PERLOP(1) Perl Programmers Reference Guide PERLOP(1)
NAME
perlop - Perl operators and precedence
SYNOPSIS
Perl operators have the following associativity and
precedence, listed from highest precedence to lowest.
Note that all 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 the
overload manpage.
DESCRIPTION
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 the perlfunc
manpage.
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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 the
arguments that follow them, and then act like a simple
TERM with regard to the preceding expression. Note that
you have to 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. See
the section on 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 the section on Quote and Quote-like Operators
toward the end of this section, as well as the section on
I/O Operators.
The Arrow Operator
Just as in C and C++, "->" is an infix dereference
operator. If the right side is either a [...] or {...}
subscript, then the left side must be either a hard or
symbolic reference to an array or hash (or a location
capable of holding a hard reference, if it's an lvalue
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(assignable)). See the perlref manpage.
Otherwise, the right side is a method name or a simple
scalar variable containing the method name, and the left
side must either be an object (a blessed reference) or a
class name (that is, a package name). See the perlobj
manpage.
Auto-increment and Auto-decrement
"++" and "--" work as in C. That is, if placed before a
variable, they increment or decrement the variable before
returning the value, and if placed after, increment or
decrement the variable after returning the value.
The auto-increment operator has a little extra builtin
magic to it. If you increment a variable that is numeric,
or that has ever been used in a numeric context, you get a
normal increment. If, however, the variable has been used
in only string contexts since it was set, and has a value
that is not the empty string and matches the pattern /^[a-
zA-Z]*[0-9]*$/, the increment is done as a string,
preserving each character within its range, with carry:
print ++($foo = '99'); # prints '100'
print ++($foo = 'a0'); # prints 'a1'
print ++($foo = 'Az'); # prints 'Ba'
print ++($foo = 'zz'); # prints 'aaa'
The auto-decrement operator is not magical.
Exponentiation
Binary "**" is the exponentiation operator. Note that it
binds even more tightly than unary minus, so -2**4 is
-(2**4), not (-2)**4. (This is implemented using C's
pow(3) function, which actually works on doubles
internally.)
Symbolic Unary Operators
Unary "!" performs logical negation, i.e., "not". See
also not for a lower precedence version of this.
Unary "-" performs arithmetic negation if the operand is
numeric. If the operand is an identifier, a string
consisting of a minus sign concatenated with the
identifier is returned. Otherwise, if the string starts
with a plus or minus, a string starting with the opposite
sign is returned. One effect of these rules is that
-bareword is equivalent to "-bareword".
Unary "~" performs bitwise negation, i.e., 1's complement.
For example, 0666 &~ 027 is 0640. (See also the section
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on Integer Arithmetic and the section on Bitwise String
Operators.)
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 the section on Terms and List
Operators (Leftward).)
Unary "\" creates a reference to whatever follows it. See
the perlref manpage. 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 interpretation.
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 $_. The return
value indicates the success of the operation. (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. This can be
is less efficient than an explicit search, because the
pattern must be compiled every time the expression is
evaluated.
Binary "!~" is just like "=~" except the return value is
negated in the logical sense.
Multiplicative Operators
Binary "*" multiplies two numbers.
Binary "/" divides two numbers.
Binary "%" computes the modulus of two numbers. Given
integer operands $a and $b: If $b is positive, then $a %
$b is $a minus the largest multiple of $b that is not
greater than $a. If $b is negative, then $a % $b is $a
minus the smallest multiple of $b that is not less than $a
(i.e. the result will be less than or equal to zero).
Note than when use integer is in scope, "%" give you
direct access to the modulus operator as implemented by
your C compiler. This operator is not as well defined for
negative operands, but it will execute faster.
Binary "x" is the repetition operator. In scalar context,
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it returns a string consisting of the left operand
repeated the number of times specified by the right
operand. In list context, if the left operand is a list
in parentheses, it repeats the list.
print '-' x 80; # print row of dashes
print "\t" x ($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 the
section on 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 the
section on Integer Arithmetic.)
Named Unary Operators
The various named unary operators are treated as functions
with one argument, with optional parentheses. These
include the filetest operators, like -f, -M, etc. See the
perlfunc manpage.
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. Examples:
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 ||:
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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)
See also the section on 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.
Binary "eq" returns true if the left argument is
stringwise equal to the right argument.
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Binary "ne" returns true if the left argument is
stringwise not equal to the right argument.
Binary "cmp" returns -1, 0, or 1 depending on whether the
left argument is stringwise less than, equal to, or
greater than the right argument.
"lt", "le", "ge", "gt" and "cmp" use the collation (sort)
order specified by the current locale if use locale is in
effect. See the perllocale manpage.
Bitwise And
Binary "&" returns its operators ANDed together bit by
bit. (See also the section on Integer Arithmetic and the
section on Bitwise String Operators.)
Bitwise Or and Exclusive Or
Binary "|" returns its operators ORed together bit by bit.
(See also the section on Integer Arithmetic and the
section on Bitwise String Operators.)
Binary "^" returns its operators XORed together bit by
bit. (See also the section on Integer Arithmetic and the
section on Bitwise String Operators.)
C-style Logical And
Binary "&&" performs a short-circuit logical AND
operation. That is, if the left operand is false, the
right operand is not even evaluated. Scalar or list
context propagates down to the right operand if it is
evaluated.
