perlop - Perl operators and precedence
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) [Toc] [Back]
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.
If any list operator (print(), etc.) or any unary operator
(chdir(), etc.) is followed by a left parenthesis as the
next token, the operator and arguments within parentheses
are taken to be of highest precedence, just like a normal
function call.
In the absence of parentheses, the precedence of list
operators such as "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
@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, "0;
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 "$foo & 255"). Then one
is added to the return value of "print" (usually 1). The
result is something like this:
1 + 1, "0; # Obviously not what you meant.
To do what you meant properly, you must write:
print(($foo & 255) + 1, "0);
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 [Toc] [Back]
""->"" 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 [Toc] [Back]
"++" 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 behaviour. Avoid
statements like:
$i = $i ++;
print ++ $i + $i ++;
Perl will not guarantee what the result of the above
statements is.
The auto-increment operator has a little extra builtin
magic to it. If you increment a variable that is numeric,
or that has ever been used in a numeric context, you get a
normal increment. If, however, the variable has been used
in only string contexts since it was set, and has a value
that is not the empty string and matches the pattern
"/^[a-zA-Z]*[0-9]*", the increment is done as a string,
preserving each character within its range, with carry:
print ++($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.
Exponentiation [Toc] [Back]
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 [Toc] [Back]
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 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 "-bareword".
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 "
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 [Toc] [Back]
Binary "=~" binds a scalar expression to a pattern match.
Certain operations search or modify the string $_ by
default. This operator makes that kind of operation work
on some other string. The right argument is a search pattern,
substitution, or transliteration. The left argument
is what is supposed to be searched, substituted, or
transliterated instead of the default $_. When used in
scalar context, the return value generally indicates the
success of the operation. Behavior in list context
depends on the particular operator. See "Regexp QuoteLike
Operators" for details.
If the right argument is an expression rather than a
search pattern, substitution, or transliteration, it is
interpreted as a search pattern at run time.
Binary "!~" is just like "=~" except the return value is
negated in the logical sense.
Multiplicative Operators [Toc] [Back]
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 $a
(i.e. the result will be less than or equal to zero).
Note that when "use integer" is in scope, "%" gives you
direct access to the modulus operator as implemented by
your C compiler. This operator is not as well defined for
negative operands, but it will execute faster.
Binary "x" is the repetition operator. In scalar context
or if the left operand is not enclosed in parentheses, it
returns a string consisting of the left operand repeated
the number of times specified by the right operand. In
list context, if the left operand is enclosed in parentheses,
it repeats the list. If the right operand is zero or
negative, it returns an empty string or an empty list,
depending on the context.
print '-' x 80; # print row of dashes
print "" x ($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 [Toc] [Back]
Binary "+" returns the sum of two numbers.
Binary "-" returns the difference of two numbers.
Binary "." concatenates two strings.
Shift Operators [Toc] [Back]
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.
Named Unary Operators [Toc] [Back]
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 ($foo * 20)
rand 10 * 20; # rand (10 * 20)
rand(10) * 20; # (rand 10) * 20
rand (10) * 20; # (rand 10) * 20
rand +(10) * 20; # rand (10 * 20)
Regarding precedence, the filetest operators, like "-f",
"-M", etc. are treated like named unary operators, but
they don't follow this functional parenthesis rule. That
means, for example, that "-f($file).".bak"" is equivalent
to "-f "$file.bak"".
See also "Terms and List Operators (Leftward)".
Relational Operators [Toc] [Back]
Binary "<" returns true if the left argument is numerically
less than the right argument.
Binary ">" returns true if the left argument is numerically
greater than the right argument.
Binary "<=" returns true if the left argument is numerically
less than or equal to the right argument.
Binary ">=" returns true if the left argument is numerically
greater than or equal to the right argument.
Binary "lt" returns true if the left argument is stringwise
less than the right argument.
Binary "gt" returns true if the left argument is stringwise
greater than the right argument.
Binary "le" returns true if the left argument is stringwise
less than or equal to the right argument.
Binary "ge" returns true if the left argument is stringwise
greater than or equal to the right argument.
Equality Operators
Binary "==" returns true if the left argument is numerically
equal to the right argument.
Binary "!=" returns true if the left argument is numerically
not equal to the right argument.
Binary "<=>" returns -1, 0, or 1 depending on whether the
left argument is numerically less than, equal to, or
greater than the right argument. If your platform supports
NaNs (not-a-numbers) as numeric values, using them
with "<=>" returns undef. NaN is not "<", "==", ">", "<="
or ">=" anything (even NaN), so those 5 return false. NaN
!= NaN returns true, as does NaN != anything else. If your
platform doesn't support NaNs then NaN is just a string
with numeric value 0.
perl -le '$a = NaN; print "No NaN support here" if $a
== $a'
perl -le '$a = NaN; print "NaN support here" if $a !=
$a'
Binary "eq" returns true if the left argument is stringwise
equal to the right argument.
