如何将整个字符串与单个正则表达式匹配为两种格式之一？

Question

I need to validate values that can have one of two formats and am trying to do so with a single regular expression but can't figure out why it doesn't work.

The first format is exactly 17 alphanumeric characters and the expression ^[A-Za-z0-9]{17}$ correctly matches the test value 5UXWX7C56BA123456 but not the shortened value 5UXWX7C56BA12345 or the lengthened value 5UXWX7C56BA1234569 .

The second format is exactly 8 alphanumeric characters followed by asterisk or underscore ansd two more alphanumeric characters. The expression ^[A-Za-z0-9]{8}[*_][A-Za-z0-9]{2}$ correctly matches the test value 5UXWX7C5*BA but not the shortened value 5UXWX7C5*B or the lengthened value 5UXWX7C5*BA1 .

However when I try to combine the expressions I get unexpected results that differ, depending on which of the sub-expressions I place first. The following snippet of code demonstrates

var pattern1 = new Regex(@"^([A-Za-z0-9]{17})|([A-Za-z0-9]{8}[*_][A-Za-z0-9]{2})$");
var pattern2 = new Regex(@"^([A-Za-z0-9]{8}[*_][A-Za-z0-9]{2})|([A-Za-z0-9]{17})$");

var values = new string[] 
{ 
    "5UXWX7C56BA12345", "5UXWX7C56BA123456", "5UXWX7C56BA1234569", 
    "5UXWX7C5*B", "5UXWX7C5*BA", "5UXWX7C5*BA1" 
};

Console.WriteLine($"Using {pattern1}\n");
Console.WriteLine($"  {"Value",-20}{"IsMatch",-9}{"Expected",-10}");
Console.WriteLine($"  {new string('-', 37)}");
values
    .Select(x => new { Value = x, Result = pattern1.IsMatch(x), ExpectedResult = x.Length == 11 || x.Length == 17 })
    .Select(x => $"  {x.Value,-20}{x.Result,-9}{x.ExpectedResult} {(x.Result == x.ExpectedResult ? "" : "UNEXPECTED")}")
    .WithEach(Console.WriteLine);

Console.WriteLine($"\n\nUsing {pattern2}\n");
Console.WriteLine($"  {"Value",-20}{"IsMatch",-9}{"Expected",-10}");
Console.WriteLine($"  {new string('-', 37)}");
values
    .Select(x => new { Value = x, Result = pattern2.IsMatch(x), ExpectedResult = x.Length == 11 || x.Length == 17 })
    .Select(x => $"  {x.Value,-20}{x.Result,-9}{x.ExpectedResult} {(x.Result == x.ExpectedResult ? "" : "UNEXPECTED")}")
    .WithEach(Console.WriteLine);

producing the following results

Using ^([A-Za-z0-9]{17})|([A-Za-z0-9]{8}[*_][A-Za-z0-9]{2})$

  Value               IsMatch  Expected  
  -------------------------------------
  5UXWX7C56BA12345    False    False 
  5UXWX7C56BA123456   True     True 
  5UXWX7C56BA1234569  True     False UNEXPECTED
  5UXWX7C5*B          False    False 
  5UXWX7C5*BA         True     True 
  5UXWX7C5*BA1        False    False 


Using ^([A-Za-z0-9]{8}[*_][A-Za-z0-9]{2})|([A-Za-z0-9]{17})$

  Value               IsMatch  Expected  
  -------------------------------------
  5UXWX7C56BA12345    False    False 
  5UXWX7C56BA123456   True     True 
  5UXWX7C56BA1234569  True     False UNEXPECTED
  5UXWX7C5*B          False    False 
  5UXWX7C5*BA         True     True 
  5UXWX7C5*BA1        True     False UNEXPECTED

I hope someone will be able to point out the error in my expressions. It seems that although I am using ^ and $ to try and force the entire line/value to be matched, that somehow when longer a match is found even though there is a further unmatched character that I would have expected to cause the entire value not to match.

Although I used LINQPad to run the snippet above I see the same results from regex101.com .

Answer 1

Your regexps are not anchored correctly:

^([A-Za-z0-9]{17})|([A-Za-z0-9]{8}[*_][A-Za-z0-9]{2})$
 ^               ^ ^                                ^                

Here, ([A-Za-z0-9]{17}) is only anchored at the start of the string (and there can be anything after that pattern) and ([A-Za-z0-9]{8}[*_][A-Za-z0-9]{2}) is only anchored at the end of the string (and there can be anything before that pattern).

The same problem is with the second pattern, you just swapped the alternatives.

Use

var pattern1 = new Regex(@"^(?:[A-Za-z0-9]{17}|[A-Za-z0-9]{8}[*_][A-Za-z0-9]{2})$");
                            ^                 ^                                ^

Otherwise, your alternatives are not anchored on both sides.

See the regex demo .