PERLOP(1) Perl Programmers Reference Guide PERLOP(1)NAMEperlop - Perl operators and precedence
SYNOPSIS
Perl operators have the following associativity and prece
dence, listed from highest precedence to lowest. Opera
tors 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 val
ues 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 behav
ing as functions because you put parentheses around the
arguments. These are all documented in the perlfunc man
page.
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 argu
ments 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. See
the Named Unary Operators entry elsewhere in this document
for more discussion of this.
Also parsed as terms are the "do {}" and "eval {}" con
structs, as well as subroutine and method calls, and the
anonymous constructors "[]" and "{}".
See also the Quote and Quote-like Operators entry else
where in this document toward the end of this section, as
well as the section on "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 assign
ment.) See the perlreftut manpage and the perlref man
page.
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 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]*\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'
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. If the operand is an identifier, a string con
sisting 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 Integer
Arithmetic entry elsewhere in this document and the Bit
wise String Operators entry elsewhere in this document.)
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 use the & operator to mask off the
excess bits.
Unary "+" has no effect whatsoever, even on strings. It
is useful syntactically for separating a function name
from a parenthesized expression that would otherwise be
interpreted as the complete list of function arguments.
(See examples above under the Terms and List Operators
(Leftward) entry elsewhere in this document.)
Unary "\" creates a reference to whatever follows it. See
the perlreftut manpage and 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 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 pat
tern, substitution, or transliteration. The left argument
is what is supposed to be searched, substituted, or
transliterated instead of the default $_. When used in
scalar context, the return value generally indicates the
success of the operation. Behavior in list context
depends on the particular operator. See the Regexp Quote-
Like Operators entry elsewhere in this document for
details.
If the right argument is an expression rather than a
search pattern, substitution, or transliteration, it is
interpreted as a search pattern at run time. This can be
less efficient than an explicit search, because the pat
tern must be compiled every time the expression is evalu
ated.
Binary "!~" is just like "=~" except the return value is
negated in the logical sense.
Multiplicative Operators
Binary "*" multiplies two numbers.
Binary "/" divides two numbers.
Binary "%" computes the modulus of two numbers. Given
integer operands "$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,
"%" gives you direct access to the modulus operator as
implemented by your C compiler. This operator is not as
well defined for negative operands, but it will execute
faster.
Binary "x" is the repetition operator. In scalar context
or if the left operand is not enclosed in parentheses, it
returns a string consisting of the left operand repeated
the number of times specified by the right operand. In
list context, if the left operand is enclosed in parenthe
ses, 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 argu
ment. Arguments should be integers. (See also the Inte
ger Arithmetic entry elsewhere in this document.)
Binary ">>" returns the value of its left argument shifted
right by the number of bits specified by the right argu
ment. Arguments should be integers. (See also the Inte
ger Arithmetic entry elsewhere in this document.)
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. 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 ($foo * 20)
rand 10 * 20; # rand (10 * 20)
rand(10) * 20; # (rand 10) * 20
rand (10) * 20; # (rand 10) * 20
rand +(10) * 20; # rand (10 * 20)
See also the section on "Terms and List Operators (Left
ward)".
Relational Operators
Binary "<" returns true if the left argument is numeri
cally less than the right argument.
Binary ">" returns true if the left argument is numeri
cally greater than the right argument.
Binary "<=" returns true if the left argument is numeri
cally less than or equal to the right argument.
Binary ">=" returns true if the left argument is numeri
cally greater than or equal to the right argument.
Binary "lt" returns true if the left argument is string
wise less than the right argument.
Binary "gt" returns true if the left argument is string
wise greater than the right argument.
Binary "le" returns true if the left argument is string
wise less than or equal to the right argument.
Binary "ge" returns true if the left argument is string
wise greater than or equal to the right argument.
Equality Operators
Binary "==" returns true if the left argument is numeri
cally equal to the right argument.
Binary "!=" returns true if the left argument is numeri
cally not equal to the right argument.
