Does a dot have to be escaped in a character class (square brackets) of a regular expression?

Yes that is true that DOT (and most other special characters) don't need to be escaped in character class.

There is no "standard" for regular expression syntax.

@BoltClock there are some: posix, posix extended, perl. See en.wikipedia.org/wiki/Regular_expression#Standards

Worth noting that modern regex lets you escape any symbol even if not required, so you can just escape stuff when unsure. Only really need to worry about brevity when a super-concise one-liner is your goal, or your expression has too many slashes that it hurts readability. Over time you'll naturally learn how to write more cleanly. And sometimes it actually makes things easier to read when you consistently escape things like ( or ? so that at a glance you can see "this is a literal" without thinking about the context of where that token is (esp. with any crazy nesting).

Suppose you were designing a regex engine and faced the question of whether a period within a character class must be escaped. The only reason for doing so would be to allow a non-escaped period in the character class to represent the meta-character ., which matches any character. But wait, what use is a character class that matches any character?