perlop - Perl operators and precedence
-=head1 SYNOPSIS
+=head1 DESCRIPTION
+
+=head2 Operator Precedence and Associativity
+
+Operator precedence and associativity work in Perl more or less like
+they do in mathematics.
+
+I<Operator precedence> means some operators are evaluated before
+others. For example, in C<2 + 4 * 5>, the multiplication has higher
+precedence so C<4 * 5> is evaluated first yielding C<2 + 20 ==
+22> and not C<6 * 5 == 30>.
+
+I<Operator associativity> defines what happens if a sequence of the
+same operators is used one after another: whether the evaluator will
+evaluate the left operations first or the right. For example, in C<8
+- 4 - 2>, subtraction is left associative so Perl evaluates the
+expression left to right. C<8 - 4> is evaluated first making the
+expression C<4 - 2 == 2> and not C<8 - 2 == 6>.
Perl operators have the following associativity and precedence,
listed from highest precedence to lowest. Operators borrowed from
left &
left | ^
left &&
- left ||
+ left || //
nonassoc .. ...
right ?:
right = += -= *= etc.
nonassoc list operators (rightward)
right not
left and
- left or xor
+ left or xor err
In the following sections, these operators are covered in precedence order.
Many operators can be overloaded for objects. See L<overload>.
-=head1 DESCRIPTION
-
=head2 Terms and List Operators (Leftward)
A TERM has the highest precedence in Perl. They include variables,
print ($foo & 255) + 1, "\n";
-probably doesn't do what you expect at first glance. See
-L<Named Unary Operators> for more discussion of this.
+probably doesn't do what you expect at first glance. The parentheses
+enclose the argument list for C<print> which is evaluated (printing
+the result of C<$foo & 255>). Then one is added to the return value
+of C<print> (usually 1). The result is something like this:
+
+ 1 + 1, "\n"; # Obviously not what you meant.
+
+To do what you meant properly, you must write:
+
+ print(($foo & 255) + 1, "\n");
+
+See L<Named Unary Operators> for more discussion of this.
Also parsed as terms are the C<do {}> and C<eval {}> constructs, as
well as subroutine and method calls, and the anonymous
=head2 Auto-increment and Auto-decrement
-"++" and "--" work as in C. That is, if placed before a variable, they
-increment or decrement the variable before returning the value, and if
-placed after, increment or decrement the variable after returning the value.
+"++" and "--" work as in C. That is, if placed before a variable,
+they increment or decrement the variable by one before returning the
+value, and if placed after, increment or decrement after returning the
+value.
+
+ $i = 0; $j = 0;
+ print $i++; # prints 0
+ print ++$j; # prints 1
+
+Note that just as in C, Perl doesn't define B<when> the variable is
+incremented or decremented. You just know it will be done sometime
+before or after the value is returned. This also means that modifying
+a variable twice in the same statement will lead to undefined behaviour.
+Avoid statements like:
+
+ $i = $i ++;
+ print ++ $i + $i ++;
+
+Perl will not guarantee what the result of the above statements is.
The auto-increment operator has a little extra builtin magic to it. If
you increment a variable that is numeric, or that has ever been used in
print ++($foo = 'Az'); # prints 'Ba'
print ++($foo = 'zz'); # prints 'aaa'
+C<undef> is always treated as numeric, and in particular is changed
+to C<0> before incrementing (so that a post-increment of an undef value
+will return C<0> rather than C<undef>).
+
The auto-decrement operator is not magical.
=head2 Exponentiation
the operand is an identifier, a string consisting of a minus sign
concatenated with the identifier is returned. Otherwise, if the string
starts with a plus or minus, a string starting with the opposite sign
-is returned. One effect of these rules is that C<-bareword> is equivalent
-to C<"-bareword">.
+is returned. One effect of these rules is that -bareword is equivalent
+to the string "-bareword".
Unary "~" performs bitwise negation, i.e., 1's complement. For
example, C<0666 & ~027> is 0640. (See also L<Integer Arithmetic> and
L<Bitwise String Operators>.) Note that the width of the result is
platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
bits wide on a 64-bit platform, so if you are expecting a certain bit
-width, remember use the & operator to mask off the excess bits.
