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perlop - Perl operators and precedence
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perlop - Perl operators and precedence
Perl operators have the following associativity and precedence,
listed from highest precedence to lowest. Operators borrowed from
C keep the same precedence relationship with each other, even where
C's precedence is slightly screwy. (This makes learning Perl easier
for C folks.) With very few exceptions, these all operate on scalar
values only, not array values.
left terms and list operators (leftward)
left ->
nonassoc ++ --
right **
right ! ~ \ and unary + and -
left =~ !~
left * / % x
left + - .
left << >>
nonassoc named unary operators
nonassoc < > <= >= lt gt le ge
nonassoc == != <=> eq ne cmp
left &
left | ^
left &&
left ||
nonassoc .. ...
right ?:
right = += -= *= etc.
left , =>
nonassoc list operators (rightward)
right not
left and
left or xor
In the following sections, these operators are covered in precedence order.
Many operators can be overloaded for objects. See the overload manpage.
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.
If any list operator (print(), etc.) or any unary operator (chdir(), etc.)
is followed by a left parenthesis as the next token, the operator and
arguments within parentheses are taken to be of highest precedence,
just like a normal function call.
In the absence of parentheses, the precedence of list operators such as
print, sort, or chmod is either very high or very low depending on
whether you are looking at the left side or the right side of the operator.
For example, in
@ary = (1, 3, sort 4, 2);
print @ary; # prints 1324
the commas on the right of the sort are evaluated before the sort,
but the commas on the left are evaluated after. In other words,
list operators tend to gobble up all arguments that follow, and
then act like a simple TERM with regard to the preceding expression.
Be careful with parentheses:
# These evaluate exit before doing the print:
print($foo, exit); # Obviously not what you want.
print $foo, exit; # Nor is this.
# These do the print before evaluating exit:
(print $foo), exit; # This is what you want.
print($foo), exit; # Or this.
print ($foo), exit; # Or even this.
Also note that
print ($foo & 255) + 1, "\n";
probably doesn't do what you expect at first glance. See
Named Unary Operators for more discussion of this.
Also parsed as terms are the do {} and eval {} constructs, as
well as subroutine and method calls, and the anonymous
constructors [] and {}.
See also Quote and Quote-like Operators toward the end of this section,
as well as I/O Operators.
``->'' is an infix dereference operator, just as it is in C
and C++. If the right side is either a [...], {...}, or a
(...) subscript, then the left side must be either a hard or
symbolic reference to an array, a hash, or a subroutine respectively.
(Or technically speaking, a location capable of holding a hard
reference, if it's an array or hash reference being used for
assignment.) See the perlreftut manpage and the perlref manpage.
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.
``++'' 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.
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.)
Unary ``!'' performs logical negation, i.e., ``not''. See also not for a lower
precedence version of this.
Unary ``-'' performs arithmetic negation if the operand is numeric. If
the operand is an identifier, a string consisting of a minus sign
concatenated with the identifier is returned. Otherwise, if the string
starts with a plus or minus, a string starting with the opposite sign
is returned. One effect of these rules is that -bareword is equivalent
to "-bareword".
Unary ``~'' performs bitwise negation, i.e., 1's complement. For
example, 0666 & ~027 is 0640. (See also Integer Arithmetic and
Bitwise String Operators.) Note that the width of the result is
platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64
bits wide on a 64-bit platform, so if you are expecting a certain bit
width, remember use the & operator to mask off the excess bits.
Unary ``+'' has no effect whatsoever, even on strings. It is useful
syntactically for separating a function name from a parenthesized expression
that would otherwise be interpreted as the complete list of function
arguments. (See examples above under Terms and List Operators (Leftward).)
Unary ``\'' 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.
Binary ``=~'' binds a scalar expression to a pattern match. Certain operations
search or modify the string $_ by default. This operator makes that kind
of operation work on some other string. The right argument is a search
pattern, substitution, or transliteration. The left argument is what is
supposed to be searched, substituted, or transliterated instead of the default
$_. When used in scalar context, the return value generally indicates the
success of the operation. Behavior in list context depends on the particular
operator. See Regexp Quote-Like Operators for details.
If the right argument is an expression rather than a search pattern,
substitution, or transliteration, it is interpreted as a search pattern at run
time. This can be 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.
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 $a (i.e. the
result will be less than or equal to zero).
