study of Regular Expressions
综述
* 原字符
* 实例
* 练习题
备注 每日一更 +代表更新 -代表还在占坑中（没有内容暂待更新） 更新内容：
- 新的元字符更新
编程依赖
使用C#作为正则表达式的学习工具，正则表达式在各种语言中都支持（自带或者相关的库） 在这里使用C#作为学习的工具。 C#中使用正则表达式可以有几种方式：
// 本次主要采用这样的实现方式（便于书写）
string[] s = { "catTheOtherText", "TheOtherTextcat",
"TheOtherText", "TheOthercatText", "ThecOtheraTextt" };
string strRegular = "^cat";
foreach( var str in s)
{
Match match = Regex.Match(str, @strRegular);
if (match.Success)
{
Console.WriteLine(str + " The Regular is " + strRegular);
}
}
这种方式比较便于理解，但是其实缺少了很多相关的设置条件，相关的设置条件，每个语言有所不同，如果有需要，需要查阅相关的手册，进行确认。如果有需要（比如设置忽略大小写，在shell里是在正则表达式之前添加参数 -i），则用如下的这种方式进行书写：
string[] s = { "catTheOtherText", "TheOtherTextcat",
"TheOtherText", "TheOthercatText", "ThecOtheraTextt" };
string strRegular = "^cat";
// 这里的RegexOptions 提供了更多正则表达式的相关选项。
// 这里的IgnoreCase 是忽略大小写的含义
Regex reg = new Regex(@strRegular, RegexOptions.IgnoreCase);
foreach( var str in s)
{
Match match = reg.Match(str);
if (match.Success)
{
Console.WriteLine(str + " The Regular is " + strRegular);
}
}
元字符
^
行的起始 如：^cat 取得行首是cat的行
string[] s = { "catTheOtherText", "TheOtherTextcat",
"TheOtherText", "TheOthercatText", "ThecOtheraTextt" };
string strRegular = "^cat";
// "catTheOtherText"
$
行的结束 如： cat$ 取得行尾是cat的行
string[] s = { "catTheOtherText", "TheOtherTextcat",
"TheOtherText", "TheOthercatText", "ThecOtheraTextt" };
string strRegular = "cat$";
// output
// "TheOtherTextcat"
[]
匹配多个字符 如[ab] 匹配a或者b
string[] s = { "aTheOtherText", "bTheOtherText",
"abTheOtherText", "TheOtheraText", "b" };
string strRegular = "[ab]";
// output
// aTheOtherText The Regular is [ab]
// bTheOtherText The Regular is [ab]
// abTheOtherText The Regular is [ab]
// TheOtheraText The Regular is [ab]
// b The Regular is [ab]
[-]
[]中的连字符，可以连接有顺序的序列 如：[1-3] 取1到3中任意__一个__数
特别的[0123456789abcdefgABCDEFG]可以写作：[0-9a-gA-G] 或[A-G0-9a-g] (顺序无关)
展开
（以后讲到） 只有-在[]中是作为有意义的字符，其他字符保留原来含义。
string[] s = { "2TheOtherText", "1TheOtherText",
"90TheOtherText", "TheOtheraText", "9", "TheOther?Text" };
string strRegular = "[0-9?]";
// 2TheOtherText The Regular is [0-9?]
// 1TheOtherText The Regular is [0-9?]
// 90TheOtherText The Regular is [0-9?]
// 9 The Regular is [0-9?]
// TheOther?Text The Regular is [0-9?]
[^]
表示除此以外 如：[^a] 取得除了有a的行
也可以使用[^0-9] 取得除了只包含0-9的其他行
string[] s = { "123456", "Text",
"?Text", "123456Text", "9" , "?",
"Text123456", "T123e45x6t" };
string strRegular = "[^0-9?]";
// Text The Regular is [^0-9?]
// ?Text The Regular is [^0-9?]
// 123456Text The Regular is [^0-9?]
// Text123456 The Regular is [^0-9?]
