Select data using a regular expression

I have a dataframe like this

import pandas as pd

df = pd.DataFrame({'a': ['abc', 'r00001', 'r00010', 'rfoo', 'r01234', 'r1234'], 'b': range(6)})

        a  b
0     abc  0
1  r00001  1
2  r00010  2
3    rfoo  3
4  r01234  4
5   r1234  5

I now want to select all columns of this dataframe where the entries in column a start with r followed by five numbers.

From here I learned how one would do this if it started just with r without the numbers:

print df.loc[df['a'].str.startswith('r'), :]

        a  b
1  r00001  1
2  r00010  2
3    rfoo  3
4  r01234  4
5   r1234  5

Something like this

print df.loc[df['a'].str.startswith(r'[r]\d{5}'), :]

does of course not work. How would one do this properly?