Biome

 Elements
The  element specifies a biome or group of biomes that a distribution can spawn in. Multiple biomes can be listed for a given distribution - each one adds to the list of valid biomes.

 elements have two attributes:

Using Regular Expressions
The regular expression capabilities are quite powerful. For example: the following two pieces of code accomplish exactly the same thing, but the version using regex strings is simpler, easier to read, and could potentially be compatible with mod-added taiga/ice/frozen biomes.

An even more useful application of the regular expression matching is this code that matches all biomes:

Weights
All biomes that match the name string are given the same weight. If a biome matches more than  element then the weights are added together. Negative weights can be used to reduce the probability for a biome.

For example, this code will match all biomes except the nether. All biomes match the first  and thus have a weight of +1, but the Nether also matches the second  and thus its total weight is 0:

The total weight for each biome (the sum of the weights from all matching  elements) is the probability that an attempt to place a distribution in that biome will succeed. A total weight of zero (or negative) means that attempts will always fail. A total weight of 1.0 (or anything greater than 1.0) means that attempts will always succeed. The number of "attempts" is determined by the base distribution frequency.  elements can make a distribution 'less' common in a biome, but not 'more' common.

List of Biomes
The following are valid vanilla biome names, as of Minecraft 1.4.5: