1835 lines
69 KiB
Plaintext
1835 lines
69 KiB
Plaintext
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=head1 NAME
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perlop - Perl operators and precedence
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=head1 SYNOPSIS
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Perl operators have the following associativity and precedence,
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listed from highest precedence to lowest. Note that all operators
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borrowed from C keep the same precedence relationship with each other,
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even where C's precedence is slightly screwy. (This makes learning
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Perl easier for C folks.) With very few exceptions, these all
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operate on scalar values only, not array values.
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left terms and list operators (leftward)
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left ->
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nonassoc ++ --
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right **
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right ! ~ \ and unary + and -
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left =~ !~
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left * / % x
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left + - .
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left << >>
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nonassoc named unary operators
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nonassoc < > <= >= lt gt le ge
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nonassoc == != <=> eq ne cmp
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left &
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left | ^
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left &&
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left ||
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nonassoc .. ...
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right ?:
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right = += -= *= etc.
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left , =>
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nonassoc list operators (rightward)
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right not
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left and
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left or xor
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In the following sections, these operators are covered in precedence order.
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Many operators can be overloaded for objects. See L<overload>.
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=head1 DESCRIPTION
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=head2 Terms and List Operators (Leftward)
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A TERM has the highest precedence in Perl. They include variables,
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quote and quote-like operators, any expression in parentheses,
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and any function whose arguments are parenthesized. Actually, there
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aren't really functions in this sense, just list operators and unary
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operators behaving as functions because you put parentheses around
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the arguments. These are all documented in L<perlfunc>.
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If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
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is followed by a left parenthesis as the next token, the operator and
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arguments within parentheses are taken to be of highest precedence,
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just like a normal function call.
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In the absence of parentheses, the precedence of list operators such as
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C<print>, C<sort>, or C<chmod> is either very high or very low depending on
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whether you are looking at the left side or the right side of the operator.
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For example, in
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@ary = (1, 3, sort 4, 2);
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print @ary; # prints 1324
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the commas on the right of the sort are evaluated before the sort, but
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the commas on the left are evaluated after. In other words, list
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operators tend to gobble up all the arguments that follow them, and
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then act like a simple TERM with regard to the preceding expression.
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Note that you have to be careful with parentheses:
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# These evaluate exit before doing the print:
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print($foo, exit); # Obviously not what you want.
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print $foo, exit; # Nor is this.
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# These do the print before evaluating exit:
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(print $foo), exit; # This is what you want.
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print($foo), exit; # Or this.
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print ($foo), exit; # Or even this.
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Also note that
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print ($foo & 255) + 1, "\n";
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probably doesn't do what you expect at first glance. See
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L<Named Unary Operators> for more discussion of this.
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Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
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well as subroutine and method calls, and the anonymous
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constructors C<[]> and C<{}>.
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See also L<Quote and Quote-like Operators> toward the end of this section,
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as well as L<"I/O Operators">.
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=head2 The Arrow Operator
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Just as in C and C++, "C<-E<gt>>" is an infix dereference operator. If the
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right side is either a C<[...]> or C<{...}> subscript, then the left side
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must be either a hard or symbolic reference to an array or hash (or
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a location capable of holding a hard reference, if it's an lvalue (assignable)).
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See L<perlref>.
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Otherwise, the right side is a method name or a simple scalar variable
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containing the method name, and the left side must either be an object
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(a blessed reference) or a class name (that is, a package name).
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See L<perlobj>.
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=head2 Auto-increment and Auto-decrement
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"++" and "--" work as in C. That is, if placed before a variable, they
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increment or decrement the variable before returning the value, and if
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placed after, increment or decrement the variable after returning the value.
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The auto-increment operator has a little extra builtin magic to it. If
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you increment a variable that is numeric, or that has ever been used in
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a numeric context, you get a normal increment. If, however, the
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variable has been used in only string contexts since it was set, and
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has a value that is not the empty string and matches the pattern
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C</^[a-zA-Z]*[0-9]*$/>, the increment is done as a string, preserving each
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character within its range, with carry:
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print ++($foo = '99'); # prints '100'
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print ++($foo = 'a0'); # prints 'a1'
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print ++($foo = 'Az'); # prints 'Ba'
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print ++($foo = 'zz'); # prints 'aaa'
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The auto-decrement operator is not magical.
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=head2 Exponentiation
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Binary "**" is the exponentiation operator. Note that it binds even more
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tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
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implemented using C's pow(3) function, which actually works on doubles
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internally.)
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=head2 Symbolic Unary Operators
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Unary "!" performs logical negation, i.e., "not". See also C<not> for a lower
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precedence version of this.
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Unary "-" performs arithmetic negation if the operand is numeric. If
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the operand is an identifier, a string consisting of a minus sign
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concatenated with the identifier is returned. Otherwise, if the string
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starts with a plus or minus, a string starting with the opposite sign
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is returned. One effect of these rules is that C<-bareword> is equivalent
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to C<"-bareword">.
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Unary "~" performs bitwise negation, i.e., 1's complement. For example,
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C<0666 &~ 027> is 0640. (See also L<Integer Arithmetic> and L<Bitwise
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String Operators>.)
