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 evalu
ate 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 Cs 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 arent
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 opera
tor and arguments within parentheses are taken to be of highest prece
dence, 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 oper
ators 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 doesnt 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 sec
tion, 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 its 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 doesnt define when the variable is incre
mented 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 + $i ++;
Perl will not guarantee what the result of the above statements is.
The auto-increment operator has a little extra builtin magic to it. If
you increment a variable that is numeric, or that has ever been used in
a numeric context, you get a normal increment. If, however, the vari
able has been used in only string contexts since it was set, and has a
value that is not the empty string and matches the pattern
"/^[a-zA-Z]*[0-9]*\z/", the increment is done as a string, preserving
each character within its range, with carry:
print ++($foo = 99); # prints 100
print ++($foo = a0); # prints a1
print ++($foo = Az); # prints Ba
print ++($foo = zz); # prints aaa
"undef" is always treated as numeric, and in particular is changed to 0
before incrementing (so that a post-increment of an undef value will
return 0 rather than "undef").
The auto-decrement operator is not magical.
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 imple
mented using Cs pow(3) function, which actually works on doubles
internally.)
Symbolic Unary Operators
Unary "!" performs logical negation, i.e., "not". See also "not" for a
lower precedence version of this.
Unary "-" performs arithmetic negation if the operand is numeric. If
the operand is an identifier, a string consisting of a minus sign con
catenated with the identifier is returned. Otherwise, if the string
starts with a plus or minus, a string starting with the opposite sign
is returned. One effect of these rules is that -bareword is equivalent
to the string "-bareword". If, however, the string begins with a non-
alphabetic character (exluding "+" or "-"), Perl will attempt to con
vert 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" isnt numeric in negation (-) at ....
Unary "~" performs bitwise negation, i.e., 1s complement. For exam
ple, "0666 & ~027" is 0640. (See also "Integer Arithmetic" and "Bit
wise String Operators".) Note that the width of the result is plat
form-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 syn
tactically for separating a function name from a parenthesized expres
sion that would otherwise be interpreted as the complete list of func
tion 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 back
slash within a string, although both forms do convey the notion of pro
tecting 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.
Behavior in list context depends on the particular operator. See "Reg
exp Quote-Like Operators" for details and perlretut for examples using
these operators.
If the right argument is an expression rather than a search pattern,
substitution, or transliteration, it is interpreted as a search pattern
at run time.
Binary "!~" is just like "=~" except the return value is negated in the
logical sense.
Multiplicative Operators
Binary "*" multiplies two numbers.
Binary "/" divides two numbers.
Binary "%" computes the modulus of two numbers. Given integer operands
$a and $b: If $b is positive, then "$a % $b" is $a minus the largest
multiple of $b that is not greater than $a. If $b is negative, then
"$a % $b" is $a minus the smallest multiple of $b that is not less than
$a (i.e. the result will be less than or equal to zero). Note that
when "use integer" is in scope, "%" gives you direct access to the mod
ulus operator as implemented by your C compiler. This operator is not
as well defined for negative operands, but it will execute faster.
Binary "x" is the repetition operator. In scalar context or if the
left operand is not enclosed in parentheses, it returns a string con
sisting of the left operand repeated the number of times specified by
the right operand. In list context, if the left operand is enclosed in
parentheses or is a list formed by "qw/STRING/", it repeats the list.
If the right operand is zero or negative, it returns an empty string or
an empty list, depending on the context.
print - x 80; # print row of dashes
print "\t" x ($tab/8), x ($tab%8); # tab over
@ones = (1) x 80; # a list of 80 1s
@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 isnt 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 inte
gers, "1 << 32" is undefined. Shifting by a negative number of bits is
also undefined.
Named Unary Operators
The various named unary operators are treated as functions with one
argument, with optional parentheses.
If any list operator (print(), etc.) or any unary operator (chdir(),
etc.) is followed by a left parenthesis as the next token, the opera
tor and arguments within parentheses are taken to be of highest prece
dence, 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 ($foo * 20)
rand 10 * 20; # rand (10 * 20)
rand(10) * 20; # (rand 10) * 20
rand (10) * 20; # (rand 10) * 20
rand +(10) * 20; # rand (10 * 20)
Regarding precedence, the filetest operators, like "-f", "-M", etc. are
treated like named unary operators, but they dont follow this func
tional parenthesis rule. That means, for example, that
"-f($file).".bak"" is equivalent to "-f "$file.bak"".
See also "Terms and List Operators (Leftward)".
Relational Operators
Binary "<" returns true if the left argument is numerically less than
the right argument.
Binary ">" returns true if the left argument is numerically greater
than the right argument.
