This uses a regular expression to strip all non-numeric character from
a string and then coerces the result to a number. This is useful for
strings that are numbers with extra formatting (e.g. $1,200.34).
Usage
extract_numeric(x)
Arguments
x
A character vector (or a factor).
Examples
extract_numeric("$1,200.34")
extract_numeric("-2%")
# The heuristic is not perfect - it won't fail for things that
# clearly aren't numbers
extract_numeric("-2-2")
extract_numeric("12abc34")
providing 
.