限定符

更新： 2008 年 7 月

限定符指定在输入中必须存在前导元素（可以是字符、组或字符类）的多少个实例才能找到匹配项。

.NET Framework 正则表达式中的限定符

下表列出了 .NET Framework 正则表达式支持的限定符。数量 n 和 m 是整数常数。有关贪婪限定符与惰性限定符之间差别的描述，请参见表后面的“贪婪限定符与惰性限定符”一节。

Dim pattern As String = "\b91*9*\b"   
Dim input As String = "99 95 919 929 9119 9219 999 9919 91119"
For Each match As Match In Regex.Matches(input, pattern)
   Console.WriteLine("'{0}' found at position {1}.", match.Value, match.Index)
Next     
' The example displays the following output:   
'       '99' found at position 0.
'       '919' found at position 6.
'       '9119' found at position 14.
'       '999' found at position 24.
'       '91119' found at position 33.

string pattern = "\b91*9*\b";   
string input = "99 95 919 929 9119 9219 999 9919 91119";
foreach (Match match in Regex.Matches(input, pattern))
   Console.WriteLine("'{0}' found at position {1}.", match.Value, match.Index);

// The example displays the following output:   
//       '99' found at position 0.
//       '919' found at position 6.
//       '9119' found at position 14.
//       '999' found at position 24.
//       '91119' found at position 33.

Dim pattern As String = "\ba(n)+\w*?\b"

Dim input As String = "Autumn is a great time for an annual announcement to all antique collectors."
For Each match As Match In Regex.Matches(input, pattern, RegexOptions.IgnoreCase)
   Console.WriteLine("'{0}' found at position {1}.", match.Value, match.Index)
Next   
' The example displays the following output:   
'       'an' found at position 27.
'       'annual' found at position 30.
'       'announcement' found at position 37.
'       'antique' found at position 57.      

string pattern = @"\ba(n)+\w*?\b";

string input = "Autumn is a great time for an annual announcement to all antique collectors.";
foreach (Match match in Regex.Matches(input, pattern, RegexOptions.IgnoreCase))
   Console.WriteLine("'{0}' found at position {1}.", match.Value, match.Index);

// The example displays the following output:   
//       'an' found at position 27.
//       'annual' found at position 30.
//       'announcement' found at position 37.
//       'antique' found at position 57.      

Dim pattern As String = "\ban?\b"
Dim input As String = "An amiable animal with a large snount and an animated nose."
For Each match As Match In Regex.Matches(input, pattern, RegexOptions.IgnoreCase)
   Console.WriteLine("'{0}' found at position {1}.", match.Value, match.Index)
Next  
' The example displays the following output:   
'       'An' found at position 0.
'       'a' found at position 23.
'       'an' found at position 42.

string pattern = @"\ban?\b";
string input = "An amiable animal with a large snount and an animated nose.";
foreach (Match match in Regex.Matches(input, pattern, RegexOptions.IgnoreCase))
   Console.WriteLine("'{0}' found at position {1}.", match.Value, match.Index);

// The example displays the following output:   
//        'An' found at position 0.
//        'a' found at position 23.
//        'an' found at position 42.

Dim pattern As String = "\b\d+\,\d{3}\b"
Dim input As String = "Sales totaled 103,524 million in January, " + _
                      "106,971 million in February, but only " + _
                      "943 million in March."
For Each match As Match In Regex.Matches(input, pattern)
   Console.WriteLine("'{0}' found at position {1}.", match.Value, match.Index)
Next     
' The example displays the following output:   
'       '103,524' found at position 14.
'       '106,971' found at position 45.

string pattern = @"\b\d+\,\d{3}\b";
string input = "Sales totaled 103,524 million in January, " + 
                      "106,971 million in February, but only " + 
                      "943 million in March.";
foreach (Match match in Regex.Matches(input, pattern))
   Console.WriteLine("'{0}' found at position {1}.", match.Value, match.Index);

//  The example displays the following output:   
//        '103,524' found at position 14.
//        '106,971' found at position 45.

 Dim pattern As String = "\b\d{2,}\b\D+"  
 Dim input As String = "7 days, 10 weeks, 300 years"
For Each match As Match In Regex.Matches(input, pattern)
   Console.WriteLine("'{0}' found at position {1}.", match.Value, match.Index)
Next 
' The example displays the following output:
'       '10 weeks, ' found at position 8.
'       '300 years' found at position 18.

string pattern = @"\b\d{2,}\b\D+";   
string input = "7 days, 10 weeks, 300 years";
foreach (Match match in Regex.Matches(input, pattern))
   Console.WriteLine("'{0}' found at position {1}.", match.Value, match.Index);

//  The example displays the following output:
//        '10 weeks, ' found at position 8.
//        '300 years' found at position 18.

