How to Use Regular Expressions in Python

09/08/2021

Contents

In this article, you will learn how to use regular expressions in Python.

Regular Expressions in Python

In Python, you can use the re module to work with regular expressions. Here are some basic steps to use regular expressions in Python:

Import the re module:

import re

Compile a regular expression pattern:

pattern = re.compile(r"your_pattern")

Use various methods available in the re module to match the pattern with a string, such as:

  • match: Determines if the regular expression matches at the beginning of the string.
  • search: Searches the string for a match to the regular expression.
  • findall: Returns all non-overlapping matches of the pattern in the string as a list.
  • finditer: Returns all non-overlapping matches of the pattern in the string as an iterator.

Here’s an example code snippet to extract all digits from a string:

import re

string = "The number is 42 and the date is 07/02/2023"
pattern = re.compile(r"\d+")

# find all digits in the string
result = pattern.findall(string)

print(result) 
# Output: ['42', '07', '02', '2023']
 

Here are some additional details about using regular expressions in Python:

Pattern syntax

The pattern syntax used in the re module is based on the Perl syntax. You can find a comprehensive list of syntax elements in the official Python documentation:
https://docs.python.org/3/library/re.html#regular-expression-syntax

Match objects

When a match is successful, you can use the group method of the match object to extract the matched string. The group method can also be called with an index argument to extract specific matching subgroups.

Flags

The re module provides several flags that modify the matching behavior of the regular expressions. Some common flags are:

  • re.IGNORECASE: Perform case-insensitive matching.
  • re.DOTALL: Make the . special character match any character including a newline.
  • re.MULTILINE: Make the ^ and $ special characters match the beginning and end of a line, rather than the entire string.

Here’s an example of using the group method and flags:

import re

string = "The number is 42 and the date is 07/02/2023"
pattern = re.compile(r"(\d+)/(\d+)/(\d+)")

# search the string for a pattern match, ignoring case
match = pattern.search(string, flags=re.IGNORECASE)

if match:
    # extract the matched string and subgroups
    matched_string = match.group()
    day = match.group(1)
    month = match.group(2)
    year = match.group(3)
    
    print(matched_string) # Output: '07/02/2023'
    print(day, month, year) # Output: '07' '02' '2023'

For more information and advanced usage, you can refer to the official documentation: https://docs.python.org/3/library/re.html

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