+width, remember to use the & operator to mask off the excess bits.
Unary "+" has no effect whatsoever, even on strings. It is useful
syntactically for separating a function name from a parenthesized expression
supposed to be searched, substituted, or transliterated instead of the default
$_. When used in scalar context, the return value generally indicates the
success of the operation. Behavior in list context depends on the particular
-operator. See L</"Regexp Quote-Like Operators"> for details.
+operator. See L</"Regexp Quote-Like Operators"> for details and
+L<perlretut> for examples using these operators.
If the right argument is an expression rather than a search pattern,
substitution, or transliteration, it is interpreted as a search pattern at run
-time. This can be less efficient than an explicit search, because the
-pattern must be compiled every time the expression is evaluated.
+time.
Binary "!~" is just like "=~" except the return value is negated in
the logical sense.
C<$a>. If C<$b> is negative, then C<$a % $b> is C<$a> minus the
smallest multiple of C<$b> that is not less than C<$a> (i.e. the
result will be less than or equal to zero).
-Note than when C<use integer> is in scope, "%" gives you direct access
+Note that when C<use integer> is in scope, "%" gives you direct access
to the modulus operator as implemented by your C compiler. This
operator is not as well defined for negative operands, but it will
execute faster.
operand is not enclosed in parentheses, it returns a string consisting
of the left operand repeated the number of times specified by the right
operand. In list context, if the left operand is enclosed in
-parentheses, it repeats the list.
+parentheses or is a list formed by C<qw/STRING/>, it repeats the list.
+If the right operand is zero or negative, it returns an empty string
+or an empty list, depending on the context.
print '-' x 80; # print row of dashes
the number of bits specified by the right argument. Arguments should
be integers. (See also L<Integer Arithmetic>.)
+Note that both "<<" and ">>" in Perl are implemented directly using
+"<<" and ">>" in C. If C<use integer> (see L<Integer Arithmetic>) is
+in force then signed C integers are used, else unsigned C integers are
+used. Either way, the implementation isn't going to generate results
+larger than the size of the integer type Perl was built with (32 bits
+or 64 bits).
+
+The result of overflowing the range of the integers is undefined
+because it is undefined also in C. In other words, using 32-bit
+integers, C<< 1 << 32 >> is undefined. Shifting by a negative number
+of bits is also undefined.
+
=head2 Named Unary Operators
The various named unary operators are treated as functions with one
-argument, with optional parentheses. These include the filetest
-operators, like C<-f>, C<-M>, etc. See L<perlfunc>.
+argument, with optional parentheses.
If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
is followed by a left parenthesis as the next token, the operator and
arguments within parentheses are taken to be of highest precedence,
-just like a normal function call. Examples:
+just like a normal function call. For example,
+because named unary operators are higher precedence than ||:
chdir $foo || die; # (chdir $foo) || die
chdir($foo) || die; # (chdir $foo) || die
chdir ($foo) || die; # (chdir $foo) || die
chdir +($foo) || die; # (chdir $foo) || die
-but, because * is higher precedence than ||:
+but, because * is higher precedence than named operators:
chdir $foo * 20; # chdir ($foo * 20)
chdir($foo) * 20; # (chdir $foo) * 20
rand (10) * 20; # (rand 10) * 20
rand +(10) * 20; # rand (10 * 20)
+Regarding precedence, the filetest operators, like C<-f>, C<-M>, etc. are
+treated like named unary operators, but they don't follow this functional
+parenthesis rule. That means, for example, that C<-f($file).".bak"> is
+equivalent to C<-f "$file.bak">.
+
See also L<"Terms and List Operators (Leftward)">.
=head2 Relational Operators
Binary "<=>" returns -1, 0, or 1 depending on whether the left
argument is numerically less than, equal to, or greater than the right
argument. If your platform supports NaNs (not-a-numbers) as numeric
-values, using them with "<=>" (or any other numeric comparison)
-returns undef.
+values, using them with "<=>" returns undef. NaN is not "<", "==", ">",
+"<=" or ">=" anything (even NaN), so those 5 return false. NaN != NaN
+returns true, as does NaN != anything else. If your platform doesn't
+support NaNs then NaN is just a string with numeric value 0.
