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PPPPEEEERRRRLLLLOOOOPPPP((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLOOOOPPPP((((1111))))
NNNNAAAAMMMMEEEE
perlop - Perl operators and precedence
SSSSYYYYNNNNOOOOPPPPSSSSIIIISSSS
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.
DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNNSSSS
TTTTeeeerrrrmmmmssss aaaannnndddd LLLLiiiisssstttt OOOOppppeeeerrrraaaattttoooorrrrssss ((((LLLLeeeeffffttttwwwwaaaarrrrdddd))))
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 _p_e_r_l_f_u_n_c
manpage.
If any list operator (_p_r_i_n_t(), etc.) or any unary operator
Page 1 (printed 6/30/95)
PPPPEEEERRRRLLLLOOOOPPPP((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLOOOOPPPP((((1111))))
(_c_h_d_i_r(), 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 _N_a_m_e_d _U_n_a_r_y _O_p_e_r_a_t_o_r_s 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 _Q_u_o_t_e _a_n_d _Q_u_o_t_e_l_i_k_e _O_p_e_r_a_t_o_r_s toward
the end of this section, as well as the section on _I/_O
_O_p_e_r_a_t_o_r_s.
TTTThhhheeee AAAArrrrrrrroooowwww OOOOppppeeeerrrraaaattttoooorrrr
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 _p_e_r_l_r_e_f
Page 2 (printed 6/30/95)
PPPPEEEERRRRLLLLOOOOPPPP((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLOOOOPPPP((((1111))))
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 _p_e_r_l_o_b_j
manpage.
AAAAuuuuttttooooiiiinnnnccccrrrreeeemmmmeeeennnntttt aaaannnndddd AAAAuuuuttttooooddddeeeeccccrrrreeeemmmmeeeennnntttt
"++" 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.
EEEExxxxppppoooonnnneeeennnnttttiiiiaaaattttiiiioooonnnn
Binary "**" is the exponentiation operator. Note that it
binds even more tightly than unary minus, so -2**4 is
-(2**4), not (-2)**4.
SSSSyyyymmmmbbbboooolllliiiicccc UUUUnnnnaaaarrrryyyy OOOOppppeeeerrrraaaattttoooorrrrssss
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.
Page 3 (printed 6/30/95)
PPPPEEEERRRRLLLLOOOOPPPP((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLOOOOPPPP((((1111))))
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 _L_i_s_t _O_p_e_r_a_t_o_r_s.)
Unary "\" creates a reference to whatever follows it. See
the _p_e_r_l_r_e_f 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.
BBBBiiiinnnnddddiiiinnnngggg OOOOppppeeeerrrraaaattttoooorrrrssss
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 $_. The return value indicates the success
of the operation. (If the right argument is an expression
rather than a search pattern, substitution, or translation,
it is interpreted as a search pattern at run time. This is
less efficient than an explicit search, since the pattern
must be compiled every time the expression is evaluated--
unless you've used /o.)
Binary "!~" is just like "=~" except the return value is
negated in the logical sense.
MMMMuuuullllttttiiiipppplllliiiiccccaaaattttiiiivvvveeee OOOOppppeeeerrrraaaattttoooorrrrssss
Binary "*" multiplies two numbers.
Binary "/" divides two numbers.
Binary "%" computes the modulus of the two numbers.
Binary "x" is the repetition operator. In a scalar context,
it returns a string consisting of the left operand repeated
the number of times specified by the right operand. In a
list context, if the left operand is a list in parens, it
repeats the list.
print '-' x 80; # print row of dashes
print "\t" x ($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
Page 4 (printed 6/30/95)
PPPPEEEERRRRLLLLOOOOPPPP((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLOOOOPPPP((((1111))))
AAAAddddddddiiiittttiiiivvvveeee OOOOppppeeeerrrraaaattttoooorrrrssss
Binary "+" returns the sum of two numbers.
Binary "-" returns the difference of two numbers.
Binary "." concatenates two strings.
SSSShhhhiiiifffftttt OOOOppppeeeerrrraaaattttoooorrrrssss
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.
NNNNaaaammmmeeeedddd UUUUnnnnaaaarrrryyyy OOOOppppeeeerrrraaaattttoooorrrrssss
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 _p_e_r_l_f_u_n_c
manpage.
If any list operator (_p_r_i_n_t(), etc.) or any unary operator
(_c_h_d_i_r(), 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 _L_i_s_t _O_p_e_r_a_t_o_r_s.
Page 5 (printed 6/30/95)
PPPPEEEERRRRLLLLOOOOPPPP((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLOOOOPPPP((((1111))))
RRRReeeellllaaaattttiiiioooonnnnaaaallll OOOOppppeeeerrrraaaattttoooorrrrssss
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.