// T123e45x6t The Regular is [^0-9?]
|
或 如 ab|ba 匹配ab和ba
如同编程中的|（或）
string[] s = { "abc", "cba",
"bca", "ab", "a"};
string strRegular = "ab|ba";
// abc The Regular is ab|ba
// cba The Regular is ab|ba
// ab The Regular is ab|ba
()
使用()包含|，限制|的作用范围 则只针对括号中的内容进行或匹配 如a(ab|ba) 匹配aab 或者aba
string[] s = { "abc", "cba",
"bca", "ab", "a", "aab"};
string strRegular = "c(ab|ba)";
// cba The Regular is c(ab|ba)
值得注意的是，以下3者等价：
string strRegular1 = "(Geoff|Jeff)(rey|ery)";
string strRegular2 = "(Geo|Je)ff(rey|ery)";
string strRegular3 = "(Geo|Je)ff(re|er)y"
下面这种方式与上面3者也等价（较为蠢）
string strOtherWayRegular = "Jeffrey|Geoffery|Jeffery|Geoffrey";
单词分界符
C#的实现不同，本例参照所选用语言的正则表达式手册。
标准中提供\<和\> 来匹配单词的首位，和末尾。
C#中则只提供\b 匹配单词分割，不特别区分首尾（看放置于哪个部分）。
模糊匹配
?
一定会被匹配到。
必须前面有一个字符, 前面字符出现__0次或1次__ 如 abca? 匹配abc或者abca
一个有歧义，其实可以将多个用括号包裹起来，这样的话也算一个 如 ab(abc)? 匹配ababc或ab
string[] s = { "abca", "abcaaa",
"abc", "abcd", "babcd"};
string strRegular = "abca?";
// abca The Regular is abca?
// abcaaa The Regular is abca?
// abc The Regular is abca?
// abcd The Regular is abca?
// babcd The Regular is abca?
string[] s = { "ababc", "abca",
"babcdeg", "cbagcd" };
string strRegular = "ab(abc)?";
// ababc The Regular is ab(abc)?
// abca The Regular is ab(abc)?
// babcdeg The Regular is ab(abc)?
+
必须前面有一个字符, 前面字符出现__1次或多次__ 如 abca? 匹配abca或者abcaa
string[] s = { "abca", "abcaaa",
"abc", "abcd", "babcd"};
string strRegular = "abca+";
// abca The Regular is abca+
// abcaaa The Regular is abca+
*
一定会被匹配到。
必须前面有一个字符, 前面字符出现__0次或多次__ 如 ba? 匹配b或者ba或者baa
string[] s = { "abca", "abcaaa",
"abc", "abcd", "babcd"};
string strRegular = "abca*";
// abca The Regular is abca*
// abcaaa The Regular is abca*
// abc The Regular is abca*
// abcd The Regular is abca*
// babcd The Regular is abca*
展开 ? *
看了上面的例子是不是对这2个有点困惑。看起来结果是一样的。其实主要在于，只有存在于明确首尾的情况时，才能使之不同。比如：
^aa?b和^aa*b
string[] s = { "ab", "aaaab",
"aab"};
string strRegular = "^aa?b";
// ab The Regular is ^aa?b
// aab The Regular is ^aa?b
string[] s = { "ab", "aaaab",
"aab"};
string strRegular = "^aa*b";
// ab The Regular is ^aa*b
// aaaab The Regular is ^aa*b
// aab The Regular is ^aa*b
练习
[][^]
'q1xText', 'Textq1',
'q2cText', 'Textq2c' 1. `q[0-9][^x]` 会匹配哪一个呢？
-
q[0-9][^x]$会匹配哪一个呢？ -
^q[0-9][^x]会匹配哪一个呢？
答案
-
q2cText The Regular is q[0-9][^x] Textq2c The Regular is q[0-9][^x]
-
Textq2c The Regular is q[0-9][^x]$
-
q2cText The Regular is ^q[0-9][^x]