Dim pattern As String = "(00\s){2,4}"
Dim input As String = "0x00 FF 00 00 18 17 FF 00 00 00 21 00 00 00 00 00"
For Each match As Match In Regex.Matches(input, pattern)
   Console.WriteLine("'{0}' found at position {1}.", match.Value, match.Index)
Next 
' The example displays the following output:
'       '00 00 ' found at position 8.
'       '00 00 00 ' found at position 23.
'       '00 00 00 00 ' found at position 35.

string pattern = @"(00\s){2,4}";
string input = "0x00 FF 00 00 18 17 FF 00 00 00 21 00 00 00 00 00";
foreach (Match match in Regex.Matches(input, pattern))
   Console.WriteLine("'{0}' found at position {1}.", match.Value, match.Index);

//  The example displays the following output:
//        '00 00 ' found at position 8.
//        '00 00 00 ' found at position 23.
//        '00 00 00 00 ' found at position 35.

 Dim pattern As String = "\b\w*?oo\w*?\b"
 Dim input As String = "woof root root rob oof woo woe"
 For Each match As Match In Regex.Matches(input, pattern, RegexOptions.IgnoreCase)
    Console.WriteLine("'{0}' found at position {1}.", match.Value, match.Index)
 Next 
 ' The example displays the following output:
'       'woof' found at position 0.
'       'root' found at position 5.
'       'root' found at position 10.
'       'oof' found at position 19.
'       'woo' found at position 23.

 string pattern = @"\b\w*?oo\w*?\b";
 string input = "woof root root rob oof woo woe";
 foreach (Match match in Regex.Matches(input, pattern, RegexOptions.IgnoreCase))
    Console.WriteLine("'{0}' found at position {1}.", match.Value, match.Index);

 //  The example displays the following output:
//        'woof' found at position 0.
//        'root' found at position 5.
//        'root' found at position 10.
//        'oof' found at position 19.
//        'woo' found at position 23.

 Dim pattern As String = "\b\w+?\b"
 Dim input As String = "Aa Bb Cc Dd Ee Ff"
For Each match As Match In Regex.Matches(input, pattern)
   Console.WriteLine("'{0}' found at position {1}.", match.Value, match.Index)
Next 
' The example displays the following output:
'       'Aa' found at position 0.
'       'Bb' found at position 3.
'       'Cc' found at position 6.
'       'Dd' found at position 9.
'       'Ee' found at position 12.
'       'Ff' found at position 15.

string pattern = @"\b\w+?\b";
string input = "Aa Bb Cc Dd Ee Ff";
foreach (Match match in Regex.Matches(input, pattern))
   Console.WriteLine("'{0}' found at position {1}.", match.Value, match.Index);

//  The example displays the following output:
//        'Aa' found at position 0.
//        'Bb' found at position 3.
//        'Cc' found at position 6.
//        'Dd' found at position 9.
//        'Ee' found at position 12.
//        'Ff' found at position 15.

Dim pattern As String = "^(\s)*(System.)??Console.Write(Line)??\(??"
Dim input As String = "System.Console.WriteLine(""Hello!"")" + vbCrLf + _
                      "Console.Write(""Hello!"")" + vbCrLf + _
                      "Console.WriteLine(""Hello!"")" + vbCrLf + _
                      "Console.ReadLine()" + vbCrLf + _
                      "   Console.WriteLine"
For Each match As Match In Regex.Matches(input, pattern, _
                                         RegexOptions.IgnorePatternWhitespace Or RegexOptions.IgnoreCase Or RegexOptions.MultiLine)
   Console.WriteLine("'{0}' found at position {1}.", match.Value, match.Index)
Next 
' The example displays the following output:
'       'System.Console.Write' found at position 0.
'       'Console.Write' found at position 36.
'       'Console.Write' found at position 61.
'       '   Console.Write' found at position 110.

string pattern = @"^(\s)*(System.)??Console.Write(Line)??\(??";
string input = "System.Console.WriteLine(\"Hello!\")\n" + 
                      "Console.Write(\"Hello!\")\n" + 
                      "Console.WriteLine(\"Hello!\")\n" + 
                      "Console.ReadLine()\n" + 
                      "   Console.WriteLine";
foreach (Match match in Regex.Matches(input, pattern, 
                                      RegexOptions.IgnorePatternWhitespace | 
                                      RegexOptions.IgnoreCase | 
                                      RegexOptions.Multiline))
   Console.WriteLine("'{0}' found at position {1}.", match.Value, match.Index);

//  The example displays the following output:
//        'System.Console.Write' found at position 0.
//        'Console.Write' found at position 36.
//        'Console.Write' found at position 61.
//        '   Console.Write' found at position 110.

