NAME¶
perlop - Perl operators and precedence
DESCRIPTION¶
Operator Precedence and Associativity¶
Operator precedence and associativity work in Perl more or less like they do in
mathematics.
Operator precedence means some operators are evaluated before others. For
example, in "2 + 4 * 5", the multiplication has higher precedence so
"4 * 5" is evaluated first yielding "2 + 20 == 22" and not
"6 * 5 == 30".
Operator associativity defines what happens if a sequence of the same
operators is used one after another: whether the evaluator will evaluate the
left operations first or the right. For example, in "8 - 4 - 2",
subtraction is left associative so Perl evaluates the expression left to
right. "8 - 4" is evaluated first making the expression "4 - 2
== 2" and not "8 - 2 == 6".
Perl operators have the following associativity and precedence, listed from
highest precedence to lowest. Operators borrowed from C keep the same
precedence relationship with each other, even where C's precedence is slightly
screwy. (This makes learning Perl easier for C folks.) With very few
exceptions, these all operate on scalar values only, not array values.
left terms and list operators (leftward)
left ->
nonassoc ++ --
right **
right ! ~ \ and unary + and -
left =~ !~
left * / % x
left + - .
left << >>
nonassoc named unary operators
nonassoc < > <= >= lt gt le ge
nonassoc == != <=> eq ne cmp ~~
left &
left | ^
left &&
left || //
nonassoc .. ...
right ?:
right = += -= *= etc.
left , =>
nonassoc list operators (rightward)
right not
left and
left or xor
In the following sections, these operators are covered in precedence order.
Many operators can be overloaded for objects. See overload.
Terms and List Operators (Leftward)¶
A TERM has the highest precedence in Perl. They include variables, quote and
quote-like operators, any expression in parentheses, and any function whose
arguments are parenthesized. Actually, there aren't really functions in this
sense, just list operators and unary operators behaving as functions because
you put parentheses around the arguments. These are all documented in
perlfunc.
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, "\n";
probably doesn't do what you expect at first glance. The parentheses enclose the
argument list for "print" which is evaluated (printing the result of
"$foo & 255"). Then one is added to the return value of
"print" (usually 1). The result is something like this:
1 + 1, "\n"; # Obviously not what you meant.
To do what you meant properly, you must write:
print(($foo & 255) + 1, "\n");
See "Named Unary Operators" for more discussion of this.
Also parsed as terms are the "do {}" and "eval {}"
constructs, as well as subroutine and method calls, and the anonymous
constructors "[]" and "{}".
See also "Quote and Quote-like Operators" toward the end of this
section, as well as "I/O Operators".
The Arrow Operator¶
""->"" is an infix dereference operator, just as it is in
C and C++. If the right side is either a "[...]", "{...}",
or a "(...)" subscript, then the left side must be either a hard or
symbolic reference to an array, a hash, or a subroutine respectively. (Or
technically speaking, a location capable of holding a hard reference, if it's
an array or hash reference being used for assignment.) See perlreftut and
perlref.
Otherwise, the right side is a method name or a simple scalar variable
containing either the method name or a subroutine reference, and the left side
must be either an object (a blessed reference) or a class name (that is, a
package name). See perlobj.
Auto-increment and Auto-decrement¶
"++" and "--" work as in C. That is, if placed before a
variable, they increment or decrement the variable by one before returning the
value, and if placed after, increment or decrement after returning the value.
$i = 0; $j = 0;
print $i++; # prints 0
print ++$j; # prints 1
Note that just as in C, Perl doesn't define
when the variable is
incremented or decremented. You just know it will be done sometime before or
after the value is returned. This also means that modifying a variable twice
in the same statement will lead to undefined behavior. Avoid statements like:
$i = $i ++;
print ++ $i + $i ++;
Perl will not guarantee what the result of the above statements is.
The auto-increment operator has a little extra builtin magic to it. If you
increment a variable that is numeric, or that has ever been used in a numeric
context, you get a normal increment. If, however, the variable has been used
in only string contexts since it was set, and has a value that is not the
empty string and matches the pattern "/^[a-zA-Z]*[0-9]*\z/", the
increment is done as a string, preserving each character within its range,
with carry:
print ++($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 implemented using
C's
pow(3) function, which actually works on doubles internally.)
Symbolic Unary Operators¶
Unary "!" performs logical negation, i.e., "not". See also
"not" for a lower precedence version of this.
Unary "-" performs arithmetic negation if the operand is numeric,
including any string that looks like a number. If the operand is an
identifier, a string consisting of a minus sign concatenated with the
identifier is returned. Otherwise, if the string starts with a plus or minus,
a string starting with the opposite sign is returned. One effect of these
rules is that -bareword is equivalent to the string "-bareword". If,
however, the string begins with a non-alphabetic character (excluding
"+" or "-"), Perl will attempt to convert the string to a
numeric and the arithmetic negation is performed. If the string cannot be
cleanly converted to a numeric, Perl will give the warning
Argument
"the string" isn't numeric in negation (-) at ....
Unary "~" performs bitwise negation, 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.
When complementing strings, if all characters have ordinal values under 256,
then their complements will, also. But if they do not, all characters will be
in either 32- or 64-bit complements, depending on your architecture. So for
example, "~"\x{3B1}"" is "\x{FFFF_FC4E}" on
32-bit machines and "\x{FFFF_FFFF_FFFF_FC4E}" on 64-bit machines.
Unary "+" has no effect whatsoever, even on strings. It is useful
syntactically for separating a function name from a parenthesized expression
that would otherwise be interpreted as the complete list of function
arguments. (See examples above under "Terms and List Operators
(Leftward)".)
Unary "\" creates a reference to whatever follows it. See perlreftut
and perlref. Do not confuse this behavior with the behavior of backslash
within a string, although both forms do convey the notion of protecting the
next thing from interpolation.
Binding Operators¶
Binary "=~" binds a scalar expression to a pattern match. Certain
operations search or modify the string $_ by default. This operator makes that
kind of operation work on some other string. The right argument is a search
pattern, substitution, or transliteration. The left argument is what is
supposed to be searched, substituted, or transliterated instead of the default
$_. When used in scalar context, the return value generally indicates the
success of the operation. The exceptions are substitution (s///) and
transliteration (y///) with the "/r" (non-destructive) option, which
cause the
return value to be the result of the substitution. Behavior
in list context depends on the particular operator. See "Regexp
Quote-Like Operators" for details and perlretut for examples using these
operators.
If the right argument is an expression rather than a search pattern,
substitution, or transliteration, it is interpreted as a search pattern at run
time. Note that this means that its contents will be interpolated twice, so
'\\' =~ q'\\';
is not ok, as the regex engine will end up trying to compile the pattern
"\", which it will consider a syntax error.
Binary "!~" is just like "=~" except the return value is
negated in the logical sense.
Binary "!~" with a non-destructive substitution (s///r) or
transliteration (y///r) is a syntax error.
Multiplicative Operators¶
Binary "*" multiplies two numbers.
Binary "/" divides two numbers.
Binary "%" is the modulo operator, which computes the division
remainder of its first argument with respect to its second argument. Given
integer operands $a and $b: If $b is positive, then "$a % $b" is $a
minus the largest multiple of $b less than or equal to $a. If $b is negative,
then "$a % $b" is $a minus the smallest multiple of $b that is not
less than $a (i.e. the result will be less than or equal to zero). If the
operands $a and $b are floating point values and the absolute value of $b
(that is "abs($b)") is less than "(UV_MAX + 1)", only the
integer portion of $a and $b will be used in the operation (Note: here
"UV_MAX" means the maximum of the unsigned integer type). If the
absolute value of the right operand ("abs($b)") is greater than or
equal to "(UV_MAX + 1)", "%" computes the floating-point
remainder $r in the equation "($r = $a - $i*$b)" where $i is a
certain integer that makes $r have the same sign as the right operand $b (
not as the left operand $a like C function "fmod()") and the
absolute value less than that of $b. Note that when "use integer" is
in scope, "%" gives you direct access to the modulo operator as
implemented by your C compiler. This operator is not as well defined for
negative operands, but it will execute faster.
Binary "x" is the repetition operator. In scalar context or if the
left operand is not enclosed in parentheses, it returns a string consisting of
the left operand repeated the number of times specified by the right operand.
In list context, if the left operand is enclosed in parentheses or is a list
formed by "qw/STRING/", it repeats the list. If the right operand is
zero or negative, it returns an empty string or an empty list, depending on
the context.
print '-' x 80; # print row of dashes
print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
@ones = (1) x 80; # a list of 80 1's
@ones = (5) x @ones; # set all elements to 5
Additive Operators¶
Binary "+" returns the sum of two numbers.
Binary "-" returns the difference of two numbers.
Binary "." concatenates two strings.
Shift Operators¶
Binary "<<" returns the value of its left argument shifted left
by the number of bits specified by the right argument. Arguments should be
integers. (See also "Integer Arithmetic".)
Binary ">>" returns the value of its left argument shifted right
by the number of bits specified by the right argument. Arguments should be
integers. (See also "Integer Arithmetic".)
Note that both "<<" and ">>" in Perl are
implemented directly using "<<" and ">>" in C.
If "use integer" (see "Integer Arithmetic") is in force
then signed C integers are used, else unsigned C integers are used. Either
way, the implementation isn't going to generate results larger than the size
of the integer type Perl was built with (32 bits or 64 bits).
The result of overflowing the range of the integers is undefined because it is
undefined also in C. In other words, using 32-bit integers, "1 <<
32" is undefined. Shifting by a negative number of bits is also
undefined.
Named Unary Operators¶
The various named unary operators are treated as functions with one argument,
with optional parentheses.
If any list operator (
print(), etc.) or any unary operator
(
chdir(), etc.) is followed by a left parenthesis as the next token,
the operator and arguments within parentheses are taken to be of highest
precedence, just like a normal function call. For example, because named unary
operators are higher precedence than ||:
chdir $foo || die; # (chdir $foo) || die
chdir($foo) || die; # (chdir $foo) || die
chdir ($foo) || die; # (chdir $foo) || die
chdir +($foo) || die; # (chdir $foo) || die
but, because * is higher precedence than named operators:
chdir $foo * 20; # chdir ($foo * 20)
chdir($foo) * 20; # (chdir $foo) * 20
chdir ($foo) * 20; # (chdir $foo) * 20
chdir +($foo) * 20; # chdir ($foo * 20)
rand 10 * 20; # rand (10 * 20)
rand(10) * 20; # (rand 10) * 20
rand (10) * 20; # (rand 10) * 20
rand +(10) * 20; # rand (10 * 20)
Regarding precedence, the filetest operators, like "-f",
"-M", etc. are treated like named unary operators, but they don't
follow this functional parenthesis rule. That means, for example, that
"-f($file).".bak"" is equivalent to "-f
"$file.bak"".
