BUG：Some bug in Balancing Group of Regular Expressions

[longxya]

Description

In the balancing group (?'g1-g2'exp), when the content matched by exp precedes the latest capture of g2, g1.Captures.Count and the actual behavior of g1 are inconsistent.

By checking the captures of the group using Group.Captures, you will find that the captures appear empty. However, when using (?(g1)yes|no) for conditional evaluation, it will match yes, indicating that there actually is a capture.

更多关于平衡组的bug，可以参考平衡组的bug·其二
For more information about this bug, please refer to Bug in Balancing Groups - Part 2

测试用例中，使用到了比较复杂的正则表达式。

复杂的正则表达式，可视化可参考正则可视化与调试工具

In the test cases, more complex regular expressions are used.

For visualizing and debugging complex regular expressions, you can refer to Regex Visualization and Debugging Tool

Reproduction Steps

using System.Text.RegularExpressions;

string input = "00123xzacvb1";
string pattern=@"\d+((?'x'[a-z-[b]]+)).(?<=(?'2-1'(?'x1'..)).{6})b(?(2)(?'Group2Captured'.)|(?'Group2NotCaptured'.))";
try
{
	Match matchInterpreted = new Regex(pattern, RegexOptions.None).Match(input);
	Console.WriteLine($"Interpreted Group2: {matchInterpreted.Groups[2].Captures.Count}");
	Console.WriteLine($"Interpreted Group2Captured: {matchInterpreted.Groups["Group2Captured"].Captures.Count>0}");
	Console.WriteLine($"Interpreted Group2NotCaptured: {matchInterpreted.Groups["Group2NotCaptured"].Captures.Count>0}");
}catch(Exception ex)
{
	Console.WriteLine($"Interpreted Exception: {ex.Message}");
}


try
{
	Match matchCompiled = new Regex(pattern, RegexOptions.Compiled).Match(input);
	Console.WriteLine($"Compiled Group2: {matchCompiled.Groups[2].Captures.Count}");
	Console.WriteLine($"Compiled Group2Captured: {matchCompiled.Groups["Group2Captured"].Captures.Count>0}");
	Console.WriteLine($"Compiled Group2NotCaptured: {matchCompiled.Groups["Group2NotCaptured"].Captures.Count>0}");
}catch(Exception ex)
{
	Console.WriteLine($"Compiled Exception: {ex.Message}");
}

Output:

Interpreted Group2: 0
Interpreted Group2Captured: True
Interpreted Group2NotCaptured: False
Compiled Group2: 0
Compiled Group2Captured: True
Compiled Group2NotCaptured: False

Expected behavior

Interpreted Group2: 1
Interpreted Group2Captured: True
Interpreted Group2NotCaptured: False
Compiled Group2: 1
Compiled Group2Captured: True
Compiled Group2NotCaptured: False

Or

Interpreted Group2: 0
Interpreted Group2Captured: False
Interpreted Group2NotCaptured: True
Compiled Group2: 0
Compiled Group2Captured: False
Compiled Group2NotCaptured: True

Actual behavior

Interpreted Group2: 0
Interpreted Group2Captured: True
Interpreted Group2NotCaptured: False
Compiled Group2: 0
Compiled Group2Captured: True
Compiled Group2NotCaptured: False

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

No response

[steveharter]

Verified the repro, including verifying generated results via [GeneratedRegex] match the interpreted and compiled results.

However, the repro should be simplified if possible so that it is easier to verify that the result is not correct and to help find the root cause.

Is the discussion at #110976 for issues with ()+? relevant here?

