Given the string ‘foo\nbar’ (where \n is a linefeed) the regular expression 
‘foo[^ ]+’ matches the complete string. The regex is compiled with REG_EXTENDED 
and REG_NEWLINE flags.

The POSIX specification at 
http://www.opengroup.org/onlinepubs/009695399/functions/regcomp.html

Says for the REG_NEWLINE flag that

“A <newline> in string shall not be matched by a period outside a bracket 
expression or by any form of a non-matching list “

For older versions of glicb (glibc-2.1.3) the behaviour of regcomp is as the 
POSIX specification. For glibc-2.5 and from at least glibc-2.3.2 this is not 
the behaviour

The following code demonstrates the issue

#include <stdio.h>
#include <sys/types.h>
#include <regex.h>

int main(int argc, char **argv)
{
  char regex[] = "foo[^ ]+";
  char text[] = "foo\nbar";
  regex_t preg;
  regmatch_t pmatch[1];
  int flags = REG_EXTENDED | REG_NEWLINE;
  int i;

  printf("About to compile regexp '%s', with flags %d\n", regex, flags);

  if(!regcomp(&preg, regex, flags))
  {  
    printf("About to search string '%s'\n", text);

    if(!regexec(&preg, text, 1, pmatch, 0))
    {
      printf("Regex matched, match text is '");
      for(i = pmatch[0].rm_so; i < pmatch[0].rm_eo; i++)
      {
        printf("%c", text[i]); 
      }
      printf("'\n");
    }
    else
    {
      printf("Regex did not match\n");
    }
    
    regfree(&preg);
  }
  else
  {
    printf("Failed to compile regex\n");
  }

  return 0;  
}

On glib-2.3.2, glibc-2.3.6 or glibc-2.5 the program gives 

About to compile regexp 'foo[^ ]+', with flags 5
About to search string 'foo
bar'
Regex matched, match text is 'foo
bar'

On glibc-2.1.3 and other C libraries such as found on Solaris 9 the output is 

About to compile regexp 'foo[^ ]+', with flags 9
About to search string 'foo
bar'
Regex did not match

Which I believe is the expected POISX behaviour.