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Wildcard Pattern Matching In C

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# 8  
Old 07-03-2016
You can "statically" pre-compile to a text file: use the regcomp command - not a C call. It creates the compiled buffer you need in a file. I typically use maybe a dozen of ptr-compiles in a simple app for validating input. You put them in an include file.

And Rudic is correct - you can compile once and then run the compiled buffer multiple times internally -- as well as pre-compile.
This User Gave Thanks to jim mcnamara For This Post:
Azrael
# 9  
Old 07-03-2016
Well, after no luck with fnmatch() and other pattern matching functions I found online I decided to give regex a try:

Code:
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <time.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <regex.h>

int main() {

   char newhold[40];

        regex_t re;
        time_t current = time(NULL);
        char day[10];
        char mon[10];
        int retval = 0;        

        strftime(day, sizeof(day), "%d", localtime(&current));
        strncat( newhold, day, 10 );
        strncat( newhold, "?", 2 );
        strftime(mon, sizeof(mon), "%b", localtime(&current));
        strncat( newhold, mon, 10 );
        strncat( newhold, "*", 2 );
        strncat( newhold, "pattern", 10 );

     if(regcomp(&re , newhold, REG_EXTENDED) != 0 ){
         return;
     }

    FILE *fp;
    fp = fopen("/var/log/apache2/other_vhosts_access.log", "r");
    char line[100];
    unsigned int i = 0;

    while(fgets(line, sizeof(line), fp)) {
        if ((retval = regexec(&re, line, 0, NULL, 0)) == 0){
        i++;
      }
    }
  printf("%s %d\n", newhold, i );
  fclose(fp);

  return 0;
}

Obviously this doesn't work. I know in the following part that "newhold" would normal have a constant defined instead of a char array. I could do that with the "pattern" section of this regex and with the "?" and "*" wildcards. However, the variables "day" and "mon" are going to be checked by the system every time the code runs. So a constant wouldn't work in this case.

Perhaps I'm going wrong in other aspects as well, but that's the biggest problem I see at the moment. Anyone know any tricks to pass variables into regex for this? I tried searching that online as well with no success. Maybe my Google-fu is just lacking?

Also I'm not finding much creating a file from the regcomp command either. I'd love to see that if anyone can provide an example.
Last edited by Azrael; 07-03-2016 at 05:05 AM..
# 10  
Old 07-03-2016
What is in /var/log/apache2/other_vhosts_access.log?

Please explain in English what you are hoping the extended regular expression you have created in newhold[] will match.

Which of the lines in /var/log/apache2/other_vhosts_access.log do you hope will be matched by your ERE?
# 11  
Old 07-03-2016
Take the contents of the string variable (newhold) you constructed, print it, and try it as a pattern for grep as a console command. You also need to check the return code from regexec in case something else biffed. It should be: REG_NOMATCH or zero. Anything else is a fatal error. regerror() is your friend.
This User Gave Thanks to jim mcnamara For This Post:
Azrael
# 12  
Old 07-03-2016
Sorry, I thought I had provided the contents of the file earlier. Here it is again with just the following for testing:

Code:
$ cat /var/log/apache2/other_vhosts_access.log
03/Jul blah pattern

I did not know regex in C and grep/egrep were so closely related. I did as Jim suggested and saw nothing was matched:

Code:
$ egrep "03?Jul*pattern" /var/log/apache2/other_vhosts_access.log | wc -l
0

I changed this to the following and it was matched:

Code:
egrep "03.Jul.*pattern" /var/log/apache2/other_vhosts_access.log | wc -l
1

Still not getting a match when I try applying this to C though:
Code:
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <time.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <regex.h>

int main() {

   char newhold[40];

        regex_t re;
        time_t current = time(NULL);
        char day[10];
        char mon[10];
        int retval = 0;

        strftime(day, sizeof(day), "%d", localtime(&current));
        strncat( newhold, day, 10 );
        strncat( newhold, ".", 2 );
        strftime(mon, sizeof(mon), "%b", localtime(&current));
        strncat( newhold, mon, 10 );
        strncat( newhold, ".*", 4 );
        strncat( newhold, "pattern", 10 );

     if(regcomp(&re , newhold, REG_NOMATCH) != 0 ){
         return 1;
     }

    FILE *fp;
    fp = fopen("/var/log/apache2/other_vhosts_access.log", "r");
    char line[100];
    unsigned int i = 0;

    while(fgets(line, sizeof(line), fp)) {
        if ((retval = regexec(&re, line, 0, NULL, 0)) == 0){
        i++;
      }
    }
  printf("%s %d\n", newhold, i );
  fclose(fp);
  return 0;
}

I also tried working with regerror(), but had trouble applying the few examples I was able to find.
# 13  
Old 07-03-2016
You're very close... In the line:
Code:
     if(regcomp(&re , newhold, REG_NOMATCH) != 0 ){

REG_NOMATCH is a defined to be a return code for regexec() indicating that it did not find a match; it is not defined to be a flag to be passed to regcomp(). If you change the above line in your code to:
Code:
     if(regcomp(&re , newhold, 0) != 0 ){

and rebuild your code, running it produces the output:
Code:
03.Jul.*pattern 1

I would, however, suggest changing:
Code:
        strftime(day, sizeof(day), "%d", localtime(&current));
        strncat( newhold, day, 10 );
        strncat( newhold, ".", 2 );
        strftime(mon, sizeof(mon), "%b", localtime(&current));
        strncat( newhold, mon, 10 );
        strncat( newhold, ".*", 4 );
        strncat( newhold, "pattern", 10 );

to:
Code:
	strftime(newhold, sizeof(newhold), "%d/%b.*pattern",
	    localtime(&current));

which gets rid of several chances to overflow the size of newhold[] and chances to unnecessarily truncate text being added in intermediate strncat() calls. If you do this and rebuild your code again, it will produce the output:
Code:
03/Jul.*pattern 1

And, then, of course, you can also get rid of the day[] and mon[] arrays.
Last edited by Don Cragun; 07-03-2016 at 07:38 PM.. Reason: Fix typo: s/get rids/gets rid/
This User Gave Thanks to Don Cragun For This Post:
Azrael
# 14  
Old 07-03-2016
Thank you Don Cragun! That does work correctly and is a lot cleaner!

I'm still interested in statically pre-compiling the regex to a text file that jim mcnamara mentioned. If anyone has any examples of that let me know.
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