FWIW: Generated C# results

// <auto-generated/>
#nullable enable
#pragma warning disable CS0162 // Unreachable code
#pragma warning disable CS0164 // Unreferenced label
#pragma warning disable CS0219 // Variable assigned but never used

namespace ConsoleApp332
{
    partial class Program
    {
        /// <remarks>
        /// Pattern:<br/>
        /// <code>\\d+((?'x'[a-z-[b]]+)).(?&lt;=(?'2-1'(?'x1'..)).{6})b(?(2)(?'Group2Captured'.)|(?'Group2NotCaptured'.))</code><br/>
        /// Options:<br/>
        /// <code>RegexOptions.IgnoreCase</code><br/>
        /// Explanation:<br/>
        /// <code>
        /// ○ Match a Unicode digit greedily at least once.<br/>
        /// ○ 1st capture group.<br/>
        ///     ○ "x" capture group.<br/>
        ///         ○ Match a character in the set [A-Za-z\u0130\u212A-[Bb]] greedily at least once.<br/>
        /// ○ Match any character other than '\n'.<br/>
        /// ○ Zero-width positive lookbehind.<br/>
        ///     ○ Match a character other than '\n' exactly 6 times right-to-left.<br/>
        ///     ○ Balancing group. Captures the 2nd capture group and uncaptures the 1st capture group.<br/>
        ///         ○ "x1" capture group.<br/>
        ///             ○ Match a character other than '\n' exactly 2 times right-to-left.<br/>
        /// ○ Match a character in the set [Bb].<br/>
        /// ○ Atomic group.<br/>
        ///     ○ Conditionally match one of two expressions depending on whether the 2nd capture group matched.<br/>
        ///         ○ Matched: "Group2Captured" capture group.<br/>
        ///             ○ Match any character other than '\n'.<br/>
        ///         ○ Not Matched: "Group2NotCaptured" capture group.<br/>
        ///             ○ Match any character other than '\n'.<br/>
        /// </code>
        /// </remarks>
        [global::System.CodeDom.Compiler.GeneratedCodeAttribute("System.Text.RegularExpressions.Generator", "10.0.11.11703")]
        private static partial global::System.Text.RegularExpressions.Regex TestRegex() => global::System.Text.RegularExpressions.Generated.TestRegex_0.Instance;
    }
}

namespace System.Text.RegularExpressions.Generated
{
    using System;
    using System.Buffers;
    using System.CodeDom.Compiler;
    using System.Collections;
    using System.ComponentModel;
    using System.Globalization;
    using System.Runtime.CompilerServices;
    using System.Text.RegularExpressions;
    using System.Threading;

    /// <summary>Custom <see cref="Regex"/>-derived type for the TestRegex method.</summary>
    [GeneratedCodeAttribute("System.Text.RegularExpressions.Generator", "10.0.11.11703")]
    file sealed class TestRegex_0 : Regex
    {
        /// <summary>Cached, thread-safe singleton instance.</summary>
        internal static readonly TestRegex_0 Instance = new();
    
        /// <summary>Initializes the instance.</summary>
        private TestRegex_0()
        {
            base.pattern = "\\d+((?'x'[a-z-[b]]+)).(?<=(?'2-1'(?'x1'..)).{6})b(?(2)(?'Group2Captured'.)|(?'Group2NotCaptured'.))";
            base.roptions = RegexOptions.IgnoreCase;
            ValidateMatchTimeout(Utilities.s_defaultTimeout);
            base.internalMatchTimeout = Utilities.s_defaultTimeout;
            base.factory = new RunnerFactory();
            base.CapNames = new Hashtable { { "0", 0 } ,  { "1", 1 } ,  { "2", 2 } ,  { "Group2Captured", 5 } ,  { "Group2NotCaptured", 6 } ,  { "x", 3 } ,  { "x1", 4 }  };
            base.capslist = new string[] {"0", "1", "2", "x", "x1", "Group2Captured", "Group2NotCaptured" };
            base.capsize = 7;
        }
    
        /// <summary>Provides a factory for creating <see cref="RegexRunner"/> instances to be used by methods on <see cref="Regex"/>.</summary>
        private sealed class RunnerFactory : RegexRunnerFactory
        {
            /// <summary>Creates an instance of a <see cref="RegexRunner"/> used by methods on <see cref="Regex"/>.</summary>
            protected override RegexRunner CreateInstance() => new Runner();
        
