PERLOP(1) Perl Programmers Reference Guide PERLOP(1)NAMEperlop - Perl operators and precedence
DESCRIPTION
Operator Precedence and Associativity
Operator precedence and associativity work in Perl more or
less like they do in mathematics.
Operator precedence means some operators are evaluated
before others. For example, in "2 + 4 * 5", the multiplica-
tion has higher precedence so "4 * 5" is evaluated first
yielding "2 + 20 == 22" and not "6 * 5 == 30".
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 "8 - 4 - 2", subtraction is left
associative so Perl evaluates the expression left to right.
"8 - 4" is evaluated first making the expression "4 - 2 ==
2" and not "8 - 2 == 6".
Perl operators have the following associativity and pre-
cedence, listed from highest precedence to lowest. Opera-
tors borrowed from C keep the same precedence relationship
with each other, even where C's precedence is slightly
screwy. (This makes learning Perl easier for C folks.)
With very few exceptions, these all operate on scalar values
only, not array values.
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left terms and list operators (leftward)
left ->
nonassoc ++ --
right **
right ! ~ \ and unary + and -
left =~ !~
left * / % x
left + - .
left << >>
nonassoc named unary operators
nonassoc < > <= >= lt gt le ge
nonassoc == != <=> eq ne cmp
left &
left | ^
left &&
left ||
nonassoc .. ...
right ?:
right = += -= *= etc.
left , =>
nonassoc list operators (rightward)
right not
left and
left or xor
In the following sections, these operators are covered in
precedence order.
Many operators can be overloaded for objects. See overload.
Terms and List Operators (Leftward)
A TERM has the highest precedence in Perl. They include
variables, quote and quote-like operators, any expression in
parentheses, and any function whose arguments are
parenthesized. Actually, there aren't really functions in
this sense, just list operators and unary operators behaving
as functions because you put parentheses around the argu-
ments. These are all documented in perlfunc.
If any list operator (print(), etc.) or any unary operator
(chdir(), etc.) is followed by a left parenthesis as the
next token, the operator and arguments within parentheses
are taken to be of highest precedence, just like a normal
function call.
In the absence of parentheses, the precedence of list opera-
tors such as "print", "sort", or "chmod" is either very high
or very low depending on whether you are looking at the left
side or the right side of the operator. For example, in
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@ary = (1, 3, sort 4, 2);
print @ary; # prints 1324
the commas on the right of the sort are evaluated before the
sort, but the commas on the left are evaluated after. In
other words, list operators tend to gobble up all arguments
that follow, and then act like a simple TERM with regard to
the preceding expression. Be careful with parentheses:
# These evaluate exit before doing the print:
print($foo, exit); # Obviously not what you want.
print $foo, exit; # Nor is this.
# These do the print before evaluating exit:
(print $foo), exit; # This is what you want.
print($foo), exit; # Or this.
print ($foo), exit; # Or even this.
Also note that
print ($foo & 255) + 1, "\n";
probably doesn't do what you expect at first glance. The
parentheses enclose the argument list for "print" which is
evaluated (printing the result of "$foo & 255"). Then one
is added to the return value of "print" (usually 1). The
result is something like this:
1 + 1, "\n"; # Obviously not what you meant.
To do what you meant properly, you must write:
print(($foo & 255) + 1, "\n");
See "Named Unary Operators" for more discussion of this.
Also parsed as terms are the "do {}" and "eval {}" con-
structs, as well as subroutine and method calls, and the
anonymous constructors "[]" and "{}".
See also "Quote and Quote-like Operators" toward the end of
this section, as well as "I/O Operators".
The Arrow Operator
""->"" is an infix dereference operator, just as it is in C
and C++. If the right side is either a "[...]", "{...}", or
a "(...)" subscript, then the left side must be either a
hard or symbolic reference to an array, a hash, or a subrou-
tine respectively. (Or technically speaking, a location
capable of holding a hard reference, if it's an array or
hash reference being used for assignment.) See perlreftut
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and perlref.
Otherwise, the right side is a method name or a simple
scalar variable containing either the method name or a sub-
routine reference, and the left side must be either an
object (a blessed reference) or a class name (that is, a
package name). See perlobj.
Auto-increment and Auto-decrement
"++" 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 when the vari-
able 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 + $i ++;
Perl will not guarantee what the result of the above state-
ments is.
The auto-increment operator has a little extra builtin magic
to it. If you increment a variable that is numeric, or that
has ever been used in a numeric context, you get a normal
increment. If, however, the variable has been used in only
string contexts since it was set, and has a value that is
not the empty string and matches the pattern
"/^[a-zA-Z]*[0-9]*\z/", the increment is done as a string,
preserving each character within its range, with carry:
print ++($foo = '99'); # prints '100'
print ++($foo = 'a0'); # prints 'a1'
print ++($foo = 'Az'); # prints 'Ba'
print ++($foo = 'zz'); # prints 'aaa'
"undef" is always treated as numeric, and in particular is
changed to 0 before incrementing (so that a post-increment
of an undef value will return 0 rather than "undef").
The auto-decrement operator is not magical.
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Exponentiation
Binary "**" is the exponentiation operator. It binds even
more tightly than unary minus, so -2**4 is -(2**4), not
(-2)**4. (This is implemented using C's pow(3) function,
which actually works on doubles internally.)
Symbolic Unary Operators
Unary "!" performs logical negation, i.e., "not". See also
"not" for a lower precedence version of this.
Unary "-" performs arithmetic negation if the operand is
numeric. If the operand is an identifier, a string consist-
ing of a minus sign concatenated with the identifier is
returned. Otherwise, if the string starts with a plus or
minus, a string starting with the opposite sign is returned.
One effect of these rules is that -bareword is equivalent to
the string "-bareword". If, however, the string begins with
a non-alphabetic character (exluding "+" or "-"), Perl will
attempt to convert the string to a numeric and the arith-
metic negation is performed. If the string cannot be cleanly
converted to a numeric, Perl will give the warning Argument
"the string" isn't numeric in negation (-) at ....
Unary "~" performs bitwise negation, i.e., 1's complement.
For example, "0666 & ~027" is 0640. (See also "Integer
Arithmetic" and "Bitwise String Operators".) Note that the
width of the result is platform-dependent: ~0 is 32 bits
wide on a 32-bit platform, but 64 bits wide on a 64-bit
platform, so if you are expecting a certain bit width,
remember 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 that would otherwise be interpreted
as the complete list of function arguments. (See examples
above under "Terms and List Operators (Leftward)".)
Unary "\" creates a reference to whatever follows it. See
perlreftut and perlref. Do not confuse this behavior with
the behavior of backslash within a string, although both
forms do convey the notion of protecting the next thing from
interpolation.
Binding Operators
Binary "=~" binds a scalar expression to a pattern match.
Certain operations search or modify the string $_ by
default. This operator makes that kind of operation work on
some other string. The right argument is a search pattern,
substitution, or transliteration. The left argument is what
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is supposed to be searched, substituted, or transliterated
instead of the default $_. When used in scalar context, the
return value generally indicates the success of the opera-
tion. Behavior in list context depends on the particular
operator. See "Regexp Quote-Like Operators" for details and
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.
Binary "!~" is just like "=~" except the return value is
negated in the logical sense.
Multiplicative Operators
Binary "*" multiplies two numbers.
Binary "/" divides two numbers.
Binary "%" computes the modulus of two numbers. Given
integer operands $a and $b: If $b is positive, then "$a %
$b" is $a minus the largest multiple of $b that is not
greater than $a. If $b is negative, then "$a % $b" is $a
minus the smallest multiple of $b that is not less than $a
(i.e. the result will be less than or equal to zero). Note
that when "use integer" is in scope, "%" gives you direct
access to the modulus operator as implemented by your C com-
piler. This operator is not as well defined for negative
operands, but it will execute faster.
