Perl程序员参考指南：提取与报告语言

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"Perl编程参考手册提供了全面的Perl编程信息，包括语言概述、更新变更、常见问题、文档目录、数据结构、语法、运算符、正则表达式、执行选项、内置函数、预定义变量、子程序、模块工作原理、模块编写与使用、模块安装指南、格式化、本地化支持、引用、数据结构介绍等多个方面，旨在帮助程序员深入理解和高效使用Perl语言。" Perl编程语言是一种强大的文本处理工具，最初设计用于报告生成和文本提取，因此得名“Practical Extraction and Report Language”。Perl具有灵活的语法和丰富的内置功能，使其在系统管理、网页开发、CGI脚本、数据库接口等多种领域都有广泛应用。 Perl的SYNOPSIS部分展示了如何运行Perl解释器。命令行选项包括： - `-s`、`-T`、`-u`、`-U`：用于特定的脚本执行模式。 - `-h`、`-v`：显示帮助和版本信息。 - `-V[:configvar]`：查看配置变量。 - `-c`、`-w`：编译检查和警告。 - `-d[:debugger]`：启用调试器。 - `-D[number/list]`：选择调试器标志。 - `-p`、`-n`、`-a`、`-Fpattern`、`-l[octal]`、`-0[octal]`：控制输入/输出行为。 - `-I<dir>`：添加搜索路径。 - `-m[−]module`、`-M[−]'module'`：加载模块。 - `-P`：使脚本看起来像C程序。 - `-S`：搜索并执行可执行文件。 - `-x[dir]`：执行脚本所在的目录。 - `-i[extension]`：就地编辑文件。 - `-e'command'`：执行单行Perl代码。 - `--`：标记命令行选项结束。 - `[programfile][argument]`：指定要执行的Perl脚本和参数。 Perl的文档被组织成多个部分，便于学习和查阅： 1. `perl`：Perl的总体概述，介绍基本概念和语言特性。 2. `perldelta`：记录了Perl版本之间的主要变化和新增功能。 3. `perlfaq`：Perl的常见问题解答，覆盖了许多使用场景。 4. `perltoc`：Perl文档的完整目录，帮助快速定位所需信息。 5. `perldata`：详细介绍了Perl中的数据类型，如标量、数组、哈希等。 6. `perlsyn`：讲解Perl的语法结构，包括语句、循环、条件判断等。 7. `perlop`：涵盖Perl的运算符及其优先级。 8. `perlre`：正则表达式的使用和高级特性。 9. `perlrun`：解析Perl的执行选项和命令行参数。 10. `perlfunc`：列出了Perl的所有内置函数。 11. `perlvar`：列出预定义的全局变量和它们的作用。 12. `perlsub`：讨论子程序（函数）的定义、调用和参数处理。 13. `perlmod`：解释Perl模块的工作原理。 14. `perlmodlib`：指导如何编写和使用Perl模块。 15. `perlmodinstall`：介绍了从CPAN安装模块的过程。 16. `perlform`：关于Perl的格式化输出。 17. `perllocale`：探讨Perl对本地化和国际化的支持。 18. `perlref`：解释如何创建和使用引用，以及引用的类型。 19. `perldsc`：为初学者介绍Perl的数据结构。通过深入阅读这些手册，开发者可以全面掌握Perl编程，提升编程技巧，解决实际问题，并利用Perl的强大功能进行高效编程。

PERLSYN(1) Perl Programmers Reference Guide PERLSYN(1)

if EXPR

unless EXPR

while EXPR

until EXPR

foreach EXPR

The if and unless modiﬁers have the expected semantics, presuming you’re a speaker of English. The

foreach modiﬁer is an iterator: For each value in EXPR, it aliases $_ to the value and executes the state-

ment. The while and until modiﬁers have the usual ‘‘while loop’’ semantics (conditional evaluated

ﬁrst), except when applied to a do−BLOCK (or to the now-deprecated do−SUBROUTINE statement), in

which case the block executes once before the conditional is evaluated. This is so that you can write loops

like:

do {

$line = <STDIN>;

...

} until $line eq ".\n";

See the do entry in the perlfunc manpage. Note also that the loop control statements described later will

NOT work in this construct, because modiﬁers don’t take loop labels. Sorry. You can always put another

block inside of it (for next) or around it (for last) to do that sort of thing. For next, just double the

braces:

do {{

next if $x == $y;

# do something here

}} until $x++ > $z;

For last, you have to be more elaborate:

LOOP: {

do {

last if $x = $y**2;

# do something here

} while $x++ <= $z;

}

CCoommppoouunndd ssttaatteemmeennttss

In Perl, a sequence of statements that deﬁnes a scope is called a block. Sometimes a block is delimited by

the ﬁle containing it (in the case of a required ﬁle, or the program as a whole), and sometimes a block is

delimited by the extent of a string (in the case of an eval).

But generally, a block is delimited by curly brackets, also known as braces. We will call this syntactic con-

struct a BLOCK.

