$ cat between.c
#include <stdio.h>

int main(void)
{
        const char *start="START", *end="END";

        while(!feof(stdin))
        {
                int c, spos=0;

                while((c=fgetc(stdin)) >= 0)
                {
                        // If it doesn't match, ignore it and start over.
                        if(c != start[spos])
                        {
                                spos=0;
                                // if it doesn't match start[n], it might
                                // match start[0], so check again.
                                if(c != start[0])
                                        continue;
                        }

                        // Increment position in the START string
                        // and check if we found a complete START.
                        if(start[++spos] == '\0') // complete START
                                break;
                }

                if(c<0) break; // No START, just EOF

                spos=0;
                // Search for END
                while((c=fgetc(stdin)) >= 0)
                {
                        if(c != end[spos])
                        {
                                int n;
                                // This time, we have to print everything
                                // we can prove didn't match.
                                fwrite(end, spos, 1, stdout);
                                spos=0;
                                if(c != end[0])
                                {
                                        fputc(c, stdout);
                                        continue;
                                }
                        }

                        if(end[++spos] == '\0') // complete END
                        {
                                // print a newline after each END is found
                                fputc('\n', stdout);
                                break;
                        }
                }
        }

        return(0);
}
$ gcc between.c
$ echo "STSTART A END B START C ENEND D" | ./a.out
 A
 C EN
$