@Ezio: Comparison of the behaviour of \letter inside/outside character classes is irrelevant. The rules for inside can be expressed simply as:

1. Letters dDsSwW are special; they represent categories as documented, and do in fact have a similar meaning outside character classes.

2. Otherwise normal Python rules for backslash escapes in string literals should be followed. This means automatically that \a -> \x07, \A -> A, \b -> backspace, \B -> B, \z -> z and \Z -> Z.

@Georg: No need to read the source, just read my initial posting: It's compiled as a zero-length matcher ("at") inside a character class ("in") i.e. a nonsense, then at runtime the illegality is deliberately ignored.