PERLOP(1)
NNAAMMEE
perlop - Perl operators and precedence
SSYYNNOOPPSSIISS
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.
DDEESSCCRRIIPPTTIIOONN
TTeerrmmss aanndd LLiisstt OOppeerraattoorrss ((LLeeffttwwaarrdd))
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, 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.
TThhee AArrrrooww OOppeerraattoorr
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.
AAuuttoo--iinnccrreemmeenntt aanndd AAuuttoo--ddeeccrreemmeenntt
"++" 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.
EExxppoonneennttiiaattiioonn
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.)
SSyymmbboolliicc UUnnaarryy OOppeerraattoorrss
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).)
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.
BBiinnddiinngg OOppeerraattoorrss
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.
MMuullttiipplliiccaattiivvee OOppeerraattoorrss
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
AAddddiittiivvee OOppeerraattoorrss
Binary "+" returns the sum of two numbers.
Binary "-" returns the difference of two numbers.
Binary "." concatenates two strings.
SShhiifftt OOppeerraattoorrss
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.)
NNaammeedd UUnnaarryy OOppeerraattoorrss
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).
RReellaattiioonnaall OOppeerraattoorrss
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.
EEqquuaalliittyy OOppeerraattoorrss
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.
BBiittwwiissee AAnndd
Binary "&" returns its operators ANDed together bit by
bit. (See also the section on Integer Arithmetic.)
BBiittwwiissee OOrr aanndd EExxcclluussiivvee OOrr
Binary "|" returns its operators ORed together bit by bit.
(See also the section on Integer Arithmetic.)
Binary "^" returns its operators XORed together bit by
bit. (See also the section on Integer Arithmetic.)
CC--ssttyyllee LLooggiiccaall AAnndd
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.
CC--ssttyyllee LLooggiiccaall OOrr
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);
RRaannggee OOppeerraattoorr
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 sseedd, aawwkk, 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 aawwkk), but it still
returns true once. If you don't want it to test the right
operand till the next evaluation (as in sseedd), 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 auto-increment 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.
CCoonnddiittiioonnaall OOppeerraattoorr
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.
AAssssiiggnnmmeenntt OOppeerraattoorrss
"=" 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;
CCoommmmaa OOppeerraattoorr
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.
LLiisstt OOppeerraattoorrss ((RRiigghhttwwaarrdd))
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).
LLooggiiccaall NNoott
Unary "not" returns the logical negation of the expression
to its right. It's the equivalent of "!" except for the
very low precedence.
LLooggiiccaall AAnndd
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.
LLooggiiccaall oorr aanndd EExxcclluussiivvee OOrr
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.
CC OOppeerraattoorrss MMiissssiinngg FFrroomm PPeerrll
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: $, @, %, and
&.)
(TYPE) Type casting operator.
QQuuoottee aanndd QQuuoottee--lliikkee OOppeerraattoorrss
While we usually think of quotes as literal values, in
Perl they function as operators, providing various kinds
of interpolating and pattern matching capabilities. Perl
provides customary quote characters for these behaviors,
but also provides a way for you to choose your quote
character for any of them. In the following table, a {}
represents any pair of delimiters you choose. Non-
bracketing delimiters use the same character fore and aft,
but the 4 sorts of brackets (round, angle, square, curly)
will all nest.
Customary Generic Meaning Interpolates
'' q{} Literal no
"" qq{} Literal yes
`` qx{} Command yes
qw{} Word list no
// m{} Pattern match yes
s{}{} Substitution yes
tr{}{} Translation no
Note that there can be whitespace between the operator and
the quoting characters, except when # is being used as the
quoting character. q#foo# is parsed as being the string
foo, which q #foo# is the operator q followed by a
comment. Its argument will be taken from the next line.
This allows you to write:
s {foo} # Replace foo
{bar} # with bar.
For constructs that do interpolation, variables beginning
with "$" 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, back-quotes do NOT
interpolate within double quotes, nor do single quotes
impede evaluation of variables when used within double
quotes.
RReeggeexxpp QQuuoottee--LLiikkee OOppeerraattoorrss
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.
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 match-only-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:
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);
# 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-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 /(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).
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:
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.
$program =~ s {
/\* # Match the opening delimiter.
.*? # Match a minimal number of characters.
\*/ # Match the closing delimiter.
} []gsx;
s/^\s*(.*?)\s*$/$1/; # trim white space
s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
Note the use of $ instead of \ in the last
example. Unlike sseedd, 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 sseedd 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 ttrr 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 $@;
II//OO OOppeerraattoorrss
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 ccsshh, 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 sseedd and aawwkk. 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, $ARGV[0] is set to "-", which when opened gives you
standard input. The @ARGV array is then processed as a
list of filenames. The loop
while (<>) {
... # code for each line
}
is equivalent to the following Perl-like pseudo code:
unshift(@ARGV, '-') unless @ARGV;
while ($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 <$foo> because that's an indirect filehandle as
explained in the previous paragraph. (In older versions
of Perl, programmers would insert curly brackets to force
interpretation as a filename glob: <${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
$file = <blurch*>;
because the latter will alternate between returning a
filename and returning FALSE.
It you're trying to do variable interpolation, it's
definitely better to use the glob() function, because the
older notation can cause people to become confused with
the indirect filehandle notation.
@files = glob("$dir/*.[ch]");
@files = glob($files[$i]);
CCoonnssttaanntt FFoollddiinngg
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.
IInntteeggeerr AArriitthhmmeettiicc
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.
FFllooaattiinngg--ppooiinntt AArriitthhmmeettiicc
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 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.