EEEEqqqquuuuaaaalllliiiittttyyyy OOOOppppeeeerrrraaaattttoooorrrrssss
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.
BBBBiiiittttwwwwiiiisssseeee AAAAnnnndddd
Binary "&" returns its operators ANDed together bit by bit.
Page 6 (printed 6/30/95)
PPPPEEEERRRRLLLLOOOOPPPP((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLOOOOPPPP((((1111))))
BBBBiiiittttwwwwiiiisssseeee OOOOrrrr aaaannnndddd EEEExxxxcccclllluuuussssiiiivvvveeee OOOOrrrr
Binary "|" returns its operators ORed together bit by bit.
Binary "^" returns its operators XORed together bit by bit.
CCCC----ssssttttyyyylllleeee LLLLooooggggiiiiccccaaaallll AAAAnnnndddd
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.
CCCC----ssssttttyyyylllleeee LLLLooooggggiiiiccccaaaallll OOOOrrrr
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);
RRRRaaaannnnggggeeee OOOOppppeeeerrrraaaattttoooorrrr
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
Page 7 (printed 6/30/95)
PPPPEEEERRRRLLLLOOOOPPPP((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLOOOOPPPP((((1111))))
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 sssseeeedddd, aaaawwwwkkkk, 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, _A_F_T_E_R 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 aaaawwwwkkkk), but it still returns true once. If
you don't want it to test the right operand till the next
evaluation (as in sssseeeedddd), 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');
Page 8 (printed 6/30/95)
PPPPEEEERRRRLLLLOOOOPPPP((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLOOOOPPPP((((1111))))
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.
CCCCoooonnnnddddiiiittttiiiioooonnnnaaaallll OOOOppppeeeerrrraaaattttoooorrrr
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.
AAAAssssssssiiiiggggmmmmeeeennnntttt OOOOppppeeeerrrraaaattttoooorrrrssss
"=" is the ordinary assignment operator.
Assignment operators work as in C. That is,
$a += 2;
is equivalent to
$a = $a + 2;
although without duplicating any side effects that
dereferencing the lvalue might trigger, such as from _t_i_e().
Other assignment operators work similarly. The following are
recognized:
**= += *= &= <<= &&=
-= /= |= >>= ||=
.= %= ^=
x=
Note that while these are grouped by family, they all have
Page 9 (printed 6/30/95)
PPPPEEEERRRRLLLLOOOOPPPP((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLOOOOPPPP((((1111))))
the precedence of assignment.
Unlike in C, the assignment operator produces a valid
lvalue. Modifying an assignment is equivalent to doing the
assignment and then modifying the variable that was assigned
to. This is useful for modifying a copy of something, like
this:
($tmp = $global) =~ tr [A-Z] [a-z];
Likewise,
($a += 2) *= 3;
is equivalent to
$a += 2;
$a *= 3;
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.
LLLLiiiisssstttt OOOOppppeeeerrrraaaattttoooorrrrssss ((((RRRRiiiigggghhhhttttwwwwaaaarrrrdddd))))
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 _L_i_s_t
_O_p_e_r_a_t_o_r_s (_L_e_f_t_w_a_r_d).
LLLLooooggggiiiiccccaaaallll NNNNooootttt
Unary "not" returns the logical negation of the expression
to its right. It's the equivalent of "!" except for the
very low precedence.
Page 10 (printed 6/30/95)
PPPPEEEERRRRLLLLOOOOPPPP((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLOOOOPPPP((((1111))))
LLLLooooggggiiiiccccaaaallll AAAAnnnndddd
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.
LLLLooooggggiiiiccccaaaallll oooorrrr aaaannnndddd EEEExxxxcccclllluuuussssiiiivvvveeee OOOOrrrr
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.
CCCC OOOOppppeeeerrrraaaattttoooorrrrssss MMMMiiiissssssssiiiinnnngggg FFFFrrrroooommmm PPPPeeeerrrrllll
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: $, @, %, and &.)
(TYPE) Type casting operator.
QQQQuuuuooootttteeee aaaannnndddd QQQQuuuuooootttteeeelllliiiikkkkeeee OOOOppppeeeerrrraaaattttoooorrrrssss
While we usually think of quotes as literal values, in Perl
they function as operators, providing various kinds of
interpolating and pattern matching capabilities. Perl
provides customary quote characters for these behaviors, but
also provides a way for you to choose your quote character
for any of them. In the following table, a {} represents
any pair of delimiters you choose. Non-bracketing
delimiters use the same character fore and aft, but the 4
sorts of brackets (round, angle, square, curly) will all
nest.