Note than when use integer is in scope, ``%'' gives you direct access
to the modulus operator as implemented by your C compiler. This
operator is not as well defined for negative operands, but it will
execute faster.
Binary ``x'' is the repetition operator. In scalar context or if the left
operand is not enclosed in parentheses, it returns a string consisting
of the left operand repeated the number of times specified by the right
operand. In list context, if the left operand is enclosed in
parentheses, it repeats the list.
print '-' x 80; # print row of dashes
print "\t" x ($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
Binary ``+'' returns the sum of two numbers.
Binary ``-'' returns the difference of two numbers.
Binary ``.'' concatenates two strings.
Binary ``<<'' returns the value of its left argument shifted left by the
number of bits specified by the right argument. Arguments should be
integers. (See also Integer Arithmetic.)
Binary ``>>'' returns the value of its left argument shifted right by
the number of bits specified by the right argument. Arguments should
be integers. (See also Integer Arithmetic.)
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 ($foo * 20)
rand 10 * 20; # rand (10 * 20)
rand(10) * 20; # (rand 10) * 20
rand (10) * 20; # (rand 10) * 20
rand +(10) * 20; # rand (10 * 20)
See also Terms and List Operators (Leftward).
Binary ``<'' returns true if the left argument is numerically less than
the right argument.
Binary ``>'' returns true if the left argument is numerically greater
than the right argument.
Binary ``<='' returns true if the left argument is numerically less than
or equal to the right argument.
Binary ``>='' returns true if the left argument is numerically greater
than or equal to the right argument.
Binary ``lt'' returns true if the left argument is stringwise less than
the right argument.
Binary ``gt'' returns true if the left argument is stringwise greater
than the right argument.
Binary ``le'' returns true if the left argument is stringwise less than
or equal to the right argument.
Binary ``ge'' returns true if the left argument is stringwise greater
than or equal to the right argument.
Binary ``=='' returns true if the left argument is numerically equal to
the right argument.
Binary ``!='' returns true if the left argument is numerically not equal
to the right argument.
Binary ``<=>'' returns -1, 0, or 1 depending on whether the left
argument is numerically less than, equal to, or greater than the right
argument. If your platform supports NaNs (not-a-numbers) as numeric
values, using them with ``<=>'' returns undef. NaN is not ``<'', ``=='', ``>'',
``<='' or ``>='' anything (even NaN), so those 5 return false. NaN != NaN
returns true, as does NaN != anything else. If your platform doesn't
support NaNs then NaN is just a string with numeric value 0.
perl -le '$a = NaN; print "No NaN support here" if $a == $a'
perl -le '$a = NaN; print "NaN support here" if $a != $a'
Binary ``eq'' returns true if the left argument is stringwise equal to
the right argument.
Binary ``ne'' returns true if the left argument is stringwise not equal
to the right argument.
Binary ``cmp'' returns -1, 0, or 1 depending on whether the left
argument is stringwise less than, equal to, or greater than the right
argument.
``lt'', ``le'', ``ge'', ``gt'' and ``cmp'' use the collation (sort) order specified
by the current locale if use locale is in effect. See the perllocale manpage.
Binary ``&'' returns its operators ANDed together bit by bit.
(See also Integer Arithmetic and Bitwise String Operators.)
Binary ``|'' returns its operators ORed together bit by bit.
(See also Integer Arithmetic and Bitwise String Operators.)
Binary ``^'' returns its operators XORed together bit by bit.
(See also Integer Arithmetic and Bitwise String Operators.)
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.
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.
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 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, just use three dots (``...'') instead of
two. In all other regards, ``...'' behaves just like ``..'' does.
The right operand is not evaluated while the operator is in the
``false'' state, and the left operand is not evaluated while the
operator is in the ``true'' state. The precedence is a little lower
than || and &&. The value returned is either the empty string for
false, or a sequence number (beginning with 1) for true. The
sequence number is reset for each range encountered. The final
sequence number in a range has the string ``E0'' appended to it, which
doesn't affect its numeric value, but gives you something to search
for if you want to exclude the endpoint. You can exclude the
beginning point by waiting for the sequence number to be greater
than 1. If either operand of scalar ``..'' is a constant expression,
that operand is 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 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 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.
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;
``='' is the ordinary assignment operator.