C-style Logical Or
Binary "||" performs a short-circuit logical OR operation.
That is, if the left operand is true, the right operand is
not even evaluated. Scalar or list context propagates
down to the right operand if it is evaluated.
The || and && operators differ from C's in that, rather
than returning 0 or 1, they return the last value
evaluated. Thus, a reasonably portable way to find out
the home directory (assuming it's not "0") 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:
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@a = @b || @c; # this is wrong
@a = scalar(@b) || @c; # really meant this
@a = @b ? @b : @c; # this works fine, though
As more readable alternatives to && and || when used for
control flow, Perl provides and and or operators (see
below). The short-circuit behavior is identical. The
precedence of "and" and "or" is much lower, however, so
that you can safely use them after a list operator without
the need for parentheses:
unlink "alpha", "beta", "gamma"
or gripe(), next LINE;
With the C-style operators that would have been written
like this:
unlink("alpha", "beta", "gamma")
|| (gripe(), next LINE);
Use "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 an array of values counting (by ones)
from the left value to the right value. This 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
}
In scalar context, ".." returns a boolean value. The
operator is bistable, like a flip-flop, and emulates the
line-range (comma) operator of sed, awk, and various
editors. Each ".." operator maintains its own boolean
state. It is false as long as its left operand is false.
Once the left operand is true, the range operator stays
true until the right operand is true, AFTER which the
range operator becomes false again. (It doesn't become
false till the next time the range operator is evaluated.
It can test the right operand and become false on the same
evaluation it became true (as in awk), but it still
returns true once. If you don't want it to test the right
operand till the next evaluation (as in sed), use three
dots ("...") instead of two.) The right operand is not
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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 implicitly compared
to the $. variable, the current line number. Examples:
As a scalar operator:
if (101 .. 200) { print; } # print 2nd hundred lines
next line if (1 .. /^$/); # skip header lines
s/^/> / if (/^$/ .. eof()); # quote body
# parse mail messages
while (<>) {
$in_header = 1 .. /^$/;
$in_body = /^$/ .. eof();
# do something based on those
} continue {
close ARGV if eof; # reset $. each file
}
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 the letters of the 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.
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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;
This is not necessarily guaranteed to contribute to the
readability of your program.
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)
Assignment Operators
"=" is the ordinary assignment operator.
Assignment operators work as in C. That is,
$a += 2;
is equivalent to
$a = $a + 2;
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although without duplicating any side effects that
dereferencing the lvalue might trigger, such as from
tie(). Other assignment operators work similarly. The
following are recognized:
**= += *= &= <<= &&=
-= /= |= >>= ||=
.= %= ^=
x=
Note that while these are grouped by family, they all have
the precedence of assignment.
Unlike in C, the assignment operator produces a valid
lvalue. Modifying an assignment is equivalent to doing
the assignment and then modifying the variable that was
assigned to. This is useful for modifying a copy of
something, like this:
($tmp = $global) =~ tr [A-Z] [a-z];
Likewise,
($a += 2) *= 3;
is equivalent to
$a += 2;
$a *= 3;
Comma Operator
Binary "," is the comma operator. In scalar context it
evaluates its left argument, throws that value away, then
evaluates its right argument and returns that value. This
is just like C's comma operator.
In list context, it's just the list argument separator,
and inserts both its arguments into the list.
The => digraph is mostly just a synonym for the comma
operator. It's useful for documenting arguments that come
in pairs. As of release 5.001, it also forces any word to
the left of it to be interpreted as a string.
List Operators (Rightward)
On the right side of a list operator, it has very low
precedence, such that it controls all comma-separated
expressions found there. The only operators with lower
precedence are the logical operators "and", "or", and
"not", which may be used to evaluate calls to list
operators without the need for extra parentheses:
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open HANDLE, "filename"
or die "Can't open: $!\n";
See also discussion of list operators in the section on
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: i.e., the right expression is evaluated only if
the left expression is true.
Logical or and Exclusive Or
Binary "or" returns the logical disjunction of the two
surrounding expressions. It's equivalent to || except for
the very low precedence. This makes it useful for control
flow
print FH $data or die "Can't write to FH: $!";
This means that it short-circuits: i.e., the right
expression is evaluated only if the left expression is
false. Due to its precedence, you should probably avoid
using this for assignment, only for control flow.
$a = $b or $c; # bug: this is wrong
($a = $b) or $c; # really means this
$a = $b || $c; # better written this way
However, when it's a list context assignment and you're
trying to use "||" for control flow, you probably need
"or" so that the assignment takes higher precedence.
@info = stat($file) || die; # oops, scalar sense of stat!
@info = stat($file) or die; # better, now @info gets its due
Then again, you could always use parentheses.
Binary "xor" returns the exclusive-OR of the two
surrounding expressions. It cannot short circuit, of
course.
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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: $, @, %, and
&.)
(TYPE) Type casting operator.
Quote and Quote-like Operators
While we usually think of quotes as literal values, in
Perl they function as operators, providing various kinds
of interpolating and pattern matching capabilities. Perl
provides customary quote characters for these behaviors,
but also provides a way for you to choose your quote
character for any of them. In the following table, a {}
represents any pair of delimiters you choose. Non-
bracketing delimiters use the same character fore and aft,
but the 4 sorts of brackets (round, angle, square, curly)
will all nest.
