This can't really work 100% without exhaustively giving every string accepted by the regex, or giving every string not accepted by the regex. Regular expressions are essentially a shorthand for doing that.
Since it's impossible to do accurately, it has to use some "common sense" via genetic programming or whatever. In your example, it probably sees that people rarely want [1-8]{1,3}, but that it's a subset of \d+ and goes with that instead.
Since it's impossible to do accurately, it has to use some "common sense" via genetic programming or whatever. In your example, it probably sees that people rarely want [1-8]{1,3}, but that it's a subset of \d+ and goes with that instead.