How Do I Search For A Pattern In Python? (In 2023)

A couple of years back when I was still a QA engineer (not a Python developer yet) I got myself into dozens of situations where I had to figure out how do I search for a pattern in Python.. A pattern in strings, a pattern in files, etc. Let’s look into possible solutions in this post.

How Do I Search For A Pattern In Python?

To search for a pattern in a string in Python, you can use the re module, which stands for regular expression. Regular expressions are a way to describe patterns in strings, and the re module provides several functions to work with them in Python.

Suppose you have a list of emails, and you want to extract the username and domain name from each email address.

The emails look like this:

jane.doe@example.com
john.smith@gmail.com

You can use the re module and regular expressions to extract the username and domain name from the email addresses like this:

import re

email = 'jane.doe@example.com'

# Extract the username and domain name using a regular expression
pattern = r'(.+)@(.+)'
match = re.search(pattern, email)

username = match.group(1)
domain = match.group(2)

print(f'Username: {username}')  # Output: 'Username: jane.doe'
print(f'Domain: {domain}')  # Output: 'Domain: example.com'

In this example, we used the re.search() function to find the first occurrence of the pattern in the email address, and then used the group() method to extract the matching substrings.

The regular expression itself uses several special characters to specify the pattern:

. matches any single character (except a newline)
+ matches one or more occurrences of the preceding character
( ) define a capturing group, which allows us to extract the matching substrings using the group() method

🚨 This is a simplified version of what you can achieve with regular expressions in Python, I’ll give you more answers to a How Do I Search For A Pattern In Python? question later in this post. If you’re interested to explore regular expressions in Python, I’ve written an extensive post just about it here.

Follow the steps below:

How Do I Search For Multiple Patterns In Python?

To search for multiple patterns in a string in Python, you can use the re module and the | operator, which stands for “or” in regular expressions.

Here’s an example of how to use the re module to search for multiple patterns in a string:

import re

# Find all occurrences of the patterns 'abc' or 'def' in the string
string = 'abcdefgabcdef'
pattern = 'abc|def'

matches = re.findall(pattern, string)
print(matches)  # Output: ['abc', 'def', 'abc', 'def']

The findall() function searches for all occurrences of the pattern in the string and returns a list of the matches.

You can also use the search() function to find the first occurrence of one of the patterns in the string:

import re

# Find the first occurrence of the patterns 'abc' or 'def' in the string
string = 'abcdefgabcdef'
pattern = 'abc|def'

match = re.search(pattern, string)
print(match.group())  # Output: 'abc'

The search() function returns a Match object, which has several methods you can use to get information about the match, such as group(), which returns the actual matched string.

🔥 Alright, that’s fairly simple to understand, but when it comes to real project situations it’s never that easy.

Here’s an example of how you might use pattern matching in Python to extract information from a string in a real-life scenario.

Suppose you have a log file that contains information about user actions on your website, and you want to extract the user’s IP address and the page they accessed from each log entry.

The log entries look like this:

10.0.0.1 - - [21/Jul/2022:16:00:59 +0000] "GET /index.html HTTP/1.1" 200 1234
10.0.0.2 - - [21/Jul/2022:16:01:23 +0000] "POST /submit.html HTTP/1.1" 200 1234
10.0.0.3 - - [21/Jul/2022:16:01:55 +0000] "GET /faq.html HTTP/1.1" 200 1234

You can use the re module and regular expressions to extract the IP addresses and page URLs from the log entries like this:

import re

log_entry = '10.0.0.1 - - [21/Jul/2022:16:00:59 +0000] "GET /index.html HTTP/1.1" 200 1234'

# Extract the IP address and the page URL using a regular expression
pattern = r'(\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}).+"(.+)"'
match = re.search(pattern, log_entry)

ip_address = match.group(1)
page_url = match.group(2)

print(f'IP address: {ip_address}')  # Output: 'IP address: 10.0.0.1'
print(f'Page URL: {page_url}')      # Output: 'Page URL: GET /index.html HTTP/1.1'

In this example, we used the re.search() function to find the first occurrence of the pattern in the log entry, and then used the group() method to extract the matching substrings.

The regular expression itself uses several special characters to specify the pattern:

\d matches any digit (0-9)
{1,3} specifies that the preceding character (in this case, \d) should be matched 1 to 3 times
\. matches a literal period (.) character
+ matches one or more occurrences of the preceding character
" matches a literal double quote character
( ) define a capturing group, which allows us to extract the matching substrings using the group() method

You can use a similar approach to extract other information from strings, such as dates, numbers, or any other pattern that you can describe with a regular expression.

I’ve used regular expressions in dozens of real project situations, check out a log parsing post I wrote years ago and a recent one but also a very useful post about parsing log files with regular expressions is here.

How Do I Find A Repeating Pattern In A String In Python?

To find a repeating pattern in a string in Python, you can use the re module and regular expressions.

Regular expressions allow you to specify patterns that can match multiple occurrences of a character, sequence of characters, or group of characters.

