Eli Zaretskii <eliz@gnu.org> writes:
>
>> +#ifdef emacs
>> +#define STR_BASE_PTR(obj)                       \
>> +    (BUFFERP (obj)? XBUFFER (obj)->text->beg :  \
>> +     STRINGP (obj)? SDATA (obj) :               \
>> +     NULL)
>

[...]

> the only test in the macro should be STRINGP.
>

Hmm, not sure I feel comfortable being that implicit.  I kept this macro
the same except for using (NILP (obj)? current_buffer->...) instead of
BUFFERP and XBUFFER.

>
> Btw, note that regex.c already has macros PTR_TO_OFFSET and
> POS_AS_IN_BUFFER which you can use.

AFAICT these are not useful for this: they give offets relative to
string1 (or string2), which would not help to compute the new value for
string1 (or string2, etc...).  Since I was looking at it, I've also
added a comment about the trick of punning the boolean result into
buffer or string base index.

------

Here is the new patch: