PERLOP(1) Perl Programmers Reference Guide PERLOP(1)
NAME
perlop - 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
multiplication 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 precedence, listed from highest precedence to lowest.
Operators borrowed from C keep the same precedence relationship with each other, even where C's precedence is
slightly screwy. (This makes learning Perl easier for C folks.) With very few exceptions, these all operate
on scalar values only, not array values.
left terms and list operators (leftward)
left ->
nonassoc ++ --
right **
right ! ~ \ and unary + and -
left =~ !~
left * / % x
left + - .
left << >>
nonassoc named unary operators
nonassoc < > <= >= lt gt le ge
nonassoc == != <=> eq ne cmp ~~
left &
left | ^
left &&
left || //
nonassoc .. ...
right ?:
right = += -= *= etc.
left , =>
nonassoc list operators (rightward)
right not
left and
left or xor
In the following sections, these operators are covered in precedence order.
Many operators can be overloaded for objects. See 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 arguments. These are all documented in perlfunc.
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 {}" constructs, 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 subroutine respectively. (Or technically speaking, a location capable of holding a hard reference,
if it's an array or hash reference being used for assignment.) See perlreftut and perlref.
Otherwise, the right side is a method name or a simple scalar variable containing either the method name or a
subroutine reference, and the left side must be either an object (a blessed reference) or a class name (that
is, a package name). See 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 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 behavior. Avoid statements like:
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.
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, that is, "not". See also "not" for a lower precedence version of this.
Unary "-" performs arithmetic negation if the operand is numeric, including any string that looks like a
number. If the operand is an identifier, a string consisting of a minus sign concatenated with the identifier
is returned. Otherwise, if the string starts with a plus or minus, a string starting with the opposite sign
is returned. One effect of these rules is that -bareword is equivalent to the string "-bareword". If,
however, the string begins with a non-alphabetic character (excluding "+" or "-"), Perl will attempt to
convert the string to a numeric and the arithmetic 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, that is, 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.
When complementing strings, if all characters have ordinal values under 256, then their complements will,
also. But if they do not, all characters will be in either 32- or 64-bit complements, depending on your
architecture. So for example, "~"\x{3B1}"" is "\x{FFFF_FC4E}" on 32-bit machines and
"\x{FFFF_FFFF_FFFF_FC4E}" on 64-bit machines.
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 is supposed to be searched,
substituted, or transliterated instead of the default $_. When used in scalar context, the return value
generally indicates the success of the operation. The exceptions are substitution (s///) and transliteration
(y///) with the "/r" (non-destructive) option, which cause the return value to be the result of the
substitution. 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
Multiplicative Operators
Binary "*" multiplies two numbers.
Binary "/" divides two numbers.
Binary "%" is the modulo operator, which computes the division remainder of its first argument with respect to
its second argument. Given integer operands $a and $b: If $b is positive, then "$a % $b" is $a minus the
largest multiple of $b less than or equal to $a. If $b is negative, then "$a % $b" is $a minus the smallest
multiple of $b that is not less than $a (that is, the result will be less than or equal to zero). If the
operands $a and $b are floating point values and the absolute value of $b (that is "abs($b)") is less than
"(UV_MAX + 1)", only the integer portion of $a and $b will be used in the operation (Note: here "UV_MAX" means
the maximum of the unsigned integer type). If the absolute value of the right operand ("abs($b)") is greater
than or equal to "(UV_MAX + 1)", "%" computes the floating-point remainder $r in the equation "($r = $a -
$i*$b)" where $i is a certain integer that makes $r have the same sign as the right operand $b (not as the
left operand $a like C function "fmod()") and the absolute value less than that of $b. Note that when "use
integer" is in scope, "%" gives you direct access to the modulo operator as implemented by your C compiler.
This operator is not as well defined for negative operands, but it will execute faster.
Binary "x" is the repetition operator. In scalar context or if the left operand is not enclosed in
parentheses, it returns a string consisting of the left operand repeated the number of times specified by the
right operand. In list context, if the left operand is enclosed in parentheses or is a list formed by
"qw/STRING/", it repeats the list. If the right operand is zero or negative, it returns an empty string or an
empty list, depending on the context.
print '-' x 80; # print row of dashes
print "\t" x ($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.
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 Arithmetic".)
Binary ">>" returns the value of its left argument shifted right by the number of bits specified by the right
argument. Arguments should be integers. (See also "Integer Arithmetic".)
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 undefined 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.
If any list operator (print(), etc.) or any unary operator (chdir(), etc.) is followed by a left parenthesis
as the next token, the operator and arguments within parentheses are taken to be of highest precedence, just
like a normal function call. For example, because named unary operators are higher precedence than "||":
chdir $foo || die; # (chdir $foo) || die
chdir($foo) || die; # (chdir $foo) || die
chdir ($foo) || die; # (chdir $foo) || die
chdir +($foo) || die; # (chdir $foo) || die
but, because * is higher precedence than named operators:
chdir $foo * 20; # chdir ($foo * 20)
chdir($foo) * 20; # (chdir $foo) * 20
chdir ($foo) * 20; # (chdir $foo) * 20
chdir +($foo) * 20; # chdir ($foo * 20)
rand 10 * 20; # rand (10 * 20)
rand(10) * 20; # (rand 10) * 20
rand (10) * 20; # (rand 10) * 20
rand +(10) * 20; # rand (10 * 20)
Regarding precedence, the filetest operators, like "-f", "-M", etc. are treated like named unary operators,
but they don't follow this functional parenthesis rule. That means, for example, that "-f($file).".bak"" is
equivalent to "-f "$file.bak"".
See also "Terms and List Operators (Leftward)".
Relational Operators
Perl operators that return true or false generally return values that can be safely used as numbers. For
example, the relational operators in this section and the equality operators in the next one return 1 for true
and a special version of the defined empty string, "", which counts as a zero but is exempt from warnings
about improper numeric conversions, just as "0 but true" is.
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 "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.
Binary "~~" does a smartmatch between its arguments. Smart matching is described in the next section.
"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified by the current locale if a legacy
"use locale" (but not "use locale ':not_characters'") is in effect. See perllocale. Do not mix these with
Unicode, only with legacy binary encodings. The standard Unicode::Collate and Unicode::Collate::Locale
modules offer much more powerful solutions to collation issues.
Smartmatch Operator
First available in Perl 5.10.1 (the 5.10.0 version behaved differently), binary "~~" does a "smartmatch"
between its arguments. This is mostly used implicitly in the "when" construct described in perlsyn, although
not all "when" clauses call the smartmatch operator. Unique among all of Perl's operators, the smartmatch
operator can recurse.
It is also unique in that all other Perl operators impose a context (usually string or numeric context) on
their operands, autoconverting those operands to those imposed contexts. In contrast, smartmatch infers
contexts from the actual types of its operands and uses that type information to select a suitable comparison
mechanism.
