Pandas: replace substring in string

Use replace with dict for replacing and regex=True:

df['url'] = df['url'].replace({'icashier.alipay.com': 'aliexpress.com'}, regex=True)
print (df)
                                          url
0  aliexpress.com/catalog/2758186/detail.aspx
1  aliexpress.com/catalog/2758186/detail.aspx
2  aliexpress.com/catalog/2758186/detail.aspx
3                                      vk.com

use str.replace to replace a substring, replace looks for exact matches unless you pass a regex pattern and param regex=True:

In [25]:
df['url'] = df['url'].str.replace('icashier.alipay.com', 'aliexpress.com')
df['url']

Out[25]:
0    aliexpress.com/catalog/2758186/detail.aspx
1    aliexpress.com/catalog/2758186/detail.aspx
2    aliexpress.com/catalog/2758186/detail.aspx
3                                        vk.com
Name: url, dtype: object

In case someone (like me) needs to replace substring in whole DataFrame:

df = df.apply(lambda col: col.str.replace('icash...', 'aliex...'))

or just in defined columns (and all others remain unchanged):

cols = ['a', 'c'] # list of all columns with value to replace
df = df.apply(lambda col: col.str.replace('icash...', 'aliex...') if col.name in cols else col)