RubyConf 2013 challenge

Came up with two:

`(\A\w|(?<=_)\w)`

`(^\D|(?<=_)\D)`

A couple questions: Is there a preferred method from a benchmarking standpoint?  Is there any other way to find the underscore within_a_variable?

Also, I can't remember the combinations offhand, but when I tested some combinations in Rspec vs. Rubular, it gave different results. On one occasion, Rspec failed, Rubular worked.