The "~~" operator compares its operands "polymorphically", determining how to compare them according to their
actual types (numeric, string, array, hash, etc.) Like the equality operators with which it shares the same
precedence, "~~" returns 1 for true and "" for false. It is often best read aloud as "in", "inside of", or
"is contained in", because the left operand is often looked for inside the right operand. That makes the
order of the operands to the smartmatch operand often opposite that of the regular match operator. In other
words, the "smaller" thing is usually placed in the left operand and the larger one in the right.
The behavior of a smartmatch depends on what type of things its arguments are, as determined by the following
table. The first row of the table whose types apply determines the smartmatch behavior. Because what
actually happens is mostly determined by the type of the second operand, the table is sorted on the right
operand instead of on the left.
Left Right Description and pseudocode
===============================================================
Any undef check whether Any is undefined
like: !defined Any
Any Object invoke ~~ overloading on Object, or die
Right operand is an ARRAY:
Left Right Description and pseudocode
===============================================================
ARRAY1 ARRAY2 recurse on paired elements of ARRAY1 and ARRAY2[2]
like: (ARRAY1[0] ~~ ARRAY2[0])
&& (ARRAY1[1] ~~ ARRAY2[1]) && ...
HASH ARRAY any ARRAY elements exist as HASH keys
like: grep { exists HASH->{$_} } ARRAY
Regexp ARRAY any ARRAY elements pattern match Regexp
like: grep { /Regexp/ } ARRAY
ARRAY HASH any ARRAY elements exist as HASH keys
like: grep { exists HASH->{$_} } ARRAY
Regexp HASH any HASH keys pattern match Regexp
like: grep { /Regexp/ } keys HASH
undef HASH always false (undef can't be a key)
like: 0 == 1
Any HASH HASH key existence
like: exists HASH->{Any}
Right operand is CODE:
Left Right Description and pseudocode
===============================================================
ARRAY CODE sub returns true on all ARRAY elements[1]
like: !grep { !CODE->($_) } ARRAY
HASH CODE sub returns true on all HASH keys[1]
like: !grep { !CODE->($_) } keys HASH
Any CODE sub passed Any returns true
like: CODE->(Any)
Right operand is a Regexp:
Left Right Description and pseudocode
===============================================================
ARRAY Regexp any ARRAY elements match Regexp
like: grep { /Regexp/ } ARRAY
HASH Regexp any HASH keys match Regexp
like: grep { /Regexp/ } keys HASH
Any Regexp pattern match
like: Any =~ /Regexp/
Other:
Left Right Description and pseudocode
===============================================================
Object Any invoke ~~ overloading on Object,
or fall back to...
Any Num numeric equality
like: Any == Num
Num nummy[4] numeric equality
like: Num == nummy
undef Any check whether undefined
like: !defined(Any)
Any Any string equality
like: Any eq Any
Notes:
1. Empty hashes or arrays match.
2. That is, each element smartmatches the element of the same index in the other array.[3]
3. If a circular reference is found, fall back to referential equality.
4. Either an actual number, or a string that looks like one.
say "some elements undefined" if undef ~~ @array;
Each operand is considered in a modified scalar context, the modification being that array and hash variables
are passed by reference to the operator, which implicitly dereferences them. Both elements of each pair are
the same:
use v5.10.1;
my %hash = (red => 1, blue => 2, green => 3,
orange => 4, yellow => 5, purple => 6,
black => 7, grey => 8, white => 9);
my @array = qw(red blue green);
say "some array elements in hash keys" if @array ~~ %hash;
say "some array elements in hash keys" if \@array ~~ \%hash;
say "red in array" if "red" ~~ @array;
say "red in array" if "red" ~~ \@array;
say "some keys end in e" if /e$/ ~~ %hash;
say "some keys end in e" if /e$/ ~~ \%hash;
Two arrays smartmatch if each element in the first array smartmatches (that is, is "in") the corresponding
element in the second array, recursively.
use v5.10.1;
my @little = qw(red blue green);
my @bigger = ("red", "blue", [ "orange", "green" ] );
if (@little ~~ @bigger) { # true!
say "little is contained in bigger";
}
Because the smartmatch operator recurses on nested arrays, this will still report that "red" is in the array.
use v5.10.1;
my @array = qw(red blue green);
my $nested_array = [[[[[[[ @array ]]]]]]];
say "red in array" if "red" ~~ $nested_array;
If two arrays smartmatch each other, then they are deep copies of each others' values, as this example
reports:
use v5.12.0;
my @a = (0, 1, 2, [3, [4, 5], 6], 7);
my @b = (0, 1, 2, [3, [4, 5], 6], 7);
if (@a ~~ @b && @b ~~ @a) {
say "a and b are deep copies of each other";
}
elsif (@a ~~ @b) {
say "a smartmatches in b";
}
elsif (@b ~~ @a) {
might have. For example:
use v5.10.1;
sub make_dogtag {
state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
my ($class, $init_fields) = @_;
die "Must supply (only) name, rank, and serial number"
unless $init_fields ~~ $REQUIRED_FIELDS;
...
}
or, if other non-required fields are allowed, use ARRAY ~~ HASH:
use v5.10.1;
sub make_dogtag {
state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
my ($class, $init_fields) = @_;
die "Must supply (at least) name, rank, and serial number"
unless [keys %{$init_fields}] ~~ $REQUIRED_FIELDS;
...
}
The smartmatch operator is most often used as the implicit operator of a "when" clause. See the section on
"Switch Statements" in perlsyn.
Smartmatching of Objects
To avoid relying on an object's underlying representation, if the smartmatch's right operand is an object that
doesn't overload "~~", it raises the exception ""Smartmatching a non-overloaded object breaks encapsulation"".
That's because one has no business digging around to see whether something is "in" an object. These are all
illegal on objects without a "~~" overload:
%hash ~~ $object
42 ~~ $object
"fred" ~~ $object
However, you can change the way an object is smartmatched by overloading the "~~" operator. This is allowed to
extend the usual smartmatch semantics. For objects that do have an "~~" overload, see overload.
Using an object as the left operand is allowed, although not very useful. Smartmatching rules take precedence
over overloading, so even if the object in the left operand has smartmatch overloading, this will be ignored.
A left operand that is a non-overloaded object falls back on a string or numeric comparison of whatever the
"ref" operator returns. That means that
$object ~~ X
does not invoke the overload method with "X" as an argument. Instead the above table is consulted as normal,
and based on the type of "X", overloading may or may not be invoked. For simple strings or numbers, in
That's because it treats $fh as a string like "IO::Handle=GLOB(0x8039e0)", then pattern matches against that.