Customary Generic Meaning Interpolates
'' q{} Literal no
"" qq{} Literal yes
`` qx{} Command yes (unless '' is delimiter)
qw{} Word list no
// m{} Pattern match yes (unless '' is delimiter)
qr{} Pattern yes (unless '' is delimiter)
s{}{} Substitution yes (unless '' is delimiter)
tr{}{} Transliteration no (but see below)
Note that there can be whitespace between the operator and
the quoting characters, except when # is being used as the
quoting character. q#foo# is parsed as being the string
foo, while q #foo# is the operator q followed by a
comment. Its argument will be taken from the next line.
This allows you to write:
s {foo} # Replace foo
{bar} # with bar.
For constructs that do interpolation, variables beginning
with "$" or "@" are interpolated, as are the following
sequences. Within a transliteration, the first ten of
these sequences may be used.
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\t tab (HT, TAB)
\n newline (NL)
\r return (CR)
\f form feed (FF)
\b backspace (BS)
\a alarm (bell) (BEL)
\e escape (ESC)
\033 octal char (ESC)
\x1b hex char (ESC)
\c[ control char
\l lowercase next char
\u uppercase next char
\L lowercase till \E
\U uppercase till \E
\E end case modification
\Q quote non-word characters till \E
If use locale is in effect, the case map used by \l, \L,
\u and \U is taken from the current locale. See the
perllocale manpage.
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 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 a Mac, these are reversed, and on systems
without line terminator, printing "\n" may emit no actual
data. In general, use "\n" when you mean a "newline" for
your system, but use the literal ASCII when you need an
exact character. For example, most networking protocols
expect and prefer a CR+LF ("\012\015" or "\cJ\cM") 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.
You cannot include a literal $ or @ within a \Q sequence.
An unescaped $ or @ interpolates the corresponding
variable, while escaping will cause the literal string \$
to be inserted. You'll need to write something like
m/\Quser\E\@\Qhost/.
Patterns are subject to an additional level of
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 above, there are no multiple levels of
interpolation. In particular, contrary to the
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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.
Most of this section is related to use of regular
expressions from Perl. Such a use may be considered from
two points of view: Perl handles a a string and a
"pattern" to RE (regular expression) engine to match, RE
engine finds (or does not find) the match, and Perl uses
the findings of RE engine for its operation, possibly
asking the engine for other matches.
RE engine has no idea what Perl is going to do with what
it finds, similarly, the rest of Perl has no idea what a
particular regular expression means to RE engine. This
creates a clean separation, and in this section we discuss
matching from Perl point of view only. The other point of
view may be found in the perlre manpage.
?PATTERN?
This is just like the /pattern/ search, except
that it matches only once between calls to the
reset() operator. This is a useful optimization
when you want to see only the first occurrence of
something in each file of a set of files, for
instance. Only ?? patterns local to the current
package are reset.
while (<>) {
if (?^$?) {
# blank line between header and body
}
} continue {
reset if eof; # clear ?? status for next file
}
This usage is vaguely deprecated, and may be
removed in some future version of Perl.
m/PATTERN/cgimosx
/PATTERN/cgimosx
Searches a string for a pattern match, and in
scalar context returns true (1) or false (''). 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
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the =~ binds rather tightly.) See also the perlre
manpage. See the perllocale manpage for
discussion of additional considerations that apply
when use locale is in effect.
Options are:
c Do not reset search position on a failed match when /g is in effect.
g Match globally, i.e., find all occurrences.
i Do case-insensitive pattern matching.
m Treat string as multiple lines.
o Compile pattern only once.
s Treat string as single line.
x Use extended regular expressions.
If "/" is the delimiter then the initial m is
optional. With the m you can use any pair of non-
alphanumeric, non-whitespace characters as
delimiters. This is particularly useful for
matching Unix path names that contain "/", to
avoid LTS (leaning toothpick syndrome). If "?" is
the delimiter, then the match-only-once rule of
?PATTERN? applies. If "'" is the delimiter, no
variable interpolation is performed on the
PATTERN.
PATTERN may contain variables, which will be
interpolated (and the pattern recompiled) every
time the pattern search is evaluated, except for
when the delimiter is a single quote. (Note that
$) and $| might not be interpolated because they
look like end-of-string tests.) If you want such
a pattern to be compiled only once, add a /o after
the trailing delimiter. This avoids expensive
run-time recompilations, and is useful when the
value you are interpolating won't change over the
life of the script. However, mentioning /o
constitutes a promise that you won't change the
variables in the pattern. If you change them,
Perl won't even notice.
If the PATTERN evaluates to the empty string, the
last successfully matched regular expression is
used instead.
If the /g option is not used, m// in a 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.
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Examples:
open(TTY, '/dev/tty');
<TTY> =~ /^y/i && foo(); # do foo if desired
if (/Version: *([0-9.]*)/) { $version = $1; }
next if m#^/usr/spool/uucp#;
# poor man's grep
$arg = shift;
while (<>) {
print if /$arg/o; # compile only once
}
if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
This last example splits $foo into the first two
words and the remainder of the line, and assigns
those three fields to $F1, $F2, and $Etc. The
conditional is true if any variables were
assigned, i.e., if the pattern matched.
