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       regcomp, regexec, regerror, regfree - POSIX regex functions


       #include <sys/types.h>
       #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 may be 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 may be the bitwise-or of one or both  of  REG_NOTBOL  and  REG_NOTEOL
       which cause changes in matching behavior described below.

              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).

   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.


       grep(1), regex(7)

       The glibc manual section, Regular Expressions


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