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Unary "+" has no effect whatsoever, even on strings. It is useful
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syntactically for separating a function name from a parenthesized expression
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that would otherwise be interpreted as the complete list of function
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arguments. (See examples above under L<Terms and List Operators (Leftward)>.)
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Unary "\" creates a reference to whatever follows it. See L<perlref>.
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Do not confuse this behavior with the behavior of backslash within a
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string, although both forms do convey the notion of protecting the next
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thing from interpretation.
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=head2 Binding Operators
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Binary "=~" binds a scalar expression to a pattern match. Certain operations
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search or modify the string $_ by default. This operator makes that kind
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of operation work on some other string. The right argument is a search
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pattern, substitution, or transliteration. The left argument is what is
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supposed to be searched, substituted, or transliterated instead of the default
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$_. The return value indicates the success of the operation. (If the
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right argument is an expression rather than a search pattern,
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substitution, or transliteration, it is interpreted as a search pattern at run
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time. This can be is less efficient than an explicit search, because the
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pattern must be compiled every time the expression is evaluated.
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Binary "!~" is just like "=~" except the return value is negated in
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the logical sense.
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=head2 Multiplicative Operators
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Binary "*" multiplies two numbers.
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Binary "/" divides two numbers.
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Binary "%" computes the modulus of two numbers. Given integer
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operands C<$a> and C<$b>: If C<$b> is positive, then C<$a % $b> is
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C<$a> minus the largest multiple of C<$b> that is not greater than
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C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
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smallest multiple of C<$b> that is not less than C<$a> (i.e. the
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result will be less than or equal to zero).
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Note than when C<use integer> is in scope, "%" give you direct access
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to the modulus operator as implemented by your C compiler. This
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operator is not as well defined for negative operands, but it will
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execute faster.
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Binary "x" is the repetition operator. In scalar context, it
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returns a string consisting of the left operand repeated the number of
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times specified by the right operand. In list context, if the left
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operand is a list in parentheses, it repeats the list.
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print '-' x 80; # print row of dashes
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print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
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@ones = (1) x 80; # a list of 80 1's
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@ones = (5) x @ones; # set all elements to 5
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=head2 Additive Operators
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Binary "+" returns the sum of two numbers.
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Binary "-" returns the difference of two numbers.
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Binary "." concatenates two strings.
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=head2 Shift Operators
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Binary "<<" returns the value of its left argument shifted left by the
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number of bits specified by the right argument. Arguments should be
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integers. (See also L<Integer Arithmetic>.)
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Binary ">>" returns the value of its left argument shifted right by
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the number of bits specified by the right argument. Arguments should
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be integers. (See also L<Integer Arithmetic>.)
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=head2 Named Unary Operators
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The various named unary operators are treated as functions with one
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argument, with optional parentheses. These include the filetest
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operators, like C<-f>, C<-M>, etc. See L<perlfunc>.
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If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
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is followed by a left parenthesis as the next token, the operator and
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arguments within parentheses are taken to be of highest precedence,
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just like a normal function call. Examples:
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chdir $foo || die; # (chdir $foo) || die
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chdir($foo) || die; # (chdir $foo) || die
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chdir ($foo) || die; # (chdir $foo) || die
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chdir +($foo) || die; # (chdir $foo) || die
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but, because * is higher precedence than ||:
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chdir $foo * 20; # chdir ($foo * 20)
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chdir($foo) * 20; # (chdir $foo) * 20
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chdir ($foo) * 20; # (chdir $foo) * 20
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chdir +($foo) * 20; # chdir ($foo * 20)
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rand 10 * 20; # rand (10 * 20)
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rand(10) * 20; # (rand 10) * 20
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rand (10) * 20; # (rand 10) * 20
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rand +(10) * 20; # rand (10 * 20)
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See also L<"Terms and List Operators (Leftward)">.
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=head2 Relational Operators
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Binary "E<lt>" returns true if the left argument is numerically less than
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the right argument.
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Binary "E<gt>" returns true if the left argument is numerically greater
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than the right argument.
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Binary "E<lt>=" returns true if the left argument is numerically less than
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or equal to the right argument.
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Binary "E<gt>=" returns true if the left argument is numerically greater
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than or equal to the right argument.
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Binary "lt" returns true if the left argument is stringwise less than
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the right argument.
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Binary "gt" returns true if the left argument is stringwise greater
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than the right argument.
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Binary "le" returns true if the left argument is stringwise less than
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or equal to the right argument.
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Binary "ge" returns true if the left argument is stringwise greater
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than or equal to the right argument.
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=head2 Equality Operators
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Binary "==" returns true if the left argument is numerically equal to
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the right argument.
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Binary "!=" returns true if the left argument is numerically not equal
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to the right argument.
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Binary "E<lt>=E<gt>" returns -1, 0, or 1 depending on whether the left
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argument is numerically less than, equal to, or greater than the right
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argument.
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Binary "eq" returns true if the left argument is stringwise equal to
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the right argument.
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Binary "ne" returns true if the left argument is stringwise not equal
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to the right argument.
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Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise
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less than, equal to, or greater than the right argument.
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"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
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by the current locale if C<use locale> is in effect. See L<perllocale>.
