正则表达式练习——将文本中的日期格式化为标准格式

//匹配文本中的日期，并将日期转换为标准格式void dateFormatEx(void){    ifstream in(".\\chapter.23.4.1.in", ios::binary);    if (!in)cerr << "no file\n";    boost::regex pat("([\\d]+) ([\\w]+) ([\\d]+)");    cout << "pattern: " << pat << '\n';    int lineno = 0;    string line;    while (getline(in, line))    {        ++lineno;        boost::smatch matches;        if (boost::regex_search(line, matches, pat))        {            string day = matches[1];            string mon = matches[2];            string year = matches[3];            line = boost::regex_replace(line, pat, year+"/"+mon+"/"+day);        }        cout << line << endl;    }}

输入语料
xxx

xxx

From: John Doe [e-mail]
To: Mary Smith [e-mail]
Subject: Saying Hello
Date: Fri, 21 Nov 1997 09:55:06 -0600
Message-ID: <[e-mail]>

This is a message just to say hello.

So, “Hello”.

From: Joe Q. Public [e-mail]
To: Mary Smith <@machine.tld:[e-mail]>, , jdoe@test .example
Date: Tue, 1 Jul 2003 10:52:37 +0200
Message-ID: <[e-mail]>

Hi everyone.

To: “Mary Smith: Personal Account” [e-mail]
From: John Doe [e-mail]
Subject: Re: Saying Hello
Date: Fri, 21 Nov 1997 11:00:00 -0600
Message-ID: [e-mail]
In-Reply-To: <[e-mail]>
References: <[e-mail]> <[e-mail]>

This is a reply to your reply.

----

输出结果