+
+ perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
+ perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
Binary "eq" returns true if the left argument is stringwise equal to
the right argument.
=head2 Bitwise And
-Binary "&" returns its operators ANDed together bit by bit.
+Binary "&" returns its operands ANDed together bit by bit.
(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
+Note that "&" has lower priority than relational operators, so for example
+the brackets are essential in a test like
+
+ print "Even\n" if ($x & 1) == 0;
+
=head2 Bitwise Or and Exclusive Or
-Binary "|" returns its operators ORed together bit by bit.
+Binary "|" returns its operands ORed together bit by bit.
(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
-Binary "^" returns its operators XORed together bit by bit.
+Binary "^" returns its operands XORed together bit by bit.
(See also L<Integer Arithmetic> and L<Bitwise String Operators>.)
+Note that "|" and "^" have lower priority than relational operators, so
+for example the brackets are essential in a test like
+
+ print "false\n" if (8 | 2) != 10;
+
=head2 C-style Logical And
Binary "&&" performs a short-circuit logical AND operation. That is,
Scalar or list context propagates down to the right operand if it
is evaluated.
-The C<||> and C<&&> operators differ from C's in that, rather than returning
-0 or 1, they return the last value evaluated. Thus, a reasonably portable
-way to find out the home directory (assuming it's not "0") might be:
+=head2 C-style Logical Defined-Or
- $home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
- (getpwuid($<))[7] || die "You're homeless!\n";
+Although it has no direct equivalent in C, Perl's C<//> operator is related
+to its C-style or. In fact, it's exactly the same as C<||>, except that it
+tests the left hand side's definedness instead of its truth. Thus, C<$a // $b>
+is similar to C<defined($a) || $b> (except that it returns the value of C<$a>
+rather than the value of C<defined($a)>) and is exactly equivalent to
+C<defined($a) ? $a : $b>. This is very useful for providing default values
+for variables. If you actually want to test if at least one of C<$a> and
+C<$b> is defined, use C<defined($a // $b)>.
+
+The C<||>, C<//> and C<&&> operators return the last value evaluated
+(unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably
+portable way to find out the home directory might be:
+
+ $home = $ENV{'HOME'} // $ENV{'LOGDIR'} //
+ (getpwuid($<))[7] // die "You're homeless!\n";
In particular, this means that you shouldn't use this
for selecting between two aggregates for assignment:
@a = scalar(@b) || @c; # really meant this
@a = @b ? @b : @c; # this works fine, though
-As more readable alternatives to C<&&> and C<||> when used for
-control flow, Perl provides C<and> and C<or> operators (see below).
-The short-circuit behavior is identical. The precedence of "and" and
-"or" is much lower, however, so that you can safely use them after a
+As more readable alternatives to C<&&>, C<//> and C<||> when used for
+control flow, Perl provides C<and>, C<err> and C<or> operators (see below).
+The short-circuit behavior is identical. The precedence of "and", "err"
+and "or" is much lower, however, so that you can safely use them after a
list operator without the need for parentheses:
unlink "alpha", "beta", "gamma"
=head2 Range Operators
Binary ".." is the range operator, which is really two different
-operators depending on the context. In list context, it returns an
-array of values counting (up by ones) from the left value to the right
+operators depending on the context. In list context, it returns a
+list of values counting (up by ones) from the left value to the right
value. If the left value is greater than the right value then it
-returns the empty array. The range operator is useful for writing
-C<foreach (1..10)> loops and for doing slice operations on arrays. In
+returns the empty list. The range operator is useful for writing
+C<foreach (1..10)> loops and for doing slice operations on arrays. In
the current implementation, no temporary array is created when the
range operator is used as the expression in C<foreach> loops, but older
versions of Perl might burn a lot of memory when you write something
# code
}
+The range operator also works on strings, using the magical auto-increment,
+see below.
