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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
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In the following sections, these operators are covered in precedence order.
Many operators can be overloaded for objects. See overload.
A TERM has the highest precedence in Perl. They include variables, quote and quote-like
operators, any expression in parentheses, and any function whose arguments are parenthesized.
Actually, there aren't really functions in this sense, just list operators and unary operators
behaving as functions because you put parentheses around the arguments. These are all documented
in perlfunc.
If any list operator (print(), etc.) or any unary operator (chdir(), etc.) is followed by a
left parenthesis as the next token, the operator and arguments within parentheses are taken to
be of highest precedence, just like a normal function call.
In the absence of parentheses, the precedence of list operators such as print, sort,
or chmod is either very high or very low depending on whether you are looking at
the left side or the right side of the operator. For example, in
@ary = (1, 3, sort 4, 2);
print @ary; # prints 1324
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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 ($foo), exit; # Or even this.
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Also note that
print ($foo & 255) + 1, "\n";
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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 perlreftut and perlref.
Otherwise, the right side is a method name or a simple scalar variable containing either the
method name or a subroutine reference, and the left side must be either an object (a blessed
reference) or a class name (that is, a package name). See perlobj.
"++" 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'
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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 perlreftut and perlref. Do not confuse this
behavior with the behavior of backslash within a string, although both forms do convey the
notion of protecting the next thing from interpolation.
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
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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.)
Note that both "<<" and ">>" in Perl are implemented directly
using "<<" and ">>" in C. If use integer (see Integer Arithmetic) is in force then signed C integers are used,
else unsigned C integers are used. Either way, the implementation isn't going to generate
results larger than the size of the integer type Perl was built with (32 bits or 64 bits).
The result of overflowing the range of the integers is undefined because it is undefined also
in C. In other words, using 32-bit integers, 1 << 32 is undefined. Shifting
by a negative number of bits is also undefined.
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 perlfunc.
If any list operator (print(), etc.) or any unary operator (chdir(), etc.) is followed by a
left parenthesis as the next token, the operator and arguments within parentheses are taken to
be of highest precedence, just like a normal function call. 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
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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 ($foo * 20)
rand 10 * 20; # rand (10 * 20)
rand(10) * 20; # (rand 10) * 20
rand (10) * 20; # (rand 10) * 20
rand +(10) * 20; # rand (10 * 20)
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See also "Terms and List Operators
(Leftward)".
Binary "<" returns true if the left argument is numerically less than the right
argument.
Binary ">" returns true if the left argument is numerically greater than the
right argument.
Binary "<=" returns true if the left argument is numerically less than or equal
to the right argument.
Binary ">=" returns true if the left argument is numerically greater than or
equal to the right argument.
Binary "lt" returns true if the left argument is stringwise less than the right
argument.
Binary "gt" returns true if the left argument is stringwise greater than the right
argument.
Binary "le" returns true if the left argument is stringwise less than or equal to
the right argument.
Binary "ge" returns true if the left argument is stringwise greater than or equal
to the right argument.
Binary "==" returns true if the left argument is numerically equal to the right
argument.
Binary "!=" returns true if the left argument is numerically not equal to the right
argument.
Binary "<=>" returns -1, 0, or 1 depending on whether the left argument is
numerically less than, equal to, or greater than the right argument. If your platform supports
NaNs (not-a-numbers) as numeric values, using them with "<=>" returns undef. NaN
is not "<", "==", ">", "<=" or
">=" anything (even NaN), so those 5 return false. NaN != NaN returns true, as does
NaN != anything else. If your platform doesn't support NaNs then NaN is just a string with
numeric value 0.
perl -le '$a = NaN; print "No NaN support here" if $a == $a'
perl -le '$a = NaN; print "NaN support here" if $a != $a'
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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 perllocale.
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";
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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
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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;
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With the C-style operators that would have been written like this:
unlink("alpha", "beta", "gamma")
|| (gripe(), next LINE);
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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 list of values counting (up by ones)
from the left value to the right value. If the left value is greater than the right value then
it returns the empty 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
}
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The range operator also works on strings, using the magical auto-increment, see below.
