perlfunc - Perl builtin functions
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NAME
perlop - Perl operators and precedence
DESCRIPTION
Operator Precedence and Associativity
Operator precedence and associativity work in Perl more or less like
they do in mathematics.
*Operator precedence* means some operators are evaluated before others.
For example, in "2 + 4 * 5", the multiplication has higher precedence so
"4 * 5" is evaluated first yielding "2 + 20 == 22" and not "6 * 5 ==
30".
*Operator associativity* defines what happens if a sequence of the same
operators is used one after another: whether the evaluator will evaluate
the left operations first or the right. For example, in "8 - 4 - 2",
subtraction is left associative so Perl evaluates the expression left to
right. "8 - 4" is evaluated first making the expression "4 - 2 == 2" and
not "8 - 2 == 6".
Perl operators have the following associativity and precedence, listed
from highest precedence to lowest. Operators borrowed from C keep the
same precedence relationship with each other, even where C's precedence
is slightly screwy. (This makes learning Perl easier for C folks.) With
very few exceptions, these all operate on scalar values only, not array
values.
left terms and list operators (leftward)
left ->
nonassoc ++ --
right **
right ! ~ \ and unary + and -
left =~ !~
left * / % x
left + - .
left << >>
nonassoc named unary operators
nonassoc < > <= >= lt gt le ge
nonassoc == != <=> eq ne cmp
left &
left | ^
left &&
left ||
nonassoc .. ...
right ?:
right = += -= *= etc.
left , =>
nonassoc list operators (rightward)
right not
left and
left or xor
In the following sections, these operators are covered in precedence
order.
Many operators can be overloaded for objects. See overload.
Terms and List Operators (Leftward)
A TERM has the highest precedence in Perl. They include variables, quote
and quote-like operators, any expression in parentheses, and any
function whose arguments are parenthesized. Actually, there aren't
really functions in this sense, just list operators and unary operators
behaving as functions because you put parentheses around the arguments.
These are all documented in perlfunc.
If any list operator (print(), etc.) or any unary operator (chdir(),
etc.) is followed by a left parenthesis as the next token, the operator
and arguments within parentheses are taken to be of highest precedence,
just like a normal function call.
In the absence of parentheses, the precedence of list operators such as
"print", "sort", or "chmod" is either very high or very low depending on
whether you are looking at the left side or the right side of the
operator. For example, in
@ary = (1, 3, sort 4, 2);
print @ary; # prints 1324
the commas on the right of the sort are evaluated before the sort, but
the commas on the left are evaluated after. In other words, list
operators tend to gobble up all arguments that follow, and then act like
a simple TERM with regard to the preceding expression. Be careful with
parentheses:
# These evaluate exit before doing the print:
print($foo, exit); # Obviously not what you want.
print $foo, exit; # Nor is this.
# These do the print before evaluating exit:
(print $foo), exit; # This is what you want.
print($foo), exit; # Or this.
print ($foo), exit; # Or even this.
Also note that
print ($foo & 255) + 1, "\n";
probably doesn't do what you expect at first glance. The parentheses
enclose the argument list for "print" which is evaluated (printing the
result of "$foo & 255"). Then one is added to the return value of
"print" (usually 1). The result is something like this:
1 + 1, "\n"; # Obviously not what you meant.
To do what you meant properly, you must write:
print(($foo & 255) + 1, "\n");
See "Named Unary Operators" for more discussion of this.
Also parsed as terms are the "do {}" and "eval {}" constructs, as well
as subroutine and method calls, and the anonymous constructors "[]" and
"{}".
See also "Quote and Quote-like Operators" toward the end of this
section, as well as "I/O Operators".
The Arrow Operator
""->"" is an infix dereference operator, just as it is in C and C++. If
the right side is either a "[...]", "{...}", or a "(...)" subscript,
then the left side must be either a hard or symbolic reference to an
array, a hash, or a subroutine respectively. (Or technically speaking, a
location capable of holding a hard reference, if it's an array or hash
reference being used for assignment.) See perlreftut and perlref.
Otherwise, the right side is a method name or a simple scalar variable
containing either the method name or a subroutine reference, and the
left side must be either an object (a blessed reference) or a class name
(that is, a package name). See perlobj.
Auto-increment and Auto-decrement
"++" and "--" work as in C. That is, if placed before a variable, they
increment or decrement the variable by one before returning the value,
and if placed after, increment or decrement after returning the value.
$i = 0; $j = 0;
print $i++; # prints 0
print ++$j; # prints 1
The auto-increment operator has a little extra builtin magic to it. If
you increment a variable that is numeric, or that has ever been used in
a numeric context, you get a normal increment. If, however, the variable
has been used in only string contexts since it was set, and has a value
that is not the empty string and matches the pattern
"/^[a-zA-Z]*[0-9]*\z/", the increment is done as a string, preserving
each character within its range, with carry:
print ++($foo = '99'); # prints '100'
print ++($foo = 'a0'); # prints 'a1'
print ++($foo = 'Az'); # prints 'Ba'
print ++($foo = 'zz'); # prints 'aaa'
"undef" is always treated as numeric, and in particular is changed to 0
before incrementing (so that a post-increment of an undef value will
return 0 rather than "undef").
The auto-decrement operator is not magical.
Exponentiation
Binary "**" is the exponentiation operator. It binds even more tightly
than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is implemented
using C's pow(3) function, which actually works on doubles internally.)
Symbolic Unary Operators
Unary "!" performs logical negation, i.e., "not". See also "not" for a
lower precedence version of this.
Unary "-" performs arithmetic negation if the operand is numeric. 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 to 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.
