下面按优先级的顺序解释每一个操作符.
如果任何列表操作符 (比如 print() ) 或任何一元操作符 (比如 chdir() ) 后面跟了一个左括号, 该操作符和它所有在括号中参数就有最高的优先级, 就象 普通的函数调用一样.
如果没有括号对, 象 print , sort , 或 chmod 这些列表操作符的优先级别要么很高, 要么很低, 取决于看向操作符的左边还是右边. 例如:
sort 右边的逗号在 sort 被计算前被计算, 而左边的逗号则在之后计算. 换句话说, 列表操作符总是要把其后的所有参数组合起来, 相对它前面的表达式整体作为一个词语. 要注意括号的使用:
也请注意
可能不会产生和一眼看上去应该有的运行结果. 参看 Named Unary Operators 查找更多有关的信息.
结构
do {}
和
eval {}
, 也被当作词语处理, 情况和调用子过程及方法一样, 匿名结构
[]
和 {}
也一样.
参看 Quote and Quotelike Operators 和 I/O Operators .
->
'' 操作符取消引用. 如果它的右边是
[...]
或 {...}
下标, 它的左边必须是对数组或哈希表的直接
或符号化引用(如果不是一个左值(可被赋值), 要是一个存放直接引用的单元).
参看
perlref
.
否则, 右边就是一个方法的名字或是一个存放方法名字的简单数值变量, 而左边必须是 一个个对象(一个 blessed reference)或是一个类名(一个包名). 参看 perlobj .
自动增量操作符还有一个小小的特别功能. 如果对一个数字变量进行增量操作, 或者
只要在数字上下文里使用, 增量的结果没有什么特别. 但如果对一个字符串变量增量,
或者是在字符串上下文里使用增量操作, 对象是满足模式 /^[a-zA-Z]*[0-9]*$/
,
那么增量操作是针对整个字符串, 结果保持在模式范围内, 操作是带进位的:
自动减量操作没有这个特性.
not
, 这是个优先级稍低的版本.
一元 ``-'' 对数字操作数是算术的负号. 作用于字符串时, 如果字符串是标识符, 返回负号
后跟标识符组成的字符串. 否则返回以负号开头的字符串. 这些规则的意思是
-bareword
就等于 ``-bareword''
.
一元 ``~'' 代表按位取反, 即 1 的补码.
一元 ``+'' 不起任何作用, 不管是对数字或字符串. 它的用处是在语法上把函数名 和后面带括号的表达式隔开, 避免后者被当做参数处理. (参看 List Operators 中的例子)
一元 ``\'' 代表引用它后面的东西. 参看 perlref . 这个含义和字符串里的反斜杠的含义不同, 虽然两者都有避免后随被解释的作用.
/o
.)
二元 ``!~'' 和 ``=~'' 的用法一样, 但返回值是逻辑上相反的.
二元 ``/'' 把两个数字相除.
二元 ``%'' 把两个数字求模.
二元 ``x'' 是重复操作符. 在数值上下文里, 返回一个字符串, 由左边的操作数 重复右边操作数指定的次数组成. 在列表上下文里, 如果左边的操作数是括号括住 的列表, 就重复这个列表.
二元 ``-'' 把两个数字相减.
二元 ``.'' 连接两个字符串.
二元 ``>>'' 把左边的参数向右移位右边参数指定的次数. 参数必须是整数.
-f
, -M
, 等等. 参看
perlfunc
.
如果任何列表操作符 ( print() , 等等.) 或者任何一元操作符 ( chdir() , 等等.) 后跟有左括号, 该操作符和括号里的参数一起具有最高的优先级, 就象普通的 函数调用一样. 例如:
但是, * 的优先级比 || 高:
参看``List Operators''.
二元 ``>'' 返回真, 如果左边的参数在数字上大于右边的参数.
二元 ``<='' 返回真, 如果左边的参数在数字上小于或等于右边的参数.
二元 ``>='' 返回真, 如果左边的参数在数字上大于或等于右边的参数.
二元 ``lt'' 返回真, 如果左边的参数在字符顺序上小于右边的参数.
二元 ``gt'' 返回真, 如果左边的参数在字符顺序上大于右边的参数.
二元 ``le'' 返回真, 如果左边的参数在字符顺序上小于或等于右边的参数.
二元 ``ge'' 返回真, 如果左边的参数在字符顺序上大于或等于右边的参数.
二元 ``!='' 返回真, 如果左边的参数在数字上不等于右边的参数.
二元 ``<=>'' 分别返回 -1, 0, 或 1, 取决于左边的参数在数字上是小于, 等于, 或大于 右边的参数.
二元 ``eq'' 返回真, 如果左边的参数在字符顺序上等于右边的参数.