In scalar context, ".." returns a boolean value. The operator is bistable, like a
flip-flop, and emulates the line-range (comma) operator of sed, awk, and various
editors. Each ".." operator maintains its own boolean state. 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
}
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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
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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');
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to get all normal letters of the English alphabet, or
$hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
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to get a hexadecimal digit, or
@z2 = ('01' .. '31'); print $z2[$mday];
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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";
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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 = $ok ? @b : @c; # oops, that's just a count!
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The operator may be assigned to if both the 2nd and 3rd arguments are legal lvalues (meaning
that you can assign to them):
($a_or_b ? $a : $b) = $c;
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Because this operator produces an assignable result, using assignments without parentheses
will get you in trouble. For example, this:
$a % 2 ? $a += 10 : $a += 2
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Really means this:
(($a % 2) ? ($a += 10) : $a) += 2
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Rather than this:
($a % 2) ? ($a += 10) : ($a += 2)
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That should probably be written more simply as:
"=" is the ordinary assignment operator.
Assignment operators work as in C. That is,
is equivalent to
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=
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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];
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Likewise,
is equivalent to
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";
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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: $!";
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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 || $c; # better written this way
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However, when it's a list-context assignment and you're trying to use "||" for
control flow, you probably need "or" so that the assignment takes higher precedence.
@info = stat($file) || die; # oops, scalar sense of stat!
@info = stat($file) or die; # better, now @info gets its due
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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*
qw{} Word list no
// m{} Pattern match yes*
qr{} Pattern yes*
s{}{} Substitution yes*
tr{}{} Transliteration no (but see below)
<<EOF here-doc yes*
* unless the delimiter is ''.
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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
is the same as
Note, however, that this does not always work for quoting Perl code:
$s = q{ if($a eq "}") ... }; # WRONG
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is a syntax error. The Text::Balanced module (from CPAN, and starting from Perl
5.8 part of the standard distribution) 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.
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The following escape sequences are available in constructs that interpolate and in
transliterations.
\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 Unicode character
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The following escape sequences are available in constructs that interpolate but not in
transliterations.
\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
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If use locale is in effect, the case map used by \l, \L,
\u and \U is taken from the current locale. See perllocale. If Unicode (for
example, \N{} or wide hex characters of 0x100 or beyond) is being used, the case
map used by \l, \L, \u and \U is as defined
by Unicode. For documentation of \N{name}, see charnames.
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.
For constructs that do interpolate, variables beginning with "$" or
"@" are interpolated. Subscripted variables such as $a[3] or
$href-{key}[0]> are also interpolated, as are array and hash slices. But method
calls such as $obj-meth> are not.
Interpolating an array or slice interpolates the elements in order, separated by the value of
$", so is equivalent to interpolating join $", @array.
"Punctuation" arrays such as @+ are only interpolated if the name is
enclosed in braces @{+}.
You cannot include a literal $ or @ within a \Q
sequence. An unescaped $ or @ interpolates the corresponding variable,
while escaping will cause the literal string \$ to be inserted. 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
}
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This usage is vaguely deprecated, which means it just might possibly be removed in some
distant future version of Perl, perhaps somewhere around the year 2168.
- m/PATTERN/cgimosx
-
- /PATTERN/cgimosx
-
Searches a string for a pattern match, and in scalar context returns true if it succeeds,
false if it fails. If no string is specified via the =~ or !~
operator, the $_ string is searched. (The string specified with =~ need not be
an lvalue--it may be the result of an expression evaluation, but remember the =~
binds rather tightly.) See also perlre.
See perllocale for
discussion of additional considerations that apply when use locale is in
effect.
Options are:
c Do not reset search position on a failed match when /g is in effect.
g Match globally, i.e., find all occurrences.
i Do case-insensitive pattern matching.
m Treat string as multiple lines.
o Compile pattern only once.
s Treat string as single line.
x Use extended regular expressions.