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=head2 Bitwise And
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Binary "&" returns its operators ANDed together bit by bit.
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(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
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=head2 Bitwise Or and Exclusive Or
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Binary "|" returns its operators ORed together bit by bit.
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(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
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Binary "^" returns its operators XORed together bit by bit.
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(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
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=head2 C-style Logical And
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Binary "&&" performs a short-circuit logical AND operation. That is,
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if the left operand is false, the right operand is not even evaluated.
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Scalar or list context propagates down to the right operand if it
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is evaluated.
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=head2 C-style Logical Or
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Binary "||" performs a short-circuit logical OR operation. That is,
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if the left operand is true, the right operand is not even evaluated.
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Scalar or list context propagates down to the right operand if it
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is evaluated.
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The C<||> and C<&&> operators differ from C's in that, rather than returning
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0 or 1, they return the last value evaluated. Thus, a reasonably portable
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way to find out the home directory (assuming it's not "0") might be:
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$home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
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(getpwuid($<))[7] || die "You're homeless!\n";
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In particular, this means that you shouldn't use this
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for selecting between two aggregates for assignment:
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@a = @b || @c; # this is wrong
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@a = scalar(@b) || @c; # really meant this
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@a = @b ? @b : @c; # this works fine, though
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As more readable alternatives to C<&&> and C<||> when used for
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control flow, Perl provides C<and> and C<or> operators (see below).
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The short-circuit behavior is identical. The precedence of "and" and
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"or" is much lower, however, so that you can safely use them after a
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list operator without the need for parentheses:
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unlink "alpha", "beta", "gamma"
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or gripe(), next LINE;
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With the C-style operators that would have been written like this:
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unlink("alpha", "beta", "gamma")
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|| (gripe(), next LINE);
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Use "or" for assignment is unlikely to do what you want; see below.
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=head2 Range Operators
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Binary ".." is the range operator, which is really two different
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operators depending on the context. In list context, it returns an
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array of values counting (by ones) from the left value to the right
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value. This is useful for writing C<foreach (1..10)> loops and for
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doing slice operations on arrays. In the current implementation, no
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temporary array is created when the range operator is used as the
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expression in C<foreach> loops, but older versions of Perl might burn
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a lot of memory when you write something like this:
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for (1 .. 1_000_000) {
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# code
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}
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In scalar context, ".." returns a boolean value. The operator is
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bistable, like a flip-flop, and emulates the line-range (comma) operator
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of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
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own boolean state. It is false as long as its left operand is false.
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Once the left operand is true, the range operator stays true until the
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right operand is true, I<AFTER> which the range operator becomes false
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again. (It doesn't become false till the next time the range operator is
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evaluated. It can test the right operand and become false on the same
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evaluation it became true (as in B<awk>), but it still returns true once.
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If you don't want it to test the right operand till the next evaluation
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(as in B<sed>), use three dots ("...") instead of two.) The right
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operand is not evaluated while the operator is in the "false" state, and
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the left operand is not evaluated while the operator is in the "true"
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state. The precedence is a little lower than || and &&. The value
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returned is either the empty string for false, or a sequence number
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(beginning with 1) for true. The sequence number is reset for each range
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encountered. The final sequence number in a range has the string "E0"
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appended to it, which doesn't affect its numeric value, but gives you
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something to search for if you want to exclude the endpoint. You can
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exclude the beginning point by waiting for the sequence number to be
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greater than 1. If either operand of scalar ".." is a constant expression,
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that operand is implicitly compared to the C<$.> variable, the current
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line number. Examples:
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As a scalar operator:
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if (101 .. 200) { print; } # print 2nd hundred lines
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next line if (1 .. /^$/); # skip header lines
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s/^/> / if (/^$/ .. eof()); # quote body
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# parse mail messages
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||
|
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 the 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;
|
||
|
|
||
|
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.
|
||
|
|
||
|
Assignment operators work as in C. That is,
|
||
|
|
||
|
$a += 2;
|
||
|
|
||
|
is equivalent to
|
||
|
|
||
|
$a = $a + 2;
|
||
|
|
||
|
although without duplicating any side effects that dereferencing the lvalue
|
||
|
might trigger, such as from tie(). Other assignment operators work similarly.
|
||
|
The following are recognized:
|
||
|
|
||
|
**= += *= &= <<= &&=
|
||
|
-= /= |= >>= ||=
|
||
|
.= %= ^=
|
||
|
x=
|
||
|
|
||
|
Note that while these are grouped by family, they all have the precedence
|
||
|
of assignment.
|
||
|
|
||
|
Unlike in C, the assignment operator produces a valid lvalue. Modifying
|
||
|
an assignment is equivalent to doing the assignment and then modifying
|
||
|
the variable that was assigned to. This is useful for modifying
|
||
|
a copy of something, like this:
|
||
|
|
||
|
($tmp = $global) =~ tr [A-Z] [a-z];
|
||
|
|
||
|
Likewise,
|
||
|
|
||
|
($a += 2) *= 3;
|
||
|
|
||
|
is equivalent to
|
||
|
|
||
|
$a += 2;
|
||
|
$a *= 3;
|
||
|
|
||
|
=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 =E<gt> 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: $, @, %, and &.)
