14.7.1.1 GNU Pattern Buffers

To compile, match, or search for a given regular expression, you must supply a pattern buffer. A pattern buffer holds one compiled regular expression.¹

You can have several different pattern buffers simultaneously, each holding a compiled pattern for a different regular expression.

regex.h defines the pattern buffer struct with the following public fields:

       unsigned char *buffer;
       unsigned long allocated;
       char *fastmap;
       char *translate;
       size_t re_nsub;
       unsigned no_sub : 1;
       unsigned not_bol : 1;
       unsigned not_eol : 1;