Bitwise And
Binary "&" returns its operands ANDed together bit by bit. (See also "Integer Arithmetic" and "Bitwise String
Operators".)
Note that "&" has lower priority than relational operators, so for example the parentheses are essential in a
test like
print "Even\n" if ($x & 1) == 0;
Bitwise Or and Exclusive Or
Binary "|" returns its operands ORed together bit by bit. (See also "Integer Arithmetic" and "Bitwise String
Operators".)
Binary "^" returns its operands XORed together bit by bit. (See also "Integer Arithmetic" and "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;
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.
Logical Defined-Or
Although it has no direct equivalent in C, Perl's "//" operator is related to its C-style or. In fact, it's
exactly the same as "||", except that it tests the left hand side's definedness instead of its truth. Thus,
"EXPR1 // EXPR2" returns the value of "EXPR1" if it's defined, otherwise, the value of "EXPR2" is returned.
("EXPR1" is evaluated in scalar context, "EXPR2" in the context of "//" itself). Usually, this is the same
result as "defined(EXPR1) ? EXPR1 : EXPR2" (except that the ternary-operator form can be used as a lvalue,
while "EXPR1 // EXPR2" cannot). This is very useful for providing default values for variables. If you
actually want to test if at least one of $a and $b is defined, use "defined($a // $b)".
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
unlink("alpha", "beta", "gamma")
|| (gripe(), next LINE);
It would be even more readable to write that this way:
unless(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 different 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 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 operator 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, even across calls to a subroutine that contains it. 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 until 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 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. Furthermore, "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:
while (<>) {
$in_header = 1 .. /^$/;
$in_body = /^$/ .. eof;
if ($in_header) {
# do something
} else { # in body
# do something else
}
} 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.
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 magical 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 specified is not in the sequence that the magical increment would produce, the sequence
goes until the next value would be longer than the final value specified.
If the initial value specified isn't part of a magical increment sequence (that is, a non-empty string
);
However, because there are many other lowercase Greek characters than just those, to match lowercase Greek
characters in a regular expression, you would use the pattern "/(?:(?=\p{Greek})\p{Lower})+/".
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 = $ok ? @b : @c; # oops, that's just a count!
The operator may be assigned to if both the 2nd and 3rd arguments are legal lvalues (meaning that you can
assign to them):
($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,
$a += 2;
is equivalent to
Unlike in C, the scalar assignment operator produces a valid lvalue. Modifying an assignment is equivalent to
doing the assignment and then modifying the variable that was assigned to. This is useful for modifying a
copy of something, like this:
($tmp = $global) =~ tr/13579/24680/;
Although as of 5.14, that can be also be accomplished this way:
use v5.14;
$tmp = ($global =~ tr/13579/24680/r);
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. These
arguments are also evaluated from left to right.
The "=>" operator is a synonym for the comma except that it causes a word on its left to be interpreted as a
string if it begins with a letter or underscore and is composed only of letters, digits and underscores. This
includes operands that might otherwise be interpreted as operators, constants, single number v-strings or
function calls. If in doubt about this behavior, the left operand can be quoted explicitly.
Otherwise, the "=>" operator behaves exactly as the comma operator or list argument separator, according to
context.
For example:
use constant FOO => "something";
my %h = ( FOO => 23 );
is equivalent to:
my %h = ("FOO", 23);
It is NOT:
my %h = ("something", 23);
The "=>" operator is helpful in documenting the correspondence between keys and values in hashes, and other
List Operators (Rightward)
On the right side of a list operator, the comma has very low precedence, such that it controls all comma-
separated expressions found there. The only operators with lower precedence are the logical operators "and",
"or", and "not", which may be used to evaluate calls to list operators without the need for parentheses:
open HANDLE, "< :utf8", "filename" or die "Can't open: $!\n";
However, some people find that code harder to read than writing it with parentheses:
open(HANDLE, "< :utf8", "filename") or die "Can't open: $!\n";
in which case you might as well just use the more customary "||" operator:
open(HANDLE, "< :utf8", "filename") || 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 surrounding expressions. It's equivalent to "&&"
except for the very low precedence. This means that it short-circuits: 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 surrounding expressions. It's equivalent to "||"
except for the very low precedence. This makes it useful for control flow:
print FH $data or die "Can't write to FH: $!";
This means that it short-circuits: the right expression is evaluated only if the left expression is false.
Due to its precedence, you must be careful to avoid using it as replacement for the "||" operator. It usually
works out better for flow control than in assignments:
$a = $b or $c; # bug: this is wrong
($a = $b) or $c; # really means this
$a = $b || $c; # better written this way
However, when it's a list-context assignment and you're trying to use "||" for control flow, you probably need
"or" so that the assignment takes higher precedence.
@info = stat($file) || die; # oops, scalar sense of stat!
@info = stat($file) or die; # better, now @info gets its due
Then again, you could always use parentheses.
Binary "xor" returns the exclusive-OR of the two surrounding expressions. It cannot short-circuit (of
course).
There is no low precedence operator for defined-OR.
C Operators Missing From Perl
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)
y{}{} 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 ASCII brackets (round,
angle, square, curly) all nest, which means that
q{foo{bar}baz}
is the same as
'foo{bar}baz'
Note, however, that this does not always work for quoting Perl code:
$s = q{ if($a eq "}") ... }; # WRONG
is a syntax error. The "Text::Balanced" module (standard as of v5.8, and from CPAN before then) 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 argument 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:
Sequence Note Description
\t tab (HT, TAB)
\n newline (NL)
\r return (CR)
\f form feed (FF)
\b backspace (BS)
\a alarm (bell) (BEL)
\e escape (ESC)
\x{263A} [1,8] hex char (example: SMILEY)
\x1b [2,8] restricted range hex char (example: ESC)
\N{name} [3] named Unicode character or character sequence
\N{U+263D} [4,8] Unicode character (example: FIRST QUARTER MOON)
\c[ [5] control char (example: chr(27))
[2] The result is the character specified by the hexadecimal number in the range 0x00 to 0xFF. See "[8]"
below for details on which character.
Only hexadecimal digits are valid following "\x". When "\x" is followed by fewer than two valid digits,
any valid digits will be zero-padded. This means that "\x7" will be interpreted as "\x07", and a lone
<\x> will be interpreted as "\x00". Except at the end of a string, having fewer than two valid digits
will result in a warning. Note that although the warning says the illegal character is ignored, it is
only ignored as part of the escape and will still be used as the subsequent character in the string. For
example:
Original Result Warns?
"\x7" "\x07" no
"\x" "\x00" no
"\x7q" "\x07q" yes
"\xq" "\x00q" yes
[3] The result is the Unicode character or character sequence given by name. See charnames.
[4] "\N{U+hexadecimal number}" means the Unicode character whose Unicode code point is hexadecimal number.
