Replace start or end of string in values of pandas column with regex

Question

With sed, I can do

$ sed 's/^/prefix/' <<EOF
> foo
> EOF
prefixfoo

When I use pandas.Series.str.replace, the regex does not work. The minimal working example

import pandas as pd

df = pd.DataFrame({"text": ["foo", "bar"]})
df.text.str.replace("^", "prefix", regex=True)

returns

0    foo
1    bar
Name: text, dtype: object

mozway · Accepted Answer · 2022-05-05 03:53:46Z

2

It works if you capture the ^:

df['text'].str.replace(r'(^)', 'prefix', regex=True)

Output:

0    prefixfoo
1    prefixbar
Name: text, dtype: object

That said the best method to prepend a string would be to concatenate:

'prefix'+df['text']

edited May 5, 2022 at 3:53

answered May 5, 2022 at 3:48

mozway

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3 Comments

Tim Biegeleisen Over a year ago

regex=True is the default behavior of str.replace and we usually don't write it explicitly.

mozway Over a year ago

@Tim which pandas version are you using? This will change to False in a future release and currently raises a warning if you don't explicitly pass True

mozway Over a year ago

"FutureWarning: The default value of regex will change from True to False in a future version. In addition, single character regular expressions will *not be treated as literal strings when regex=True.*"

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