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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. In this case, only the g and c
flags on the empty pattern is honoured - the other flags are taken from the original
pattern. If no match has previously succeeded, this will (silently) act instead as a genuine
empty pattern (which will always match).
If the /g option is not used, m// in list context returns a
list consisting of the subexpressions matched by the 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 $Etc. The conditional is true if any variables
were assigned, i.e., if the pattern matched.
The /g modifier specifies global pattern matching--that is, matching as many
times as possible within the string. How it behaves depends on the context. In list context,
it returns a list of the substrings matched by any capturing parentheses in the regular
expression. If there are no parentheses, it returns a list of all the matched strings, as if
there were parentheses around the whole pattern.
In scalar context, each execution of m//g finds the next match, returning
true if it matches, and false if there is no further match. The position after the last
match can be read or set using the pos() function; see perlfunc/pos. A failed match
normally resets the search position to the beginning of the string, but you can avoid that
by adding the /c modifier (e.g. m//gc). Modifying the target
string also resets the search position.
You can intermix m//g matches with m/\G.../g, where \G
is a zero-width assertion that matches the exact position where the previous m//g,
if any, left off. Without the /g modifier, the \G assertion still
anchors at pos(), but the match is of course only attempted once. Using \G
without /g on a target string that has not previously had a /g
match applied to it is the same as using the \A assertion to match the
beginning of the string. Note also that, currently, \G is only properly
supported when anchored at the very beginning of the pattern.
Examples:
# list context
($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
# scalar context
$/ = "";
while (defined($paragraph = <>)) {
while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
$sentences++;
}
}
print "$sentences\n";
# using m//gc with \G
$_ = "ppooqppqq";
while ($i++ < 2) {
print "1: '";
print $1 while /(o)/gc; print "', pos=", pos, "\n";
print "2: '";
print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
print "3: '";
print $1 while /(p)/gc; print "', pos=", pos, "\n";
}
print "Final: '$1', pos=",pos,"\n" if /\G(.)/;
|
|
The last example should print:
1: 'oo', pos=4
2: 'q', pos=5
3: 'pp', pos=7
1: '', pos=7
2: 'q', pos=8
3: '', pos=8
Final: 'q', pos=8
|
|
Notice that the final match matched q instead of p, which a
match without the \G anchor would have done. Also note that the final match did
not update pos -- pos is only updated on a /g match.
If the final match did indeed match p, 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
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 perlre 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:
To capture a command's STDOUT but discard its STDERR:
$output = `cmd 2>/dev/null`;
|
|
To capture a command's STDERR but discard its STDOUT (ordering is important here):
$output = `cmd 2>&1 1>/dev/null`;
|
|
To exchange a command's STDOUT and STDERR in order to capture the STDERR but leave its
STDOUT to come out the old STDERR:
$output = `cmd 3>&1 1>&2 2>&3 3>&-`;
|
|
To read both a command's STDOUT and its STDERR separately, it's easiest and safest to
redirect them separately to files, and then read from those files when the program is done:
system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
|
|
Using single-quote as a delimiter protects the command from Perl's double-quote
interpolation, passing it on to the shell instead:
$perl_info = qx(ps $$); # that's Perl's $$
$shell_info = qx'ps $$'; # that's the new shell's $$
|
|
How that string gets evaluated is entirely subject to the command interpreter on your
system. On most platforms, you will have to protect shell metacharacters if you want them
treated literally. This is in practice difficult to do, as it's unclear how to escape which
characters. See perlsec for
a clean and safe example of a manual fork() and exec() to emulate backticks safely.
On some platforms (notably DOS-like ones), the shell may not be capable of dealing with
multiline commands, so putting newlines in the string may not get you what you want. You may
be able to evaluate multiple commands in a single line by separating them with the command
separator character, if your shell supports that (e.g. ; on many Unix shells; &
on the Windows NT cmd shell).
Beginning with v5.6.0, Perl will attempt to flush all files opened for output before
starting the child process, but this may not be supported on some platforms (see perlport). To be safe, you may
need to set $| ($AUTOFLUSH in English) or call the autoflush()
method of IO::Handle on any open handles.
