Regex in Python

In Python, regular expressions (regex) are supported through the built-in re module.

The regular expression is a sequence of characters with specialized syntax that we can use to find or match characters or pattern matching within strings.

Here's a overview of regex in Python:

Importing the module:

Syntax:

# import regular expression module using `re` module
import re

Compiling a regular expression pattern:

Syntax:

# compile pattern
pattern = re.compile(r'pattern value')

We can use the "r" prefix before the pattern string to treat it as a raw string, which helps to avoid accidental escape characters.

Regular Expression Functions:

re.match(): The `match` function tries to match a pattern at the beginning of a string.
re.search(): The `search` function searches for the first occurrence of a pattern in a string.
re.findall(): The `findall` function returns all occurrences of a pattern in a string as a list of strings.
re.finditer(): This `finditer` function returns an iterator yielding match objects for all non-overlapping matches of a pattern in a string.
re.fullmatch(): The `fullmatch()` function is used to match the whole string with a regex pattern.

Using regex methods:

search():

Searches for the first occurrence of a pattern in a string.

Example:

# import regular expression module using `re` module
import re

# compile pattern
pattern = re.compile(r'single-line')

stringValue = 'This is a single-line string'
result = pattern.search(stringValue)
print(result.group()) # Output: single-line

newPattern = re.compile(r'new')
checkForNonExistingValue = newPattern.search(stringValue)
print(checkForNonExistingValue) # Output: None

Another Way:

# Another Way
import re

newPattern = r"Hi"
text = "Hi, My name is Alice"

search = re.search(newPattern, text)

print(search.group()) # Output: Hi

if search:
    print("Pattern Match found:", search.group()) # Pattern Match found: Hi
else:
    print("No match")

match():

Checks if the pattern matches at the beginning of the string.

Example:

# import regular expression module using `re` module
import re

# compile pattern
pattern = re.compile(r'This')

string = 'This is a single-line string'
result = pattern.match(string)

# Printing the result
print(result.group()) # Output: This

Another Way:

# Another Way
import re

newPattern = r"Hi"
text = "Hi, My name is Alice"

match = re.match(newPattern, text)

print(match.group()) # Output: Hi

if match:
    print("Pattern Match found:", match.group()) # Pattern Match found: Hi
else:
    print("No match")

findall():

Finds all occurrences of the pattern in the string and returns them as a list.

Example:

# import regular expression module using `re` module
import re

# compile pattern
pattern = re.compile(r'e')

string = 'This is a single-line string'

# Apply findall function
result = pattern.findall(string)

# Printing the result
print(result) # Output: ['e', 'e']

Another Way:

# Another Way
import re

newPattern = r"e"
text = "Hi, My name is Alice"

findall = re.findall(newPattern, text)

print(findall) # Output: ['e', 'e']

if findall:
    print("Pattern Match found:", findall) # Pattern Match found: ['e', 'e']
else:
    print("No match")

finditer():

Returns an iterator yielding match objects over all non-overlapping matches in the string.

Example:

# import regular expression module using `re` module
import re

# Defining regular expression pattern  
pattern = re.compile(r'is')

string = 'This is a single-line string'
matches = pattern.finditer(string)

# Printing the matches using loop over iterator
for match in matches:
    print("Match found at index:", match.start())
    # Match found at index: 2
    # Match found at index: 5

fullmatch():

fullmatch() function is used to match the whole string with a regex pattern.

Example:

import re

pattern = r"Hi, My Name is Alice"
text = "Hi, My Name is Alice"

matches = re.fullmatch(pattern, text)

print("Match found:", matches.group()) # Output: Match found: Hi, My Name is Alice

Using match objects:

After applying a regex method, we can access the match object properties like "group()", "start()", and "end()" to get the matched "text", "start index", and "end index" respectively.

Example:

# import regular expression module using `re` module
import re

# Defining regular expression pattern  
pattern = re.compile(r'single-line')

#Declaring string value
string = 'This is a single-line string'

# Searching for the first occurrence
result = pattern.search(string)
print(result.group())  # Output: 'single-line'

# Finding all occurrences
result = pattern.findall(string)
print(result)  # Output: ['single-line']

# Matching at the beginning
result = pattern.match(string)
print(result)  # Output: None

Note:

Regular expressions can be complex and powerful tools for string manipulation and pattern matching in Python.

Understanding the syntax and usage is essential for effective text processing.

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