In Regex, . refers to any character, be it a number, an aplhabet character, or any other special character. * means zero or more times.
In regex in general, ^ is negation only at the beginning of a character class. Unless CMake is doing something really funky (to the point where calling their pattern matching language "regex" could be regarded as misleading or incorrect) I'm guessing the fact that it worked for you was an isolated accident.
regex - How to match "any character" in regular expression? - Stack ...
In and other regex flavors, outside a character class ([...]), + acts as a quantifier meaning "one or more, but as many as possible, occurrences of the quantified pattern*.
By putting ^ at the beginning of your regex and $ at the end, you ensure that no other characters are allowed before or after your regex. For example, the regex [0-9] matches the strings "9" as well as "A9B", but the regex ^[0-9]$ only matches "9".
regex - What does ?= mean in a regular expression? - Stack Overflow
regex - Regular Expressions: Is there an AND operator? - Stack Overflow
For reference, from regular-expressions.info/dot.html: "JavaScript and VBScript do not have an option to make the dot match line break characters. In those languages, you can use a character class such as [\s\S] to match any character. This character matches a character that is either a whitespace character (including line break characters), or a character that is not a whitespace character ...