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REGEX(3) Linux Programmer's Manual REGEX(3)


     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 zero 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.
            Do  not report position of matches.  The nmatch and pmatch argu-
            ments 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  immedi-
            ately  before  a  newline, regardless of whether eflags contains
 POSIX regex matching
     regexec() is used to match a null-terminated string against the precom-
     piled  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  subex-
     pression  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
         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() returns zero for a successful compilation or  an  error  code
     for failure.
     regexec()  returns zero for a successful match or REG_NOMATCH for fail-


     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
     allbox; lbw20 lb lb l l  l.   Interface Attribute Value  T{  regcomp(),
     regexec()  T}   Thread safety  MT-Safe locale T{ regerror() T}   Thread
     safety  MT-Safe env T{ regfree() T}   Thread safety  MT-Safe


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


     grep(1), regex(7)
     The glibc manual section, Regular Expressions


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     description  of  the project, information about reporting bugs, and the
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GNU 2017-09-15 REGEX(3)

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