Binary "<=>" returns -1, 0, or 1 depending on whether the
left argument is numerically less than, equal to, or
greater than the right argument. If your platform sup
ports NaNs (not-a-numbers) as numeric values, using them
with "<=>" returns undef. NaN is not "<", "==", ">", "<="
or ">=" anything (even NaN), so those 5 return false. NaN
!= NaN returns true, as does NaN != anything else. If your
platform doesn't support NaNs then NaN is just a string
with numeric value 0.
perl -le '$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 string
wise equal to the right argument.
Binary "ne" returns true if the left argument is string
wise 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 Integer Arithmetic entry elsewhere in
this document and the Bitwise String Operators entry else
where in this document.)
Bitwise Or and Exclusive Or
Binary "|" returns its operators ORed together bit by bit.
(See also the Integer Arithmetic entry elsewhere in this
document and the Bitwise String Operators entry elsewhere
in this document.)
Binary "^" returns its operators XORed together bit by
bit. (See also the Integer Arithmetic entry elsewhere in
this document and the Bitwise String Operators entry else
where in this document.)
C-style Logical And
Binary "&&" performs a short-circuit logical AND opera
tion. 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:
@a = @b || @c; # this is wrong
@a = scalar(@b) || @c; # really meant this
@a = @b ? @b : @c; # this works fine, though
As more readable alternatives to "&&" and "||" when used
for control flow, Perl provides "and" and "or" operators
(see below). The short-circuit behavior is identical.
The precedence of "and" and "or" is much lower, however,
so that you can safely use them after a list operator
without the need for parentheses:
unlink "alpha", "beta", "gamma"
or gripe(), next LINE;
With the C-style operators that would have been written
like this:
unlink("alpha", "beta", "gamma")
|| (gripe(), next LINE);
Using "or" for assignment is unlikely to do what you want;
see below.
Range Operators
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 (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 array. 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 expres
sion 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 edi
tors. Each ".." operator maintains its own boolean state.
It is false as long as its left operand is false. Once
the left operand is true, the range operator stays true
until the right operand is true, AFTER which the range
operator becomes false again. It doesn't become false
till the next time the range operator is evaluated. It
can test the right operand and become false on the same
evaluation it became true (as in awk), but it still
returns true once. If you don't want it to test the right
operand till the next evaluation, as in sed, just use
three dots ("...") instead of two. In all other regards,
"..." behaves just like ".." does.
The right operand is not evaluated while the operator is
in the "false" state, and the left operand is not evalu
ated 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 end
point. 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 mag
ical auto-increment algorithm if the operands are strings.
You can say
@alphabet = ('A' .. 'Z');
to get all normal 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 spec
ified 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.
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 = $a + 2;
although without duplicating any side effects that deref
erencing the lvalue might trigger, such as from tie().
Other assignment operators work similarly. The following
are recognized:
**= += *= &= <<= &&=
-= /= |= >>= ||=
.= %= ^=
x=
Although these are grouped by family, they all have the
precedence of assignment.
Unlike in C, the scalar assignment operator produces a
valid lvalue. Modifying an assignment is equivalent to
doing the assignment and then modifying the variable that
was assigned to. This is useful for modifying a copy of
something, like this:
($tmp = $global) =~ tr [A-Z] [a-z];
Likewise,
($a += 2) *= 3;
is equivalent to
$a += 2;
$a *= 3;
Similarly, a list assignment in list context produces the
list of lvalues assigned to, and a list assignment in
scalar context returns the number of elements produced by
the expression on the right hand side of the assignment.
Comma Operator
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 opera
tors without the need for extra parentheses:
open HANDLE, "filename"
or die "Can't open: $!\n";
See also discussion of list operators in the Terms and
List Operators (Leftward) entry elsewhere in this docu
ment.
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-cir
cuits: 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 expres
sion 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 surround
ing 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 char
acter 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 (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)
Non-bracketing delimiters use the same character fore and
aft, but the four sorts of brackets (round, angle, square,
curly) will all nest, which means that
q{foo{bar}baz}
is the same as
'foo{bar}baz'
Note, however, that this does not always work for quoting
Perl code:
$s = q{ if($a eq "}") ... }; # WRONG
is a syntax error. The "Text::Balanced" module on CPAN is
able to do this properly.
There can be whitespace between the operator and the quot
ing characters, except when "#" is being used as the quot
ing 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.
For constructs that do interpolate, variables beginning
with ""$"" or ""@"" are interpolated, as are the following
escape sequences. Within a transliteration, the first
eleven of these sequences may be used.
\t tab (HT, TAB)
\n newline (NL)
\r return (CR)
\f form feed (FF)
\b backspace (BS)
\a alarm (bell) (BEL)
\e escape (ESC)
\033 octal char (ESC)
\x1b hex char (ESC)
\x{263a} wide hex char (SMILEY)
\c[ control char (ESC)
\N{name} named char
\l lowercase next char
\u uppercase next char
\L lowercase till \E
\U uppercase till \E
\E end case modification
\Q quote non-word characters till \E
If "use locale" is in effect, the case map used by "\l",
"\L", "\u" and "\U" is taken from the current locale. See
the perllocale manpage. For documentation of "\N{name}",
see the charnames manpage.
All systems use the virtual ""\n"" to represent a line
terminator, called a "newline". There is no such thing as
an unvarying, physical newline character. It is only an
illusion that the operating system, device drivers, C
libraries, and Perl all conspire to preserve. Not all
systems read ""\r"" as ASCII CR and ""\n"" as ASCII LF.
For example, on a Mac, these are reversed, and on systems
without line terminator, printing ""\n"" may emit no
actual data. In general, use ""\n"" when you mean a "new
line" for your system, but use the literal ASCII when you
need an exact character. For example, most networking
protocols expect and prefer a CR+LF (""\015\012"" or
""\cM\cJ"") for line terminators, and although they often
accept just ""\012"", they seldom tolerate just ""\015"".
If you get in the habit of using ""\n"" for networking,
you may be burned some day.
You cannot include a literal "$" or "@" within a "\Q"
sequence. An unescaped "$" or "@" interpolates the corre
sponding variable, while escaping will cause the literal
string "\$" to be inserted. You'll need to write some
thing like "m/\Quser\E\@\Qhost/".
Patterns are subject to an additional level of interpreta
tion as a regular expression. This is done as a second
pass, after variables are interpolated, so that regular
expressions may be incorporated into the pattern from the
variables. If this is not what you want, use "\Q" to
interpolate a variable literally.
Apart from the behavior described above, Perl does not
expand multiple levels of interpolation. In particular,
contrary to the expectations of shell programmers, back-
quotes do NOT interpolate within double quotes, nor do
single quotes impede evaluation of variables when used
within double quotes.
Regexp Quote-Like Operators
Here are the quote-like operators that apply to pattern
matching and related activities.
?PATTERN?
This is just like the "/pattern/" search, except
that it matches only once between calls to the
reset() operator. This is a useful optimization
when you want to see only the first occurrence of
something in each file of a set of files, for
instance. Only "??" patterns local to the cur
rent package are reset.
while (<>) {
if (?^$?) {
# blank line between header and body
}
} continue {
reset if eof; # clear ?? status for next file
}
This usage is vaguely deprecated, which means it
just might possibly be removed in some distant
future version of Perl, perhaps somewhere around
the year 2168.
m/PATTERN/cgimosx
/PATTERN/cgimosx
Searches a string for a pattern match, and in
scalar context returns true if it succeeds, false
if it fails. If no string is specified via the
"=~" or "!~" operator, the $_ string is searched.
(The string specified with "=~" need not be an
lvalue--it may be the result of an expression
evaluation, but remember the "=~" binds rather
tightly.) See also the perlre manpage. See the
perllocale manpage for discussion of additional
considerations that apply when "use locale" is in
effect.
Options are:
c Do not reset search position on a failed match when /g is in effect.
g Match globally, i.e., find all occurrences.
i Do case-insensitive pattern matching.
m Treat string as multiple lines.
o Compile pattern only once.
s Treat string as single line.
x Use extended regular expressions.
If "/" is the delimiter then the initial "m" is
optional. With the "m" you can use any pair of
non-alphanumeric, non-whitespace characters as
delimiters. This is particularly useful for
matching path names that contain "/", to avoid LTS
(leaning toothpick syndrome). If "?" is the
delimiter, then the match-only-once rule of "?PAT
TERN?" applies. If "'" is the delimiter, no
interpolation is performed on the PATTERN.
