I have the following .clang-format configuration file:

IncludeBlocks: Regroup
IncludeCategories:
- Regex: '^<[a-z_]+>$'          #1
  Priority: 1
- Regex: '.*\/libalpha.h[">]$'  #2.1
  Priority: 2
  SortPriority: 1
- Regex: '.*\/betalib.h[">]$'   #2.2
  Priority: 2
  SortPriority: 2
- Regex: '.*\/gamma.h[">]$'     #2.3
  Priority: 2
  SortPriority: 3
- Regex: '.*\/delta.h[">]$'     #2.4
  Priority: 2
  SortPriority: 4
- Regex: '.*\/omega.h[">]$'     #2.5
  Priority: 2
  SortPriority: 5
- Regex: '^<.+'                 #3
  Priority: 3
- Regex: '.*'                   #4
  Priority: 4

The intention is the following:

• category 1 groups all the C++ standard library's headers, such as string and unordered_map by only allowing lowercase characters and underscores for the name, as well as requiring <> instead of ""
• categories 2.1 through 2.5 group specific filenames that are under include subdirectories; these can be included using either <> or "", and should be listed under a specific order, given by SortPriority, and without empty lines between them
• category 3 groups all headers included using <> which weren't captured by category 1
• category 4 groups all headers included using "" which aren't the specific ones captured by categories 2.*

When applying this configuration to the following c++ file:

#include <array>
#include <format>
#include <unordered_map>

#include <abc/delta.h>
#include <iostream>
#include <x1.h>
#include <xyz/betalib.h>

#include <abc/gamma.h>
#include "l.h"
#include "y.h"
#include <x2.h>

#include <abc/libalpha.h>

#include "g.h"

#include <string>
#include <fstream>
#include <vector>
#include <x3.h>

#include <abc/omega.h>

#include <map>


int main() { return 0; }

using the command

clang-format.exe main.cpp

with both files in the current directory, I expected the result to be

#include <array>
#include <format>
#include <fstream>
#include <iostream>
#include <map>
#include <string>
#include <unordered_map>
#include <vector>

#include <abc/libalpha.h>
#include <xyz/betalib.h>
#include <abc/gamma.h>
#include <abc/delta.h>
#include <abc/omega.h>

#include <x1.h>
#include <x2.h>
#include <x3.h>

#include "g.h"
#include "l.h"
#include "y.h"

int main() { return 0; }

Instead, I get

#include <abc/libalpha.h>

#include <array>
#include <format>
#include <fstream>
#include <iostream>
#include <map>
#include <string>
#include <unordered_map>
#include <vector>

#include <xyz/betalib.h>
#include <abc/gamma.h>

#include <x1.h>
#include <x2.h>
#include <x3.h>

#include "g.h"
#include "l.h"
#include "y.h"

#include <abc/delta.h>
#include <abc/omega.h>

int main() { return 0; }

where both the first include and the last 2 includes are not in the place I would expect them to.

I can achieve the result that I want using this .clang-format configuration file:

IncludeBlocks: Regroup
IncludeCategories:
- Regex: '^<[a-z_]+>$'          #1
  Priority: 1
- Regex: '.*\/libalpha.h[">]$'  #2.1
  Priority: 2
  SortPriority: 2
- Regex: '.*\/betalib.h[">]$'   #2.2
  Priority: 2
  SortPriority: 3
- Regex: '.*\/gamma.h[">]$'     #2.3
  Priority: 2
  SortPriority: 4
- Regex: '.*\/delta.h[">]$'     #2.4
  Priority: 2
  SortPriority: 5
- Regex: '.*\/omega.h[">]$'     #2.5
  Priority: 2
  SortPriority: 6
- Regex: '^<.+'                 #2.6
  Priority: 7
- Regex: '.*'                   #2.7
  Priority: 8

but this feels very unintuitive, and it isn't explained at all in the documentation that SortPriority supercedes Priority when used by moving categories with Priority 2 and SortPriority 1 above categories with Priority 1 and categories with Priority 2 and SortPriority 4 and 5 below categories with Priority 4.

I tested in Windows using versions 16.0.6, 17.0.6, 18.1.8, 19.1.7, and 20.1.0, and in Linux versions 20.1.0 and Debian-14.0.6.

Did I understand the configuration incorrectly, or is this a bug in clang-format?