Binding Operators
Binary "=~" binds a scalar expression to a pattern match. Certain
operations search or modify the string $_ by default. This operator
makes that kind of operation work on some other string. The right
argument is a search pattern, substitution, or transliteration. The left
argument is what is supposed to be searched, substituted, or
transliterated instead of the default $_. When used in scalar context,
the return value generally indicates the success of the operation.
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.
Binary "!~" is just like "=~" except the return value is negated in the
logical sense.
Multiplicative Operators
Binary "*" multiplies two numbers.
Binary "/" divides two numbers.
Binary "%" computes the modulus of two numbers. Given integer operands
$a and $b: If $b is positive, then "$a % $b" is $a minus the largest
multiple of $b that is not greater than $a. If $b is negative, then "$a
% $b" is $a minus the smallest multiple of $b that is not less than $a
(i.e. the result will be less than or equal to zero). Note that when
"use integer" is in scope, "%" gives you direct access to the modulus
operator as implemented by your C compiler. This operator is not as well
defined for negative operands, but it will execute faster.
Binary "x" is the repetition operator. In scalar context or if the left
operand is not enclosed in parentheses, it returns a string consisting
of the left operand repeated the number of times specified by the right
operand. In list context, if the left operand is enclosed in
parentheses, it repeats the list. If the right operand is zero or
negative, it returns an empty string or an empty list, depending on the
context.
print '-' x 80; # print row of dashes
print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
@ones = (1) x 80; # a list of 80 1's
@ones = (5) x @ones; # set all elements to 5
Additive Operators
Binary "+" returns the sum of two numbers.
Binary "-" returns the difference of two numbers.
Binary "." concatenates two strings.
Shift Operators
Binary "<<" returns the value of its left argument shifted left by the
number of bits specified by the right argument. Arguments should be
integers. (See also "Integer Arithmetic".)
Binary ">>" returns the value of its left argument shifted right by the
number of bits specified by the right argument. Arguments should be
integers. (See also "Integer Arithmetic".)
Note that both "<<" and ">>" in Perl are implemented directly using "<<"
and ">>" in C. If "use integer" (see "Integer Arithmetic") is in force
then signed C integers are used, else unsigned C integers are used.
Either way, the implementation isn't going to generate results larger
than the size of the integer type Perl was built with (32 bits or 64
bits).
The result of overflowing the range of the integers is undefined because
it is undefined also in C. In other words, using 32-bit integers, "1 <<
32" is undefined. Shifting by a negative number of bits is also
undefined.
Named Unary Operators
The various named unary operators are treated as functions with one
argument, with optional parentheses.
If any list operator (print(), etc.) or any unary operator (chdir(),
etc.) is followed by a left parenthesis as the next token, the operator
and arguments within parentheses are taken to be of highest precedence,
just like a normal function call. For example, because named unary
operators are higher precedence than ||:
chdir $foo || die; # (chdir $foo) || die
chdir($foo) || die; # (chdir $foo) || die
chdir ($foo) || die; # (chdir $foo) || die
chdir +($foo) || die; # (chdir $foo) || die
but, because * is higher precedence than named operators:
chdir $foo * 20; # chdir ($foo * 20)
chdir($foo) * 20; # (chdir $foo) * 20
chdir ($foo) * 20; # (chdir $foo) * 20
chdir +($foo) * 20; # chdir ($foo * 20)
rand 10 * 20; # rand (10 * 20)
rand(10) * 20; # (rand 10) * 20
rand (10) * 20; # (rand 10) * 20
rand +(10) * 20; # rand (10 * 20)
Regarding precedence, the filetest operators, like "-f", "-M", etc. are
treated like named unary operators, but they don't follow this
functional parenthesis rule. That means, for example, that
"-f($file).".bak"" is equivalent to "-f "$file.bak"".
See also "Terms and List Operators (Leftward)".
Relational Operators
Binary "<" returns true if the left argument is numerically less than
the right argument.
Binary ">" returns true if the left argument is numerically greater than
the right argument.
Binary "<=" returns true if the left argument is numerically less than
or equal to the right argument.
Binary ">=" returns true if the left argument is numerically greater
than or equal to the right argument.
Binary "lt" returns true if the left argument is stringwise less than
the right argument.
Binary "gt" returns true if the left argument is stringwise greater than
the right argument.
Binary "le" returns true if the left argument is stringwise less than or
equal to the right argument.
Binary "ge" returns true if the left argument is stringwise greater than
or equal to the right argument.
Equality Operators
Binary "==" returns true if the left argument is numerically equal to
the right argument.
Binary "!=" returns true if the left argument is numerically not equal
to the right argument.
Binary "<=>" returns -1, 0, or 1 depending on whether the left argument
is numerically less than, equal to, or greater than the right argument.
If your platform supports NaNs (not-a-numbers) as numeric values, using
them with "<=>" returns undef. NaN is not "<", "==", ">", "<=" or ">="
anything (even NaN), so those 5 return false. NaN != NaN returns true,
as does NaN != anything else. If your platform doesn't support NaNs then
NaN is just a string with numeric value 0.
perl -le '$a = NaN; print "No NaN support here" if $a == $a'
perl -le '$a = NaN; print "NaN support here" if $a != $a'
Binary "eq" returns true if the left argument is stringwise equal to the
right argument.
Binary "ne" returns true if the left argument is stringwise not equal to
the right argument.
Binary "cmp" returns -1, 0, or 1 depending on whether the left argument
is stringwise less than, equal to, or greater than the right argument.
"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order
specified by the current locale if "use locale" is in effect. See
perllocale.