Binary "<=" returns true if the left argument is numerically less than
or equal to the right argument.
Binary ">=" returns true if the left argument is numerically greater
than or equal to the right argument.
Binary "lt" returns true if the left argument is stringwise less than
the right argument.
Binary "gt" returns true if the left argument is stringwise greater
than the right argument.
Binary "le" returns true if the left argument is stringwise less than
or equal to the right argument.
Binary "ge" returns true if the left argument is stringwise greater
than or equal to the right argument.
Equality Operators
Binary "==" returns true if the left argument is numerically equal to
the right argument.
Binary "!=" returns true if the left argument is numerically not equal
to the right argument.
Binary "<=>" returns -1, 0, or 1 depending on whether the left argument
is numerically less than, equal to, or greater than the right argument.
If your platform supports NaNs (not-a-numbers) as numeric values, using
them with "<=>" returns undef. NaN is not "<", "==", ">", "<=" or ">="
anything (even NaN), so those 5 return false. NaN != NaN returns true,
as does NaN != anything else. If your platform doesnt 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 != $a
Binary "eq" returns true if the left argument is stringwise equal to
the right argument.
Binary "ne" returns true if the left argument is stringwise not equal
to the right argument.
Binary "cmp" returns -1, 0, or 1 depending on whether the left argument
is stringwise less than, equal to, or greater than the right argument.
"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order speci
fied by the current locale if "use locale" is in effect. See perllo
cale.
Bitwise And
Binary "&" returns its operands ANDed together bit by bit. (See also
"Integer Arithmetic" and "Bitwise String Operators".)
Note that "&" has lower priority than relational operators, so for
example the brackets are essential in a test like
print "Even\n" if ($x & 1) == 0;
Bitwise Or and Exclusive Or
Binary "|" returns its operands ORed together bit by bit. (See also
"Integer Arithmetic" and "Bitwise String 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.
The "||" and "&&" operators return the last value evaluated (unlike Cs
"||" 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 "Youre homeless!\n";
In particular, this means that you shouldnt 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 oper
ator without the need for parentheses:
unlink "alpha", "beta", "gamma"
or gripe(), next LINE;
With the C-style operators that would have been written like this:
unlink("alpha", "beta", "gamma")
|| (gripe(), next LINE);
Using "or" for assignment is unlikely to do what you want; see below.
Range Operators
Binary ".." is the range operator, which is really two different opera
tors 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 imple
mentation, 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-incre
ment, see below.
In scalar context, ".." returns a boolean value. The operator is
bistable, like a flip-flop, and emulates the line-range (comma) opera
tor 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 doesnt 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 dont 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 doesnt 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 num
ber (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 $. as described in the previous paragraph, the compar
ison is "int(EXPR) == int($.)" which is only an issue when $. is set
to a floating point value and you are not reading from a file. Fur
thermore, "span" .. "spat" or "2.18 .. 3.14" will not do what you want
in scalar context because each of the operands are evaluated using
their integer representation.
Examples:
As a scalar operator:
if (101 .. 200) { print; } # print 2nd hundred lines, short for
# if ($. == 101 .. $. == 200) ...
next LINE if (1 .. /^$/); # skip header lines, short for
# ... if ($. == 1 .. /^$/);
# (typically in a loop labeled LINE)
s/^/> / if (/^$/ .. eof()); # quote body
# parse mail messages
while (<>) {
$in_header = 1 .. /^$/;
$in_body = /^$/ .. eof;
if ($in_header) {
# ...
} else { # in body
# ...
}
} continue {
close ARGV if eof; # reset $. each file
}
Heres a simple example to illustrate the difference between the two
range operators:
@lines = (" - Foo",
"01 - Bar",
"1 - Baz",
" - Quux");
foreach (@lines) {
if (/0/ .. /1/) {
print "$_\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 speci
fied.
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 argu
ment 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 argu
ment, whichever is selected.
$a = $ok ? $b : $c; # get a scalar
@a = $ok ? @b : @c; # get an array
$a = $ok ? @b : @c; # oops, thats just a count!
The operator may be assigned to if both the 2nd and 3rd arguments are
legal lvalues (meaning that you can assign to them):
($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
$a = $a + 2;
although without duplicating any side effects that dereferencing the
lvalue might trigger, such as from tie(). Other assignment operators
work similarly. The following are recognized:
**= += *= &= <<= &&=
-= /= |= >>= ||=
.= %= ^=
x=
Although these are grouped by family, they all have the precedence of
assignment.
Unlike in C, the scalar assignment operator produces a valid lvalue.
