PERLOP(1) PERLOP(1)
perlop - Perl operators and precedence
Perl operators have the following associativity and precedence, listed
from highest precedence to lowest. Note that all operators borrowed from
C keep the same precedence relationship with each other, even where C's
precedence is slightly screwy. (This makes learning Perl easier for C
folks.) With very few exceptions, these all operate on scalar values
only, not array values.
left terms and list operators (leftward)
left ->
nonassoc ++ --
right **
right ! ~ \ and unary + and -
left =~ !~
left * / % x
left + - .
left << >>
nonassoc named unary operators
nonassoc < > <= >= lt gt le ge
nonassoc == != <=> eq ne cmp
left &
left | ^
left &&
left ||
nonassoc .. ...
right ?:
right = += -= *= etc.
left , =>
nonassoc list operators (rightward)
right not
left and
left or xor
In the following sections, these operators are covered in precedence
order.
Terms and List Operators (Leftward)
A TERM has the highest precedence in Perl. They includes variables,
quote and quote-like operators, any expression in parentheses, and any
function whose arguments are parenthesized. Actually, there aren't
really functions in this sense, just list operators and unary operators
behaving as functions because you put parentheses around the arguments.
These are all documented in the perlfunc manpage.
If any list operator (print(), etc.) or any unary operator (chdir(),
etc.) is followed by a left parenthesis as the next token, the operator
and arguments within parentheses are taken to be of highest precedence,
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just like a normal function call.
In the absence of parentheses, the precedence of list operators such as
print, sort, or chmod is either very high or very low depending on
whether you are looking at the left side or the right side of the
operator. For example, in
@ary = (1, 3, sort 4, 2);
print @ary; # prints 1324
the commas on the right of the sort are evaluated before the sort, but
the commas on the left are evaluated after. In other words, list
operators tend to gobble up all the arguments that follow them, and then
act like a simple TERM with regard to the preceding expression. Note
that you have to be careful with parentheses:
# These evaluate exit before doing the print:
print($foo, exit); # Obviously not what you want.
print $foo, exit; # Nor is this.
# These do the print before evaluating exit:
(print $foo), exit; # This is what you want.
print($foo), exit; # Or this.
print ($foo), exit; # Or even this.
Also note that
print ($foo & 255) + 1, "\n";
probably doesn't do what you expect at first glance. See the section on
Named Unary Operators for more discussion of this.
Also parsed as terms are the do {} and eval {} constructs, as well as
subroutine and method calls, and the anonymous constructors [] and {}.
See also the section on Quote and Quote-like Operators toward the end of
this section, as well as the section on I/O Operators.
The Arrow Operator [Toc] [Back]
Just as in C and C++, "->" is an infix dereference operator. If the
right side is either a [...] or {...} subscript, then the left side must
be either a hard or symbolic reference to an array or hash (or a location
capable of holding a hard reference, if it's an lvalue (assignable)).
See the perlref manpage.
Otherwise, the right side is a method name or a simple scalar variable
containing the method name, and the left side must either be an object (a
blessed reference) or a class name (that is, a package name). See the
perlobj manpage.
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Auto-increment and Auto-decrement
"++" and "--" work as in C. That is, if placed before a variable, they
increment or decrement the variable before returning the value, and if
placed after, increment or decrement the variable after returning the
value.
The auto-increment operator has a little extra builtin magic to it. If
you increment a variable that is numeric, or that has ever been used in a
numeric context, you get a normal increment. If, however, the variable
has been used in only string contexts since it was set, and has a value
that is not null and matches the pattern /^[a-zA-Z]*[0-9]*$/, the
increment is done as a string, preserving each character within its
range, with carry:
print ++($foo = '99'); # prints '100'
print ++($foo = 'a0'); # prints 'a1'
print ++($foo = 'Az'); # prints 'Ba'
print ++($foo = 'zz'); # prints 'aaa'
The auto-decrement operator is not magical.
