list context, this isn't important because you automatically -get them all anyway. However, in scalar context the operator returns -the next value each time it's called, or C<undef> when the list has -run out. As with filehandle reads, an automatic C<defined> is -generated when the glob occurs in the test part of a C<while>, -because legal glob returns (e.g. a file called F<0>) would otherwise -terminate the loop. Again, C<undef> is returned only once. So if -you're expecting a single value from a glob, it is much better to -say - - ($file) = <blurch*>; - -than - - $file = <blurch*>; - -because the latter will alternate between returning a filename and -returning false. - -It you're trying to do variable interpolation, it's definitely better -to use the glob() function, because the older notation can cause people -to become confused with the indirect filehandle notation. - - @files = glob("$dir/*.[ch]"); - @files = glob($files[$i]); - -=head2 Constant Folding - -Like C, Perl does a certain amount of expression evaluation at -compile time whenever it determines that all arguments to an -operator are static and have no side effects. In particular, string -concatenation happens at compile time between literals that don't do -variable substitution. Backslash interpolation also happens at -compile time. You can say - - 'Now is the time for all' . "\n" . - 'good men to come to.' - -and this all reduces to one string internally. Likewise, if -you say - - foreach $file (@filenames) { - if (-s $file > 5 + 100 * 2**16) { } - } - -the compiler will precompute the number which that expression -represents so that the interpreter won't have to. - -=head2 Bitwise String Operators - -Bitstrings of any size may be manipulated by the bitwise operators -(C<~ | & ^>). - -If the operands to a binary bitwise op are strings of different -sizes, B<|> and B<^> ops act as though the shorter operand had -additional zero bits on the right, while the B<&> op acts as though -the longer operand were truncated to the length of the shorter. -The granularity for such extension or truncation is one or more -bytes. - - # ASCII-based examples - print "j p \n" ^ " a h"; # prints "JAPH\n" - print "JA" | " ph\n"; # prints "japh\n" - print "japh\nJunk" & '_____'; # prints "JAPH\n"; - print 'p N$' ^ " E<H\n"; # prints "Perl\n"; - -If you are intending to manipulate bitstrings, be certain that -you're supplying bitstrings: If an operand is a number, that will imply -a B<numeric> bitwise operation. You may explicitly show which type of -operation you intend by using C<""> or C<0+>, as in the examples below. - - $foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF) - $foo = '150' | 105 ; # yields 255 - $foo = 150 | '105'; # yields 255 - $foo = '150' | '105'; # yields string '155' (under ASCII) - - $baz = 0+$foo & 0+$bar; # both ops explicitly numeric - $biz = "$foo" ^ "$bar"; # both ops explicitly stringy - -See L<perlfunc/vec> for information on how to manipulate individual bits -in a bit vector. - -=head2 Integer Arithmetic - -By default, Perl assumes that it must do most of its arithmetic in -floating point. But by saying - - use integer; - -you may tell the compiler that it's okay to use integer operations -(if it feels like it) from here to the end of the enclosing BLOCK. -An inner BLOCK may countermand this by saying - - no integer; - -which lasts until the end of that BLOCK. Note that this doesn't -mean everything is only an integer, merely that Perl may use integer -operations if it is so inclined. For example, even under C<use -integer>, if you take the C<sqrt(2)>, you'll still get C<1.4142135623731> -or so. - -Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<", -and ">>") always produce integral results. (But see also -L<Bitwise String Operators>.) However, C<use integer> still has meaning for -them. By default, their results are interpreted as unsigned integers, but -if C<use integer> is in effect, their results are interpreted -as signed integers. For example, C<~0> usually evaluates to a large -integral value. However, C<use integer; ~0> is C<-1> on twos-complement -machines. - -=head2 Floating-point Arithmetic - -While C<use integer> provides integer-only arithmetic, there is no -analogous mechanism to provide automatic rounding or truncation to a -certain number of decimal places. For rounding to a certain number -of digits, sprintf() or printf() is usually the easiest route. -See L<perlfaq4>. - -Floating-point numbers are only approximations to what a mathematician -would call real numbers. There are infinitely more reals than floats, -so some corners must be cut. For example: - - printf "%.20g\n", 123456789123456789; - # produces 123456789123456784 - -Testing for exact equality of floating-point equality or inequality is -not a good idea. Here's a (relatively expensive) work-around to compare -whether two floating-point numbers are equal to a particular number of -decimal places. See Knuth, volume II, for a more robust treatment of -this topic. - - sub fp_equal { - my ($X, $Y, $POINTS) = @_; - my ($tX, $tY); - $tX = sprintf("%.${POINTS}g", $X); - $tY = sprintf("%.${POINTS}g", $Y); - return $tX eq $tY; - } - -The POSIX module (part of the standard perl distribution) implements -ceil(), floor(), and other mathematical and trigonometric functions. -The Math::Complex module (part of the standard perl distribution) -defines mathematical functions that work on both the reals and the -imaginary numbers. Math::Complex not as efficient as POSIX, but -POSIX can't work with complex numbers. - -Rounding in financial applications can have serious implications, and -the rounding method used should be specified precisely. In these -cases, it probably pays not to trust whichever system rounding is -being used by Perl, but to instead implement the rounding function you -need yourself. - -=head2 Bigger Numbers - -The standard Math::BigInt and Math::BigFloat modules provide -variable-precision arithmetic and overloaded operators, although -they're currently pretty slow. At the cost of some space and -considerable speed, they avoid the normal pitfalls associated with -limited-precision representations. - - use Math::BigInt; - $x = Math::BigInt->new('123456789123456789'); - print $x * $x; - - # prints +15241578780673678515622620750190521 - -There are several modules that let you calculate with (bound only by -memory and cpu-time) unlimited or fixed precision. There are also -some non-standard modules that provide faster implementations via -external C libraries. - -Here is a short, but incomplete summary: - - Math::Fraction big, unlimited fractions like 9973 / 12967 - Math::String treat string sequences like numbers - Math::FixedPrecision calculate with a fixed precision - Math::Currency for currency calculations - Bit::Vector manipulate bit vectors fast (uses C) - Math::BigIntFast Bit::Vector wrapper for big numbers - Math::Pari provides access to the Pari C library - Math::BigInteger uses an external C library - Math::Cephes uses external Cephes C library (no big numbers) - Math::Cephes::Fraction fractions via the Cephes library - Math::GMP another one using an external C library - -Choose wisely. - -=cut |