Modifying an assignment is equivalent to doing the assignment and then
modifying the variable that was assigned to. This is useful for modi
fying a copy of something, like this:
($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 lval
ues 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 argu
ment and returns that value. This is just like Cs comma operator.
In list context, its 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 (con
sisting entirely of word characters) to its left to be interpreted as a
string (as of 5.001). This includes words that might otherwise be con
sidered a constant or function call.
use constant FOO => "something";
my %h = ( FOO => 23 );
is equivalent to:
my %h = ("FOO", 23);
It is NOT:
my %h = ("something", 23);
If the argument on the left is not a word, it is first interpreted as
an expression, and then the string value of that is used.
The "=>" operator is helpful in documenting the correspondence between
keys and values in hashes, and other paired elements in lists.
%hash = ( $key => $value );
login( $username => $password );
List Operators (Rightward)
On the right side of a list operator, it has very low precedence, such
that it controls all comma-separated expressions found there. The only
operators with lower precedence are the logical operators "and", "or",
and "not", which may be used to evaluate calls to list operators with
out the need for extra parentheses:
open HANDLE, "filename"
or die "Cant 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. Its the equivalent of "!" except for the very low precedence.
Logical And
Binary "and" returns the logical conjunction of the two surrounding
expressions. Its equivalent to && except for the very low precedence.
This means that it short-circuits: i.e., the right expression is evalu
ated only if the left expression is true.
Logical or and Exclusive Or
Binary "or" returns the logical disjunction of the two surrounding
expressions. Its equivalent to || except for the very low precedence.
This makes it useful for control flow
print FH $data or die "Cant write to FH: $!";
This means that it short-circuits: i.e., the right expression is evalu
ated 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 || $c; # better written this way
However, when its a list-context assignment and youre trying to use
"||" for control flow, you probably need "or" so that the assignment
takes higher precedence.
@info = stat($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 expres
sions. It cannot short circuit, of course.
C Operators Missing From Perl
Here is what C has that Perl doesnt:
unary & Address-of operator. (But see the "\" operator for taking a
reference.)
unary * Dereference-address operator. (Perls prefix dereferencing
operators are typed: $, @, %, and &.)
(TYPE) Type-casting operator.
Quote and Quote-like Operators
While we usually think of quotes as literal values, in Perl they func
tion as operators, providing various kinds of interpolating and pattern
matching capabilities. Perl provides customary quote characters for
these behaviors, but also provides a way for you to choose your quote
character for any of them. In the following table, a "{}" represents
any pair of delimiters you choose.
Customary Generic Meaning Interpolates
q{} Literal no
"" qq{} Literal yes
qx{} Command yes*
qw{} Word list no
// m{} Pattern match yes*
qr{} Pattern yes*
s{}{} Substitution yes*
tr{}{} Transliteration no (but see below)
<{key}[0]" are also interpolated, as are array and hash slices.
But method calls such as "$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 "$" or "@" interpolates the corresponding variable, while
escaping will cause the literal string "\$" to be inserted. Youll
need to write something like "m/\Quser\E\@\Qhost/".
Patterns are subject to an additional level of interpretation as a reg
ular expression. This is done as a second pass, after variables are
interpolated, so that regular expressions may be incorporated into the
pattern from the variables. If this is not what you want, use "\Q" to
interpolate a variable literally.
Apart from the behavior described above, Perl does not expand multiple
levels of interpolation. In particular, contrary to the expectations
of shell programmers, back-quotes do NOT interpolate within double
quotes, nor do single quotes impede evaluation of variables when used
within double quotes.
Regexp Quote-Like Operators
Here are the quote-like operators that apply to pattern matching and
related activities.
?PATTERN?
This is just like the "/pattern/" search, except that it
matches only once between calls to the reset() operator. This
is a useful optimization when you want to see only the first
occurrence of something in each file of a set of files, for
instance. Only "??" patterns local to the current package are
reset.
while (<>) {
if (?^$?) {
# 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-whites
pace 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 delim
iter, 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 evalu
ated, 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 com
piled only once, add a "/o" after the trailing delimiter. This
avoids expensive run-time recompilations, and is useful when
the value you are interpolating wont change over the life of
the script. However, mentioning "/o" constitutes a promise
that you wont change the variables in the pattern. If you
change them, Perl wont even notice. See also
"qr/STRING/imosx".
If the PATTERN evaluates to the empty string, the last success
fully 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 parenthe
ses in the pattern, i.e., ($1, $2, $3...). (Note that here $1
etc. are also set, and that this differs from Perl 4s behav
ior.) When there are no parentheses in the pattern, the return
value is the list "(1)" for success. With or without parenthe
ses, an empty list is returned upon failure.