|
||
|
|
||
|
=item (TYPE)
|
||
|
|
||
|
Type casting operator.
|
||
|
|
||
|
=back
|
||
|
|
||
|
=head2 Quote and Quote-like Operators
|
||
|
|
||
|
While we usually think of quotes as literal values, in Perl they
|
||
|
function as operators, providing various kinds of interpolating and
|
||
|
pattern matching capabilities. Perl provides customary quote characters
|
||
|
for these behaviors, but also provides a way for you to choose your
|
||
|
quote character for any of them. In the following table, a C<{}> represents
|
||
|
any pair of delimiters you choose. Non-bracketing delimiters use
|
||
|
the same character fore and aft, but the 4 sorts of brackets
|
||
|
(round, angle, square, curly) will all nest.
|
||
|
|
||
|
Customary Generic Meaning Interpolates
|
||
|
'' q{} Literal no
|
||
|
"" qq{} Literal yes
|
||
|
`` qx{} Command yes (unless '' is delimiter)
|
||
|
qw{} Word list no
|
||
|
// m{} Pattern match yes (unless '' is delimiter)
|
||
|
qr{} Pattern yes (unless '' is delimiter)
|
||
|
s{}{} Substitution yes (unless '' is delimiter)
|
||
|
tr{}{} Transliteration no (but see below)
|
||
|
|
||
|
Note that there can be whitespace between the operator and the quoting
|
||
|
characters, except when C<#> is being used as the quoting character.
|
||
|
C<q#foo#> is parsed as being the string C<foo>, while C<q #foo#> is the
|
||
|
operator C<q> followed by a comment. Its argument will be taken from the
|
||
|
next line. This allows you to write:
|
||
|
|
||
|
s {foo} # Replace foo
|
||
|
{bar} # with bar.
|
||
|
|
||
|
For constructs that do interpolation, variables beginning with "C<$>"
|
||
|
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 (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)
|
||
|
\c[ control 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>.
|
||
|
|
||
|
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<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 above, there are no 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.
|
||
|
|
||
|
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<perlre>.
|
||
|
|
||
|
=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, and may be removed in some future
|
||
|
version of Perl.
|
||
|
|
||
|
=item m/PATTERN/cgimosx
|
||
|
|
||
|
=item /PATTERN/cgimosx
|
||
|
|
||
|
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 $_ string is searched. (The string specified with
|
||
|
C<=~> need not be an lvalue--it may be the result of an expression
|
||
|
evaluation, but remember the C<=~> binds rather tightly.) See also
|
||
|
L<perlre>.
|
||
|
See L<perllocale> for discussion of additional considerations that apply
|
||
|
when C<use locale> is in effect.
|
||
|
|
||
|
Options are:
|
||
|
|
||
|
c Do not reset search position on a failed match when /g is in effect.
|
||
|
g Match globally, i.e., find all occurrences.
|
||
|
i Do case-insensitive pattern matching.
|
||
|
m Treat string as multiple lines.
|
||
|
o Compile pattern only once.
|
||
|
s Treat string as single line.
|
||
|
x Use extended regular expressions.
|
||
|
|
||
|
If "/" is the delimiter then the initial C<m> is optional. With the C<m>
|
||
|
you can use any pair of non-alphanumeric, non-whitespace characters
|
||
|
as delimiters. This is particularly useful for matching Unix path names
|
||
|
that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is
|
||
|
the delimiter, then the match-only-once rule of C<?PATTERN?> applies.
|
||
|
If "'" is the delimiter, no variable interpolation is performed on the
|
||
|
PATTERN.
|
||
|
|
||
|
PATTERN may contain variables, which will be interpolated (and the
|
||
|
pattern recompiled) every time the pattern search is evaluated, except
|
||
|
for when the delimiter is a single quote. (Note that C<$)> and C<$|>
|
||
|
might not be 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.
|
||
|
|
||
|
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 a 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
|
||
|
$Etc. The conditional is true if any variables were assigned, i.e., if
|
||
|
the pattern matched.
|
||
|
|
||
|
The C</g> modifier specifies global pattern matching--that is, matching
|
||
|
as many times as possible within the string. How it behaves depends on
|
||
|
the context. In list context, it returns a list of all the
|
||
|
substrings matched by all the parentheses in the regular expression.
|
||
|
If there are no parentheses, it returns a list of all the matched
|
||
|
strings, as if there were parentheses around the whole pattern.
|
||
|
|
||
|
In scalar context, each execution of C<m//g> finds the next match,
|
||
|
returning TRUE if it matches, and FALSE if there is no further match.
|
||
|
The position after the last match can be read or set using the pos()
|
||
|
function; see L<perlfunc/pos>. A failed match normally resets the
|
||
|
search position to the beginning of the string, but you can avoid that
|
||
|
by adding the C</c> modifier (e.g. C<m//gc>). Modifying the target
|
||
|
string also resets the search position.
|
||
|
|
||
|
You can intermix C<m//g> matches with C<m/\G.../g>, where C<\G> is a
|
||
|
zero-width assertion that matches the exact position where the previous
|
||
|
C<m//g>, if any, left off. The C<\G> assertion is not supported without
|
||
|
the C</g> modifier; currently, without C</g>, C<\G> behaves just like
|
||
|
C<\A>, but that's accidental and may change in the future.