Beware that some command shells may place restrictions on the length of the command line.
You must ensure your strings don't exceed this limit after any necessary interpolations. See
the platform-specific release notes for more details about your particular environment.
Using this operator can lead to programs that are difficult to port, because the shell
commands called vary between systems, and may in fact not be present at all. As one example,
the type command under the POSIX shell is very different from the type
command under DOS. That doesn't mean you should go out of your way to avoid backticks when
they're the right way to get something done. Perl was made to be a glue language, and one of
the things it glues together is commands. Just understand what you're getting yourself into.
See "I/O Operators" for more discussion.
- qw/STRING/
-
Evaluates to a list of the words extracted out of STRING, using embedded whitespace as
the word delimiters. It can be understood as being roughly equivalent to:
the difference being that it generates a real list at compile time. So this expression:
is semantically equivalent to the list:
Some frequently seen examples:
use POSIX qw( setlocale localeconv )
@EXPORT = qw( foo bar baz );
|
|
A common mistake is to try to separate the words with comma or to put comments into a
multi-line qw-string. For this reason, the use warnings pragma and
the -w switch (that is, the $^W variable) produces warnings if the
STRING contains the "," or the "#" character.
- s/PATTERN/REPLACEMENT/egimosx
-
Searches a string for a pattern, and if found, replaces that pattern with the replacement
text and returns the number of substitutions made. Otherwise it returns false (specifically,
the empty string).
If no string is specified via the =~ or !~ operator, the $_
variable is searched and modified. (The string specified with =~ must be scalar
variable, an array element, a hash element, or an assignment to one of those, i.e., an
lvalue.)
If the delimiter chosen is a single quote, no interpolation is done on either the PATTERN
or the REPLACEMENT. Otherwise, if the PATTERN contains a $ that looks like a variable rather
than an end-of-string test, the variable will be interpolated into the pattern at run-time.
If you want the pattern compiled only once the first time the variable is interpolated, use
the /o option. If the pattern evaluates to the empty string, the last
successfully executed regular expression is used instead. See perlre for further explanation
on these. See perllocale
for discussion of additional considerations that apply when use locale is in
effect.
Options are:
e Evaluate the right side as an expression.
g Replace globally, i.e., all occurrences.
i Do case-insensitive pattern matching.
m Treat string as multiple lines.
o Compile pattern only once.
s Treat string as single line.
x Use extended regular expressions.
|
|
Any non-alphanumeric, non-whitespace delimiter may replace the slashes. If single quotes
are used, no interpretation is done on the replacement string (the /e modifier
overrides this, however). Unlike Perl 4, Perl 5 treats backticks as normal delimiters; the
replacement text is not evaluated as a command. If the PATTERN is delimited by bracketing
quotes, the REPLACEMENT has its own pair of quotes, which may or may not be bracketing
quotes, e.g., s(foo)(bar) or s<foo>/bar/. A /e
will cause the replacement portion to be treated as a full-fledged Perl expression and
evaluated right then and there. It is, however, syntax checked at compile-time. A second e
modifier will cause the replacement portion to be evaled 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 example. Unlike sed, we use the \<digit>
form in only the left hand side. Anywhere else it's $<digit>.
Occasionally, you can't use just a /g to get all the changes to occur that
you might want. Here are two common cases:
# put commas in the right places in an integer
1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;
# expand tabs to 8-column spacing
1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
|
|
- tr/SEARCHLIST/REPLACEMENTLIST/cds
-
- y/SEARCHLIST/REPLACEMENTLIST/cds
-
Transliterates all occurrences of the characters found in the search list with the
corresponding character in the replacement list. It returns the number of characters
replaced or deleted. If no string is specified via the =~ or !~ operator, the $_ string is
transliterated. (The string specified with =~ must be a scalar variable, an array element, a
hash element, or an assignment to one of those, i.e., an lvalue.)