[5] The character following "\c" is mapped to some other character as shown in the table:
Sequence Value
\c@ chr(0)
\cA chr(1)
\ca chr(1)
\cB chr(2)
\cb chr(2)
...
\cZ chr(26)
\cz chr(26)
\c[ chr(27)
\c] chr(29)
\c^ chr(30)
\c? chr(127)
In other words, it's the character whose code point has had 64 xor'd with its uppercase. "\c?" is DELETE
because "ord("@") ^ 64" is 127, and "\c@" is NULL because the ord of "@" is 64, so xor'ing 64 itself
produces 0.
Also, "\c\X" yields " chr(28) . "X"" for any X, but cannot come at the end of a string, because the
backslash would be parsed as escaping the end quote.
On ASCII platforms, the resulting characters from the list above are the complete set of ASCII controls.
This isn't the case on EBCDIC platforms; see "OPERATOR DIFFERENCES" in perlebcdic for the complete list of
what these sequences mean on both ASCII and EBCDIC platforms.
Use of any other character following the "c" besides those listed above is discouraged, and some are
deprecated with the intention of removing those in a later Perl version. What happens for any of these
other characters currently though, is that the value is derived by xor'ing with the seventh bit, which is
64.
To get platform independent controls, you can use "\N{...}".
may give unintended results. (For example, in a regular expression it may be confused with a
backreference; see "Octal escapes" in perlrebackslash.) Starting in Perl 5.14, you may use "\o{}"
instead, which avoids all these problems. Otherwise, it is best to use this construct only for ordinals
"\077" and below, remembering to pad to the left with zeros to make three digits. For larger ordinals,
either use "\o{}", or convert to something else, such as to hex and use "\x{}" instead.
Having fewer than 3 digits may lead to a misleading warning message that says that what follows is
ignored. For example, "\128" in the ASCII character set is equivalent to the two characters "\n8", but
the warning "Illegal octal digit '8' ignored" will be thrown. If "\n8" is what you want, you can avoid
this warning by padding your octal number with 0's: "\0128".
[8] Several constructs above specify a character by a number. That number gives the character's position in
the character set encoding (indexed from 0). This is called synonymously its ordinal, code position, or
code point. Perl works on platforms that have a native encoding currently of either ASCII/Latin1 or
EBCDIC, each of which allow specification of 256 characters. In general, if the number is 255 (0xFF,
0377) or below, Perl interprets this in the platform's native encoding. If the number is 256 (0x100,
0400) or above, Perl interprets it as a Unicode code point and the result is the corresponding Unicode
character. For example "\x{50}" and "\o{120}" both are the number 80 in decimal, which is less than 256,
so the number is interpreted in the native character set encoding. In ASCII the character in the 80th
position (indexed from 0) is the letter "P", and in EBCDIC it is the ampersand symbol "&". "\x{100}" and
"\o{400}" are both 256 in decimal, so the number is interpreted as a Unicode code point no matter what the
native encoding is. The name of the character in the 256th position (indexed by 0) in Unicode is "LATIN
CAPITAL LETTER A WITH MACRON".
There are a couple of exceptions to the above rule. "\N{U+hex number}" is always interpreted as a Unicode
code point, so that "\N{U+0050}" is "P" even on EBCDIC platforms. And if "use encoding" is in effect, the
number is considered to be in that encoding, and is translated from that into the platform's native
encoding if there is a corresponding native character; otherwise to Unicode.
NOTE: Unlike C and other languages, Perl has no "\v" escape sequence for the vertical tab (VT - ASCII 11), but
you may use "\ck" or "\x0b". ("\v" does have meaning in regular expression patterns in Perl, see perlre.)
The following escape sequences are available in constructs that interpolate, but not in transliterations.
\l lowercase next character only
\u titlecase (not uppercase!) next character only
\L lowercase all characters till \E or end of string
\U uppercase all characters till \E or end of string
\F foldcase all characters till \E or end of string
\Q quote (disable) pattern metacharacters till \E or
end of string
\E end either case modification or quoted section
(whichever was last seen)
See "quotemeta" in perlfunc for the exact definition of characters that are quoted by "\Q".
"\L", "\U", "\F", and "\Q" can stack, in which case you need one "\E" for each. For example:
say"This \Qquoting \ubusiness \Uhere isn't quite\E done yet,\E is it?";
This quoting\ Business\ HERE\ ISN\'T\ QUITE\ done\ yet\, is it?
If "use locale" is in effect (but not "use locale ':not_characters'"), the case map used by "\l", "\L", "\u",
and "\U" is taken from the current locale. See perllocale. If Unicode (for example, "\N{}" or code points of
0x100 or beyond) is being used, the case map used by "\l", "\L", "\u", and "\U" is as defined by Unicode.
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 $", @array". "Punctuation" arrays such as "@*" are usually interpolated
only if the name is enclosed in braces "@{*}", but the arrays @_, "@+", and "@-" are interpolated even without
braces.
For double-quoted strings, the quoting from "\Q" is applied after interpolation and escapes are processed.
"abc\Qfoo\tbar$s\Exyz"
is equivalent to
"abc" . quotemeta("foo\tbar$s") . "xyz"
For the pattern of regex operators ("qr//", "m//" and "s///"), the quoting from "\Q" is applied after
interpolation is processed, but before escapes are processed. This allows the pattern to match literally
(except for "$" and "@"). For example, the following matches:
'\s\t' =~ /\Q\s\t/
Because "$" or "@" trigger interpolation, you'll need to use something like "/\Quser\E\@\Qhost/" to match them
literally.
Patterns are subject to an additional level of interpretation as a regular expression. This is done as a
second pass, after variables are interpolated, so that regular expressions may be incorporated into the
pattern from the variables. If this is not what you want, use "\Q" to interpolate a variable literally.
Apart from the behavior described above, Perl does not expand multiple levels of interpolation. In
particular, contrary to the expectations of shell programmers, back-quotes do NOT interpolate within double
quotes, nor do single quotes impede evaluation of variables when used within double quotes.
Regexp Quote-Like Operators
Here are the quote-like operators that apply to pattern matching and related activities.
qr/STRING/msixpodual
This operator quotes (and possibly compiles) its STRING as a regular expression. STRING is
interpolated 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/msixpodual" expression. The returned value is a normalized version of the original pattern.
It magically differs from a string containing the same characters: "ref(qr/x/)" returns "Regexp";
however, dereferencing it is not well defined (you currently get the normalized version of the
original pattern, but this may change).
For example,
$rex = qr/my.STRING/is;
print $rex; # prints (?si-xm:my.STRING)
s/$rex/foo/;
is equivalent to
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
optimizations, but none would be triggered in the above example if we did not use qr() operator.)