Examples:
open(TTY, /dev/tty);
=~ /^y/i && foo(); # do foo if desired
if (/Version: *([0-9.]*)/) { $version = $1; }
next if m#^/usr/spool/uucp#;
# poor mans grep
$arg = shift;
while (<>) {
print if /$arg/o; # compile only once
}
if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
This last example splits $foo into the first two words and the
remainder of the line, and assigns those three fields to $F1,
$F2, and $Etc. The conditional is true if any variables were
assigned, i.e., if the pattern matched.
The "/g" modifier specifies global pattern matching--that is,
matching as many times as possible within the string. How it
behaves depends on the context. In list context, it returns a
list of the substrings matched by any capturing parentheses in
the regular expression. If there are no parentheses, it
returns a list of all the matched strings, as if there were
parentheses around the whole pattern.
In scalar context, each execution of "m//g" finds the next
match, returning true if it matches, and false if there is no
further match. The position after the last match can be read
or set using the pos() function; see "pos" in perlfunc. A
failed match normally resets the search position to the begin
ning of the string, but you can avoid that by adding the "/c"
modifier (e.g. "m//gc"). Modifying the target string also
resets the search position.
You can intermix "m//g" matches with "m/\G.../g", where "\G" is
a zero-width assertion that matches the exact position where
the previous "m//g", if any, left off. Without the "/g" modi
fier, the "\G" assertion still anchors at pos(), but the match
is of course only attempted once. Using "\G" without "/g" on a
target string that has not previously had a "/g" match applied
to it is the same as using the "\A" assertion to match the
beginning of the string. Note also that, currently, "\G" is
only properly supported when anchored at the very beginning of
the pattern.
Examples:
# list context
($one,$five,$fifteen) = (uptime =~ /(\d+\.\d+)/g);
# scalar context
$/ = "";
while (defined($paragraph = <>)) {
while ($paragraph =~ /[a-z][")]*[.!?]+[")]*\s/g) {
$sentences++;
}
}
print "$sentences\n";
# using m//gc with \G
$_ = "ppooqppqq";
while ($i++ < 2) {
print "1: ";
print $1 while /(o)/gc; print ", pos=", pos, "\n";
print "2: ";
print $1 if /\G(q)/gc; print ", pos=", pos, "\n";
print "3: ";
print $1 while /(p)/gc; print ", pos=", pos, "\n";
}
print "Final: $1, pos=",pos,"\n" if /\G(.)/;
The last example should print:
1: oo, pos=4
2: q, pos=5
3: pp, pos=7
1: , pos=7
2: q, pos=8
3: , pos=8
Final: q, pos=8
Notice that the final match matched "q" instead of "p", which a
match without the "\G" anchor would have done. Also note that
the final match did not update "pos" -- "pos" is only updated
on a "/g" match. If the final match did indeed match "p", its
a good bet that youre 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.
$_ = <&1;
To capture a commands STDOUT but discard its STDERR:
$output = cmd 2>/dev/null;
To capture a commands STDERR but discard its STDOUT (ordering
is important here):
$output = cmd 2>&1 1>/dev/null;
To exchange a commands 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 commands STDOUT and its STDERR separately, its
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
Perls double-quote interpolation, passing it on to the shell
instead:
$perl_info = qx(ps $$); # thats Perls $$
$shell_info = qxps $$; # thats the new shells $$
How that string gets evaluated is entirely subject to the com
mand 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 its
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 want. You may be able
to evaluate multiple commands in a single line by separating
them with the command separator character, if your shell sup
ports that (e.g. ";" on many Unix shells; "&" on the Windows NT
"cmd" shell).
Beginning with v5.6.0, Perl will attempt to flush all files
opened for output before starting the child process, but this
may not be supported on some platforms (see perlport). To be
safe, you may need to set $| ($AUTOFLUSH in English) or call
the "autoflush()" method of "IO::Handle" on any open handles.
Beware that some command shells may place restrictions on the
length of the command line. You must ensure your strings dont
exceed this limit after any necessary interpolations. See the
platform-specific release notes for more details about your
particular environment.
Using this operator can lead to programs that are difficult to
port, because the shell commands called vary between systems,
and may in fact not be present at all. As one example, the
"type" command under the POSIX shell is very different from the
"type" command under DOS. That doesnt mean you should go out
of your way to avoid backticks when theyre 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 under
stand what youre 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 under
stood 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 rea
son, the "use warnings" pragma and the -w switch (that is, the
$^W variable) produces warnings if the STRING contains the ","
or the "#" character.
s/PATTERN/REPLACEMENT/egimosx
Searches a string for a pattern, and if found, replaces that
pattern with the replacement text and returns the number of
substitutions made. Otherwise it returns false (specifically,
the empty string).
