PERLOP(1)PERLOP(1)NAMEperlop - Perl operators and precedence
SYNOPSIS
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.)
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)
left not
left and
left or xor
In the following sections, these operators are covered in precedence
order.
DESCRIPTIONS
Terms and List Operators (Leftward)
Any TERM is of highest precedence of Perl. These includes variables,
quote and quotelike operators, any expression in parentheses, and any
function whose arguments are parenthesized. Actually, there aren't
really functions in this sense, just list operators and unary operators
behaving as functions because you put parentheses around the arguments.
These are all documented in the perlfunc manpage.
If any list operator (print(), etc.) or any unary operator (chdir(),
etc.) is followed by a left parenthesis as the next token, the
operator and arguments within parentheses are taken to be of highest
precedence, just like a normal function call.
In the absence of parentheses, the precedence of list operators such as
print, sort, or chmod is either very high or very low depending on
whether you look at the left side of operator or the right side of it.
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 parens:
# 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 Quotelike Operators toward the end of
this section, as well as the section on I/O Operators.
The Arrow Operator
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.
Autoincrement and Autodecrement
"++" 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 autoincrement operator has a little extra built-in 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 only been used in 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 autodecrement operator is not magical.
Exponentiation
Binary "**" is the exponentiation operator. Note that it binds even
more tightly than unary minus, so -2**4 is -(2**4), not (-2)**4.
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.
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 List
Operators.)
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
Binary "=~" binds an 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 $_. The return value indicates the success of the
operation. (If the right argument is an expression rather than a
search pattern, substitution, or translation, it is interpreted as a
search pattern at run time. This is less efficient than an explicit
search, since the pattern must be compiled every time the expression is
evaluated--unless you've used /o.)
Binary "!~" is just like "=~" except the return value is negated in the
logical sense.
Multiplicative Operators
Binary "*" multiplies two numbers.
Binary "/" divides two numbers.
Binary "%" computes the modulus of the two numbers.
Binary "x" is the repetition operator. In a scalar context, it returns
a string consisting of the left operand repeated the number of times
specified by the right operand. In a list context, if the left operand
is a list in parens, it repeats the list.
print '-' x 80; # print row of dashes
print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
@ones = (1) x 80; # a list of 80 1's
@ones = (5) x @ones; # set all elements to 5
Additive Operators
Binary "+" returns the sum of two numbers.
Binary "-" returns the difference of two numbers.
Binary "." concatenates two strings.
Shift Operators
Binary "<<" returns the value of its left argument shifted left by the
number of bits specified by the right argument. Arguments should be
integers.
Binary ">>" returns the value of its left argument shifted right by the
number of bits specified by the right argument. Arguments should be
integers.
Named Unary Operators
The various named unary operators are treated as functions with one
argument, with optional parentheses. These include the filetest
operators, like -f, -M, etc. See the perlfunc manpage.
If any list operator (print(), etc.) or any unary operator (chdir(),
etc.) is followed by a left parenthesis as the next token, the
operator and arguments within parentheses are taken to be of highest
precedence, just like a normal function call. 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 List Operators.
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.
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.
Bitwise And
Binary "&" returns its operators ANDed together bit by bit.
Bitwise Or and Exclusive Or
Binary "⎪" returns its operators ORed together bit by bit.
Binary "^" returns its operators XORed together bit by bit.
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 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
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
}
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 $_ 100 times
@foo = @foo[$[ .. $#foo]; # an expensive no-op
@foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
The range operator (in a list context) makes use of the magical
autoincrement algorithm if the operaands 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.
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. Scalar or list context propagates downward into the 2nd
or 3rd argument, whichever is selected. 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;
Note that this is not guaranteed to contribute to the readability of
your program.
Assigment Operators
"=" is the ordinary assignment operator.
Assignment operators work as in C. That is,
$a += 2;
is equivalent to
$a = $a + 2;
although without duplicating any side effects that dereferencing the
lvalue might trigger, such as from tie(). Other assignment operators
work similarly. The following are recognized:
**= += *= &= <<= &&=
-= /= ⎪= >>= ⎪⎪=
.= %= ^=
x=
Note that while these are grouped by family, they all have the
precedence of assignment.
Unlike in C, the assignment operator produces a valid lvalue.