Options (specified by the following modifiers) are:
m Treat string as multiple lines.
s Treat string as single line. (Make . match a newline)
i Do case-insensitive pattern matching.
x Use extended regular expressions.
p When matching preserve a copy of the matched string so
that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
o Compile pattern only once.
a ASCII-restrict: Use ASCII for \d, \s, \w; specifying two a's
further restricts /i matching so that no ASCII character will
match a non-ASCII one
l Use the locale
u Use Unicode rules
d Use Unicode or native charset, as in 5.12 and earlier
If a precompiled pattern is embedded in a larger pattern then the effect of "msixpluad" will be
propagated appropriately. The effect the "o" modifier has is not propagated, being restricted to
those patterns explicitly using it.
The last four modifiers listed above, added in Perl 5.14, control the character set semantics, but
"/a" is the only one you are likely to want to specify explicitly; the other three are selected
automatically by various pragmas.
See perlre for additional information on valid syntax for STRING, and for a detailed look at the
semantics of regular expressions. In particular, all modifiers except the largely obsolete "/o" are
further explained in "Modifiers" in perlre. "/o" is described in the next section.
m/PATTERN/msixpodualgc
/PATTERN/msixpodualgc
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.
Options are as described in "qr//" above; in addition, the following match process modifiers are
available:
except for when the delimiter is a single quote. (Note that $(, $), and $| are not interpolated
because they look like end-of-string tests.) Perl will not recompile the pattern unless an
interpolated variable that it contains changes. You can force Perl to skip the test and never
recompile by adding a "/o" (which stands for "once") after the trailing delimiter. Once upon a time,
Perl would recompile regular expressions unnecessarily, and this modifier was useful to tell it not to
do so, in the interests of speed. But now, the only reasons to use "/o" are either:
1. The variables are thousands of characters long and you know that they don't change, and you need
to wring out the last little bit of speed by having Perl skip testing for that. (There is a
maintenance penalty for doing this, as mentioning "/o" constitutes a promise that you won't change
the variables in the pattern. If you do change them, Perl won't even notice.)
2. you want the pattern to use the initial values of the variables regardless of whether they change
or not. (But there are saner ways of accomplishing this than using "/o".)
The bottom line is that using "/o" is almost never a good idea.
The empty pattern //
If the PATTERN evaluates to the empty string, the last successfully matched regular expression is used
instead. In this case, only the "g" and "c" flags on the empty pattern are honored; the other flags
are taken from the original pattern. If no match has previously succeeded, this will (silently) act
instead as a genuine empty pattern (which will always match).
Note that it's possible to confuse Perl into thinking "//" (the empty regex) is really "//" (the
defined-or operator). Perl is usually pretty good about this, but some pathological cases might
trigger this, such as "$a///" (is that "($a) / (//)" or "$a // /"?) and "print $fh //" ("print $fh(//"
or "print($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 "m" (so "//" becomes "m//").
Matching in list context
If the "/g" option is not used, "m//" in list context returns a list consisting of the subexpressions
matched by the parentheses in the pattern, that is, ($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")
|| die "can't access /dev/tty: $!";
<TTY> =~ /^y/i && foo(); # do foo if desired
if (/Version: *([0-9.]*)/) { $version = $1; }
next if m#^/usr/spool/uucp#;
# poor man's grep
$arg = shift;
while (<>) {
print if /$arg/o; # compile only once (no longer needed!)
}
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 (for example, "m//gc").
Modifying the target string also resets the search position.
\G assertion
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()" as it was at the start of the operation (see "pos" in perlfunc),
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);
# scalar context
local $/ = "";
while ($paragraph = <>) {
while ($paragraph =~ /\p{Ll}['")]*[.!?]+['")]*\s/g) {
$sentences++;
}
}
say $sentences;
Here's another way to check for sentences in a paragraph:
my $sentence_rx = qr{
(?: (?<= ^ ) | (?<= \s ) ) # after start-of-string or whitespace
\p{Lu} # capital letter
.*? # a bunch of anything
(?<= \S ) # that ends in non-whitespace
(?<! \b [DMS]r ) # but isn't a common abbreviation
(?<! \b Mrs )
(?<! \b Sra )
(?<! \b St )
[.?!] # followed by a sentence ender
(?= $ | \s ) # in front of end-of-string or whitespace
}sx;
local $/ = "";
while (my $paragraph = <>) {
say "NEW PARAGRAPH";
my $count = 0;
while ($paragraph =~ /($sentence_rx)/g) {
printf "\tgot sentence %d: <%s>\n", ++$count, $1;
}
}
Here's how to use "m//gc" with "\G":
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 a very old (pre-5.6.0)
version of 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 different actions depending on which regexp matched. Each regexp
tries to match where the previous one leaves off.
$_ = <<'EOL';
$url = URI::URL->new( "http://example.com/" ); die if $url eq "xXx";
EOL
LOOP: {
print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
print(" lowercase"), redo LOOP if /\G\p{Ll}+\b[,.;]?\s*/gc;
print(" UPPERCASE"), redo LOOP if /\G\p{Lu}+\b[,.;]?\s*/gc;
print(" Capitalized"), redo LOOP if /\G\p{Lu}\p{Ll}+\b[,.;]?\s*/gc;
print(" MiXeD"), redo LOOP if /\G\pL+\b[,.;]?\s*/gc;
print(" alphanumeric"), redo LOOP if /\G[\p{Alpha}\pN]+\b[,.;]?\s*/gc;
print(" line-noise"), redo LOOP if /\G\W+/gc;
print ". That's all!\n";
}
Here is the output (split into several lines):
line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE
line-noise lowercase line-noise lowercase line-noise lowercase
lowercase line-noise lowercase lowercase line-noise lowercase
lowercase line-noise MiXeD line-noise. That's all!
m?PATTERN?msixpodualgc
?PATTERN?msixpodualgc
This is just like the "m/PATTERN/" search, except that it matches only once between calls to the
reset() operator. This is a useful optimization when you want to see only the first occurrence of
something in each file of a set of files, for instance. Only "m??" patterns local to the current
package are reset.
while (<>) {
if (m?^$?) {
# blank line between header and body
}
} continue {
reset if eof; # clear m?? status for next file
}
s/PATTERN/REPLACEMENT/msixpodualgcer
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 the "/r" (non-destructive) option is used then it runs the substitution on a copy of the string and
instead of returning the number of substitutions, it returns the copy whether or not a substitution
occurred. The original string is never changed when "/r" is used. The copy will always be a plain
string, even if the input is an object or a tied variable.
If no string is specified via the "=~" or "!~" operator, the $_ variable is searched and modified.
Unless the "/r" option is used, the string specified must be a scalar variable, an array element, a
hash element, or an assignment to one of those; that is, some sort of scalar lvalue.
