Perl编程全指南：从入门到精通

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"Perl编程参考手册" Perl编程参考手册是一份详尽的资源，涵盖了Perl编程语言的各个方面，是开发者必备的工具书。手册被划分为多个部分，以方便开发者根据需要查阅。以下是对各部分的详细介绍： 1. **perl** - 这个部分提供了Perl的概述，包括它的基本概念、用途和历史。 2. **perldelta** - perldelta详细阐述了Perl自上一版本以来所做的所有改动和更新，帮助开发者跟踪语言的发展。 3. **perlfaq** - Perl FAQ解答了许多常见的Perl编程问题，是解决疑惑的好帮手。 4. **perltoc** - Perltoc是一个文档目录，列出了所有Perl文档的结构，方便查找特定主题。 5. **perldata** - 这一部分讲解了Perl的数据结构，如标量、数组、哈希等，以及如何操作它们。 6. **perlsyn** - 介绍了Perl的语法，包括变量、流程控制、函数定义等。 7. **perlop** - 对Perl的运算符和优先级进行了详细解释，有助于理解运算符的工作方式。 8. **perlre** - 这里深入讨论了Perl的正则表达式，包括模式匹配、替换和捕获组等。 9. **perlrun** - 描述了如何执行Perl程序，以及可用的命令行选项。 10. **perlfunc** - 列出了Perl内置的函数及其用法。 11. **perlvar** - 详述了Perl预定义的变量，如$!, @_, %ENV等。 12. **perlsub** - 介绍了如何定义和使用子程序（子例程）。 13. **perlmod** - 解释了Perl模块的工作原理，包括模块的加载和使用。 14. **perlmodlib** - 指导开发者编写和使用Perl模块，以及如何从CPAN（ Comprehensive Perl Archive Network）安装模块。 15. **perlmodinstall** - 提供了在CPAN上安装模块的具体步骤和技巧。 16. **perlform** - 讨论了Perl的格式化输出，包括如何创建和使用格式。 17. **perllocale** - 描述了Perl对本地化和国际化支持，包括字符集和日期格式。 18. **perlref** - 详细介绍了Perl的引用，以及如何使用它们来构建复杂的数据结构。 19. **perldsc** - 作为Perl数据结构的入门指南，帮助初学者理解列表、数组、哈希等。 20. **perllol** - 专门讲解列表的列表（列表的嵌套），是处理复杂数据结构的重要部分。 21. **perltoot** - 一个面向对象编程的教程，介绍了Perl中的面向对象特性。 22. **perlobj** - 更深入地探讨了Perl的对象系统。 23. **perltie** - 展示了如何将简单变量与对象绑定，实现隐藏的面向对象功能。 24. **perlbot** - 提供了一些Perl面向对象编程的技巧和示例。 25. **perlipc** - 讲解了进程间通信（IPC），包括管道、信号、套接字等。 26. **perldebug** - 介绍了如何在Perl中进行调试，包括内建的调试器和调试技巧。 27. **perldiag** - 列出了Perl的诊断消息，帮助理解编译错误。 28. **perlsec** - 关注Perl的安全问题，提供安全编程的建议。 29. **perltrap** - 警告开发者可能遇到的陷阱和常见错误。 30. **perlport** - 指导开发者编写跨平台兼容的Perl代码。 31. **perlstyle** - 提供了Perl编程的风格指南，旨在提高代码的可读性和一致性。 32. **perlpod** - 介绍了Perl的POD（Plain Old Documentation）格式，用于编写简洁的文档。 33. **perlbook** - 提供有关Perl书籍的信息，推荐进一步学习的资源。 34. **perlembed** - 说明了如何在C或C++应用程序中嵌入Perl。 35. **perlapio** - 描述了Perl内部的I/O抽象接口，用于低级别的输入输出操作。 36. **perlxs** - 介绍Perl XS API，用于编写C语言扩展。 37. **perlxstut** - XS的教程，帮助开发者更深入地理解和使用XS。 38. **perlguts** - 针对编写Perl扩展的内部函数进行了介绍，适合高级用户。 39. **perlcall** - 讲解了从C调用Perl代码的约定和方法。 40. **perlhist** - 记录了Perl的历史版本变更，可供开发者了解其发展轨迹。这些手册章节提供了从基础到高级的Perl编程知识，无论你是初学者还是经验丰富的开发者，都能从中受益。通过这些资源，你可以全面掌握Perl编程的各个方面，提升你的编程技能和解决问题的能力。

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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