When our creative writing group researched the haiku, we found many articles on the Internet, most of which observed that the modern English haiku couldn't be forced into the Japanese 5-7-5 format. In Japanese, for example, a word can be split between lines to make the verse conform which would be very clumsy in English! In the Japanese form, the rule is not about syllables in our sense of the word. The rule is 5-7-5 moras, of which there may be more than one in a syllable. We found this quite informative, though it would be an advantage to know Japanese to understand it fully!