Bitwise And
Binary "&" returns its operands ANDed together bit by bit. (See also
"Integer Arithmetic" and "Bitwise String Operators".)
Note that "&" has lower priority than relational operators, so for
example the brackets are essential in a test like
print "Even\n" if ($x & 1) == 0;
Bitwise Or and Exclusive Or
Binary "|" returns its operands ORed together bit by bit. (See also
"Integer Arithmetic" and "Bitwise String Operators".)
Binary "^" returns its operands XORed together bit by bit. (See also
"Integer Arithmetic" and "Bitwise String Operators".)
Note that "|" and "^" have lower priority than relational operators, so
for example the brackets are essential in a test like
print "false\n" if (8 | 2) != 10;
C-style Logical And
Binary "&&" performs a short-circuit logical AND operation. That is, if
the left operand is false, the right operand is not even evaluated.
Scalar or list context propagates down to the right operand if it is
evaluated.
C-style Logical Or
Binary "||" performs a short-circuit logical OR operation. That is, if
the left operand is true, the right operand is not even evaluated.
Scalar or list context propagates down to the right operand if it is
evaluated.
The "||" and "&&" operators return the last value evaluated (unlike C's
"||" and "&&", which return 0 or 1). Thus, a reasonably portable way to
find out the home directory might be:
$home = $ENV{'HOME'} || $ENV{'LOGDIR'} ||
(getpwuid($<))[7] || die "You're homeless!\n";
In particular, this means that you shouldn't use this for selecting
between two aggregates for assignment:
@a = @b || @c; # this is wrong
@a = scalar(@b) || @c; # really meant this
@a = @b ? @b : @c; # this works fine, though
As more readable alternatives to "&&" and "||" when used for control
flow, Perl provides "and" and "or" operators (see below). The
short-circuit behavior is identical. The precedence of "and" and "or" is
much lower, however, so that you can safely use them after a list
operator without the need for parentheses:
unlink "alpha", "beta", "gamma"
or gripe(), next LINE;
With the C-style operators that would have been written like this:
unlink("alpha", "beta", "gamma")
|| (gripe(), next LINE);
Using "or" for assignment is unlikely to do what you want; see below.
Range Operators
Binary ".." is the range operator, which is really two different
operators depending on the context. In list context, it returns a list
of values counting (up by ones) from the left value to the right value.
If the left value is greater than the right value then it returns the
empty list. The range operator is useful for writing "foreach (1..10)"
loops and for doing slice operations on arrays. In the current
implementation, no temporary array is created when the range operator is
used as the expression in "foreach" loops, but older versions of Perl
might burn a lot of memory when you write something like this:
for (1 .. 1_000_000) {
# code
}
The range operator also works on strings, using the magical
auto-increment, see below.
In scalar context, ".." returns a boolean value. The operator is
bistable, like a flip-flop, and emulates the line-range (comma) operator
of sed, awk, and various editors. Each ".." operator maintains its own
boolean state. 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 considered true if it is equal ("==") to the current input line
number (the $. variable).
To be pedantic, the comparison is actually "int(EXPR) == int(EXPR)", but
that is only an issue if you use a floating point expression; when
implicitly using $. as described in the previous paragraph, the
comparison is "int(EXPR) == int($.)" which is only an issue when $. is
set to a floating point value and you are not reading from a file.
Furthermore, "span" .. "spat" or "2.18 .. 3.14" will not do what you
want in scalar context because each of the operands are evaluated using
their integer representation.
Examples:
As a scalar operator:
if (101 .. 200) { print; } # print 2nd hundred lines, short for
# if ($. == 101 .. $. == 200) ...
next line if (1 .. /^$/); # skip header lines, short for
# ... if ($. == 1 .. /^$/);
s/^/> / if (/^$/ .. eof()); # quote body
# parse mail messages
while (<>) {
$in_header = 1 .. /^$/;
$in_body = /^$/ .. eof;
if ($in_header) {
# ...
} else { # in body
# ...
}
} 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 English alphabet, or
$hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
to get a hexadecimal digit, or
@z2 = ('01' .. '31'); print $z2[$mday];
to get dates with leading zeros. If the final value specified is not in
the sequence that the magical increment would produce, the sequence goes
until the next value would be longer than the final value specified.
Because each operand is evaluated in integer form, "2.18 .. 3.14" will
return two elements in list context.
@list = (2.18 .. 3.14); # same as @list = (2 .. 3);
Conditional Operator
Ternary "?:" is the conditional operator, just as in C. It works much
like an if-then-else. If the argument before the ? is true, the argument
before the : is returned, otherwise the argument after the : is
returned. For example:
printf "I have %d dog%s.\n", $n,
($n == 1) ? '' : "s";
Scalar or list context propagates downward into the 2nd or 3rd argument,
whichever is selected.
$a = $ok ? $b : $c; # get a scalar
@a = $ok ? @b : @c; # get an array
$a = $ok ? @b : @c; # oops, that's just a count!
The operator may be assigned to if both the 2nd and 3rd arguments are
legal lvalues (meaning that you can assign to them):
($a_or_b ? $a : $b) = $c;
Because this operator produces an assignable result, using assignments
without parentheses will get you in trouble. For example, this:
$a % 2 ? $a += 10 : $a += 2
Really means this:
(($a % 2) ? ($a += 10) : $a) += 2
Rather than this:
($a % 2) ? ($a += 10) : ($a += 2)
That should probably be written more simply as:
$a += ($a % 2) ? 10 : 2;
Assignment Operators
"=" is the ordinary assignment operator.