Here’s an example of how you can use regular expressions to find a repeating pattern in a string:

import re

# Find all occurrences of strings that start with 'a' and end with 'c',
# and have any number of characters in between
string = 'abc abcdefg abcd abc123 abc! a123bc zys'
pattern = r'a.+c'

matches = re.findall(pattern, string)
print(matches)  # Output: ['abc abcdefg abcd abc123 abc! a123bc']

In this example, we used the . character, which matches any single character, and the + character, which matches one or more occurrences of the preceding character.

This allows us to match strings that have any number of characters between the ‘a’ and the ‘c’.

We also used the r prefix before the pattern string to create a “raw” string, which helps to avoid issues with escape characters.

Now, let’s take a look at a real-life scenario.. 👇🏻

Here’s a simple example of how you might use regular expressions in Python to search for a pattern that starts with a specific character and ends with a specific character in a real-life scenario.

Suppose you have a list of file names, and you want to extract the file extension from each file name.

The file names look like this:

document.txt
presentation.ppt
spreadsheet.xls

You can use the re module and regular expressions to extract the file extensions from the file names like this:

import re

filename = 'document.txt'

# Extract the file extension using a regular expression
pattern = r'\.([a-zA-Z0-9]+)$'
match = re.search(pattern, filename)

extension = match.group(1)

print(f'File extension: {extension}')  # Output: 'File extension: txt'

In this example, we used the re.search() function to find the first occurrence of the pattern in the file name, and then used the group() method to extract the matching substring.

The regular expression itself uses several special characters to specify the pattern:

\. matches a literal period (.) character
[a-zA-Z0-9] matches any letter or digit
+ matches one or more occurrences of the preceding character
$ matches the end of the string
( ) define a capturing group, which allows us to extract the matching substring using the group() method

As I said before, with these examples, you can use a similar approach to extract other information from strings that starts with a specific character and ends with a specific character, such as dates, numbers, or any other pattern that you can describe with a regular expression.

How Do I Search Multiple Strings In A File In Python?

To search for multiple strings in a file in Python, you can use the in operator to check if each string is present in the file.

Here’s an example of how you can do this:

# Open the file in read mode
with open('file.txt', 'r') as f:
    # Read the file contents into a string
    contents = f.read()

# Search for each string in the file contents
strings = ['string1', 'string2', 'string3']
for s in strings:
    if s in contents:
        print(f'Found "{s}" in the file')
    else:
        print(f'Did not find "{s}" in the file')

This will search for each string in the strings list in the file contents and print a message indicating whether or not the string was found.

If you want to search for multiple patterns in a file, you can use the re module and regular expressions, as described earlier in the post.

For example:

import re

# Open the file in read mode
with open('file.txt', 'r') as f:
    # Read the file contents into a string
    contents = f.read()

# Search for each pattern in the file contents
patterns = [r'pattern1', r'pattern2', r'pattern3']
for pattern in patterns:
    if re.search(pattern, contents):
        print(f'Found pattern "{pattern}" in the file')
    else:
        print(f'Did not find pattern "{pattern}" in the file')

This will search for each pattern in the patterns list in the file contents and print a message indicating whether or not the pattern was found.

🚨 Here’s an example of how you might use the in operator to search for multiple strings in a file in a real-life scenario.

Suppose you have a file that contains a list of employee records, and you want to search the file for records that match certain criteria, such as the employee’s name, job title, or salary.

The records are separated by newline characters and have the following format:

id: 123
name: John Smith
title: Manager
salary: $75,000

id: 456
name: Jane Doe
title: Developer
salary: $65,000

id: 789
name: Bob Johnson
title: Salesperson
salary: $50,000

You can use the in operator to search for specific strings in the file like this:

# Open the file in read mode
with open('employees.txt', 'r') as f:
    # Read the file contents into a string
    contents = f.read()

# Search for records that match the criteria
criteria = ['name: John Smith', 'title: Manager', 'salary: $75,000']
if all(c in contents for c in criteria):
    print('Found a record matching the criteria')
else:
    print('Did not find a record matching the criteria')

This will search for records that contain all of the strings in the criteria list, and print a message indicating whether or not a matching record was found.

If you want to search for records that match any of the criteria, you can use the any() function instead of the all() function:

if any(c in contents for c in criteria):
    print('Found a record matching one or more of the criteria')
else:
    print('Did not find a record matching any of the criteria')

This will search for records that contain any of the strings in the criteria list, and print a message indicating whether or not a matching record was found.

You can also use the re module and regular expressions to search for patterns in the file, as described in a previous answer.

For example:

import re

# Open the file in read mode
with open('employees.txt', 'r') as f:
    # Read the file contents into a string
    contents = f.read()

# Search for records that match the criteria
criteria = [r'name: .+', r'title: .+', r'salary: \$.+']
if all(re.search(c, contents) for c in criteria):
    print('Found a record matching the criteria')
else:
    print('Did not find a record matching the criteria')

This will search for records that contain patterns that match all of the regular expressions in the criteria list, and print a message indicating whether or not a matching record was found.

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