请问，这个正则表达式如何写

[drse23432]

string sentence = "×××××";

一个单词，有5个字母，如何只找出第3个字母，或者只找出第一个字母，其他的不找。因为，如果一个匹配结果太多的话，再去遍历寻找，效率会低。

[dalmeeme]

找出第3个字母：

		string s = @"Let us do morning-execises!";
		MatchCollection matches = Regex.Matches(s, @"(?<=[,;'\-\s]+[A-Za-z]{2})[A-Za-z]");
		foreach (Match match in matches)
			Response.Write(match.Value + "<br/>");

[dalmeeme]

找出第一个字母

		string s = @"Let us do morning-execises!";
		MatchCollection matches = Regex.Matches(s, @"(?<=^|[,;'\-\s]+)[A-Za-z]");
		foreach (Match match in matches)
			Response.Write(match.Value + "<br/>");

找出第三个字母（更正）

string s = @"Let us do morning-execises!";
        MatchCollection matches = Regex.Matches(s, @"(?<=^|[,;'\-\s]+[A-Za-z]{2})[A-Za-z]");
        foreach (Match match in matches)
            Response.Write(match.Value + "<br/>");

[EnForGrass]

[Quote=引用 2 楼 dalmeeme 的回复:]
找出第一个字母
C# code
string s = @"Let us do morning-execises!";
MatchCollection matches = Regex.Matches(s, @"(?<=^|[,;'\-\s]+)[A-Za-z]");
foreach (Match match in matches)
……
[/Quote]
支持一个

            string ss = @"Let us do morning-execises!";
            MatchCollection matches = Regex.Matches(ss, @"(?<=^|[\W]+)[A-Za-z]");
            foreach (Match match in matches)
                  MessageBox.Show(match.Value);