 Dim pattern As String = "\b(\w{3,}?\.){2}?\w{3,}?\b"
 Dim input As String = "www.microsoft.com msdn.microsoft.com mywebsite mycompany.com"
For Each match As Match In Regex.Matches(input, pattern)
   Console.WriteLine("'{0}' found at position {1}.", match.Value, match.Index)
Next     
' The example displays the following output:
'       'www.microsoft.com' found at position 0.
'       'msdn.microsoft.com' found at position 18.

string pattern = @"\b(\w{3,}?\.){2}?\w{3,}?\b";
string input = "www.microsoft.com msdn.microsoft.com mywebsite mycompany.com";
foreach (Match match in Regex.Matches(input, pattern))
   Console.WriteLine("'{0}' found at position {1}.", match.Value, match.Index);

//  The example displays the following output:
//        'www.microsoft.com' found at position 0.
//        'msdn.microsoft.com' found at position 18.

Dim pattern As String = "\b[A-Z](\w*?\s*?){1,10}[.!?]"
Dim input As String = "Hi. I am writing a short note. Its purpose is " + _
                      "to test a regular expression that attempts to find " + _
                      "sentences with ten or fewer words. Most sentences " + _
                      "in this note are short."
For Each match As Match In Regex.Matches(input, pattern)
   Console.WriteLine("'{0}' found at position {1}.", match.Value, match.Index)
Next 
' The example displays the following output:
'       'Hi.' found at position 0.
'       'I am writing a short note.' found at position 4.
'       'Most sentences in this note are short.' found at position 132.

string pattern = @"\b[A-Z](\w*?\s*?){1,10}[.!?]";
string input = "Hi. I am writing a short note. Its purpose is " + 
                      "to test a regular expression that attempts to find " + 
                      "sentences with ten or fewer words. Most sentences " + 
                      "in this note are short.";
foreach (Match match in Regex.Matches(input, pattern))
   Console.WriteLine("'{0}' found at position {1}.", match.Value, match.Index);

//  The example displays the following output:
//        'Hi.' found at position 0.
//        'I am writing a short note.' found at position 4.
//        'Most sentences in this note are short.' found at position 132.

贪婪限定符与惰性限定符

一些限定符有两种版本：

• 贪婪版本。

  贪婪限定符尝试尽可能多地匹配它所应用到的元素。

• 非贪婪（惰性）版本。

  非贪婪限定符尝试尽可能少地匹配它所应用到的元素。

为了演示两者之间的差别，请考虑一个非常简单的正则表达式，它用于从信用卡号之类的数字串中提取最后四位数。使用 * 贪婪限定符的正则表达式版本为 \b.*([0-9]{4})\b。但是，如果有一个字符串包含两个卡号，那么该正则表达式只成功显示第二个卡号的最后四位数，如下面的示例所示。

Dim greedyPattern As String = "\b.*([0-9]{4})\b"
Dim input1 As String = "1112223333 3992991999"
For Each match As Match In Regex.Matches(input1, greedypattern)
   Console.WriteLine("Account ending in ******{0}.", match.Groups(1).Value)
Next
' The example displays the following output:
'       Account ending in ******1999.

string greedyPattern = @"\b.*([0-9]{4})\b";
string input1 = "1112223333 3992991999";
foreach (Match match in Regex.Matches(input1, greedyPattern))
   Console.WriteLine("Account ending in ******{0}.", match.Groups[1].Value);

// The example displays the following output:
//       Account ending in ******1999.

这不是所需要的行为。该正则表达式未能匹配第一个卡号，因为 * 限定符尝试在整个字符串中尽可能多地匹配前面的元素，因此它在字符串的末尾找到了匹配项。

但是采用 *? 惰性限定符的对应正则表达式可产生期望的行为，如下面的示例所示。

Dim lazyPattern As String = "\b.*?([0-9]{4})\b"
Dim input2 As String = "1112223333 3992991999"
For Each match As Match In Regex.Matches(input2, lazypattern)
   Console.WriteLine("Account ending in ******{0}.", match.Groups(1).Value)
Next     
' The example displays the following output:
'       Account ending in ******3333.
'       Account ending in ******1999.

string lazyPattern = @"\b.*?([0-9]{4})\b";
string input2 = "1112223333 3992991999";
foreach (Match match in Regex.Matches(input2, lazyPattern))
   Console.WriteLine("Account ending in ******{0}.", match.Groups[1].Value);

// The example displays the following output:
//       Account ending in ******3333.
//       Account ending in ******1999.

在大多数情况下，带有贪婪限定符和惰性限定符的正则表达式将返回相同的匹配项。在与句点 (. ) 元字符（该元字符匹配任何字符）一起使用时，它们多数情况下会返回不同的结果。

请参见

其他资源

正则表达式语言元素

修订记录