import re
subject = 'I have a _cat, here is a pic {cat_pic}. Another_pic {cat_figure}'
regex = re.compile(r'{[^}]*}|(_)')
def myreplacement(m):
    if m.group(1):
        return ""
    else:
        return m.group(0)
replaced = regex.sub(myreplacement, subject)
print(replaced)