Using a variable within a regular expression in Pandas str.contains()

Question

I'm attempting to select rows from a dataframe using the pandas str.contains() function with a regular expression that contains a variable as shown below.

df = pd.DataFrame(["A test Case","Another Testing Case"], columns=list("A"))
variable = "test"
df[df["A"].str.contains(r'\b' + variable + '\b', regex=True, case=False)] #Returns nothing

While the above returns nothing, the following returns the appropriate row as expected

df[df["A"].str.contains(r'\btest\b', regex=True, case=False)] #Returns values as expected

Any help would be appreciated.

Perhaps your issue is that you are concatenating the raw strings to a standard string?? Maybe try fr'\b{variable}\b' — nicholishen
– nicholishen, Commented Dec 4, 2018 at 22:05

cs95 · Accepted Answer · 2018-12-04 22:05:57Z

23

Both word boundary characters must be inside raw strings. Why not use some sort of string formatting instead? String concatenation as a rule is generally discouraged.

df[df["A"].str.contains(fr'\b{variable}\b', regex=True, case=False)] 
# Or, 
# df[df["A"].str.contains(r'\b{}\b'.format(variable), regex=True, case=False)] 

             A
0  A test Case

answered Dec 4, 2018 at 22:05

cs95

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3 Comments

Erfan Over a year ago

How would you do this if you had the specify the amount of characters, since that happens with [0-9]{3}, for example if you want a pattern of three numbers. Was facing this problem just yet, so just used string concatenation which solved it, and f-string didnt work.

cs95 Over a year ago

@Erfan the standard method is to escape the curly braces. If memory serves, that would be {{3}}.

törzsmókus Over a year ago

wow, I never realized that f-strings and r-strings may be combined

hamidreza zarrabi · Accepted Answer · 2021-05-04 14:25:38Z

0

Following command work for me:
df.query('text.str.contains(@variable)')

answered May 4, 2021 at 14:25

hamidreza zarrabi

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Comments

Phanish Chava · Accepted Answer · 2019-07-25 16:33:24Z

-1

I had the exact same problem when parsing a 'variable' to str.contains(variable).

Try using str.contains(variable, regex=False)

It worked for me perfectly.

answered Jul 25, 2019 at 16:33

Phanish Chava

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1 Comment

Andrea Moro Over a year ago

Clearly the opposite as the OP requested.

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