Effective way to find `hashtags` in raw text in Go

main.go

func ParseHashtags(input string) string {
	const minHashtagLength = 3

	var foundHashtags []string
searchHashtag:
	for {
		hashtagIndex := strings.Index(input, "#")
		if hashtagIndex < 0 {
			break // no hashtags found
		}

		// if string is not starts from #, we should trim allowed chars for prefix
		if hashtagIndex > 0 {
			lastSpace := max(0, strings.LastIndexFunc(input[:hashtagIndex], unicode.IsSpace))

			for _, r := range input[lastSpace:hashtagIndex] {
				if unicode.IsLetter(r) || unicode.IsDigit(r) {
					input = input[hashtagIndex+1:] // found hashtag is invalid

					continue searchHashtag
				}
			}
		}

		// make the '#' as first character!
		input = input[hashtagIndex:]

		// squash sequence of '#' characters
		hashtagsCount := 0
		for _, r := range input {
			if r != '#' {
				break
			}
			hashtagsCount++
		}
		input = input[hashtagsCount-1:]

		// validate hashtag body (we can drop first byte because of it is '#')
		bodyEnds := 0
		for i, r := range input[1:] {
			// first char after '#' must be letter
			if i == 0 && !unicode.IsLetter(r) {
				input = input[1:]
				continue searchHashtag
			}

			if unicode.IsLetter(r) || unicode.IsDigit(r) {
				bodyEnds = i + utf8.RuneLen(r) // hashtag must ends on letter or digit
			} else if r != '_' && r != '.' && r != '-' {
				break
			}
		}

		if bodyEnds >= minHashtagLength {
			foundHashtags = append(foundHashtags, input[:bodyEnds+1])
		}
		input = input[bodyEnds+1:]
	}

	return strings.Join(foundHashtags, " ")
}