Binary "ne" returns true if the left argument is stringwise
not equal to the right argument.
Binary "cmp" returns -1, 0, or 1 depending on whether the
left argument is stringwise less than, equal to, or
greater than the right argument.
"lt", "le", "ge", "gt" and "cmp" use the collation (sort)
order specified by the current locale if "use locale" is
in effect. See perllocale.
Bitwise And [Toc] [Back]
Binary "&" returns its operands ANDed together bit by bit.
(See also "Integer Arithmetic" and "Bitwise String Operators".)
Note that "&" has lower priority than relational operators,
so for example the brackets are essential in a test
like
print "Even0 if ($x & 1) == 0;
Bitwise Or and Exclusive Or [Toc] [Back]
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 "false0 if (8 | 2) != 10;
C-style Logical And [Toc] [Back]
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 [Toc] [Back]
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!0;
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 precedence of "and" and "or" is much lower, however,
so that you can safely use them after a list operator
without the need for parentheses:
unlink "alpha", "beta", "gamma"
or gripe(), next LINE;
With the C-style operators that would have been written
like this:
unlink("alpha", "beta", "gamma")
|| (gripe(), next LINE);
Using "or" for assignment is unlikely to do what you want;
see below.
Range Operators [Toc] [Back]
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.
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
point expression; when implicitly using $. as
described in the previous paragraph, the comparison is
"int(EXPR) == int($.)" which is only an issue when $. is
set to a floating point value and you are not reading from
a file. Furthermore, "span" .. "spat" or "2.18 .. 3.14"
will not do what you want in scalar context because each
of the operands are evaluated using their integer representation.
Examples:
As a scalar operator:
if (101 .. 200) { print; } # print 2nd hundred lines,
short for
# if ($. == 101 .. $. ==
200) ...
next line if (1 .. /^$/); # skip header lines, short
for
# ... if ($. == 1 ..
/^$/);
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 $. each
file
}
Here's a simple example to illustrate the difference
between the two range operators:
@lines = (" - Foo",
"01 - Bar",
"1 - Baz",
" - Quux");
foreach(@lines)
{
if (/0/ .. /1/)
{
print "$_0;
}
}
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.
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 [Toc] [Back]
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.0, $n,
($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 [Toc] [Back]
"=" is the ordinary assignment operator.
Assignment operators work as in C. That is,
$a += 2;
is equivalent to
$a = $a + 2;
although without duplicating any side effects that dereferencing
the lvalue might trigger, such as from tie().
Other assignment operators work similarly. The following
are recognized:
**= += *= &= <<= &&=
-= /= |= >>= ||=
.= %= ^=
x=
Although these are grouped by family, they all have the
precedence of assignment.
Unlike in C, the scalar assignment operator produces a
valid lvalue. Modifying an assignment is equivalent to
doing the assignment and then modifying the variable that
was assigned to. This is useful for modifying a copy of
something, like this:
($tmp = $global) =~ tr [A-Z] [a-z];
Likewise,
($a += 2) *= 3;
is equivalent to
$a += 2;
$a *= 3;
Similarly, a list assignment in list context produces the
list of lvalues assigned to, and a list assignment in
scalar context returns the number of elements produced by
the expression on the right hand side of the assignment.
Comma Operator [Toc] [Back]
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 to its left to be interpreted as a string (as of
5.001). It is helpful in documenting the correspondence
between keys and values in hashes, and other paired elements
in lists.
List Operators (Rightward) [Toc] [Back]
On the right side of a list operator, it has very low
precedence, such that it controls all comma-separated
expressions found there. The only operators with lower
precedence are the logical operators "and", "or", and
"not", which may be used to evaluate calls to list operators
without the need for extra parentheses:
open HANDLE, "filename"
or die "Can't open: $!0;
See also discussion of list operators in "Terms and List
Operators (Leftward)".
Logical Not [Toc] [Back]
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 [Toc] [Back]
Binary "and" returns the logical conjunction of the two
surrounding expressions. It's equivalent to && except for
the very low precedence. This means that it
short-circuits: i.e., the right expression is evaluated
only if the left expression is true.
Logical or and Exclusive Or [Toc] [Back]
Binary "or" returns the logical disjunction of the two
surrounding expressions. It's equivalent to || except for
the very low precedence. This makes it useful for control
flow
print FH $data or die "Can't write to FH:
$!";
This means that it short-circuits: i.e., the right expression
is evaluated only if the left expression is false.
