Sentence tokenization w/o relying on punctuations and capitalizations
The only thing I can think of is to use a statistical classifier based on words that typically start or end sentences. This will not necessarily work in your example (I think only a full grammatical analysis would be able to identify sentence boundaries in that case), but you might get some way towards your goal.
Simply build a list of words that typically come at the beginning of a sentence. Words like the or this will probably be quite high on that list; count how many times the word occurs in your training text, and how many of these times it is at the beginning of a sentence. Then do the same for the end -- here you should never get the, as it cannot end a sentence in any but the most contrived examples.
With these two lists, go through your text and work out if you have a word that is likely to end a sentence followed by one that is likely to start one; if yes, you have a candidate for a potential sentence boundary. In your example, this would be likely to start a sentence, and sentence would be likely to be the sentence-final word. Obviously it depends on your data whether it works or not. If you're feeling adventurous, use parts-of-speech tags instead of the actual words; then your lists will be much shorter, and it should probably still work just as well.
However, you might find that you also get phrase boundaries (as each sentence will start with a phrase, and the end of the last phrase of a sentence will also coincide with the end of the sentence). It is hard to predict whether it will work without actually trying it out, but it should be quick and easy to implement and is better than nothing.
Is there a possible approach for extracting sentences from paragraphs / sentence tokenization for paragraphs that doesn't have any punctuations and/or all lowercased? We have a specific need for being able to split paragraphs into sentences while expecting the worst case that paragraph inputted are improper.
Example:
this is a sentence this is a sentence this is a sentence this is a sentence this is a sentence
into
["this is a sentence", "this is a sentence", "this is a sentence", "this is a sentence", "this is a sentence"]
The sentence tokenizer that we have tried so far seems to rely on punctuations and true casing:
Using nltk.sent_tokenize
"This is a sentence. This is a sentence. This is a sentence"
into
['This is a sentence.', 'This is a sentence.', 'This is a sentence']