sscanf

sscanf.c

#include <stdio.h>

int main (int argc, char **argv) {
  char str[] = "image%06d.png", buf[100];
  int num;

  sscanf (str,"%s", buf);
  printf ("%s\n", buf);
  
  return 0;
}

/*
juzna@juzna-pc:/tmp$ gcc tst.c && ./a.out 
image%06d.png
*/