"""
 palindrome.py
 A recursive function to recognize a palindrome.
 Jim M | GPL | Nov 2011
"""

def clean_string(string):
    """Return string as lowercase, keeping only letters.
       >>> clean_string("The cat.")
       'thecat'
    """
    result = ""
    for char in string.lower():
        if char in 'abcdefghijklmnopqrstuvwxyz':
            result = result + char
    return result

def palindrome(string):
    """Retrun true if string is a palindrome.
       >>> palindrome('cat')
       False
       >>> palindrome('mom')
       True
       >>> palindrome('')
       True
    """
    if len(string) < 2:
        return True
    if string[0] != string[-1]: 
        return False
    else:
        return palindrome(string[1:-1])

def main():
    while True:
        phrase = raw_input('What is the phrase? (nothing to quit) ')
        if not phrase: 
            print "... bye"
            return
        phrase = clean_string(phrase)
        print "Your string cleaned up is '%s'." % phrase
        if palindrome(phrase):
            print "Yup, it's a palindrome."
        else:
            print "Nope, it isn't a palindrome."

if __name__ == "__main__":
    import doctest
    doctest.testmod()
    main()