Notational Compatibility with Other Regex Flavors
The ampersand has no special meaning in character classes in any other regular expression flavors discussed in this tutorial. The ampersand is simply a literal, and repeating it just adds needless duplicates. All these flavors treat[1234&&3456] as identical to [&123456].
Strictly speaking, this means that the character class intersection syntax is incompatible with the majority of other regex flavors. But in practice there's no difference, because there is no point in using two ampersands in a character class when you just want to add a literal ampersand. A single ampersand is still treated as a literal by Java and Ruby.