PATTERN may contain variables, which will be
interpolated (and the pattern recompiled) every
time the pattern search is evaluated, except for
when the delimiter is a single quote. (Note that
"$(", "$)", and "$|" are not interpolated because
they look like end-of-string tests.) If you want
such a pattern to be compiled only once, add a
"/o" after the trailing delimiter. This avoids
expensive run-time recompilations, and is useful
when the value you are interpolating won't change
over the life of the script. However, mentioning
"/o" constitutes a promise that you won't change
the variables in the pattern. If you change them,
Perl won't even notice. See also the section on
"qr/STRING/imosx".
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 list con
text returns a list consisting of the subexpres
sions matched by the parentheses in the pattern,
i.e., ("$1", "$2", "$3"...). (Note that here "$1"
etc. are also set, and that this differs from Perl
4's behavior.) When there are no parentheses in
the pattern, the return value is the list "(1)"
for success. With or without parentheses, an
empty list is returned upon failure.
Examples:
open(TTY, '/dev/tty');
<TTY> =~ /^y/i && foo(); # do foo if desired
if (/Version: *([0-9.]*)/) { $version = $1; }
next if m#^/usr/spool/uucp#;
# poor man's grep
$arg = shift;
while (<>) {
print if /$arg/o; # compile only once
}
if (($F1, $F2, $Etc) = ($foo =~ /^(\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 $Etc. The
conditional is true if any variables were
assigned, i.e., if the pattern matched.
The "/g" modifier specifies global pattern match
ing--that is, matching as many times as possible
within the string. How it behaves depends on the
context. In list context, it returns a list of
the substrings matched by any capturing parenthe
ses in the regular expression. If there are no
parentheses, it returns a list of all the matched
strings, as if there were parentheses around the
whole pattern.
In scalar context, each execution of "m//g" finds
the next match, returning true if it matches, and
false if there is no further match. The position
after the last match can be read or set using the
pos() function; see the pos entry in the perlfunc
manpage. A failed match normally resets the
search position to the beginning of the string,
but you can avoid that by adding the "/c" modifier
(e.g. "m//gc"). Modifying the target string also
resets the search position.
You can intermix "m//g" matches with "m/\G.../g",
where "\G" is a zero-width assertion that matches
the exact position where the previous "m//g", if
any, left off. Without the "/g" modifier, the
"\G" assertion still anchors at pos(), but the
match is of course only attempted once. Using
"\G" without "/g" on a target string that has not
previously had a "/g" match applied to it is the
same as using the "\A" assertion to match the
beginning of the string.
Examples:
# list context
($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
# scalar context
$/ = "";
while (defined($paragraph = <>)) {
while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
$sentences++;
}
}
print "$sentences\n";
# using m//gc with \G
$_ = "ppooqppqq";
while ($i++ < 2) {
print "1: '";
print $1 while /(o)/gc; print "', pos=", pos, "\n";
print "2: '";
print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
print "3: '";
print $1 while /(p)/gc; print "', pos=", pos, "\n";
}
print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
The last example should print:
1: 'oo', pos=4
2: 'q', pos=5
3: 'pp', pos=7
1: '', pos=7
2: 'q', pos=8
3: '', pos=8
Final: 'q', pos=8
Notice that the final match matched "q" instead of
"p", which a match without the "\G" anchor would
have done. Also note that the final match did not
update "pos" -- "pos" is only updated on a "/g"
match. If the final match did indeed match "p",
it's a good bet that you're running an older
(pre-5.6.0) Perl.
A useful idiom for "lex"-like scanners is
"/\G.../gc". You can combine several regexps like
this to process a string part-by-part, doing dif
ferent actions depending on which regexp matched.
Each regexp tries to match where the previous one
leaves off.
$_ = <<'EOL';
$url = new URI::URL "http://www/"; die if $url eq "xXx";
EOL
LOOP:
{
print(" digits"), redo LOOP if /\G\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 rep
resents a backslash unless followed by the delim
iter 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
qr/STRING/imosx
This operator quotes (and possibly compiles) its
STRING as a regular expression. STRING is inter
polated the same way as PATTERN in "m/PATTERN/".