Assignment operators work as in C. That is,
$a += 2;
is equivalent to
$a = $a + 2;
although without duplicating any side effects that dereferencing the
lvalue might trigger, such as from tie(). Other assignment operators
work similarly. The following are recognized:
**= += *= &= <<= &&=
-= /= |= >>= ||=
.= %= ^=
x=
Although these are grouped by family, they all have the precedence of
assignment.
Unlike in C, the scalar assignment operator produces a valid lvalue.
Modifying an assignment is equivalent to doing the assignment and then
modifying the variable that was assigned to. This is useful for
modifying a copy of something, like this:
($tmp = $global) =~ tr [A-Z] [a-z];
Likewise,
($a += 2) *= 3;
is equivalent to
$a += 2;
$a *= 3;
Similarly, a list assignment in list context produces the list of
lvalues assigned to, and a list assignment in scalar context returns the
number of elements produced by the expression on the right hand side of
the assignment.
Comma Operator
Binary "," is the comma operator. In scalar context it evaluates its
left argument, throws that value away, then evaluates its right argument
and returns that value. This is just like C's comma operator.
In list context, it's just the list argument separator, and inserts both
its arguments into the list.
The "=>" operator is a synonym for the comma, but forces any word to its
left to be interpreted as a string (as of 5.001). It is helpful in
documenting the correspondence between keys and values in hashes, and
other paired elements in lists.
List Operators (Rightward)
On the right side of a list operator, it has very low precedence, such
that it controls all comma-separated expressions found there. The only
operators with lower precedence are the logical operators "and", "or",
and "not", which may be used to evaluate calls to list operators without
the need for extra parentheses:
open HANDLE, "filename"
or die "Can't open: $!\n";
See also discussion of list operators in "Terms and List Operators
(Leftward)".
Logical Not
Unary "not" returns the logical negation of the expression to its right.
It's the equivalent of "!" except for the very low precedence.
Logical And
Binary "and" returns the logical conjunction of the two surrounding
expressions. It's equivalent to && except for the very low precedence.
This means that it short-circuits: i.e., the right expression is
evaluated only if the left expression is true.
Logical or and Exclusive Or
Binary "or" returns the logical disjunction of the two surrounding
expressions. It's equivalent to || except for the very low precedence.
This makes it useful for control flow
print FH $data or die "Can't write to FH: $!";
This means that it short-circuits: i.e., the right expression is
evaluated only if the left expression is false. Due to its precedence,
you should probably avoid using this for assignment, only for control
flow.
$a = $b or $c; # bug: this is wrong
($a = $b) or $c; # really means this
$a = $b || $c; # better written this way
However, when it's a list-context assignment and you're trying to use
"||" for control flow, you probably need "or" so that the assignment
takes higher precedence.
@info = stat($file) || die; # oops, scalar sense of stat!
@info = stat($file) or die; # better, now @info gets its due
Then again, you could always use parentheses.
Binary "xor" returns the exclusive-OR of the two surrounding
expressions. It cannot short circuit, of course.
C Operators Missing From Perl
Here is what C has that Perl doesn't:
unary & Address-of operator. (But see the "\" operator for taking a
reference.)
unary * Dereference-address operator. (Perl's prefix dereferencing
operators are typed: $, @, %, and &.)
(TYPE) Type-casting operator.
Quote and Quote-like Operators
While we usually think of quotes as literal values, in Perl they
function as operators, providing various kinds of interpolating and
pattern matching capabilities. Perl provides customary quote characters
for these behaviors, but also provides a way for you to choose your
quote character for any of them. In the following table, a "{}"
represents any pair of delimiters you choose.
Customary Generic Meaning Interpolates
'' q{} Literal no
"" qq{} Literal yes
`` qx{} Command yes*
qw{} Word list no
// m{} Pattern match yes*
qr{} Pattern yes*
s{}{} Substitution yes*
tr{}{} Transliteration no (but see below)
<<EOF here-doc yes*
* unless the delimiter is ''.
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 (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.
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
NOTE: Unlike C and other languages, Perl has no \v escape sequence for
the vertical tab (VT - ASCII 11).
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
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 "$" or "@" interpolates the corresponding variable, while
escaping will cause the literal string "\$" to be inserted. You'll need
to write something like "m/\Quser\E\@\Qhost/".
Patterns are subject to an additional level of interpretation as a
regular expression. This is done as a second pass, after variables are
interpolated, so that regular expressions may be incorporated into the
pattern from the variables. If this is not what you want, use "\Q" to
interpolate a variable literally.
Apart from the behavior described above, Perl does not expand multiple
levels of interpolation. In particular, contrary to the expectations of
shell programmers, back-quotes do *NOT* interpolate within double
quotes, nor do single quotes impede evaluation of variables when used
within double quotes.
Regexp Quote-Like Operators
Here are the quote-like operators that apply to pattern matching and
related activities.
?PATTERN?
This is just like the "/pattern/" search, except that it matches
only once between calls to the reset() operator. This is a
useful optimization when you want to see only the first
occurrence of something in each file of a set of files, for
instance. Only "??" patterns local to the current package are
reset.
while (<>) {
if (?^$?) {
# 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 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.
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 $Etc. The conditional is true if any variables were
assigned, i.e., if the pattern matched.
The "/g" modifier specifies global pattern matching--that is,
matching as many times as possible within the string. How it
behaves depends on the context. In list context, it returns a
list of the substrings matched by any capturing parentheses in
the regular expression. If there are no parentheses, it returns
a list of all the matched strings, as if there were parentheses
around the whole pattern.
In scalar context, each execution of "m//g" finds the next
match, returning true if it matches, and false if there is no
further match. The position after the last match can be read or
set using the pos() function; see "pos" in perlfunc. A failed
match normally resets the search position to the beginning of
the string, but you can avoid that by adding the "/c" modifier
(e.g. "m//gc"). Modifying the target string also resets the
search position.