+
In scalar context, ".." returns a boolean value. The operator is
bistable, like a flip-flop, and emulates the line-range (comma) operator
of B<sed>, B<awk>, and various editors. Each ".." operator maintains its
doesn't affect its numeric value, but gives you something to search
for if you want to exclude the endpoint. You can exclude the
beginning point by waiting for the sequence number to be greater
-than 1. If either operand of scalar ".." is a constant expression,
-that operand is implicitly compared to the C<$.> variable, the
-current line number. Examples:
+than 1.
+
+If either operand of scalar ".." is a constant expression,
+that operand is considered true if it is equal (C<==>) to the current
+input line number (the C<$.> variable).
+
+To be pedantic, the comparison is actually C<int(EXPR) == int(EXPR)>,
+but that is only an issue if you use a floating point expression; when
+implicitly using C<$.> as described in the previous paragraph, the
+comparison is C<int(EXPR) == int($.)> which is only an issue when C<$.>
+is set to a floating point value and you are not reading from a file.
+Furthermore, C<"span" .. "spat"> or C<2.18 .. 3.14> will not do what
+you want in scalar context because each of the operands are evaluated
+using their integer representation.
+
+Examples:
As a scalar operator:
- if (101 .. 200) { print; } # print 2nd hundred lines
- next line if (1 .. /^$/); # skip header lines
+ if (101 .. 200) { print; } # print 2nd hundred lines, short for
+ # if ($. == 101 .. $. == 200) ...
+ next line if (1 .. /^$/); # skip header lines, short for
+ # ... if ($. == 1 .. /^$/);
s/^/> / if (/^$/ .. eof()); # quote body
# parse mail messages
while (<>) {
$in_header = 1 .. /^$/;
- $in_body = /^$/ .. eof();
- # do something based on those
+ $in_body = /^$/ .. eof;
+ if ($in_header) {
+ # ...
+ } else { # in body
+ # ...
+ }
} continue {
- close ARGV if eof; # reset $. each file
+ close ARGV if eof; # reset $. each file
}
-As a list operator:
+Here's a simple example to illustrate the difference between
+the two range operators:
+
+ @lines = (" - Foo",
+ "01 - Bar",
+ "1 - Baz",
+ " - Quux");
+
+ foreach(@lines)
+ {
+ if (/0/ .. /1/)
+ {
+ print "$_\n";
+ }
+ }
+
+This program will print only the line containing "Bar". If
+the range operator is changed to C<...>, it will also print the
+"Baz" line.
+
+And now some examples as a list operator:
for (101 .. 200) { print; } # print $_ 100 times
@foo = @foo[0 .. $#foo]; # an expensive no-op
@alphabet = ('A' .. 'Z');
-to get all normal letters of the alphabet, or
+to get all normal letters of the English alphabet, or
$hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
goes until the next value would be longer than the final value
specified.
+Because each operand is evaluated in integer form, C<2.18 .. 3.14> will
+return two elements in list context.
+
+ @list = (2.18 .. 3.14); # same as @list = (2 .. 3);
+
=head2 Conditional Operator
Ternary "?:" is the conditional operator, just as in C. It works much
In list context, it's just the list argument separator, and inserts
both its arguments into the list.
-The => digraph is mostly just a synonym for the comma operator. It's useful for
-documenting arguments that come in pairs. As of release 5.001, it also forces
-any word to the left of it to be interpreted as a string.
+The C<< => >> operator is a synonym for the comma, but forces any word
+(consisting entirely of word characters) to its left to be interpreted
+as a string (as of 5.001). This includes words that might otherwise be
+considered a constant or function call.
+
+ use constant FOO => "something";
+
+ my %h = ( FOO => 23 );
+
+is equivalent to:
+
+ my %h = ("FOO", 23);
+
+It is I<NOT>:
+
+ my %h = ("something", 23);
+
+If the argument on the left is not a word, it is first interpreted as
+an expression, and then the string value of that is used.
+
+The C<< => >> operator is helpful in documenting the correspondence
+between keys and values in hashes, and other paired elements in lists.
+
+ %hash = ( $key => $value );
+ login( $username => $password );
=head2 List Operators (Rightward)
precedence. This means that it short-circuits: i.e., the right
expression is evaluated only if the left expression is true.
-=head2 Logical or and Exclusive Or
+=head2 Logical or, Defined or, and Exclusive Or
Binary "or" returns the logical disjunction of the two surrounding
expressions. It's equivalent to || except for the very low precedence.
