Number pattern does not match decimals without a leading numeric value.

[erichiggins]

The current number pattern used by Backbone.Validation is /^-?(?:\d+|\d{1,3}(?:,\d{3})+)(?:\.\d+)?$/.
Here are some tests which prove that it does not work as expected.

> var pattern = /^-?(?:\d+|\d{1,3}(?:,\d{3})+)(?:\.\d+)?$/;
> "0.5".match(pattern);
["0.5"]

> ".5".match(pattern);
null

JavaScript treats these the same, however:

> Number("0.5");
0.5
> Number(".5");
.5

When binding to a View, such as a form, it's common for users to enter decimals without leading zeros. This current pattern does not sufficiently accomodate that, and puts the burden on developers to handle it.