If "'" is used as the delimiter, no interpolation
is done. Returns a Perl value which may be used
instead of the corresponding "/STRING/imosx"
expression.
For example,
$rex = qr/my.STRING/is;
s/$rex/foo/;
is equivalent to
s/my.STRING/foo/is;
The result may be used as a subpattern in a match:
$re = qr/$pattern/;
$string =~ /foo${re}bar/; # can be interpolated in other patterns
$string =~ $re; # or used standalone
$string =~ /$re/; # or this way
Since Perl may compile the pattern at the moment
of execution of qr() operator, using qr() may have
speed advantages in some situations, notably if
the result of qr() is used standalone:
sub match {
my $patterns = shift;
my @compiled = map qr/$_/i, @$patterns;
grep {
my $success = 0;
foreach my $pat (@compiled) {
$success = 1, last if /$pat/;
}
$success;
} @_;
}
Precompilation of the pattern into an internal
representation at the moment of qr() avoids a need
to recompile the pattern every time a match
"/$pat/" is attempted. (Perl has many other
internal optimizations, but none would be trig
gered 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.
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 redirec
tions 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 STD
OUT (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 $$'; # that's the new shell's $$
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 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 charac
ter, 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 the perlport manpage). 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 restric
tions 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 back
ticks 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 dis
cussion.
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 difference being that it generates a real list
at compile time. 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 "$^W" vari
able) produces warnings if the STRING contains the
"," or the "#" character.
s/PATTERN/REPLACEMENT/egimosx
Searches a string for a pattern, and if found,
replaces that pattern with the replacement text
and returns the number of substitutions made.
Otherwise it returns false (specifically, the
empty string).
If no string is specified via the "=~" or "!~"
operator, the "$_" variable is searched and modi
fied. (The string specified with "=~" must be
scalar variable, an array element, a hash element,
or an assignment to one of those, i.e., an
lvalue.)
If the delimiter chosen is a single quote, no
interpolation is done on either the PATTERN or the
REPLACEMENT. Otherwise, if the PATTERN contains a
$ that looks like a variable rather than an end-
of-string test, the variable will be interpolated
into the pattern at run-time. If you want the
pattern compiled only once the first time the
variable is interpolated, use the "/o" option. If
the pattern evaluates to the empty string, the
last successfully executed regular expression is
used instead. See 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
replace the slashes. If single quotes are used,
no interpretation is done on the replacement
string (the "/e" modifier overrides this, how
ever). Unlike Perl 4, Perl 5 treats backticks as
normal delimiters; the replacement text is not
evaluated as a command. If the PATTERN is delim
ited by bracketing quotes, the REPLACEMENT has its
own pair of quotes, which may or may not be brack
eting 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
$count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
$_ = 'abc123xyz';
s/\d+/$&*2/e; # yields 'abc246xyz'
s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
s/%(.)/$percent{$1}/g; # change percent escapes; no /e
s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
s/^=(\w+)/&pod($1)/ge; # use function call
# expand variables in $_, but dynamics only, using
# symbolic dereferencing
s/\$(\w+)/${$1}/g;
# Add one to the value of any numbers in the string
s/(\d+)/1 + $1/eg;
# This will expand any embedded scalar variable
# (including lexicals) in $_ : First $1 is interpolated
# to the variable name, and then evaluated
s/(\$\w+)/$1/eeg;
# Delete (most) C comments.
$program =~ s {
/\* # Match the opening delimiter.
.*? # Match a minimal number of characters.
\*/ # Match the closing delimiter.
} []gsx;
s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively
for ($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 exam
ple. 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;
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 speci
fied 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 that "tr" does not do regular expression
character classes such as "\d" or "[:lower:]".
The <tr> operator is not equivalent to the tr(1)
utility. If you want to map strings between
lower/upper cases, see the lc entry in the perl
func manpage and the uc entry in the perlfunc man
page, and in general consider using the "s" opera
tor if you need regular expressions.
Note also that the whole range idea is rather
unportable between character sets--and even within
character sets they may cause results you probably
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). Any
thing else is unsafe. If in doubt, spell out the
character sets in full.