|
||
|
|
||
|
Examples:
|
||
|
|
||
|
# list context
|
||
|
($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
|
||
|
|
||
|
# scalar context
|
||
|
{
|
||
|
local $/ = "";
|
||
|
while (defined($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";
|
||
|
}
|
||
|
|
||
|
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
|
||
|
|
||
|
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 /(tcl|rexx|python)/; # :-)
|
||
|
$baz = "\n"; # a one-character string
|
||
|
|
||
|
=item qr/PATTERN/imosx
|
||
|
|
||
|
Quote-as-a-regular-expression operator. I<STRING> is interpolated the
|
||
|
same way as I<PATTERN> in C<m/PATTERN/>. If "'" is used as the
|
||
|
delimiter, no variable 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 I<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. (Note that 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. 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:
|
||
|
|
||
|
$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<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).
|
||
|
|
||
|
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/
|
||
|
|
||
|
Returns a list of the words extracted out of STRING, using embedded
|
||
|
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.
|
||
|
However do not rely on this as in a future release it could be changed to
|
||
|
be exactly equivalent to the list
|
||
|
|
||
|
('foo', 'bar', 'baz')
|
||
|
|
||
|
Which in a scalar context would result in C<'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<-w>
|
||
|
switch produce 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 variable 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<sE<lt>fooE<gt>/bar/>. A C</e> will cause the
|
||
|
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.
|
||
|
|
||
|
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;
|
||
|
|
||
|
# /e's can even nest; this will expand
|
||
|
# any embedded scalar variable (including lexicals) in $_
|
||
|
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 \E<lt>I<digit>E<gt> form in only the left hand side.
|
||
|
Anywhere else it's $E<lt>I<digit>E<gt>.
|
||
|
|
||
|
Occasionally, you can't use just a C</g> to get all the changes
|
||
|
to occur. Here are two common cases:
|
||
|
|
||
|
# put commas in the right places in an integer
|
||
|
1 while s/(.*\d)(\d\d\d)/$1,$2/g; # perl4
|
||
|
1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; # perl5
|
||
|
|
||
|
# expand tabs to 8-column spacing
|
||
|
1 while s/\t+/' ' x (length($&)*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 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 = $host) =~ tr/a-z/A-Z/;
|
||
|
|
||
|
tr/a-zA-Z/ /cs; # change non-alphas to single space
|
||
|
|
||
|
tr [\200-\377]
|
||
|
[\000-\177]; # delete 8th bit
|
||
|
|
||
|
If multiple transliterations are given for a character, only the first one is used:
|
||
|
|
||
|
tr/AAA/XYZ/
|
||
|
|
||
|
will transliterate any A to X.
|
||
|
|
||
|
Note that because the transliteration table is built at compile time, neither
|
||
|
the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
|
||
|
interpolation. That means that if you want to use variables, you must use
|
||
|
an eval():
|
||
|
|
||
|
eval "tr/$oldlist/$newlist/";
|
||
|
die $@ if $@;
|
||
|
|
||
|
eval "tr/$oldlist/$newlist/, 1" or die $@;
|
||
|
|
||
|
=back
|
||
|
|
||
|
=head2 Gory details of parsing quoted constructs
|
||
|
|
||
|
When presented with something which may have several different
|
||
|
interpretations, Perl uses the principle B<DWIM> (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.
|
||
|
|
||
|
The most important detail of Perl parsing rules is the first one
|
||
|
discussed below; when processing a quoted construct, Perl I<first>
|
||
|
finds the end of the construct, then it interprets the contents of the
|
||
|
construct. If you understand this rule, you may skip the rest of this
|
||
|
section on the first reading. The other rules would
|
||
|
contradict user's expectations much less frequently than the first one.
|
||
|
|
||
|
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
|
||
|
a multichar delimiter
|
||
|
C<"\nEOF\n"> of C<<<EOF> construct, C</> which terminates C<qq/> construct,
|
||
|
C<]> which terminates C<qq[> construct, or C<E<gt>> which terminates a
|
||
|
fileglob started with C<<>.
|
||
|
|
||
|
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. When searching for multichar delimiter
|
||
|
no skipping is performed.
|
||
|
|
||
|
For constructs with 3-part delimiters (C<s///> etc.) the search is
|
||
|
repeated once more.
|
||
|
|
||
|
During this search no attention is paid to the semantic of the construct,
|
||
|
thus:
|
||
|
|
||
|
"$hash{"$foo/$bar"}"
|
||
|
|
||
|
or:
|
||
|
|
||
|
m/
|
||
|
bar # NOT a comment, this slash / terminated m//!
|
||
|
/x
|
||
|
|
||
|
do not form legal quoted expressions, the quoted part ends on the first C<">
|
||
|
and C</>, and the rest happens to be a syntax error. Note that since the slash
|
||
|
which terminated C<m//> was followed by a C<SPACE>, the above is not C<m//x>,
|
||
|
but rather C<m//> with no 'x' switch. So the embedded C<#> is interpreted
|
||
|
as a literal C<#>.
