import re

# detect 'IMG' in upper/lower case allowing for
# zero or more spaces between the < and the 'I'
img = '< *[iI][mM][gG] '

# allow any character up to the 'ALT' or 'alt' before >
alt = img + '.*[aA][lL][tT].*>'

# open file and read it into list
filename = raw_input('Enter a filename to search ')
inf = open(filename,'r')
lines = inf.readlines()

# if the line has an IMG tag and no ALT inside
# add our message as an HTML comment
for i in range(len(lines)):
  if re.search(img,lines[i]) and not \
     re.search(alt,lines[i]):
        lines[i] += '<!-- PROVIDE ALT TAGS ON IMAGES! -->\n'

# Now write the altered file and tidy up.
inf.close()
outf = open(filename,'w')
outf.writelines(lines)
outf.close()