Options:
c Complement the SEARCHLIST.
d Delete found but unreplaced characters.
s Squash duplicate replaced characters.
If the "/c" modifier is specified, the SEARCHLIST
character set is complemented. If the "/d" modi
fier is specified, any characters specified by
SEARCHLIST not found in REPLACEMENTLIST are
deleted. (Note that this is slightly more flexi
ble 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 sin
gle instance of the character.
If the "/d" modifier is used, the REPLACEMENTLIST
is always interpreted exactly as specified. Oth
erwise, 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 = $host) =~ tr/a-z/A-Z/;
tr/a-zA-Z/ /cs; # change non-alphas to single space
tr [\200-\377]
[\000-\177]; # delete 8th bit
If multiple transliterations are given for a char
acter, only the first one is used:
tr/AAA/XYZ/
will transliterate any A to X.
Because the transliteration table is built at com
pile time, neither the SEARCHLIST nor the REPLACE
MENTLIST are subjected to double quote interpola
tion. That means that if you want to use vari
ables, you must use an eval():
eval "tr/$oldlist/$newlist/";
die $@ if $@;
eval "tr/$oldlist/$newlist/, 1" or die $@;
Gory details of parsing quoted constructs
When presented with something that might have several dif
ferent interpretations, Perl uses the DWIM (that's "Do
What I Mean") principle to pick the most probable inter
pretation. This strategy is so successful that Perl pro
grammers often do not suspect the ambivalence of what they
write. But from time to time, Perl's notions differ sub
stantially from what the author honestly meant.
This section hopes to clarify how Perl handles quoted con
structs. 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 dis
cussed 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 per
forms different numbers of passes, from one to five, but
these passes are always performed in the same order.
Finding the end
The first pass is finding the end of the quoted con
struct, whether it be a multicharacter delimiter
""\nEOF\n"" in the "<<EOF" construct, a "/" that ter
minates a "qq//" construct, a "]" which terminates
"qq[]" construct, or a ">" which terminates a fileglob
started with "<".
When searching for single-character non-pairing delim
iters, such as "/", combinations of "\\" and "\/" are
skipped. However, when searching for single-character
pairing delimiter like "[", combinations of "\\",
"\]", and "\[" are all skipped, and nested "[", "]"
are skipped as well. When searching for multicharac
ter delimiters, nothing is skipped.
For constructs with three-part delimiters ("s///",
"y///", and "tr///"), the search is repeated once
more.
During this search no attention is paid to the seman
tics of the construct. Thus:
"$hash{"$foo/$bar"}"
or:
m/
bar # NOT a comment, this slash / terminated m//!
/x
do not form legal quoted expressions. The quoted
part ends on the first """ and "/", and the rest hap
pens to be a syntax error. Because the slash that
terminated "m//" was followed by a "SPACE", the exam
ple above is not "m//x", but rather "m//" with no "/x"
modifier. So the embedded "#" is interpreted as a
literal "#".
Removal of backslashes before delimiters
During the second pass, text between the starting and
ending delimiters is copied to a safe location, and
the "\" is removed from combinations consisting of "\"
and delimiter--or delimiters, meaning both starting
and ending delimiters will should these differ. This
removal does not happen for multi-character delim
iters. Note that the combination "\\" is left intact,
just as it was.
Starting from this step no information about the
delimiters is used in parsing.
Interpolation
The next step is interpolation in the text obtained,
which is now delimiter-independent. There are four
different cases.
""<<'EOF'"", ""m''"", ""s'''"", ""tr///"", ""y///""
No interpolation is performed.
""''"", ""q//""
The only interpolation is removal of "\" from
pairs "\\".
"""""""", ""``"", ""qq//"", ""qx//"", ""<file*glob>""
"\Q", "\U", "\u", "\L", "\l" (possibly paired with
"\E") are converted to corresponding Perl con
structs. Thus, ""$foo\Qbaz$bar"" is converted to
"$foo . (quotemeta("baz" . $bar))" internally.
The other combinations are replaced with appropri
ate 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 con
verted 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 opera
tions. 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 pro
tected 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 " . $b -> {c};
Most of the time, the longest possible text that
does not include spaces between components and
which contains matching braces or brackets.
because the outcome may be determined by voting
based on heuristic estimators, the result is not
strictly predictable. Fortunately, it's usually
correct for ambiguous cases.