Binary "x" is the repetition operator. In scalar context or
if the left operand is not enclosed in parentheses, it
returns a string consisting of the left operand repeated the
number of times specified by the right operand. In list
context, if the left operand is enclosed in parentheses or
is a list formed by "qw/STRING/", it repeats the list. If
the right operand is zero or negative, it returns an empty
string or an empty list, depending on the context.
print '-' x 80; # print row of dashes
print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
@ones = (1) x 80; # a list of 80 1's
@ones = (5) x @ones; # set all elements to 5
Additive Operators
Binary "+" returns the sum of two numbers.
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Binary "-" returns the difference of two numbers.
Binary "." concatenates two strings.
Shift Operators
Binary "<<" returns the value of its left argument shifted
left by the number of bits specified by the right argument.
Arguments should be integers. (See also "Integer Arith-
metic".)
Binary ">>" returns the value of its left argument shifted
right by the number of bits specified by the right argument.
Arguments should be integers. (See also "Integer Arith-
metic".)
Note that both "<<" and ">>" in Perl are implemented
directly using "<<" and ">>" in C. If "use integer" (see
"Integer Arithmetic") is in force then signed C integers are
used, else unsigned C integers are used. Either way, the
implementation isn't going to generate results larger than
the size of the integer type Perl was built with (32 bits or
64 bits).
The result of overflowing the range of the integers is unde-
fined because it is undefined also in C. In other words,
using 32-bit integers, "1 << 32" is undefined. Shifting by
a negative number of bits is also undefined.
Named Unary Operators
The various named unary operators are treated as functions
with one 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. For example, because named unary operators
are higher precedence than ||:
chdir $foo || die; # (chdir $foo) || die
chdir($foo) || die; # (chdir $foo) || die
chdir ($foo) || die; # (chdir $foo) || die
chdir +($foo) || die; # (chdir $foo) || die
but, because * is higher precedence than named operators:
chdir $foo * 20; # chdir ($foo * 20)
chdir($foo) * 20; # (chdir $foo) * 20
chdir ($foo) * 20; # (chdir $foo) * 20
chdir +($foo) * 20; # chdir ($foo * 20)
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rand 10 * 20; # rand (10 * 20)
rand(10) * 20; # (rand 10) * 20
rand (10) * 20; # (rand 10) * 20
rand +(10) * 20; # rand (10 * 20)
Regarding precedence, the filetest operators, like "-f",
"-M", etc. are treated like named unary operators, but they
don't follow this functional parenthesis rule. That means,
for example, that "-f($file).".bak"" is equivalent to "-f
"$file.bak"".
See also "Terms and List Operators (Leftward)".
Relational Operators
Binary "<" returns true if the left argument is numerically
less than the right argument.
Binary ">" returns true if the left argument is numerically
greater than the right argument.
Binary "<=" returns true if the left argument is numerically
less than or equal to the right argument.
Binary ">=" returns true if the left argument is numerically
greater than or equal to the right argument.
Binary "lt" returns true if the left argument is stringwise
less than the right argument.
Binary "gt" returns true if the left argument is stringwise
greater than the right argument.
Binary "le" returns true if the left argument is stringwise
less than or equal to the right argument.
Binary "ge" returns true if the left argument is stringwise
greater than or equal to the right argument.
Equality Operators
Binary "==" returns true if the left argument is numerically
equal to the right argument.
Binary "!=" returns true if the left argument is numerically
not equal to the right argument.
Binary "<=>" returns -1, 0, or 1 depending on whether the
left argument is numerically less than, equal to, or greater
than the right argument. If your platform supports NaNs
(not-a-numbers) as numeric values, using them with "<=>"
returns undef. NaN is not "<", "==", ">", "<=" or ">="
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anything (even NaN), so those 5 return false. NaN != NaN
returns true, as does NaN != anything else. If your platform
doesn't support NaNs then NaN is just a string with numeric
value 0.
perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
Binary "eq" returns true if the left argument is stringwise
equal to the right argument.
Binary "ne" returns true if the left argument is stringwise
not equal to the right argument.
Binary "cmp" returns -1, 0, or 1 depending on whether the
left argument is stringwise less than, equal to, or greater
than the right argument.
"lt", "le", "ge", "gt" and "cmp" use the collation (sort)
order specified by the current locale if "use locale" is in
effect. See perllocale.
Bitwise And
Binary "&" returns its operands ANDed together bit by bit.
(See also "Integer Arithmetic" and "Bitwise String Opera-
tors".)
Note that "&" has lower priority than relational operators,
so for example the brackets are essential in a test like
print "Even\n" if ($x & 1) == 0;
Bitwise Or and Exclusive Or
Binary "|" returns its operands ORed together bit by bit.
(See also "Integer Arithmetic" and "Bitwise String Opera-
tors".)
Binary "^" returns its operands XORed together bit by bit.
(See also "Integer Arithmetic" and "Bitwise String Opera-
tors".)
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;
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C-style Logical And
Binary "&&" performs a short-circuit logical AND operation.
That is, if the left operand is false, the right operand is
not even evaluated. Scalar or list context propagates down
to the right operand if it is evaluated.
C-style Logical Or
Binary "||" performs a short-circuit logical OR operation.
That is, if the left operand is true, the right operand is
not even evaluated. Scalar or list context propagates down
to the right operand if it is evaluated.
The "||" and "&&" operators return the last value evaluated
(unlike C's "||" and "&&", 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 = @b || @c; # this is wrong
@a = scalar(@b) || @c; # really meant this
@a = @b ? @b : @c; # this works fine, though
As more readable alternatives to "&&" and "||" when used for
control flow, Perl provides "and" and "or" operators (see
below). The short-circuit behavior is identical. The pre-
cedence of "and" and "or" is much lower, however, so that
you can safely use them after a list operator without the
need for parentheses:
unlink "alpha", "beta", "gamma"
or gripe(), next LINE;
With the C-style operators that would have been written like
this:
unlink("alpha", "beta", "gamma")
|| (gripe(), next LINE);
Using "or" for assignment is unlikely to do what you want;
see below.
Range Operators
Binary ".." is the range operator, which is really two dif-
ferent operators depending on the context. In list context,
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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 list. The
range operator is useful for writing "foreach (1..10)" loops
and for doing slice operations on arrays. In the current
implementation, no temporary array is created when the range
operator is used as the expression in "foreach" loops, but
older versions of Perl might burn a lot of memory when you
write something like this:
for (1 .. 1_000_000) {
# code
}
The range operator also works on strings, using the magical
auto-increment, see below.
In scalar context, ".." returns a boolean value. The opera-
tor is bistable, like a flip-flop, and emulates the line-
range (comma) operator of sed, awk, and various editors.
Each ".." operator maintains its own boolean state. It is
false as long as its left operand is false. Once the left
operand is true, the range operator stays true until the
right operand is true, AFTER which the range operator
becomes false again. It doesn't become false till the next
time the range operator is evaluated. It can test the right
operand and become false on the same evaluation it became
true (as in awk), but it still returns true once. If you
don't want it to test the right operand till the next
evaluation, as in sed, just use three dots ("...") instead
of two. In all other regards, "..." behaves just like ".."
does.
The right operand is not evaluated while the operator is in
the "false" state, and the left operand is not evaluated
while the operator is in the "true" state. The precedence
is a little lower than || and &&. The value returned is
either the empty string for false, or a sequence number
(beginning with 1) for true. The sequence number is reset
for each range encountered. The final sequence number in a
range has the string "E0" appended to it, which doesn't
affect its numeric value, but gives you something to search
for if you want to exclude the endpoint. You can exclude
the beginning point by waiting for the sequence number to be
greater than 1.
If either operand of scalar ".." is a constant expression,
that operand is considered true if it is equal ("==") to the
current input line number (the $. variable).