See also "Terms and List Operators (Leftward)".
Relational Operators¶
Binary "<" returns true if the left argument is numerically less
than the right argument.
Binary ">" returns true if the left argument is numerically greater
than the right argument.
Binary "<=" returns true if the left argument is numerically less
than or equal to the right argument.
Binary ">=" returns true if the left argument is numerically
greater than or equal to the right argument.
Binary "lt" returns true if the left argument is stringwise less than
the right argument.
Binary "gt" returns true if the left argument is stringwise greater
than the right argument.
Binary "le" returns true if the left argument is stringwise less than
or equal to the right argument.
Binary "ge" returns true if the left argument is stringwise greater
than or equal to the right argument.
Equality Operators¶
Binary "==" returns true if the left argument is numerically equal to
the right argument.
Binary "!=" returns true if the left argument is numerically not equal
to the right argument.
Binary "<=>" returns -1, 0, or 1 depending on whether the left
argument is numerically less than, equal to, or greater than the right
argument. If your platform supports NaNs (not-a-numbers) as numeric values,
using them with "<=>" returns undef. NaN is not
"<", "==", ">", "<=" or
">=" anything (even NaN), so those 5 return false. NaN != NaN
returns true, as does NaN != anything else. If your platform doesn't support
NaNs then NaN is just a string with numeric value 0.
perl -le '$a = "NaN"; print "No NaN support here" if $a == $a'
perl -le '$a = "NaN"; print "NaN support here" if $a != $a'
Binary "eq" returns true if the left argument is stringwise equal to
the right argument.
Binary "ne" returns true if the left argument is stringwise not equal
to the right argument.
Binary "cmp" returns -1, 0, or 1 depending on whether the left
argument is stringwise less than, equal to, or greater than the right
argument.
Binary "~~" does a smart match between its arguments. Smart matching
is described in "Smart matching in detail" in perlsyn.
"lt", "le", "ge", "gt" and
"cmp" use the collation (sort) order specified by the current locale
if "use locale" is in effect. See perllocale.
Bitwise And¶
Binary "&" returns its operands ANDed together bit by bit. (See
also "Integer Arithmetic" and "Bitwise String Operators".)
Note that "&" has lower priority than relational operators, so for
example the brackets are essential in a test like
print "Even\n" if ($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.
C-style Logical Defined-Or¶
Although it has no direct equivalent in C, Perl's "//" operator is
related to its C-style or. In fact, it's exactly the same as "||",
except that it tests the left hand side's definedness instead of its truth.
Thus, "$a // $b" is similar to "defined($a) || $b" (except
that it returns the value of $a rather than the value of
"defined($a)") and yields the same result as "defined($a) ? $a
: $b" (except that the ternary-operator form can be used as a lvalue,
while "$a // $b" cannot). This is very useful for providing default
values for variables. If you actually want to test if at least one of $a and
$b is defined, use "defined($a // $b)".
The "||", "//" and "&&" operators return
the last value evaluated (unlike C's "||" and
"&&", which return 0 or 1). Thus, a reasonably portable way
to find out the home directory might be:
$home = $ENV{HOME}
// $ENV{LOGDIR}
// (getpwuid($<))[7]
// die "You're homeless!\n";
In particular, this means that you shouldn't use this for selecting between two
aggregates for assignment:
@a = @b || @c; # this is wrong
@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 the "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¶
Binary ".." is the range operator, which is really two different
operators depending on the context. In list context, it returns a list of
values counting (up by ones) from the left value to the right value. If the
left value is greater than the right value then it returns the empty list. The
range operator is useful for writing "foreach (1..10)" loops and for
doing slice operations on arrays. In the current implementation, no temporary
array is created when the range operator is used as the expression in
"foreach" loops, but older versions of Perl might burn a lot of
memory when you write something like this:
for (1 .. 1_000_000) {
# code
}
The range operator also works on strings, using the magical auto-increment, see
below.
In scalar context, ".." returns a boolean value. The operator is
bistable, like a flip-flop, and emulates the line-range (comma) operator of
sed,
awk, and various editors. Each ".." operator
maintains its own boolean state, even across calls to a subroutine that
contains it. It is false as long as its left operand is false. Once the left
operand is true, the range operator stays true until the right operand is
true,
AFTER which the range operator becomes false again. It doesn't
become false till the next time the range operator is evaluated. It can test
the right operand and become false on the same evaluation it became true (as
in
awk), but it still returns true once. If you don't want it to test
the right operand until the next evaluation, as in
sed, just use three
dots ("...") instead of two. In all other regards, "..."
behaves just like ".." does.
The right operand is not evaluated while the operator is in the
"false" state, and the left operand is not evaluated while the
operator is in the "true" state. The precedence is a little lower
than || and &&. The value returned is either the empty string for
false, or a sequence number (beginning with 1) for true. The sequence number
is reset for each range encountered. The final sequence number in a range has
the string "E0" appended to it, which doesn't affect its numeric
value, but gives you something to search for if you want to exclude the
endpoint. You can exclude the beginning point by waiting for the sequence
number to be greater than 1.
If either operand of scalar ".." is a constant expression, that
operand is considered true if it is equal ("==") to the current
input line number (the $. variable).
To be pedantic, the comparison is actually "int(EXPR) == int(EXPR)",
but that is only an issue if you use a floating point expression; when
implicitly using $. as described in the previous paragraph, the comparison is
"int(EXPR) == int($.)" which is only an issue when $. is set to a
floating point value and you are not reading from a file. Furthermore,
"span" .. "spat" or "2.18 .. 3.14" will not do
what you want in scalar context because each of the operands are evaluated
using their integer representation.
Examples:
As a scalar operator:
if (101 .. 200) { print; } # print 2nd hundred lines, short for
# if ($. == 101 .. $. == 200) { print; }
next LINE if (1 .. /^$/); # skip header lines, short for
# next LINE 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) {
# do something
} else { # in body
# do something else
}
} continue {
close ARGV if eof; # reset $. each file
}
Here's a simple example to illustrate the difference between the two range
operators:
@lines = (" - Foo",
"01 - Bar",
"1 - Baz",
" - Quux");
foreach (@lines) {
if (/0/ .. /1/) {
print "$_\n";
}
}
This program will print only the line containing "Bar". If the range
operator is changed to "...", it will also print the "Baz"
line.
And now some examples as a list operator:
for (101 .. 200) { print; } # print $_ 100 times
@foo = @foo[0 .. $#foo]; # an expensive no-op
@foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
The range operator (in list context) makes use of the magical auto-increment
algorithm if the operands are strings. You can say
@alphabet = ("A" .. "Z");
to get all normal letters of the English alphabet, or
$hexdigit = (0 .. 9, "a" .. "f")[$num & 15];
to get a hexadecimal digit, or
@z2 = ("01" .. "31"); print $z2[$mday];
to get dates with leading zeros.
If the final value specified is not in the sequence that the magical increment
would produce, the sequence goes until the next value would be longer than the
final value specified.
If the initial value specified isn't part of a magical increment sequence (that
is, a non-empty string matching "/^[a-zA-Z]*[0-9]*\z/"), only the
initial value will be returned. So the following will only return an alpha:
use charnames "greek";
my @greek_small = ("\N{alpha}" .. "\N{omega}");
To get the 25 traditional lowercase Greek letters, including both sigmas, you
could use this instead:
use charnames "greek";
my @greek_small = map { chr }
ord "\N{alpha}" .. ord "\N{omega}";
However, because there are
many other lowercase Greek characters than
just those, to match lowercase Greek characters in a regular expression, you
would use the pattern "/(?:(?=\p{Greek})\p{Lower})+/".
Because each operand is evaluated in integer form, "2.18 .. 3.14" will
return two elements in list context.
@list = (2.18 .. 3.14); # same as @list = (2 .. 3);
Conditional Operator¶
Ternary "?:" is the conditional operator, just as in C. It works much
like an if-then-else. If the argument before the ? is true, the argument
before the : is returned, otherwise the argument after the : is returned. For
example:
printf "I have %d dog%s.\n", $n,
($n == 1) ? "" : "s";
Scalar or list context propagates downward into the 2nd or 3rd argument,
whichever is selected.
$a = $ok ? $b : $c; # get a scalar
@a = $ok ? @b : @c; # get an array
$a = $ok ? @b : @c; # oops, that's just a count!
The operator may be assigned to if both the 2nd and 3rd arguments are legal
lvalues (meaning that you can assign to them):
($a_or_b ? $a : $b) = $c;
Because this operator produces an assignable result, using assignments without
parentheses will get you in trouble. For example, this:
$a % 2 ? $a += 10 : $a += 2
Really means this:
(($a % 2) ? ($a += 10) : $a) += 2
Rather than this:
($a % 2) ? ($a += 10) : ($a += 2)
That should probably be written more simply as:
$a += ($a % 2) ? 10 : 2;
Assignment Operators¶
"=" is the ordinary assignment operator.
Assignment operators work as in C. That is,
$a += 2;
is equivalent to
$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 = $global) =~ tr [0-9] [a-j];
Likewise,
($a += 2) *= 3;
is equivalent to
$a += 2;
$a *= 3;
Similarly, a list assignment in list context produces the list of lvalues
assigned to, and a list assignment in scalar context returns the number of
elements produced by the expression on the right hand side of the assignment.
The Triple-Dot Operator¶
The triple-dot operator, "...", sometimes called the "whatever
operator", the "yada-yada operator", or the "
et
cetera" operator, is a placeholder for code. Perl parses it without
error, but when you try to execute a whatever, it throws an exception with the
text "Unimplemented":
sub unimplemented { ... }
eval { unimplemented() };
if ($@ eq "Unimplemented" ) {
say "Oh look, an exception--whatever.";
}
You can only use the triple-dot operator to stand in for a complete statement.
These examples of the triple-dot work:
{ ... }
sub foo { ... }
...;
eval { ... };
sub foo {
my ($self) = shift;
...;
}
do {
my $variable;
...;
say "Hurrah!";
} while $cheering;
The yada-yada--or whatever--cannot stand in for an expression that is part of a
larger statement since the "..." is also the three-dot version of
the binary range operator (see "Range Operators"). These examples of
the whatever operator are still syntax errors:
print ...;
open(PASSWD, ">", "/dev/passwd") or ...;
if ($condition && ...) { say "Hello" }
There are some cases where Perl can't immediately tell the difference between an
expression and a statement. For instance, the syntax for a block and an
anonymous hash reference constructor look the same unless there's something in
the braces that give Perl a hint. The whatever is a syntax error if Perl
doesn't guess that the "{ ... }" is a block. In that case, it
doesn't think the "..." is the whatever because it's expecting an
expression instead of a statement:
my @transformed = map { ... } @input; # syntax error
You can use a ";" inside your block to denote that the "{ ...
