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Perl 5 是一种功能强大的文本处理语言，广泛用于系统管理、网络编程、脚本编写以及Web开发等场景。它的语法独特，结合了C、sed、awk等语言的特点，被誉为“实用提取和报告语言”（Practical Extraction and Reporting Language）。本教程将深入探讨Perl 5的基础知识、核心概念和常见问题，帮助你掌握这一强大工具。让我们从基础开始。在Perl中，每行代码被称为一个语句，以分号结尾。变量是存储数据的地方，它们的命名遵循特定的规则，如`$scalar`、`@array`和`%hash`分别代表标量、数组和哈希。Perl 5 提供了丰富的内建函数，例如`print`用于输出，`chomp`用于删除字符串末尾的换行符，`length`计算字符串长度，而`split`则用于分割字符串。字符串操作是Perl的核心，支持模式匹配和替换，例如`=~`操作符配合正则表达式进行匹配。例如，`$str =~ s/old/new/;`会将`$str`中的第一个“old”替换为“new”。Perl还提供了文件操作功能，如打开、读取、写入和关闭文件，`open`函数用于打开文件，`<FILEHANDLE>`用于读取，`print FILEHANDLE`用于写入，而`close`用于关闭文件。数组和哈希是Perl中处理多值的常用数据结构。数组是一系列按顺序排列的元素集合，通过索引访问，例如`$array[0]`。哈希则由键值对组成，通过键来访问值，例如`$hash{'key'}`。Perl的数组和哈希操作十分灵活，可以方便地进行遍历、添加和删除元素。 Perl的控制流程包括条件语句（if/else）和循环（for/while）。例如，`if ($condition) { ... }`用于判断条件，`while ($expression) { ... }`则用于重复执行一段代码，直到表达式的值变为假。Perl还支持开关语句（switch/case），但需借助模块`Switch`实现。函数是可重用代码块，可以接受参数并返回值。Perl支持子例程定义，如`sub myFunction { ... }`。模块机制允许导入其他文件中的函数和变量，使用`use`或`require`关键字。Perl的模块生态非常丰富，涵盖各种用途，如CGI处理、数据库连接、网络编程等。异常处理在Perl中通过`eval`函数实现。当`eval`内的代码出错时，错误信息会被捕获并存储在全局变量`$@`中。你可以用`die`函数抛出错误并终止程序，或者使用`warn`打印警告信息而不终止程序。 Perl 5 还有一个重要的特性是正则表达式。它提供了强大的文本匹配和替换功能，支持多种预定义字符类、重复、选择、分组等高级操作。正则表达式在Perl中无处不在，是解决许多文本处理问题的关键。 Perl社区维护了一份详细的《perl FAQ》（常问问题集），涵盖了从语言基础到高级特性的各种问题，是学习和解决问题的重要资源。常见问题包括变量作用域、引用、模块使用、文件处理、正则表达式等。通过深入阅读和实践，你可以逐步提升Perl编程技能。在实际项目中，了解如何有效地使用Perl模块，如DBI（数据库接口）进行数据库操作，LWP（Library for WWW in Perl）进行网络请求，以及XML::Parser解析XML文档，都是提高效率的关键。 Perl 5 是一款功能强大的语言，尤其在文本处理和脚本编写领域有着广泛的应用。通过深入学习《Perl 5教程》和查阅《perl FAQ》，你将能够熟练掌握Perl编程，解决各种实际问题。在实践中不断探索和积累，你将成为Perl编程的专家。

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<HTML> <HEAD> <TITLE>perlfaq4 </TITLE> </HEAD> <BODY background="gback.jpg"> <H1>perlfaq4 - perl常问问题集，第四篇</H1> <p> <p><H2><A NAME="INDEX">目录 </A></H2>  <UL> <LI><A HREF="#NAME">NAME</A> <LI><A HREF="#DESCRIPTION">DESCRIPTION</A> <LI><A HREF="#Data_Numbers">Data: Numbers</A> <UL> <LI><A HREF="#Why_am_I_getting_long_decimals_">Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?</A> <LI><A HREF="#Why_isn_t_my_octal_data_interpre">Why isn't my octal data interpreted correctly?</A> <LI><A HREF="#Does_perl_have_a_round_function_">Does perl have a round function? What about ceil() and floor()?</A> <LI><A HREF="#How_do_I_convert_bits_into_ints_">How do I convert bits into ints?</A> <LI><A HREF="#How_do_I_multiply_matrices_">How do I multiply matrices?</A> <LI><A HREF="#How_do_I_perform_an_operation_on">How do I perform an operation on a series of integers?</A> <LI><A HREF="#How_can_I_output_Roman_numerals_">How can I output Roman numerals?</A> <LI><A HREF="#Why_aren_t_my_random_numbers_ran">Why aren't my random numbers random?</A> </UL> <LI><A HREF="#Data_Dates">Data: Dates</A> <UL> <LI><A HREF="#How_do_I_find_the_week_of_the_ye">How do I find the week-of-the-year/day-of-the-year?</A> <LI><A HREF="#How_can_I_compare_two_date_strin">How can I compare two date strings?</A> <LI><A HREF="#How_can_I_take_a_string_and_turn">How can I take a string and turn it into epoch seconds?</A> <LI><A HREF="#How_can_I_find_the_Julian_Day_">How can I find the Julian Day?