Yes: "X."
Yes: "I shall test this."
Yes: "It worked!?"
Yes: "It worked?!"
Yes: "In this problem you are to write four methods."
Yes: "Is THIS a valid sentence?"
Yes: "That is valid!"
Yes: "A a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a."
Yes: "Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx."
Yes: "Xxxxxxx xxxxxxxxxxxxxxx Xxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx."
No: "In this problem you are to write four methods. Right!?"
No: "One. Two."
No: "It worked!!"
No: "x."
No: "'I shall test this.'"
No: "Some extra blanks are  here."
No: "Some words. after the period"
No: "The sentence must end with a period, question mark, exclamation mark, or one of the two strings \"!?\" and \"?!\"."