@info = stat($file) || die; # oops, scalar sense of stat!
@info = stat($file) or die; # better, now @info gets its due
-Then again, you could always use parentheses.
+Then again, you could always use parentheses.
+
+Binary "err" is equivalent to C<//>--it's just like binary "or", except it tests
+its left argument's definedness instead of its truth. There are two ways to
+remember "err": either because many functions return C<undef> on an B<err>or,
+or as a sort of correction: C<$a=($b err 'default')>
Binary "xor" returns the exclusive-OR of the two surrounding expressions.
It cannot short circuit, of course.
Customary Generic Meaning Interpolates
'' q{} Literal no
"" qq{} Literal yes
- `` qx{} Command yes (unless '' is delimiter)
+ `` qx{} Command yes*
qw{} Word list no
- // m{} Pattern match yes (unless '' is delimiter)
- qr{} Pattern yes (unless '' is delimiter)
- s{}{} Substitution yes (unless '' is delimiter)
+ // m{} Pattern match yes*
+ qr{} Pattern yes*
+ s{}{} Substitution yes*
tr{}{} Transliteration no (but see below)
+ <<EOF here-doc yes*
+
+ * unless the delimiter is ''.
Non-bracketing delimiters use the same character fore and aft, but the four
sorts of brackets (round, angle, square, curly) will all nest, which means
$s = q{ if($a eq "}") ... }; # WRONG
-is a syntax error. The C<Text::Balanced> module on CPAN is able to do this
-properly.
+is a syntax error. The C<Text::Balanced> module (from CPAN, and
+starting from Perl 5.8 part of the standard distribution) is able
+to do this properly.
There can be whitespace between the operator and the quoting
characters, except when C<#> is being used as the quoting character.
s {foo} # Replace foo
{bar} # with bar.
-For constructs that do interpolate, variables beginning with "C<$>"
-or "C<@>" are interpolated, as are the following escape sequences. Within
-a transliteration, the first eleven of these sequences may be used.
+The following escape sequences are available in constructs that interpolate
+and in transliterations.
\t tab (HT, TAB)
\n newline (NL)
\x1b hex char (ESC)
\x{263a} wide hex char (SMILEY)
\c[ control char (ESC)
- \N{name} named char
+ \N{name} named Unicode character
+
+B<NOTE>: Unlike C and other languages, Perl has no \v escape sequence for
+the vertical tab (VT - ASCII 11).
+
+The following escape sequences are available in constructs that interpolate
+but not in transliterations.
\l lowercase next char
\u uppercase next char
\E end case modification
\Q quote non-word characters till \E
-If C<use locale> is in effect, the case map used by C<\l>, C<\L>, C<\u>
-and C<\U> is taken from the current locale. See L<perllocale>. For
-documentation of C<\N{name}>, see L<charnames>.
+If C<use locale> is in effect, the case map used by C<\l>, C<\L>,
+C<\u> and C<\U> is taken from the current locale. See L<perllocale>.
+If Unicode (for example, C<\N{}> or wide hex characters of 0x100 or
+beyond) is being used, the case map used by C<\l>, C<\L>, C<\u> and
+C<\U> is as defined by Unicode. For documentation of C<\N{name}>,
+see L<charnames>.
All systems use the virtual C<"\n"> to represent a line terminator,
called a "newline". There is no such thing as an unvarying, physical
C<"\015">. If you get in the habit of using C<"\n"> for networking,
you may be burned some day.
+For constructs that do interpolate, variables beginning with "C<$>"
+or "C<@>" are interpolated. Subscripted variables such as C<$a[3]> or
+C<< $href->{key}[0] >> are also interpolated, as are array and hash slices.
+But method calls such as C<< $obj->meth >> are not.
+
+Interpolating an array or slice interpolates the elements in order,
+separated by the value of C<$">, so is equivalent to interpolating
+C<join $", @array>. "Punctuation" arrays such as C<@+> are only
+interpolated if the name is enclosed in braces C<@{+}>.
+
You cannot include a literal C<$> or C<@> within a C<\Q> sequence.
