Python, string,

def reverse_letters(input_str: str) -> str:
    Reverses letters in a given string without adjusting the position of the words
    >>> reverse_letters('The cat in the hat')
    'ehT tac ni eht tah'
    >>> reverse_letters('The quick brown fox jumped over the lazy dog.')
    'ehT kciuq nworb xof depmuj revo eht yzal .god'
    >>> reverse_letters('Is this true?')
    'sI siht ?eurt'
    >>> reverse_letters("I   love       Python")
    'I evol nohtyP'
    return " ".join([word[::-1] for word in input_str.split()])

if __name__ == "__main__":
    import doctest