In the following sections, these operators are covered in precedence order.
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
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:
Also note that
probably doesn't do what you expect at first glance. See Named Unary Operators for more discussion of this.
Also parsed as terms are the do {} and eval {} constructs, as
well as subroutine and method calls, and the anonymous
constructors [] and {}.
See also Quote and Quotelike Operators toward the end of this section, as well as O Operators''.
->'' 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 .
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:
The autodecrement operator is not magical.
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 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.
/o.)
Binary ``!~'' is just like ``=~'' except the return value is negated in the logical sense.
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.
Binary ``-'' returns the difference of two numbers.
Binary ``.'' concatenates two strings.
Binary ``>>'' returns the value of its left argument shifted right by the number of bits specified by the right argument. Arguments should be integers.
-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:
but, because * is higher precedence than ||:
See also ``Terms and List Operators (Leftward)''.
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.
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.
Binary ``^'' returns its operators XORed together bit by bit.
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:
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:
With the C-style operators that would have been written like this:
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:
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:
As a list operator:
The range operator (in a list context) makes use of the magical autoincrement algorithm if the operands are strings. You can say
to get all the letters of the alphabet, or
to get a hexadecimal digit, or
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.
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):
This is not necessarily guaranteed to contribute to the readability of your program.
Assignment operators work as in C. That is,
is equivalent to
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:
Likewise,
is equivalent to
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.
See also discussion of list operators in Terms and List Operators (Leftward) .
Binary ``xor'' returns the exclusive-OR of the two surrounding expressions. It cannot short circuit, of course.
{} 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.
For constructs that do interpolation, variables beginning with ``$'' or ``@''
are interpolated, as are the following sequences:
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/ 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.
=~ 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:
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 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:
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:
See I/O Operators for more discussion.
Some frequently seen examples:
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 last successfully executed regular
expression is used instead. See the perlre manpage
for further explanation on these.
Options are:
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:
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:
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:
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:
If multiple translations are given for a character, only the first one is used:
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():
$/ 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//
. (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 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:
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 open for details on this.
If a <FILEHANDLE> is used in a context that is looking for a list, a list consisting of all the input lines is returned, one line per list element. It's easy to make a LARGE data space this way, so use with care.
The null filehandle <> is special and can be used to emulate the
behavior of sed and awk. Input from <> comes either from
standard input, or from each file listed on the command line. Here's
how it works: the first time <> is evaluated, the @ARGV array is
checked, and if it is null, $ARGV[0] is set to ``-'', which when opened
gives you standard input. The @ARGV array is then processed as a list
of filenames. The loop
is equivalent to the following Perl-like pseudo code:
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:
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:
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:
is equivalent to
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:
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).
A glob only evaluates its (embedded) argument 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
than
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.
and this all reduces to one string internally. Likewise, if you say
the compiler will pre-compute the number that expression represents so that the interpreter won't have to.
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
which lasts until the end of that BLOCK.