The in Operator

The simplest way to check if a substring exists within a string is the in operator.

sentence = "Python is a powerful programming language"

print("Python" in sentence)
print("java" in sentence)
print("powerful" in sentence)

# Using 'not in'
print("java" not in sentence)

text = "Python is Great"
search = "python"

# Case-insensitive search
if search.lower() in text.lower():
    print("Found it!")

The find() Method

find() returns the index of the first occurrence of a substring. If not found, it returns -1 (instead of raising an error).

text = "Hello, World! Hello, Python!"

print(text.find("Hello"))     # First occurrence
print(text.find("World"))     # Position of "World"
print(text.find("Java"))      # Not found

Searching from a Starting Position

You can specify where to start searching.

text = "Hello, World! Hello, Python!"

# Find second "Hello" by starting after the first
first = text.find("Hello")
second = text.find("Hello", first + 1)
print(f"First 'Hello' at index: {first}")
print(f"Second 'Hello' at index: {second}")

email = "user@example.com"
at_pos = email.find("@")

if at_pos != -1:
    username = email[:at_pos]
    domain = email[at_pos + 1:]
    print(f"Username: {username}")
    print(f"Domain: {domain}")
else:
    print("Invalid email")

find() vs index()

text = "Python programming"

# find() returns -1
print(text.find("Java"))

# index() raises ValueError
# text.index("Java")  # ValueError: substring not found

# Both work the same when substring IS found
print(text.find("prog"))
print(text.index("prog"))

Searching from the Right: rfind() and rindex()

rfind() and rindex() search from the right (end) of the string and return the index of the last occurrence.

path = "/home/user/documents/report.pdf"

# Find last "/" to get the filename
last_slash = path.rfind("/")
filename = path[last_slash + 1:]
print(f"Filename: {filename}")

# Find last "." to get the extension
last_dot = path.rfind(".")
extension = path[last_dot + 1:]
print(f"Extension: {extension}")

text = "the cat sat on the mat near the hat"
search = "the"

positions = []
start = 0
while True:
    pos = text.find(search, start)
    if pos == -1:
        break
    positions.append(pos)
    start = pos + 1

print(f"'{search}' found at positions: {positions}")

Advanced replace()

Building on what you learned in Lesson 1, here are more advanced uses of replace().

# Censoring words
message = "The password is secret123"
censored = message.replace("secret123", "*****")
print(censored)

# Normalizing whitespace
messy = "too   many     spaces"
while "  " in messy:
    messy = messy.replace("  ", " ")
print(messy)

# Converting separators
csv_data = "name,age,city"
tab_data = csv_data.replace(",", "\t")
print(tab_data)

Introduction to Pattern Matching

While find() and replace() work great for exact matches, sometimes you need to search for patterns rather than exact text. Python has a module called re (regular expressions) for this, which you will explore in later modules.

For now, here are some common pattern-checking techniques using the methods you already know:

# Check if a string contains only digits
pin = "1234"
print(f"Is PIN all digits? {pin.isdigit()}")

# Check if input looks like an email
email = "user@example.com"
has_at = "@" in email
has_dot = "." in email
print(f"Looks like email? {has_at and has_dot}")