regex example

10 Mar 2012

/*
 * File: regex.c
 * 
 * This is a sample regex usage, gets three arguments. It is a grep
 * like tool.  
 * -f <FILE> -> A file to be read if omited or - stdin is read 
 * -p <PATTERN> -> The pattern to be matched agains every line on FILE 
 * -v -> Invert the match, as like grep -v.
 *
 * Compiling: gcc -o preg regex.c 
 */
 

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <regex.h>
#include <unistd.h>
#include <getopt.h>



int main(int argc, char **argv)
{
        regex_t reg;
        char pattern[256];
        int status;
        char buf[256];
        FILE *fp = NULL;
        int opt;
        int inverted = 0;
        
        while ((opt = getopt(argc, argv, "vp:f:")) != -1) {
                switch (opt) {
                case 'v':
                        inverted = 1;
                        break;
                case 'p':
                        strncpy(pattern, optarg, 256);
                        break;
                case 'f':
                        if (optarg[0] == '-') {
                                fp = stdin;
                        } else {
                                fp = fopen(optarg, "r");
                                if (!fp) {
                                        perror("fopen");
                                        exit(EXIT_FAILURE);
                                }
                        }
                        break;
                }
                /* printf("%c\n", opt); */
        }

        if (!fp)
                fp = stdin;

        status = regcomp(&reg, pattern, REG_EXTENDED | REG_NOSUB);
        if (status != 0)
        {
                fprintf(stderr, "Compiling the regular expression \"%s\" failed.\n", pattern);
                exit(EXIT_FAILURE);
        }


        while (fgets(buf, 256, fp)) {
                status = regexec(&reg, buf,
                                 /* nmatch = */ 0,
                                 /* pmatch = */ NULL,
                                 /* eflags = */ 0);

                if (inverted ? status : !status) {
                        printf("%s", buf);
                }
        }

        return 0;
}