A character range may be specified with a hyphen, so tr/A-J/0-9/ does the
same replacement as tr/ACEGIBDFHJ/0246813579/. For sed devotees, y
is provided as a synonym for tr. If the SEARCHLIST is delimited by bracketing
quotes, the REPLACEMENTLIST has its own pair of quotes, which may or may not be bracketing
quotes, e.g., tr[A-Z][a-z] or tr(+\-*/)/ABCD/.
Note 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 perlfunc/lc and perlfunc/uc, and in general
consider using the s operator 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). Anything else is unsafe. If in doubt, spell out the character sets in full.
Options:
c Complement the SEARCHLIST.
d Delete found but unreplaced characters.
s Squash duplicate replaced characters.
|
|
If the /c modifier is specified, the SEARCHLIST character set is
complemented. If the /d modifier is specified, any characters specified by
SEARCHLIST not found in REPLACEMENTLIST are deleted. (Note that this is slightly more
flexible than the behavior of some tr programs, which delete anything they find in
the SEARCHLIST, period.) If the /s modifier is specified, sequences of
characters that were transliterated to the same character are squashed down to a single
instance of the character.
If the /d modifier is used, the REPLACEMENTLIST is always interpreted
exactly as specified. Otherwise, if the REPLACEMENTLIST is shorter than the SEARCHLIST, the
final character is replicated till it is long enough. If the REPLACEMENTLIST is empty, the
SEARCHLIST is replicated. This latter is useful for counting characters in a class or for
squashing character sequences in a class.
Examples:
$ARGV[1] =~ tr/A-Z/a-z/; # canonicalize to lower case
$cnt = tr/*/*/; # count the stars in $_
$cnt = $sky =~ tr/*/*/; # count the stars in $sky
$cnt = tr/0-9//; # count the digits in $_
tr/a-zA-Z//s; # bookkeeper -> bokeper
($HOST = $host) =~ tr/a-z/A-Z/;
tr/a-zA-Z/ /cs; # change non-alphas to single space
tr [\200-\377]
[\000-\177]; # delete 8th bit
|
|
If multiple transliterations are given for a character, only the first one is used:
will transliterate any A to X.
Because the transliteration table is built at compile time, neither the SEARCHLIST nor
the REPLACEMENTLIST are subjected to double quote interpolation. That means that if you want
to use variables, you must use an eval():
eval "tr/$oldlist/$newlist/";
die $@ if $@;
eval "tr/$oldlist/$newlist/, 1" or die $@;
|
|
- <<EOF
-
A line-oriented form of quoting is based on the shell "here-document" syntax.
Following a << you specify a string to terminate the quoted material, and
all lines following the current line down to the terminating string are the value of the
item. The terminating string may be either an identifier (a word), or some quoted text. If
quoted, the type of quotes you use determines the treatment of the text, just as in regular
quoting. An unquoted identifier works like double quotes. There must be no space between the
<< and the identifier, unless the identifier is quoted. (If you put a
space it will be treated as a null identifier, which is valid, and matches the first empty
line.) The terminating string must appear by itself (unquoted and with no surrounding
whitespace) on the terminating line.
print <<EOF;
The price is $Price.
EOF
print << "EOF"; # same as above
The price is $Price.
EOF
print << `EOC`; # execute commands
echo hi there
echo lo there
EOC
print <<"foo", <<"bar"; # you can stack them
I said foo.
foo
I said bar.
bar
myfunc(<< "THIS", 23, <<'THAT');
Here's a line
or two.
THIS
and here's another.
THAT
|
|
Just don't forget that you have to put a semicolon on the end to finish the statement, as
Perl doesn't know you're not going to try to do this:
print <<ABC
179231
ABC
+ 20;
|
|
If you want your here-docs to be indented with the rest of the code, you'll need to
remove leading whitespace from each line manually:
($quote = <<'FINIS') =~ s/^\s+//gm;
The Road goes ever on and on,
down from the door where it began.
