前两天面试一个程序员，自己说工作中用到过正则表达式，也比较熟悉，问他要使用正则表达式需要引用那个命名空间，使用哪些类，居然吱吱唔唔答不上来，让他写一个验证电话号码的正则表达式也写不出来，实在是很奇怪这种程序员是怎么工作了两三年的。
言归正传，下面介绍下.net中正则表达式中的使用。
要在.net中使用正则表达式，需要引用System.Text.RegularExpressions 命名空间。新建一个正则表达式类：
string pattern = " some_pattern " ; // 正则表达式字符串 Regex regex = new Regex(pattern);
使用正则表达式匹配字符串
string input = " some_input " ;Match match = regex.Match(input);MatchCollection matches = regex.Matches(input); bool isMatch = regex.IsMatch(input);
Match方法返回单个的精确匹配结果，Matches返回所有的匹配结果的一个Match类的集合，IsMatch方法返回是否能够匹配输入字符串的一个bool结果。
Match类是一个保持匹配结果的类，它有一个成员Groups，是一个保存Group class的集合类。
Group 表示单个捕获组的结果。由于存在数量词，一个捕获组可以在单个匹配中捕获零个、一个或更多的字符串，因此 Group 提供 Capture 对象的集合。
Capture 表示单个成功捕获中的一个子字符串。
Group从Capture继承，表示单个捕获组的最后一个字符串。
即对于一个Group 类的实例对象group:
int captureCount = group.Captures.Count;
则group.Value与group.Captures[captureCount - 1].Value是相等的。
以下是几个正则表达式的使用样例:
使用正则表达式检查字符串是否具有表示货币值的正确格式。
代码 using System; using System.Text.RegularExpressions; public class Test{ public static void Main () { // Define a regular expression for currency values. Regex rx = new Regex( @" ^-?\d+(\.\d{2})?$ " ); // Define some test strings. string [] tests = { " -42 " , " 19.99 " , " 0.001 " , " 100 USD " , " .34 " , " 0.34 " , " 1,052.21 " }; // Check each test string against the regular expression. foreach ( string test in tests) { if (rx.IsMatch(test)) { Console.WriteLine( " {0} is a currency value. " , test); } else { Console.WriteLine( " {0} is not a currency value. " , test); } } } } // The example displays the following output to the console: // -42 is a currency value. // 19.99 is a currency value. // 0.001 is not a currency value. // 100 USD is not a currency value. // .34 is not a currency value. // 0.34 is a currency value. // 1,052.21 is not a currency value.
使用正则表达式检查字符串中重复出现的词。
using System; using System.Text.RegularExpressions; public class Test{ public static void Main () { // Define a regular expression for repeated words. Regex rx = new Regex( @" \b(?<word>\w+)\s+(\k<word>)\b " , RegexOptions.Compiled | RegexOptions.IgnoreCase); // Define a test string. string text = " The the quick brown fox fox jumped over the lazy dog
dog. " ; // Find matches. MatchCollection matches = rx.Matches(text); // Report the number of matches found. Console.WriteLine( " {0} matches found in:\n {1} " , matches.Count, text); // Report on each match. foreach (Match match in matches) { GroupCollection groups = match.Groups; Console.WriteLine( " '{0}' repeated at positions {1} and {2} " , groups[ " word " ].Value, groups[ 0 ].Index, groups[ 1 ].Index); } }} // The example produces the following output to the console: // 3 matches found in: // The the quick brown fox fox jumped over the lazy dog dog. // 'The' repeated at positions 0 and 4 // 'fox' repeated at positions 20 and 25 // 'dog' repeated at positions 50 and 54
使用 Capture 对象在控制台中显示每个正则表达式匹配项组的成员。
代码 string text = " One fish two fish red fish blue fish " ; string pat = @" (?<1>\w+)\s+(?<2>fish)\s* " ; // Compile the regular expression. Regex r = new Regex(pat, RegexOptions.IgnoreCase); // Match the regular expression pattern against a text string. Match m = r.Match(text); while (m.Success) { // Display the first match and its capture set. System.Console.WriteLine( " Match=[ " + m + " ] " ); CaptureCollection cc = m.Captures; foreach (Capture c in cc) { System.Console.WriteLine( " Capture=[ " + c + " ] " ); } // Display Group1 and its capture set. Group g1 = m.Groups[ 1 ]; System.Console.WriteLine( " Group1=[ " + g1 + " ] " ); foreach (Capture c1 in g1.Captures) { System.Console.WriteLine( " Capture1=[ " + c1 + " ] " ); } // Display Group2 and its capture set. Group g2 = m.Groups[ 2 ]; System.Console.WriteLine( " Group2=[ " + g2 + " ] " ); foreach (Capture c2 in g2.Captures) { System.Console.WriteLine( " Capture2=[ " + c2 + " ] " ); } // Advance to the next match. m = m.NextMatch();} // The example displays the following output: // Match=[One fish ] // Capture=[One fish ] // Group1=[One] // Capture1=[One] // Group2=[fish] // Capture2=[fish] // Match=[two fish ] // Capture=[two fish ] // Group1=[two] // Capture1=[two] // Group2=[fish] // Capture2=[fish] // Match=[red fish ] // Capture=[red fish ] // Group1=[red] // Capture1=[red] // Group2=[fish] // Capture2=[fish] // Match=[blue fish] // Capture=[blue fish] // Group1=[blue] // Capture1=[blue] // Group2=[fish] // Capture2=[fish]