Customary Generic Meaning Interpolates
'' q{} Literal no
"" qq{} Literal yes
`` qx{} Command yes
qw{} Word list no
// m{} Pattern match yes
s{}{} Substitution yes
tr{}{} Translation no
Page 11 (printed 6/30/95)
PPPPEEEERRRRLLLLOOOOPPPP((((1111)))) UUUUNNNNIIIIXXXX SSSSyyyysssstttteeeemmmm VVVV ((((RRRReeeelllleeeeaaaasssseeee 0000....0000 PPPPaaaattttcccchhhhlllleeeevvvveeeellll 00000000)))) PPPPEEEERRRRLLLLOOOOPPPP((((1111))))
For constructs that do interpolation, variables beginning
with "$ 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 _N_O_T 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 _r_e_s_e_t()
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,
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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 _p_e_r_l_r_e 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
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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 $Etc. The
conditional is true if any variables were assigned,
i.e. if the pattern matched.
The /g modifier specifies global pattern matching--
that is, matching as many times as possible within
the string. How it behaves depends on the context.
In a list context, it returns a list of all the
substrings matched by all the parentheses in the
regular expression. If there are no parentheses, it
returns a list of all the matched strings, as if
there were parentheses around the whole pattern.
In a scalar context, m//g iterates through the
string, returning TRUE each time it matches, and
FALSE when it eventually runs out of matches. (In
other words, it remembers where it left off last
time and restarts the search at that point. You can
actually find the current match position of a string
using the _p_o_s() function--see the _p_e_r_l_f_u_n_c manpage.)
If you modify the string in any way, the match
position is reset to the beginning. Examples:
# list context
($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/
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'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/
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 _O_p_e_r_a_t_o_r_s 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
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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 _p_e_r_l_r_e 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
_e_v_a_l()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|;
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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 sssseeeedddd, we only use the \<_d_i_g_i_t> form in the
left hand side. Anywhere else it's $<_d_i_g_i_t>.
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
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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
sssseeeedddd 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 ttttrrrr 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 = $host) =~ tr/a-z/A-Z/;
tr/a-zA-Z/ /cs; # change non-alphas to single space
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tr [\200-\377]
[\000-\177]; # delete 8th bit
Note that because the translation table is built at
compile time, neither the SEARCHLIST nor the
REPLACEMENTLIST are subjected to double quote
interpolation. That means that if you want to use
variables, you must use an _e_v_a_l():
eval "tr/$oldlist/$newlist/";
die $@ if $@;
eval "tr/$oldlist/$newlist/, 1" or die $@;
IIII////OOOO OOOOppppeeeerrrraaaattttoooorrrrssss
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
_p_e_r_l_v_a_r manpage for the interpretation of $?). Unlike in
ccccsssshhhh, 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. _I_f _a_n_d _O_N_L_Y _i_f the input symbol is the
only thing inside the conditional of a while loop, the value
is automatically assigned to the variable $_. (This may
seem like an odd thing to you, but you'll use the construct
in almost every Perl script you write.) Anyway, the
following lines are equivalent to each other:
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while ($_ = <STDIN>) { print; }
while (<STDIN>) { print; }
for (;<STDIN>;) { print; }
print while $_ = <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 _o_p_e_n() 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 _L_A_R_G_E data
space this way, so use with care.
The null filehandle <> is special and can be used to emulate
the behavior of sssseeeedddd and aaaawwwwkkkk. 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 $ARGV. It also uses filehandle _A_R_G_V
internally--<> is just a synonym for <ARGV>, which is
magical. (The pseudo code above doesn't work because it
treats <ARGV> as non-magical.)
You can modify @ARGV before the first <> as long as the
array ends up containing the list of filenames you really
want. Line numbers ($.) continue as if the input were one
big happy file. (But see example under _e_o_f() for how to
reset line numbers on each file.)
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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 _c_s_h(1) on your machine.) Of course, the shortest way
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to do the above is:
chmod 0644, <*.c>;
Because globbing invokes a shell, it's often faster to call
_r_e_a_d_d_i_r() yourself and just do your own _g_r_e_p() on the
filenames. Furthermore, due to its current implementation
of using a shell, the _g_l_o_b() routine may get "Arg list too
long" errors (unless you've installed _t_c_s_h(1L) as /_b_i_n/_c_s_h).
CCCCoooonnnnssssttttaaaannnntttt FFFFoooollllddddiiiinnnngggg
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.
IIIInnnntttteeeeggggeeeerrrr aaaarrrriiiitttthhhhmmmmeeeettttiiiicccc
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.
Page 22 (printed 6/30/95)