如何使正则表达式只捕获命名组

Question

根据Regex文档，使用RegexOptions.ExplicitCapture使得正则表达式只匹配命名组，如(?<groupName>...);但在行动上它做了一些有点不同的事情。

考虑的几行代码：

static void Main(string[] args) { 
    Regex r = new Regex(
     @"(?<code>^(?<l1>[\d]{2})/(?<l2>[\d]{3})/(?<l3>[\d]{2})$|^(?<l1>[\d]{2})/(?<l2>[\d]{3})$|(?<l1>^[\d]{2}$))" 
     , RegexOptions.ExplicitCapture 
    ); 
    var x = r.Match("32/123/03"); 
    r.GetGroupNames().ToList().ForEach(gn => { 
     Console.WriteLine("GroupName:{0,5} --> Value: {1}", gn, x.Groups[gn].Success ? x.Groups[gn].Value : ""); 
    }); 
} 

当你运行这段代码，你会看到的结果中包含一个名为组，而我没有在我的正则表达式命名0组！

GroupName: 0 --> Value: 32/123/03 
GroupName: code --> Value: 32/123/03 
GroupName: l1 --> Value: 32 
GroupName: l2 --> Value: 123 
GroupName: l3 --> Value: 03 
Press any key to continue . . . 

请问有人请向我解释这种行为？

Answer 1

您总是有组0：这是整场比赛。基于定义组的开括号的序数位置，编号组相对于1。正则表达式（格式为清楚起见）：

(?<code> 
^
    (?<l1> [\d]{2}) 
/
    (?<l2> [\d]{3}) 
/
    (?<l3> [\d]{2}) 
    $ 
| 
^
    (?<l1>[\d]{2}) 
/
    (?<l2>[\d]{3}) 
    $ 
| 
    (?<l1> ^[\d]{2} $) 
) 

你的表达会原路返回，所以你可能会考虑简化您的正则表达式。这可能是更清晰，更高效：

static Regex rxCode = new Regex(@" 
^     # match start-of-line, followed by 
    (?<code>    # a mandatory group ('code'), consisting of 
    (?<g1> \d\d)  # - 2 decimal digits ('g1'), followed by 
    (     # - an optional group, consisting of 
    /    # - a literal '/', followed by 
     (?<g2> \d\d\d) # - 3 decimal digits ('g2'), followed by 
     (    # - an optional group, consisting of 
     /   #  - a literal '/', followed by 
     (?<g3> \d\d) #  - 2 decimal digits ('g3') 
    )?    #  - END: optional group 
    )?     # - END: optional group 
)     # - END: named group ('code'), followed by 
    $     # - end-of-line 
" , RegexOptions.IgnorePatternWhitespace|RegexOptions.ExplicitCapture); 

一旦你的，这样的事情：

string[] texts = { "12" , "12/345" , "12/345/67" , } ; 

foreach (string text in texts) 
{ 
    Match m = rxCode.Match(text) ; 
    Console.WriteLine("{0}: match was {1}" , text , m.Success ? "successful" : "NOT successful") ; 
    if (m.Success) 
    { 
    Console.WriteLine(" code: {0}" , m.Groups["code"].Value) ; 
    Console.WriteLine(" g1: {0}" , m.Groups["g1"].Value) ; 
    Console.WriteLine(" g2: {0}" , m.Groups["g2"].Value) ; 
    Console.WriteLine(" g3: {0}" , m.Groups["g3"].Value) ; 
    } 
} 

产生预期

12: match was successful 
    code: 12 
    g1: 12 
    g2: 
    g3: 
12/345: match was successful 
    code: 12/345 
    g1: 12 
    g2: 345 
    g3: 
12/345/67: match was successful 
    code: 12/345/67 
    g1: 12 
    g2: 345 
    g3: 67 

Answer 2

命名组

^(?<l1>[\d]{2})/(?<l2>[\d]{3})/(?<l3>[\d]{2})$|^(?<l1>[\d]{2})/(?<l2>[\d]{3})$|(?<l1>^[\d]{2}$) 

试试这个（我从你的正则表达式中删除第一组） - see demo