import re
log = '2016-10-13 17:04:50 - info - uri:"GET x/y/z" ip:1.1.1.1 Rs:{"data": "blah blah"}'
text = re.compile(r"Rs:({[^}]*})").search(log)
if text:
print(text.group(1))
print(re.findall(r"Rs:({[^}]*})", log))
aW1wb3J0IHJlCmxvZyA9ICcyMDE2LTEwLTEzIDE3OjA0OjUwIC0gaW5mbyAtIHVyaToiR0VUIHgveS96IiBpcDoxLjEuMS4xIFJzOnsiZGF0YSI6ICJibGFoIGJsYWgifScKdGV4dCA9IHJlLmNvbXBpbGUociJSczooe1tefV0qfSkiKS5zZWFyY2gobG9nKQppZiB0ZXh0OgoJcHJpbnQodGV4dC5ncm91cCgxKSkKcHJpbnQocmUuZmluZGFsbChyIlJzOih7W159XSp9KSIsIGxvZykp