""?RE?"", ""/RE/"", ""m/RE/"", ""s/RE/foo/"",
Processing of "\Q", "\U", "\u", "\L", "\l", and
interpolation happens (almost) as with "qq//" con
structs, but the substitution of "\" followed by
RE-special chars (including "\") is not performed.
Moreover, inside "(?{BLOCK})", "(?# comment )",
and a "#"-comment in a "//x"-regular expression,
no processing is performed whatsoever. This is
the first step at which the presence of the "//x"
modifier is relevant.
Interpolation has several quirks: "$|", "$(", and
"$)" are not 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:
"/${arr[0-9]}/" is interpreted as array element
"-9", not as a regular expression from the vari
able "$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.
It is at this step that "\1" is begrudgingly con
verted to "$1" in the replacement text of "s///"
to correct the incorrigible sed hackers who
haven't picked up the saner idiom yet. A warning
is emitted if the "use warnings" pragma or the -w
command-line flag (that is, the "$^W" variable)
was set.
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 fin
ish 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 spe
cial 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 modi
fier is "mx", and after backslash-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-whites
pace choices.
This step is the last one for all constructs except
regular expressions, which are processed further.
Interpolation of regular expressions
Previous steps were performed during the compilation
of Perl code, but this one happens at run
time--although it may be optimized to be calculated at
compile time if appropriate. After preprocessing
described above, and possibly after evaluation if
catenation, joining, casing translation, or metaquot
ing are involved, the resulting string is passed to
the RE engine for compilation.
Whatever happens in the RE engine might be better dis
cussed in the perlre manpage, but for the sake of con
tinuity, 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 automa
ton.
Backslashed characters are either replaced with corre
sponding 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 com
ments 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 con
struct 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 "{}"-delim
ited 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 the re man
page" pragma, as well as Perl's -Dr command-line
switch documented in the Command 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 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 inter
preted as an external command, and the output of that com
mand is the value of the backtick string, like in a shell.
In scalar context, a single string consisting of all out
put 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 the perlvar
manpage for the interpretation of "$?"). Unlike in csh,
no translation is done on the return data--newlines remain
newlines. Unlike in any of the shells, single quotes do
not hide variable names in the command from interpreta
tion. To pass a literal dollar-sign through to the shell
you need to hide it with a backslash. The generalized
form of backticks is "qx//". (Because backticks always
undergo shell expansion as well, see the perlsec manpage
for security concerns.)
In scalar context, evaluating a filehandle in angle brack
ets yields the next line from that file (the newline, if
any, included), or "undef" at end-of-file or on error.
When "$/" 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 vari
able, 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" state
ment (even if disguised as a "for(;;)" loop), the value is
automatically assigned to the global variable $_, destroy
ing 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 $_; ... }
In other boolean contexts, "<filehandle>" without an
explicit "defined" test or comparison elicit a warning if
the "use warnings" pragma or the -w command-line switch
(the "$^W" variable) is in effect.
The filehandles STDIN, STDOUT, and STDERR are predefined.
(The 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 the perlopentut manpage and 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 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 the readline entry in the perlfunc manpage.
The null filehandle <> is special: it can be used to emu
late 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 actu
ally work. It really does shift the @ARGV array and put
the current filename into the $ARGV variable. It also
uses filehandle ARGV internally--<> is just a synonym for
<ARGV>, which is magical. (The pseudo code above doesn't
work because it treats <ARGV> as non-magical.)
You can modify @ARGV before the first <> as long as the
array ends up containing the list of filenames you really
want. Line numbers ("$.") continue as though the input
were one big happy file. See the example in the eof entry
in the perlfunc manpage 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/) { $verbose++ }
# ... # other switches
}
while (<>) {
# ... # code for each line
}
The <> symbol will return "undef" for end-of-file only
once. If you call it again after this, it will assume you
are processing another @ARGV list, and if you haven't set
@ARGV, will read input from STDIN.