}" is a block and not a hash reference constructor. Now the whatever
works:
my @transformed = map {; ... } @input; # ; disambiguates
my @transformed = map { ...; } @input; # ; disambiguates
Comma Operator¶
Binary "," is the comma operator. In scalar context it evaluates its
left argument, throws that value away, then evaluates its right argument and
returns that value. This is just like C's comma operator.
In list context, it's just the list argument separator, and inserts both its
arguments into the list. These arguments are also evaluated from left to
right.
The "=>" operator is a synonym for the comma except that it causes
its left operand to be interpreted as a string if it begins with a letter or
underscore and is composed only of letters, digits and underscores. This
includes operands that might otherwise be interpreted as operators, constants,
single number v-strings or function calls. If in doubt about this behavior,
the left operand can be quoted explicitly.
Otherwise, the "=>" operator behaves exactly as the comma operator
or list argument separator, according to context.
For example:
use constant FOO => "something";
my %h = ( FOO => 23 );
is equivalent to:
my %h = ("FOO", 23);
It is
NOT:
my %h = ("something", 23);
The "=>" operator is helpful in documenting the correspondence
between keys and values in hashes, and other paired elements in lists.
%hash = ( $key => $value );
login( $username => $password );
List Operators (Rightward)¶
On the right side of a list operator, the comma has very low precedence, such
that it controls all comma-separated expressions found there. The only
operators with lower precedence are the logical operators "and",
"or", and "not", which may be used to evaluate calls to
list operators without the need for extra parentheses:
open HANDLE, "< $file"
or die "Can't open $file: $!\n";
See also discussion of list operators in "Terms and List Operators
(Leftward)".
Logical Not¶
Unary "not" returns the logical negation of the expression to its
right. It's the equivalent of "!" except for the very low
precedence.
Logical And¶
Binary "and" returns the logical conjunction of the two surrounding
expressions. It's equivalent to "&&" except for the very low
precedence. This means that it short-circuits: the right expression is
evaluated only if the left expression is true.
Logical or, Defined or, and Exclusive Or¶
Binary "or" returns the logical disjunction of the two surrounding
expressions. It's equivalent to "||" except for the very low
precedence. This makes it useful for control flow:
print FH $data or die "Can't write to FH: $!";
This means that it short-circuits: the right expression is evaluated only if the
left expression is false. Due to its precedence, you must be careful to avoid
using it as replacement for the "||" operator. It usually works out
better for flow control than in assignments:
$a = $b or $c; # bug: this is wrong
($a = $b) or $c; # really means this
$a = $b || $c; # better written this way
However, when it's a list-context assignment and you're trying to use
"||" for control flow, you probably need "or" so that the
assignment takes higher precedence.
@info = stat($file) || die; # oops, scalar sense of stat!
@info = stat($file) or die; # better, now @info gets its due
Then again, you could always use parentheses.
Binary "xor" returns the exclusive-OR of the two surrounding
expressions. It cannot short-circuit (of course).
C Operators Missing From Perl¶
Here is what C has that Perl doesn't:
- unary &
- Address-of operator. (But see the "\" operator
for taking a reference.)
- unary *
- Dereference-address operator. (Perl's prefix 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 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)
y{}{} Transliteration no (but see below)
<<EOF here-doc yes*
* unless the delimiter is ''.
Non-bracketing delimiters use the same character fore and aft, but the four
sorts of ASCII brackets (round, angle, square, curly) all nest, which means
that
q{foo{bar}baz}
is the same as
'foo{bar}baz'
Note, however, that this does not always work for quoting Perl code:
$s = q{ if($a eq "}") ... }; # WRONG
is a syntax error. The "Text::Balanced" module (standard as of v5.8,
and from CPAN before then) is able to do this properly.
There can be whitespace between the operator and the quoting characters, except
when "#" is being used as the quoting character. "q#foo#"
is parsed as the string "foo", while "q #foo#" is the
operator "q" followed by a comment. Its argument will be taken from
the next line. This allows you to write:
s {foo} # Replace foo
{bar} # with bar.
The following escape sequences are available in constructs that interpolate, and
in transliterations:
Sequence Note Description
\t tab (HT, TAB)
\n newline (NL)
\r return (CR)
\f form feed (FF)
\b backspace (BS)
\a alarm (bell) (BEL)
\e escape (ESC)
\x{263A} [1,8] hex char (example: SMILEY)
\x1b [2,8] restricted range hex char (example: ESC)
\N{name} [3] named Unicode character or character sequence
\N{U+263D} [4,8] Unicode character (example: FIRST QUARTER MOON)
\c[ [5] control char (example: chr(27))
\o{23072} [6,8] octal char (example: SMILEY)
\033 [7,8] restricted range octal char (example: ESC)
- [1]
- The result is the character specified by the hexadecimal
number between the braces. See "[8]" below for details on which
character.
Only hexadecimal digits are valid between the braces. If an invalid
character is encountered, a warning will be issued and the invalid
character and all subsequent characters (valid or invalid) within the
braces will be discarded.
If there are no valid digits between the braces, the generated character is
the NULL character ("\x{00}"). However, an explicit empty brace
("\x{}") will not cause a warning (currently).
- [2]
- The result is the character specified by the hexadecimal
number in the range 0x00 to 0xFF. See "[8]" below for details on
which character.
Only hexadecimal digits are valid following "\x". When
"\x" is followed by fewer than two valid digits, any valid
digits will be zero-padded. This means that "\x7" will be
interpreted as "\x07", and a lone <\x> will be interpreted
as "\x00". Except at the end of a string, having fewer than two
valid digits will result in a warning. Note that although the warning says
the illegal character is ignored, it is only ignored as part of the escape
and will still be used as the subsequent character in the string. For
example:
Original Result Warns?
"\x7" "\x07" no
"\x" "\x00" no
"\x7q" "\x07q" yes
"\xq" "\x00q" yes
- [3]
- The result is the Unicode character or character sequence
given by name. See charnames.
- [4]
- "\N{U+hexadecimal number}" means the
Unicode character whose Unicode code point is hexadecimal
number.
- [5]
- The character following "\c" is mapped to some
other character as shown in the table:
Sequence Value
\c@ chr(0)
\cA chr(1)
\ca chr(1)
\cB chr(2)
\cb chr(2)
...
\cZ chr(26)
\cz chr(26)
\c[ chr(27)
\c] chr(29)
\c^ chr(30)
\c? chr(127)
Also, "\c\ X" yields " chr(28) .
"X"" for any X, but cannot come at the end of
a string, because the backslash would be parsed as escaping the end quote.
On ASCII platforms, the resulting characters from the list above are the
complete set of ASCII controls. This isn't the case on EBCDIC platforms;
see "OPERATOR DIFFERENCES" in perlebcdic for the complete list
of what these sequences mean on both ASCII and EBCDIC platforms.
Use of any other character following the "c" besides those listed
above is discouraged, and some are deprecated with the intention of
removing those in Perl 5.16. What happens for any of these other
characters currently though, is that the value is derived by inverting the
7th bit (0x40).
To get platform independent controls, you can use "\N{...}".
- [6]
- The result is the character specified by the octal number
between the braces. See "[8]" below for details on which
character.
If a character that isn't an octal digit is encountered, a warning is
raised, and the value is based on the octal digits before it, discarding
it and all following characters up to the closing brace. It is a fatal
error if there are no octal digits at all.
- [7]
- The result is the character specified by the three-digit
octal number in the range 000 to 777 (but best to not use above 077, see
next paragraph). See "[8]" below for details on which character.
Some contexts allow 2 or even 1 digit, but any usage without exactly three
digits, the first being a zero, may give unintended results. (For example,
see "Octal escapes" in perlrebackslash.) Starting in Perl 5.14,
you may use "\o{}" instead, which avoids all these problems.
Otherwise, it is best to use this construct only for ordinals
"\077" and below, remembering to pad to the left with zeros to
make three digits. For larger ordinals, either use "\o{}" , or
convert to something else, such as to hex and use "\x{}"
instead.
Having fewer than 3 digits may lead to a misleading warning message that
says that what follows is ignored. For example, "\128" in the
ASCII character set is equivalent to the two characters "\n8",
but the warning "Illegal octal digit '8' ignored" will be
thrown. To avoid this warning, make sure to pad your octal number with
0's: "\0128".
- [8]
- Several constructs above specify a character by a number.
That number gives the character's position in the character set encoding
(indexed from 0). This is called synonymously its ordinal, code position,
or code point. Perl works on platforms that have a native encoding
currently of either ASCII/Latin1 or EBCDIC, each of which allow
specification of 256 characters. In general, if the number is 255 (0xFF,
0377) or below, Perl interprets this in the platform's native encoding. If
the number is 256 (0x100, 0400) or above, Perl interprets it as a Unicode
code point and the result is the corresponding Unicode character. For
example "\x{50}" and "\o{120}" both are the number 80
in decimal, which is less than 256, so the number is interpreted in the
native character set encoding. In ASCII the character in the 80th position
(indexed from 0) is the letter "P", and in EBCDIC it is the
ampersand symbol "&". "\x{100}" and
"\o{400}" are both 256 in decimal, so the number is interpreted
as a Unicode code point no matter what the native encoding is. The name of
the character in the 100th position (indexed by 0) in Unicode is
"LATIN CAPITAL LETTER A WITH MACRON".
There are a couple of exceptions to the above rule. "\N{U+ hex
number}" is always interpreted as a Unicode code point, so that
"\N{U+0050}" is "P" even on EBCDIC platforms. And if
"use encoding" is in effect, the number is considered to be
in that encoding, and is translated from that into the platform's native
encoding if there is a corresponding native character; otherwise to
Unicode.
NOTE: Unlike C and other languages, Perl has no "\v" escape
sequence for the vertical tab (VT - ASCII 11), but you may use "\ck"
or "\x0b". ("\v" does have meaning in regular expression
patterns in Perl, see perlre.)
The following escape sequences are available in constructs that interpolate, but
not in transliterations.