You can intermix "m//g" matches with "m/\G.../g", where "\G" is
a zero-width assertion that matches the exact position where the
previous "m//g", if any, left off. Without the "/g" modifier,
the "\G" assertion still anchors at pos(), but the match is of
course only attempted once. Using "\G" without "/g" on a target
string that has not previously had a "/g" match applied to it is
the same as using the "\A" assertion to match the beginning of
the string. Note also that, currently, "\G" is only properly
supported when anchored at the very beginning of the pattern.
Examples:
# list context
($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
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 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:
$output = `cmd 2>&1`;
To capture a command's STDOUT but discard its STDERR:
$output = `cmd 2>/dev/null`;
To capture a command's STDERR but discard its STDOUT (ordering
is important here):
$output = `cmd 2>&1 1>/dev/null`;
To exchange a command's STDOUT and STDERR in order to capture
the STDERR but leave its STDOUT to come out the old STDERR:
$output = `cmd 3>&1 1>&2 2>&3 3>&-`;
To read both a command's STDOUT and its STDERR separately, it's
easiest to redirect them separately to files, and then read from
those files when the program is done:
system("program args 1>program.stdout 2>program.stderr");
Using single-quote as a delimiter protects the command from
Perl's double-quote interpolation, passing it on to the shell
instead:
$perl_info = qx(ps $$); # that's Perl's $$
$shell_info = qx'ps $$'; # that's the new shell's $$
How that string gets evaluated is entirely subject to the
command interpreter on your system. On most platforms, you will
have to protect shell metacharacters if you want them treated
literally. This is in practice difficult to do, as it's unclear
how to escape which characters. See perlsec for a clean and safe
example of a manual fork() and exec() to emulate backticks
safely.
On some platforms (notably DOS-like ones), the shell may not be
capable of dealing with multiline commands, so putting newlines
in the string may not get you what you want. You may be able to
evaluate multiple commands in a single line by separating them
with the command separator character, if your shell supports
that (e.g. ";" on many Unix shells; "&" on the Windows NT "cmd"
shell).
Beginning with v5.6.0, Perl will attempt to flush all files
opened for output before starting the child process, but this
may not be supported on some platforms (see perlport). To be
safe, you may need to set $| ($AUTOFLUSH in English) or call the
"autoflush()" method of "IO::Handle" on any open handles.
Beware that some command shells may place restrictions on the
length of the command line. You must ensure your strings don't
exceed this limit after any necessary interpolations. See the
platform-specific release notes for more details about your
particular environment.
Using this operator can lead to programs that are difficult to
port, because the shell commands called vary between systems,
and may in fact not be present at all. As one example, the
"type" command under the POSIX shell is very different from the
"type" command under DOS. That doesn't mean you should go out of
your way to avoid backticks when they're the right way to get
something done. Perl was made to be a glue language, and one of
the things it glues together is commands. Just understand what
you're getting yourself into.
See "I/O Operators" for more discussion.
qw/STRING/
Evaluates to a list of the words extracted out of STRING, using
embedded whitespace as the word delimiters. It can be understood
as being roughly equivalent to:
split(' ', q/STRING/);
the differences being that it generates a real list at compile
time, and in scalar context it returns the last element in the
list. So this expression:
qw(foo bar baz)
is semantically equivalent to the list:
'foo', 'bar', 'baz'
Some frequently seen examples:
use POSIX qw( setlocale localeconv )
@EXPORT = qw( foo bar baz );
A common mistake is to try to separate the words with comma or
to put comments into a multi-line "qw"-string. For this reason,
the "use warnings" pragma and the -w switch (that is, the $^W
variable) produces warnings if the STRING contains the "," or
the "#" character.
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 "eval"ed
before being run as a Perl expression.
Examples:
s/\bgreen\b/mauve/g; # don't change wintergreen
$path =~ s|/usr/bin|/usr/local/bin|;
s/Login: $foo/Login: $bar/; # run-time pattern
($foo = $bar) =~ s/this/that/; # copy first, then change
$count = ($paragraph =~ s/Mister\b/Mr./g); # get change-count
$_ = 'abc123xyz';
s/\d+/$&*2/e; # yields 'abc246xyz'
s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
s/%(.)/$percent{$1}/g; # change percent escapes; no /e
s/%(.)/$percent{$1} || $&/ge; # expr now, so /e
s/^=(\w+)/&pod($1)/ge; # use function call
# expand variables in $_, but dynamics only, using
# symbolic dereferencing
s/\$(\w+)/${$1}/g;
# Add one to the value of any numbers in the string
s/(\d+)/1 + $1/eg;
# This will expand any embedded scalar variable
# (including lexicals) in $_ : First $1 is interpolated
# to the variable name, and then evaluated
s/(\$\w+)/$1/eeg;
# Delete (most) C comments.
$program =~ s {
/\* # Match the opening delimiter.
.*? # Match a minimal number of characters.
\*/ # Match the closing delimiter.
} []gsx;
s/^\s*(.*?)\s*$/$1/; # trim white space in $_, expensively
for ($variable) { # trim white space in $variable, cheap
s/^\s+//;
s/\s+$//;
}
s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
Note the use of $ instead of \ in the last 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 "lc" in perlfunc and "uc" in perlfunc,
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 = $host) =~ tr/a-z/A-Z/;
tr/a-zA-Z/ /cs; # change non-alphas to single space
tr [\200-\377]
[\000-\177]; # delete 8th bit
If multiple transliterations are given for a character, only the
first one is used:
tr/AAA/XYZ/
will transliterate any A to X.