The /g modifier specifies global pattern
matching--that is, matching as many times as
possible within the string. How it behaves
depends on the context. In list context, it
returns a list of all the substrings matched by
all the parentheses in the regular expression. If
there are no parentheses, it returns a list of all
the matched strings, as if there were parentheses
around the whole pattern.
In scalar context, each execution of m//g finds
the next match, returning TRUE if it matches, and
FALSE if there is no further match. The position
after the last match can be read or set using the
pos() function; see the pos entry in the perlfunc
manpage. A failed match normally resets the
search position to the beginning of the string,
but you can avoid that by adding the /c modifier
(e.g. m//gc). Modifying the target string also
resets the search position.
You can intermix m//g matches with m/\G.../g,
where \G is a zero-width assertion that matches
the exact position where the previous m//g, if
any, left off. The \G assertion is not supported
without the /g modifier; currently, without /g, \G
behaves just like \A, but that's accidental and
may change in the future.
Examples:
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# list context
($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
# scalar context
{
local $/ = "";
while (defined($paragraph = <>)) {
while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
$sentences++;
}
}
}
print "$sentences\n";
# using m//gc with \G
$_ = "ppooqppqq";
while ($i++ < 2) {
print "1: '";
print $1 while /(o)/gc; print "', pos=", pos, "\n";
print "2: '";
print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
print "3: '";
print $1 while /(p)/gc; print "', pos=", pos, "\n";
}
The last example should print:
1: 'oo', pos=4
2: 'q', pos=5
3: 'pp', pos=7
1: '', pos=7
2: 'q', pos=8
3: '', pos=8
A useful idiom for lex-like scanners is /\G.../gc.
You can combine several regexps like this to
process a string part-by-part, doing different
actions depending on which regexp matched. Each
regexp tries to match where the previous one
leaves off.
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$_ = <<'EOL';
$url = new URI::URL "http://www/"; die if $url eq "xXx";
EOL
LOOP:
{
print(" digits"), redo LOOP if /\G\d+\b[,.;]?\s*/gc;
print(" lowercase"), redo LOOP if /\G[a-z]+\b[,.;]?\s*/gc;
print(" UPPERCASE"), redo LOOP if /\G[A-Z]+\b[,.;]?\s*/gc;
print(" Capitalized"), redo LOOP if /\G[A-Z][a-z]+\b[,.;]?\s*/gc;
print(" MiXeD"), redo LOOP if /\G[A-Za-z]+\b[,.;]?\s*/gc;
print(" alphanumeric"), redo LOOP if /\G[A-Za-z0-9]+\b[,.;]?\s*/gc;
print(" line-noise"), redo LOOP if /\G[^A-Za-z0-9]+/gc;
print ". That's all!\n";
}
Here is the output (split into several lines):
line-noise lowercase line-noise lowercase UPPERCASE line-noise
UPPERCASE line-noise lowercase line-noise lowercase line-noise
lowercase lowercase line-noise lowercase lowercase line-noise
MiXeD line-noise. That's all!
q/STRING/
'STRING'
A single-quoted, literal string. A backslash
represents a backslash unless followed by the
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 /(tcl|rexx|python)/; # :-)
$baz = "\n"; # a one-character string
qr/PATTERN/imosx
Quote-as-a-regular-expression operator. STRING is
interpolated the same way as PATTERN in
m/PATTERN/. If "'" is used as the delimiter, no
variable interpolation is done. Returns a Perl
value which may be used instead of the
corresponding /STRING/imosx expression.
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For example,
$rex = qr/my.STRING/is;
s/$rex/foo/;
is equivalent to
s/my.STRING/foo/is;
The result may be used as a subpattern in a match:
$re = qr/$pattern/;
$string =~ /foo${re}bar/; # can be interpolated in other patterns
$string =~ $re; # or used standalone
$string =~ /$re/; # or this way
Since Perl may compile the pattern at the moment
of execution of qr() operator, using qr() may have
speed advantages in some situations, notably if
the result of qr() is used standalone:
sub match {
my $patterns = shift;
my @compiled = map qr/$_/i, @$patterns;
grep {
my $success = 0;
foreach my $pat @compiled {
$success = 1, last if /$pat/;
}
$success;
} @_;
}
Precompilation of the pattern into an internal
representation at the moment of qr() avoids a need
to recompile the pattern every time a match /$pat/
is attempted. (Note that Perl has many other
internal optimizations, but none would be
triggered in the above example if we did not use
qr() operator.)
Options are:
i Do case-insensitive pattern matching.
m Treat string as multiple lines.
o Compile pattern only once.
s Treat string as single line.
x Use extended regular expressions.
See the perlre manpage for additional information
on valid syntax for STRING, and for a detailed
look at the semantics of regular expressions.
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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. In list context, returns a list of lines
(however you've defined lines with $/ or
$INPUT_RECORD_SEPARATOR).
