TBH I would say that Regex is overkill there. And your pattern match can be improved:
def somefun(<<num>> <> rest) when num in ?0..?9, do: call_bar(rest)
def somefun(""), do: bar()
def somefun(bin) when is_binary(bin), do: foo()
The problem with regular expressions the use backtracking engine (and PCRE is such engine) is that it can explode to O(n^2) with some expressions (namely nested wildcard matches). And such cases have been spotted on the wild with pretty simple expressions.