            /// <summary>Provides the runner that contains the custom logic implementing the specified regular expression.</summary>
            private sealed class Runner : RegexRunner
            {
                /// <summary>Scan the <paramref name="inputSpan"/> starting from base.runtextstart for the next match.</summary>
                /// <param name="inputSpan">The text being scanned by the regular expression.</param>
                protected override void Scan(ReadOnlySpan<char> inputSpan)
                {
                    // Search until we can't find a valid starting position, we find a match, or we reach the end of the input.
                    while (TryFindNextPossibleStartingPosition(inputSpan) &&
                           !TryMatchAtCurrentPosition(inputSpan) &&
                           base.runtextpos != inputSpan.Length)
                    {
                        base.runtextpos++;
                        if (Utilities.s_hasTimeout)
                        {
                            base.CheckTimeout();
                        }
                    }
                }
        
                /// <summary>Search <paramref name="inputSpan"/> starting from base.runtextpos for the next location a match could possibly start.</summary>
                /// <param name="inputSpan">The text being scanned by the regular expression.</param>
                /// <returns>true if a possible match was found; false if no more matches are possible.</returns>
                private bool TryFindNextPossibleStartingPosition(ReadOnlySpan<char> inputSpan)
                {
                    int pos = base.runtextpos;
                    
                    // Any possible match is at least 5 characters.
                    if (pos <= inputSpan.Length - 5)
                    {
                        // The pattern begins with a Unicode digit.
                        // Find the next occurrence. If it can't be found, there's no match.
                        int i = inputSpan.Slice(pos).IndexOfAnyDigit();
                        if (i >= 0)
                        {
                            base.runtextpos = pos + i;
                            return true;
                        }
                    }
                    
                    // No match found.
                    base.runtextpos = inputSpan.Length;
                    return false;
                }
        
                /// <summary>Determine whether <paramref name="inputSpan"/> at base.runtextpos is a match for the regular expression.</summary>
                /// <param name="inputSpan">The text being scanned by the regular expression.</param>
                /// <returns>true if the regular expression matches at the current position; otherwise, false.</returns>
                private bool TryMatchAtCurrentPosition(ReadOnlySpan<char> inputSpan)
                {
                    int pos = base.runtextpos;
                    int matchStart = pos;
                    int capture_starting_pos = 0;
                    int capture_starting_pos1 = 0;
                    int capture_starting_pos2 = 0;
                    int capture_starting_pos3 = 0;
                    int capture_starting_pos4 = 0;
                    int capture_starting_pos5 = 0;
                    int charloop_capture_pos = 0;
                    int charloop_capture_pos1 = 0;
                    int charloop_starting_pos = 0, charloop_ending_pos = 0;
                    int charloop_starting_pos1 = 0, charloop_ending_pos1 = 0;
                    ReadOnlySpan<char> slice = inputSpan.Slice(pos);
                    
                    // Match a Unicode digit greedily at least once.
                    //{
                        charloop_starting_pos = pos;
                        
                        int iteration = 0;
                        while ((uint)iteration < (uint)slice.Length && char.IsDigit(slice[iteration]))
                        {
                            iteration++;
                        }
                        
                        if (iteration == 0)
                        {
                            UncaptureUntil(0);
                            return false; // The input didn't match.
                        }
                        
                        slice = slice.Slice(iteration);
                        pos += iteration;
                        
                        charloop_ending_pos = pos;
                        charloop_starting_pos++;
                        goto CharLoopEnd;
                        
                        CharLoopBacktrack:
                        UncaptureUntil(charloop_capture_pos);
                        
                        if (Utilities.s_hasTimeout)
                        {
                            base.CheckTimeout();
                        }
                        