\l lowercase next character only
\u titlecase (not uppercase!) next character only
\L lowercase all characters till \E seen
\U uppercase all characters till \E seen
\Q quote non-word characters till \E
\E end either case modification or quoted section
(whichever was last seen)
"\L", "\U", and "\Q" can stack, in which case you
need one "\E" for each. For example:
say "This \Qquoting \ubusiness \Uhere isn't quite\E done yet,\E is it?";
This quoting\ Business\ HERE\ ISN\'T\ QUITE\ done\ yet\, is it?
If "use locale" is in effect, the case map used by "\l",
"\L", "\u", and "\U" is taken from the current
locale. See perllocale. If Unicode (for example, "\N{}" or code
points of 0x100 or beyond) is being used, the case map used by "\l",
"\L", "\u", and "\U" is as defined by Unicode.
That means that case-mapping a single character can sometimes produce several
characters.
All systems use the virtual "\n" 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 Perl all conspire to preserve. Not all systems read
"\r" as ASCII CR and "\n" as ASCII LF. For example, on the
ancient Macs (pre-MacOS X) of yesteryear, these used to be reversed, and on
systems without line terminator, printing "\n" might emit no actual
data. In general, use "\n" when you mean a "newline" for
your system, but use the literal ASCII when you need an exact character. For
example, most networking protocols expect and prefer a CR+LF
("\015\012" or "\cM\cJ") for line terminators, and
although they often accept just "\012", they seldom tolerate just
"\015". If you get in the habit of using "\n" for
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 $",
@array". "Punctuation" arrays such as "@*" are
usually interpolated only if the name is enclosed in braces "@{*}",
but the arrays @_, "@+", and "@-" are interpolated even
without braces.
For double-quoted strings, the quoting from "\Q" is applied after
interpolation and escapes are processed.
"abc\Qfoo\tbar$s\Exyz"
is equivalent to
"abc" . quotemeta("foo\tbar$s") . "xyz"
For the pattern of regex operators ("qr//", "m//" and
"s///"), the quoting from "\Q" is applied after
interpolation is processed, but before escapes are processed. This allows the
pattern to match literally (except for "$" and "@"). For
example, the following matches:
'\s\t' =~ /\Q\s\t/
Because "$" or "@" trigger interpolation, you'll need to use
something like "/\Quser\E\@\Qhost/" to match them literally.
Patterns are subject to an additional level of interpretation as a regular
expression. This is done as a second pass, after variables are interpolated,
so that regular expressions may be incorporated into the pattern from the
variables. If this is not what you want, use "\Q" to interpolate a
variable literally.
Apart from the behavior described above, Perl does not expand multiple levels of
interpolation. In particular, contrary to the expectations of shell
programmers, back-quotes do
NOT interpolate within double quotes, nor
do single quotes impede evaluation of variables when used within double
quotes.
Regexp Quote-Like Operators¶
Here are the quote-like operators that apply to pattern matching and related
activities.
- qr/STRING/msixpodual
- This operator quotes (and possibly compiles) its
STRING as a regular expression. STRING is interpolated the
same way as PATTERN in "m/PATTERN/". If "'" is
used as the delimiter, no interpolation is done. Returns a Perl value
which may be used instead of the corresponding
"/STRING/msixpodual" expression. The returned value is a
normalized version of the original pattern. It magically differs from a
string containing the same characters: "ref(qr/x/)" returns
"Regexp", even though dereferencing the result returns undef.
For example,
$rex = qr/my.STRING/is;
print $rex; # prints (?si-xm:my.STRING)
s/$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 the
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 (specified by the following modifiers) are:
m Treat string as multiple lines.
s Treat string as single line. (Make . match a newline)
i Do case-insensitive pattern matching.
x Use extended regular expressions.
p When matching preserve a copy of the matched string so
that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be defined.
o Compile pattern only once.
l Use the locale
u Use Unicode rules
a Use ASCII for \d, \s, \w; specifying two a's further restricts
/i matching so that no ASCII character will match a non-ASCII
one
d Use Unicode or native charset, as in 5.12 and earlier
If a precompiled pattern is embedded in a larger pattern then the effect of
"msixpluad" will be propagated appropriately. The effect the
"o" modifier has is not propagated, being restricted to those
patterns explicitly using it.
The last four modifiers listed above, added in Perl 5.14, control the
character set semantics.
See perlre for additional information on valid syntax for STRING, and for a
detailed look at the semantics of regular expressions. In particular, all
the modifiers execpt "/o" are further explained in
"Modifiers" in perlre. "/o" is described in the next
section.
- m/PATTERN/msixpodualgc
- /PATTERN/msixpodualgc
- Searches a string for a pattern match, and in scalar
context returns true if it succeeds, false if it fails. If no string is
specified via the "=~" or "!~" operator, the $_ string
is searched. (The string specified with "=~" need not be an
lvalue--it may be the result of an expression evaluation, but remember the
"=~" binds rather tightly.) See also perlre.
Options are as described in "qr//" above; in addition, the
following match process modifiers are available:
g Match globally, i.e., find all occurrences.
c Do not reset search position on a failed match when /g is in effect.
If "/" is the delimiter then the initial "m" is
optional. With the "m" you can use any pair of non-whitespace
(ASCII) characters as delimiters. This is particularly useful for matching
path names that contain "/", to avoid LTS (leaning toothpick
syndrome). If "?" is the delimiter, then a match-only-once rule
applies, described in "m?PATTERN?" below. If "'" is
the delimiter, no interpolation is performed on the PATTERN. When using a
character valid in an identifier, whitespace is required after the
"m".
PATTERN may contain variables, which will be interpolated every time the
pattern search is evaluated, except for when the delimiter is a single
quote. (Note that $(, $), and $| are not interpolated because they look
like end-of-string tests.) Perl will not recompile the pattern unless an
interpolated variable that it contains changes. You can force Perl to skip
the test and never recompile by adding a "/o" (which stands for
"once") after the trailing delimiter. Once upon a time, Perl
would recompile regular expressions unnecessarily, and this modifier was
useful to tell it not to do so, in the interests of speed. But now, the
only reasons to use "/o" are either:
- 1.
- The variables are thousands of characters long and you know
that they don't change, and you need to wring out the last little bit of
speed by having Perl skip testing for that. (There is a maintenance
penalty for doing this, as mentioning "/o" constitutes a promise
that you won't change the variables in the pattern. If you change them,
Perl won't even notice.)
- 2.
- you want the pattern to use the initial values of the
variables regardless of whether they change or not. (But there are saner
ways of accomplishing this than using "/o".)
- The empty pattern //
- If the PATTERN evaluates to the empty string, the last
successfully matched regular expression is used instead. In this
case, only the "g" and "c" flags on the empty pattern
are honored; the other flags are taken from the original pattern. If no
match has previously succeeded, this will (silently) act instead as a
genuine empty pattern (which will always match).
Note that it's possible to confuse Perl into thinking "//" (the
empty regex) is really "//" (the defined-or operator). Perl is
usually pretty good about this, but some pathological cases might trigger
this, such as "$a///" (is that "($a) / (//)" or
"$a // /"?) and "print $fh //" ("print
$fh(//" or "print($fh //"?). In all of these examples, Perl
will assume you meant defined-or. If you meant the empty regex, just use
parentheses or spaces to disambiguate, or even prefix the empty regex with
an "m" (so "//" becomes "m//").
- Matching in list context
- If the "/g" option is not used, "m//"
in list context returns a list consisting of the subexpressions matched by
the parentheses in the pattern, 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")
|| die "can't access /dev/tty: $!";
<TTY> =~ /^y/i && foo(); # do foo if desired
if (/Version: *([0-9.]*)/) { $version = $1; }
next if m#^/usr/spool/uucp#;
# poor man's grep
$arg = shift;
while (<>) {
print if /$arg/o; # compile only once (no longer needed!)
}
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 $Etc. The
conditional is true if any variables were assigned; that is, if the
pattern matched.
The "/g" modifier specifies global pattern matching--that is,
matching as many times as possible within the string. How it behaves
depends on the context. In list context, it returns a list of the
substrings matched by any capturing parentheses in the regular expression.
If there are no parentheses, it returns a list of all the matched strings,
as if there were parentheses around the whole pattern.
In scalar context, each execution of "m//g" finds the next match,
returning true if it matches, and false if there is no further match. The
position after the last match can be read or set using the
"pos()" function; see "pos" in perlfunc. A failed
match normally resets the search position to the beginning of the string,
but you can avoid that by adding the "/c" modifier (e.g.
"m//gc"). Modifying the target string also resets the search
position.
- \G assertion
- You can intermix "m//g" matches with
"m/\G.../g", where "\G" is a zero-width assertion that
matches the exact position where the previous "m//g", if any,
left off. Without the "/g" modifier, the "\G"
assertion still anchors at "pos()" as it was at the start of the
operation (see "pos" in perlfunc), but the match is of course
only attempted once. Using "\G" without "/g" on a
target string that has not previously had a "/g" match applied
to it is the same as using the "\A" assertion to match the
beginning of the string. Note also that, currently, "\G" is only
properly supported when anchored at the very beginning of the pattern.
Examples:
# list context
($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
# scalar context
local $/ = "";
while ($paragraph = <>) {
while ($paragraph =~ /\p{Ll}['")]*[.!?]+['")]*\s/g) {
$sentences++;
}
}
say $sentences;
Here's another way to check for sentences in a paragraph:
my $sentence_rx = qr{
(?: (?<= ^ ) | (?<= \s ) ) # after start-of-string or whitespace
\p{Lu} # capital letter
.*? # a bunch of anything
(?<= \S ) # that ends in non-whitespace
(?<! \b [DMS]r ) # but isn't a common abbreviation
(?<! \b Mrs )
(?<! \b Sra )
(?<! \b St )
[.?!] # followed by a sentence ender
(?= $ | \s ) # in front of end-of-string or whitespace
}sx;
local $/ = "";
while (my $paragraph = <>) {
say "NEW PARAGRAPH";
my $count = 0;
while ($paragraph =~ /($sentence_rx)/g) {
printf "\tgot sentence %d: <%s>\n", ++$count, $1;
}
}
Here's how to use "m//gc" with "\G":
$_ = "ppooqppqq";
while ($i++ < 2) {
print "1: '";
print $1 while /(o)/gc; print "', pos=", pos, "\n";
print "2: '";
print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
print "3: '";
print $1 while /(p)/gc; print "', pos=", pos, "\n";
}
print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
The last example should print:
1: 'oo', pos=4
2: 'q', pos=5
3: 'pp', pos=7
1: '', pos=7
2: 'q', pos=8
3: '', pos=8
Final: 'q', pos=8
Notice that the final match matched "q" instead of "p",
which a match without the "\G" anchor would have done. Also note
that the final match did not update "pos". "pos" is
only updated on a "/g" match. If the final match did indeed
match "p", it's a good bet that you're running a very old
(pre-5.6.0) version of Perl.