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 and safest to redirect
them separately to files, and then read from those
files when the program is done:
system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
Using single-quote as a delimiter protects the
command from Perl's double-quote interpolation,
passing it on to the shell instead:
$perl_info = qx(ps $$); # that's Perl's $$
$shell_info = qx'ps $$'; # that's the new shell's $$
Note that how the string gets evaluated is
entirely subject to the command interpreter on
your system. On most platforms, you will have to
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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 the perlsec manpage 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).
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 the section on I/O Operators for more
discussion.
qw/STRING/
Returns a list of the words extracted out of
STRING, using embedded whitespace as the word
delimiters. It is exactly equivalent to
split(' ', q/STRING/);
This equivalency means that if used in scalar
context, you'll get split's (unfortunate) scalar
context behavior, complete with mysterious
warnings. However do not rely on this as in a
future release it could be changed to be exactly
equivalent to the list
('foo', 'bar', 'baz')
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Which in a scalar context would result in '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 -w switch produce
warnings if the STRING contains the "," or the "#"
character.
s/PATTERN/REPLACEMENT/egimosx
Searches a string for a pattern, and if found,
replaces that pattern with the replacement text
and returns the number of substitutions made.
Otherwise it returns false (specifically, the
empty string).
If no string is specified via the =~ or !~
operator, the $_ variable is searched and
modified. (The string specified with =~ must be
scalar variable, an array element, a hash element,
or an assignment to one of those, i.e., an
lvalue.)
If the delimiter chosen is a single quote, no
variable 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 the perlre
manpage for further explanation on these. See the
perllocale manpage for discussion of additional
considerations that apply when use locale is in
effect.
Options are:
e Evaluate the right side as an expression.
g Replace globally, i.e., all occurrences.
i Do case-insensitive pattern matching.
m Treat string as multiple lines.
o Compile pattern only once.
s Treat string as single line.
x Use extended regular expressions.
Any non-alphanumeric, non-whitespace delimiter may
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replace the slashes. If single quotes are used,
no interpretation is done on the replacement
string (the /e modifier overrides this, however).
Unlike Perl 4, Perl 5 treats backticks as normal
delimiters; the replacement text is not evaluated
as a command. If the PATTERN is delimited by
bracketing quotes, the 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
interpreted as a full-fledged Perl expression and
eval()ed right then and there. It is, however,
syntax checked at compile-time.
Examples:
s/\bgreen\b/mauve/g; # don't change wintergreen
$path =~ s|/usr/bin|/usr/local/bin|;
s/Login: $foo/Login: $bar/; # run-time pattern
($foo = $bar) =~ s/this/that/; # copy first, then change
$count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
$_ = 'abc123xyz';
s/\d+/$&*2/e; # yields 'abc246xyz'
s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
s/%(.)/$percent{$1}/g; # change percent escapes; no /e
s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
s/^=(\w+)/&pod($1)/ge; # use function call
# expand variables in $_, but dynamics only, using
# symbolic dereferencing
s/\$(\w+)/${$1}/g;
# /e's can even nest; this will expand
# any embedded scalar variable (including lexicals) in $_
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 white space in $_, expensively
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for ($variable) { # trim white space 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. Here are two common cases:
# put commas in the right places in an integer
1 while s/(.*\d)(\d\d\d)/$1,$2/g; # perl4
1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; # perl5
# expand tabs to 8-column spacing
1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
tr/SEARCHLIST/REPLACEMENTLIST/cds
y/SEARCHLIST/REPLACEMENTLIST/cds
Transliterates all occurrences of the characters
found in the search list with the corresponding
character in the replacement list. It returns the
number of characters replaced or deleted. If no
string is specified via the =~ or !~ operator, the
$_ string is transliterated. (The string
specified with =~ must be a scalar variable, an
array element, a hash element, or an assignment to
one of those, i.e., an lvalue.)
A character range may be specified with a hyphen,
so tr/A-J/0-9/ does the same replacement as
tr/ACEGIBDFHJ/0246813579/. For sed devotees, y is
provided as a synonym for tr. If the SEARCHLIST
is delimited by bracketing quotes, the
REPLACEMENTLIST has its own pair of quotes, which
may or may not be bracketing quotes, e.g., tr[A-
Z][a-z] or tr(+\-*/)/ABCD/.
Note 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.
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Options:
c Complement the SEARCHLIST.
d Delete found but unreplaced characters.
s Squash duplicate replaced characters.
If the /c modifier is specified, the SEARCHLIST
character set is complemented. If the /d modifier
is specified, any characters specified by
SEARCHLIST not found in REPLACEMENTLIST are
deleted. (Note that this is slightly more
flexible than the behavior of some tr programs,
which delete anything they find in the SEARCHLIST,
period.) If the /s modifier is specified,
sequences of characters that were transliterated
to the same character are squashed down to a
single instance of the character.
If the /d modifier is used, the REPLACEMENTLIST is
always interpreted exactly as specified.
Otherwise, if the REPLACEMENTLIST is shorter than
the SEARCHLIST, the final character is replicated
till it is long enough. If the REPLACEMENTLIST is
empty, the SEARCHLIST is replicated. This latter
is useful for counting characters in a class or
for squashing character sequences in a class.