To be pedantic, the comparison is actually "int(EXPR) ==
int(EXPR)", but that is only an issue if you use a floating
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point expression; when implicitly using $. as described in
the previous paragraph, the comparison is "int(EXPR) ==
int($.)" which is only an issue when $. is set to a floating
point value and you are not reading from a file. Further-
more, "span" .. "spat" or "2.18 .. 3.14" will not do what
you want in scalar context because each of the operands are
evaluated using their integer representation.
Examples:
As a scalar operator:
if (101 .. 200) { print; } # print 2nd hundred lines, short for
# if ($. == 101 .. $. == 200) ...
next LINE if (1 .. /^$/); # skip header lines, short for
# ... if ($. == 1 .. /^$/);
# (typically in a loop labeled LINE)
s/^/> / if (/^$/ .. eof()); # quote body
# parse mail messages
while (<>) {
$in_header = 1 .. /^$/;
$in_body = /^$/ .. eof;
if ($in_header) {
# ...
} else { # in body
# ...
}
} continue {
close ARGV if eof; # reset $. each file
}
Here's a simple example to illustrate the difference between
the two range operators:
@lines = (" - Foo",
"01 - Bar",
"1 - Baz",
" - Quux");
foreach (@lines) {
if (/0/ .. /1/) {
print "$_\n";
}
}
This program will print only the line containing "Bar". If
the range operator is changed to "...", it will also print
the "Baz" line.
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And now some examples 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 magi-
cal auto-increment algorithm if the operands are strings.
You can say
@alphabet = ('A' .. 'Z');
to get all normal letters of the English 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 speci-
fied 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.
Because each operand is evaluated in integer form, "2.18 ..
3.14" will return two elements in list context.
@list = (2.18 .. 3.14); # same as @list = (2 .. 3);
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 = $ok ? @b : @c; # oops, that's just a count!
The operator may be assigned to if both the 2nd and 3rd
arguments are legal lvalues (meaning that you can assign to
them):
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($a_or_b ? $a : $b) = $c;
Because this operator produces an assignable result, using
assignments without parentheses will get you in trouble.
For example, this:
$a % 2 ? $a += 10 : $a += 2
Really means this:
(($a % 2) ? ($a += 10) : $a) += 2
Rather than this:
($a % 2) ? ($a += 10) : ($a += 2)
That should probably be written more simply as:
$a += ($a % 2) ? 10 : 2;
Assignment Operators
"=" is the ordinary assignment operator.
Assignment operators work as in C. That is,
$a += 2;
is equivalent to
$a = $a + 2;
although without duplicating any side effects that dere-
ferencing the lvalue might trigger, such as from tie().
Other assignment operators work similarly. The following are
recognized:
**= += *= &= <<= &&=
-= /= |= >>= ||=
.= %= ^=
x=
Although these are grouped by family, they all have the pre-
cedence of assignment.
Unlike in C, the scalar assignment operator produces a valid
lvalue. Modifying an assignment is equivalent to doing the
assignment and then modifying the variable that was assigned
to. This is useful for modifying a copy of something, like
this:
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($tmp = $global) =~ tr [A-Z] [a-z];
Likewise,
($a += 2) *= 3;
is equivalent to
$a += 2;
$a *= 3;
Similarly, a list assignment in list context produces the
list of lvalues assigned to, and a list assignment in scalar
context returns the number of elements produced by the
expression on the right hand side of the assignment.
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 "=>" 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 func-
tion call.
use constant FOO => "something";
my %h = ( FOO => 23 );
is equivalent to:
my %h = ("FOO", 23);
It is 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 "=>" operator is helpful in documenting the correspon-
dence between keys and values in hashes, and other paired
elements in lists.
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%hash = ( $key => $value );
login( $username => $password );
List Operators (Rightward)
On the right side of a list operator, it has very low pre-
cedence, such that it controls all comma-separated expres-
sions 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 "Terms and List
Operators (Leftward)".
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.
Logical And
Binary "and" returns the logical conjunction of the two sur-
rounding 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.
Logical or and Exclusive Or
Binary "or" returns the logical disjunction of the two sur-
rounding 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 expres-
sion 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 || $c; # better written this way
However, when it's a list-context assignment and you're try-
ing to use "||" for control flow, you probably need "or" so
that the assignment takes higher precedence.
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@info = stat($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.
C Operators Missing From Perl
Here is what C has that Perl doesn't:
unary & Address-of operator. (But see the "\" operator for
taking a reference.)
unary * Dereference-address operator. (Perl's prefix dere-
ferencing operators are typed: $, @, %, and &.)
(TYPE) Type-casting operator.
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 pro-
vides customary quote characters for these behaviors, but
also provides a way for you to choose your quote character
for any of them. In the following table, a "{}" represents
any pair of delimiters you choose.
Customary Generic Meaning Interpolates
'' q{} Literal no
"" qq{} Literal yes
`` qx{} Command yes*
qw{} Word list no
// 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 that
q{foo{bar}baz}
is the same as
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'foo{bar}baz'
Note, however, that this does not always work for quoting
Perl code:
$s = q{ if($a eq "}") ... }; # WRONG
is a syntax error. The "Text::Balanced" module (from CPAN,
and starting from Perl 5.8 part of the standard distribu-
tion) is able to do this properly.
There can be whitespace between the operator and the quoting
characters, except when "#" is being used as the quoting
character. "q#foo#" is parsed as the string "foo", while "q
#foo#" is the operator "q" followed by a comment. Its argu-
ment will be taken from the next line. This allows you to
write:
s {foo} # Replace foo
{bar} # with bar.
The following escape sequences are available in constructs
that interpolate and in transliterations.
\t tab (HT, TAB)
\n newline (NL)
\r return (CR)
\f form feed (FF)
\b backspace (BS)
\a alarm (bell) (BEL)
\e escape (ESC)
\033 octal char (ESC)
\x1b hex char (ESC)
\x{263a} wide hex char (SMILEY)
\c[ control char (ESC)
\N{name} named Unicode character
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
\L lowercase till \E
\U uppercase till \E
\E end case modification
\Q quote non-word characters till \E
If "use locale" is in effect, the case map used by "\l",
"\L", "\u" and "\U" is taken from the current locale. See
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perllocale. If Unicode (for example, "\N{}" or wide hex
characters of 0x100 or beyond) is being used, the case map
used by "\l", "\L", "\u" and "\U" is as defined by Unicode.
For documentation of "\N{name}", see charnames.
All systems use the virtual "\n" to represent a line termi-
nator, called a "newline". There is no such thing as an
unvarying, physical newline character. It is only an illu-
sion that the operating system, device drivers, C libraries,
and Perl all conspire to preserve. Not all systems read
"\r" as ASCII CR and "\n" as ASCII LF. For example, on a
Mac, these are reversed, and on systems without line termi-
nator, printing "\n" may emit no actual data. In general,
use "\n" when you mean a "newline" for your system, but use
the literal ASCII when you need an exact character. For
example, most networking protocols expect and prefer a CR+LF
("\015\012" or "\cM\cJ") for line terminators, and although
they often accept just "\012", they seldom tolerate just
"\015". If you get in the habit of using "\n" for network-
ing, you may be burned some day.
For constructs that do interpolate, variables beginning with
""$"" or ""@"" are interpolated. Subscripted variables such
as $a[3] or "$href->{key}[0]" are also interpolated, as are
array and hash slices. But method calls such as "$obj->meth"
are not.
Interpolating an array or slice interpolates the elements in
order, separated by the value of $", so is equivalent to
interpolating "join $", @array". "Punctuation" arrays
such as "@+" are only interpolated if the name is enclosed
in braces "@{+}".
You cannot include a literal "$" or "@" within a "\Q"
sequence. An unescaped "$" or "@" interpolates the
corresponding variable, while escaping will cause the
literal string "\$" to be inserted. You'll need to write
something like "m/\Quser\E\@\Qhost/".