A useful idiom for "lex"-like scanners is "/\G.../gc".
You can combine several regexps like this to process a string
part-by-part, doing different actions depending on which regexp matched.
Each regexp tries to match where the previous one leaves off.
$_ = <<'EOL';
$url = URI::URL->new( "http://example.com/" ); die if $url eq "xXx";
EOL
LOOP: {
print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
print(" lowercase"), redo LOOP if /\G\p{Ll}+\b[,.;]?\s*/gc;
print(" UPPERCASE"), redo LOOP if /\G\p{Lu}+\b[,.;]?\s*/gc;
print(" Capitalized"), redo LOOP if /\G\p{Lu}\p{Ll}+\b[,.;]?\s*/gc;
print(" MiXeD"), redo LOOP if /\G\pL+\b[,.;]?\s*/gc;
print(" alphanumeric"), redo LOOP if /\G[\p{Alpha}\pN]+\b[,.;]?\s*/gc;
print(" line-noise"), redo LOOP if /\G\W+/gc;
print ". That's all!\n";
}
Here is the output (split into several lines):
line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE
line-noise lowercase line-noise lowercase line-noise lowercase
lowercase line-noise lowercase lowercase line-noise lowercase
lowercase line-noise MiXeD line-noise. That's all!
- m?PATTERN?msixpodualgc
- ?PATTERN?msixpodualgc
- This is just like the "m/PATTERN/" search, except
that it matches only once between calls to the reset() operator.
This is a useful optimization when you want to see only the first
occurrence of something in each file of a set of files, for instance. Only
"m??" patterns local to the current package are reset.
while (<>) {
if (m?^$?) {
# blank line between header and body
}
} continue {
reset if eof; # clear m?? status for next file
}
Another example switched the first "latin1" encoding it finds to
"utf8" in a pod file:
s//utf8/ if m? ^ =encoding \h+ \K latin1 ?x;
The match-once behavior is controlled by the match delimiter being
"?"; with any other delimiter this is the normal "m//"
operator.
For historical reasons, the leading "m" in "m?PATTERN?"
is optional, but the resulting "?PATTERN?" syntax is deprecated,
will warn on usage and might be removed from a future stable release of
Perl (without further notice!).
- s/PATTERN/REPLACEMENT/msixpodualgcer
- Searches a string for a pattern, and if found, replaces
that pattern with the replacement text and returns the number of
substitutions made. Otherwise it returns false (specifically, the empty
string).
If the "/r" (non-destructive) option is used then it runs the
substitution on a copy of the string and instead of returning the number
of substitutions, it returns the copy whether or not a substitution
occurred. The original string is never changed when "/r" is
used. The copy will always be a plain string, even if the input is an
object or a tied variable.
If no string is specified via the "=~" or "!~" operator,
the $_ variable is searched and modified. Unless the "/r" option
is used, the string specified must be a scalar variable, an array element,
a hash element, or an assignment to one of those; that is, some sort of
scalar lvalue.
If the delimiter chosen is a single quote, no interpolation is done on
either the PATTERN or the REPLACEMENT. Otherwise, if the PATTERN contains
a $ that looks like a variable rather than an end-of-string test, the
variable will be interpolated into the pattern at run-time. If you want
the pattern compiled only once the first time the variable is
interpolated, use the "/o" option. If the pattern evaluates to
the empty string, the last successfully executed regular expression is
used instead. See perlre for further explanation on these.
Options are as with m// with the addition of the following replacement
specific options:
e Evaluate the right side as an expression.
ee Evaluate the right side as a string then eval the result.
r Return substitution and leave the original string untouched.
Any non-whitespace delimiter may replace the slashes. Add space after the
"s" when using a character allowed in identifiers. If single
quotes are used, no interpretation is done on the replacement string (the
"/e" modifier overrides this, however). Unlike Perl 4, Perl 5
treats backticks as normal delimiters; the replacement text is not
evaluated as a command. If the PATTERN is delimited by bracketing quotes,
the REPLACEMENT has its own pair of quotes, which may or may not be
bracketing quotes, e.g., "s(foo)(bar)" or
"s<foo>/bar/". A "/e" will cause the replacement
portion to be treated as a full-fledged Perl expression and evaluated
right then and there. It is, however, syntax checked at compile-time. A
second "e" modifier will cause the replacement portion to be
"eval"ed before being run as a Perl expression.
Examples:
s/\bgreen\b/mauve/g; # don't change wintergreen
$path =~ s|/usr/bin|/usr/local/bin|;
s/Login: $foo/Login: $bar/; # run-time pattern
($foo = $bar) =~ s/this/that/; # copy first, then change
($foo = "$bar") =~ s/this/that/; # convert to string, copy, then change
$foo = $bar =~ s/this/that/r; # Same as above using /r
$foo = $bar =~ s/this/that/r
=~ s/that/the other/r; # Chained substitutes using /r
@foo = map { s/this/that/r } @bar # /r is very useful in maps
$count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
$_ = 'abc123xyz';
s/\d+/$&*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
$_ = 'abc123xyz';
$a = s/abc/def/r; # $a is 'def123xyz' and
# $_ remains 'abc123xyz'.
# expand variables in $_, but dynamics only, using
# symbolic dereferencing
s/\$(\w+)/${$1}/g;
# Add one to the value of any numbers in the string
s/(\d+)/1 + $1/eg;
# Titlecase words in the last 30 characters only
substr($str, -30) =~ s/\b(\p{Alpha}+)\b/\u\L$1/g;
# This will expand any embedded scalar variable
# (including lexicals) in $_ : First $1 is interpolated
# to the variable name, and then evaluated
s/(\$\w+)/$1/eeg;
# Delete (most) C comments.
$program =~ s {
/\* # Match the opening delimiter.
.*? # Match a minimal number of characters.
\*/ # Match the closing delimiter.
} []gsx;
s/^\s*(.*?)\s*$/$1/; # trim whitespace in $_, expensively
for ($variable) { # trim whitespace in $variable, cheap
s/^\s+//;
s/\s+$//;
}
s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
Note the use of $ instead of \ in the last example. Unlike sed, we
use the \< digit> form in only the left hand side. Anywhere
else it's $< digit>.
Occasionally, you can't use just a "/g" to get all the changes to
occur that you might want. Here are two common cases:
# put commas in the right places in an integer
1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
# expand tabs to 8-column spacing
1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
"s///le" is treated as a substitution followed by the
"le" operator, not the "/le" flags. This may change in
a future version of Perl. It produces a warning if warnings are enabled.
To disambiguate, use a space or change the order of the flags:
s/foo/bar/ le 5; # "le" infix operator
s/foo/bar/el; # "e" and "l" flags
Quote-Like Operators¶
- q/STRING/
- 'STRING'
- 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 = '\n'; # a two-character string
- qq/STRING/
- "STRING"
- A double-quoted, interpolated string.
$_ .= qq
(*** The previous line contains the naughty word "$1".\n)
if /\b(tcl|java|python)\b/i; # :-)
$baz = "\n"; # a one-character string
- qx/STRING/
- `STRING`
- A string which is (possibly) interpolated and then executed
as a system command with "/bin/sh" or its equivalent. Shell
wildcards, pipes, and redirections will be honored. The collected standard
output of the command is returned; standard error is unaffected. In scalar
context, it comes back as a single (potentially multi-line) string, or
undef if the command failed. In list context, returns a list of lines
(however you've defined lines with $/ or $INPUT_RECORD_SEPARATOR), or an
empty list if the command failed.
Because backticks do not affect standard error, use shell file descriptor
syntax (assuming the shell supports this) if you care to address this. To
capture a command's STDERR and STDOUT together:
$output = `cmd 2>&1`;
To capture a command's STDOUT but discard its STDERR:
$output = `cmd 2>/dev/null`;
To capture a command's STDERR but discard its STDOUT (ordering is important
here):
$output = `cmd 2>&1 1>/dev/null`;
To exchange a command's STDOUT and STDERR in order to capture the STDERR but
leave its STDOUT to come out the old STDERR:
$output = `cmd 3>&1 1>&2 2>&3 3>&-`;
To read both a command's STDOUT and its STDERR separately, it's easiest to
redirect them separately to files, and then read from those files when the
program is done:
system("program args 1>program.stdout 2>program.stderr");
The STDIN filehandle used by the command is inherited from Perl's STDIN. For
example:
open(SPLAT, "stuff") || die "can't open stuff: $!";
open(STDIN, "<&SPLAT") || die "can't dupe SPLAT: $!";
print STDOUT `sort`;
will print the sorted contents of the file named "stuff".
Using single-quote as a delimiter protects the command from Perl's
double-quote interpolation, passing it on to the shell instead:
$perl_info = qx(ps $$); # that's Perl's $$
$shell_info = qx'ps $$'; # that's the new shell's $$
How that string gets evaluated is entirely subject to the command
interpreter on your system. On most platforms, you will have to protect
shell metacharacters if you want them treated literally. This is in
practice difficult to do, as it's unclear how to escape which characters.
See perlsec for a clean and safe example of a manual fork() and
exec() to emulate backticks safely.
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 supports that (e.g. ";" on many Unix shells;
"&" on the Windows NT "cmd" shell).
Beginning with v5.6.0, Perl will attempt to flush all files opened for
output before starting the child process, but this may not be supported on
some platforms (see perlport). To be safe, you may need to set $|
($AUTOFLUSH in English) or call the "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 don't exceed this limit after
any necessary interpolations. See the platform-specific release notes for
more details about your particular environment.
Using this operator can lead to programs that are difficult to port, because
the shell commands called vary between systems, and may in fact not be
present at all. As one example, the "type" command under the
POSIX shell is very different from the "type" command under DOS.
That doesn't mean you should go out of your way to avoid backticks when
they're the right way to get something done. Perl was made to be a glue
language, and one of the things it glues together is commands. Just
understand what you're getting yourself into.
See "I/O Operators" for more discussion.
- qw/STRING/
- Evaluates to a list of the words extracted out of STRING,
using embedded whitespace as the word delimiters. It can be understood as
being roughly equivalent to:
split(" ", q/STRING/);
the differences being that it generates a real list at compile time, and in
scalar context it returns the last element in the list. So this
expression:
qw(foo bar baz)
is semantically equivalent to the list:
"foo", "bar", "baz"
Some frequently seen examples:
use POSIX qw( setlocale localeconv )
@EXPORT = qw( foo bar baz );
A common mistake is to try to separate the words with comma or to put
comments into a multi-line "qw"-string. For this reason, the
"use warnings" pragma and the -w switch (that is, the $^W
variable) produces warnings if the STRING contains the "," or
the "#" character.
