Perl内置变量详解：预定义和匹配信息

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在Perl编程语言中，预定义变量是已预先设定并由解释器在运行时自动管理的特殊变量，它们在编写代码时提供便利，简化了输入处理、模式匹配以及执行特定操作。本资源主要关注Perl中的几个关键预定义变量及其用途。 1. **$ARG**：默认输入和模式搜索空间。当你通过命令行参数传递数据给脚本时，$ARG 变量存储的是第一个参数。这个变量通常用于获取命令行输入的第一个元素，但其行为受命令行参数数量的影响。在循环中，`while(<>)` 或 `while(defined($_=<>){})` 是一种常见的用法，表示读取标准输入直到没有更多的数据。 2. **$_**：这是一个特殊的隐式变量，它在很多场景下作为默认的输入处理对象。在模式匹配时，$_ 代表当前输入行或捕获的模式。例如，`/def/; print "$`:$&:$'\n"` 这段代码中，$_ 在正则表达式匹配到 "def" 后，$`、$& 和 $' 分别代表预匹配（前导字符串）、匹配的字符串和后跟字符串。 3. **$a, $b**: 当在 sort() 函数中使用时，这些变量属于特殊包变量，用于指定排序规则。例如，`@articles = sort { $a cmp $b } @files;` 中，$a 和 $b 会根据预定义的比较操作符进行排序。 4. **$<digits>**: 这些变量与括号匹配的子模式有关，保存上一次正则表达式匹配中的子组内容。比如，如果你有一个复杂的模式匹配，$<1>, $<2>, 等等，可以访问相应的子字符串。 5. **$MATCH, $&, $PREMATCH, $POSTMATCH**: 这些变量分别对应于匹配到的字符串（$MATCH）、整个匹配（$&）、匹配前的文本（$PREMATCH）和匹配后的文本（$POSTMATCH）。它们在处理复杂模式匹配时非常有用，尤其是在处理前后文本上下文时。 6. **local $_**：此语句允许你在局部作用域内更改 $_ 的值，这对于临时改变输入处理或模式匹配行为很有用。 7. **$LAST_PATTERN, $+**: $LAST_PATTERN 可能是指最后一个成功模式匹配中的整个匹配，而 $+ 保存的是最后一个括号匹配中的内容。这两个变量有助于处理多重选择模式的情况，不知道具体哪个子模式匹配时，它们提供了额外的信息。总结来说，Perl 的预定义变量是编写高效和灵活代码的重要工具，它们简化了输入处理和模式匹配的逻辑，使得程序员能够更专注于业务逻辑。理解和熟练掌握这些变量的用法对于编写高效、可维护的Perl脚本至关重要。

Perl Predefined Variables

Variable Description Example

$ARG

The default input and pattern-searching space.

(Mnemonic: underline is understood in certain

operations.)

while (<>) {...} #equiv. only in while

while (defined($_ = <>)) {...}

Special package variables when using sort(), see

sort.

@articles = sort {$a cmp $b} @files;

$<digits>

Contains the sub-pattern from the corresponding

set of capturing parentheses from the last pattern

match. (Mnemonic: like \digits.)

$MATCH

The string matched by the last successful pattern

match. (Mnemonic: like & in some editors.)

$PREMATCH

The string preceding whatever was matched by the

last successful pattern match. (Mnemonic: ` often

precedes a quoted string.)

$POSTMATCH

$’

The string following whatever was matched by the

last successful pattern match. (Mnemonic: ' often

follows a quoted string.)

local $_ = 'abcdefghi';

/def/;

print "$` : $& : $'", "";

# prints abc : def : ghi

$LAST_PATTERN

_MATCH

The text matched by the last bracket of the last

successful search pattern.

This is useful if you don't know which one of a set

of alternative patterns matched. (Mnemonic: be

positive and forward looking.)

/Version: (.*)|Revision: (.*)/

($rev = $+);

$^N

The text matched by the used group most-recently

closed (i.e. the group with the rightmost closing

parenthesis) of the last successful search pattern.

(Mnemonic: the (possibly) Nested parenthesis that

most recently closed.)

$v = "sep:2:match";

$v =~ /(?:(\d)(?{ $a = $^N }))/;

print $a; # prints 2

@LAST_MATCH_END

This array holds the offsets of the ends of the last

successful submatches in the currently active

dynamic scope.

$+[0] is the offset into the string of

the end of the entire match.

$+[1] is the offset past where $1

ends.

You can use $#+ to determine how many

subgroups were in the last successful

match.

Set to a non-zero integer value to do multi-line

matching within a string, 0 (or undefined) to tell

Perl that it can assume that strings contain a single

line, for the purpose of optimizing pattern

matches. (Mnemonic: * matches multiple things.)

Use of $* is deprecated in modern

Perl, supplanted by the /s and /m

modifiers on pattern matching.

HANDLE->

input_line_number(EXPR)

$INPUT_LINE_NUMBER

$NR

Current line number for the last filehandle

accessed. (Mnemonic: many programs use "." to

mean the current line number.)

IO::Handle->

input_record_separator

(EXPR)

$INPUT_RECORD

_SEPARATOR

$RS

The input record separator, newline by default.

Setting $/ to a reference to an integer, scalar

containing an integer, or scalar that's convertible

to an integer will attempt to read records instead of

lines, with the maximum record size being the

referenced integer. (Mnemonic: / delimits line

boundaries when quoting poetry.)

local $/; # enable "slurp" mode

local $_ = <FH>; # whole file now here

HANDLE->

autoflush(EXPR)

$OUTPUT_AUTOFLUSH

If set to nonzero, forces a flush right away and

after every write or print on the currently selected

output channel. Default is 0. (Mnemonic: when

you want your pipes to be piping hot.)

IO::Handle->

output_field_separator

(EXPR)

$OUTPUT_FIELD

_SEPARATOR

$OFS

The output field separator for the print operator. If

defined, this value is printed between each of

print's arguments. Default is undef. (Mnemonic:

what is printed when there is a "," in your print

statement.)

@arr = (1,2,3);

$, = “ - ”

print @arr; # prints 1 – 2 - 3

IO::Handle->

output_record_separator

(EXPR)

$OUTPUT_RECORD

_SEPARATOR

$ORS

The output record separator for the print operator.

Default is undef.

(Mnemonic: you set $\ instead of adding "\n" at

the end of the print.)

@arr = (1, 2, “baz”);

$\ = “\t”

foreach (@arr) { print }

# prints 1 [tab] 2 [tab] baz

peter@catonmat.net

http://www.catonmat.net

good coders code, great reuse

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Perl内置变量详解：预定义和匹配信息

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