Because the transliteration table is built at compile time,
neither the SEARCHLIST nor the REPLACEMENTLIST are subjected to
double quote interpolation. That means that if you want to use
variables, you must use an eval():
eval "tr/$oldlist/$newlist/";
die $@ if $@;
eval "tr/$oldlist/$newlist/, 1" or die $@;
<<EOF A line-oriented form of quoting is based on the shell
"here-document" syntax. Following a "<<" you specify a string to
terminate the quoted material, and all lines following the
current line down to the terminating string are the value of the
item. The terminating string may be either an identifier (a
word), or some quoted text. 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.
Gory details of parsing quoted constructs
When presented with something that might have several different
interpretations, Perl uses the DWIM (that's "Do What I Mean") principle
to pick the most probable interpretation. This strategy is so successful
that Perl programmers often do not suspect the ambivalence of what they
write. But from time to time, Perl's notions differ substantially from
what the author honestly meant.
This section hopes to clarify how Perl handles quoted constructs.
Although the most common reason to learn this is to unravel labyrinthine
regular expressions, because the initial steps of parsing are the same
for all quoting operators, they are all discussed together.
The most important Perl parsing rule is the first one discussed below:
when processing a quoted construct, Perl first finds the end of that
construct, then interprets its contents. If you understand this rule,
you may skip the rest of this section on the first reading. The other
rules are likely to contradict the user's expectations much less
frequently than this first one.
Some passes discussed below are performed concurrently, but because
their results are the same, we consider them individually. For different
quoting constructs, Perl performs different numbers of passes, from one
to 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:
"$hash{"$foo/$bar"}"
or:
m/
bar # NOT a comment, this slash / terminated m//!
/x
do not form legal quoted expressions. The quoted part ends on the
first """ and "/", and the rest happens to be a syntax error.
Because the slash that terminated "m//" was followed by a "SPACE",
the example above is not "m//x", but rather "m//" with no "/x"
modifier. So the embedded "#" is interpreted as a literal "#".
Removal of backslashes before delimiters
During the second pass, text between the starting and ending
delimiters is copied to a safe location, and the "\" is removed from
combinations consisting of "\" and delimiter--or delimiters, meaning
both starting and ending delimiters will should these differ. This
removal does not happen for multi-character delimiters. Note that
the combination "\\" is left intact, just as it was.
Starting from this step no information about the delimiters is used
in parsing.
Interpolation
The next step is interpolation in the text obtained, which is now
delimiter-independent. There are four different cases.
"<<'EOF'", "m''", "s'''", "tr///", "y///"
No interpolation is performed.
'', "q//"
The only interpolation is removal of "\" from pairs "\\".
"", ``, "qq//", "qx//", "<file*glob>"
"\Q", "\U", "\u", "\L", "\l" (possibly paired with "\E") are
converted to corresponding Perl constructs. Thus,
"$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:
"a " . $b . " -> {c}";
or:
"a " . $b -> {c};
Most of the time, the longest possible text that does not
include spaces between components and which contains matching
braces or brackets. because the outcome may be determined by
voting based on heuristic estimators, the result is not strictly
predictable. Fortunately, it's usually correct for ambiguous
cases.
"?RE?", "/RE/", "m/RE/", "s/RE/foo/",
Processing of "\Q", "\U", "\u", "\L", "\l", and interpolation
happens (almost) as with "qq//" 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:
m m ^ a \s* b mmx;
In the RE above, which is intentionally obfuscated for
illustration, the delimiter is "m", the modifier is "mx", and
after 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 "Command Switches" in perlrun.
Optimization of regular expressions
This step is listed for completeness only. Since it does not change
semantics, details of this step are not documented and are subject
to change without notice. This step is performed over the finite
automaton that was generated during the previous pass.
It is at this stage that "split()" silently optimizes "/^/" to mean
"/^/m".
I/O Operators
There are several I/O operators you should know about.
A string enclosed by backticks (grave accents) first undergoes
double-quote interpolation. It is then interpreted as an external
command, and the output of that command is the value of the backtick
string, like in a shell. In scalar context, a single string consisting
of all output is returned. In list context, a list of values is
returned, one per line of output. (You can set $/ to use a different
line terminator.) The command is executed each time the pseudo-literal
is evaluated. The status value of the command is returned in $? (see
perlvar for the interpretation of $?). Unlike in csh, no translation is
done on the return data--newlines remain newlines. Unlike in any of the
shells, single quotes do not hide variable names in the command from
interpretation. To pass a literal dollar-sign through to the shell you
need to hide it with a backslash. The generalized form of backticks is
"qx//". (Because backticks always undergo shell expansion as well, see
perlsec for security concerns.)
In scalar context, evaluating a filehandle in angle brackets yields the
next line from that file (the newline, if any, included), or "undef" at
end-of-file or on error. When $/ is set to "undef" (sometimes known as
file-slurp mode) and the file is empty, it returns '' the first time,
followed by "undef" subsequently.
Ordinarily you must assign the returned value to a variable, but there
is one situation where an automatic assignment happens. If and only if
the input symbol is the only thing inside the conditional of a "while"
statement (even if disguised as a "for(;;)" loop), the value is
automatically assigned to the global variable $_, destroying whatever
was there previously. (This may seem like an odd thing to you, but
you'll use the construct in almost every Perl script you write.) The $_
variable is not implicitly localized. You'll have to put a "local $_;"
before the loop if you want that to happen.
