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       regcomp, regexec, regerror, regfree - POSIX regex functions


       #include <regex.h>

       int regcomp(regex_t *preg, const char *regex, int cflags);

       int regexec(const regex_t *preg, const char *string, size_t nmatch,
                   regmatch_t pmatch[], int eflags);

       size_t regerror(int errcode, const regex_t *preg, char *errbuf,
                       size_t errbuf_size);

       void regfree(regex_t *preg);


   POSIX regex compiling
       regcomp()  is  used  to  compile  a  regular  expression  into a form that is suitable for
       subsequent regexec() searches.

       regcomp() is supplied with preg, a pointer to a pattern  buffer  storage  area;  regex,  a
       pointer  to  the  null-terminated  string  and cflags, flags used to determine the type of

       All regular expression searching  must  be  done  via  a  compiled  pattern  buffer,  thus
       regexec()  must  always  be  supplied  with the address of a regcomp() initialized pattern

       cflags is the bitwise-or of zero or more of the following:

              Use POSIX Extended Regular Expression syntax when interpreting regex.  If not  set,
              POSIX Basic Regular Expression syntax is used.

              Do not differentiate case.  Subsequent regexec() searches using this pattern buffer
              will be case insensitive.

              Do not report position of matches.  The nmatch and pmatch  arguments  to  regexec()
              are ignored if the pattern buffer supplied was compiled with this flag set.

              Match-any-character operators don't match a newline.

              A nonmatching list ([^...])  not containing a newline does not match a newline.

              Match-beginning-of-line  operator  (^) matches the empty string immediately after a
              newline, regardless of whether eflags, the execution flags of  regexec(),  contains

              Match-end-of-line  operator  ($)  matches  the  empty  string  immediately before a
              newline, regardless of whether eflags contains REG_NOTEOL.

   POSIX regex matching
       regexec() is used to match  a  null-terminated  string  against  the  precompiled  pattern
       buffer, preg.  nmatch and pmatch are used to provide information regarding the location of
       any matches.  eflags is the bitwise-or of zero or more of the following flags:

              The match-beginning-of-line operator always fails to match (but see the compilation
              flag REG_NEWLINE above).  This flag may be used when different portions of a string
              are passed to regexec() and the beginning of the string should not  be  interpreted
              as the beginning of the line.

              The  match-end-of-line operator always fails to match (but see the compilation flag
              REG_NEWLINE above).

              Use pmatch[0] on the input string, starting  at  byte  pmatch[0].rm_so  and  ending
              before  byte pmatch[0].rm_eo.  This allows matching embedded NUL bytes and avoids a
              strlen(3) on large strings.  It does not use nmatch on input, and does  not  change
              REG_NOTBOL or REG_NEWLINE processing.  This flag is a BSD extension, not present in

   Byte offsets
       Unless REG_NOSUB was set for the compilation of the pattern  buffer,  it  is  possible  to
       obtain  match  addressing information.  pmatch must be dimensioned to have at least nmatch
       elements.  These are filled in by regexec() with substring match addresses.   The  offsets
       of  the  subexpression  starting at the ith open parenthesis are stored in pmatch[i].  The
       entire regular expression's match addresses are stored in pmatch[0].  (Note that to return
       the  offsets  of  N  subexpression  matches,  nmatch  must  be  at least N+1.)  Any unused
       structure elements will contain the value -1.

       The regmatch_t structure which is the type of pmatch is defined in <regex.h>.

           typedef struct {
               regoff_t rm_so;
               regoff_t rm_eo;
           } regmatch_t;

       Each rm_so element that is not -1 indicates the start offset of the next largest substring
       match  within  the  string.   The  relative  rm_eo element indicates the end offset of the
       match, which is the offset of the first character after the matching text.

   POSIX error reporting
       regerror() is used to turn the error codes that can be  returned  by  both  regcomp()  and
       regexec() into error message strings.

       regerror()  is  passed  the  error code, errcode, the pattern buffer, preg, a pointer to a
       character string buffer, errbuf, and the size  of  the  string  buffer,  errbuf_size.   It
       returns  the  size  of  the  errbuf  required to contain the null-terminated error message
       string.  If both errbuf and errbuf_size are nonzero, errbuf is filled in  with  the  first
       errbuf_size - 1 characters of the error message and a terminating null byte ('\0').

   POSIX pattern buffer freeing
       Supplying regfree() with a precompiled pattern buffer, preg will free the memory allocated
       to the pattern buffer by the compiling process, regcomp().


       regcomp() returns zero for a successful compilation or an error code for failure.

       regexec() returns zero for a successful match or REG_NOMATCH for failure.


       The following errors can be returned by regcomp():

              Invalid use of back reference operator.

              Invalid use of pattern operators such as group or list.

              Invalid use of repetition operators such as using '*' as the first character.

              Un-matched brace interval operators.

              Un-matched bracket list operators.

              Invalid collating element.

              Unknown character class name.

              Nonspecific error.  This is not defined by POSIX.2.

              Trailing backslash.

              Un-matched parenthesis group operators.

              Invalid use of the range operator; for example,  the  ending  point  of  the  range
              occurs prior to the starting point.

              Compiled  regular  expression requires a pattern buffer larger than 64 kB.  This is
              not defined by POSIX.2.

              The regex routines ran out of memory.

              Invalid back reference to a subexpression.


       For an explanation of the terms used in this section, see attributes(7).

       │InterfaceAttributeValue          │
       │regcomp(), regexec() │ Thread safety │ MT-Safe locale │
       │regerror()           │ Thread safety │ MT-Safe env    │
       │regfree()            │ Thread safety │ MT-Safe        │


       POSIX.1-2001, POSIX.1-2008.


       #include <stdint.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <regex.h>

       #define ARRAY_SIZE(arr) (sizeof((arr)) / sizeof((arr)[0]))

       static const char *const str =
               "1) John Driverhacker;\n2) John Doe;\n3) John Foo;\n";
       static const char *const re = "John.*o";

       int main(void)
           static const char *s = str;
           regex_t     regex;
           regmatch_t  pmatch[1];
           regoff_t    off, len;

           if (regcomp(&regex, re, REG_NEWLINE))

           printf("String = \"%s\"\n", str);

           for (int i = 0; ; i++) {
               if (regexec(&regex, s, ARRAY_SIZE(pmatch), pmatch, 0))

               off = pmatch[0].rm_so + (s - str);
               len = pmatch[0].rm_eo - pmatch[0].rm_so;
               printf("#%d:\n", i);
               printf("offset = %jd; length = %jd\n", (intmax_t) off,
                       (intmax_t) len);
               printf("substring = \"%.*s\"\n", len, s + pmatch[0].rm_so);

               s += pmatch[0].rm_eo;



       grep(1), regex(7)

       The glibc manual section, Regular Expressions


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