Patterns are subject to an additional level of interpreta-
tion as a regular expression. This is done as a second
pass, after variables are interpolated, so that regular
expressions may be incorporated into the pattern from the
variables. If this is not what you want, use "\Q" to inter-
polate a variable literally.
Apart from the behavior described above, Perl does not
expand multiple levels of interpolation. In particular,
contrary to the expectations of shell programmers, back-
quotes do NOT interpolate within double quotes, nor do sin-
gle quotes impede evaluation of variables when used within
double quotes.
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Regexp Quote-Like Operators
Here are the quote-like operators that apply to pattern
matching and related activities.
?PATTERN?
This is just like the "/pattern/" search, except
that it matches only once between calls to the
reset() operator. This is a useful optimization
when you want to see only the first occurrence of
something in each file of a set of files, for
instance. Only "??" patterns local to the current
package are reset.
while (<>) {
if (?^$?) {
# blank line between header and body
}
} continue {
reset if eof; # clear ?? status for next file
}
This usage is vaguely deprecated, which means it
just might possibly be removed in some distant
future version of Perl, perhaps somewhere around the
year 2168.
m/PATTERN/cgimosx
/PATTERN/cgimosx
Searches a string for a pattern match, and in scalar
context returns true if it succeeds, false if it
fails. If no string is specified via the "=~" or
"!~" operator, the $_ string is searched. (The
string specified with "=~" need not be an lvalue--it
may be the result of an expression evaluation, but
remember the "=~" binds rather tightly.) See also
perlre. See perllocale for discussion of additional
considerations that apply when "use locale" is in
effect.
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 "m" is
optional. With the "m" you can use any pair of
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non-alphanumeric, non-whitespace characters as del-
imiters. This is particularly useful for matching
path names that contain "/", to avoid LTS (leaning
toothpick syndrome). If "?" is the delimiter, then
the match-only-once rule of "?PATTERN?" applies. If
"'" is the delimiter, no interpolation is performed
on the PATTERN.
PATTERN may contain variables, which will be inter-
polated (and the pattern recompiled) every time the
pattern search is evaluated, except for when the
delimiter is a single quote. (Note that $(, $), and
$| are not interpolated because they look like end-
of-string tests.) If you want such a pattern to be
compiled only once, add a "/o" after the trailing
delimiter. This avoids expensive run-time recompi-
lations, and is useful when the value you are inter-
polating won't change over the life of the script.
However, mentioning "/o" constitutes a promise that
you won't change the variables in the pattern. If
you change them, Perl won't even notice. See also
"qr/STRING/imosx".
If the PATTERN evaluates to the empty string, the
last successfully matched regular expression is used
instead. In this case, only the "g" and "c" flags on
the empty pattern is honoured - the other flags are
taken from the original pattern. If no match has
previously succeeded, this will (silently) act
instead as a genuine empty pattern (which will
always match).
If the "/g" option is not used, "m//" in list con-
text returns a list consisting of the subexpressions
matched by the parentheses in the pattern, i.e.,
($1, $2, $3...). (Note that here $1 etc. are also
set, and that this differs from Perl 4's behavior.)
When there are no parentheses in the pattern, the
return value is the list "(1)" for success. With or
without parentheses, an empty list is returned upon
failure.
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#;
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# 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 $Etc. The con-
ditional is true if any variables were assigned,
i.e., if the pattern matched.
The "/g" modifier specifies global pattern
matching--that is, matching as many times as possi-
ble within the string. How it behaves depends on
the context. In list context, it returns a list of
the substrings matched by any capturing parentheses
in the regular expression. If there are no
parentheses, it returns a list of all the matched
strings, as if there were parentheses around the
whole pattern.
In scalar context, each execution of "m//g" finds
the next match, returning true if it matches, and
false if there is no further match. The position
after the last match can be read or set using the
pos() function; see "pos" in perlfunc. A failed
match normally resets the search position to the
beginning of the string, but you can avoid that by
adding the "/c" modifier (e.g. "m//gc"). Modifying
the target string also resets the search position.
You can intermix "m//g" matches with "m/\G.../g",
where "\G" is a zero-width assertion that matches
the exact position where the previous "m//g", if
any, left off. Without the "/g" modifier, the "\G"
assertion still anchors at pos(), but the match is
of course only attempted once. Using "\G" without
"/g" on a target string that has not previously had
a "/g" match applied to it is the same as using the
"\A" assertion to match the beginning of the string.
Note also that, currently, "\G" is only properly
supported when anchored at the very beginning of the
pattern.
Examples:
# list context
($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
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# scalar context
$/ = "";
while (defined($paragraph = <>)) {
while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
$sentences++;
}
}
print "$sentences\n";
# using m//gc with \G
$_ = "ppooqppqq";
while ($i++ < 2) {
print "1: '";
print $1 while /(o)/gc; print "', pos=", pos, "\n";
print "2: '";
print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
print "3: '";
print $1 while /(p)/gc; print "', pos=", pos, "\n";
}
print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
The last example should print:
1: 'oo', pos=4
2: 'q', pos=5
3: 'pp', pos=7
1: '', pos=7
2: 'q', pos=8
3: '', pos=8
Final: 'q', pos=8
Notice that the final match matched "q" instead of
"p", which a match without the "\G" anchor would
have done. Also note that the final match did not
update "pos" -- "pos" is only updated on a "/g"
match. If the final match did indeed match "p", it's
a good bet that you're running an older (pre-5.6.0)
Perl.
A useful idiom for "lex"-like scanners is
"/\G.../gc". You can combine several regexps like
this to process a string part-by-part, doing dif-
ferent actions depending on which regexp matched.
Each regexp tries to match where the previous one
leaves off.
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$_ = <<'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!
q/STRING/
'STRING'
A single-quoted, literal string. A backslash
represents a backslash unless followed by the delim-
iter or another backslash, in which case the delim-
iter or backslash is interpolated.
$foo = q!I said, "You said, 'She said it.'"!;
$bar = q('This is it.');
$baz = '\n'; # a two-character string
qq/STRING/
"STRING"
A double-quoted, interpolated string.
$_ .= qq
(*** The previous line contains the naughty word "$1".\n)
if /\b(tcl|java|python)\b/i; # :-)
$baz = "\n"; # a one-character string
qr/STRING/imosx
This operator quotes (and possibly compiles) its
STRING as a regular expression. STRING is interpo-
lated the same way as PATTERN in "m/PATTERN/". If
"'" is used as the delimiter, no interpolation is
done. Returns a Perl value which may be used
instead of the corresponding "/STRING/imosx" expres-
sion.
For example,
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$rex = qr/my.STRING/is;
s/$rex/foo/;
is equivalent to
s/my.STRING/foo/is;
The result may be used as a subpattern in a match:
$re = qr/$pattern/;
$string =~ /foo${re}bar/; # can be interpolated in other patterns
$string =~ $re; # or used standalone
$string =~ /$re/; # or this way
Since Perl may compile the pattern at the moment of
execution of qr() operator, using qr() may have
speed advantages in some situations, notably if the
result of qr() is used standalone:
sub match {
my $patterns = shift;
my @compiled = map qr/$_/i, @$patterns;
grep {
my $success = 0;
foreach my $pat (@compiled) {
$success = 1, last if /$pat/;
}
$success;
} @_;
}
Precompilation of the pattern into an internal
representation at the moment of qr() avoids a need
to recompile the pattern every time a match "/$pat/"
is attempted. (Perl has many other internal optimi-
zations, 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 perlre for additional information on valid syn-
tax for STRING, and for a detailed look at the
semantics of regular expressions.
qx/STRING/
`STRING`
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A string which is (possibly) interpolated and then
executed as a system command with "/bin/sh" or its
equivalent. Shell wildcards, pipes, and redirec-
tions will be honored. The collected standard out-
put of the command is returned; standard error is
unaffected. In scalar context, it comes back as a
single (potentially multi-line) string, or undef if
the command failed. In list context, returns a list
of lines (however you've defined lines with $/ or
$INPUT_RECORD_SEPARATOR), or an empty list if the
command failed.
