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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 compilation.

       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 buffer.

       cflags may be the bitwise-or of one or more of the following:

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

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

              Support  for  substring  addressing  of matches is not required.
              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 REG_NOTBOL.

              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 substring match addressing  information.   pmatch
       must be dimensioned to have at least nmatch elements.  These are filled
       in by regexec() with substring match addresses.  Any  unused  structure
       elements will contain the value -1.

       The  regmatch_t  structure  which  is  the type of pmatch is defined in

           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.

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

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


       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.

              Non specific error.  This is not defined by POSIX.2.

              Trailing backslash.

              Un-matched parenthesis group operators.

              Invalid use of the range operator, e.g., the ending point of the
              range occurs prior to the starting point.

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

              The regex routines ran out of memory.

              Invalid back reference to a subexpression.




       grep(1), regex(7), GNU regex manual


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