Assignment operators work as in C. That is,
$a += 2;
is equivalent to
$a = $a + 2;
although without duplicating any side effects that dereferencing the lvalue
might trigger, such as from tie(). Other assignment operators work similarly.
The following are recognized:
**= += *= &= <<= &&=
-= /= |= >>= ||=
.= %= ^=
x=
Although these are grouped by family, they all have the precedence
of assignment.
Unlike in C, the scalar assignment operator produces a valid lvalue.
Modifying an assignment is equivalent to doing the assignment and
then modifying the variable that was assigned to. This is useful
for modifying a copy of something, like this:
($tmp = $global) =~ tr [A-Z] [a-z];
Likewise,
($a += 2) *= 3;
is equivalent to
$a += 2;
$a *= 3;
Similarly, a list assignment in list context produces the list of
lvalues assigned to, and a list assignment in scalar context returns
the number of elements produced by the expression on the right hand
side of the assignment.
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.
On the right side of a list operator, it has very low precedence,
such that it controls all comma-separated expressions found there.
The only operators with lower precedence are the logical operators
``and'', ``or'', and ``not'', which may be used to evaluate calls to list
operators without the need for extra parentheses:
open HANDLE, "filename"
or die "Can't open: $!\n";
See also discussion of list operators in Terms and List Operators (Leftward).
Unary ``not'' returns the logical negation of the expression to its right.
It's the equivalent of ``!'' except for the very low precedence.
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.
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.
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.
While we usually think of quotes as literal values, in Perl they
function as operators, providing various kinds of interpolating and
pattern matching capabilities. Perl provides customary quote characters
for these behaviors, but also provides a way for you to choose your
quote character for any of them. In the following table, a {} represents
any pair of delimiters you choose.
Customary Generic Meaning Interpolates
'' q{} Literal no
"" qq{} Literal yes
`` qx{} Command yes (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 quoting
characters, except when # is being used as the quoting character.
q#foo# is parsed as the string foo, while q #foo# is the
operator q followed by a comment. Its argument will be taken
from the next line. This allows you to write:
s {foo} # Replace foo
{bar} # with bar.
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 ``newline'' for your system, but use the literal ASCII when you
need an exact character. For example, most networking protocols expect
and prefer a CR+LF ("\015\012" or "\cM\cJ") for line terminators,
and although they often accept just "\012", they seldom tolerate just
"\015". If you get in the habit of using "\n" for networking,
you may be burned some day.
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 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.
Here are the quote-like operators that apply to pattern
matching and related activities.
- ?PATTERN?
-
This is just like the
/pattern/ search, except that it matches only
once between calls to the reset() operator. This is a useful
optimization when you want to see only the first occurrence of
something in each file of a set of files, for instance. Only ??
patterns local to the current package are reset.
while (<>) {
if (?^$?) {
# blank line between header and body
}
} continue {
reset if eof; # clear ?? status for next file
}
This usage is vaguely deprecated, which means it just might possibly
be removed in some distant future version of Perl, perhaps somewhere
around the year 2168.
- m/PATTERN/cgimosx
-
- /PATTERN/cgimosx
-
Searches a string for a pattern match, and in scalar context returns
true if it succeeds, false if it fails. If no string is specified
via the
=~ or !~ operator, the $_ string is searched. (The
string specified with =~ need not be an lvalue--it may be the
result of an expression evaluation, but remember the =~ binds
rather tightly.) See also 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 ?PATTERN? applies.
If ``''' is the delimiter, no interpolation is performed on the PATTERN.
PATTERN may contain variables, which will be interpolated (and the
pattern recompiled) every time the pattern search is evaluated, except
for when the delimiter is a single quote. (Note that $(, $), and
$| are not interpolated because they look like end-of-string tests.)
If you want such a pattern to be compiled only once, add a /o after
the trailing delimiter. This avoids expensive run-time recompilations,
and is useful when the value you are interpolating won't change over
the life of the script. However, mentioning /o constitutes a promise
that you won't change the variables in the pattern. If you change them,
Perl won't even notice. See also qr/STRING/imosx.
If the PATTERN evaluates to the empty string, the last
successfully matched regular expression is used instead.
If the /g option is not used, m// in list context returns a
list consisting of the subexpressions matched by the parentheses in the
pattern, i.e., ($1, $2, $3...). (Note that here $1 etc. are
also set, and that this differs from Perl 4's behavior.) When there are
no parentheses in the pattern, the return value is the list (1) for
success. With or without parentheses, an empty list is returned upon
failure.