Exponentiation [Toc] [Back]
Binary "**" is the exponentiation operator. Note that it binds even more
tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is
implemented using C's pow(3) function, which actually works on doubles
internally.)
Symbolic Unary Operators [Toc] [Back]
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. (See also the
section on Integer Arithmetic.)
Unary "+" has no effect whatsoever, even on strings. It is useful
syntactically for separating a function name from a parenthesized
expression that would otherwise be interpreted as the complete list of
function arguments. (See examples above under the section on Terms and
List Operators (Leftward).)
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Unary "\" creates a reference to whatever follows it. See the perlref
manpage. Do not confuse this behavior with the behavior of backslash
within a string, although both forms do convey the notion of protecting
the next thing from interpretation.
Binding Operators [Toc] [Back]
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 translation. The left
argument is what is supposed to be searched, substituted, or translated
instead of the default $_. The return value indicates the success of the
operation. (If the right argument is an expression rather than a search
pattern, substitution, or translation, it is interpreted as a search
pattern at run time. This can be is less efficient than an explicit
search, because the pattern must be compiled every time the expression is
evaluated.
Binary "!~" is just like "=~" except the return value is negated in the
logical sense.
Multiplicative Operators [Toc] [Back]
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 $a (i.e.
the result will be less than or equal to zero).
Binary "x" is the repetition operator. In a scalar context, it returns a
string consisting of the left operand repeated the number of times
specified by the right operand. In a list context, if the left operand
is a list in parentheses, it repeats the list.
print '-' x 80; # print row of dashes
print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
@ones = (1) x 80; # a list of 80 1's
@ones = (5) x @ones; # set all elements to 5
Additive Operators [Toc] [Back]
Binary "+" returns the sum of two numbers.
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Binary "-" returns the difference of two numbers.
Binary "." concatenates two strings.
Shift Operators [Toc] [Back]
Binary "<<" returns the value of its left argument shifted left by the
number of bits specified by the right argument. Arguments should be
integers. (See also the section on Integer Arithmetic.)
Binary ">>" returns the value of its left argument shifted right by the
number of bits specified by the right argument. Arguments should be
integers. (See also the section on Integer Arithmetic.)
Named Unary Operators [Toc] [Back]
The various named unary operators are treated as functions with one
argument, with optional parentheses. These include the filetest
operators, like -f, -M, etc. See the perlfunc manpage.
If any list operator (print(), etc.) or any unary operator (chdir(),
etc.) is followed by a left parenthesis as the next token, the operator
and arguments within parentheses are taken to be of highest precedence,
just like a normal function call. Examples:
chdir $foo || die; # (chdir $foo) || die
chdir($foo) || die; # (chdir $foo) || die
chdir ($foo) || die; # (chdir $foo) || die
chdir +($foo) || die; # (chdir $foo) || die
but, because * is higher precedence than ||:
chdir $foo * 20; # chdir ($foo * 20)
chdir($foo) * 20; # (chdir $foo) * 20
chdir ($foo) * 20; # (chdir $foo) * 20
chdir +($foo) * 20; # chdir ($foo * 20)
rand 10 * 20; # rand (10 * 20)
rand(10) * 20; # (rand 10) * 20
rand (10) * 20; # (rand 10) * 20
rand +(10) * 20; # rand (10 * 20)
See also the section on Terms and List Operators (Leftward).
Relational Operators
Binary "<" returns true if the left argument is numerically less than the
right argument.
Binary ">" returns true if the left argument is numerically greater than
the right argument.
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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 [Toc] [Back]
Binary "==" returns true if the left argument is numerically equal to the
right argument.
Binary "!=" returns true if the left argument is numerically not equal to
the right argument.
Binary "<=>" returns -1, 0, or 1 depending on whether the left argument
is numerically less than, equal to, or greater than the right argument.
Binary "eq" returns true if the left argument is stringwise equal to the
right argument.
Binary "ne" returns true if the left argument is stringwise not equal to
the right argument.