</A> <LI><A HREF="#Does_Perl_have_a_year_2000_probl">Does Perl have a year 2000 problem?</A> </UL> <LI><A HREF="#Data_Strings">Data: Strings</A> <UL> <LI><A HREF="#How_do_I_validate_input_">How do I validate input?</A> <LI><A HREF="#How_do_I_unescape_a_string_">How do I unescape a string?</A> <LI><A HREF="#How_do_I_remove_consecutive_pair">How do I remove consecutive pairs of characters?</A> <LI><A HREF="#How_do_I_expand_function_calls_i">How do I expand function calls in a string?</A> <LI><A HREF="#How_do_I_find_matching_nesting_a">How do I find matching/nesting anything?</A> <LI><A HREF="#How_do_I_reverse_a_string_">How do I reverse a string?</A> <LI><A HREF="#How_do_I_expand_tabs_in_a_string">How do I expand tabs in a string?</A> <LI><A HREF="#How_do_I_reformat_a_paragraph_">How do I reformat a paragraph?</A> <LI><A HREF="#How_can_I_access_change_the_firs">How can I access/change the first N letters of a string?</A> <LI><A HREF="#How_do_I_change_the_Nth_occurren">How do I change the Nth occurrence of something?</A> <LI><A HREF="#How_can_I_count_the_number_of_oc">How can I count the number of occurrences of a substring within a string?</A> <LI><A HREF="#How_do_I_capitalize_all_the_word">How do I capitalize all the words on one line?</A> <LI><A HREF="#How_can_I_split_a_character_de">How can I split a [character] delimited string except when inside [character]? (Comma-separated files)</A> <LI><A HREF="#How_do_I_strip_blank_space_from_">How do I strip blank space from the beginning/end of a string?</A> <LI><A HREF="#How_do_I_extract_selected_column">How do I extract selected columns from a string?</A> <LI><A HREF="#How_do_I_find_the_soundex_value_">How do I find the soundex value of a string?</A> <LI><A HREF="#How_can_I_expand_variables_in_te">How can I expand variables in text strings?</A> <LI><A HREF="#What_s_wrong_with_always_quoting">What's wrong with always quoting "$vars"?</A> <LI><A HREF="#Why_don_t_my_HERE_documents_wo">Why don't my <<HERE documents work?</A> </UL> <LI><A HREF="#Data_Arrays">Data: Arrays</A> <UL> <LI><A HREF="#What_is_the_difference_between_">What is the difference between $array[1] and @array[1]?</A> <LI><A HREF="#How_can_I_extract_just_the_uniqu">How can I extract just the unique elements of an array?</A> <LI><A HREF="#How_can_I_tell_whether_an_array_">How can I tell whether an array contains a certain element?</A> <LI><A HREF="#How_do_I_compute_the_difference_">How do I compute the difference of two arrays? How do I compute the intersection of two arrays?</A> <LI><A HREF="#How_do_I_find_the_first_array_el">How do I find the first array element for which a condition is true?</A> <LI><A HREF="#How_do_I_handle_linked_lists_">How do I handle linked lists?</A> <LI><A HREF="#How_do_I_handle_circular_lists_">How do I handle circular lists?</A> <LI><A HREF="#How_do_I_shuffle_an_array_random">How do I shuffle an array randomly?</A> <LI><A HREF="#How_do_I_process_modify_each_ele">How do I process/modify each element of an array?</A> <LI><A HREF="#How_do_I_select_a_random_element">How do I select a random element from an array?