                        if (charloop_starting_pos >= charloop_ending_pos ||
                            (charloop_ending_pos = inputSpan.Slice(charloop_starting_pos, charloop_ending_pos - charloop_starting_pos).LastIndexOfAny(Utilities.s_nonAscii_53BE860A7BB3C901EBE8EECDBB69D761C6C74DF0564F8B7A7926DECC0EC263B1)) < 0)
                        {
                            UncaptureUntil(0);
                            return false; // The input didn't match.
                        }
                        charloop_ending_pos += charloop_starting_pos;
                        pos = charloop_ending_pos;
                        slice = inputSpan.Slice(pos);
                        
                        CharLoopEnd:
                        charloop_capture_pos = base.Crawlpos();
                    //}
                    
                    // Advance the next matching position.
                    if (base.runtextpos < pos)
                    {
                        base.runtextpos = pos;
                    }
                    
                    // 1st capture group.
                    //{
                        capture_starting_pos = pos;
                        
                        // "x" capture group.
                        //{
                            capture_starting_pos1 = pos;
                            
                            // Match a character in the set [A-Za-z\u0130\u212A-[Bb]] greedily at least once.
                            //{
                                charloop_starting_pos1 = pos;
                                
                                int iteration1 = slice.IndexOfAnyExcept(Utilities.s_nonAscii_53BE860A7BB3C901EBE8EECDBB69D761C6C74DF0564F8B7A7926DECC0EC263B1);
                                if (iteration1 < 0)
                                {
                                    iteration1 = slice.Length;
                                }
                                
                                if (iteration1 == 0)
                                {
                                    goto CharLoopBacktrack;
                                }
                                
                                slice = slice.Slice(iteration1);
                                pos += iteration1;
                                
                                charloop_ending_pos1 = pos;
                                charloop_starting_pos1++;
                                goto CharLoopEnd1;
                                
                                CharLoopBacktrack1:
                                UncaptureUntil(charloop_capture_pos1);
                                
                                if (Utilities.s_hasTimeout)
                                {
                                    base.CheckTimeout();
                                }
                                
                                if (charloop_starting_pos1 >= charloop_ending_pos1 ||
                                    (charloop_ending_pos1 = inputSpan.Slice(charloop_starting_pos1, charloop_ending_pos1 - charloop_starting_pos1).LastIndexOfAnyExcept('\n')) < 0)
                                {
                                    goto CharLoopBacktrack;
                                }
                                charloop_ending_pos1 += charloop_starting_pos1;
                                pos = charloop_ending_pos1;
                                slice = inputSpan.Slice(pos);
                                
                                CharLoopEnd1:
                                charloop_capture_pos1 = base.Crawlpos();
                            //}
                            
                            base.Capture(3, capture_starting_pos1, pos);
                            
                            goto CaptureSkipBacktrack;
                            
                            CaptureBacktrack:
                            goto CharLoopBacktrack1;
                            
                            CaptureSkipBacktrack:;
                        //}
                        
                        base.Capture(1, capture_starting_pos, pos);
                        
                        goto CaptureSkipBacktrack1;
                        
                        CaptureBacktrack1:
                        goto CaptureBacktrack;
                        
                        CaptureSkipBacktrack1:;
                    //}
                    
                    // Match any character other than '\n'.
                    if (slice.IsEmpty || slice[0] == '\n')
                    {
                        goto CaptureBacktrack1;
                    }
                    
                    pos++;
                    slice = inputSpan.Slice(pos);
                    // Zero-width positive lookbehind.
                    {
                        slice = inputSpan.Slice(pos);
                        int positivelookbehind_starting_pos = pos;
                        
                        if (Utilities.s_hasTimeout)
                        {
                            base.CheckTimeout();
                        }
                        
                        // Match a character other than '\n' exactly 6 times right-to-left.
                        {
                            for (int i = 0; i < 6; i++)
                            {
                                if ((uint)(pos - 1) >= inputSpan.Length || inputSpan[pos - 1] == '\n')
                                {
                                    goto CaptureBacktrack1;
                                }
                                pos--;
                            }
                        }
                        