- tr/SEARCHLIST/REPLACEMENTLIST/cdsr
- y/SEARCHLIST/REPLACEMENTLIST/cdsr
- Transliterates all occurrences of the characters found in
the search list with the corresponding character in the replacement list.
It returns the number of characters replaced or deleted. If no string is
specified via the "=~" or "!~" operator, the $_ string
is transliterated.
If the "/r" (non-destructive) option is present, a new copy of the
string is made and its characters transliterated, and this copy is
returned no matter whether it was modified or not: the original string is
always left unchanged. The new copy is always a plain string, even if the
input string is an object or a tied variable.
Unless the "/r" option is used, the string specified with
"=~" must be a scalar variable, an array element, a hash
element, or an assignment to one of those; in other words, an lvalue.
A character range may be specified with a hyphen, so "tr/A-J/0-9/"
does the same replacement as "tr/ACEGIBDFHJ/0246813579/". For
sed devotees, "y" is provided as a synonym for
"tr". If the SEARCHLIST is delimited by bracketing quotes, the
REPLACEMENTLIST has its own pair of quotes, which may or may not be
bracketing quotes; for example, "tr[aeiouy][yuoiea]" or
"tr(+\-*/)/ABCD/".
Note that "tr" does not do regular expression character
classes such as "\d" or "\pL". The "tr"
operator is not equivalent to the tr(1) utility. If you want to map
strings between lower/upper cases, see "lc" in perlfunc and
"uc" in perlfunc, and in general consider using the
"s" operator if you need regular expressions. The
"\U", "\u", "\L", and "\l"
string-interpolation escapes on the right side of a substitution operator
will perform correct case-mappings, but "tr[a-z][A-Z]" will not
(except sometimes on legacy 7-bit data).
Note also that the whole range idea is rather unportable between character
sets--and even within character sets they may cause results you probably
didn't expect. A sound principle is to use only ranges that begin from and
end at either alphabets of equal case (a-e, A-E), or digits (0-4).
Anything else is unsafe. If in doubt, spell out the character sets in
full.
Options:
c Complement the SEARCHLIST.
d Delete found but unreplaced characters.
s Squash duplicate replaced characters.
r Return the modified string and leave the original string
untouched.
If the "/c" modifier is specified, the SEARCHLIST character set is
complemented. If the "/d" modifier is specified, any characters
specified by SEARCHLIST not found in REPLACEMENTLIST are deleted. (Note
that this is slightly more flexible than the behavior of some tr
programs, which delete anything they find in the SEARCHLIST, period.) If
the "/s" modifier is specified, sequences of characters that
were transliterated to the same character are squashed down to a single
instance of the character.
If the "/d" modifier is used, the REPLACEMENTLIST is always
interpreted exactly as specified. Otherwise, if the REPLACEMENTLIST is
shorter than the SEARCHLIST, the final character is replicated till it is
long enough. If the REPLACEMENTLIST is empty, the SEARCHLIST is
replicated. This latter is useful for counting characters in a class or
for squashing character sequences in a class.
Examples:
$ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case ASCII
$cnt = tr/*/*/; # count the stars in $_
$cnt = $sky =~ tr/*/*/; # count the stars in $sky
$cnt = tr/0-9//; # count the digits in $_
tr/a-zA-Z//s; # bookkeeper -> bokeper
($HOST = $host) =~ tr/a-z/A-Z/;
$HOST = $host =~ tr/a-z/A-Z/r; # same thing
$HOST = $host =~ tr/a-z/A-Z/r # chained with s///r
=~ s/:/ -p/r;
tr/a-zA-Z/ /cs; # change non-alphas to single space
@stripped = map tr/a-zA-Z/ /csr, @original;
# /r with map
tr [\200-\377]
[\000-\177]; # wickedly delete 8th bit
If multiple transliterations are given for a character, only the first one
is used:
tr/AAA/XYZ/
will transliterate any A to X.
Because the transliteration table is built at compile time, neither the
SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote
interpolation. That means that if you want to use variables, you must use
an eval():
eval "tr/$oldlist/$newlist/";
die $@ if $@;
eval "tr/$oldlist/$newlist/, 1" or die $@;
- <<EOF
- A line-oriented form of quoting is based on the shell
"here-document" syntax. Following a "<<" you
specify a string to terminate the quoted material, and all lines following
the current line down to the terminating string are the value of the item.
The terminating string may be either an identifier (a word), or some quoted
text. An unquoted identifier works like double quotes. There may not be a
space between the "<<" and the identifier, unless the
identifier is explicitly quoted. (If you put a space it will be treated as
a null identifier, which is valid, and matches the first empty line.) The
terminating string must appear by itself (unquoted and with no surrounding
whitespace) on the terminating line.
If the terminating string is quoted, the type of quotes used determine the
treatment of the text.
- Double Quotes
- Double quotes indicate that the text will be interpolated
using exactly the same rules as normal double quoted strings.
print <<EOF;
The price is $Price.
EOF
print << "EOF"; # same as above
The price is $Price.
EOF
- Single Quotes
- Single quotes indicate the text is to be treated literally
with no interpolation of its content. This is similar to single quoted
strings except that backslashes have no special meaning, with
"\\" being treated as two backslashes and not one as they would
in every other quoting construct.
Just as in the shell, a backslashed bareword following the
"<<" means the same thing as a single-quoted string does:
$cost = <<'VISTA'; # hasta la ...
That'll be $10 please, ma'am.
VISTA
$cost = <<\VISTA; # Same thing!
That'll be $10 please, ma'am.
VISTA
This is the only form of quoting in perl where there is no need to worry
about escaping content, something that code generators can and do make
good use of.
- Backticks
- The content of the here doc is treated just as it would be
if the string were embedded in backticks. Thus the content is interpolated
as though it were double quoted and then executed via the shell, with the
results of the execution returned.
print << `EOC`; # execute command and get results
echo hi there
EOC
It is possible to stack multiple here-docs in a row:
print <<"foo", <<"bar"; # you can stack them
I said foo.
foo
I said bar.
bar
myfunc(<< "THIS", 23, <<'THAT');
Here's a line
or two.
THIS
and here's another.
THAT
Just don't forget that you have to put a semicolon on the end to finish the
statement, as Perl doesn't know you're not going to try to do this:
print <<ABC
179231
ABC
+ 20;
If you want to remove the line terminator from your here-docs, use
"chomp()".
chomp($string = <<'END');
This is a string.
END
If you want your here-docs to be indented with the rest of the code, you'll need
to remove leading whitespace from each line manually:
($quote = <<'FINIS') =~ s/^\s+//gm;
The Road goes ever on and on,
down from the door where it began.
FINIS
If you use a here-doc within a delimited construct, such as in
"s///eg", the quoted material must come on the lines following the
final delimiter. So instead of
s/this/<<E . 'that'
the other
E
. 'more '/eg;
you have to write
s/this/<<E . 'that'
. 'more '/eg;
the other
E
If the terminating identifier is on the last line of the program, you must be
sure there is a newline after it; otherwise, Perl will give the warning
Can't find string terminator "END" anywhere before EOF....
Additionally, quoting rules for the end-of-string identifier are unrelated to
Perl's quoting rules. "q()", "qq()", and the like are not
supported in place of '' and "", and the only interpolation is for
backslashing the quoting character:
print << "abc\"def";
testing...
abc"def
Finally, quoted strings cannot span multiple lines. The general rule is that the
identifier must be a string literal. Stick with that, and you should be
safe.
Gory details of parsing quoted constructs¶
When presented with something that might have several different interpretations,
Perl uses the
DWIM (that's "Do What I Mean") principle to
pick the most probable interpretation. This strategy is so successful that
Perl programmers often do not suspect the ambivalence of what they write. But
from time to time, Perl's notions differ substantially from what the author
honestly meant.
This section hopes to clarify how Perl handles quoted constructs. Although the
most common reason to learn this is to unravel labyrinthine regular
expressions, because the initial steps of parsing are the same for all quoting
operators, they are all discussed together.
The most important Perl parsing rule is the first one discussed below: when
processing a quoted construct, Perl first finds the end of that construct,
then interprets its contents. If you understand this rule, you may skip the
rest of this section on the first reading. The other rules are likely to
contradict the user's expectations much less frequently than this first one.
Some passes discussed below are performed concurrently, but because their
results are the same, we consider them individually. For different quoting
constructs, Perl performs different numbers of passes, from one to four, but
these passes are always performed in the same order.
- Finding the end
- The first pass is finding the end of the quoted construct,
where the information about the delimiters is used in parsing. During this
search, text between the starting and ending delimiters is copied to a
safe location. The text copied gets delimiter-independent.
If the construct is a here-doc, the ending delimiter is a line that has a
terminating string as the content. Therefore "<<EOF" is
terminated by "EOF" immediately followed by "\n" and
starting from the first column of the terminating line. When searching for
the terminating line of a here-doc, nothing is skipped. In other words,
lines after the here-doc syntax are compared with the terminating string
line by line.
For the constructs except here-docs, single characters are used as starting
and ending delimiters. If the starting delimiter is an opening punctuation
(that is "(", "[", "{", or
"<"), the ending delimiter is the corresponding closing
punctuation (that is ")", "]", "}", or
">"). If the starting delimiter is an unpaired character like
"/" or a closing punctuation, the ending delimiter is same as
the starting delimiter. Therefore a "/" terminates a
"qq//" construct, while a "]" terminates
"qq[]" and "qq]]" constructs.
When searching for single-character delimiters, escaped delimiters and
"\\" are skipped. For example, while searching for terminating
"/", combinations of "\\" and "\/" are
skipped. If the delimiters are bracketing, nested pairs are also skipped.
For example, while searching for closing "]" paired with the
opening "[", combinations of "\\", "\]", and
"\[" are all skipped, and nested "[" and "]"
are skipped as well. However, when backslashes are used as the delimiters
(like "qq\\" and "tr\\\"), nothing is skipped. During
the search for the end, backslashes that escape delimiters are removed
(exactly speaking, they are not copied to the safe location).
For constructs with three-part delimiters ("s///",
"y///", and "tr///"), the search is repeated once
more. If the first delimiter is not an opening punctuation, three
delimiters must be same such as "s!!!" and "tr)))", in
which case the second delimiter terminates the left part and starts the
right part at once. If the left part is delimited by bracketing
punctuation (that is "()", "[]", "{}", or
"<>"), the right part needs another pair of delimiters
such as "s(){}" and "tr[]//". In these cases,
whitespace and comments are allowed between both parts, though the comment
must follow at least one whitespace character; otherwise a character
expected as the start of the comment may be regarded as the starting
delimiter of the right part.