If the delimiter chosen is a single quote, no interpolation is done on either the PATTERN or the
REPLACEMENT. Otherwise, if the PATTERN contains a $ that looks like a variable rather than an end-of-
string test, the variable will be interpolated into the pattern at run-time. If you want the pattern
compiled only once the first time the variable is interpolated, use the "/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.
Options are as with m// with the addition of the following replacement specific options:
e Evaluate the right side as an expression.
ee Evaluate the right side as a string then eval the result.
r Return substitution and leave the original string untouched.
Any non-whitespace delimiter may replace the slashes. Add space after the "s" when using a character
allowed in identifiers. 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
delimiters; the replacement text is not evaluated as a command. If the PATTERN is delimited by
bracketing quotes, the REPLACEMENT has its own pair of quotes, which may or may not be bracketing
quotes, for example, "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 = $bar) =~ s/this/that/; # copy first, then change
($foo = "$bar") =~ s/this/that/; # convert to string, copy, then change
$foo = $bar =~ s/this/that/r; # Same as above using /r
$foo = $bar =~ s/this/that/r
=~ s/that/the other/r; # Chained substitutes using /r
@foo = map { s/this/that/r } @bar # /r is very useful in maps
$count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
# 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 + $1/eg;
# Titlecase words in the last 30 characters only
substr($str, -30) =~ s/\b(\p{Alpha}+)\b/\u\L$1/g;
# 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;
"s///le" is treated as a substitution followed by the "le" operator, not the "/le" flags. This may
change in a future version of Perl. It produces a warning if warnings are enabled. To disambiguate,
use a space or change the order of the flags:
s/foo/bar/ le 5; # "le" infix operator
s/foo/bar/el; # "e" and "l" flags
Quote-Like Operators
q/STRING/
'STRING'
(*** The previous line contains the naughty word "$1".\n)
if /\b(tcl|java|python)\b/i; # :-)
$baz = "\n"; # a one-character string
qx/STRING/
`STRING`
A string which is (possibly) interpolated and then executed as a system command with "/bin/sh" or its
equivalent. Shell wildcards, pipes, and redirections will be honored. The collected standard output of
the command is returned; standard error is unaffected. In scalar context, it comes back as a single
(potentially multi-line) string, or undef if the command failed. In list context, returns a list of lines
(however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
Because backticks do not affect standard error, use shell file descriptor syntax (assuming the shell
supports this) if you care to address this. To capture a command's STDERR and STDOUT together:
$output = `cmd 2>&1`;
To capture a command's STDOUT but discard its STDERR:
$output = `cmd 2>/dev/null`;
To capture a command's STDERR but discard its STDOUT (ordering is important here):
$output = `cmd 2>&1 1>/dev/null`;
To exchange a command's STDOUT and STDERR in order to capture the STDERR but leave its STDOUT to come out
the old STDERR:
$output = `cmd 3>&1 1>&2 2>&3 3>&-`;
To read both a command's STDOUT and its STDERR separately, it's easiest 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");
The STDIN filehandle used by the command is inherited from Perl's STDIN. For example:
open(SPLAT, "stuff") || die "can't open stuff: $!";
open(STDIN, "<&SPLAT") || die "can't dupe SPLAT: $!";
print STDOUT `sort`;
will print the sorted contents of the file named "stuff".
Using single-quote as a delimiter protects the command from Perl's double-quote interpolation, passing it
on to the shell instead:
$perl_info = qx(ps $$); # that's Perl's $$
$shell_info = qx'ps $$'; # that's the new shell's $$
How that string gets evaluated is entirely subject to the command interpreter on your system. On most
platforms, you will have to protect shell metacharacters if you want them treated literally. This is in
practice difficult to do, as it's unclear how to escape which characters. See perlsec for a clean and
safe example of a manual fork() and exec() to emulate backticks safely.
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
delimiters. It can be understood as being roughly equivalent to:
split(" ", q/STRING/);
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)
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.
tr/SEARCHLIST/REPLACEMENTLIST/cdsr
y/SEARCHLIST/REPLACEMENTLIST/cdsr
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.
If the "/r" (non-destructive) option is present, a new copy of the string is made and its characters
transliterated, and this copy is returned no matter whether it was modified or not: the original string is
always left unchanged. The new copy is always a plain string, even if the input string is an object or a
tied variable.
Unless the "/r" option is used, the string specified with "=~" must be a scalar variable, an array
element, a hash element, or an assignment to one of those; in other words, 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 SEARCHLIST
is delimited by bracketing quotes, the REPLACEMENTLIST has its own pair of quotes, which may or may not be
bracketing quotes; for example, "tr[aeiouy][yuoiea]" or "tr(+\-*/)/ABCD/".
Note that "tr" does not do regular expression character classes such as "\d" or "\pL". The "tr" operator
c Complement the SEARCHLIST.
d Delete found but unreplaced characters.
s Squash duplicate replaced characters.
r Return the modified string and leave the original string
untouched.
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 SEARCHLIST, the final character is replicated till it is long
enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is replicated. This latter is useful for
counting characters in a class or for squashing character sequences in a class.
Examples:
$ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case ASCII
$cnt = tr/*/*/; # count the stars in $_
$cnt = $sky =~ tr/*/*/; # count the stars in $sky
$cnt = tr/0-9//; # count the digits in $_
tr/a-zA-Z//s; # bookkeeper -> bokeper
($HOST = $host) =~ tr/a-z/A-Z/;
$HOST = $host =~ tr/a-z/A-Z/r; # same thing
$HOST = $host =~ tr/a-z/A-Z/r # chained with s///r
=~ s/:/ -p/r;
tr/a-zA-Z/ /cs; # change non-alphas to single space
@stripped = map tr/a-zA-Z/ /csr, @original;
# /r with map
tr [\200-\377]
[\000-\177]; # wickedly delete 8th bit
If multiple transliterations are given for a character, only the first one is used:
tr/AAA/XYZ/
will transliterate any A to X.
Because the transliteration table is built at compile time, neither the SEARCHLIST nor the REPLACEMENTLIST
are subjected to double quote interpolation. That means that if you want to use variables, you must use
an eval():
identifier is explicitly 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.
If the terminating string is quoted, the type of quotes used determine the treatment of the text.
Double Quotes
Double quotes indicate that the text will be interpolated using exactly the same rules as normal
double quoted strings.
print <<EOF;
The price is $Price.
EOF
print << "EOF"; # same as above
The price is $Price.
EOF
Single Quotes
Single quotes indicate the text is to be treated literally with no interpolation of its content. This
is similar to single quoted strings except that backslashes have no special meaning, with "\\" being
treated as two backslashes and not one as they would in every other quoting construct.
Just as in the shell, a backslashed bareword following the "<<" means the same thing as a single-
quoted string does:
$cost = <<'VISTA'; # hasta la ...
