# matches regardless of upper- or lowercase

search_str = input('grep: ').lower()    # make this lowercase
filename   = input('in  : ')

textfile   = open(filename, 'r')
line_num = 0

for line in textfile:
    line_str  = line.strip()
    line_num += 1

    # search for string in lowercase version of line
    if line_str.lower().find(search_str) >= 0:
        # but print the original version of line
        print(line_num, line_str)

textfile.close()