The following compound statements may be used to control ﬂow:

if (EXPR) BLOCK

if (EXPR) BLOCK else BLOCK

if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK

LABEL while (EXPR) BLOCK

LABEL while (EXPR) BLOCK continue BLOCK

LABEL for (EXPR; EXPR; EXPR) BLOCK

LABEL foreach VAR (LIST) BLOCK

LABEL BLOCK continue BLOCK

Note that, unlike C and Pascal, these are deﬁned in terms of BLOCKs, not statements. This means that the

16 perl 5.005, patch 02 20/Jul/98

PERLSYN(1) Perl Programmers Reference Guide PERLSYN(1)

curly brackets are required--no dangling statements allowed. If you want to write conditionals without

curly brackets there are several other ways to do it. The following all do the same thing:

if (!open(FOO)) { die "Can’t open $FOO: $!"; }

die "Can’t open $FOO: $!" unless open(FOO);

open(FOO) or die "Can’t open $FOO: $!"; # FOO or bust!

open(FOO) ? ’hi mom’ : die "Can’t open $FOO: $!";

# a bit exotic, that last one

The if statement is straightforward. Because BLOCKs are always bounded by curly brackets, there is

never any ambiguity about which if an else goes with. If you use unless in place of if, the sense of

the test is reversed.

The while statement executes the block as long as the expression is true (does not evaluate to the null

string ("")or0or "0"). The LABEL is optional, and if present, consists of an identiﬁer followed by a

colon. The LABEL identiﬁes the loop for the loop control statements next, last, and redo. If the

LABEL is omitted, the loop control statement refers to the innermost enclosing loop. This may include

dynamically looking back your call-stack at run time to ﬁnd the LABEL. Such desperate behavior triggers a

warning if you use the −−ww ﬂag.

If there is a continue BLOCK, it is always executed just before the conditional is about to be evaluated

again, just like the third part of a for loop in C. Thus it can be used to increment a loop variable, even

when the loop has been continued via the next statement (which is similar to the C continue state-

ment).

LLoooopp CCoonnttrrooll

The next command is like the continue statement in C; it starts the next iteration of the loop:

LINE: while (<STDIN>) {

next LINE if /ˆ#/; # discard comments

...

}

The last command is like the break statement in C (as used in loops); it immediately exits the loop in

question. The continue block, if any, is not executed:

LINE: while (<STDIN>) {

last LINE if /ˆ$/; # exit when done with header

...

}

The redo command restarts the loop block without evaluating the conditional again. The continue

block, if any, is not executed. This command is normally used by programs that want to lie to themselves

about what was just input.

For example, when processing a ﬁle like /etc/termcap. If your input lines might end in backslashes to indi-

cate continuation, you want to skip ahead and get the next record.

while (<>) {

chomp;

if (s/\\$//) {

$_ .= <>;

redo unless eof();

}

# now process $_

}

which is Perl short-hand for the more explicitly written version:

20/Jul/98 perl 5.005, patch 02 17

PERLSYN(1) Perl Programmers Reference Guide PERLSYN(1)

one that avoids the problem you get into if you explicitly test for end-of-ﬁle on an interactive ﬁle descriptor

causing your program to appear to hang.

$on_a_tty = -t STDIN && -t STDOUT;

sub prompt { print "yes? " if $on_a_tty }

for ( prompt(); <STDIN>; prompt() ) {

# do something

}

FF oorreeaacchhLLooooppss

The foreach loop iterates over a normal list value and sets the variable VA R to be each element of the list

in turn. If the variable is preceded with the keyword my, then it is lexically scoped, and is therefore visible

only within the loop. Otherwise, the variable is implicitly local to the loop and regains its former value

upon exiting the loop. If the variable was previously declared with my, it uses that variable instead of the

global one, but it’s still localized to the loop. (Note that a lexically scoped variable can cause problems if

you have subroutine or format declarations within the loop which refer to it.)

The foreach keyword is actually a synonym for the for keyword, so you can use foreach for read-

ability or for for brevity. (Or because the Bourne shell is more familiar to you than csh, so writing for

comes more naturally.) If VA R is omitted, $_ is set to each value. If any element of LIST is an lvalue, you

can modify it by modifying VAR inside the loop. That’s because the foreach loop index variable is an

implicit alias for each item in the list that you’re looping over.

If any part of LIST is an array, foreach will get very confused if you add or remove elements within the

loop body, for example with splice. So don’t do that.

foreach probably won’t do what you expect if VAR is a tied or other special variable. Don’t do that

either.

Examples:

for (@ary) { s/foo/bar/ }

foreach my $elem (@elements) {

$elem *= 2;

}

for $count (10,9,8,7,6,5,4,3,2,1,’BOOM’) {

print $count, "\n"; sleep(1);

}

for (1..15) { print "Merry Christmas\n"; }

foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {

print "Item: $item\n";

}

Here’s how a C programmer might code up a particular algorithm in Perl:

20/Jul/98 perl 5.005, patch 02 19

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