</A> <LI><A HREF="#How_do_I_permute_N_elements_of_a">How do I permute N elements of a list?</A> <LI><A HREF="#How_do_I_sort_an_array_by_anyth">How do I sort an array by (anything)?</A> <LI><A HREF="#How_do_I_manipulate_arrays_of_bi">How do I manipulate arrays of bits?</A> <LI><A HREF="#Why_does_defined_return_true_o">Why does defined() return true on empty arrays and hashes?</A> </UL> <LI><A HREF="#Data_Hashes_Associative_Arrays">Data: Hashes (Associative Arrays)</A> <UL> <LI><A HREF="#How_do_I_process_an_entire_hash_">How do I process an entire hash?</A> <LI><A HREF="#What_happens_if_I_add_or_remove_">What happens if I add or remove keys from a hash while iterating over it?</A> <LI><A HREF="#How_do_I_look_up_a_hash_element_">How do I look up a hash element by value?</A> <LI><A HREF="#How_can_I_know_how_many_entries_">How can I know how many entries are in a hash?</A> <LI><A HREF="#How_do_I_sort_a_hash_optionally">How do I sort a hash (optionally by value instead of key)?</A> <LI><A HREF="#How_can_I_always_keep_my_hash_so">How can I always keep my hash sorted?</A> <LI><A HREF="#What_s_the_difference_between_d">What's the difference between "delete" and "undef" with hashes?</A> <LI><A HREF="#Why_don_t_my_tied_hashes_make_th">Why don't my tied hashes make the defined/exists distinction?</A> <LI><A HREF="#How_do_I_reset_an_each_operati">How do I reset an each() operation part-way through?</A> <LI><A HREF="#How_can_I_get_the_unique_keys_fr">How can I get the unique keys from two hashes?</A> <LI><A HREF="#How_can_I_store_a_multidimension">How can I store a multidimensional array in a DBM file?</A> <LI><A HREF="#How_can_I_make_my_hash_remember_">How can I make my hash remember the order I put elements into it?</A> <LI><A HREF="#Why_does_passing_a_subroutine_an">Why does passing a subroutine an undefined element in a hash create it?</A> <LI><A HREF="#How_can_I_make_the_Perl_equivale">How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?</A> <LI><A HREF="#How_can_I_use_a_reference_as_a_h">How can I use a reference as a hash key?</A> </UL> <LI><A HREF="#Data_Misc">Data: Misc</A> <UL> <LI><A HREF="#How_do_I_handle_binary_data_corr">How do I handle binary data correctly?</A> <LI><A HREF="#How_do_I_determine_whether_a_sca">How do I determine whether a scalar is a number/whole/integer/float?</A> <LI><A HREF="#How_do_I_keep_persistent_data_ac">How do I keep persistent data across program calls?</A> <LI><A HREF="#How_do_I_print_out_or_copy_a_rec">How do I print out or copy a recursive data structure?</A> <LI><A HREF="#How_do_I_define_methods_for_ever">How do I define methods for every class/object?</A> <LI><A HREF="#How_do_I_verify_a_credit_card_ch">How do I verify a credit card checksum?</A> </UL> <LI><A HREF="#AUTHOR_AND_COPYRIGHT">AUTHOR AND COPYRIGHT</A> </UL>  <HR> <P> <H2><A NAME="NAME">NAME </A></H2> perlfaq4 - Data Manipulation ($Revision: 1.19 $, $Date: 1997/04/24 22:43:57 $) <P> <P> <HR> <H2><A NAME="DESCRIPTION">DESCRIPTION </A></H2> The section of the <FONT SIZE=-1>FAQ</FONT> answers question related to the manipulation of data as num