Replace 'a' with "xyz"

/*
    Given a char array with words in it, find all 'a' characters and
    replace with some string, say "xyz". Modify the input array, do
    not create a copy of the array.

    Date: 110515, Sun 15 May 2011 05:19:16 PM IST
    Link: http://stackoverflow.com/questions/5931945/
*/

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

int main() {
    char arr[1024];
    char search='a', *replace="xyz";

    fgets(arr,1023/strlen(replace),stdin);
    if (arr[strlen(arr)-1]=='\n')
        arr[strlen(arr)-1]='\0';
    puts(arr);

    int i, j, k, offset, target;
    for (i=offset=0; i<strlen(arr); ++i)
        if (arr[i]==search)
            offset+=strlen(replace)-1;

    target = strlen(arr)+offset;
    for (i=strlen(arr); i>=0; --i) {
        if (arr[i]!=search)
            arr[target--] = arr[i];
        else {
            target -= strlen(replace);
            strncpy(&arr[target+1],replace,strlen(replace));
        }
    }

    puts(arr);

    return 0;
}