Result of Regex.Match("wtfb",@"(.)()+?b") is not same as new Regex(@"(.)()+?b",RegexOptions.Compiled).Match("wtfb")

[longxya]

Description

If pattern includes ()+?, then Regex.Match(input,pattern) will match differnet new Regex(pattern ,RegexOptions.Compiled).Match(input)

Some discussions in GitHub Discussion #110976

Reproduction Steps

using System.Text.RegularExpressions;

string input = "wtfb";
string pattern="^(.)+()+?b";
Match matchInterpreted = new Regex(pattern, RegexOptions.None).Match(input);
Match matchCompiled = new Regex(pattern, RegexOptions.Compiled).Match(input);

Console.WriteLine($"Interpreted: {matchInterpreted.Value}");
Console.WriteLine($"Compiled: {matchCompiled.Value}");

Output:

Interpreted: b
Compiled: wtfb

Expected behavior

Output:

Interpreted: wtfb
Compiled: wtfb

Actual behavior

Output:

Interpreted: b
Compiled: wtfb

It looks like ()+? would make nearest outside brackets start at where the empty loop began, and this effect maybe gradually spread to group 0 or not.

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

No response

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[DL444]

Interpreted: b

Typo in the expected behavior. It is expected that the interpreted result matches compiled.

[steveharter]

Verified - the interpreted mode has a different result than both compiled and generated via [GeneratedRegex] and .NET v8, v9 and current v10 have the same issue.

I'll mark this as a bug and for v10 however with "help wanted".

cc @stephentoub

[longxya]

Do I need to submit new issues for other regular expression problems that appear to be bugs? Such as:

Using new Regex("(.)(?'2-1'(?'-1'.))", RegexOptions.Compiled).Matches("wtf") will throw an exception "Index was outside the bounds of the array".

• For regularexpression (?'2-1'(?'1'.)) , the interpreted mode has a different result than both compiled and generated via [GeneratedRegex].

[ovidiucosteanet]

Hello! I'd like to create a PR for this. Can you please assign the issue to me?

[stephentoub]

@ovidiucosteanet, if you'd like to give it a go, we'd welcome the help. Thanks.

[longxya]

@stephentoub @ovidiucosteanet
Similar regular expressions can cause infinite matching

Maybe it is related to this issue

using System;
using System.Text.RegularExpressions;

string pattern = @"(()+?){2}";
//pattern = @"(()+?)+";
//pattern = @"(()+?)*";
//pattern = @"(()+?){1,2}";
string input = "WTF123a1";
int timeOut = 10;
Regex regex = new Regex(pattern, RegexOptions.None, TimeSpan.FromMilliseconds(timeOut));
var mhes = regex.Matches(input);
try
{
	//var matchCount = mhes.Count;// will throw TimeOutException if memory is enough
	for (var i = 0; i < 1000; i++)
	{
		Console.WriteLine(mhes[i].Index + " , " + mhes[i].Length);
	}
}catch(Exception e)
{
	Console.WriteLine("Interpreted : "+e.Message);
}

Output:

0 , 3
0 , 3
0 , 3
0 , 3
0 , 3
0 , 3
  ·
  ·
  ·

And I do not know why changing the quantifier to something like {2,100} would directly throw an exception System.OutOfMemoryException

using System;
using System.Text.RegularExpressions;

string pattern = @"(()+?){2,100}";
string input = "WTF123a1";
int timeOut = 10;
Regex regex = new Regex(pattern, RegexOptions.None, TimeSpan.FromMilliseconds(timeOut));
var mhes = regex.Matches(input);
try
{
	for (var i = 0; i < 1000; i++)
	{
		Console.WriteLine(mhes[i].Index + " , " + mhes[i].Length);
	}
}catch(Exception e)
{
	Console.WriteLine("Interpreted : "+e.Message);
}

Output:

Interpreted : Exception of type 'System.OutOfMemoryException' was thrown.