|
||
|
|
||
|
=item Removal of backslashes before delimiters
|
||
|
|
||
|
During the second pass the text between the starting delimiter and
|
||
|
the ending delimiter is copied to a safe location, and the C<\> is
|
||
|
removed from combinations consisting of C<\> and delimiter(s) (both starting
|
||
|
and ending delimiter if they differ).
|
||
|
|
||
|
The removal does not happen for multi-char delimiters.
|
||
|
|
||
|
Note that the combination C<\\> is left as it was!
|
||
|
|
||
|
Starting from this step no information about the delimiter(s) is used in the
|
||
|
parsing.
|
||
|
|
||
|
=item Interpolation
|
||
|
|
||
|
Next step is interpolation in the obtained delimiter-independent text.
|
||
|
There are four different cases.
|
||
|
|
||
|
=over
|
||
|
|
||
|
=item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
|
||
|
|
||
|
No interpolation is performed.
|
||
|
|
||
|
=item C<''>, C<q//>
|
||
|
|
||
|
The only interpolation is removal of C<\> from pairs C<\\>.
|
||
|
|
||
|
=item C<"">, C<``>, C<qq//>, C<qx//>, C<<file*globE<gt>>
|
||
|
|
||
|
C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> (possibly paired with C<\E>) are converted
|
||
|
to corresponding Perl constructs, thus C<"$foo\Qbaz$bar"> is converted to :
|
||
|
|
||
|
$foo . (quotemeta("baz" . $bar));
|
||
|
|
||
|
Other combinations of C<\> with following chars are substituted with
|
||
|
appropriate expansions.
|
||
|
|
||
|
Let it be stressed that I<whatever is between C<\Q> and C<\E>> is interpolated
|
||
|
in the usual way. Say, C<"\Q\\E"> has no C<\E> inside: it has C<\Q>, C<\\>,
|
||
|
and C<E>, thus the result is the same as for C<"\\\\E">. Generally speaking,
|
||
|
having backslashes between C<\Q> and C<\E> may lead to counterintuitive
|
||
|
results. So, C<"\Q\t\E"> is converted to:
|
||
|
|
||
|
quotemeta("\t")
|
||
|
|
||
|
which is the same as C<"\\\t"> (since TAB is not alphanumerical). 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 internally converted to the C<join> and
|
||
|
C<.> Perl operations, thus C<"$foo >>> '@arr'"> becomes:
|
||
|
|
||
|
$foo . " >>> '" . (join $", @arr) . "'";
|
||
|
|
||
|
All the operations in the above are performed simultaneously left-to-right.
|
||
|
|
||
|
Since the result of "\Q STRING \E" has all the metacharacters quoted
|
||
|
there is no way to insert a literal C<$> or C<@> inside a C<\Q\E> pair: if
|
||
|
protected by C<\> C<$> will be quoted to became "\\\$", if not, it is
|
||
|
interpreted as starting an interpolated scalar.
|
||
|
|
||
|
Note also that the interpolating code needs to make a decision on where the
|
||
|
interpolated scalar ends. For instance, whether C<"a $b -E<gt> {c}"> means:
|
||
|
|
||
|
"a " . $b . " -> {c}";
|
||
|
|
||
|
or:
|
||
|
|
||
|
"a " . $b -> {c};
|
||
|
|
||
|
I<Most of the time> the decision is to take the longest possible text which
|
||
|
does not include spaces between components and contains matching
|
||
|
braces/brackets. Since the outcome may be determined by I<voting> based
|
||
|
on heuristic estimators, the result I<is not strictly predictable>, but
|
||
|
is usually correct for the ambiguous cases.
|
||
|
|
||
|
=item C<?RE?>, C</RE/>, C<m/RE/>, C<s/RE/foo/>,
|
||
|
|
||
|
Processing of C<\Q>, C<\U>, C<\u>, C<\L>, C<\l> and interpolation happens
|
||
|
(almost) as with C<qq//> constructs, but I<the substitution of C<\> followed by
|
||
|
RE-special chars (including C<\>) is not performed>! Moreover,
|
||
|
inside C<(?{BLOCK})>, C<(?# comment )>, and C<#>-comment of
|
||
|
C<//x>-regular expressions no processing is performed at all.
|
||
|
This is the first step where presence of the C<//x> switch is relevant.
|
||
|
|
||
|
Interpolation has several quirks: C<$|>, C<$(> and C<$)> are not interpolated, and
|
||
|
constructs C<$var[SOMETHING]> are I<voted> (by several different estimators)
|
||
|
to be an array element or C<$var> followed by a RE alternative. This is
|
||
|
the place where the notation C<${arr[$bar]}> comes handy: C</${arr[0-9]}/>
|
||
|
is interpreted as an array element C<-9>, not as a regular expression from
|
||
|
variable C<$arr> followed by a digit, which is the interpretation of
|
||
|
C</$arr[0-9]/>. Since voting among different estimators may be performed,
|
||
|
the result I<is not predictable>.
|
||
|
|
||
|
It is on this step that C<\1> is converted to C<$1> in the replacement
|
||
|
text of C<s///>.