Binary "cmp" returns -1, 0, or 1 depending on whether the left argument
is stringwise less than, equal to, or greater than the right argument.
"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified
by the current locale if use locale is in effect. See the perllocale
manpage.
Bitwise And [Toc] [Back]
Binary "&" returns its operators ANDed together bit by bit. (See also
the section on Integer Arithmetic.)
Bitwise Or and Exclusive Or
Binary "|" returns its operators ORed together bit by bit. (See also the
section on Integer Arithmetic.)
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Binary "^" returns its operators XORed together bit by bit. (See also
the section on Integer Arithmetic.)
C-style Logical And [Toc] [Back]
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 [Toc] [Back]
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";
As more readable alternatives to && and ||, 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);
Range Operator [Toc] [Back]
Binary ".." is the range operator, which is really two different
operators depending on the context. In a list context, it returns an
array of values counting (by ones) from the left value to the right
value. This is useful for writing for (1..10) loops and for doing slice
operations on arrays. Be aware that under the current implementation, a
temporary array is created, so you'll burn a lot of memory if you write
something like this:
for (1 .. 1_000_000) {
# code
}
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In a scalar context, ".." returns a boolean value. The operator is
bistable, like a flip-flop, and emulates the line-range (comma) operator
of sed, awk, and various editors. Each ".." operator maintains its own
boolean state. It is false as long as its left operand is false. Once
the left operand is true, the range operator stays true until the right
operand is true, AFTER which the range operator becomes false again. (It
doesn't become false till the next time the range operator is evaluated.
It can test the right operand and become false on the same evaluation it
became true (as in awk), but it still returns true once. If you don't
want it to test the right operand till the next evaluation (as in sed),
use three dots ("...") instead of two.) The right operand is not
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 null 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 numeric literal, 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
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 a list context) makes use of the magical autoincrement
algorithm if the operands are strings. You can say
@alphabet = ('A' .. 'Z');
to get all the letters of the alphabet, or
$hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
to get a hexadecimal digit, or
@z2 = ('01' .. '31'); print $z2[$mday];
to get dates with leading zeros. If the final value specified is not in
the sequence that the magical increment would produce, the sequence goes
until the next value would be longer than the final value specified.
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Conditional Operator [Toc] [Back]
Ternary "?:" is the conditional operator, just as in C. It works much
like an if-then-else. If the argument before the ? is true, the argument
before the : is returned, otherwise the argument after the : is
returned. For example:
printf "I have %d dog%s.\n", $n,
($n == 1) ? '' : "s";
Scalar or list context propagates downward into the 2nd or 3rd argument,
whichever is selected.
$a = $ok ? $b : $c; # get a scalar
@a = $ok ? @b : @c; # get an array
$a = $ok ? @b : @c; # oops, that's just a count!
The operator may be assigned to if both the 2nd and 3rd arguments are
legal lvalues (meaning that you can assign to them):
($a_or_b ? $a : $b) = $c;
This is not necessarily guaranteed to contribute to the readability of
your program.
Assignment Operators
"=" is the ordinary assignment operator.
Assignment operators work as in C. That is,
$a += 2;
is equivalent to
$a = $a + 2;
although without duplicating any side effects that dereferencing the
lvalue might trigger, such as from tie(). Other assignment operators
work similarly. The following are recognized:
**= += *= &= <<= &&=
-= /= |= >>= ||=
.= %= ^=
x=
Note that while these are grouped by family, they all have the precedence
of assignment.
Unlike in C, the assignment operator produces a valid lvalue. Modifying
an assignment is equivalent to doing the assignment and then modifying
the variable that was assigned to. This is useful for modifying a copy
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of something, like this:
($tmp = $global) =~ tr [A-Z] [a-z];
Likewise,
($a += 2) *= 3;
is equivalent to
$a += 2;
$a *= 3;
Comma Operator [Toc] [Back]
Binary "," is the comma operator. In a 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 a list context, it's just the list argument separator, and inserts
both its arguments into the list.