The following lines are equivalent:
while (defined($_ = <STDIN>)) { print; }
while ($_ = <STDIN>) { print; }
while (<STDIN>) { print; }
for (;<STDIN>;) { print; }
print while defined($_ = <STDIN>);
print while ($_ = <STDIN>);
print while <STDIN>;
This also behaves similarly, but avoids $_ :
while (my $line = <STDIN>) { print $line }
In these loop constructs, the assigned value (whether assignment is
automatic or explicit) is then tested to see whether it is defined. The
defined test avoids problems where line has a string value that would be
treated as false by Perl, for example a "" or a "0" with no trailing
newline. If you really mean for such values to terminate the loop, they
should be tested for explicitly:
while (($_ = <STDIN>) ne '0') { ... }
while (<STDIN>) { last unless $_; ... }
In other boolean contexts, "<*filehandle*>" without an explicit
"defined" test or comparison elicit a warning if the "use warnings"
pragma or the -w command-line switch (the $^W variable) is in effect.
The filehandles STDIN, STDOUT, and STDERR are predefined. (The
filehandles "stdin", "stdout", and "stderr" will also work except in
packages, where they would be interpreted as local identifiers rather
than global.) Additional filehandles may be created with the open()
function, amongst others. See perlopentut and "open" in perlfunc for
details on this.
If a <FILEHANDLE> is used in a context that is looking for a list, a
list comprising all input lines is returned, one line per list element.
It's easy to grow to a rather large data space this way, so use with
care.
<FILEHANDLE> may also be spelled "readline(*FILEHANDLE)". See "readline"
in perlfunc.
The null filehandle <> is special: it can be used to emulate the
behavior of sed and awk. Input from <> comes either from standard input,
or from each file listed on the command line. Here's how it works: the
first time <> is evaluated, the @ARGV array is checked, and if it is
empty, $ARGV[0] is set to "-", which when opened gives you standard
input. The @ARGV array is then processed as a list of filenames. The
loop
while (<>) {
... # code for each line
}
is equivalent to the following Perl-like pseudo code:
unshift(@ARGV, '-') unless @ARGV;
while ($ARGV = shift) {
open(ARGV, $ARGV);
while (<ARGV>) {
... # code for each line
}
}
except that it isn't so cumbersome to say, and will actually work. It
really does shift the @ARGV array and put the current filename into the
$ARGV variable. It also uses filehandle *ARGV* internally--<> is just a
synonym for <ARGV>, which is magical. (The pseudo code above doesn't
work because it treats <ARGV> as non-magical.)
You can modify @ARGV before the first <> as long as the array ends up
containing the list of filenames you really want. Line numbers ($.)
continue as though the input were one big happy file. See the example in
"eof" in perlfunc for how to reset line numbers on each file.
If you want to set @ARGV to your own list of files, go right ahead. This
sets @ARGV to all plain text files if no @ARGV was given:
@ARGV = grep { -f && -T } glob('*') unless @ARGV;
You can even set them to pipe commands. For example, this automatically
filters compressed arguments through gzip:
@ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;
If you want to pass switches into your script, you can use one of the
Getopts modules or put a loop on the front like this:
while ($_ = $ARGV[0], /^-/) {
shift;
last if /^--$/;
if (/^-D(.*)/) { $debug = $1 }
if (/^-v/) { $verbose++ }
# ... # other switches
}
while (<>) {
# ... # code for each line
}
The <> symbol will return "undef" for end-of-file only once. If you call
it again after this, it will assume you are processing another @ARGV
list, and if you haven't set @ARGV, will read input from STDIN.
If what the angle brackets contain is a simple scalar variable (e.g.,
<$foo>), then that variable contains the name of the filehandle to input
from, or its typeglob, or a reference to the same. For example:
$fh = \*STDIN;
$line = <$fh>;
If what's within the angle brackets is neither a filehandle nor a simple
scalar variable containing a filehandle name, typeglob, or typeglob
reference, it is interpreted as a filename pattern to be globbed, and
either a list of filenames or the next filename in the list is returned,
depending on context. This distinction is determined on syntactic
grounds alone. That means "<$x>" is always a readline() from an indirect
handle, but "<$hash{key}>" is always a glob(). That's because $x is a
simple scalar variable, but $hash{key} is not--it's a hash element.
One level of double-quote interpretation is done first, but you can't
say "<$foo>" because that's an indirect filehandle as explained in the
previous paragraph. (In older versions of Perl, programmers would insert
curly brackets to force interpretation as a filename glob: "<${foo}>".
These days, it's considered cleaner to call the internal function
directly as "glob($foo)", which is probably the right way to have done
it in the first place.) For example:
while (<*.c>) {
chmod 0644, $_;
}
is roughly equivalent to:
open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
while (<FOO>) {
chomp;
chmod 0644, $_;
}
except that the globbing is actually done internally using the standard
"File::Glob" extension. Of course, the shortest way to do the above is:
chmod 0644, <*.c>;
A (file)glob evaluates its (embedded) argument only when it is starting
a new list. All values must be read before it will start over. In list
context, this isn't important because you automatically get them all
anyway. However, in scalar context the operator returns the next value
each time it's called, or "undef" when the list has run out. As with
filehandle reads, an automatic "defined" is generated when the glob
occurs in the test part of a "while", because legal glob returns (e.g. a
file called 0) would otherwise terminate the loop. Again, "undef" is
returned only once. So if you're expecting a single value from a glob,
it is much better to say
($file) = <blurch*>;
than
$file = <blurch*>;
because the latter will alternate between returning a filename and
returning false.
If you're trying to do variable interpolation, it's definitely better to
use the glob() function, because the older notation can cause people to
become confused with the indirect filehandle notation.