If no string is specified via the "=~" or "!~" operator, the $_
variable is searched and modified. (The string specified with
"=~" must be scalar variable, an array element, a hash element,
or an assignment to one of those, i.e., an lvalue.)
If the delimiter chosen is a single quote, no 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. See perllocale for
discussion of additional considerations that apply when "use
locale" is in effect.
Options are:
e Evaluate the right side as an expression.
g Replace globally, i.e., all occurrences.
i Do case-insensitive pattern matching.
m Treat string as multiple lines.
o Compile pattern only once.
s Treat string as single line.
x Use extended regular expressions.
Any non-alphanumeric, non-whitespace delimiter may replace the
slashes. If single quotes are used, no interpretation is done
on the replacement string (the "/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 REPLACE
MENT has its own pair of quotes, which may or may not be brack
eting quotes, e.g., "s(foo)(bar)" or "s/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" modi
fier will cause the replacement portion to be "eval"ed before
being run as a Perl expression.
Examples:
s/\bgreen\b/mauve/g; # dont change wintergreen
$path =~ s|/usr/bin|/usr/local/bin|;
s/Login: $foo/Login: $bar/; # run-time pattern
($foo = $bar) =~ s/this/that/; # copy first, then change
$count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
$_ = abc123xyz;
s/\d+/$&*2/e; # yields abc246xyz
s/\d+/sprintf("%5d",$&)/e; # yields abc 246xyz
s/\w/$& x 2/eg; # yields aabbcc 224466xxyyzz
s/%(.)/$percent{$1}/g; # change percent escapes; no /e
s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
s/^=(\w+)/&pod($1)/ge; # use function call
# expand variables in $_, but dynamics only, using
# symbolic dereferencing
s/\$(\w+)/${$1}/g;
# Add one to the value of any numbers in the string
s/(\d+)/1 + $1/eg;
# 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 \ form in only the left hand side. Any
where else its $.
Occasionally, you cant use just a "/g" to get all the changes
to occur that you might want. Here are two common cases:
# put commas in the right places in an integer
1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
# expand tabs to 8-column spacing
1 while s/\t+/ x (length($&)*8 - length($)%8)/e;
tr/SEARCHLIST/REPLACEMENTLIST/cds
y/SEARCHLIST/REPLACEMENTLIST/cds
Transliterates all occurrences of the characters found in the
search list with the corresponding character in the replacement
list. It returns the number of characters replaced or deleted.
If no string is specified via the =~ or !~ operator, the $_
string is transliterated. (The string specified with =~ must
be a scalar variable, an array element, a hash element, or an
assignment to one of those, i.e., an lvalue.)
A character range may be specified with a hyphen, so
"tr/A-J/0-9/" does the same replacement as "tr/ACEG
IBDFHJ/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, e.g., "tr[A-Z][a-z]" or
"tr(+\-*/)/ABCD/".
Note that "tr" does not do regular expression character classes
such as "\d" or "[:lower:]". The operator is not equiva
lent to the tr(1) utility. If you want to map strings between
lower/upper cases, see "lc" in perlfunc and "uc" in perlfunc,
and in general consider using the "s" operator if you need reg
ular expressions.
Note also that the whole range idea is rather unportable
between character sets--and even within character sets they may
cause results you probably didnt expect. A sound principle is
to use only ranges that begin from and end at either alphabets
of equal case (a-e, A-E), or digits (0-4). Anything else is
unsafe. If in doubt, spell out the character sets in full.
Options:
c Complement the SEARCHLIST.
d Delete found but unreplaced characters.
s Squash duplicate replaced characters.
If the "/c" modifier is specified, the SEARCHLIST character set
is complemented. If the "/d" modifier is specified, any char
acters 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 charac
ter.
If the "/d" modifier is used, the REPLACEMENTLIST is always
interpreted exactly as specified. Otherwise, if the REPLACE
MENTLIST is shorter than the SEARCHLIST, the final character is
replicated till it is long enough. If the REPLACEMENTLIST is
empty, the SEARCHLIST is replicated. This latter is useful for
counting characters in a class or for squashing character
sequences in a class.
Examples:
$ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
$cnt = tr/*/*/; # count the stars in $_
$cnt = $sky =~ tr/*/*/; # count the stars in $sky
$cnt = tr/0-9//; # count the digits in $_
tr/a-zA-Z//s; # bookkeeper -> bokeper
($HOST = $host) =~ tr/a-z/A-Z/;
tr/a-zA-Z/ /cs; # change non-alphas to single space
tr [\200-\377]
[\000-\177]; # delete 8th bit
If multiple transliterations are given for a character, only
the first one is used:
tr/AAA/XYZ/
will transliterate any A to X.