|
||
|
|
||
|
Note that absence 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. Since C</> is equivalent to C<\/> inside a regular expression, this
|
||
|
does not matter unless the delimiter is a special character for the RE engine,
|
||
|
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 above RE, 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>).
|
||
|
|
||
|
=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 C<quotemeta()>ing are involved) the resulting I<string> is passed to RE
|
||
|
engine for compilation.
|
||
|
|
||
|
Whatever happens in the RE engine is better be discussed in L<perlre>,
|
||
|
but for the sake of continuity let us do it here.
|
||
|
|
||
|
This is another step where presence of the C<//x> switch is relevant.
|
||
|
The RE engine scans the string left-to-right, and converts it to a finite
|
||
|
automaton.
|
||
|
|
||
|
Backslashed chars are either substituted by corresponding literal
|
||
|
strings (as with C<\{>), or generate special nodes of the finite automaton
|
||
|
(as with C<\b>). Characters which are 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 is ignored (as is whitespace and C<#>-style comments if
|
||
|
C<//x> is present).
|
||
|
|
||
|
Note that the parsing of the construct C<[...]> is performed using
|
||
|
rather different rules than for the rest of the regular expression.
|
||
|
The terminator of this construct is found using the same rules as for
|
||
|
finding a terminator of a C<{}>-delimited construct, the only exception
|
||
|
being that C<]> immediately following C<[> is considered as if preceded
|
||
|
by a backslash. Similarly, the terminator of C<(?{...})> is found using
|
||
|
the same rules as for finding a terminator of a C<{}>-delimited construct.
|
||
|
|
||
|
It is possible to inspect both the string given to RE engine, and the
|
||
|
resulting finite automaton. See arguments C<debug>/C<debugcolor>
|
||
|
of C<use L<re>> directive, and/or B<-Dr> option of Perl in
|
||
|
L<perlrun/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. This step is performed over the finite automaton generated
|
||
|
during the previous pass.
|
||
|
|
||
|
However, in older versions of Perl C<L<split>> used to silently
|
||
|
optimize C</^/> to mean C</^/m>. This behaviour, though present
|
||
|
in current versions of Perl, may be deprecated in future.
|
||
|
|
||
|
=back
|
||
|
|
||
|
=head2 I/O Operators
|
||
|
|
||
|
There are several I/O operators you should know about.
|
||
|
|
||
|
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 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
|
||
|
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 $ 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 a scalar context, evaluating a filehandle in angle brackets yields the
|
||
|
next line from that file (newline, if any, included), or C<undef> at
|
||
|
end-of-file. When C<$/> is set to C<undef> (i.e. file slurp mode),
|
||
|
and the file is empty, it returns C<''> the first time, followed by
|
||
|
C<undef> subsequently.
|
||
|
|
||
|
Ordinarily you must assign the returned value to a variable, but there is one
|
||
|
situation where an automatic assignment happens. I<If and ONLY if> the
|
||
|
input symbol is the only thing inside the conditional of a C<while> or
|
||
|
C<for(;;)> loop, the value is automatically assigned to the variable
|
||
|
C<$_>. 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($_ = <STDIN>)) { print; }
|
||
|
while ($_ = <STDIN>) { print; }
|
||
|
while (<STDIN>) { print; }
|
||
|
for (;<STDIN>;) { print; }
|
||
|
print while defined($_ = <STDIN>);
|
||
|
print while ($_ = <STDIN>);
|
||
|
print while <STDIN>;
|
||
|
|
||
|
and this also behaves similarly, but avoids the use of $_ :
|
||
|
|
||
|
while (my $line = <STDIN>) { print $line }
|
||
|
|
||
|
If you really mean such values to terminate the loop they should be
|
||
|
tested for explicitly:
|
||
|
|
||
|
while (($_ = <STDIN>) ne '0') { ... }
|
||
|
while (<STDIN>) { last unless $_; ... }
|
||
|
|
||
|
In other boolean contexts, C<E<lt>I<filehandle>E<gt>> without explicit C<defined>
|
||
|
test or comparison will solicit a warning if C<-w> is in effect.
|
||
|
|
||
|
The filehandles STDIN, STDOUT, and STDERR are predefined. (The
|
||
|
filehandles C<stdin>, C<stdout>, and C<stderr> will also work except in
|
||
|
packages, where they would be interpreted as local identifiers rather
|
||
|
than global.) Additional filehandles may be created with the open()
|
||
|
function. See L<perlfunc/open> for details on this.
|
||
|
|
||
|
If a E<lt>FILEHANDLEE<gt> is used in a context that is looking for a list, a
|
||
|
list consisting of all the input lines is returned, one line per list
|
||
|
element. It's easy to make a I<LARGE> data space this way, so use with
|
||
|
care.
|
||
|
|
||
|
E<lt>FILEHANDLEE<gt> may also be spelt readline(FILEHANDLE). See
|
||
|
L<perlfunc/readline>.