Because backticks do not affect standard error, use
shell file descriptor syntax (assuming the shell
supports this) if you care to address this. To cap-
ture 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 to redirect them separately
to files, and then read from those files when the
program is done:
system("program args 1>program.stdout 2>program.stderr");
Using single-quote as a delimiter protects the com-
mand from Perl's double-quote interpolation, passing
it on to the shell instead:
$perl_info = qx(ps $$); # that's Perl's $$
$shell_info = qx'ps $$'; # that's the new shell's $$
How that string gets evaluated is entirely subject
to the command interpreter on your system. On most
platforms, you will have to protect shell
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metacharacters if you want them treated literally.
This is in practice difficult to do, as it's unclear
how to escape which characters. See perlsec for a
clean and safe example of a manual fork() and exec()
to emulate backticks safely.
On some platforms (notably DOS-like ones), the shell
may not be capable of dealing with multiline com-
mands, so putting newlines in the string may not get
you what you want. You may be able to evaluate mul-
tiple commands in a single line by separating them
with the command separator character, if your shell
supports that (e.g. ";" on many Unix shells; "&" on
the Windows NT "cmd" shell).
Beginning with v5.6.0, Perl will attempt to flush
all files opened for output before starting the
child process, but this may not be supported on some
platforms (see perlport). To be safe, you may need
to set $| ($AUTOFLUSH in English) or call the "auto-
flush()" method of "IO::Handle" on any open handles.
Beware that some command shells may place restric-
tions 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 "type" command under
the POSIX shell is very different from the "type"
command under DOS. That doesn't mean you should go
out of your way to avoid backticks when they're the
right way to get something done. Perl was made to
be a glue language, and one of the things it glues
together is commands. Just understand what you're
getting yourself into.
See "I/O Operators" for more discussion.
qw/STRING/
Evaluates to a list of the words extracted out of
STRING, using embedded whitespace as the word delim-
iters. It can be understood as being roughly
equivalent to:
split(' ', q/STRING/);
the differences being that it generates a real list
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at compile time, and in scalar context it returns
the last element in the list. So this expression:
qw(foo bar baz)
is semantically equivalent to the list:
'foo', 'bar', 'baz'
Some frequently seen examples:
use POSIX qw( setlocale localeconv )
@EXPORT = qw( foo bar baz );
A common mistake is to try to separate the words
with comma or to put comments into a multi-line
"qw"-string. For this reason, the "use warnings"
pragma and the -w switch (that is, the $^W variable)
produces warnings if the STRING contains the "," or
the "#" character.
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 "=~" or "!~"
operator, the $_ variable is searched and modified.
(The string specified with "=~" must be scalar vari-
able, 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 inter-
polation is done on either the PATTERN or the
REPLACEMENT. Otherwise, if the PATTERN contains a $
that looks like a variable rather than an end-of-
string test, the variable will be interpolated into
the pattern at run-time. If you want the pattern
compiled only once the first time the variable is
interpolated, use the "/o" option. If the pattern
evaluates to the empty string, the last successfully
executed regular expression is used instead. See
perlre for further explanation on these. See perllo-
cale for discussion of additional considerations
that apply when "use locale" is in effect.
Options are:
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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 "/e" modifier overrides this, however). Unlike
Perl 4, Perl 5 treats backticks as normal delim-
iters; the replacement text is not evaluated as a
command. If the PATTERN is delimited by bracketing
quotes, the REPLACEMENT has its own pair of quotes,
which may or may not be bracketing quotes, e.g.,
"s(foo)(bar)" or "s<foo>/bar/". A "/e" will cause
the replacement portion to be treated as a full-
fledged Perl expression and evaluated right then and
there. It is, however, syntax checked at
compile-time. A second "e" modifier will cause the
replacement portion to be "eval"ed before being run
as a Perl expression.
Examples:
s/\bgreen\b/mauve/g; # don't change wintergreen
$path =~ s|/usr/bin|/usr/local/bin|;
s/Login: $foo/Login: $bar/; # run-time pattern
($foo = $bar) =~ s/this/that/; # copy first, then change
$count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
$_ = 'abc123xyz';
s/\d+/$&*2/e; # yields 'abc246xyz'
s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
s/%(.)/$percent{$1}/g; # change percent escapes; no /e
s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
s/^=(\w+)/&pod($1)/ge; # use function call
# expand variables in $_, but dynamics only, using
# symbolic dereferencing
s/\$(\w+)/${$1}/g;
# Add one to the value of any numbers in the string
s/(\d+)/1 + $1/eg;
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# This will expand any embedded scalar variable
# (including lexicals) in $_ : First $1 is interpolated
# to the variable name, and then evaluated
s/(\$\w+)/$1/eeg;
# Delete (most) C comments.
$program =~ s {
/\* # Match the opening delimiter.
.*? # Match a minimal number of characters.
\*/ # Match the closing delimiter.
} []gsx;
s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
for ($variable) { # trim whitespace 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 sed, we use the \<digit> form in only the
left hand side. Anywhere else it's $<digit>.
Occasionally, you can't use just a "/g" to get all
the changes to occur that you might want. Here are
two common cases:
# put commas in the right places in an integer
1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
# expand tabs to 8-column spacing
1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
tr/SEARCHLIST/REPLACEMENTLIST/cds
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
"tr/A-J/0-9/" does the same replacement as
"tr/ACEGIBDFHJ/0246813579/". For sed devotees, "y"
is provided as a synonym for "tr". If the SEAR-
CHLIST is delimited by bracketing quotes, the
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REPLACEMENTLIST has its own pair of quotes, which
may or may not be bracketing quotes, e.g.,
"tr[A-Z][a-z]" or "tr(+\-*/)/ABCD/".
Note that "tr" does not do regular expression char-
acter classes such as "\d" or "[:lower:]". The <tr>
operator is not equivalent to the tr(1) utility. If
you want to map strings between lower/upper cases,
see "lc" in perlfunc and "uc" in perlfunc, and in
general consider using the "s" operator if you need
regular expressions.
Note also that the whole range idea is rather
unportable between character sets--and even within
character sets they may cause results you probably
didn't expect. A sound principle is to use only
ranges that begin from and end at either alphabets
of equal case (a-e, A-E), or digits (0-4). Anything
else is unsafe. If in doubt, spell out the charac-
ter sets in full.
Options:
c Complement the SEARCHLIST.
d Delete found but unreplaced characters.
s Squash duplicate replaced characters.
If the "/c" modifier is specified, the SEARCHLIST
character set is complemented. If the "/d" modifier
is specified, any characters specified by SEARCHLIST
not found in REPLACEMENTLIST are deleted. (Note that
this is slightly more flexible than the behavior of
some tr programs, which delete anything they find in
the SEARCHLIST, period.) If the "/s" modifier is
specified, sequences of characters that were
transliterated to the same character are squashed
down to a single instance of the character.
If the "/d" modifier is used, the REPLACEMENTLIST is
always interpreted exactly as specified. Otherwise,
if the REPLACEMENTLIST is shorter than the SEAR-
CHLIST, 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 $_
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$cnt = $sky =~ tr/*/*/; # count the stars in $sky
$cnt = tr/0-9//; # count the digits in $_
tr/a-zA-Z//s; # bookkeeper -> bokeper
($HOST = $host) =~ tr/a-z/A-Z/;
tr/a-zA-Z/ /cs; # change non-alphas to single space
tr [\200-\377]
[\000-\177]; # delete 8th bit
If multiple transliterations are given for a charac-
ter, only the first one is used:
tr/AAA/XYZ/
will transliterate any A to X.