Examples:
open(TTY, '/dev/tty');
<TTY> =~ /^y/i && foo(); # do foo if desired
if (/Version: *([0-9.]*)/) { $version = $1; }
next if m#^/usr/spool/uucp#;
# poor man's grep
$arg = shift;
while (<>) {
print if /$arg/o; # compile only once
}
if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
This last example splits $foo into the first two words and the
remainder of the line, and assigns those three fields to $F1, $F2, and
$Etc. The conditional is true if any variables were assigned, i.e., if
the pattern matched.
The /g modifier specifies global pattern matching--that is,
matching as many times as possible within the string. How it behaves
depends on the context. In list context, it returns a list of the
substrings matched by any capturing parentheses in the regular
expression. If there are no parentheses, it returns a list of all
the matched strings, as if there were parentheses around the whole
pattern.
In scalar context, each execution of m//g finds the next match,
returning true if it matches, and false if there is no further match.
The position after the last match can be read or set using the pos()
function; see pos in the perlfunc manpage. A failed match normally resets the
search position to the beginning of the string, but you can avoid that
by adding the /c modifier (e.g. m//gc). Modifying the target
string also resets the search position.
You can intermix m//g matches with m/\G.../g, where \G is a
zero-width assertion that matches the exact position where the previous
m//g, if any, left off. Without the /g modifier, the \G assertion
still anchors at pos(), but the match is of course only attempted once.
Using \G without /g on a target string that has not previously had a
/g match applied to it is the same as using the \A assertion to match
the beginning of the string.
Examples:
# list context
($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 different actions depending on which regexp matched. Each
regexp tries to match where the previous one leaves off.
$_ = <<'EOL';
$url = new URI::URL "http://www/";; die if $url eq "xXx";
EOL
LOOP:
{
print(" digits"), redo LOOP if /\G\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 /\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 interpolated the same way as PATTERN
in
m/PATTERN/. If ``''' is used as the delimiter, no interpolation
is done. Returns a Perl value which may be used instead of the
corresponding /STRING/imosx expression.
For example,
$rex = qr/my.STRING/is;
s/$rex/foo/;
is equivalent to
s/my.STRING/foo/is;
The result may be used as a subpattern in a match:
$re = qr/$pattern/;
$string =~ /foo${re}bar/; # can be interpolated in other patterns
$string =~ $re; # or used standalone
$string =~ /$re/; # or this way
Since Perl may compile the pattern at the moment of execution of qr()
operator, using qr() may have speed advantages in some situations,
notably if the result of qr() is used standalone:
sub match {
my $patterns = shift;
my @compiled = map qr/$_/i, @$patterns;
grep {
my $success = 0;
foreach my $pat (@compiled) {
$success = 1, last if /$pat/;
}
$success;
} @_;
}
Precompilation of the pattern into an internal representation at
the moment of qr() avoids a need to recompile the pattern every
time a match /$pat/ is attempted. (Perl has many other internal
optimizations, but none would be triggered in the above example if
we did not use qr() operator.)
Options are:
i Do case-insensitive pattern matching.
m Treat string as multiple lines.
o Compile pattern only once.
s Treat string as single line.
x Use extended regular expressions.
See 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 redirections will be honored. The collected standard
output of the command is returned; standard error is unaffected. In
scalar context, it comes back as a single (potentially multi-line)
string, or undef if the command failed. In list context, returns a
list of lines (however you've defined lines with $/ or
$INPUT_RECORD_SEPARATOR), or an empty list if the command failed.
Because backticks do not affect standard error, use shell file descriptor
syntax (assuming the shell supports this) if you care to address this.
To capture a command's STDERR and STDOUT together:
$output = `cmd 2>&1`;
To capture a command's STDOUT but discard its STDERR:
$output = `cmd 2>/dev/null`;
To capture a command's STDERR but discard its STDOUT (ordering is
important here):
$output = `cmd 2>&1 1>/dev/null`;
To exchange a command's STDOUT and STDERR in order to capture the STDERR
but leave its STDOUT to come out the old STDERR:
$output = `cmd 3>&1 1>&2 2>&3 3>&-`;
To read both a command's STDOUT and its STDERR separately, it's easiest
and saf | 