|
||
|
|
||
|
The null filehandle E<lt>E<gt> is special and can be used to emulate the
|
||
|
behavior of B<sed> and B<awk>. Input from E<lt>E<gt> comes either from
|
||
|
standard input, or from each file listed on the command line. Here's
|
||
|
how it works: the first time E<lt>E<gt> is evaluated, the @ARGV array is
|
||
|
checked, and if it is empty, C<$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 array @ARGV and put the current filename into variable
|
||
|
$ARGV. It also uses filehandle I<ARGV> internally--E<lt>E<gt> is just a
|
||
|
synonym for E<lt>ARGVE<gt>, which is magical. (The pseudo code above
|
||
|
doesn't work because it treats E<lt>ARGVE<gt> as non-magical.)
|
||
|
|
||
|
You can modify @ARGV before the first E<lt>E<gt> as long as the array ends up
|
||
|
containing the list of filenames you really want. Line numbers (C<$.>)
|
||
|
continue as if the input were one big happy file. (But see example
|
||
|
under C<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/) { $verbose++ }
|
||
|
# ... # other switches
|
||
|
}
|
||
|
|
||
|
while (<>) {
|
||
|
# ... # code for each line
|
||
|
}
|
||
|
|
||
|
The E<lt>E<gt> symbol will return C<undef> for end-of-file only once.
|
||
|
If you call it again after this it will assume you are processing another
|
||
|
@ARGV list, and if you haven't set @ARGV, will input from STDIN.
|
||
|
|
||
|
If the string inside the angle brackets is a reference to a scalar
|
||
|
variable (e.g., E<lt>$fooE<gt>), 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<E<lt>$xE<gt>> is always a readline from
|
||
|
an indirect handle, but C<E<lt>$hash{key}E<gt>> 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<E<lt>$fooE<gt>> because that's an indirect filehandle as explained
|
||
|
in the previous paragraph. (In older versions of Perl, programmers
|
||
|
would insert curly brackets to force interpretation as a filename glob:
|
||
|
C<E<lt>${foo}E<gt>>. 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.) Example:
|
||
|
|
||
|
while (<*.c>) {
|
||
|
chmod 0644, $_;
|
||
|
}
|
||
|
|
||
|
is equivalent to
|
||
|
|
||
|
open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
|
||
|
while (<FOO>) {
|
||
|
chop;
|
||
|
chmod 0644, $_;
|
||
|
}
|
||
|
|
||
|
In fact, it's currently implemented that way. (Which means it will not
|
||
|
work on filenames with spaces in them unless you have csh(1) on your
|
||
|
machine.) Of course, the shortest way to do the above is:
|
||
|
|
||
|
chmod 0644, <*.c>;
|
||
|
|
||
|
Because globbing invokes a shell, it's often faster to call readdir() yourself
|
||
|
and do your own grep() on the filenames. Furthermore, due to its current
|
||
|
implementation of using a shell, the glob() routine may get "Arg list too
|
||
|
long" errors (unless you've installed tcsh(1L) as F</bin/csh>).
|
||
|
|
||
|
A glob evaluates its (embedded) argument only when it is starting a new
|
||
|
list. All values must be read before it will start over. In a list
|
||
|
context this isn't important, because you automatically get them all
|
||
|
anyway. In scalar context, however, the operator returns the next value
|
||
|
each time it is called, or a C<undef> value if you've just run out. As
|
||
|
for filehandles an automatic C<defined> is generated when the glob
|
||
|
occurs in the test part of a C<while> or C<for> - because legal glob returns
|
||
|
(e.g. a file called F<0>) would otherwise terminate the loop.
|
||
|
Again, C<undef> is returned only once. So if you're expecting a single value
|
||
|
from a glob, it is much better to say
|
||
|
|
||
|
($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 interpretation 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 $file > 5 + 100 * 2**16) { }
|
||
|
}
|
||
|
|
||
|
the compiler will precompute the number that
|
||
|
expression represents so that the interpreter
|
||
|
won't have to.
|
||
|
|
||
|
=head2 Bitwise String Operators
|
||
|
|
||
|
Bitstrings of any size may be manipulated by the bitwise operators
|
||
|
(C<~ | & ^>).
|
||
|
|
||
|
If the operands to a binary bitwise op are strings of different sizes,
|
||
|
B<|> and B<^> ops will act as if the shorter operand had additional
|
||
|
zero bits on the right, while the B<&> op will act as if the longer
|
||
|
operand were truncated to the length of the shorter. Note that the
|
||
|
granularity for such extension or truncation is one or more I<bytes>.
|
||
|
|
||
|
# ASCII-based examples
|
||
|
print "j p \n" ^ " a h"; # prints "JAPH\n"
|
||
|
print "JA" | " ph\n"; # prints "japh\n"
|
||
|
print "japh\nJunk" & '_____'; # prints "JAPH\n";
|
||
|
print 'p N$' ^ " E<H\n"; # prints "Perl\n";
|
||
|
|
||
|
If you are intending to manipulate bitstrings, you should 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
|
||
|
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.
|
||
|
|
||
|
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. However, 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 -1 on twos-complement machines.
|
||
|
|
||
|
=head2 Floating-point Arithmetic
|
||
|
|
||
|
While C<use integer> provides integer-only arithmetic, there is no
|
||
|
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
|
||
|
distribution) defines a number of mathematical functions that can also
|
||
|
work on real 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.
|
||
|
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
|