Week 8 - Strings & Regular Expressions

Strings and Regular Expressions¶

Lesson Objectives¶

Understand strings as sequences and practice accessing individual characters
Learn string slicing and manipulation techniques
Recognize the immutability of strings and explore methods for creating modified strings
Utilize string comparison operations for alphabetic ordering
Implement essential string methods and apply them in reading and writing files
Gain familiarity with regular expressions to search, replace, and validate patterns within text

1. Review of Strings are Sequences¶

Explanation:
Strings in Python are sequences (specifically a sequence of characters strung together), which means they can be indexed to access individual characters. Unlike integers or booleans, strings can hold multiple characters in a particular order.

Code Example:

city = "Eureka"
first_letter = city[0]  # Accessing first character
second_letter = city[1]  # Accessing second character
print("First letter:", first_letter)
print("Second letter:", second_letter)

# Negative index
last_letter = city[-1]
print("Last letter:", last_letter)

NOTE: The [-1] syntax is specific to interpreted languages like Python.

2. String Slicing¶

Explanation:
String slicing enables access to parts of a string. The syntax string[start:end] captures a substring from the starting index up to, but not including, the ending index.

Code Example:

sentence = "Programming is fun!"
print("First 11 characters:", sentence[:11])
print("Characters from index 5 to 9:", sentence[5:9])
print("Last 3 characters:", sentence[-3:])

3. String Immutability¶

Explanation:
Strings are immutable, meaning that their characters cannot be changed directly. Instead, we create a new string by modifying or combining parts of the original.

Code Example:

greeting = "Goodbye"
# Attempting to change the first letter results in an error
greeting[0] = 'H'

# So the correct way is to create a new string using slicing.
new_greeting = "H" + greeting[1:]
print("New greeting:", new_greeting)

4. Review of String Comparison¶

Explanation:
Strings can be compared using relational operators such as <, >, and ==. Python uses ASCII values for comparison, which places uppercase letters before lowercase letters.

Code Example:

def compare_to_word(word):
    if word < "apple":
        print(f"{word} comes before apple.")
    elif word > "apple":
        print(f"{word} comes after apple.")
    else:
        print("It's apple!")

compare_to_word("banana")
compare_to_word("Apple")

5. Review of String Methods¶

Explanation:
Strings come with various built-in methods, including upper(), lower(), find(), and replace(), which allow transformations and searches within strings.

Code Example:

message = "Hello, World!"
print("Uppercase:", message.upper())
print("Lowercase:", message.lower())
print("Position of 'World':", message.find("World"))
print("Replacing 'World' with 'Universe':", message.replace("World", "Universe"))

6. Reading and Writing Files¶

Explanation:
Reading and writing text files involves opening a file, processing its content, and saving any modifications. We often use loops to handle each line of the file.

Code Example:

# Writing to a file
with open("greeting.txt", "w") as writer:
    writer.write("Hello, World!\nWelcome to Python programming.")

# Reading from a file
with open("greeting.txt", "r") as reader:
    for line in reader:
        print(line.strip())

7. Introduction to Regular Expressions¶

Explanation:
Regular expressions (regex) are used to identify specific patterns in strings. They enable powerful operations like search and replace for advanced text processing.

Code Example:

import re

text = "Welcome to the world of Python programming!"
pattern = r"\bworld\b"  # Matches 'world' as a whole word \b is a word boundary
match = re.search(pattern, text)
if match:
    print("Found 'world' at index:", match.start())
else:
    print("'world' not found")

8. String Substitution with Regex¶

Explanation:
String substitution replaces parts of a string based on a pattern. Regex substitutions use the sub() method to replace matches with a specified replacement.

Code Example:

text = "The color of the sky is colorless."
pattern = r"\bcolor\b"
new_text = re.sub(pattern, "colour", text)
print("Modified text:", new_text)

Exercises¶

Slice and Reorder:
Write a function half_mirror(str_in) that takes a string input and creates a mirrored version of its first half. If the string has an odd number of characters, consider only the first half (ignoring the middle character). Return the mirrored half as a new string. Example: Input: “PythonProgramming”, Output: “rPnohtyP”
Find and Count Patterns:
Create a function count_pattern_occurrences(text, pattern) that counts how many times a specified pattern appears in a given text, using regular expressions.
Example: For pattern = r"\bthe\b", text = "The cat chased the mouse." should return 2.
File Content Replacer:
Write a program that reads from a file and replaces all occurrences of a user-specified word with another word. Save the modified text to a new file.
Example: Replace "sky" with "ocean" in a file named nature.txt and save the results in new_nature.txt.