Because the transliteration table is built at compile time,
neither the SEARCHLIST nor the REPLACEMENTLIST are subjected to
double quote interpolation. That means that if you want to use
variables, you must use an eval():
eval "tr/$oldlist/$newlist/";
die $@ if $@;
eval "tr/$oldlist/$newlist/, 1" or die $@;
<" which terminates a fileglob
started with "<".
When searching for single-character non-pairing delimiters, such as
"/", combinations of "\\" and "\/" are skipped. However, when
searching for single-character pairing delimiter like "[", combina
tions of "\\", "\]", and "\[" are all skipped, and nested "[", "]"
are skipped as well. When searching for multicharacter delimiters,
nothing is skipped.
For constructs with three-part delimiters ("s///", "y///", and
"tr///"), the search is repeated once more.
During this search no attention is paid to the semantics of the
construct. Thus:
"$hash{"$foo/$bar"}"
or:
m/
bar # NOT a comment, this slash / terminated m//!
/x
do not form legal quoted expressions. The quoted part ends on the
first """ and "/", and the rest happens to be a syntax error.
Because the slash that terminated "m//" was followed by a "SPACE",
the example above is not "m//x", but rather "m//" with no "/x" mod
ifier. So the embedded "#" is interpreted as a literal "#".
Also no attention is paid to "\c\" during this search. Thus the
second "\" in "qq/\c\/" is interpreted as a part of "\/", and the
following "/" is not recognized as a delimiter. Instead, use
"\034" or "\x1c" at the end of quoted constructs.
Removal of backslashes before delimiters
During the second pass, text between the starting and ending delim
iters is copied to a safe location, and the "\" is removed from
combinations consisting of "\" and delimiter--or delimiters, mean
ing both starting and ending delimiters will should these differ.
This removal does not happen for multi-character delimiters. Note
that the combination "\\" is left intact, just as it was.
Starting from this step no information about the delimiters is used
in parsing.
Interpolation
The next step is interpolation in the text obtained, which is now
delimiter-independent. There are four different cases.
"<"
"\Q", "\U", "\u", "\L", "\l" (possibly paired with "\E") are
converted to corresponding Perl constructs. Thus,
"$foo\Qbaz$bar" is converted to "$foo . (quotemeta("baz" .
$bar))" internally. The other combinations are replaced with
appropriate expansions.
Let it be stressed that whatever falls between "\Q" and "\E" is
interpolated in the usual way. 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, back
slashes 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 interpo
lated 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, its usually correct for
ambiguous cases.
"?RE?", "/RE/", "m/RE/", "s/RE/foo/",
Processing of "\Q", "\U", "\u", "\L", "\l", and interpolation
happens (almost) as with "qq//" constructs, but the substitu
tion of "\" followed by RE-special chars (including "\") is not
performed. 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 has several quirks: $|, $(, and $) are not inter
polated, 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 vari
able $arr followed by a digit, which would be the interpreta
tion of "/$arr[0-9]/". Since voting among different estimators
may occur, the result is not predictable.
It is at this step that "\1" is begrudgingly converted to $1 in
the replacement text of "s///" to correct the incorrigible sed
hackers who havent picked up the saner idiom yet. A warning
is emitted if the "use warnings" pragma or the -w command-line
flag (that is, the $^W variable) was set.
The lack of processing of "\\" creates specific 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 charac
ter 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 illus
tration, the delimiter is "m", the modifier is "mx", and after
backslash-removal the RE is the same as for "m/ ^ a \s* b /mx".
Theres more than one reason youre encouraged to restrict your
delimiters to non-alphanumeric, non-whitespace choices.
This step is the last one for all constructs except regular expres
sions, which are processed further.
Interpolation of regular expressions
Previous steps were performed during the compilation of Perl code,
but this one happens at run time--although it may be optimized to
be calculated at compile time if appropriate. After preprocessing
described above, and possibly after evaluation if catenation, join
ing, 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 lit
eral 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 "[" is treated as
though preceded by a backslash. Similarly, the terminator of
"(?{...})" is found using the same rules as for finding the termi
nator of a "{}"-delimited construct.
It is possible to inspect both the string given to RE engine and
the resulting finite automaton. See the arguments "debug"/"debug
color" in the "use re" pragma, as well as Perls -Dr command-line
switch documented in "Command Switches" in perlrun.
Optimization of regular expressions
This step is listed for completeness only. Since it does not
change semantics, details of this step are not documented and are
subject to change without notice. This step is performed over the
finite automaton that was generated during the previous pass.
It is at this stage that "split()" silently optimizes "/^/" to mean
"/^/m".