An unescaped C<$> or C<@> interpolates the corresponding variable,
while escaping will cause the literal string C<\$> to be inserted.
reset if eof; # clear ?? status for next file
}
-This usage is vaguely depreciated, which means it just might possibly
+This usage is vaguely deprecated, which means it just might possibly
be removed in some distant future version of Perl, perhaps somewhere
around the year 2168.
and is useful when the value you are interpolating won't change over
the life of the script. However, mentioning C</o> constitutes a promise
that you won't change the variables in the pattern. If you change them,
-Perl won't even notice. See also L<"qr//">.
+Perl won't even notice. See also L<"qr/STRING/imosx">.
If the PATTERN evaluates to the empty string, the last
-I<successfully> matched regular expression is used instead.
+I<successfully> matched regular expression is used instead. In this
+case, only the C<g> and C<c> flags on the empty pattern is honoured -
+the other flags are taken from the original pattern. If no match has
+previously succeeded, this will (silently) act instead as a genuine
+empty pattern (which will always match).
+
+Note that it's possible to confuse Perl into thinking C<//> (the empty
+regex) is really C<//> (the defined-or operator). Perl is usually pretty
+good about this, but some pathological cases might trigger this, such as
+C<$a///> (is that C<($a) / (//)> or C<$a // />?) and C<print $fh //>
+(C<print $fh(//> or C<print($fh //>?). In all of these examples, Perl
+will assume you meant defined-or. If you meant the empty regex, just
+use parentheses or spaces to disambiguate, or even prefix the empty
+regex with an C<m> (so C<//> becomes C<m//>).
If the C</g> option is not used, C<m//> in list context returns a
list consisting of the subexpressions matched by the parentheses in the
still anchors at pos(), but the match is of course only attempted once.
Using C<\G> without C</g> on a target string that has not previously had a
C</g> match applied to it is the same as using the C<\A> assertion to match
-the beginning of the string.
+the beginning of the string. Note also that, currently, C<\G> is only
+properly supported when anchored at the very beginning of the pattern.
Examples:
$output = `cmd 3>&1 1>&2 2>&3 3>&-`;
To read both a command's STDOUT and its STDERR separately, it's easiest
-and safest to redirect them separately to files, and then read from those
-files when the program is done:
+to redirect them separately to files, and then read from those files
+when the program is done:
- system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
+ system("program args 1>program.stdout 2>program.stderr");
Using single-quote as a delimiter protects the command from Perl's
double-quote interpolation, passing it on to the shell instead:
split(' ', q/STRING/);
-the difference being that it generates a real list at compile time. So
+the differences being that it generates a real list at compile time, and
+in scalar context it returns the last element in the list. So
this expression:
qw(foo bar baz)
\*/ # Match the closing delimiter.
} []gsx;
- s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively
+ s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
- for ($variable) { # trim white space in $variable, cheap
+ for ($variable) { # trim whitespace in $variable, cheap
s/^\s+//;
s/\s+$//;
}
eval "tr/$oldlist/$newlist/, 1" or die $@;
+=item <<EOF
+
+A line-oriented form of quoting is based on the shell "here-document"
+syntax. Following a C<< << >> you specify a string to terminate
+the quoted material, and all lines following the current line down to
+the terminating string are the value of the item. The terminating
+string may be either an identifier (a word), or some quoted text. If
+quoted, the type of quotes you use determines the treatment of the
+text, just as in regular quoting. An unquoted identifier works like
+double quotes. There must be no space between the C<< << >> and
+the identifier, unless the identifier is quoted. (If you put a space it
+will be treated as a null identifier, which is valid, and matches the first
+empty line.) The terminating string must appear by itself (unquoted and
+with no surrounding whitespace) on the terminating line.
+
+ print <<EOF;
+ The price is $Price.
+ EOF
+
+ print << "EOF"; # same as above
+ The price is $Price.
+ EOF
+
+ print << `EOC`; # execute commands
+ echo hi there
+ echo lo there
+ EOC
+
+ print <<"foo", <<"bar"; # you can stack them
+ I said foo.
+ foo
+ I said bar.
