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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.(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_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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