If angle brackets contain is a simple scalar variable
(e.g., <$foo>), then that variable contains the name of
the filehandle to input from, or its typeglob, or a refer
ence to the same. For example:
$fh = \*STDIN;
$line = <$fh>;
If what's within the angle brackets is neither a filehan
dle 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.) 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, $_;
}
except that the globbing is actually done internally using
the standard "File::Glob" extension. Of course, the
shortest way to do the above is:
chmod 0644, <*.c>;
A (file)glob evaluates its (embedded) argument only when
it is starting a new list. All values must be read before
it will start over. In list context, this isn't important
because you automatically get them all anyway. However,
in scalar context the operator returns the next value each
time it's called, or "undef" when the list has run out.
As with filehandle reads, an automatic "defined" is gener
ated when the glob occurs in the test part of a "while",
because legal glob returns (e.g. a file called 0) would
otherwise terminate the loop. Again, "undef" is returned
only once. So if you're expecting a single value from a
glob, it is much better to say
($file) = <blurch*>;
than
$file = <blurch*>;
because the latter will alternate between returning a
filename and returning false.
It you're trying to do variable interpolation, it's defi
nitely better to use the glob() function, because the
older notation can cause people to become confused with
the indirect filehandle notation.
@files = glob("$dir/*.[ch]");
@files = glob($files[$i]);
Constant Folding
Like C, Perl does a certain amount of expression evalua
tion at compile time whenever it determines that all argu
ments 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 $file > 5 + 100 * 2**16) { }
}
the compiler will precompute the number which that expres
sion represents so that the interpreter won't have to.
Bitwise String Operators
Bitstrings of any size may be manipulated by the bitwise
operators ("~ | & ^").
If the operands to a binary bitwise op are strings of dif
ferent sizes, | and ^ ops act as though the shorter
operand had additional zero bits on the right, while the &
op acts as though the longer operand were truncated to the
length of the shorter. The granularity for such extension
or truncation is one or more bytes.
# ASCII-based examples
print "j p \n" ^ " a h"; # prints "JAPH\n"
print "JA" | " ph\n"; # prints "japh\n"
print "japh\nJunk" & '_____'; # prints "JAPH\n";
print 'p N$' ^ " 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 num
ber, 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 (if it feels like it) 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 only an integer, merely that
Perl may use integer operations if it is so inclined. 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 the Bitwise String Operators entry elsewhere
in this document.) However, "use integer" still has mean
ing for them. By default, their results are interpreted
as unsigned integers, but if "use integer" is in effect,
their results are interpreted as signed integers. For
example, "~0" usually evaluates to a large integral value.
However, "use integer; ~0" is "-1" on twos-complement
machines.
Floating-point Arithmetic
While "use integer" provides integer-only arithmetic,
there is no 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
the perlfaq4 manpage.
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 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 impli
cations, and the rounding method used should be specified
precisely. In these cases, it probably pays not to trust
whichever system rounding is being used by Perl, but to
instead implement the rounding function you need yourself.
Bigger Numbers
The standard Math::BigInt and Math::BigFloat modules pro
vide variable-precision arithmetic and overloaded opera
tors, although they're currently pretty slow. At the cost
of some space and considerable speed, they avoid the nor
mal pitfalls associated with limited-precision representa
tions.
use Math::BigInt;
$x = Math::BigInt->new('123456789123456789');
print $x * $x;
# prints +15241578780673678515622620750190521
There are several modules that let you calculate with
(bound only by memory and cpu-time) unlimited or fixed
precision. There are also some non-standard modules that
provide faster implementations via external C libraries.
Here is a short, but incomplete summary:
Math::Fraction big, unlimited fractions like 9973 / 12967
Math::String treat string sequences like numbers
Math::FixedPrecision calculate with a fixed precision
Math::Currency for currency calculations
Bit::Vector manipulate bit vectors fast (uses C)
Math::BigIntFast Bit::Vector wrapper for big numbers
Math::Pari provides access to the Pari C library
Math::BigInteger uses an external C library
Math::Cephes uses external Cephes C library (no big numbers)
Math::Cephes::Fraction fractions via the Cephes library
Math::GMP another one using an external C library
Choose wisely.
2001-03-18 perl v5.6.1 PERLOP(1)