That'll be $10 please, ma'am.
VISTA
$cost = <<\VISTA; # Same thing!
That'll be $10 please, ma'am.
VISTA
This is the only form of quoting in perl where there is no need to worry about escaping content,
something that code generators can and do make good use of.
Backticks
The content of the here doc is treated just as it would be if the string were embedded in backticks.
Thus the content is interpolated as though it were double quoted and then executed via the shell, with
the results of the execution returned.
print << `EOC`; # execute command and get results
echo hi there
EOC
It is possible to stack multiple here-docs in a row:
print <<"foo", <<"bar"; # you can stack them
I said foo.
foo
I said bar.
bar
ABC
+ 20;
If you want to remove the line terminator from your here-docs, use "chomp()".
chomp($string = <<'END');
This is a string.
END
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 "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 Can't find string terminator "END" anywhere before EOF....
Additionally, quoting rules for the end-of-string identifier are unrelated 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 different 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 constructs. Although the most common reason to learn
this is to unravel labyrinthine regular expressions, because the initial steps of parsing are the same for all
The first pass is finding the end of the quoted construct, where the information about the delimiters is
used in parsing. During this search, text between the starting and ending delimiters is copied to a safe
location. The text copied gets delimiter-independent.
If the construct is a here-doc, the ending delimiter is a line that has a terminating string as the
content. Therefore "<<EOF" is terminated by "EOF" immediately followed by "\n" and starting from the first
column of the terminating line. When searching for the terminating line of a here-doc, nothing is
skipped. In other words, lines after the here-doc syntax are compared with the terminating string line by
line.
For the constructs except here-docs, single characters are used as starting and ending delimiters. If the
starting delimiter is an opening punctuation (that is "(", "[", "{", or "<"), the ending delimiter is the
corresponding closing punctuation (that is ")", "]", "}", or ">"). If the starting delimiter is an
unpaired character like "/" or a closing punctuation, the ending delimiter is same as the starting
delimiter. Therefore a "/" terminates a "qq//" construct, while a "]" terminates "qq[]" and "qq]]"
constructs.
When searching for single-character delimiters, escaped delimiters and "\\" are skipped. For example,
while searching for terminating "/", combinations of "\\" and "\/" are skipped. If the delimiters are
bracketing, nested pairs are also skipped. For example, while searching for closing "]" paired with the
opening "[", combinations of "\\", "\]", and "\[" are all skipped, and nested "[" and "]" are skipped as
well. However, when backslashes are used as the delimiters (like "qq\\" and "tr\\\"), nothing is skipped.
During the search for the end, backslashes that escape delimiters or backslashes are removed (exactly
speaking, they are not copied to the safe location).
For constructs with three-part delimiters ("s///", "y///", and "tr///"), the search is repeated once more.
If the first delimiter is not an opening punctuation, three delimiters must be same such as "s!!!" and
"tr)))", in which case the second delimiter terminates the left part and starts the right part at once.
If the left part is delimited by bracketing punctuation (that is "()", "[]", "{}", or "<>"), the right
part needs another pair of delimiters such as "s(){}" and "tr[]//". In these cases, whitespace and
comments are allowed between both parts, though the comment must follow at least one whitespace character;
otherwise a character expected as the start of the comment may be regarded as the starting delimiter of
the right part.
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\" (multichar control char syntax) 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.
Interpolation
The only interpolation is removal of "\" from pairs of "\\". Therefore "-" in "tr'''" and "y'''" is
treated literally as a hyphen and no character range is available. "\1" in the replacement of "s'''"
does not work as $1.
"tr///", "y///"
No variable interpolation occurs. String modifying combinations for case and quoting such as "\Q",
"\U", and "\E" are not recognized. The other escape sequences such as "\200" and "\t" and backslashed
characters such as "\\" and "\-" are converted to appropriate literals. The character "-" is treated
specially and therefore "\-" is treated as a literal "-".
"", "``", "qq//", "qx//", "<file*glob>", "<<"EOF""
"\Q", "\U", "\u", "\L", "\l", "\F" (possibly paired with "\E") are converted to corresponding Perl
constructs. Thus, "$foo\Qbaz$bar" is converted to "$foo . (quotemeta("baz" . $bar))" internally. The
other escape sequences such as "\200" and "\t" and backslashed characters such as "\\" and "\-" are
replaced with appropriate expansions.
Let it be stressed that whatever falls between "\Q" and "\E" is interpolated in the usual way.
Something 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 internally to the "join" and "." catenation operations.
Thus, "$foo XXX '@arr'" becomes:
$foo . " XXX '" . (join $", @arr) . "'";
All operations above are performed simultaneously, left to right.
Because the result of "\Q STRING \E" has all metacharacters quoted, there is no way to insert a
literal "$" or "@" inside a "\Q\E" pair. If protected by "\", "$" will be quoted to became "\\\$"; if
not, it is interpreted as the start of an interpolated scalar.
Note also that the interpolation code needs to make a decision on where the interpolated scalar ends.
For instance, whether "a $b -> {c}" really means:
"a " . $b . " -> {c}";
or:
"a " . $b -> {c};
Most of the time, the longest possible text that does not include spaces between components and which
contains matching braces or brackets. because the outcome may be determined by voting based on
heuristic estimators, the result is not strictly predictable. Fortunately, it's usually correct for
ambiguous cases.
the replacement of "s///"
Processing of "\Q", "\U", "\u", "\L", "\l", "\F" and interpolation happens as with "qq//" constructs.
However any other combinations of "\" followed by a character are not substituted but only skipped, in
order to parse them as regular expressions at the following step. As "\c" is skipped at this step,
"@" of "\c@" in RE is possibly treated as an array symbol (for example @foo), even though the same
text in "qq//" gives interpolation of "\c@".
Moreover, inside "(?{BLOCK})", "(?# comment )", and a "#"-comment in a "//x"-regular expression, no
processing is performed whatsoever. This is the first step at which the presence of the "//x"
modifier is relevant.
Interpolation in patterns has several quirks: $|, $(, $), "@+" and "@-" are not interpolated, and
constructs $var[SOMETHING] are voted (by several different estimators) to be either an array element
or $var followed by an RE alternative. This is where the notation "${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.
The lack of processing of "\\" creates specific restrictions on the post-processed text. If the
delimiter 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
delimiter 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 delimiter-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 delimiters to non-alphanumeric, non-whitespace
choices.
This step is the last one for all constructs except regular expressions, which are processed further.
parsing 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 concatenation, 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 discussed 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 automaton.