Due to its precedence, you should probably avoid using
this for assignment, only for control flow.
$a = $b or $c; # bug: this is wrong
($a = $b) or $c; # really means this
$a = $b || $c; # better written this way
However, when it's a list-context assignment and you're
trying to use "||" for control flow, you probably need
"or" so that the assignment takes higher precedence.
@info = stat($file) || die; # oops, scalar sense
of stat!
@info = stat($file) or die; # better, now @info
gets its due
Then again, you could always use parentheses.
Binary "xor" returns the exclusive-OR of the two surrounding
expressions. It cannot short circuit, of course.
C Operators Missing From Perl [Toc] [Back]
Here is what C has that Perl doesn't:
unary & Address-of operator. (But see the "
for taking a reference.)
unary * Dereference-address operator. (Perl's prefix
dereferencing operators are typed: $, @, %, and
&.)
(TYPE) Type-casting operator.
Quote and Quote-like Operators [Toc] [Back]
While we usually think of quotes as literal values, in
Perl they function as operators, providing various kinds
of interpolating and pattern matching capabilities. Perl
provides customary quote characters for these behaviors,
but also provides a way for you to choose your quote character
for any of them. In the following table, a "{}"
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
'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 distribution)
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.
tab (HT, TAB)
nreturn ((CR)
form feed (FF)
backspace (BS)
alarm (bell) (BEL)
\ escape (ESC)
33 octal char (ESC)
b hex char (ESC)
} wide hex char (SMILEY)
ame} named Unicode character
NOTE: Unlike C and other languages, Perl has no escape
sequence for the vertical tab (VT - ASCII 11).
The following escape sequences are available in constructs
that interpolate but not in transliterations.
lowercase next char
uppercase next char
lowercase till U uppercase till end case modification
Q quote non-word characters till
If "use locale" is in effect, the case map used by "
" "" and "U" is taken from the current locale. See
perllocale. If Unicode (for example, "" or wide hex
characters of 0x100 or beyond) is being used, the case map
used by " " "" and "U" is as defined by Unicode.
For documentation of "ame}", see charnames.
All systems use the virtual "0 to represent a line terminator,
called a "newline". There is no such thing as an
unvarying, physical newline character. It is only an
illusion that the operating system, device drivers, C
libraries, and"PaslASCIIcCRsande"0oasrASCIIeLF.NoForll
systems read "
example, on a Mac, these are reversed, and on systems
without line terminator, printing "0 may emit no actual
data. In general, use "0 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 (" 15 12" or " line terminators, and although they often accept just
" 12", they seldom tolerate just " 15". If you get in
the habit of using "0 for networking, 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 "$" or "@" interpolates the corresponding
variable, while escaping will cause the literal
string " to be inserted. You'll need to write something
like "m/Quser\@Qhost/".
Patterns are subject to an additional level of interpretation
as a regular expression. This is done as a second
pass, after variables are interpolated, so that regular
expressions may be incorporated into the pattern from the
variables. If this is not what you want, use "Q" to
interpolate a variable literally.
Apart from the behavior described above, Perl does not
expand multiple levels of interpolation. In particular,
contrary to the expectations of shell programmers, backquotes
do NOT interpolate within double quotes, nor do
single quotes impede evaluation of variables when used
within double quotes.
Regexp Quote-Like Operators [Toc] [Back]
Here are the quote-like operators that apply to pattern
matching and related activities.
?PATTERN?
This is just like the "/pattern/" search, except
that it matches only once between calls to the
reset() operator. This is a useful optimization
when you want to see only the first occurrence of
something in each file of a set of files, for
instance. Only "??" patterns local to the current
package are reset.
while (<>) {
if (?^$?) {
# 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
non-alphanumeric, non-whitespace characters as
delimiters. This is particularly useful for
matching path names that contain "/", to avoid LTS
(leaning toothpick syndrome). If "?" is the
delimiter, then the match-only-once rule of "?PATTERN?"
applies. If "'" is the delimiter, no
interpolation is performed on the PATTERN.
PATTERN may contain variables, which will be
interpolated (and the pattern recompiled) every
time the pattern search is evaluated, except for
when the delimiter is a single quote. (Note that
$(, $), 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 recompilations, and is useful when
the value you are interpolating 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 context
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#;
# poor man's grep
$arg = shift;
while (<>) {
print if /$arg/o; # compile only
once
}
if (($F1, $F2, $Etc) = ($foo =~ /^()(.*)/))
This last example splits $foo into the first two
words and the remainder of the line, and assigns
those three fields to $F1, $F2, and $Etc. The
conditional is true if any variables were
assigned, i.e., if the pattern matched.