FINIS
|
|
If you use a here-doc within a delimited construct, such as in s///eg, the
quoted material must come on the lines following the final delimiter. So instead of
s/this/<<E . 'that'
the other
E
. 'more '/eg;
|
|
you have to write
s/this/<<E . 'that'
. 'more '/eg;
the other
E
|
|
If the terminating identifier is on the last line of the program, you must be sure there
is a newline after it; otherwise, Perl will give the warning Can't find string terminator
"END" anywhere before EOF....
Additionally, the quoting rules for the identifier are not related to Perl's quoting
rules -- q(), qq(), and the like are not supported in place of ''
and "", and the only interpolation is for backslashing the quoting
character:
print << "abc\"def";
testing...
abc"def
|
|
Finally, quoted strings cannot span multiple lines. The general rule is that the
identifier must be a string literal. Stick with that, and you should be safe.
When presented with something that might have several different interpretations, Perl uses
the DWIM (that's "Do What I Mean") principle to pick the most probable
interpretation. This strategy is so successful that Perl programmers often do not suspect the
ambivalence of what they write. But from time to time, Perl's notions differ substantially from
what the author honestly meant.
This section hopes to clarify how Perl handles quoted constructs. Although the most common
reason to learn this is to unravel labyrinthine regular expressions, because the initial steps
of parsing are the same for all quoting operators, they are all discussed together.
The most important Perl parsing rule is the first one discussed below: when processing a
quoted construct, Perl first finds the end of that construct, then interprets its contents. If
you understand this rule, you may skip the rest of this section on the first reading. The other
rules are likely to contradict the user's expectations much less frequently than this first one.
Some passes discussed below are performed concurrently, but because their results are the
same, we consider them individually. For different quoting constructs, Perl performs different
numbers of passes, from one to five, but these passes are always performed in the same order.
- Finding the end
-
The first pass is finding the end of the quoted construct, whether it be a multicharacter
delimiter "\nEOF\n" in the <<EOF construct, a /
that terminates a qq// construct, a ] which terminates qq[]
construct, or a > which terminates a fileglob started with <.
When searching for single-character non-pairing delimiters, such as /,
combinations of \\ and \/ are skipped. However, when searching for
single-character pairing delimiter like [, combinations of \\, \],
and \[ are all skipped, and nested [, ] are skipped
as well. When searching for multicharacter delimiters, nothing is skipped.
For constructs with three-part delimiters (s///, y///, and tr///),
the search is repeated once more.
During this search no attention is paid to the semantics of the construct. Thus:
or:
m/
bar # NOT a comment, this slash / terminated m//!
/x
|
|
do not form legal quoted expressions. The quoted part ends on the first "
and /, and the rest happens to be a syntax error. Because the slash that
terminated m// was followed by a SPACE, the example above is not m//x,
but rather m// with no /x modifier. So the embedded #
is interpreted as a literal #.
- 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 delimiters. Note that the
combination \\ is left intact, just as it was.
Starting from this step no information about the delimiters is used in parsing.
- Interpolation
-
The next step is interpolation in the text obtained, which is now delimiter-independent.
There are four different cases.
<<'EOF', m'',
s''', tr///, y///
- No interpolation is performed.
'', q//
- The only interpolation is removal of
\ from pairs \\.
"", ``,
qq//, qx//, <file*glob>
-
\Q, \U, \u, \L, \l
(possibly paired with \E) are converted to corresponding Perl constructs.
Thus, "$foo\Qbaz$bar" is converted to $foo .
(quotemeta("baz" . $bar)) internally. The other combinations are
replaced with appropriate expansions.
Let it be stressed that whatever falls between \Q and \E
is interpolated in the usual way. Something like "\Q\\E" has no \E
inside. instead, it has \Q, \\, and E, so the
result is the same as for "\\\\E". As a general rule, backslashes
between \Q and \E may lead to counterintuitive results. So, "\Q\t\E"
is converted to quotemeta("\t"), which is the same as "\\\t"
(since TAB is not alphanumeric). Note also that:
$str = '\t';
return "\Q$str";
|
|
may be closer to the conjectural intention of the writer of "\Q\t\E".