Examples:
$ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
$cnt = tr/*/*/; # count the stars in $_
$cnt = $sky =~ tr/*/*/; # count the stars in $sky
$cnt = tr/0-9//; # count the digits in $_
tr/a-zA-Z//s; # bookkeeper -> bokeper
($HOST = $host) =~ tr/a-z/A-Z/;
tr/a-zA-Z/ /cs; # change non-alphas to single space
tr [\200-\377]
[\000-\177]; # delete 8th bit
If multiple transliterations are given for a
character, only the first one is used:
tr/AAA/XYZ/
will transliterate any A to X.
Note that because the transliteration table is
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built at compile time, neither the SEARCHLIST nor
the REPLACEMENTLIST are subjected to double quote
interpolation. That means that if you want to use
variables, you must use an eval():
eval "tr/$oldlist/$newlist/";
die $@ if $@;
eval "tr/$oldlist/$newlist/, 1" or die $@;
Gory details of parsing quoted constructs
When presented with something which may have several
different interpretations, Perl uses the principle DWIM
(expanded to Do What I Mean - not what I wrote) to pick up
the most probable interpretation of the source. This
strategy is so successful that Perl users usually do not
suspect ambivalence of what they write. However, time to
time Perl's ideas differ from what the author meant.
The target of this section is to clarify the Perl's way of
interpreting quoted constructs. The most frequent reason
one may have to want to know the details discussed in this
section is hairy regular expressions. However, the first
steps of parsing are the same for all Perl quoting
operators, so here they are discussed together.
The most important detail of Perl parsing rules is the
first one discussed below; when processing a quoted
construct, Perl first finds the end of the construct, then
it interprets the contents of the construct. If you
understand this rule, you may skip the rest of this
section on the first reading. The other rules would
contradict user's expectations much less frequently than
the first one.
Some of the passes discussed below are performed
concurrently, but as far as results are the same, we
consider them one-by-one. For different quoting
constructs Perl performs different number of passes, from
one to five, but they are always performed in the same
order.
Finding the end
First pass is finding the end of the quoted
construct, be it a multichar delimiter "\nEOF\n" of
<<EOF construct, / which terminates qq/ construct, ]
which terminates qq[ construct, or > which terminates
a fileglob started with <.
When searching for one-char non-matching delimiter,
such as /, combinations \\ and \/ are skipped. When
searching for one-char matching delimiter, such as ],
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combinations \\, \] and \[ are skipped, and nested [,
] are skipped as well. When searching for multichar
delimiter no skipping is performed.
For constructs with 3-part delimiters (s/// etc.) the
search is repeated once more.
During this search no attention is paid to the
semantic of the construct, thus:
"$hash{"$foo/$bar"}"
or:
m/
bar # NOT a comment, this slash / terminated m//!
/x
do not form legal quoted expressions, the quoted part
ends on the first " and /, and the rest happens to be
a syntax error. Note that since the slash which
terminated m// was followed by a SPACE, the above is
not m//x, but rather m// with no 'x' switch. So the
embedded # is interpreted as a literal #.
Removal of backslashes before delimiters
During the second pass the text between the starting
delimiter and the ending delimiter is copied to a
safe location, and the \ is removed from combinations
consisting of \ and delimiter(s) (both starting and
ending delimiter if they differ).
The removal does not happen for multi-char
delimiters.
Note that the combination \\ is left as it was!
Starting from this step no information about the
delimiter(s) is used in the parsing.
Interpolation
Next step is interpolation in the obtained delimiter-
independent text. There are four different cases.
<<'EOF', m'', s''', tr///, y///
No interpolation is performed.
'', q// The only interpolation is removal of \ from
pairs \\.
", ``, qq//, qx//, <file*glob>
\Q, \U, \u, \L, \l (possibly paired with \E) are
converted to corresponding Perl constructs, thus
"$foo\Qbaz$bar" is converted to :
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$foo . (quotemeta("baz" . $bar));
Other combinations of \ with following chars are
substituted with appropriate expansions.
Let it be stressed that whatever is between \Q
and \E is interpolated in the usual way. Say,
"\Q\\E" has no \E inside: it has \Q, \\, and E,
thus the result is the same as for "\\\\E".
Generally speaking, having 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
alphanumerical). 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 internally
converted to the join and . Perl operations,
thus "$foo >> '@arr'"> becomes:
$foo . " >>> '" . (join $", @arr) . "'";
All the operations in the above are performed
simultaneously left-to-right.
Since the result of "\Q STRING \E" has all the
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 starting an
interpolated scalar.
Note also that the interpolating code needs to
make a decision on where the interpolated scalar
ends. For instance, whether "a $b -> {c}" means:
"a " . $b . " -> {c}";
or:
"a " . $b -> {c};
Most of the time the decision is to take the
longest possible text which does not include
spaces between components and contains matching
braces/brackets. Since the outcome may be
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determined by voting based on heuristic
estimators, the result is not strictly
predictable, but is usually correct for the
ambiguous cases.
?RE?, /RE/, m/RE/, s/RE/foo/,
Processing of \Q, \U, \u, \L, \l and
interpolation happens (almost) as with qq//
constructs, but the substitution of \ followed
by RE-special chars (including \) is not
performed! Moreover, inside (?{BLOCK}), (?#
comment ), and #-comment of //x-regular
expressions no processing is performed at all.
This is the first step where presence of the //x
switch is relevant.
