src/NLog/Targets/FileArchiveHandlers/BaseFileArchiveHandler.cs (1)

267-287: Make digit parsing robust and simpler (avoid char.IsNumber + nullable math quirks).

Use char.IsDigit and an int accumulator to avoid surprises with non-ASCII numerals and nullable arithmetic.

Apply:

-            private static bool TryParseStartSequenceNumber(string archiveFileName, int seqStartIndex, out int archiveSequenceNo)
-            {
-                int? parsedSequenceNo = null;
-
-                int startIndex = seqStartIndex;
-                for (int i = startIndex; i < archiveFileName.Length; ++i)
-                {
-                    char chr = archiveFileName[i];
-                    if (!char.IsNumber(chr))
-                    {
-                        if (!parsedSequenceNo.HasValue)
-                            continue;
-                        break;
-                    }
-
-                    parsedSequenceNo = parsedSequenceNo > 0 ? parsedSequenceNo * 10 : 0;
-                    parsedSequenceNo += (chr - '0');
-                }
-                archiveSequenceNo = parsedSequenceNo ?? 0;
-                return parsedSequenceNo.HasValue;
-            }
+            private static bool TryParseStartSequenceNumber(string archiveFileName, int seqStartIndex, out int archiveSequenceNo)
+            {
+                int value = 0;
+                bool started = false;
+                for (int i = seqStartIndex; i < archiveFileName.Length; ++i)
+                {
+                    char chr = archiveFileName[i];
+                    if (!char.IsDigit(chr))
+                    {
+                        if (!started)
+                            continue;
+                        break;
+                    }
+                    started = true;
+                    checked { value = (value * 10) + (chr - '0'); }
+                }
+                archiveSequenceNo = value;
+                return started;
+            }

If overflow is a concern for pathological filenames, catch OverflowException and fall back to returning false.

src/NLog/Targets/FileArchiveHandlers/BaseFileArchiveHandler.cs (1)

271-296: Parse the last digit run before extension to robustly extract seq-no.

Current logic aborts when encountering multiple numeric groups (e.g., date + seq), disabling seq ordering. Parse the final contiguous digit group before the extension instead.

-                int? parsedSequenceNo = null;
-
-                bool foundSequenceNo = false;
-
-                int startIndex = seqStartIndex;
-                for (int i = startIndex; i < archiveFileName.Length; ++i)
-                {
-                    char chr = archiveFileName[i];
-                    if (!char.IsNumber(chr))
-                    {
-                        foundSequenceNo = parsedSequenceNo.HasValue;
-                        continue;
-                    }
-                    else if (foundSequenceNo)
-                    {
-                        var fileExt = archiveFileName.LastIndexOf('.');
-                        if (fileExt > 0 && fileExt < i)
-                            break;      // Ignore bonus sequence-number, when found inside file-extension
-                        archiveSequenceNo = 0;
-                        return false;   // Abort bonus sequence-number found, so cannot trust anything
-                    }
-
-                    parsedSequenceNo = parsedSequenceNo > 0 ? parsedSequenceNo * 10 : 0;
-                    parsedSequenceNo += (chr - '0');
-                }
-                archiveSequenceNo = parsedSequenceNo ?? 0;
-                return parsedSequenceNo.HasValue;
+                int fileExt = archiveFileName.LastIndexOf('.');
+                int endIndex = fileExt > 0 ? fileExt : archiveFileName.Length;
+                int lastStart = -1, lastLen = 0, curStart = -1, curLen = 0;
+                for (int i = Math.Max(0, seqStartIndex); i < endIndex; ++i)
+                {
+                    char ch = archiveFileName[i];
+                    if (char.IsDigit(ch))
+                    {
+                        if (curLen == 0) curStart = i;
+                        curLen++;
+                    }
+                    else if (curLen > 0)
+                    {
+                        lastStart = curStart;
+                        lastLen = curLen;
+                        curStart = -1;
+                        curLen = 0;
+                    }
+                }
+                if (curLen > 0)
+                {
+                    lastStart = curStart;
+                    lastLen = curLen;
+                }
+                if (lastLen > 0)
+                {
+                    int value = 0;
+                    for (int i = lastStart; i < lastStart + lastLen; i++)
+                        value = (value * 10) + (archiveFileName[i] - '0');
+                    archiveSequenceNo = value;
+                    return true;
+                }
+                archiveSequenceNo = 0;
+                return false;

src/NLog/Targets/FileArchiveHandlers/BaseFileArchiveHandler.cs (1)

267-296: Sequence parsing fails for common "{date}_{seq}" names; parse the last digit run before extension

Current TryParseStartSequenceNumber aborts when a second digit run appears (e.g., "App_20250905_12.txt"), so the seq "12" isn’t recognized and cleanup may fall back to timestamps, reintroducing the bug from #5986 in mixed date+seq patterns.

Refactor to take the last contiguous digit run between seqStartIndex and the file extension.

Apply:

-            private static bool TryParseStartSequenceNumber(string archiveFileName, int seqStartIndex, out int archiveSequenceNo)
-            {
-                int? parsedSequenceNo = null;
-
-                bool foundSequenceNo = false;
-
-                int startIndex = seqStartIndex;
-                for (int i = startIndex; i < archiveFileName.Length; ++i)
-                {
-                    char chr = archiveFileName[i];
-                    if (!char.IsNumber(chr))
-                    {
-                        foundSequenceNo = parsedSequenceNo.HasValue;
-                        continue;
-                    }
-                    else if (foundSequenceNo)
-                    {
-                        var fileExt = archiveFileName.LastIndexOf('.');
-                        if (fileExt > 0 && fileExt < i)
-                            break;      // Ignore bonus sequence-number, when found inside file-extension
-                        archiveSequenceNo = 0;
-                        return false;   // Abort bonus sequence-number found, so cannot trust anything
-                    }
-
-                    parsedSequenceNo = parsedSequenceNo > 0 ? parsedSequenceNo * 10 : 0;
-                    parsedSequenceNo += (chr - '0');
-                }
-                archiveSequenceNo = parsedSequenceNo ?? 0;
-                return parsedSequenceNo.HasValue;
-            }
+            private static bool TryParseStartSequenceNumber(string archiveFileName, int seqStartIndex, out int archiveSequenceNo)
+            {
+                archiveSequenceNo = 0;
+                if (seqStartIndex < 0 || seqStartIndex >= archiveFileName.Length)
+                    return false;
+
+                // Limit scan to filename part (exclude extension)
+                int endExclusive = archiveFileName.LastIndexOf('.');
+                if (endExclusive <= seqStartIndex)
+                    endExclusive = archiveFileName.Length;
+
+                // Capture the last contiguous digit run between seqStartIndex and endExclusive
+                int lastRunStart = -1, lastRunEnd = -1;
+                bool inRun = false;
+                for (int i = seqStartIndex; i < endExclusive; ++i)
+                {
+                    char ch = archiveFileName[i];
+                    if (char.IsDigit(ch))
+                    {
+                        if (!inRun) { inRun = true; lastRunStart = i; }
+                        lastRunEnd = i + 1;
+                    }
+                    else
+                    {
+                        if (inRun) { inRun = false; }
+                    }
+                }
+
+                if (lastRunStart >= 0 && lastRunEnd > lastRunStart)
+                {
+                    int value = 0;
+                    for (int i = lastRunStart; i < lastRunEnd; ++i)
+                        value = (value * 10) + (archiveFileName[i] - '0');
+                    archiveSequenceNo = value;
+                    return true;
+                }
+                return false;
+            }