                        // Balancing group. Captures the 2nd capture group and uncaptures the 1st capture group.
                        {
                            capture_starting_pos2 = pos;
                            
                            if (!base.IsMatched(1))
                            {
                                goto CaptureBacktrack1;
                            }
                            
                            // "x1" capture group.
                            {
                                capture_starting_pos3 = pos;
                                
                                // Match a character other than '\n' exactly 2 times right-to-left.
                                {
                                    for (int i = 0; i < 2; i++)
                                    {
                                        if ((uint)(pos - 1) >= inputSpan.Length || inputSpan[pos - 1] == '\n')
                                        {
                                            goto CaptureBacktrack1;
                                        }
                                        pos--;
                                    }
                                }
                                
                                base.Capture(4, capture_starting_pos3, pos);
                            }
                            
                            base.TransferCapture(2, 1, capture_starting_pos2, pos);
                        }
                        
                        pos = positivelookbehind_starting_pos;
                        slice = inputSpan.Slice(pos);
                    }
                    
                    // Match a character in the set [Bb].
                    if (slice.IsEmpty || ((slice[0] | 0x20) != 'b'))
                    {
                        goto CaptureBacktrack1;
                    }
                    
                    // Conditionally match one of two expressions depending on whether the 2nd capture group matched.
                    {
                        pos++;
                        slice = inputSpan.Slice(pos);
                        if (base.IsMatched(2))
                        {
                            // The 2nd capture group captured a value.  Match the first branch.
                            // "Group2Captured" capture group.
                            {
                                capture_starting_pos4 = pos;
                                
                                // Match any character other than '\n'.
                                if (slice.IsEmpty || slice[0] == '\n')
                                {
                                    goto CaptureBacktrack1;
                                }
                                
                                pos++;
                                slice = inputSpan.Slice(pos);
                                base.Capture(5, capture_starting_pos4, pos);
                            }
                            
                        }
                        else
                        {
                            // Otherwise, match the second branch.
                            // "Group2NotCaptured" capture group.
                            {
                                capture_starting_pos5 = pos;
                                
                                // Match any character other than '\n'.
                                if (slice.IsEmpty || slice[0] == '\n')
                                {
                                    goto CaptureBacktrack1;
                                }
                                
                                pos++;
                                slice = inputSpan.Slice(pos);
                                base.Capture(6, capture_starting_pos5, pos);
                            }
                            
                        }
                    }
                    
                    // The input matched.
                    base.runtextpos = pos;
                    base.Capture(0, matchStart, pos);
                    return true;
                    
                    // <summary>Undo captures until it reaches the specified capture position.</summary>
                    [MethodImpl(MethodImplOptions.AggressiveInlining)]
                    void UncaptureUntil(int capturePosition)
                    {
                        while (base.Crawlpos() > capturePosition)
                        {
                            base.Uncapture();
                        }
                    }
                }
            }
        }

    }
    
    /// <summary>Helper methods used by generated <see cref="Regex"/>-derived implementations.</summary>
    [GeneratedCodeAttribute("System.Text.RegularExpressions.Generator", "10.0.11.11703")]
    file static class Utilities
    {
        /// <summary>Default timeout value set in <see cref="AppContext"/>, or <see cref="Regex.InfiniteMatchTimeout"/> if none was set.</summary>
        internal static readonly TimeSpan s_defaultTimeout = AppContext.GetData("REGEX_DEFAULT_MATCH_TIMEOUT") is TimeSpan timeout ? timeout : Regex.InfiniteMatchTimeout;
        
        /// <summary>Whether <see cref="s_defaultTimeout"/> is non-infinite.</summary>
        internal static readonly bool s_hasTimeout = s_defaultTimeout != Regex.InfiniteMatchTimeout;
        