I/O Operators
There are several I/O operators you should know about.
A string enclosed by backticks (grave accents) first 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 termina
tor.) The command is executed each time the pseudo-literal is evalu
ated. 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 auto
matically assigned to the global variable $_, destroying whatever was
there previously. (This may seem like an odd thing to you, but youll
use the construct in almost every Perl script you write.) The $_ vari
able is not implicitly localized. Youll have to put a "local $_;"
before the loop if you want that to happen.
The following lines are equivalent:
while (defined($_ = )) { print; }
while ($_ = ) { print; }
while () { print; }
for (;;) { print; }
print while defined($_ = );
print while ($_ = );
print while ;
This also behaves similarly, but avoids $_ :
while (my $line = ) { print $line }
In these loop constructs, the assigned value (whether assignment is
automatic or explicit) is then tested to see whether it is defined.
The defined test avoids problems where line has a string value that
would be treated as false by Perl, for example a "" or a "0" with no
trailing newline. If you really mean for such values to terminate the
loop, they should be tested for explicitly:
while (($_ = ) ne 0) { ... }
while () { last unless $_; ... }
In other boolean contexts, "" without an explicit "defined"
test or comparison elicit a warning if the "use warnings" pragma or the
-w command-line switch (the $^W variable) is in effect.
The filehandles STDIN, STDOUT, and STDERR are predefined. (The file
handles "stdin", "stdout", and "stderr" will also work except in pack
ages, where they would be interpreted as local identifiers rather than
global.) Additional filehandles may be created with the open() func
tion, amongst others. See perlopentut and "open" in perlfunc for
details on this.
If a is used in a context that is looking for a list, a
list comprising all input lines is returned, one line per list element.
Its easy to grow to a rather large data space this way, so use with
care.
may also be spelled "readline(*FILEHANDLE)". See "read
line" in perlfunc.
The null filehandle <> is special: it can be used to emulate the behav
ior of sed and awk. Input from <> comes either from standard input, or
from each file listed on the command line. Heres how it works: the
first time <> is evaluated, the @ARGV array is checked, and if it is
empty, $ARGV[0] is set to "-", which when opened gives you standard
input. The @ARGV array is then processed as a list of filenames. The
loop
while (<>) {
... # code for each line
}
is equivalent to the following Perl-like pseudo code:
unshift(@ARGV, -) unless @ARGV;
while ($ARGV = shift) {
open(ARGV, $ARGV);
while () {
... # code for each line
}
}
except that it isnt so cumbersome to say, and will actually work. It
really does shift the @ARGV array and put the current filename into the
$ARGV variable. It also uses filehandle ARGV internally--<> is just a
synonym for , which is magical. (The pseudo code above doesnt
work because it treats as non-magical.)
You can modify @ARGV before the first <> as long as the array ends up
containing the list of filenames you really want. Line numbers ($.)
continue as though the input were one big happy file. See the example
in "eof" in perlfunc for how to reset line numbers on each file.
If you want to set @ARGV to your own list of files, go right ahead.
This sets @ARGV to all plain text files if no @ARGV was given:
@ARGV = grep { -f && -T } glob(*) unless @ARGV;
You can even set them to pipe commands. For example, this automati
cally filters compressed arguments through gzip:
@ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
If you want to pass switches into your script, you can use one of the
Getopts modules or put a loop on the front like this:
while ($_ = $ARGV[0], /^-/) {
shift;
last if /^--$/;
if (/^-D(.*)/) { $debug = $1 }
if (/^-v/) { $verbose++ }
# ... # other switches
}
while (<>) {
# ... # code for each line
}
The <> symbol will return "undef" for end-of-file only once. If you
call it again after this, it will assume you are processing another
@ARGV list, and if you havent set @ARGV, will read input from STDIN.
If what the angle brackets contain is a simple scalar variable (e.g.,
<$foo>), then that variable contains the name of the filehandle to
input from, or its typeglob, or a reference to the same. For example:
$fh = \*STDIN;
$line = <$fh>;
If whats within the angle brackets is neither a filehandle nor a sim
ple 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 syn
tactic grounds alone. That means "<$x>" is always a readline() from an
indirect handle, but "<$hash{key}>" is always a glob(). Thats because
$x is a simple scalar variable, but $hash{key} is not--its a hash ele
ment. Even "<$x >" (note the extra space) is treated as "glob("$x ")",
not "readline($x)".
