#!/bin/bash
echo '{"base":"EUR","date":"2016-03-01","rates":{"USD":1.0872}}' | grep -oP '(?<="USD":)\d+(?:\.\d+)?'
echo '{"base":"EUR","date":"2016-03-01","rates":{"USD":.0872}}' | grep -oP '(?<="USD":)\d*\.?\d+'
echo '{"base":"EUR","date":"2016-03-01","rates":{"USD":-.0872}}' | grep -oP '(?<="USD":)-?\d*\.?\d+'