String Handling and Regular Expressions

This lesson covers string handling — the manipulation and processing of text data — and regular expressions — powerful pattern-matching tools used for searching, validating, and transforming strings. These are essential skills for A-Level Computer Science.

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String Basics

A string is a sequence of characters. In most programming languages, strings are immutable — once created, they cannot be changed in place. Any operation that appears to modify a string actually creates a new string.

Common String Operations

Operation	Description	Python Example	Result
Length	Number of characters	len("Hello")	5
Indexing	Access a single character	"Hello"[1]	"e"
Slicing	Extract a substring	"Hello"[1:4]	"ell"
Concatenation	Join two strings	"Hi" + " there"	"Hi there"
Repetition	Repeat a string	"Ha" * 3	"HaHaHa"
Membership	Check if substring exists	"ell" in "Hello"	True
Upper/Lower	Change case	"Hello".upper()	"HELLO"
Strip	Remove whitespace	" Hi ".strip()	"Hi"
Split	Break into a list	"a,b,c".split(",")	["a","b","c"]
Join	Combine a list into a string	",".join(["a","b"])	"a,b"
Replace	Replace occurrences	"cat".replace("c","b")	"bat"
Find	Find position of substring	"Hello".find("ll")	2

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String Traversal

You can iterate through every character in a string using a loop:

Pseudocode

text = "COMPUTER"
FOR i = 0 TO LENGTH(text) - 1
    OUTPUT text[i]
NEXT i

Python

text = "COMPUTER"
for char in text:
    print(char)

# With index
for i in range(len(text)):
    print(f"Index {i}: {text[i]}")

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String Conversion

Converting between strings and other data types is essential for input/output operations.

# String to integer
age = int("17")

# String to float
price = float("9.99")

# Integer to string
text = str(42)

# Character to ASCII code
code = ord("A")      # 65

# ASCII code to character
char = chr(65)       # "A"

ASCII and Character Codes

Character	ASCII Code	Character	ASCII Code
'0'	48	'A'	65
'9'	57	'Z'	90
' '	32	'a'	97
'!'	33	'z'	122

Exam Tip: You should know the ASCII ranges for digits (48-57), uppercase letters (65-90), and lowercase letters (97-122). This knowledge is useful for character manipulation and simple encryption algorithms like Caesar cipher.

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Practical String Processing

Example: Validating an Email Address (Without Regex)

def is_valid_email(email: str) -> bool:
    if email.count("@") != 1:
        return False
    local, domain = email.split("@")
    if len(local) == 0 or len(domain) == 0:
        return False
    if "." not in domain:
        return False
    if domain.startswith(".") or domain.endswith("."):
        return False
    return True

Example: Caesar Cipher

def caesar_encrypt(text: str, shift: int) -> str:
    result = ""
    for char in text:
        if char.isalpha():
            base = ord('A') if char.isupper() else ord('a')
            shifted = (ord(char) - base + shift) % 26 + base
            result += chr(shifted)
        else:
            result += char
    return result

print(caesar_encrypt("Hello World", 3))  # Khoor Zruog

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Regular Expressions