One level of double-quote interpretation is done first, but you cant
say "<$foo>" because thats an indirect filehandle as explained in the
previous paragraph. (In older versions of Perl, programmers would
insert curly brackets to force interpretation as a filename glob:
"<${foo}>". These days, its 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 () {
chomp;
chmod 0644, $_;
}
except that the globbing is actually done internally using the standard
"File::Glob" extension. Of course, the shortest way to do the above
is:
chmod 0644, <*.c>;
A (file)glob evaluates its (embedded) argument only when it is starting
a new list. All values must be read before it will start over. In
list context, this isnt important because you automatically get them
all anyway. However, in scalar context the operator returns the next
value each time its called, or "undef" when the list has run out. As
with filehandle reads, an automatic "defined" is generated when the
glob occurs in the test part of a "while", because legal glob returns
(e.g. a file called 0) would otherwise terminate the loop. Again,
"undef" is returned only once. So if youre expecting a single value
from a glob, it is much better to say
($file) = ;
than
$file = ;
because the latter will alternate between returning a filename and
returning false.
If youre trying to do variable interpolation, its definitely better
to use the glob() function, because the older notation can cause people
to become confused with the indirect filehandle notation.
@files = glob("$dir/*.[ch]");
@files = glob($files[$i]);
Constant Folding
Like C, Perl does a certain amount of expression evaluation at compile
time whenever it determines that all arguments to an operator are
static and have no side effects. In particular, string concatenation
happens at compile time between literals that dont do variable substi
tution. Backslash interpolation also happens at compile time. You can
say
Now is the time for all . "\n" .
good men to come to.
and this all reduces to one string internally. Likewise, if you say
foreach $file (@filenames) {
if (-s $file > 5 + 100 * 2**16) { }
}
the compiler will precompute the number which that expression repre
sents so that the interpreter wont have to.
No-ops
Perl doesnt officially have a no-op operator, but the bare constants 0
and 1 are special-cased to not produce a warning in a void context, so
you can for example safely do
1 while foo();
Bitwise String Operators
Bitstrings of any size may be manipulated by the bitwise operators ("~
| & ^").
If the operands to a binary bitwise op are strings of different sizes,
| and ^ ops act as though the shorter operand had additional zero bits
on the right, while the & op acts as though the longer operand were
truncated to the length of the shorter. The granularity for such
extension or truncation is one or more bytes.
# ASCII-based examples
print "j p \n" ^ " a h"; # prints "JAPH\n"
print "JA" | " ph\n"; # prints "japh\n"
print "japh\nJunk" & _____; # prints "JAPH\n";
print p N$ ^ " E>") always produce integral results. (But see also "Bitwise String
Operators".) However, "use integer" still has meaning for them. By
default, their results are interpreted as unsigned integers, but if
"use integer" is in effect, their results are interpreted as signed
integers. For example, "~0" usually evaluates to a large integral
value. However, "use integer; ~0" is "-1" on twos-complement machines.
Floating-point Arithmetic
While "use integer" provides integer-only arithmetic, there is no anal
ogous mechanism to provide automatic rounding or truncation to a cer
tain number of decimal places. For rounding to a certain number of
digits, sprintf() or printf() is usually the easiest route. See perl
faq4.
Floating-point numbers are only approximations to what a mathematician
would call real numbers. There are infinitely more reals than floats,
so some corners must be cut. For example:
printf "%.20g\n", 123456789123456789;
# produces 123456789123456784
Testing for exact equality of floating-point equality or inequality is
not a good idea. Heres a (relatively expensive) work-around to com
pare whether two floating-point numbers are equal to a particular num
ber of decimal places. See Knuth, volume II, for a more robust treat
ment 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
cant 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 and Math::BigFloat modules provide variable-
precision arithmetic and overloaded operators, although theyre cur
rently pretty slow. At the cost of some space and considerable speed,
they avoid the normal pitfalls associated with limited-precision repre
sentations.
use Math::BigInt;
$x = Math::BigInt->new(123456789123456789);
print $x * $x;
# prints +15241578780673678515622620750190521
There are several modules that let you calculate with (bound only by
memory and cpu-time) unlimited or fixed precision. There are also some
non-standard modules that provide faster implementations via external C
libraries.
Here is a short, but incomplete summary:
Math::Fraction big, unlimited fractions like 9973 / 12967
Math::String treat string sequences like numbers
Math::FixedPrecision calculate with a fixed precision
Math::Currency for currency calculations
Bit::Vector manipulate bit vectors fast (uses C)
Math::BigIntFast Bit::Vector wrapper for big numbers
Math::Pari provides access to the Pari C library
Math::BigInteger uses an external C library
Math::Cephes uses external Cephes C library (no big numbers)
Math::Cephes::Fraction fractions via the Cephes library
Math::GMP another one using an external C library
Choose wisely.
perl v5.8.8 2008-04-25 PERLOP(1)
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