If your float is always expressed in decimal notation something like
>>> import re
>>> re.findall("\d+\.\d+", "Current Level: 13.4 db.")
['13.4']
may suffice.
A more robust version would be:
>>> re.findall(r"[-+]?\d*\.\d+|\d+", "Current Level: -13.2 db or 14.2 or 3")
['-13.2', '14.2', '3']
If you want to validate user input, you could alternatively also check for a float by stepping to it directly:
user_input = "Current Level: 1e100 db"
for token in user_input.split():
try:
# if this succeeds, you have your (first) float
print float(token), "is a float"
except ValueError:
print token, "is something else"
# => Would print ...
#
# Current is something else
# Level: is something else
# 1e+100 is a float
# db is something else