During this search no attention is paid to the semantics of the construct.
Thus:
"$hash{"$foo/$bar"}"
or:
m/
bar # NOT a comment, this slash / terminated m//!
/x
do not form legal quoted expressions. The quoted part ends on the first
""" and "/", and the rest happens to be a syntax
error. Because the slash that terminated "m//" was followed by a
"SPACE", the example above is not "m//x", but rather
"m//" with no "/x" modifier. So the embedded
"#" is interpreted as a literal "#".
Also no attention is paid to "\c\" (multichar control char syntax)
during this search. Thus the second "\" in "qq/\c\/"
is interpreted as a part of "\/", and the following
"/" is not recognized as a delimiter. Instead, use
"\034" or "\x1c" at the end of quoted constructs.
- Interpolation
- The next step is interpolation in the text obtained, which
is now delimiter-independent. There are multiple cases.
- "<<'EOF'"
- No interpolation is performed. Note that the combination
"\\" is left intact, since escaped delimiters are not available
for here-docs.
- "m''", the pattern of "s'''"
- No interpolation is performed at this stage. Any
backslashed sequences including "\\" are treated at the stage to
"parsing regular expressions".
- '', "q//", "tr'''", "y'''",
the replacement of "s'''"
- The only interpolation is removal of "\" from
pairs of "\\". Therefore "-" in "tr'''" and
"y'''" is treated literally as a hyphen and no character range
is available. "\1" in the replacement of "s'''" does
not work as $1.
- "tr///", "y///"
- No variable interpolation occurs. String modifying
combinations for case and quoting such as "\Q", "\U",
and "\E" are not recognized. The other escape sequences such as
"\200" and "\t" and backslashed characters such as
"\\" and "\-" are converted to appropriate literals.
The character "-" is treated specially and therefore
"\-" is treated as a literal "-".
- "", "``", "qq//",
"qx//", "<file*glob>",
"<<"EOF""
- "\Q", "\U", "\u",
"\L", "\l" (possibly paired with "\E") are
converted to corresponding Perl constructs. Thus,
"$foo\Qbaz$bar" is converted to "$foo .
(quotemeta("baz" . $bar))" internally. The other escape
sequences such as "\200" and "\t" and backslashed
characters such as "\\" and "\-" are replaced with
appropriate expansions.
Let it be stressed that whatever falls between
"\Q" and "\E" is
interpolated in the usual way. Something like "\Q\\E" has no
"\E" inside. instead, it has "\Q", "\\", and
"E", so the result is the same as for "\\\\E". As a
general rule, backslashes between "\Q" and "\E" may
lead to counterintuitive results. So, "\Q\t\E" is converted to
"quotemeta("\t")", which is the same as
"\\\t" (since TAB is not alphanumeric). Note also that:
$str = '\t';
return "\Q$str";
may be closer to the conjectural intention of the writer of
"\Q\t\E".
Interpolated scalars and arrays are converted internally to the
"join" and "." catenation operations. Thus, "$foo
XXX '@arr'" becomes:
$foo . " XXX '" . (join $", @arr) . "'";
All operations above are performed simultaneously, left to right.
Because the result of "\Q STRING \E" has all metacharacters
quoted, there is no way to insert a literal "$" or "@"
inside a "\Q\E" pair. If protected by "\",
"$" will be quoted to became "\\\$"; if not, it is
interpreted as the start of an interpolated scalar.
Note also that the interpolation code needs to make a decision on where the
interpolated scalar ends. For instance, whether "a $b -> {c}"
really means:
"a " . $b . " -> {c}";
or:
"a " . $b -> {c};
Most of the time, the longest possible text that does not include spaces
between components and which contains matching braces or brackets. because
the outcome may be determined by voting based on heuristic estimators, the
result is not strictly predictable. Fortunately, it's usually correct for
ambiguous cases.
- the replacement of "s///"
- Processing of "\Q", "\U",
"\u", "\L", "\l", and interpolation happens
as with "qq//" constructs.
It is at this step that "\1" is begrudgingly converted to $1 in
the replacement text of "s///", in order to correct the
incorrigible sed hackers who haven't 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.
- "RE" in "?RE?", "/RE/",
"m/RE/", "s/RE/foo/",
- Processing of "\Q", "\U",
"\u", "\L", "\l", "\E", and
interpolation happens (almost) as with "qq//" constructs.
Processing of "\N{...}" is also done here, and compiled into an
intermediate form for the regex compiler. (This is because, as mentioned
below, the regex compilation may be done at execution time, and
"\N{...}" is a compile-time construct.)
However any other combinations of "\" followed by a character are
not substituted but only skipped, in order to parse them as regular
expressions at the following step. As "\c" is skipped at this
step, "@" of "\c@" in RE is possibly treated as an
array symbol (for example @foo), even though the same text in
"qq//" gives interpolation of "\c@".
Moreover, inside "(?{BLOCK})", "(?# comment )", and a
"#"-comment in a "//x"-regular expression, no
processing is performed whatsoever. This is the first step at which the
presence of the "//x" modifier is relevant.
Interpolation in patterns has several quirks: $|, $(, $), "@+" and
"@-" are not interpolated, and constructs $var[SOMETHING] are
voted (by several different estimators) to be either an array element or
$var followed by an RE alternative. This is where the notation
"${arr[$bar]}" comes handy: "/${arr[0-9]}/" is
interpreted as array element "-9", not as a regular expression
from the variable $arr followed by a digit, which would be the
interpretation of "/$arr[0-9]/". Since voting among different
estimators may occur, the result is not predictable.
The lack of processing of "\\" creates specific restrictions on
the post-processed text. If the delimiter is "/", one cannot get
the combination "\/" into the result of this step. "/"
will finish the regular expression, "\/" will be stripped to
"/" on the previous step, and "\\/" will be left as
is. Because "/" is equivalent to "\/" inside a regular
expression, this does not matter unless the delimiter happens to be
character special to the RE engine, such as in "s*foo*bar*",
"m[foo]", or "?foo?"; or an alphanumeric char, as in:
m m ^ a \s* b mmx;
In the RE above, which is intentionally obfuscated for illustration, the
delimiter is "m", the modifier is "mx", and after
delimiter-removal the RE is the same as for "m/ ^ a \s* b /mx".
There's more than one reason you're encouraged to restrict your delimiters
to non-alphanumeric, non-whitespace choices.
This step is the last one for all constructs except regular expressions, which
are processed further.
- parsing regular expressions
- Previous steps were performed during the compilation of
Perl code, but this one happens at run time, although it may be optimized
to be calculated at compile time if appropriate. After preprocessing
described above, and possibly after evaluation if concatenation, joining,
casing translation, or metaquoting are involved, the resulting
string is passed to the RE engine for compilation.
Whatever happens in the RE engine might be better discussed in perlre, but
for the sake of continuity, we shall do so here.
This is another step where the presence of the "//x" modifier is
relevant. The RE engine scans the string from left to right and converts
it to a finite automaton.
Backslashed characters are either replaced with corresponding literal
strings (as with "\{"), or else they generate special nodes in
the finite automaton (as with "\b"). Characters special to the
RE engine (such as "|") generate corresponding nodes or groups
of nodes. "(?#...)" comments are ignored. All the rest is either
converted to literal strings to match, or else is ignored (as is
whitespace and "#"-style comments if "//x" is
present).
Parsing of the bracketed character class construct, "[...]", is
rather different than the rule used for the rest of the pattern. The
terminator of this construct is found using the same rules as for finding
the terminator of a "{}"-delimited construct, the only exception
being that "]" immediately following "[" is treated as
though preceded by a backslash. Similarly, the terminator of
"(?{...})" is found using the same rules as for finding the
terminator of a "{}"-delimited construct.
It is possible to inspect both the string given to RE engine and the
resulting finite automaton. See the arguments
"debug"/"debugcolor" in the "use re" pragma,
as well as Perl's -Dr command-line switch documented in
"Command Switches" in perlrun.
- Optimization of regular expressions
- This step is listed for completeness only. Since it does
not change semantics, details of this step are not documented and are
subject to change without notice. This step is performed over the finite
automaton that was generated during the previous pass.
It is at this stage that "split()" silently optimizes
"/^/" to mean "/^/m".
I/O Operators¶
There are several I/O operators you should know about.
A string enclosed by backticks (grave accents) first undergoes double-quote
interpolation. It is then interpreted as an external command, and the output
of that command is the value of the backtick string, like in a shell. In
scalar context, a single string consisting of all output is returned. In list
context, a list of values is returned, one per line of output. (You can set $/
to use a different line terminator.) The command is executed each time the
pseudo-literal is evaluated. The status value of the command is returned in $?
(see perlvar for the interpretation of $?). Unlike in
csh, no
translation is done on the return data--newlines remain newlines. Unlike in
any of the shells, single quotes do not hide variable names in the command
from interpretation. To pass a literal dollar-sign through to the shell you
need to hide it with a backslash. The generalized form of backticks is
"qx//". (Because backticks always undergo shell expansion as well,
see perlsec for security concerns.)
In scalar context, evaluating a filehandle in angle brackets yields the next
line from that file (the newline, if any, included), or "undef" at
end-of-file or on error. When $/ is set to "undef" (sometimes known
as file-slurp mode) and the file is empty, it returns '' the first time,
followed by "undef" subsequently.
Ordinarily you must assign the returned value to a variable, but there is one
situation where an automatic assignment happens. If and only if the input
symbol is the only thing inside the conditional of a "while"
statement (even if disguised as a "for(;;)" loop), the value is
automatically assigned to the global variable $_, destroying whatever was
there previously. (This may seem like an odd thing to you, but you'll use the
construct in almost every Perl script you write.) The $_ variable is not
implicitly localized. You'll have to put a "local $_;" before the
loop if you want that to happen.
The following lines are equivalent:
while (defined($_ = <STDIN>)) { print; }
while ($_ = <STDIN>) { print; }
while (<STDIN>) { print; }
for (;<STDIN>;) { print; }
print while defined($_ = <STDIN>);
print while ($_ = <STDIN>);
print while <STDIN>;
This also behaves similarly, but avoids $_ :
while (my $line = <STDIN>) { print $line }
In these loop constructs, the assigned value (whether assignment is automatic or
explicit) is then tested to see whether it is defined. The defined test avoids
problems where line has a string value that would be treated as false by Perl,
for example a "" or a "0" with no trailing newline. If you
really mean for such values to terminate the loop, they should be tested for
explicitly:
while (($_ = <STDIN>) ne '0') { ... }
while (<STDIN>) { last unless $_; ... }
In other boolean contexts, "<filehandle>" without an explicit
"defined" test or comparison elicits a warning if the "use
warnings" pragma or the
-w command-line switch (the $^W variable)
is in effect.