+ bar
+
+ myfunc(<< "THIS", 23, <<'THAT');
+ Here's a line
+ or two.
+ THIS
+ and here's another.
+ THAT
+
+Just don't forget that you have to put a semicolon on the end
+to finish the statement, as Perl doesn't know you're not going to
+try to do this:
+
+ print <<ABC
+ 179231
+ ABC
+ + 20;
+
+If you want your here-docs to be indented with the
+rest of the code, you'll need to remove leading whitespace
+from each line manually:
+
+ ($quote = <<'FINIS') =~ s/^\s+//gm;
+ The Road goes ever on and on,
+ down from the door where it began.
+ FINIS
+
+If you use a here-doc within a delimited construct, such as in C<s///eg>,
+the quoted material must come on the lines following the final delimiter.
+So instead of
+
+ s/this/<<E . 'that'
+ the other
+ E
+ . 'more '/eg;
+
+you have to write
+
+ s/this/<<E . 'that'
+ . 'more '/eg;
+ the other
+ E
+
+If the terminating identifier is on the last line of the program, you
+must be sure there is a newline after it; otherwise, Perl will give the
+warning B<Can't find string terminator "END" anywhere before EOF...>.
+
+Additionally, the quoting rules for the identifier are not related to
+Perl's quoting rules -- C<q()>, C<qq()>, and the like are not supported
+in place of C<''> and C<"">, and the only interpolation is for backslashing
+the quoting character:
+
+ print << "abc\"def";
+ testing...
+ abc"def
+
+Finally, quoted strings cannot span multiple lines. The general rule is
+that the identifier must be a string literal. Stick with that, and you
+should be safe.
+
=back
=head2 Gory details of parsing quoted constructs
quoting constructs, Perl performs different numbers of passes, from
one to five, but these passes are always performed in the same order.
-=over
+=over 4
=item Finding the end
the example above is not C<m//x>, but rather C<m//> with no C</x>
modifier. So the embedded C<#> is interpreted as a literal C<#>.
+Also no attention is paid to C<\c\> during this search.
+Thus the second C<\> in C<qq/\c\/> is interpreted as a part of C<\/>,
+and the following C</> is not recognized as a delimiter.
+Instead, use C<\034> or C<\x1c> at the end of quoted constructs.
+
=item Removal of backslashes before delimiters
During the second pass, text between the starting and ending
The next step is interpolation in the text obtained, which is now
delimiter-independent. There are four different cases.
-=over
+=over 4
=item C<<<'EOF'>, C<m''>, C<s'''>, C<tr///>, C<y///>
In the RE above, which is intentionally obfuscated for illustration, the
delimiter is C<m>, the modifier is C<mx>, and after backslash-removal the
-RE is the same as for C<m/ ^ a s* b /mx>). There's more than one
+RE is the same as for C<m/ ^ a \s* b /mx>. There's more than one
reason you're encouraged to restrict your delimiters to non-alphanumeric,
non-whitespace choices.
A string enclosed by backticks (grave accents) first undergoes
double-quote interpolation. It is then interpreted as an external
command, and the output of that command is the value of the
-pseudo-literal, j
-string consisting of all output is returned. In list context, a
-list of values is returned, one per line of output. (You can set
-C<$/> to use a different line terminator.) The command is executed
-each time the pseudo-literal is evaluated. The status value of the
-command is returned in C<$?> (see L<perlvar> for the interpretation
-of C<$?>). Unlike in B<csh>, no translation is done on the return
-data--newlines remain newlines. Unlike in any of the shells, single
-quotes do not hide variable names in the command from interpretation.
-To pass a literal dollar-sign through to the shell you need to hide
-it with a backslash. The generalized form of backticks is C<qx//>.
-(Because backticks always undergo shell expansion as well, see
-L<perlsec> for security concerns.)
+backtick string, like in a shell. In scalar context, a single string
+consisting of all output is returned. In list context, a list of
+values is returned, one per line of output. (You can set C<$/> to use
+a different line terminator.) The command is executed each time the
+pseudo-literal is evaluated. The status value of the command is
+returned in C<$?> (see L<perlvar> for the interpretation of C<$?>).