Because the transliteration table is built at com-
pile time, neither the SEARCHLIST nor the REPLA-
CEMENTLIST are subjected to double quote interpola-
tion. That means that if you want to use variables,
you must use an eval():
eval "tr/$oldlist/$newlist/";
die $@ if $@;
eval "tr/$oldlist/$newlist/, 1" or die $@;
<<EOF A line-oriented form of quoting is based on the
shell "here-document" syntax. Following a "<<" you
specify a string to terminate the quoted material,
and all lines following the current line down to the
terminating string are the value of the item. The
terminating string may be either an identifier (a
word), or some quoted text. If quoted, the type of
quotes you use determines the treatment of the text,
just as in regular quoting. An unquoted identifier
works like double quotes. There must be no space
between the "<<" and the identifier, unless the
identifier is quoted. (If you put a space it will
be treated as a null identifier, which is valid, and
matches the first empty line.) The terminating
string must appear by itself (unquoted and with no
surrounding whitespace) on the terminating line.
print <<EOF;
The price is $Price.
EOF
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print << "EOF"; # same as above
The price is $Price.
EOF
print << `EOC`; # execute commands
echo hi there
echo lo there
EOC
print <<"foo", <<"bar"; # you can stack them
I said foo.
foo
I said bar.
bar
myfunc(<< "THIS", 23, <<'THAT');
Here's a line
or two.
THIS
and here's another.
THAT
Just don't forget that you have to put a semicolon
on the end to finish the statement, as Perl doesn't
know you're not going to try to do this:
print <<ABC
179231
ABC
+ 20;
If you want your here-docs to be indented with the
rest of the code, you'll need to remove leading whi-
tespace from each line manually:
($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 "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
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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
Can't find string terminator "END" anywhere before
EOF....
Additionally, the quoting rules for the identifier
are not related to Perl's quoting rules -- "q()",
"qq()", and the like are not supported in place of
'' and "", and the only interpolation is for
backslashing the quoting character:
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.
Gory details of parsing quoted constructs
When presented with something that might have several dif-
ferent interpretations, Perl uses the DWIM (that's "Do What
I Mean") principle to pick the most probable interpretation.
This strategy is so successful that Perl programmers often
do not suspect the ambivalence of what they write. But from
time to time, Perl's notions differ substantially from what
the author honestly meant.
This section hopes to clarify how Perl handles quoted con-
structs. Although the most common reason to learn this is to
unravel labyrinthine regular expressions, because the ini-
tial steps of parsing are the same for all quoting opera-
tors, they are all discussed together.
The most important Perl parsing rule is the first one dis-
cussed below: when processing a quoted construct, Perl first
finds the end of that construct, then interprets its con-
tents. If you understand this rule, you may skip the rest
of this section on the first reading. The other rules are
likely to contradict the user's expectations much less fre-
quently than this first one.
Some passes discussed below are performed concurrently, but
because their results are the same, we consider them
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individually. For different quoting constructs, Perl per-
forms different numbers of passes, from one to five, but
these passes are always performed in the same order.
Finding the end
The first pass is finding the end of the quoted con-
struct, whether it be a multicharacter delimiter
"\nEOF\n" in the "<<EOF" construct, a "/" that ter-
minates a "qq//" construct, a "]" which terminates
"qq[]" construct, or a ">" which terminates a fileglob
started with "<".
When searching for single-character non-pairing delim-
iters, such as "/", combinations of "\\" and "\/" are
skipped. However, when searching for single-character
pairing delimiter like "[", combinations of "\\", "\]",
and "\[" are all skipped, and nested "[", "]" are
skipped as well. When searching for multicharacter del-
imiters, nothing is skipped.
For constructs with three-part delimiters ("s///",
"y///", and "tr///"), the search is repeated once more.
During this search no attention is paid to the semantics
of the construct. Thus:
"$hash{"$foo/$bar"}"
or:
m/
bar # NOT a comment, this slash / terminated m//!
/x
do not form legal quoted expressions. The quoted part
ends on the first """ and "/", and the rest happens to
be a syntax error. Because the slash that terminated
"m//" was followed by a "SPACE", the example above is
not "m//x", but rather "m//" with no "/x" modifier. So
the embedded "#" is interpreted as a literal "#".
Also no attention is paid to "\c\" during this search.
Thus the second "\" in "qq/\c\/" is interpreted as a
part of "\/", and the following "/" is not recognized as
a delimiter. Instead, use "\034" or "\x1c" at the end of
quoted constructs.
Removal of backslashes before delimiters
During the second pass, text between the starting and
ending delimiters is copied to a safe location, and the
"\" is removed from combinations consisting of "\" and
delimiter--or delimiters, meaning both starting and
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ending delimiters will should these differ. This removal
does not happen for multi-character delimiters. Note
that the combination "\\" is left intact, just as it
was.
Starting from this step no information about the delim-
iters is used in parsing.
Interpolation
The next step is interpolation in the text obtained,
which is now delimiter-independent. There are four dif-
ferent cases.
"<<'EOF'", "m''", "s'''", "tr///", "y///"
No interpolation is performed.
'', "q//"
The only interpolation is removal of "\" from pairs
"\\".
"", ``, "qq//", "qx//", "<file*glob>"
"\Q", "\U", "\u", "\L", "\l" (possibly paired with
"\E") are converted to corresponding Perl con-
structs. Thus, "$foo\Qbaz$bar" is converted to
"$foo . (quotemeta("baz" . $bar))" internally. The
other combinations are replaced with appropriate
expansions.
Let it be stressed that whatever falls between "\Q"
and "\E" is interpolated in the usual way. Some-
thing like "\Q\\E" has no "\E" inside. instead, it
has "\Q", "\\", and "E", so the result is the same
as for "\\\\E". As a general rule, backslashes
between "\Q" and "\E" may lead to counterintuitive
results. So, "\Q\t\E" is converted to
"quotemeta("\t")", which is the same as "\\\t"
(since TAB is not alphanumeric). Note also that:
$str = '\t';
return "\Q$str";
may be closer to the conjectural intention of the
writer of "\Q\t\E".
Interpolated scalars and arrays are converted inter-
nally to the "join" and "." catenation operations.
Thus, "$foo XXX '@arr'" becomes:
$foo . " XXX '" . (join $", @arr) . "'";
All operations above are performed simultaneously,
left to right.
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Because the result of "\Q STRING \E" has all meta-
characters quoted, there is no way to insert a
literal "$" or "@" inside a "\Q\E" pair. If pro-
tected by "\", "$" will be quoted to became "\\\$";
if not, it is interpreted as the start of an inter-
polated scalar.
Note also that the interpolation code needs to make
a decision on where the interpolated scalar ends.
For instance, whether "a $b -> {c}" really means:
"a " . $b . " -> {c}";
or:
"a " . $b -> {c};
Most of the time, the longest possible text that
does not include spaces between components and which
contains matching braces or brackets. because the
outcome may be determined by voting based on heuris-
tic estimators, the result is not strictly predict-
able. Fortunately, it's usually correct for ambigu-
ous cases.
"?RE?", "/RE/", "m/RE/", "s/RE/foo/",
Processing of "\Q", "\U", "\u", "\L", "\l", and
interpolation happens (almost) as with "qq//" con-
structs, but the substitution of "\" followed by
RE-special chars (including "\") is not performed.
Moreover, inside "(?{BLOCK})", "(?# comment )", and
a "#"-comment in a "//x"-regular expression, no pro-
cessing is performed whatsoever. This is the first
step at which the presence of the "//x" modifier is
relevant.
