To compile, match, or search for a given regular expression, you must supply a pattern buffer. A pattern buffer holds one compiled regular expression.(4)
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 as follows:
/* Space that holds the compiled pattern. It is declared as
`unsigned char *' because its elements are
sometimes used as array indexes. */
unsigned char *buffer;
/* Number of bytes to which `buffer' points. */
unsigned long allocated;
/* Number of bytes actually used in `buffer'. */
unsigned long used;
/* Syntax setting with which the pattern was compiled. */
reg_syntax_t syntax;
/* Pointer to a fastmap, if any, otherwise zero. re_search uses
the fastmap, if there is one, to skip over impossible
starting points for matches. */
char *fastmap;
/* Either a translate table to apply to all characters before
comparing them, or zero for no translation. The translation
is applied to a pattern when it is compiled and to a string
when it is matched. */
char *translate;
/* Number of subexpressions found by the compiler. */
size_t re_nsub;
/* Zero if this pattern cannot match the empty string, one else.
Well, in truth it's used only in `re_search_2', to see
whether or not we should use the fastmap, so we don't set
this absolutely perfectly; see `re_compile_fastmap' (the
`duplicate' case). */
unsigned can_be_null : 1;
/* If REGS_UNALLOCATED, allocate space in the `regs' structure
for `max (RE_NREGS, re_nsub + 1)' groups.
If REGS_REALLOCATE, reallocate space if necessary.
If REGS_FIXED, use what's there. */
#define REGS_UNALLOCATED 0
#define REGS_REALLOCATE 1
#define REGS_FIXED 2
unsigned regs_allocated : 2;
/* Set to zero when `regex_compile' compiles a pattern; set to one
by `re_compile_fastmap' if it updates the fastmap. */
unsigned fastmap_accurate : 1;
/* If set, `re_match_2' does not return information about
subexpressions. */
unsigned no_sub : 1;
/* If set, a beginning-of-line anchor doesn't match at the
beginning of the string. */
unsigned not_bol : 1;
/* Similarly for an end-of-line anchor. */
unsigned not_eol : 1;
/* If true, an anchor at a newline matches. */
unsigned newline_anchor : 1;
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