        /// <summary>Finds the next index of any character that matches a Unicode digit.</summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        internal static int IndexOfAnyDigit(this ReadOnlySpan<char> span)
        {
            int i = span.IndexOfAnyExcept(Utilities.s_asciiExceptDigits);
            if ((uint)i < (uint)span.Length)
            {
                if (char.IsAscii(span[i]))
                {
                    return i;
                }
        
                do
                {
                    if (char.IsDigit(span[i]))
                    {
                        return i;
                    }
                    i++;
                }
                while ((uint)i < (uint)span.Length);
            }
        
            return -1;
        }
        
        /// <summary>Supports searching for characters in or not in "\0\u0001\u0002\u0003\u0004\u0005\u0006\a\b\t\n\v\f\r\u000e\u000f\u0010\u0011\u0012\u0013\u0014\u0015\u0016\u0017\u0018\u0019\u001a\u001b\u001c\u001d\u001e\u001f !\"#$%&amp;'()*+,-./:;&lt;=&gt;?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~\u007f".</summary>
        internal static readonly SearchValues<char> s_asciiExceptDigits = SearchValues.Create("\0\u0001\u0002\u0003\u0004\u0005\u0006\a\b\t\n\v\f\r\u000e\u000f\u0010\u0011\u0012\u0013\u0014\u0015\u0016\u0017\u0018\u0019\u001a\u001b\u001c\u001d\u001e\u001f !\"#$%&'()*+,-./:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~\u007f");
        
        /// <summary>Supports searching for characters in or not in "ACDEFGHIJKLMNOPQRSTUVWXYZacdefghijklmnopqrstuvwxyzİK".</summary>
        internal static readonly SearchValues<char> s_nonAscii_53BE860A7BB3C901EBE8EECDBB69D761C6C74DF0564F8B7A7926DECC0EC263B1 = SearchValues.Create("ACDEFGHIJKLMNOPQRSTUVWXYZacdefghijklmnopqrstuvwxyzİK");
    }
}

[longxya]

Is the discussion at #110976 for issues with ()+? relevant here?

Discussion at #110976 for issues with ()+? is not relevant here. They are two separate problems, I think.

The main problem of the current issue is not the inconsistency between the interpreted mode and the compiled regular expression match results.
However, for the regular expressions raised in the current issue, they do sometimes match inconsistently.

In mircosoft's document Balancing group definitions, when this Balancing grouping construct has the following format:
(?<name1-name2>subexpression)

The balancing group definition deletes the definition of name2 and stores the interval between name2 and name1 in name1.

But, what happens if match of subexpression comes before name2(Groups["name2"].Index)?
Both in interpreted and compiled mode, name1 will capture an negative value, but Groups["name1"].Captures is empty.
In this situation, if use (?(name1)yes|no), or \k'name1', (?'-name1'), it will provoke a series of problems:

• If the capture group contains both normal content and negative length content, a fake pop phenomenon occurs during the balancing group pop operation. When the capture group first captures normal content and then captures negative length content, performing the balancing group pop operation will actually only pop the negative length content, leaving the normal content intact. However, querying the captures of the group using Group.Captures will show that the capture group has no captures left.
  case: (?'2'\d+)([a-z-[b]]+).(?<=(?'2-1'..).{6})b(?(2)(?'Group2Captured'.)|(?'Group2NotCaptured'.))(?'2')(?'-2')(?'-2')\2 match 00123xzacvb100123
• For a group with negative length capture, performing a balancing group pop operation, if you consider the negative length capture as part of the captures and continue popping until all captures (including negative length captures) are cleared, then the first attempt to retrieve the group information via Match.Group[groupName] or Match.Group[groupNumber] will throw an exception. On the second attempt, it will return null, and all subsequent groups after this group will also be null.
  case: (?'2'\d+)([a-z-[b]]+).(?<=(?'2-1'..).{6})b(?(2)(?'Group2Captured'.)|(?'Group2NotCaptured'.))(?'2')(?'-2')(?'-2')(?'-2') match 00123xzacvb1
• If a group with recent negative length capture is back-referenced, it will result in a match failure in interpreter mode. In compiled mode, if the content matched by exp is exactly L characters ahead of the latest capture of g2, it will cause the current input index to move L characters in the opposite direction of the match.
  case: \d+([a-z-[b]]+).(?<=(?'2-1'.).{9})b(?(2)(?'Group2Captured'.)|(?'Group2NotCaptured'.))\2 match 00123xzacvb1
• However, if the content matched by exp is exactly two characters ahead of the latest capture of name2, name1 will behave quite "normally". But not entirely normal, after the push and pop operations on Group name1 with (?'name1')(?'-name1'), the properties of name1 are inconsistent before and after the operations.
  case: \d+([a-z-[b]]+).(?<=(?'2-1'..).{7})b(?(2)(?'Group2Captured'.)|(?'Group2NotCaptured'.))(?'2')(?'-2')(?(2)(?'Group2Captured'.)|(?'Group2NotCaptured'.)) match 00123xzacvb21
• etc...

[ericstj]

@stephentoub what is your take on this behavior? Is it expected and we just need to document it better, or should we look into balancing groups deeper to see if there is a bug? The docs mention that the balancing group deletes the previously defined group - is that why this behavior is observed?

FWIW I tested the repo on a few different framework versions and it behaves consistently. I also wasn't sure about group numbering, so I added logging to that. Here's the updated source and output:

using System.Text.RegularExpressions;

string input = "00123xzacvb1";
string pattern=@"\d+((?'x'[a-z-[b]]+)).(?<=(?'2-1'(?'x1'..)).{6})b(?(2)(?'Group2Captured'.)|(?'Group2NotCaptured'.))";
try
{
	Match match = new Regex(pattern, RegexOptions.None).Match(input);
	for (int i = 0; i < match.Groups.Count; i++)
		Console.WriteLine($"Group{i} : '{match.Groups[i]}' : Success {match.Groups[i].Success} : {match.Groups[i].Captures.Count}");
	Console.WriteLine($"Group2Captured: {match.Groups["Group2Captured"].Captures.Count>0}");
	Console.WriteLine($"Group2NotCaptured: {match.Groups["Group2NotCaptured"].Captures.Count>0}");
}
catch(Exception ex)
{
	Console.WriteLine($"Interpreted Exception: {ex.Message}");
}

Output:

Group0 : '00123xzacvb1' : Success True : 1
Group1 : '' : Success False : 0
Group2 : '' : Success False : 0
Group3 : 'xzac' : Success True : 1
Group4 : '12' : Success True : 1
Group5 : '1' : Success True : 1
Group6 : '' : Success False : 0
Group2Captured: True
Group2NotCaptured: False

[longxya]

@stephentoub @steveharter @ericstj
A new discovery:

In Compiled mode

using System;
using System.Text.RegularExpressions;

string input = "00123xzacvb1";
string pattern=@"\d+((?'x'[a-z-[b]]+)).(?<=(?'2-1'(?'x1'..)).{6})b(?(2)(?'Group2Captured'.)|(?'Group2NotCaptured'.))(?'2')(?'-2')(?<=(?'wtf'\2))";
try
{
	Match match = new Regex(pattern, RegexOptions.Compiled).Match(input);
	try
	{
		for (int i = 0; i < match.Groups.Count; i++)
		{
			Console.Write($"Group{i} : Name:'{match.Groups[i].Name}' : \t");
			Console.WriteLine($"Value:'{match.Groups[i]}' :\t Success {match.Groups[i].Success} : {match.Groups[i].Captures.Count}");
		}
	}catch(Exception ex)
	{
		Console.WriteLine(ex.Message);
	}
}
catch(Exception ex)
{
	Console.WriteLine($"Interpreted Exception: {ex.Message}");
}