Interpolation has several quirks: $|, $(, and $) are
not interpolated, and constructs $var[SOMETHING] are
voted (by several different estimators) to be either
an array element or $var followed by an RE alterna-
tive. This is where the notation "${arr[$bar]}"
comes handy: "/${arr[0-9]}/" is interpreted as array
element "-9", not as a regular expression from the
variable $arr followed by a digit, which would be
the interpretation of "/$arr[0-9]/". Since voting
among different estimators may occur, the result is
not predictable.
It is at this step that "\1" is begrudgingly con-
verted to $1 in the replacement text of "s///" to
correct the incorrigible sed hackers who haven't
picked up the saner idiom yet. A warning is emitted
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if the "use warnings" pragma or the -w command-line
flag (that is, the $^W variable) was set.
The lack of processing of "\\" creates specific res-
trictions on the post-processed text. If the delim-
iter is "/", one cannot get the combination "\/"
into the result of this step. "/" will finish the
regular expression, "\/" will be stripped to "/" on
the previous step, and "\\/" will be left as is.
Because "/" is equivalent to "\/" inside a regular
expression, this does not matter unless the delim-
iter happens to be character special to the RE
engine, such as in "s*foo*bar*", "m[foo]", or
"?foo?"; or an alphanumeric char, as in:
m m ^ a \s* b mmx;
In the RE above, which is intentionally obfuscated
for illustration, the delimiter is "m", the modifier
is "mx", and after backslash-removal the RE is the
same as for "m/ ^ a \s* b /mx". There's more than
one reason you're encouraged to restrict your delim-
iters to non-alphanumeric, non-whitespace choices.
This step is the last one for all constructs except reg-
ular expressions, which are processed further.
Interpolation of regular expressions
Previous steps were performed during the compilation of
Perl code, but this one happens at run time--although it
may be optimized to be calculated at compile time if
appropriate. After preprocessing described above, and
possibly after evaluation if catenation, joining, casing
translation, or metaquoting are involved, the resulting
string is passed to the RE engine for compilation.
Whatever happens in the RE engine might be better dis-
cussed in perlre, but for the sake of continuity, we
shall do so here.
This is another step where the presence of the "//x"
modifier is relevant. The RE engine scans the string
from left to right and converts it to a finite automa-
ton.
Backslashed characters are either replaced with
corresponding literal strings (as with "\{"), or else
they generate special nodes in the finite automaton (as
with "\b"). Characters special to the RE engine (such
as "|") generate corresponding nodes or groups of nodes.
"(?#...)" comments are ignored. All the rest is either
converted to literal strings to match, or else is
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ignored (as is whitespace and "#"-style comments if
"//x" is present).
Parsing of the bracketed character class construct,
"[...]", is rather different than the rule used for the
rest of the pattern. The terminator of this construct is
found using the same rules as for finding the terminator
of a "{}"-delimited construct, the only exception being
that "]" immediately following "[" is treated as though
preceded by a backslash. Similarly, the terminator of
"(?{...})" is found using the same rules as for finding
the terminator of a "{}"-delimited construct.
It is possible to inspect both the string given to RE
engine and the resulting finite automaton. See the
arguments "debug"/"debugcolor" in the "use re" pragma,
as well as Perl's -Dr command-line switch documented in
"Command Switches" in perlrun.
Optimization of regular expressions
This step is listed for completeness only. Since it
does not change semantics, details of this step are not
documented and are subject to change without notice.
This step is performed over the finite automaton that
was generated during the previous pass.
It is at this stage that "split()" silently optimizes
"/^/" to mean "/^/m".
I/O Operators
There are several I/O operators you should know about.
A string enclosed by backticks (grave accents) first under-
goes double-quote interpolation. It is then interpreted as
an external command, and the output of that command is the
value of the backtick string, like in a shell. In scalar
context, a single string consisting of all output is
returned. In list context, a list of values is returned,
one per line of output. (You can set $/ to use a different
line terminator.) The command is executed each time the
pseudo-literal is evaluated. The status value of the com-
mand is returned in $? (see perlvar for the interpretation
of $?). Unlike in csh, no translation is done on the return
data--newlines remain newlines. Unlike in any of the
shells, single quotes do not hide variable names in the com-
mand from interpretation. To pass a literal dollar-sign
through to the shell you need to hide it with a backslash.
The generalized form of backticks is "qx//". (Because back-
ticks always undergo shell expansion as well, see perlsec
for security concerns.)
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In scalar context, evaluating a filehandle in angle brackets
yields the next line from that file (the newline, if any,
included), or "undef" at end-of-file or on error. When $/
is set to "undef" (sometimes known as file-slurp mode) and
the file is empty, it returns '' the first time, followed by
"undef" subsequently.
Ordinarily you must assign the returned value to a variable,
but there is one situation where an automatic assignment
happens. If and only if the input symbol is the only thing
inside the conditional of a "while" statement (even if dis-
guised as a "for(;;)" loop), the value is automatically
assigned to the global variable $_, destroying whatever was
there previously. (This may seem like an odd thing to you,
but you'll use the construct in almost every Perl script you
write.) The $_ variable is not implicitly localized. You'll
have to put a "local $_;" before the loop if you want that
to happen.
The following lines are equivalent:
while (defined($_ = <STDIN>)) { print; }
while ($_ = <STDIN>) { print; }
while (<STDIN>) { print; }
for (;<STDIN>;) { print; }
print while defined($_ = <STDIN>);
print while ($_ = <STDIN>);
print while <STDIN>;
This also behaves similarly, but avoids $_ :
while (my $line = <STDIN>) { print $line }
In these loop constructs, the assigned value (whether
assignment is automatic or explicit) is then tested to see
whether it is defined. The defined test avoids problems
where line has a string value that would be treated as false
by Perl, for example a "" or a "0" with no trailing newline.
If you really mean for such values to terminate the loop,
they should be tested for explicitly:
while (($_ = <STDIN>) ne '0') { ... }
while (<STDIN>) { last unless $_; ... }
In other boolean contexts, "<filehandle>" without an expli-
cit "defined" test or comparison elicit a warning if the
"use warnings" pragma or the -w command-line switch (the $^W
variable) is in effect.
The filehandles STDIN, STDOUT, and STDERR are predefined.
(The filehandles "stdin", "stdout", and "stderr" will also
work except in packages, where they would be interpreted as
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local identifiers rather than global.) Additional filehan-
dles may be created with the open() function, amongst oth-
ers. See perlopentut and "open" in perlfunc for details on
this.
If a <FILEHANDLE> is used in a context that is looking for a
list, a list comprising all input lines is returned, one
line per list element. It's easy to grow to a rather large
data space this way, so use with care.
<FILEHANDLE> may also be spelled "readline(*FILEHANDLE)".
See "readline" in perlfunc.
The null filehandle <> is special: it can be used to emulate
the behavior of sed and awk. Input from <> comes either
from standard input, or from each file listed on the command
line. Here's how it works: the first time <> is evaluated,
the @ARGV array is checked, and if it is empty, $ARGV[0] is
set to "-", which when opened gives you standard input. The
@ARGV array is then processed as a list of filenames. The
loop
while (<>) {
... # code for each line
}
is equivalent to the following Perl-like pseudo code:
unshift(@ARGV, '-') unless @ARGV;
while ($ARGV = shift) {
open(ARGV, $ARGV);
while (<ARGV>) {
... # code for each line
}
}
except that it isn't so cumbersome to say, and will actually
work. It really does shift the @ARGV array and put the
current filename into the $ARGV variable. It also uses
filehandle ARGV internally--<> is just a synonym for <ARGV>,
which is magical. (The pseudo code above doesn't work
because it treats <ARGV> as non-magical.)