The => digraph is mostly just a synonym for the comma operator. It's
useful for documenting arguments that come in pairs. As of release
5.001, it also forces any word to the left of it to be interpreted as a
string.
List Operators (Rightward) [Toc] [Back]
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 the section on Terms and List
Operators (Leftward).
Logical Not [Toc] [Back]
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 [Toc] [Back]
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
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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 means that it short-circuits: i.e., the right expression is
evaluated only if the left expression is false.
Binary "xor" returns the exclusive-OR of the two surrounding expressions.
It cannot short circuit, of course.
C Operators Missing From Perl [Toc] [Back]
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: $, @, %, and &.)
(TYPE) Type casting operator.
Quote and Quote-like Operators [Toc] [Back]
While we usually think of quotes as literal values, in Perl they function
as operators, providing various kinds of interpolating and pattern
matching capabilities. Perl provides customary quote characters for
these behaviors, but also provides a way for you to choose your quote
character for any of them. In the following table, a {} represents any
pair of delimiters you choose. Non-bracketing delimiters use the same
character fore and aft, but the 4 sorts of brackets (round, angle,
square, curly) will all nest.
Customary Generic Meaning Interpolates
'' q{} Literal no
"" qq{} Literal yes
`` qx{} Command yes
qw{} Word list no
// m{} Pattern match yes
s{}{} Substitution yes
tr{}{} Translation no
Note that there can be whitespace between the operator and the quoting
characters, except when # is being used as the quoting character. q#foo#
is parsed as being the string foo, which q #foo# is the operator q
followed by a comment. Its argument will be taken from the next line.
This allows you to write:
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s {foo} # Replace foo
{bar} # with bar.
For constructs that do interpolation, variables beginning with "$" or "@"
are interpolated, as are the following sequences:
\t tab (HT, TAB)
\n newline (LF, NL)
\r return (CR)
\f form feed (FF)
\b backspace (BS)
\a alarm (bell) (BEL)
\e escape (ESC)
\033 octal char
\x1b hex char
\c[ control char
\l lowercase next char
\u uppercase next char
\L lowercase till \E
\U uppercase till \E
\E end case modification
\Q quote regexp metacharacters till \E
If use locale is in effect, the case map used by \l, \L, \u and <\U> is
taken from the current locale. See the perllocale manpage.
Patterns are subject to an additional level of interpretation as a
regular expression. This is done as a second pass, after variables are
interpolated, so that regular expressions may be incorporated into the
pattern from the variables. If this is not what you want, use \Q to
interpolate a variable literally.
Apart from the above, there are no multiple levels of interpolation. In
particular, contrary to the expectations of shell programmers, backquotes
do NOT interpolate within double quotes, nor do single quotes
impede evaluation of variables when used within double quotes.
Regexp Quote-Like Operators [Toc] [Back]
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.
This usage is vaguely deprecated, and may be removed in some
future version of Perl.
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m/PATTERN/cgimosx
/PATTERN/cgimosx
Searches a string for a pattern match, and in a scalar context
returns true (1) or false (''). If no string is specified via
the =~ or !~ operator, the $_ string is searched. (The string
specified with =~ need not be an lvalue--it may be the result of
an expression evaluation, but remember the =~ binds rather
tightly.) See also the perlre manpage. See the perllocale
manpage for discussion of additional considerations which apply
when use locale is in effect.
Options are:
c Do not reset search position on a failed match when /g is in effect.
g Match globally, i.e., find all occurrences.
i Do case-insensitive pattern matching.
m Treat string as multiple lines.
o Compile pattern only once.
s Treat string as single line.
x Use extended regular expressions.
If "/" is the delimiter then the initial m is optional. With the
m you can use any pair of non-alphanumeric, non-whitespace
characters as delimiters. This is particularly useful for
matching Unix path names that contain "/", to avoid LTS (leaning
toothpick syndrome). If "?" is the delimiter, then the matchonly-once
rule of ?PATTERN? applies.
