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标准库（三）正则表达式

我什么都布吉岛 发布时间：2021-11-09 10:02:16 ，浏览量：1

一、正则表达式是什么？

正则表达式是一种描述字符串的一种手段。正则表达式像是一系列字符串特征的描述，比如我们需要在一系列文本中找出xxx@foxmail.com的字符串，我们就可以通过描述正则表达式这个特征找出所有的email地址。正则表达式在很多地方都有应用，如文本合法性表达，网络爬虫，Web网表单验证等等。

二、正则表达式库简介

C++11开始支持正则表达式，大大提升了字符串处理能力。头文件定义在中，详细的参考手册见这里，下面是对手册的一些简单整理。

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正则表达式组件
Main classes
basic_regex
sub_match
match_results
Algorithm
regex_match
regex_search
regex_replace
iterators
regex_iterator
regex_token_iterator
Exceptions
regex_error
Traits
regex_traits
Constants
synatax_option_type
match_flag_type
error_type
2.1 Main classes 类含义basic_regex正则表达式对象sub_match代表由子表达式匹配的字符序列match_results代表一个正则表达式匹配，包括所有子表达式匹配

我们知道，正则表达式是对一个字符串的描述，假如我们需要匹配多个信息时，显然是一个描述是不够的，需要额外的子表达式。如：zhangsan 10086我们的目的是要找出两个信息，一个是名字，另一个是电话。

  • 正则对象basic_regex std::basic_regex是构造正则表达式的模板，其定义如下：
template  class basic_regex;

可以看出构造一个正则对象只需要提供一个字符类型charT即可，一些通用的字符类型已经被定义方便使用：

TypeDefinitionregexbasic_regexwregexbasic_regex

那么模板实例化成类后，该如何构造？

basic_regex();
explicit basic_regex( const CharT* s,  flag_type f = std::regex_constants::ECMAScript );
basic_regex( const CharT* s, std::size_t count,flag_type f = std::regex_constants::ECMAScript );
basic_regex( const basic_regex& other );
basic_regex( basic_regex&& other );
template
explicit basic_regex( const std::basic_string& str,
                      flag_type f = std::regex_constants::ECMAScript );
template
basic_regex( ForwardIt first, ForwardIt last,
             flag_type f = std::regex_constants::ECMAScript );
basic_regex( std::initializer_list init,
             flag_type f = std::regex_constants::ECMAScript );

默认情况下，构造出来的正则表达式语法是ECMAScript，构造函数有：

  • 默认构造函数；
  • C风格字符串；
  • 指定长度的C风格字符串；
  • 拷贝构造和移动构造函数；
  • std::string；
  • 范围迭代器构造（满足前向迭代器）；
  • 初始化列表

表达式属性可以是：

  • 正则表达式语法 ECMAScript egrep awk等
  • 表达式行为控制 大小写敏感 是否加速（牺牲构造时间） 是否子匹配 默认范围是全闭
其他方法含义r1=re替换r1中的正则表达式r1.assign(re,f)重新给定正则表达式及选项r.mark_count()子表达式数量r.flags()返回r的标志集

注意：构造函数和赋值操作都会抛出regex_error异常（见表3）。

  • std::match_result 用于存储匹配或者搜索结果的容器。容器的元素是子匹配，一般而言，他只能被算法regex_search regex_match 或者iterator所改变，如果成功则返回true且第一个子匹配为整个序列；反之则返回false。
template class match_results;
namespace pmr {
    template 
    using match_results = std::match_results;
}

同样有一些方便使用点的类实例：

TypeDefinitionstd::cmatchstd::match_resultsstd::wcmatchstd::match_resultsstd::smatchstd::match_resultsstd::wsmatchstd::match_resultsstd::pmr::cmatch (C++17)std::pmr::match_resultsstd::pmr::wcmatch (C++17)std::pmr::match_resultsstd::pmr::smatch(C++17)std::pmr::match_resultsstd::pmr::wsmatch (C++17)std::pmr::match_results

具体地，smatch的操作及含义如下：

smatch操作含义m.ready()是否通过regex_match或regex_search设置m，如果没设置，m操作未定义m.size()匹配失败返回0，否则返回最近一次匹配的正则表达式中子表达式的数目m.empty()m.size()=0返回true;反之falsem.prefix()一个ssub_match对象表示匹配之前的序列m.suffix()一个ssub_match对象匹配之后的序列m.format(…)正则表达式替换操作m.length(n）第n个匹配的子表达式的大小m.position(n)第n个匹配的子表达式距离序列开始的距离m.str(n)第n个匹配的子表达式匹配的stringm[n]对应第n个子表达式的ssub_match对象m.begin(),m.end()m.cbegin(),m.cend()表示m中sub_match元素范围迭代器

注：在接受一个引索操作中，n的默认值位0且必须小用户m.size()。第一个子匹配（引索为0）表示整个匹配。特别提一下，suffix是匹配到的元素之后的序列，一般用在对表达式重复搜寻上！

while(std::regex_search(s,sm,re2))
{
    for(auto x:sm)
    {
        std::cout
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