Modifying an assignment is equivalent to doing the assignment and then
modifying the variable that was assigned to. This is useful for
modifying a copy of something, like this:
($tmp = $global) =~ tr [A-Z] [a-z];
Likewise,
($a += 2) *= 3;
is equivalent to
$a += 2;
$a *= 3;
Comma Operator
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 simply a synonym for the comma operator. It's useful
for documenting arguments that come in pairs.
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 the section on 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 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
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: $, @, %, and &.)
(TYPE) Type casting operator.
Quote and Quotelike Operators
While we usually think of quotes as literal values, in Perl they
function as operators, providing various kinds of interpolating and
pattern matching capabilities. Perl provides customary quote
characters for these behaviors, but also provides a way for you to
choose your quote character for any of them. In the following table, a
{} represents any pair of delimiters you choose. Non-bracketing
delimiters use the same character fore and aft, but the 4 sorts of
brackets (round, angle, square, curly) will all nest.
Customary Generic Meaning Interpolates
'' q{} Literal no
"" qq{} Literal yes
`` qx{} Command yes
qw{} Word list no
// m{} Pattern match yes
s{}{} Substitution yes
tr{}{} Translation no
For constructs that do interpolation, variables beginning with "$ or
"@" are interpolated, as are the following sequences:
\t tab
\n newline
\r return
\f form feed
\v vertical tab, whatever that is
\b backspace
\a alarm (bell)
\e escape
\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
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.
?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 only want to see 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.
m/PATTERN/gimosx
/PATTERN/gimosx
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.
Options are:
g Match globally, i.e. find all occurrences.
i Do case-insensitive pattern matching.
m Treat string as multiple lines.
o Only compile pattern 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).
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 most recently
executed (and successfully compiled) 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:
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 $Etc. The conditional is true if any variables were
assigned, i.e. if the pattern matched.
The /g modifier specifies global pattern matching--that is,
matching as many times as possible within the string. How it
behaves depends on the context. In a list context, it returns
a list of all the substrings matched by all the parentheses in
the regular expression. If there are no parentheses, it
returns a list of all the matched strings, as if there were
parentheses around the whole pattern.
In a scalar context, m//g iterates through the string,
returning TRUE each time it matches, and FALSE when it
eventually runs out of matches. (In other words, it remembers
where it left off last time and restarts the search at that
point. You can actually find the current match position of a
string using the pos() function--see the perlfunc manpage.) If
you modify the string in any way, the match position is reset
to the beginning. Examples:
# list context
($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);
# scalar context
$/ = ""; $* = 1; # $* deprecated in Perl 5
while ($paragraph = <>) {
while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
$sentences++;
}
}
print "$sentences\n";
q/STRING/
'STRING'
A single-quoted, literal string. Backslashes are ignored,
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.');
qq/STRING/
"STRING"
A double-quoted, interpolated string.
$_ .= qq
(*** The previous line contains the naughty word "$1".\n)
if /(tcl⎪rexx⎪python)/; # :-)
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 };
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 );
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 (0).
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 only want
the pattern compiled once the first time the variable is
interpolated, use the /o option. If the pattern evaluates to a
null string, the most recently executed (and successfully
compiled) regular expression is used instead. See the perlre
manpage for further explanation on these.
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 Only compile pattern 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). If backquotes are used, the replacement string is a
command to execute whose output will be used as the actual
replacement text. 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.
$program =~ s {
/\* (?# Match the opening delimiter.)
.*? (?# Match a minimal number of characters.)
\*/ (?# Match the closing delimiter.)
} []gsx;
s/^\s*(.*?)\s*$/$1/; # trim white space
s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
Note the use of $ instead of \ in the last example. Unlike
sed, we only use the \<digit> form in the left hand side.
Anywhere else it's $<digit>.
Occasionally, you can't just use 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, 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[A-Z][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
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
pseudo-literal, 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 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//.
Evaluating a filehandle in angle brackets yields the next line from
that file (newline included, so it's never false until end of file, at
which time an undefined value is returned). 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 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.
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:
unshift(@ARGV, '-') if $#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.
If the string inside angle brackets is not a filehandle, 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 <$foo> because that's an indirect filehandle as explained in the
previous paragraph. You could insert curly brackets to force
interpretation as a filename glob: <${foo}>. (Alternately, you can
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 just 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).
Constant Folding
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 pre-compute the number that expression represents so
that the interpreter won't have to.
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 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.
3rd Berkeley DistributionPERLOP(1)