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 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 "["
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 undergoes double-quote interpolation. It is then
interpreted as an external command, and the output of that command is the value of the backtick string, like
in a shell. In scalar context, a single string consisting of all output is returned. In list context, a list
of values is returned, one per line of output. (You can set $/ 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
$? (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
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 "qx//". (Because backticks always undergo shell expansion as
well, see 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 "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 disguised 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 assigns to a lexical variable instead of to $_:
while (my $line = <STDIN>) { print $line }
In these loop constructs, the assigned value (whether assignment is automatic or explicit) is then tested to
see whether it is defined. The defined test avoids problems where the line has a string value that would be
treated as false by Perl; for example a "" or a "0" with no trailing newline. If you really mean for such
values to terminate the loop, they should be tested for explicitly:
while (($_ = <STDIN>) ne '0') { ... }
while (<STDIN>) { last unless $_; ... }
In other boolean contexts, "<FILEHANDLE>" without an explicit "defined" test or comparison elicits 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 local identifiers rather than global.)
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 $ARGV variable. It also uses filehandle ARGV internally. <> is just a
synonym for <ARGV>, which is magical. (The pseudo code above doesn't work because it treats <ARGV> as non-
magical.)
Since the null filehandle uses the two argument form of "open" in perlfunc it interprets special characters,
so if you have a script like this:
while (<>) {
print;
}
and call it with "perl dangerous.pl 'rm -rfv *|'", it actually opens a pipe, executes the "rm" command and
reads "rm"'s output from that pipe. If you want all items in @ARGV to be interpreted as file names, you can
use the module "ARGV::readonly" from CPAN.
You can modify @ARGV before the first <> as long as the array ends up containing the list of filenames you
really want. Line numbers ($.) 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:
@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 }
contains the name of the filehandle to input from, or its typeglob, or a reference to the same. For example:
$fh = \*STDIN;
$line = <$fh>;
If what's within the angle brackets is neither a filehandle nor a simple scalar variable containing a
filehandle name, typeglob, or typeglob reference, it is interpreted as a filename pattern to be globbed, and
either a list of filenames or the next filename in the list is returned, depending on context. This
distinction is determined on syntactic grounds alone. That means "<$x>" is always a readline() from an
indirect handle, but "<$hash{key}>" is always a glob(). That's because $x is a simple scalar variable, 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 "<$foo>" because that's an indirect
filehandle as explained in the previous paragraph. (In older versions of Perl, programmers would insert curly
brackets to force interpretation as a filename glob: "<${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, $_;
}
except that the globbing is actually done internally using the standard "File::Glob" extension. Of course,
the shortest way to do the above is:
chmod 0644, <*.c>;
A (file)glob evaluates its (embedded) argument only when it is starting a new list. All values must be read
before it will start over. In list context, this isn't important because you automatically get them all
anyway. However, in scalar context the operator returns the next value each time it's called, or "undef" when
the list has run out. As with filehandle reads, an automatic "defined" is generated when the glob occurs in
the test part of a "while", because legal glob returns (for example, a file called 0) would otherwise
terminate the loop. Again, "undef" is returned only once. So if you're expecting a single value from a glob,
it is much better to say
($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 definitely better to use the glob() function, because the
older notation can cause people to become confused with the indirect filehandle notation.
. 'good men to come to.'
and this all reduces to one string internally. Likewise, if you say
foreach $file (@filenames) {
if (-s $file > 5 + 100 * 2**16) { }
}
the compiler precomputes the number which that expression represents so that the interpreter won't have to.
No-ops
Perl doesn't officially have a no-op operator, but the bare constants 0 and 1 are special-cased not to produce
a warning in void context, so you can for example safely do
1 while foo();
Bitwise String Operators
Bitstrings of any size may be manipulated by the bitwise operators ("~ | & ^").
If the operands to a binary bitwise op are strings of different sizes, | and ^ ops act as though the shorter
operand had additional zero bits on the right, while the & op acts as though the longer operand were truncated
to the length of the shorter. The granularity for such extension or truncation is one or more bytes.
# ASCII-based examples
print "j p \n" ^ " a h"; # prints "JAPH\n"
print "JA" | " ph\n"; # prints "japh\n"
print "japh\nJunk" & '_____'; # prints "JAPH\n";
print 'p N$' ^ " E<H\n"; # prints "Perl\n";
If you are intending to manipulate bitstrings, be certain that you're supplying bitstrings: If an operand is a
number, that will imply a numeric bitwise operation. You may explicitly 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" ^ "$bar"; # both ops explicitly stringy
See "vec" in perlfunc for information on how to manipulate individual bits in a bit vector.
Integer Arithmetic
By default, Perl assumes that it must do most of its arithmetic in floating point. But by saying
use integer;
you may tell the compiler to use integer operations (see integer for a detailed explanation) from here to the
end of the enclosing BLOCK. An inner BLOCK may countermand this by saying
no integer;
which lasts until the end of that BLOCK. Note that this doesn't mean everything is an integer, merely that
sprintf() or printf() is usually the easiest route. See perlfaq4.
Floating-point numbers are only approximations to what a mathematician would call real numbers. There are
infinitely more reals than floats, so some corners must be cut. For example:
printf "%.20g\n", 123456789123456789;
# produces 123456789123456784
Testing for exact floating-point equality or inequality is not a good idea. Here's a (relatively expensive)
work-around to compare whether two floating-point numbers are equal to a particular number of decimal places.
See Knuth, volume II, for a more robust treatment of this topic.
sub fp_equal {
my ($X, $Y, $POINTS) = @_;
my ($tX, $tY);
$tX = sprintf("%.${POINTS}g", $X);
$tY = sprintf("%.${POINTS}g", $Y);
return $tX eq $tY;
}
The POSIX module (part of the standard perl distribution) implements ceil(), floor(), and other mathematical
and trigonometric functions. The Math::Complex module (part of the standard perl distribution) defines
mathematical functions that work on both the reals and the imaginary numbers. Math::Complex not as efficient
as POSIX, but POSIX can't work with complex numbers.
Rounding in financial applications can have serious implications, and the rounding method used should be
specified precisely. In these cases, it probably pays not to trust whichever system rounding is being used by
Perl, but to instead implement the rounding function you need yourself.
Bigger Numbers
The standard "Math::BigInt", "Math::BigRat", and "Math::BigFloat" modules, along with the "bigint", "bigrat",
and "bitfloat" pragmas, 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 5.010;
use bigint; # easy interface to Math::BigInt
$x = 123456789123456789;
say $x * $x;
+15241578780673678515622620750190521
Or with rationals:
use 5.010;
use bigrat;
$a = 3/22;
$b = 4/6;
say "a/b is ", $a/$b;
say "a*b is ", $a*$b;
a/b is 9/44
a*b is 1/11
Several modules 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.
Math::Cephes::Fraction fractions via the Cephes library
Math::GMP another one using an external C library
Choose wisely.
perl v5.16.3 2013-03-04 PERLOP(1)
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