The "/g" modifier specifies global pattern matching--that
is, matching as many times as possible
within the string. How it behaves depends on the
context. In list context, it returns a list of
the substrings matched by any capturing parentheses
in the regular expression. If there are no
parentheses, it returns a list of all the matched
strings, as if there were parentheses around the
whole pattern.
In scalar context, each execution of "m//g" finds
the next match, returning true if it matches, and
false if there is no further match. The position
after the last match can be read or set using the
pos() function; see "pos" in perlfunc. A failed
match normally resets the search position to the
beginning of the string, but you can avoid that by
adding the "/c" modifier (e.g. "m//gc"). Modifying
the target string also resets the search position.
You can intermix "m//g" matches with "m/G.../g",
where "G" is a zero-width assertion that matches
the exact position where the previous "m//g", if
any, left off. Without the "/g" modifier, the
"G" assertion still anchors at pos(), but the
match is of course only attempted once. Using
"G" without "/g" on a target string that has not
previously had a "/g" match applied to it is the
same as using the "0 beginning of
the string. Note also that, currently,
"G" is only properly supported when
anchored at the very beginning of the pattern.
Examples:
# list context
($one,$five,$fifteen) = (`uptime` =~
/(+.+)/g);
# scalar context
$/ = "";
while (defined($paragraph = <>)) {
while ($paragraph =~ /[az]['")]*[.!?]+['")]*g) {
$sentences++;
}
}
print "$sentences0;
# using m//gc with G
$_ = "ppooqppqq";
while ($i++ < 2) {
print "1: '";
print $1 while /(o)/gc; print "', pos=",
pos, "0;
print "2: '";
print $1 if /G(q)/gc; print "', pos=",
pos, "0;
print "3: '";
print $1 while /(p)/gc; print "', pos=",
pos, "0;
}
print "Final: '$1', pos=",pos,"0 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 different
actions depending on which regexp matched.
Each regexp tries to match where the previous one
leaves off.
$_ = <<'EOL';
$url = new URI::URL "http://www/"; die if
$url eq "xXx"; ,
EOL .
, LOOP: ;
. { ]
, print(" digits"), redo LOOP if /G+?
. print(" lowercase"), redo LOOP if /G[az]+; , g
] . print(" UPPERCASE"), redo LOOP if /G[AZ]+? , ; ;
/ . ] print(" Capitalized"), redo LOOP if /G[AZ][ga-z;+?,
c ] /. print(" MiXeD"), redo LOOP if /G[AZa-z]+? g;
/ c] print(" alphanumeric"), redo LOOP if /G[AZa-z0-9]+?
c / print(" line-noise"), redo LOOP if
/G[^A-Za-z0-9]+/gc;
c print ". That's all!0;
; }
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 delimiter
or another backslash, in which case the
delimiter or backslash is interpolated.
$foo = q!I said, "You said, 'She said it.'"!;
$bar = q('This is it.');
$baz = '0; # a two-character
string
qq/STRING/
"STRING"
A double-quoted, interpolated string.
t
c
PERLOP(1) Perl Programmers Rlference Guide PERLOP(1)
|
j
$_ .= qq a
(*** The previovs line contains the naughty
word "$1".0 a
if /|
$baz = "0; p # a one-character
string y
t
qr/STRING/imosx h
This operator quoteso(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/imosx"
expression.
For example,
$rex = qr/my.STRING/is;
s/$rex/foo/;
is equivalent to
s/my.STRING/foo/is;
The result may be used as a subpattern in a match:
$re = qr/$pattern/;
$string =~ /foo${re}bar/; # can be interpolated in other patterns
$string =~ $re; # or used standalone
$string =~ /$re/; # or this way
Since Perl may compile the pattern at the moment
of execution of qr() operator, using qr() may have
speed advantages in some situations, notably if
the result of qr() is used standalone:
sub match {
my $patterns = shift;
my @compiled = map qr/$_/i, @$patterns;
grep {
my $success = 0;
foreach my $pat (@compiled) {
$success = 1, last if /$pat/;
}
$success;
} @_;
}
Precompilation of the pattern into an internal
representation at the moment of qr() avoids a need
to recompile the pattern every time a match
"/$pat/" is attempted. (Perl has many other
internal optimizations, but none would be triggered
in the above example if we did not use qr()
operator.)
Options are:
i Do case-insensitive pattern matching.
m Treat string as multiple lines.
o Compile pattern only once.
s Treat string as single line.
x Use extended regular expressions.
See perlre for additional information on valid
syntax for STRING, and for a detailed look at the
semantics of regular expressions.
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");
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.
On some platforms (notably DOS-like ones), the
shell may not be capable of dealing with multiline
commands, so putting newlines in the string may
not get you what you w
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