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Effortlessly Check If a Pandas String Contains a Specific Pattern

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How to Check if a Python String Contains a Substring

If you’re new to programming or come from a programming language other than Python, you may be looking for the best way to check whether a string contains another string in Python. Identifying such substrings comes in handy when you’re working with text content from a file or after you’ve received user input. You may want to perform different actions in your program depending on whether a substring is present or not.

In this tutorial, you’ll focus on the most Pythonic way to tackle this task, using the membership operator in. Additionally, you’ll learn how to identify the right string methods for related, but different, use cases. Finally, you’ll also learn how to find substrings in pandas columns. This is helpful if you need to search through data from a CSV file. You could use the approach that you’ll learn in the next section, but if you’re working with tabular data, it’s best to load the data into a pandas DataFrame and search for substrings in pandas.

How to Confirm That a Python String Contains Another String

If you need to check whether a string contains a substring, use Python’s membership operator in. In Python, this is the recommended way to confirm the existence of a substring in a string:

raw_file_content = """Hi there and welcome.
This is a special hidden file with a SECRET secret.
I don't want to tell you The Secret,
but I do want to secretly tell you that I have one."""


"secret" in raw_file_content

The in membership operator gives you a quick and readable way to check whether a substring is present in a string. You may notice that the line of code almost reads like English.

If you want to check whether the substring is not in the string, then you can use not in:

"secret" not in raw_file_content

When you use in, the expression returns a Boolean value:

  • True if Python found the substring
  • False if Python didn’t find the substring

You can use this intuitive syntax in conditional statements to make decisions in your code:

if "secret" in raw_file_content:
    print("Found!")

In this code snippet, you use the membership operator to check whether "secret" is a substring of raw_file_content. If it is, then you’ll print a message to the terminal.

Generalize Your Check by Removing Case Sensitivity

Sometimes you may want to perform a case-insensitive search for a substring. To achieve this, you can convert both the string and the substring to lowercase using the lower() method before applying the membership operator in. This ensures that the search is not affected by the case of the characters:

raw_file_content = "This is a sample string"
substring = "sample"


if substring.lower() in raw_file_content.lower():
    print("Found!")

Now, regardless of whether the substring is “sample” or “SAMPLE”, the code will still print “Found!” if the substring exists within the string.

Learn More About the Substring

Sometimes, you may need more information about the substring within the string, such as its index or the number of occurrences. In these cases, you can use additional string methods to obtain the desired information.

To find the index of the first occurrence of a substring within a string, you can use the find() method:

raw_file_content = "This is a sample string"
substring = "sample"


index = raw_file_content.find(substring)


if index != -1:
    print(f"Found at index {index}")

If the substring is found, the code will print the index of the first occurrence. Otherwise, it will not print anything.

To count the number of occurrences of a substring within a string, you can use the count() method:

raw_file_content = "This is a sample string"
substring = "s"


count = raw_file_content.count(substring)
print(f"Number of occurrences: {count}")

In this example, the code will count the number of occurrences of the substring “s” within the string “This is a sample string” and print the result.

Find a Substring With Conditions Using Regex

If you need to find a substring with specific conditions, such as matching a pattern or containing certain characters, you can use the re module in Python. This module provides support for regular expressions, which are powerful patterns used for string matching.

Here’s an example that demonstrates how to find substrings that start with “a” and end with “c” within a string:

import re


raw_file_content = "abc ac a-c"
pattern = r"a.c"


matches = re.findall(pattern, raw_file_content)


for match in matches:
    print(f"Match: {match}")

In this code, the regular expression pattern a.c matches any three-character substring that starts with “a” and ends with “c”. The findall() function in the re module returns a list of all non-overlapping matches. The code then loops through the matches and prints each one.

Find a Substring in a pandas DataFrame Column

If you’re working with tabular data in pandas, you can easily find substrings in specific columns of a DataFrame using the str.contains() method. This method checks whether each value in the specified column contains a specified substring and returns a boolean Series indicating the result.

Here’s an example that demonstrates how to find rows in a DataFrame where the “Name” column contains the substring “John”:

import pandas as pd


data = {
    "Name": ["John Smith", "Jane Doe", "Alex Johnson", "Sam Johnson"],
    "Age": [30, 25, 35, 40]
}


df = pd.DataFrame(data)


substring = "John"
filtered_df = df[df["Name"].str.contains(substring)]


print(filtered_df)

In this code, a DataFrame df is created with two columns: “Name” and “Age”. The code then uses the str.contains() method to check whether each value in the “Name” column contains the substring “John”. The resulting boolean Series is used to filter the DataFrame and only keep rows where the condition is true. The filtered DataFrame is then printed.

Key Takeaways

Understanding how to check if a Python string contains a substring is an essential skill for many programming tasks. In this tutorial, you’ve learned the most Pythonic way to accomplish this using the in membership operator. You’ve also learned how to generalize your check by removing case sensitivity, how to obtain more information about the substring within the string, how to find substrings with specific conditions using regular expressions, and how to find substrings in pandas DataFrame columns.

By mastering these techniques, you’ll be well-equipped to handle substring searches in Python and effectively manipulate text data in your programs.