PATTERN may contain variables, which will be interpolated (and
the pattern recompiled) every time the pattern search is
evaluated. (Note that $) and $| might not be interpolated
because they look like end-of-string tests.) If you want such a
pattern to be compiled only once, add a /o after the trailing
delimiter. This avoids expensive run-time recompilations, and is
useful when the value you are interpolating won't change over the
life of the script. However, mentioning /o constitutes a promise
that you won't change the variables in the pattern. If you
change them, Perl won't even notice.
If the PATTERN evaluates to a null string, the last successfully
executed regular expression is used instead.
If used in a context that requires a list value, a pattern match
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.) If the match fails, a null array is returned. If the
match succeeds, but there were no parentheses, a list value of
(1) is returned.
Examples:
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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 $Etc. The conditional is true if any variables were
assigned, i.e., if the pattern matched.
The /g modifier specifies global pattern matching--that is,
matching as many times as possible within the string. How it
behaves depends on the context. In a list context, it returns a
list of all the substrings matched by all the parentheses in the
regular expression. If there are no parentheses, it returns a
list of all the matched strings, as if there were parentheses
around the whole pattern.
In a scalar context, m//g iterates through the string, returning
TRUE each time it matches, and FALSE when it eventually runs out
of matches. (In other words, it remembers where it left off last
time and restarts the search at that point. You can actually
find the current match position of a string or set it using the
pos() function; see the pos entry in the perlfunc manpage.) A
failed match normally resets the search position to the beginning
of the string, but you can avoid that by adding the /c modifier
(e.g. m//gc). Modifying the target string also resets the search
position.
You can intermix m//g matches with m/\G.../g, where \G is a
zero-width assertion that matches the exact position where the
previous m//g, if any, left off. The \G assertion is not
supported without the /g modifier; currently, without /g, \G
behaves just like \A, but that's accidental and may change in the
future.
Examples:
# list context
($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
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# scalar context
$/ = ""; $* = 1; # $* deprecated in modern perls
while (defined($paragraph = <>)) {
while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
$sentences++;
}
}
print "$sentences\n";
# using m//gc with \G
$_ = "ppooqppqq";
while ($i++ < 2) {
print "1: '";
print $1 while /(o)/gc; print "', pos=", pos, "\n";
print "2: '";
print $1 if /\G(q)/gc; print "', pos=", pos, "\n";
print "3: '";
print $1 while /(p)/gc; print "', pos=", pos, "\n";
}
The last example should print:
1: 'oo', pos=4
2: 'q', pos=5
3: 'pp', pos=7
1: '', pos=7
2: 'q', pos=8
3: '', pos=8
A useful idiom for lex-like scanners is /\G.../gc. You can
combine several regexps like this to process a string part-bypart,
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):
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PERLOP(1) PERLOP(1)
line-noise lowercase line-noise lowercase UPPERCASE line-noise
UPPERCASE line-noise lowercase line-noise lowercase line-noise
lowercase lowercase line-noise lowercase lowercase line-noise
MiXeD line-noise. That's all!
q/STRING/
'STRING'
A single-quoted, literal string. A backslash represents a
backslash unless followed by the delimiter or another backslash,
in which case the delimiter or backslash is interpolated.
$foo = q!I said, "You said, 'She said it.'"!;
$bar = q('This is it.');
$baz = '\n'; # a two-character string
qq/STRING/
"STRING"
A double-quoted, interpolated string.
$_ .= qq
(*** The previous line contains the naughty word "$1".\n)
if /(tcl|rexx|python)/; # :-)
$baz = "\n"; # a one-character string
qx/STRING/
`STRING`
A string which is interpolated and then executed as a system
command. The collected standard output of the command is
returned. In scalar context, it comes back as a single
(potentially multi-line) string. In list context, returns a list
of lines (however you've defined lines with $/ or
$INPUT_RECORD_SEPARATOR).
