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POSIX API Compatibility Functions

#include <boost/cregex.hpp>
#include <boost/regex.h>

The following functions are available for users who need a POSIX compatible C library, they are available in both Unicode and narrow character versions, the standard POSIX API names are macros that expand to one version or the other depending upon whether UNICODE is defined or not.

Important: Note that all the symbols defined here are enclosed inside namespace boost when used in C++ programs, unless you use #include <boost/regex.h> instead - in which case the symbols are still defined in namespace boost, but are made available in the global namespace as well.

The functions are defined as:

extern "C" {
int regcompA(regex_tA*, const char*, int);
unsigned int regerrorA(int, const regex_tA*, char*, unsigned int);
int regexecA(const regex_tA*, const char*, unsigned int, regmatch_t*, int);
void regfreeA(regex_tA*);

int regcompW(regex_tW*, const wchar_t*, int);
unsigned int regerrorW(int, const regex_tW*, wchar_t*, unsigned int);
int regexecW(const regex_tW*, const wchar_t*, unsigned int, regmatch_t*, int);
void regfreeW(regex_tW*);

#ifdef UNICODE
#define regcomp regcompW
#define regerror regerrorW
#define regexec regexecW
#define regfree regfreeW
#define regex_t regex_tW
#define regcomp regcompA
#define regerror regerrorA
#define regexec regexecA
#define regfree regfreeA
#define regex_t regex_tA

All the functions operate on structure regex_t, which exposes two public members:

unsigned int re_nsub this is filled in by regcomp and indicates the number of sub-expressions contained in the regular expression.

const TCHAR* re_endp points to the end of the expression to compile when the flag REG_PEND is set.

Footnote: regex_t is actually a #define - it is either regex_tA or regex_tW depending upon whether UNICODE is defined or not, TCHAR is either char or wchar_t again depending upon the macro UNICODE.


regcomp takes a pointer to a regex_t, a pointer to the expression to compile and a flags parameter which can be a combination of:

  REG_EXTENDED Compiles modern regular expressions. Equivalent to regbase::char_classes | regbase::intervals | regbase::bk_refs.  
  REG_BASIC Compiles basic (obsolete) regular expression syntax. Equivalent to regbase::char_classes | regbase::intervals | regbase::limited_ops | regbase::bk_braces | regbase::bk_parens | regbase::bk_refs.  
  REG_NOSPEC All characters are ordinary, the expression is a literal string.  
  REG_ICASE Compiles for matching that ignores character case.  
  REG_NOSUB Has no effect in this library.  
  REG_NEWLINE When this flag is set a dot does not match the newline character.  
  REG_PEND When this flag is set the re_endp parameter of the regex_t structure must point to the end of the regular expression to compile.  
  REG_NOCOLLATE When this flag is set then locale dependent collation for character ranges is turned off.  
, , ,
When this flag is set, then escape sequences are permitted in bracket expressions (character sets).  
  REG_NEWLINE_ALT  When this flag is set then the newline character is equivalent to the alternation operator |.  
  REG_PERL  Compiles Perl like regular expressions.  
  REG_AWK A shortcut for awk-like behavior: REG_EXTENDED | REG_ESCAPE_IN_LISTS  
  REG_GREP A shortcut for grep like behavior: REG_BASIC | REG_NEWLINE_ALT  
  REG_EGREP  A shortcut for egrep like behavior: REG_EXTENDED | REG_NEWLINE_ALT  


regerror takes the following parameters, it maps an error code to a human readable string:

  int code The error code.  
  const regex_t* e The regular expression (can be null).  
  char* buf The buffer to fill in with the error message.  
  unsigned int buf_size The length of buf.  

If the error code is OR'ed with REG_ITOA then the message that results is the printable name of the code rather than a message, for example "REG_BADPAT". If the code is REG_ATIO then e must not be null and e->re_pend must point to the printable name of an error code, the return value is then the value of the error code. For any other value of code, the return value is the number of characters in the error message, if the return value is greater than or equal to buf_size then regerror will have to be called again with a larger buffer.


regexec finds the first occurrence of expression e within string buf. If len is non-zero then *m is filled in with what matched the regular expression, m[0] contains what matched the whole string, m[1] the first sub-expression etc, see regmatch_t in the header file declaration for more details. The eflags parameter can be a combination of:

  REG_NOTBOL Parameter buf does not represent the start of a line.  
  REG_NOTEOL Parameter buf does not terminate at the end of a line.  
  REG_STARTEND The string searched starts at buf + pmatch[0].rm_so and ends at buf + pmatch[0].rm_eo.  


Finally regfree frees all the memory that was allocated by regcomp.

Footnote: this is an abridged reference to the POSIX API functions, it is provided for compatibility with other libraries, rather than an API to be used in new code (unless you need access from a language other than C++). This version of these functions should also happily coexist with other versions, as the names used are macros that expand to the actual function names.

Revised 24 Oct 2003

Copyright John Maddock 1998- 2003

Use, modification and distribution are subject to the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at