Rule 6 Each atom matches according to its type. If the atom doesn't match, or doesn't allow a match of the rest of the regular expression, the Engine backtracks to Rule 5 and tries the next choice for the atom's quantity. The order of characters in the list doesn't matter. However, if you want to match a caret, don't put it first. Another option is to backslash these characters.
Most characters loose their meta-ness inside square brackets.
Since each character in the brackets is interpreted individually, it is meaningless to specify alternation with the vertical bar. A vertical bar is simply a vertical bar in a character class. Note that paired parentheses are numbered by counting left parentheses from the left. Priority is given to backreferences in this case. Any other backslashed character matches itself, and so does any character not mentioned above.