$today = qx{ date };
Note that how the 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. 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).
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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.
Also realize that using this operator frequently leads to
unportable programs.
See the section on I/O Operators for more discussion.
qw/STRING/
Returns a list of the words extracted out of STRING, using
embedded whitespace as the word delimiters. It is exactly
equivalent to
split(' ', q/STRING/);
Some frequently seen examples:
use POSIX qw( setlocale localeconv )
@EXPORT = qw( foo bar baz );
A common mistake is to try to separate the words with comma or to
put comments into a multi-line qw-string. For this reason the -w
switch produce warnings if the STRING contains the "," or the "#"
character.
s/PATTERN/REPLACEMENT/egimosx
Searches a string for a pattern, and if found, replaces that
pattern with the replacement text and returns the number of
substitutions made. Otherwise it returns false (specifically,
the empty string).
If no string is specified via the =~ or !~ operator, the $_
variable is searched and modified. (The string specified with =~
must be a scalar variable, an array element, a hash element, or
an assignment to one of those, i.e., an lvalue.)
If the delimiter chosen is single quote, no variable
interpolation is done on either the PATTERN or the REPLACEMENT.
Otherwise, if the PATTERN contains a $ that looks like a variable
rather than an end-of-string test, the variable will be
interpolated into the pattern at run-time. If you want the
pattern compiled only once the first time the variable is
interpolated, use the /o option. If the pattern evaluates to a
null string, the last successfully executed regular expression is
used instead. See the perlre manpage for further explanation on
these. See the perllocale manpage for discussion of additional
considerations which apply when use locale is in effect.
Options are:
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PERLOP(1) PERLOP(1)
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 interpreter as a full-fledged Perl expression and
eval()ed right then and there. It is, however, syntax checked at
compile-time.
Examples:
s/\bgreen\b/mauve/g; # don't change wintergreen
$path =~ s|/usr/bin|/usr/local/bin|;
s/Login: $foo/Login: $bar/; # run-time pattern
($foo = $bar) =~ s/this/that/;
$count = ($paragraph =~ s/Mister\b/Mr./g);
$_ = 'abc123xyz';
s/\d+/$&*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
# /e's can even nest; this will expand
# simple embedded variables in $_
s/(\$\w+)/$1/eeg;
# Delete C comments.
$program =~ s {
/\* # Match the opening delimiter.
.*? # Match a minimal number of characters.
\*/ # Match the closing delimiter.
} []gsx;
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PERLOP(1) PERLOP(1)
s/^\s*(.*?)\s*$/$1/; # trim white space
s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
Note the use of $ instead of \ in the last example. Unlike sed,
we use the \<digit> form in only the left hand side. Anywhere
else it's $<digit>.
Occasionally, you can't use just a /g to get all the changes to
occur. Here are two common cases:
# put commas in the right places in an integer
1 while s/(.*\d)(\d\d\d)/$1,$2/g; # perl4
1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g; # perl5
# expand tabs to 8-column spacing
1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;
tr/SEARCHLIST/REPLACEMENTLIST/cds
y/SEARCHLIST/REPLACEMENTLIST/cds
Translates 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
translated. (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.) 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[AZ][a-z]
or tr(+-*/)/ABCD/.
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 translated 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
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PERLOP(1) PERLOP(1)
character is replicated till it is long enough. If the
REPLACEMENTLIST is null, 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 = $host) =~ tr/a-z/A-Z/;
tr/a-zA-Z/ /cs; # change non-alphas to single space
tr [\200-\377]
[\000-\177]; # delete 8th bit
If multiple translations are given for a character, only the
first one is used:
tr/AAA/XYZ/
will translate any A to X.