The filehandles STDIN, STDOUT, and STDERR are predefined. (The filehandles
"stdin", "stdout", and "stderr" will also work
except in packages, where they would be interpreted as local identifiers
rather than global.) Additional filehandles may be created with the
open() function, amongst others. See perlopentut and "open"
in perlfunc for details on this.
If a <FILEHANDLE> is used in a context that is looking for a list, a list
comprising all input lines is returned, one line per list element. It's easy
to grow to a rather large data space this way, so use with care.
<FILEHANDLE> may also be spelled "readline(*FILEHANDLE)". See
"readline" in perlfunc.
The null filehandle <> is special: it can be used to emulate the behavior
of
sed and
awk. Input from <> comes either from standard
input, or from each file listed on the command line. Here's how it works: the
first time <> is evaluated, the @ARGV array is checked, and if it is
empty, $ARGV[0] is set to "-", which when opened gives you standard
input. The @ARGV array is then processed as a list of filenames. The loop
while (<>) {
... # code for each line
}
is equivalent to the following Perl-like pseudo code:
unshift(@ARGV, '-') unless @ARGV;
while ($ARGV = shift) {
open(ARGV, $ARGV);
while (<ARGV>) {
... # code for each line
}
}
except that it isn't so cumbersome to say, and will actually work. It really
does shift the @ARGV array and put the current filename into the $ARGV
variable. It also uses filehandle
ARGV internally. <> is just a
synonym for <ARGV>, which is magical. (The pseudo code above doesn't
work because it treats <ARGV> as non-magical.)
Since the null filehandle uses the two argument form of "open" in
perlfunc it interprets special characters, so if you have a script like this:
while (<>) {
print;
}
and call it with "perl dangerous.pl 'rm -rfv *|'", it actually opens a
pipe, executes the "rm" command and reads "rm"'s output
from that pipe. If you want all items in @ARGV to be interpreted as file
names, you can use the module "ARGV::readonly" from CPAN.
You can modify @ARGV before the first <> as long as the array ends up
containing the list of filenames you really want. Line numbers ($.) continue
as though the input were one big happy file. See the example in
"eof" in perlfunc for how to reset line numbers on each file.
If you want to set @ARGV to your own list of files, go right ahead. This sets
@ARGV to all plain text files if no @ARGV was given:
@ARGV = grep { -f && -T } glob('*') unless @ARGV;
You can even set them to pipe commands. For example, this automatically filters
compressed arguments through
gzip:
@ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
If you want to pass switches into your script, you can use one of the Getopts
modules or put a loop on the front like this:
while ($_ = $ARGV[0], /^-/) {
shift;
last if /^--$/;
if (/^-D(.*)/) { $debug = $1 }
if (/^-v/) { $verbose++ }
# ... # other switches
}
while (<>) {
# ... # code for each line
}
The <> symbol will return "undef" for end-of-file only once. If
you call it again after this, it will assume you are processing another @ARGV
list, and if you haven't set @ARGV, will read input from STDIN.
If what the angle brackets contain is a simple scalar variable (e.g.,
<$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 what's within the angle brackets is neither a filehandle nor a simple scalar
variable containing a filehandle name, typeglob, or typeglob reference, it is
interpreted as a filename pattern to be globbed, and either a list of
filenames or the next filename in the list is returned, depending on context.
This distinction is determined on syntactic grounds alone. That means
"<$x>" is always a
readline() from an indirect handle,
but "<$hash{key}>" is always a
glob(). That's because
$x is a simple scalar variable, but $hash{key} is not--it's a hash element.
Even "<$x >" (note the extra space) is treated as
"glob("$x ")", not "readline($x)".
One level of double-quote interpretation is done first, but you can't say
"<$foo>" because that's an indirect filehandle as explained in
the previous paragraph. (In older versions of Perl, programmers would insert
curly brackets to force interpretation as a filename glob:
"<${foo}>". These days, it's considered cleaner to call the
internal function directly as "glob($foo)", which is probably the
right way to have done it in the first place.) For example:
while (<*.c>) {
chmod 0644, $_;
}
is roughly equivalent to:
open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
while (<FOO>) {
chomp;
chmod 0644, $_;
}
except that the globbing is actually done internally using the standard
"File::Glob" extension. Of course, the shortest way to do the above
is:
chmod 0644, <*.c>;
A (file)glob evaluates its (embedded) argument only when it is starting a new
list. All values must be read before it will start over. In list context, this
isn't important because you automatically get them all anyway. However, in
scalar context the operator returns the next value each time it's called, or
"undef" when the list has run out. As with filehandle reads, an
automatic "defined" is generated when the glob occurs in the test
part of a "while", because legal glob returns (e.g. a file called
0) would otherwise terminate the loop. Again, "undef" is
returned only once. So if you're expecting a single value from a glob, it is
much better to say
($file) = <blurch*>;
than
$file = <blurch*>;
because the latter will alternate between returning a filename and returning
false.
If you're trying to do variable interpolation, it's definitely better to use the
glob() function, because the older notation can cause people to become
confused with the indirect filehandle notation.
@files = glob("$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 don't do variable substitution. Backslash interpolation
also happens at compile time. You can say
'Now is the time for all' . "\n" .
'good men to come to.'
and this all reduces to one string internally. Likewise, if you say
foreach $file (@filenames) {
if (-s $file > 5 + 100 * 2**16) { }
}
the compiler will precompute the number which that expression represents so that
the interpreter won't have to.
No-ops¶
Perl doesn't officially have a no-op operator, but the bare constants 0 and 1
are special-cased to not produce a warning in a void context, so you can for
example safely do
1 while foo();
Bitwise String Operators¶
Bitstrings of any size may be manipulated by the bitwise operators ("~ |
& ^").
If the operands to a binary bitwise op are strings of different sizes,
|
and
^ ops act as though the shorter operand had additional zero bits on
the right, while the
& op acts as though the longer operand were
truncated to the length of the shorter. The granularity for such extension or
truncation is one or more bytes.
# ASCII-based examples
print "j p \n" ^ " a h"; # prints "JAPH\n"
print "JA" | " ph\n"; # prints "japh\n"
print "japh\nJunk" & '_____'; # prints "JAPH\n";
print 'p N$' ^ " E<H\n"; # prints "Perl\n";
If you are intending to manipulate bitstrings, be certain that you're supplying
bitstrings: If an operand is a number, that will imply a
numeric
bitwise operation. You may explicitly show which type of operation you intend
by using "" or "0+", as in the examples below.
$foo = 150 | 105; # yields 255 (0x96 | 0x69 is 0xFF)
$foo = '150' | 105; # yields 255
$foo = 150 | '105'; # yields 255
$foo = '150' | '105'; # yields string '155' (under ASCII)
$baz = 0+$foo & 0+$bar; # both ops explicitly numeric
$biz = "$foo" ^ "$bar"; # both ops explicitly stringy
See "vec" in perlfunc for information on how to manipulate individual
bits in a bit vector.
Integer Arithmetic¶
By default, Perl assumes that it must do most of its arithmetic in floating
point. But by saying
use integer;
you may tell the compiler to use integer operations (see integer for a detailed
explanation) from here to the end of the enclosing BLOCK. An inner BLOCK may
countermand this by saying
no integer;
which lasts until the end of that BLOCK. Note that this doesn't mean everything
is an integer, merely that Perl will use integer operations for arithmetic,
comparison, and bitwise operators. For example, even under "use
integer", if you take the sqrt(2), you'll still get 1.4142135623731 or
so.
Used on numbers, the bitwise operators ("&", "|",
"^", "~", "<<", and ">>")
always produce integral results. (But see also "Bitwise String
Operators".) However, "use integer" still has meaning for them.
By default, their results are interpreted as unsigned integers, but if
"use integer" is in effect, their results are interpreted as signed
integers. For example, "~0" usually evaluates to a large integral
value. However, "use integer; ~0" is "-1" on
two's-complement machines.
Floating-point Arithmetic¶
While "use integer" provides integer-only arithmetic, there is no
analogous mechanism to provide automatic rounding or truncation to a certain
number of decimal places. For rounding to a certain number of digits,
sprintf() or
printf() is usually the easiest route. See
perlfaq4.
Floating-point numbers are only approximations to what a mathematician would
call real numbers. There are infinitely more reals than floats, so some
corners must be cut. For example:
printf "%.20g\n", 123456789123456789;
# produces 123456789123456784
Testing for exact floating-point equality or inequality is not a good idea.
Here's a (relatively expensive) work-around to compare whether two
floating-point numbers are equal to a particular number of decimal places. See
Knuth, volume II, for a more robust treatment of this topic.
sub fp_equal {
my ($X, $Y, $POINTS) = @_;
my ($tX, $tY);
$tX = sprintf("%.${POINTS}g", $X);
$tY = sprintf("%.${POINTS}g", $Y);
return $tX eq $tY;
}
The POSIX module (part of the standard perl distribution) implements
ceil(),
floor(), and other mathematical and trigonometric
functions. The Math::Complex module (part of the standard perl distribution)
defines mathematical functions that work on both the reals and the imaginary
numbers. Math::Complex not as efficient as POSIX, but POSIX can't work with
complex numbers.
Rounding in financial applications can have serious implications, and the
rounding method used should be specified precisely. In these cases, it
probably pays not to trust whichever system rounding is being used by Perl,
but to instead implement the rounding function you need yourself.
Bigger Numbers¶
The standard "Math::BigInt", "Math::BigRat", and
"Math::BigFloat" modules, along with the "bigint",
"bigrat", and "bitfloat" pragmas, provide
variable-precision arithmetic and overloaded operators, although they're
currently pretty slow. At the cost of some space and considerable speed, they
avoid the normal pitfalls associated with limited-precision representations.
use 5.010;
use bigint; # easy interface to Math::BigInt
$x = 123456789123456789;
say $x * $x;
+15241578780673678515622620750190521
Or with rationals:
use 5.010;
use bigrat;
$a = 3/22;
$b = 4/6;
say "a/b is ", $a/$b;
say "a*b is ", $a*$b;
a/b is 9/44
a*b is 1/11
Several modules let you calculate with (bound only by memory and CPU time)
unlimited or fixed precision. There are also some non-standard modules that
provide faster implementations via external C libraries.
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.