10 Mar Python Regular Expressions
Python’s re module allows you to search, match, split, and replace text using Regular Expressions (RegEx). It’s powerful for pattern-matching tasks like validating emails, extracting substrings, or replacing text. Below are clear examples with code snippets to help you practice.
RegEx Metacharacters
In Python, regex metacharacters are special symbols that define search patterns, making regular expressions powerful for string matching and manipulation.
RegEx Sets
A set in regex is a collection of characters enclosed within square brackets [], where each character represents a possible match.
Basics of Python RegEx
- Import module: import re
- Common functions:
- re.match() → Checks for a match at the beginning of a string.
- re.search() → Searches for a match anywhere in the string.
- re.findall() → Returns all matches as a list.
- re.split() → Splits string by pattern.
- re.sub() → Replaces matches with new text.
RegEx Examples
Let us see some Regular Expression examples in Python:
1. Match at Start using the match() function
Check for a match at the beginning of a string using the match() function:
# Match at start of a string in Python RegExp using match()
import re
pattern = "^c" # ^ metacharacter is for start of string
txt = "cricket"
# The match() checks for a regular expression match only at the very beginning of the string
res = re.match(pattern, txt)
if res:
print("Search is successful")
else:
print("Search is not successful")
Output
Search is successful
2. Match at the end of a string
Check for a match at the end of a string using the search() function.
# Match at end of a string in Python RegExp
import re
pattern = "worldcup$" # $ metacharacter is for end of string
txt = "cricket worldcup"
# The search() will scan the whole string for a match
res = re.search(pattern, txt)
if res:
print("Search is successful")
else:
print("Search is not successful")
Output
Search is successful
3. Search Anywhere using the search() function
Search for a match anywhere in the string using the search() method:
# Search anywhere using the search() function in Python RegExp
# Find a string that starts with "The", has anything in between, and then contains "Cup"
import re
txt1 = "The Cricket World Cup"
res = re.search("^The.*Cup", txt1)
if res:
print("Search is sucessful")
else:
print("Search is not sucessful")
txt2 = "The11Cup"
res2 = re.search("^The.*Cup", txt2)
if res2:
print("Search is successful")
else:
print("Search is not successful")
Output
Search is successful Search is successful
4. Find All Matches using the findall() function
To find and return all matches as a list, use the findall() function in Python:
# Find all matches using the findall() function in Python Regex
# Find all occurrences of "sun" or "fun" in the text
import re
txt = "sun fun run sam fan"
# [sf] -> Match either s or f at this position
# un -> Followed immediately by "un"
matches = re.findall("[sf]un", txt)
print(matches)
Output
['sun', 'fun']
5. Search for vowels in a list with Python Regex findall()
To search for vowels, use the findall() function in Python.
# Search for vowels in a list with Python Regex findall()
import re
words = ["apple", "banana", "sky", "cricket"]
pattern ="[aeiou]"
# findall() returns all matches in a string
for w in words:
vowels = re.findall(pattern, w)
print(f"{w}:{vowels}")
Output
apple:['a', 'e'] banana:['a', 'a', 'a'] sky:[] cricket:['i', 'e']
6. Split a string at each whitespace character with Python Regex split()
To split a string, use the split() function in Python.
# Split a string at each whitespace character with Python Regex split()
import re
txt = "World Wide Web"
# We can also use re.split("\\s", txt)
res = re.split(r"\s", txt)
print(res)
Output
['World', 'Wide', 'Web']
7. Replace a text using the sub() function in Python Regex
To replace matches with new text, use the sub() function in Python:
import re txt = "T20 World Cup" res = re.sub(r"T20", "ODI", txt) print(res)
Output
ODI World Cup
We replaced T20 with ODI in the above example.
8. Replace part of a string that matches a regex pattern using the sub() function
To replace a part of a string that matches a pattern, use the sub() function:
# Replace part of a string that matches a regex pattern using the sub() function in Python Regex
import re
txt = "cricket worldcup"
res = re.sub("[aeiou]", "*", txt)
print(res)
Output
cr*ck*t w*rldc*p
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