Note that because the translation table is built at compile time,
neither the SEARCHLIST nor the REPLACEMENTLIST are subjected to
double quote interpolation. That means that if you want to use
variables, you must use an eval():
eval "tr/$oldlist/$newlist/";
die $@ if $@;
eval "tr/$oldlist/$newlist/, 1" or die $@;
I/O Operators
There are several I/O operators you should know about. A string is
enclosed by backticks (grave accents) first undergoes variable
substitution just like a double quoted string. It is then interpreted as
a command, and the output of that command is the value of the pseudoliteral,
like in a shell. In a scalar context, a single string
consisting of all the output is returned. In a list context, a list of
values is returned, one for each line of output. (You can set $/ to use
a different line terminator.) The command is executed each time the
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PERLOP(1) PERLOP(1)
pseudo-literal is evaluated. The status value of the command is returned
in $? (see the perlvar manpage for the interpretation of $?). Unlike in
csh, no translation is done on the return data--newlines remain newlines.
Unlike in any of the shells, single quotes do not hide variable names in
the command from interpretation. To pass a $ through to the shell you
need to hide it with a backslash. The generalized form of backticks is
qx//. (Because backticks always undergo shell expansion as well, see the
perlsec manpage for security concerns.)
Evaluating a filehandle in angle brackets yields the next line from that
file (newline, if any, included), or undef at end of file. Ordinarily
you must assign that value to a variable, but there is one situation
where an automatic assignment happens. If and ONLY if the input symbol
is the only thing inside the conditional of a while or for(;;) loop, the
value is automatically assigned to the variable $_. The assigned value
is then tested to see if it is defined. (This may seem like an odd thing
to you, but you'll use the construct in almost every Perl script you
write.) Anyway, the following lines are equivalent to each other:
while (defined($_ = <STDIN>)) { print; }
while (<STDIN>) { print; }
for (;<STDIN>;) { print; }
print while defined($_ = <STDIN>);
print while <STDIN>;
The filehandles STDIN, STDOUT, and STDERR are predefined. (The
filehandles stdin, stdout, and stderr will also work except in packages,
where they would be interpreted as local identifiers rather than global.)
Additional filehandles may be created with the open() function. See the
open() entry in the perlfunc manpage for details on this.
If a <FILEHANDLE> is used in a context that is looking for a list, a list
consisting of all the input lines is returned, one line per list element.
It's easy to make a LARGE data space this way, so use with care.
The null filehandle <> is special and can be used to emulate the behavior
of sed and awk. Input from <> comes either from standard input, or from
each file listed on the command line. Here's how it works: the first
time <> is evaluated, the @ARGV array is checked, and if it is null,
$ARGV[0] is set to "-", which when opened gives you standard input. The
@ARGV array is then processed as a list of filenames. The loop
while (<>) {
... # code for each line
}
is equivalent to the following Perl-like pseudo code:
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PERLOP(1) PERLOP(1)
unshift(@ARGV, '-') unless @ARGV;
while ($ARGV = shift) {
open(ARGV, $ARGV);
while (<ARGV>) {
... # code for each line
}
}
except that it isn't so cumbersome to say, and will actually work. It
really does shift array @ARGV and put the current filename into variable
$ARGV. It also uses filehandle ARGV internally--<> is just a synonym for
<ARGV>, which is magical. (The pseudo code above doesn't work because it
treats <ARGV> as non-magical.)
You can modify @ARGV before the first <> as long as the array ends up
containing the list of filenames you really want. Line numbers ($.)
continue as if the input were one big happy file. (But see example under
eof() for how to reset line numbers on each file.)
If you want to set @ARGV to your own list of files, go right ahead. 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/) { $verbose++ }
... # other switches
}
while (<>) {
... # code for each line
}
The <> symbol will return FALSE only once. If you call it again after
this it will assume you are processing another @ARGV list, and if you
haven't set @ARGV, will input from STDIN.
