Simplified strong/emphasis regexps

(?<=\S|^) becomes (?<!\s)
(?=\S|$)  becomes (?!\s)
(?<!\s)(?<!\*) becomes (?<![\s*])
(?<!\s)(?<!_)  becomes (?<![\s_])
(?!\s)(?![\.,:;]\s) becomes (?![\.,:;]?\s)