Here are five possible code completions for the given function:

**Option 1: Simple Loop**
```c
static UINT ExtractRunLengthRegularFgBg(const BYTE* pbOrderHdr, const BYTE* pbEnd, UINT32* advance)
{
    UINT runLength = 0;
    while (pbOrderHdr < pbEnd) {
        runLength++;
        pbOrderHdr++;
    }
    *advance = runLength;
    return runLength;
}
```

**Option 2: Using Pointer Arithmetic**
```c
static UINT ExtractRunLengthRegularFgBg(const BYTE* pbOrderHdr, const BYTE* pbEnd, UINT32* advance)
{
    UINT runLength = 0;
    while (pbOrderHdr < pbEnd) {
        runLength++;
        pbOrderHdr++;
    }
    *advance = (UINT32)(pbOrderHdr - (const BYTE*)pbOrderHdr);
    return runLength;
}
```

**Option 3: Using std::distance**
```c
#include <iterator>

static UINT ExtractRunLengthRegularFgBg(const BYTE* pbOrderHdr, const BYTE* pbEnd, UINT32* advance)
{
    UINT runLength = 0;
    runLength = std::distance(pbOrderHdr, pbEnd);
    *advance = runLength;
    return runLength;
}
```

**Option 4: Using std::count**
```c
#include <algorithm>

static UINT ExtractRunLengthRegularFgBg(const BYTE* pbOrderHdr, const BYTE* pbEnd, UINT32* advance)
{
    UINT runLength = 0;
    runLength = std::count(pbOrderHdr, pbEnd, *pbOrderHdr);
    *advance = runLength;
    return runLength;
}
```

**Option 5: Using std::accumulate**
```c
#include <numeric>

static UINT ExtractRunLengthRegularFgBg(const BYTE* pbOrderHdr, const BYTE* pbEnd, UINT32* advance)
{
    UINT runLength = 0;
    runLength = std::accumulate(pbOrderHdr, pbEnd, 0, [](UINT acc, BYTE) { return acc + 1; });
    *advance = runLength;
    return runLength;
}
```

Note: These examples assume that `UINT` and `UINT32` are defined as unsigned integer types, and `BYTE` is defined as an unsigned char type.