Interpolation has several quirks: $|, $( and $)
are not interpolated, and constructs
$var[SOMETHING] are voted (by several different
estimators) to be an array element or $var
followed by a RE alternative. This is the place
where the notation ${arr[$bar]} comes handy:
/${arr[0-9]}/ is interpreted as an array element
-9, not as a regular expression from variable
$arr followed by a digit, which is the
interpretation of /$arr[0-9]/. Since voting
among different estimators may be performed, the
result is not predictable.
It is on this step that \1 is converted to $1 in
the replacement text of s///.
Note that absence 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. Since / is equivalent to \/
inside a regular expression, this does not
matter unless the delimiter is a special
character for the RE engine, as in s*foo*bar*,
m[foo], or ?foo?, or an alphanumeric char, as
in:
m m ^ a \s* b mmx;
In the above RE, which is intentionally
obfuscated for illustration, the delimiter is m,
the modifier is mx, and after backslash-removal
the RE is the same as for m/ ^ a s* b /mx).
This step is the last one for all the constructs
except regular expressions, which are processed
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further.
Interpolation of regular expressions
All the previous steps were performed during the
compilation of Perl code, this one happens in run
time (though it may be optimized to be calculated at
compile time if appropriate). After all the
preprocessing performed above (and possibly after
evaluation if catenation, joining, up/down-casing and
quotemeta()ing are involved) the resulting string is
passed to RE engine for compilation.
Whatever happens in the RE engine is better be
discussed in the perlre manpage, but for the sake of
continuity let us do it here.
This is another step where presence of the //x switch
is relevant. The RE engine scans the string left-to-
right, and converts it to a finite automaton.
Backslashed chars are either substituted by
corresponding literal strings (as with \{), or
generate special nodes of the finite automaton (as
with \b). Characters which are 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 is ignored (as is whitespace and #-style
comments if //x is present).
Note that the parsing of the construct [...] is
performed using rather different rules than for the
rest of the regular expression. The terminator of
this construct is found using the same rules as for
finding a terminator of a {}-delimited construct, the
only exception being that ] immediately following [
is considered as if preceded by a backslash.
Similarly, the terminator of (?{...}) is found using
the same rules as for finding a terminator of a
{}-delimited construct.
It is possible to inspect both the string given to RE
engine, and the resulting finite automaton. See
arguments debug/debugcolor of use the re manpage
directive, and/or -Dr option of Perl in the Switches
entry in the perlrun manpage.
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. This step
is performed over the finite automaton generated
during the previous pass.
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However, in older versions of Perl the split manpage
used to silently optimize /^/ to mean /^/m. This
behaviour, though present in current versions of
Perl, may be deprecated in future.
I/O Operators
There are several I/O operators you should know about.
A string enclosed by backticks (grave accents) first
undergoes variable substitution just like a double quoted
string. It is then interpreted as a command, and the
output of that command is the value of the pseudo-literal,
like in a shell. In scalar context, a single string
consisting of all the output is returned. In list
context, a list of values is returned, one for each 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 the perlvar manpage 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 $
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 the
perlsec manpage for security concerns.)
In a scalar context, evaluating a filehandle in angle
brackets yields the next line from that file (newline, if
any, included), or undef at end-of-file. When $/ is set
to undef (i.e. 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 or
for(;;) loop, the value is automatically assigned to the
variable $_. In these loop constructs, the assigned value
(whether assignment is automatic or explicit) is then
tested to see if it is defined. The defined test avoids
problems where line has a string value that would be
treated as false by perl e.g. "" or "0" with no trailing
newline. (This may seem like an odd thing to you, but
you'll use the construct in almost every Perl script you
write.) Anyway, the following lines are equivalent to each
other:
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while (defined($_ = <STDIN>)) { print; }
while ($_ = <STDIN>) { print; }
while (<STDIN>) { print; }
for (;<STDIN>;) { print; }
print while defined($_ = <STDIN>);
print while ($_ = <STDIN>);
print while <STDIN>;
and this also behaves similarly, but avoids the use of $_
:
while (my $line = <STDIN>) { print $line }
If you really mean such values to terminate the loop they
should be tested for explicitly:
while (($_ = <STDIN>) ne '0') { ... }
while (<STDIN>) { last unless $_; ... }
In other boolean contexts, <filehandle> without explicit
defined test or comparison will solicit a warning if -w 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. See
the open entry in the perlfunc manpage for details on
this.
If a <FILEHANDLE> is used in a context that is looking for
a list, a list consisting of all the input lines is
returned, one line per list element. It's easy to make a
LARGE data space this way, so use with care.
<FILEHANDLE> may also be spelt readline(FILEHANDLE). See
the readline entry in the perlfunc manpage.
The null filehandle <> is special and 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, $ARGV[0] is set to "-", which when opened gives you
standard input. The @ARGV array is then processed as a
list of filenames. The loop
while (<>) {
... # code for each line
}
is equivalent to the following Perl-like pseudo code:
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unshift(@ARGV, '-') unless @ARGV;
while ($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 array @ARGV and put
the current filename into variable $ARGV. It also uses
filehandle ARGV internally--<> is just a synonym for
<ARGV>, which is magical. (The pseudo code above doesn't
work because it treats <ARGV> as non-magical.)