+Unlike in B<csh>, no translation is done on the return data--newlines
+remain newlines. Unlike in any of the shells, single quotes do not
+hide variable names in the command from interpretation. To pass a
+literal dollar-sign through to the shell you need to hide it with a
+backslash. The generalized form of backticks is C<qx//>. (Because
+backticks always undergo shell expansion as well, see L<perlsec> for
+security concerns.)
In scalar context, evaluating a filehandle in angle brackets yields
the next line from that file (the newline, if any, included), or
the value is automatically assigned to the global variable $_,
destroying whatever was there previously. (This may seem like an
odd thing to you, but you'll use the construct in almost every Perl
-script you write.) The $_ variables is not implicitly localized.
+script you write.) The $_ variable is not implicitly localized.
You'll have to put a C<local $_;> before the loop if you want that
to happen.
If you call it again after this, it will assume you are processing another
@ARGV list, and if you haven't set @ARGV, will read input from STDIN.
-If angle brackets contain is a simple scalar variable (e.g.,
+If what the angle brackets contain is a simple scalar variable (e.g.,
<$foo>), then that variable contains the name of the
filehandle to input from, or its typeglob, or a reference to the
same. For example:
grounds alone. That means C<< <$x> >> is always a readline() from
an indirect handle, but C<< <$hash{key}> >> is always a glob().
That's because $x is a simple scalar variable, but C<$hash{key}> is
-not--it's a hash element.
+not--it's a hash element. Even C<< <$x > >> (note the extra space)
+is treated as C<glob("$x ")>, not C<readline($x)>.
One level of double-quote interpretation is done first, but you can't
say C<< <$foo> >> because that's an indirect filehandle as explained
open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
while (<FOO>) {
- chop;
+ chomp;
chmod 0644, $_;
}
because the latter will alternate between returning a filename and
returning false.
-It you're trying to do variable interpolation, it's definitely better
+If you're trying to do variable interpolation, it's definitely better
to use the glob() function, because the older notation can cause people
to become confused with the indirect filehandle notation.
the compiler will precompute the number which that expression
represents so that the interpreter won't have to.
+=head2 No-ops
+
+Perl doesn't officially have a no-op operator, but the bare constants
+C<0> and C<1> are special-cased to not produce a warning in a void
+context, so you can for example safely do
+
+ 1 while foo();
+
=head2 Bitwise String Operators
Bitstrings of any size may be manipulated by the bitwise operators
or so.
Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<",
-and ">>") always produce integral results. (But see also L<Bitwise
-String Operators>.) However, C<use integer> still has meaning for
+and ">>") always produce integral results. (But see also
+L<Bitwise String Operators>.) However, C<use integer> still has meaning for
them. By default, their results are interpreted as unsigned integers, but
if C<use integer> is in effect, their results are interpreted
as signed integers. For example, C<~0> usually evaluates to a large
The standard Math::BigInt and Math::BigFloat modules provide
variable-precision arithmetic and overloaded operators, although
-they're currently pretty slow. At the cost of some space and
+they're currently pretty slow. At the cost of some space and
considerable speed, they avoid the normal pitfalls associated with
limited-precision representations.
# prints +15241578780673678515622620750190521
-The non-standard modules SSLeay::BN and Math::Pari provide
-equivalent functionality (and much more) with a substantial
-performance savings.
+There are several modules that let you calculate with (bound only by
+memory and cpu-time) unlimited or fixed precision. There are also
+some non-standard modules that provide faster implementations via
+external C libraries.
+
+Here is a short, but incomplete summary:
+
+ Math::Fraction big, unlimited fractions like 9973 / 12967
+ Math::String treat string sequences like numbers
+ Math::FixedPrecision calculate with a fixed precision
+ Math::Currency for currency calculations
+ Bit::Vector manipulate bit vectors fast (uses C)
+ Math::BigIntFast Bit::Vector wrapper for big numbers
+ Math::Pari provides access to the Pari C library
+ Math::BigInteger uses an external C library
+ Math::Cephes uses external Cephes C library (no big numbers)
+ Math::Cephes::Fraction fractions via the Cephes library
+ Math::GMP another one using an external C library
+
+Choose wisely.
=cut
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