You should not sprintf() that string as a pattern: you should just use it directly. sprintf() would convert the \n to a newline character, but regexp knows that \n means newline.
It is not clear what your question is? You are matching an exact string in the file. If it is found at all, the return of the 'match' would be an exact copy of the string, so there does not appear to be a purpose in doing the regexp ?
Are you trying to locate the index of the first occurrence of that string in the file? If so you could just strfind() [though in that case you would want the sprintf() version of it.]