You can modify @ARGV before the first <> as long as the
array ends up containing the list of filenames you really
want. Line numbers ($.) continue as though the input were
one big happy file. See the example in "eof" in perlfunc
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:
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@ARGV = grep { -f && -T } glob('*') unless @ARGV;
You can even set them to pipe commands. For example, this
automatically filters compressed arguments through gzip:
@ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
If you want to pass switches into your script, you can use
one of the Getopts modules or put a loop on the front like
this:
while ($_ = $ARGV[0], /^-/) {
shift;
last if /^--$/;
if (/^-D(.*)/) { $debug = $1 }
if (/^-v/) { $verbose++ }
# ... # other switches
}
while (<>) {
# ... # code for each line
}
The <> symbol will return "undef" for end-of-file only once.
If you call it again after this, it will assume you are pro-
cessing another @ARGV list, and if you haven't set @ARGV,
will read input from STDIN.
If what the angle brackets contain is a simple scalar vari-
able (e.g., <$foo>), then that variable contains the name of
the filehandle to input from, or its typeglob, or a refer-
ence 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 "<$x>" is always a
readline() from an indirect handle, but "<$hash{key}>" is
always a glob(). That's because $x is a simple scalar vari-
able, but $hash{key} is not--it's a hash element. Even "<$x
>" (note the extra space) is treated as "glob("$x ")", not
"readline($x)".
One level of double-quote interpretation is done first, but
you can't say "<$foo>" because that's an indirect filehandle
as explained in the previous paragraph. (In older versions
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of Perl, programmers would insert curly brackets to force
interpretation as a filename glob: "<${foo}>". These days,
it's considered cleaner to call the internal function
directly as "glob($foo)", which is probably the right way to
have done it in the first place.) For example:
while (<*.c>) {
chmod 0644, $_;
}
is roughly equivalent to:
open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
while (<FOO>) {
chomp;
chmod 0644, $_;
}
except that the globbing is actually done internally using
the standard "File::Glob" extension. Of course, the shor-
test way to do the above is:
chmod 0644, <*.c>;
A (file)glob evaluates its (embedded) argument only when it
is starting a new list. All values must be read before it
will start over. In list context, this isn't important
because you automatically get them all anyway. However, in
scalar context the operator returns the next value each time
it's called, or "undef" when the list has run out. As with
filehandle reads, an automatic "defined" is generated when
the glob occurs in the test part of a "while", because legal
glob returns (e.g. a file called 0) would otherwise ter-
minate the loop. Again, "undef" is returned only once. So
if you're expecting a single value from a glob, it is much
better to say
($file) = <blurch*>;
than
$file = <blurch*>;
because the latter will alternate between returning a
filename and returning false.
If you're trying to do variable interpolation, it's defin-
itely better to use the glob() function, because the older
notation can cause people to become confused with the
indirect filehandle notation.
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@files = glob("$dir/*.[ch]");
@files = glob($files[$i]);
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 partic-
ular, string concatenation happens at compile time between
literals that don't do variable substitution. Backslash
interpolation also happens at compile time. You can say
'Now is the time for all' . "\n" .
'good men to come to.'
and this all reduces to one string internally. Likewise, if
you say
foreach $file (@filenames) {
if (-s $file > 5 + 100 * 2**16) { }
}
the compiler will precompute the number which that expres-
sion represents so that the interpreter won't have to.
No-ops
Perl doesn't officially have a no-op operator, but the bare
constants 0 and 1 are special-cased to not produce a warning
in a void context, so you can for example safely do
1 while foo();
Bitwise String Operators
Bitstrings of any size may be manipulated by the bitwise
operators ("~ | & ^").
If the operands to a binary bitwise op are strings of dif-
ferent sizes, | and ^ ops act as though the shorter operand
had additional zero bits on the right, while the & op acts
as though the longer operand were truncated to the length of
the shorter. The granularity for such extension or trunca-
tion is one or more bytes.
# ASCII-based examples
print "j p \n" ^ " a h"; # prints "JAPH\n"
print "JA" | " ph\n"; # prints "japh\n"
print "japh\nJunk" & '_____'; # prints "JAPH\n";
print 'p N$' ^ " E<H\n"; # prints "Perl\n";
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If you are intending to manipulate bitstrings, be certain
that you're supplying bitstrings: If an operand is a number,
that will imply a numeric bitwise operation. You may expli-
citly show which type of operation you intend by using "" or
"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 "vec" in perlfunc for information on how to manipulate
individual bits in a bit vector.
Integer Arithmetic
By default, Perl assumes that it must do most of its arith-
metic in floating point. But by saying
use integer;
you may tell the compiler that it's okay to use integer
operations (if it feels like it) from here to the end of the
enclosing BLOCK. An inner BLOCK may countermand this by say-
ing
no integer;
which lasts until the end of that BLOCK. Note that this
doesn't mean everything is only an integer, merely that Perl
may use integer operations if it is so inclined. For exam-
ple, even under "use integer", if you take the sqrt(2),
you'll still get 1.4142135623731 or so.
Used on numbers, the bitwise operators ("&", "|", "^", "~",
"<<", and ">>") always produce integral results. (But see
also "Bitwise String Operators".) However, "use integer"
still has meaning for them. By default, their results are
interpreted as unsigned integers, but if "use integer" is in
effect, their results are interpreted as signed integers.
For example, "~0" usually evaluates to a large integral
value. However, "use integer; ~0" is "-1" on twos-
complement machines.
Floating-point Arithmetic
While "use integer" provides integer-only arithmetic, there
is no analogous mechanism to provide automatic rounding or
truncation to a certain number of decimal places. For
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rounding to a certain number of digits, sprintf() or
printf() is usually the easiest route. See perlfaq4.
Floating-point numbers are only approximations to what a
mathematician would call real numbers. There are infinitely
more reals than floats, so some corners must be cut. For
example:
printf "%.20g\n", 123456789123456789;
# produces 123456789123456784
Testing for exact equality of floating-point equality or
inequality is not a good idea. Here's a (relatively expen-
sive) work-around to compare whether two floating-point
numbers are equal to a particular number of decimal places.
See Knuth, volume II, for a more robust treatment of this
topic.
sub fp_equal {
my ($X, $Y, $POINTS) = @_;
my ($tX, $tY);
$tX = sprintf("%.${POINTS}g", $X);
$tY = sprintf("%.${POINTS}g", $Y);
return $tX eq $tY;
}
The POSIX module (part of the standard perl distribution)
implements ceil(), floor(), and other mathematical and tri-
gonometric functions. The Math::Complex module (part of the
standard perl distribution) defines mathematical functions
that work on both the reals and the imaginary numbers.
Math::Complex not as efficient as POSIX, but POSIX can't
work with complex numbers.
Rounding in financial applications can have serious implica-
tions, and the rounding method used should be specified pre-
cisely. 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.
Bigger Numbers
The standard Math::BigInt and Math::BigFloat modules provide
variable-precision arithmetic and overloaded operators,
although they're currently pretty slow. At the cost of some
space and considerable speed, they avoid the normal pitfalls
associated with limited-precision representations.
use Math::BigInt;
$x = Math::BigInt->new('123456789123456789');
print $x * $x;
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# prints +15241578780673678515622620750190521
There are several modules that let you calculate with (bound
only by memory and cpu-time) unlimited or fixed precision.
There are also some non-standard modules that provide faster
implementations via external C libraries.
Here is a short, but incomplete summary:
Math::Fraction big, unlimited fractions like 9973 / 12967
Math::String treat string sequences like numbers
Math::FixedPrecision calculate with a fixed precision
Math::Currency for currency calculations
Bit::Vector manipulate bit vectors fast (uses C)
Math::BigIntFast Bit::Vector wrapper for big numbers
Math::Pari provides access to the Pari C library
Math::BigInteger uses an external C library
Math::Cephes uses external Cephes C library (no big numbers)
Math::Cephes::Fraction fractions via the Cephes library
Math::GMP another one using an external C library
Choose wisely.
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