You can modify @ARGV before the first <> as long as the
array ends up containing the list of filenames you really
want. Line numbers ($.) continue as if the input were
one big happy file. (But see example under eof 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/) { $verbose++ }
# ... # other switches
}
while (<>) {
# ... # code for each line
}
The <> symbol will return undef for end-of-file only once.
If you call it again after this it will assume you are
processing another @ARGV list, and if you haven't set
@ARGV, will input from STDIN.
If the string inside the angle brackets is a reference to
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a scalar variable (e.g., <$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 $hash{key} is not--it's a hash
element.
One level of double-quote interpretation is done first,
but you can't say <$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.) Example:
while (<*.c>) {
chmod 0644, $_;
}
is equivalent to
open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
while (<FOO>) {
chop;
chmod 0644, $_;
}
In fact, it's currently implemented that way. (Which
means it will not work on filenames with spaces in them
unless you have csh(1) on your machine.) Of course, the
shortest way to do the above is:
chmod 0644, <*.c>;
Because globbing invokes a shell, it's often faster to
call readdir() yourself and do your own grep() on the
filenames. Furthermore, due to its current implementation
of using a shell, the glob() routine may get "Arg list too
long" errors (unless you've installed tcsh(1L) as
/bin/csh).
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A glob evaluates its (embedded) argument only when it is
starting a new list. All values must be read before it
will start over. In a list context this isn't important,
because you automatically get them all anyway. In scalar
context, however, the operator returns the next value each
time it is called, or a undef value if you've just run
out. As for filehandles an automatic defined is generated
when the glob occurs in the test part of a while or for -
because legal glob returns (e.g. a file called 0) would
otherwise terminate the loop. Again, undef is returned
only once. So if you're expecting a single value from a
glob, it is much better to say
($file) = <blurch*>;
than
$file = <blurch*>;
because the latter will alternate between returning a
filename and returning FALSE.
It you're trying to do variable interpolation, it's
definitely better to use the glob() function, because the
older notation can cause people to become confused with
the indirect filehandle notation.
@files = glob("$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 interpretation 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 $file > 5 + 100 * 2**16) { }
}
the compiler will precompute the number that expression
represents so that the interpreter won't have to.
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Bitwise String Operators
Bitstrings of any size may be manipulated by the bitwise
operators (~ | & ^).
If the operands to a binary bitwise op are strings of
different sizes, | and ^ ops will act as if the shorter
operand had additional zero bits on the right, while the &
op will act as if the longer operand were truncated to the
length of the shorter. Note that the granularity for such
extension or truncation is one or more bytes.
# ASCII-based examples
print "j p \n" ^ " a h"; # prints "JAPH\n"
print "JA" | " ph\n"; # prints "japh\n"
print "japh\nJunk" & '_____'; # prints "JAPH\n";
print 'p N$' ^ " E<H\n"; # prints "Perl\n";
If you are intending to manipulate bitstrings, you should
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" ^ "$bar"; # both ops explicitly stringy
See the vec entry in the perlfunc manpage for information
on how to manipulate individual bits in a bit vector.
Integer Arithmetic
By default Perl assumes that it must do most of its
arithmetic in floating point. But by saying
use integer;
you may tell the compiler that it's okay to use integer
operations from here to the end of the enclosing BLOCK.
An inner BLOCK may countermand this by saying
no integer;
which lasts until the end of that BLOCK.
The bitwise operators ("&", "|", "^", "~", "<<", and ">>")
always produce integral results. (But see also the
section on Bitwise String Operators.) However, use
integer still has meaning for them. By default, their
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results are interpreted as unsigned integers. However, if
use integer is in effect, their results are interpreted as
signed integers. For example, ~0 usually evaluates to a
large integral value. However, use integer; ~0 is -1 on
twos-complement machines.
Floating-point Arithmetic
While use integer provides integer-only arithmetic, there
is no similar ways to provide rounding or truncation at a
certain number of decimal places. For rounding to a
certain number of digits, sprintf() or printf() is usually
the easiest route.
Floating-point numbers are only approximations to what a
mathematician would call real numbers. There are
infinitely more reals than floats, so some corners must be
cut. For example:
printf "%.20g\n", 123456789123456789;
# produces 123456789123456784
Testing for exact equality of floating-point equality or
inequality is not a good idea. Here's a (relatively
expensive) work-around to compare whether two floating-
point numbers are equal to a particular number of decimal
places. See Knuth, volume II, for a more robust treatment
of this topic.
sub fp_equal {
my ($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 a number of other
mathematical and trigonometric functions. The
Math::Complex module (part of the standard perl
distribution) defines a number of mathematical functions
that can also work on real 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.
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Bigger Numbers
The standard Math::BigInt and Math::BigFloat modules
provide variable precision arithmetic and overloaded
operators. At the cost of some space and considerable
speed, they avoid the normal pitfalls associated with
limited-precision representations.
use Math::BigInt;
$x = Math::BigInt->new('123456789123456789');
print $x * $x;
# prints +15241578780673678515622620750190521
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Source: OpenBSD 2.6 man pages. Copyright: Portions are copyrighted by BERKELEY
SOFTWARE DESIGN, INC., The Regents of the University of California, Massachusetts
Institute of Technology, Free Software Foundation, FreeBSD Inc., and others. |
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