Interpolated scalars and arrays are converted internally to the join and
. catenation operations. Thus, "$foo XXX '@arr'"
becomes:
$foo . " XXX '" . (join $", @arr) . "'";
|
|
All operations above are performed simultaneously, left to right.
Because the result of "\Q STRING \E" has all metacharacters
quoted, there is no way to insert a literal $ or @ inside a \Q\E
pair. If protected by \, $ will be quoted to became "\\\$";
if not, it is interpreted as the start of an interpolated scalar.
Note also that the interpolation code needs to make a decision on where the
interpolated scalar ends. For instance, whether "a $b -> {c}"
really means:
or:
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// constructs, 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
variable $arr followed by a digit, which would be the interpretation of /$arr[0-9]/.
Since voting among different estimators may occur, the result is not predictable.
It is at this step that \1 is begrudgingly converted to $1
in the replacement text of s/// to correct the incorrigible sed
hackers who 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 finish the regular expression, \/
will be stripped to / on the previous step, and \\/ will be
left as is. Because / is equivalent to \/ inside a regular
expression, this does not matter unless the delimiter happens to be character special to
the RE engine, such as in s*foo*bar*, m[foo], or ?foo?;
or an alphanumeric char, as in:
In the RE above, which is intentionally obfuscated for illustration, the delimiter is
m, the modifier is mx, and after backslash-removal the RE is
the same as for m/ ^ a s* b /mx). There's more than one reason you're
encouraged to restrict your delimiters to non-alphanumeric, non-whitespace choices.
This step is the last one for all constructs except regular expressions, which are
processed further.
- 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 metaquoting are involved, the resulting string is passed to
the RE engine for compilation.
Whatever happens in the RE engine might be better discussed in perlre, but for the sake of
continuity, we shall do so here.
This is another step where the presence of the //x modifier is relevant. The
RE engine scans the string from left to right and converts it to a finite automaton.
Backslashed characters are either replaced with corresponding literal strings (as with \{),
or else they generate special nodes in the finite automaton (as with \b).
Characters special to the RE engine (such as |) generate corresponding nodes or
groups of nodes. (?#...) comments are ignored. All the rest is either converted
to literal strings to match, or else is ignored (as is whitespace and #-style
comments if //x is present).
Parsing of the bracketed character class construct, [...], is rather
different than the rule used for the rest of the pattern. The terminator of this construct
is found using the same rules as for finding the terminator of a {}-delimited
construct, the only exception being that ] immediately following [
is treated as though preceded by a backslash. Similarly, the terminator of (?{...})
is found using the same rules as for finding the terminator of a {}-delimited
construct.
It is possible to inspect both the string given to RE engine and the resulting finite
automaton. See the arguments debug/debugcolor in the use re pragma, as well as Perl's -Dr
command-line switch documented in perlrun/"Command
Switches".
- 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.
There are several I/O operators you should know about.
A string enclosed by backticks (grave accents) first undergoes double-quote interpolation. It
is then interpreted as an external command, and the output of that command is the value of the
backtick string, like in a shell. In scalar context, a single string consisting of all output is
returned. In list context, a list of values is returned, one per line of output. (You can set $/
to use a different line terminator.) The command is executed each time the pseudo-literal is
evaluated. The status value of the command is returned in $? (see perlvar for the interpretation of $?).
Unlike in csh, no translation is done on the return data--newlines remain newlines.
Unlike in any of the shells, single quotes do not hide variable names in the command from
interpretation. To pass a literal dollar-sign through to the shell you need to hide it with a
backslash. The generalized form of backticks is qx//. (Because backticks always
undergo shell expansion as well, see perlsec
for security concerns.)
In scalar context, evaluating a filehandle in angle brackets yields the next line from that
file (the newline, if any, included), or undef at end-of-file or on error. When $/
is set to undef (sometimes known as file-slurp mode) and the file is empty, it
returns '' the first time, followed by undef subsequently.
Ordinarily you must assign the returned value to a |
