Python Core | VoidX Academy

18. Python RegEx

Data Processing

Python RegEx (Regular Expressions)

A Regular Expression (RegEx) is a sequence of characters that forms a search pattern. It is an extremely powerful tool used for searching, matching, and manipulating text strings based on specific patterns, rather than exact words.

Python has a built-in package called re, which can be used to work with Regular Expressions. You simply import it to start parsing strings for complex patterns like emails, phone numbers, or specific word boundaries.

Importing the re Module

Before you can use regular expressions, you must import the re module. Let's do a quick test to check if a string starts with "The" and ends with "Spain".

import re

txt = "The rain in Spain"

# Search the string to see if it starts with "The" and ends with "Spain"
x = re.search("^The.*Spain$", txt)

if x:
    print("YES! We have a match!")
else:
    print("No match")

OutputYES! We have a match!

The findall() Function

The findall() function returns a list containing all matches. If no matches are found, it returns an empty list. It searches the entire string and extracts every occurrence of the pattern.

import re

txt = "The rain in Spain falls mainly in the plain"

# Find all occurrences of 'ai'
x = re.findall("ai", txt)
print(x)
print(f"Found {len(x)} matches.")

# Find something that doesn't exist
y = re.findall("Portugal", txt)
print(y)

Output['ai', 'ai', 'ai', 'ai'] Found 4 matches. []

The search() Function

The search() function searches the string for a match, and returns a Match object if there is a match. If there is more than one match, only the first occurrence of the match will be returned.

import re

txt = "The rain in Spain"
x = re.search("\s", txt)  # \s searches for a whitespace character

print(f"The first white-space character is located at position: {x.start()}")

OutputThe first white-space character is located at position: 3

The split() Function

The split() function returns a list where the string has been split at each match. This is far more powerful than the standard string .split() method because you can split on complex patterns.

import re

txt = "The rain in Spain"

# Split at each white-space character
x = re.split("\s", txt)
print(x)

# Split only at the first occurrence (maxsplit=1)
y = re.split("\s", txt, 1)
print(y)

Output['The', 'rain', 'in', 'Spain'] ['The', 'rain in Spain']

The sub() Function

The sub() function replaces the matches with the text of your choice. It is short for "substitute".

import re

txt = "The rain in Spain"

# Replace every white-space character with the number 9
x = re.sub("\s", "9", txt)
print(x)

# Replace only the first 2 occurrences
y = re.sub("\s", "9", txt, 2)
print(y)

OutputThe9rain9in9Spain The9rain9in Spain

Match Object Methods

A Match Object is an object containing information about the search and the result. It has properties and methods to extract details about the match.

.span() returns a tuple containing the start and end positions of the match.
.string returns the original string passed into the function.
.group() returns the exact part of the string where there was a match.

import re

txt = "The rain in Spain"
x = re.search(r"\bS\w+", txt)  # Find a word that starts with 'S'

print(x.span())
print(x.string)
print(x.group())

Output(12, 17) The rain in Spain Spain

💡Best Practice: Always use raw strings (prefix the string with r, like r"pattern") when writing regular expressions. This stops Python from misinterpreting escape characters (like \n or \b) before the regex engine even sees them.

Common Metacharacters

Metacharacters are characters with a special meaning in RegEx.

[] - A set of characters (e.g. "[a-m]")
\ - Signals a special sequence or escapes special characters (e.g. "\d" for digits)
. - Any character except newline (e.g. "he..o")
^ - Starts with (e.g. "^hello")
$ - Ends with (e.g. "world$")
* - Zero or more occurrences
+ - One or more occurrences

import re
txt = "hello planet 123"

print(re.findall("[a-e]", txt))  # Find all letters a through e
print(re.findall("\d", txt))     # Find all digits
print(bool(re.search("^hello", txt))) # Does it start with hello?

Output['e', 'a', 'e'] ['1', '2', '3'] True

18. Python RegEx

Data Processing

Python RegEx (Regular Expressions)

Importing the re Module

Before you can use regular expressions, you must import the re module. Let's do a quick test to check if a string starts with "The" and ends with "Spain".

import re

txt = "The rain in Spain"

# Search the string to see if it starts with "The" and ends with "Spain"
x = re.search("^The.*Spain$", txt)

if x:
    print("YES! We have a match!")
else:
    print("No match")

OutputYES! We have a match!

The findall() Function

The findall() function returns a list containing all matches. If no matches are found, it returns an empty list. It searches the entire string and extracts every occurrence of the pattern.

import re

txt = "The rain in Spain falls mainly in the plain"

# Find all occurrences of 'ai'
x = re.findall("ai", txt)
print(x)
print(f"Found {len(x)} matches.")

# Find something that doesn't exist
y = re.findall("Portugal", txt)
print(y)

Output['ai', 'ai', 'ai', 'ai'] Found 4 matches. []

The search() Function

import re

txt = "The rain in Spain"
x = re.search("\s", txt)  # \s searches for a whitespace character

print(f"The first white-space character is located at position: {x.start()}")

OutputThe first white-space character is located at position: 3

The split() Function

import re

txt = "The rain in Spain"

# Split at each white-space character
x = re.split("\s", txt)
print(x)

# Split only at the first occurrence (maxsplit=1)
y = re.split("\s", txt, 1)
print(y)

Output['The', 'rain', 'in', 'Spain'] ['The', 'rain in Spain']

The sub() Function

The sub() function replaces the matches with the text of your choice. It is short for "substitute".

import re

txt = "The rain in Spain"

# Replace every white-space character with the number 9
x = re.sub("\s", "9", txt)
print(x)

# Replace only the first 2 occurrences
y = re.sub("\s", "9", txt, 2)
print(y)

OutputThe9rain9in9Spain The9rain9in Spain

Match Object Methods

A Match Object is an object containing information about the search and the result. It has properties and methods to extract details about the match.

.span() returns a tuple containing the start and end positions of the match.
.string returns the original string passed into the function.
.group() returns the exact part of the string where there was a match.

import re

txt = "The rain in Spain"
x = re.search(r"\bS\w+", txt)  # Find a word that starts with 'S'

print(x.span())
print(x.string)
print(x.group())

Output(12, 17) The rain in Spain Spain

Common Metacharacters

Metacharacters are characters with a special meaning in RegEx.

[] - A set of characters (e.g. "[a-m]")
\ - Signals a special sequence or escapes special characters (e.g. "\d" for digits)
. - Any character except newline (e.g. "he..o")
^ - Starts with (e.g. "^hello")
$ - Ends with (e.g. "world$")
* - Zero or more occurrences
+ - One or more occurrences

import re
txt = "hello planet 123"

print(re.findall("[a-e]", txt))  # Find all letters a through e
print(re.findall("\d", txt))     # Find all digits
print(bool(re.search("^hello", txt))) # Does it start with hello?

Output['e', 'a', 'e'] ['1', '2', '3'] True

18. Python RegEx

Python RegEx (Regular Expressions)

Importing the re Module

The findall() Function

The search() Function

The split() Function

The sub() Function

Match Object Methods

Common Metacharacters

Knowledge Check

18. Python RegEx

Python RegEx (Regular Expressions)

Importing the re Module

The findall() Function

The search() Function

The split() Function

The sub() Function

Match Object Methods

Common Metacharacters

Knowledge Check