Pandas Extract Number from String

Give it a regex capture group:

df.A.str.extract('(\d+)')

Gives you:

0      1
1    NaN
2     10
3    100
4      0
Name: A, dtype: object

(\d+) is a regex capturing group, and \d+ specifies a regex pattern that matches only digits. Note that this will only work for whole numbers and not floats.