@files = glob("$dir/*.[ch]");
@files = glob($files[$i]);
Constant Folding
Like C, Perl does a certain amount of expression evaluation at compile
time whenever it determines that all arguments to an operator are static
and have no side effects. In particular, string concatenation happens at
compile time between literals that don't do variable substitution.
Backslash interpolation also happens at compile time. You can say
'Now is the time for all' . "\n" .
'good men to come to.'
and this all reduces to one string internally. Likewise, if you say
foreach $file (@filenames) {
if (-s $file > 5 + 100 * 2**16) { }
}
the compiler will precompute the number which that expression represents
so that the interpreter won't have to.
Bitwise String Operators
Bitstrings of any size may be manipulated by the bitwise operators ("~ |
& ^").
If the operands to a binary bitwise op are strings of different sizes, |
and ^ ops act as though the shorter operand had additional zero bits on
the right, while the & op acts as though the longer operand were
truncated to the length of the shorter. The granularity for such
extension or truncation is one or more bytes.
# ASCII-based examples
print "j p \n" ^ " a h"; # prints "JAPH\n"
print "JA" | " ph\n"; # prints "japh\n"
print "japh\nJunk" & '_____'; # prints "JAPH\n";
print 'p N$' ^ " E<H\n"; # prints "Perl\n";
If you are intending to manipulate bitstrings, be certain that you're
supplying bitstrings: If an operand is a number, that will imply a
numeric bitwise operation. You may explicitly show which type of
operation you intend by using "" or "0+", as in the examples below.
$foo = 150 | 105 ; # yields 255 (0x96 | 0x69 is 0xFF)
$foo = '150' | 105 ; # yields 255
$foo = 150 | '105'; # yields 255
$foo = '150' | '105'; # yields string '155' (under ASCII)
$baz = 0+$foo & 0+$bar; # both ops explicitly numeric
$biz = "$foo" ^ "$bar"; # both ops explicitly stringy
See "vec" in perlfunc for information on how to manipulate individual
bits in a bit vector.
Integer Arithmetic
By default, Perl assumes that it must do most of its arithmetic in
floating point. But by saying
use integer;
you may tell the compiler that it's okay to use integer operations (if
it feels like it) from here to the end of the enclosing BLOCK. An inner
BLOCK may countermand this by saying
no integer;
which lasts until the end of that BLOCK. Note that this doesn't mean
everything is only an integer, merely that Perl may use integer
operations if it is so inclined. For example, even under "use integer",
if you take the sqrt(2), you'll still get 1.4142135623731 or so.
Used on numbers, the bitwise operators ("&", "|", "^", "~", "<<", and
">>") always produce integral results. (But see also "Bitwise String
Operators".) However, "use integer" still has meaning for them. By
default, their results are interpreted as unsigned integers, but if "use
integer" is in effect, their results are interpreted as signed integers.
For example, "~0" usually evaluates to a large integral value. However,
"use integer; ~0" is -1 on twos-complement machines.
Floating-point Arithmetic
While "use integer" provides integer-only arithmetic, there is no
analogous mechanism to provide automatic rounding or truncation to a
certain number of decimal places. For rounding to a certain number of
digits, sprintf() or printf() is usually the easiest route. See
perlfaq4.
Floating-point numbers are only approximations to what a mathematician
would call real numbers. There are infinitely more reals than floats, so
some corners must be cut. For example:
printf "%.20g\n", 123456789123456789;
# produces 123456789123456784
Testing for exact equality of floating-point equality or inequality is
not a good idea. Here's a (relatively expensive) work-around to compare
whether two floating-point numbers are equal to a particular number of
decimal places. See Knuth, volume II, for a more robust treatment of
this topic.
sub fp_equal {
my ($X, $Y, $POINTS) = @_;
my ($tX, $tY);
$tX = sprintf("%.${POINTS}g", $X);
$tY = sprintf("%.${POINTS}g", $Y);
return $tX eq $tY;
}
The POSIX module (part of the standard perl distribution) implements
ceil(), floor(), and other mathematical and trigonometric functions. The
Math::Complex module (part of the standard perl distribution) defines
mathematical functions that work on both the reals and the imaginary
numbers. Math::Complex not as efficient as POSIX, but POSIX can't work
with complex numbers.
Rounding in financial applications can have serious implications, and
the rounding method used should be specified precisely. In these cases,
it probably pays not to trust whichever system rounding is being used by
Perl, but to instead implement the rounding function you need yourself.
Bigger Numbers
The standard Math::BigInt and Math::BigFloat modules provide
variable-precision arithmetic and overloaded operators, although they're
currently pretty slow. At the cost of some space and considerable speed,
they avoid the normal pitfalls associated with limited-precision
representations.
use Math::BigInt;
$x = Math::BigInt->new('123456789123456789');
print $x * $x;
# prints +15241578780673678515622620750190521
There are several modules that let you calculate with (bound only by
memory and cpu-time) unlimited or fixed precision. There are also some
non-standard modules that provide faster implementations via external C
libraries.
Here is a short, but incomplete summary:
Math::Fraction big, unlimited fractions like 9973 / 12967
Math::String treat string sequences like numbers
Math::FixedPrecision calculate with a fixed precision
Math::Currency for currency calculations
Bit::Vector manipulate bit vectors fast (uses C)
Math::BigIntFast Bit::Vector wrapper for big numbers
Math::Pari provides access to the Pari C library
Math::BigInteger uses an external C library
Math::Cephes uses external Cephes C library (no big numbers)
Math::Cephes::Fraction fractions via the Cephes library
Math::GMP another one using an external C library
Choose wisely.
]]>
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