Output：

Group0 : Name:'0' :     Value:'00123xzacvb1' :     Success True : 1
Group1 : Name:'1' :     Value:'' :     Success False : 0
Group2 : Name:'2' :     Value:'' :     Success False : 0
Group3 : Name:'x' :     Value:'xzac' :     Success True : 1
Group4 : Name:'x1' :     Value:'12' :     Success True : 1
Group5 : Name:'Group2Captured' :     Value:'1' :     Success True : 1
Group6 : Name:'Group2NotCaptured' :     Value:'' :     Success False : 0
Group7 : Name:'wtf' :     Index and length must refer to a location within the string. (Parameter 'length')

---

---

Two of the old discoveries:

using System;
using System.Text.RegularExpressions;

string input = "00123xzacvb1";
string pattern=@"\d+((?'x'[a-z-[b]]+)).(?<=(?'2-1'(?'x1'..)).{6})b(?(2)(?'Group2Captured'.)|(?'Group2NotCaptured'.))(?'2')(?'-2')(?<=(?'wtf'\2))(?'-2')";
try
{
	Match match = new Regex(pattern, RegexOptions.Compiled).Match(input);
	
	for (int i = 0; i < match.Groups.Count; i++)
	{
		try
		{
			Console.Write($"Group{i} : Name:'{match.Groups[i].Name}' : \t");
			Console.WriteLine($"Value:'{match.Groups[i]}' :\t Success {match.Groups[i].Success} : {match.Groups[i].Captures.Count}");
		}catch(Exception ex)
		{
			Console.WriteLine(ex.Message);
		}
	}
	
	Console.WriteLine("---------------------------Second trying---------------------------");
	
	for (int i = 0; i < match.Groups.Count; i++)
	{
		try
		{
			Console.Write($"Group{i} : Name:'{match.Groups[i].Name}' : \t");
			Console.WriteLine($"Value:'{match.Groups[i]}' :\t Success {match.Groups[i].Success} : {match.Groups[i].Captures.Count}");
		}catch(Exception ex)
		{
			Console.WriteLine(ex.Message);
		}
	}
}
catch(Exception ex)
{
	Console.WriteLine($"Interpreted Exception: {ex.Message}");
}

Output:

Group0 : Name:'0' :     Value:'00123xzacvb1' :     Success True : 1
Index was outside the bounds of the array.
Object reference not set to an instance of an object.
Object reference not set to an instance of an object.
Object reference not set to an instance of an object.
Object reference not set to an instance of an object.
Object reference not set to an instance of an object.
Object reference not set to an instance of an object.
---------------------------Second trying---------------------------
Group0 : Name:'0' :     Value:'00123xzacvb1' :     Success True : 1
Group1 : Name:'1' :     Value:'' :     Success False : 0
Object reference not set to an instance of an object.
Object reference not set to an instance of an object.
Object reference not set to an instance of an object.
Object reference not set to an instance of an object.
Object reference not set to an instance of an object.
Object reference not set to an instance of an object.

---

using System;
using System.Text.RegularExpressions;

string input = "00123xzacvb1";
string pattern=@"\d+((?'x'[a-z-[b]]+)).(?<=(?'2-1'(?'x1'..)).{6})b(?(2)(?'Group2Captured'.)|(?'Group2NotCaptured'.))(?'2')(?'-2')(?<=(?'wtf'\2)..)";
try
{
	Match match = new Regex(pattern, RegexOptions.None).Match(input);
	Console.WriteLine($"Interpreted Match Success: {match.Success}");
	match = new Regex(pattern, RegexOptions.Compiled).Match(input);
	Console.WriteLine($"Complied Match Success: {match.Success}");
}
catch(Exception ex)
{
	Console.WriteLine($"Interpreted Exception: {ex.Message}");
}

Outputt:

Interpreted Match Success: False
Complied Match Success: True

[steveharter]

Due to the new findings in "Object reference not set to an instance of an object." with compiled, I'll mark this as a bug. The original issue is likely a bug as well, perhaps the same root cause.

The GroupsCollection is reporting a count of 8 which should be 1, at least that is what the value is for the interpreted results.

GroupsCollection[2] is null causing the null ref. However, on the first pass, GroupsCollection[1] throws IndexOutOfRangeException.