If the string inside the angle brackets is a reference to a scalar
variable (e.g., <$foo>), then that variable contains the name of the
filehandle to input from, or a reference to the same. For example:
$fh = \*STDIN;
$line = <$fh>;
If the string inside angle brackets is not a filehandle or a scalar
variable containing a filehandle name or reference, then 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. One level of $ interpretation is done first, but you can't say
<$foo> because that's an indirect filehandle as explained in the previous
paragraph. (In older versions of Perl, programmers would insert curly
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PERLOP(1) PERLOP(1)
brackets to force interpretation as a filename glob: <${foo}>. These
days, it's considered cleaner to call the internal function directly as
glob($foo), which is probably the right way to have done it in the first
place.) Example:
while (<*.c>) {
chmod 0644, $_;
}
is equivalent to
open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
while (<FOO>) {
chop;
chmod 0644, $_;
}
In fact, it's currently implemented that way. (Which means it will not
work on filenames with spaces in them unless you have csh(1) on your
machine.) Of course, the shortest way to do the above is:
chmod 0644, <*.c>;
Because globbing invokes a shell, it's often faster to call readdir()
yourself and do your own grep() on the filenames. Furthermore, due to
its current implementation of using a shell, the glob() routine may get
"Arg list too long" errors (unless you've installed tcsh(1L) as
/bin/csh).
A glob evaluates its (embedded) argument only when it is starting a new
list. All values must be read before it will start over. In a list
context this isn't important, because you automatically get them all
anyway. In a scalar context, however, the operator returns the next
value each time it is called, or a FALSE value if you've just run out.
Again, FALSE is returned only once. So if you're expecting a single
value from a glob, it is much better to say
($file) = <blurch*>;
than
$file = <blurch*>;
because the latter will alternate between returning a filename and
returning FALSE.
It you're trying to do variable interpolation, it's definitely better to
use the glob() function, because the older notation can cause people to
become confused with the indirect filehandle notation.
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@files = glob("$dir/*.[ch]");
@files = glob($files[$i]);
Constant Folding [Toc] [Back]
Like C, Perl does a certain amount of expression evaluation at compile
time, whenever it determines that all of the arguments to an operator are
static and have no side effects. In particular, string concatenation
happens at compile time between literals that don't do variable
substitution. Backslash interpretation also happens at compile time.
You can say
'Now is the time for all' . "\n" .
'good men to come to.'
and this all reduces to one string internally. Likewise, if you say
foreach $file (@filenames) {
if (-s $file > 5 + 100 * 2**16) { ... }
}
the compiler will precompute the number that expression represents so
that the interpreter won't have to.
Integer Arithmetic [Toc] [Back]
By default Perl assumes that it must do most of its arithmetic in
floating point. But by saying
use integer;
you may tell the compiler that it's okay to use integer operations from
here to the end of the enclosing BLOCK. An inner BLOCK may countermand
this by saying
no integer;
which lasts until the end of that BLOCK.
The bitwise operators ("&", "|", "^", "~", "<<", and ">>") always produce
integral results. However, use integer still has meaning for them. By
default, their results are interpreted as unsigned integers. However, if
use integer is in effect, their results are interpreted as signed
integers. For example, ~0 usually evaluates to a large integral value.
However, use integer; ~0 is -1.
Floating-point Arithmetic [Toc] [Back]
While use integer provides integer-only arithmetic, there is no similar
ways to provide rounding or truncation at a certain number of decimal
places. For rounding to a certain number of digits, sprintf() or
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PERLOP(1) PERLOP(1)
printf() is usually the easiest route.
The POSIX module (part of the standard perl distribution) implements
ceil(), floor(), and a number of other mathematical and trigonometric
functions. The Math::Complex module (part of the standard perl
distribution) defines a number of mathematical functions that can also
work on real numbers. Math::Complex not as efficient as POSIX, but POSIX
can't work with complex numbers.
Rounding in financial applications can have serious implications, and the
rounding method used should be specified precisely. In these cases, it
probably pays not to trust whichever system rounding is being used by
Perl, but to instead implement the rounding function you need yourself.
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PPPPaaaaggggeeee 22226666 [ Back ]
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