二元 ``ne'' 返回真, 如果左边的参数在字符顺序上不等于右边的参数.
二元 ``cmp'' 分别返回 -1, 0, 或 1, 取决于左边的参数在字符顺序上是小于, 等于, 或大于 右边的参数.
二元 ``^'' 返回两个操作数按位进行 异或 操作的结果
||
和 &&
与 C 的不同之处是返回值不一定是
0 或 1, 而是最后的计算结果. 因此, 能正确找出 home 目录(不会是 ``0'')的方法应该是:
为了增加程序的可读性, Perl 提供了 ``and'' 和 ``or'' 操作符(见下). 快速操作的特点 是一样的. 但 ``and'' 和 ``or'' 的优先级要低很多, 所以在不用括号也可以安全地用在 列表操作符后:
用 C 风格操作符就要写成这样:
for (1..10)
循环, 或者是对数组做分片操作. 由于当前的实现方式中
使用了临时数组, 所以下面的代码会浪费很多内存:
在数值上下文里, ``..'' 返回一个布尔值. 操作符是对位的, 象开关一样, 功能 类似 sed, awk 等其它编辑器里的行范围(逗号) 操作符. 每个 ``..'' 操作符会维护自己的布尔值状态. 如果左边的操作数是假, 状态为假. 如果左操作数为真, 状态就是真, 直到右边的操作数也为真, 状态变回假. (在范围操作符被再次计算时, 状态才变回假. 它可以测试右操作数并在变成真值的 计算过程中变成假值(象在 awk 里一样), 但还是会变到真值一次. 如果不想等到下一次计算才测试右操作数(象在 sed 里一样), 可以用三点 (``...'') 代替两点.) 当操作符处于 ``假'' 状态时, 右操作数不会被计算, 而当状态为 ``真'' 时, 左操作数不会被计算. 操作符的优先级稍低于 || 和 &&. 返回值是空字符串 代表假值, 或是一串数字(从 1 开始)代表真值. 当范围到达时这串数字被复位。 范围里最后一个数字有追加的字符串 ``E0'', 但不影响它 的数字值, 用来提供到达尾部的信息. 如果要忽略开始点, 可以等待大于 1 的数字. 如果 ``..'' 的左右操作数都不是数字, 操作数隐含地和当前行号变量 $. 进行比较. 例子:
作为数值操作符:
作为列表操作符:
如果操作数是字符串, 范围操作符(在列表上下文里)使用特别的增量算法. 例如用
表示所有英文字母, 或者
表示所有16进制数字, 或者
打印两位格式的日期. 如果最后指定的值不在算法能产生的范围内, 那么一直计算到 下一个值的长度比最后的值大为止.
数值上下文或列表上下文会被传送给选中的参数.
如果第2,3参数都是合法的左值, 操作符可以被赋值:
但这样写的程序读起来不太容易.
赋值符和 C 里的作用一样. 即
等同于
但不象 tie() 那样会带来反引用左值的副作用. 其他赋值操作符的使用是类似的:
**= += *= &= <<= &&= -= /= |= >>= ||= .= %= ^= x=
要注意这些操作符的优先级比等号高.
和 C 不同, 赋值操作符产生一个有效的左值. 对赋值进行修改等于先赋值, 再修改 用于赋值的变量. 如:
还有
等同于
In a list context, it's just the list argument separator, and inserts both its arguments into the list.
The => digraph is mostly just a synonym for the comma operator. It's useful for documenting arguments that come in pairs. As of release 5.001, it also forces any word to the left of it to be interpreted as a string.
See also discussion of list operators in List Operators (Leftward).
Binary ``xor'' returns the exclusive-OR of the two surrounding expressions. It cannot short circuit, of course.
{}
represents
any pair of delimiters you choose. Non-bracketing delimiters use
the same character fore and aft, but the 4 sorts of brackets
(round, angle, square, curly) will all nest.
For constructs that do interpolation, variables beginning with ``$
'' 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). If
backquotes are used, the replacement string is a command to execute
whose output will be used as the actual replacement text. If the
PATTERN is delimited by bracketing quotes, the REPLACEMENT has its own
pair of quotes, which may or may not be bracketing quotes, e.g.
s(foo)(bar)
or s<foo>/bar/
. A /e
will cause the
replacement portion to be interpreter as a full-fledged Perl expression
and
eval()
ed right then and there. It is, however, syntax checked at
compile-time.
Examples:
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() :
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 <$foo>
because that's an indirect filehandle as explained in the previous
paragraph. In older version of Perl, programmers would insert curly
brackets to force interpretation as a filename glob: <${foo}>
.
These days, it's consdired 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 notatin.
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.