Extract an IP address from a string.

extract_ip_address.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// http://stackoverflow.com/a/792645
static
int isValidIp4 (char *str) {
    int segs = 0;   /* Segment count. */
    int chcnt = 0;  /* Character count within segment. */
    int accum = 0;  /* Accumulator for segment. */

    /* Catch NULL pointer. */

    if (str == NULL)
        return 0;

    /* Process every character in string. */

    while (*str != '\0') {
        /* Segment changeover. */

        if (*str == '.') {
            /* Must have some digits in segment. */

            if (chcnt == 0)
                return 0;

            /* Limit number of segments. */

            if (++segs == 4)
                return 0;

            /* Reset segment values and restart loop. */

            chcnt = accum = 0;
            str++;
            continue;
        }
 

        /* Check numeric. */

        if ((*str < '0') || (*str > '9'))
            break;

        /* Accumulate and check segment. */

        if ((accum = accum * 10 + *str - '0') > 255)
            return 0;

        /* Advance other segment specific stuff and continue loop. */

        chcnt++;
        str++;
    }

    /* Check enough segments and enough characters in last segment. */

    if (segs != 3)
        return 0;

    if (chcnt == 0)
        return 0;

    /* Address okay. */

    return 1;
}

static
char * find_end_of_ip(char *ip){

	char *p = ip;

	while (*p != '\0') {
		//printf("p[%c]\n",*p);
		if (isdigit(*p)){
			p++;
		}
		else if (*p == '.'){
			p++;
		}
		else 
			return p;
	}

	return p;
}

static
char * extract_ip(char *ip){
	char *d = strdup(ip);

	////printf("dup[%s]\n", d);
	char *end = find_end_of_ip(d);
	*end = '\0';
	return d;
}

int main(){
	char *lines[] = {
		"this has the address 1.2.3.4 in it",
		"I connected from 22.44.55.66 today",
		"test 1.2.3.4",
		"1.2.3.4",
		0
	};

	char **line = lines;
	char  *p;

	while( *line ){
		printf("Testing [%s]\n", *line );
		p = *line;

		while( *p ){
			//printf("input [%s]\n", p );
			if (isValidIp4( p )){
				printf("VALID!!\n");

				char *ipaddress = extract_ip(p);

				printf("%s\n", ipaddress);
				free(ipaddress);
				break;
			}

			p++;
		}

		line++;
	}
}