Basic Character Classes

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Basic Character Classes

PatternDescriptionExample Matches
[abc]Matches any single character in the bracketsa, b, or c
[^abc]Matches any single character NOT in the bracketsd, 1, ! (not a, b, or c)
[a-z]Matches any character in the range a to za, b, c, …, z
[A-Z]Matches any character in the range A to ZA, B, C, …, Z
[0-9]Matches any digit0, 1, 2, …, 9
[a-zA-Z]Matches any letter (lowercase or uppercase)a, B, z, Y
[a-z0-9]Matches any lowercase letter or digita, 5, z, 9
[A-Z0-9]Matches any uppercase letter or digitA, 5, Z, 9

1. [abc] – Matches any single character in the brackets

Description: Matches exactly one character that is either ‘a’, ‘b’, or ‘c’

Example 1: Find vowels in a word

python

import re
text = "hello world"
result = re.findall(r'[aeiou]', text)
print(result)  # ['e', 'o', 'o']
# Matches all vowels in the text

Example 2: Find specific punctuation

python

text = "Hello! How are you? I'm fine."
result = re.findall(r'[!?.]', text)
print(result)  # ['!', '?', '.']
# Matches specific punctuation marks

Example 3: Find digits 1, 2, or 3

python

text = "Phone: 123-456-7890"
result = re.findall(r'[123]', text)
print(result)  # ['1', '2', '3']
# Matches only digits 1, 2, and 3

Example 4: Find specific letters (case-sensitive)

python

text = "Apple Banana cherry"
result = re.findall(r'[AB]', text)
print(result)  # ['A', 'B']
# Matches only uppercase A and B

2. [^abc] – Matches any single character NOT in the brackets

Description: Matches any character except ‘a’, ‘b’, or ‘c’

Example 1: Find non-vowels

python

text = "hello world"
result = re.findall(r'[^aeiou]', text)
print(result)  # ['h', 'l', 'l', ' ', 'w', 'r', 'l', 'd']
# All characters except vowels

Example 2: Find non-digits

python

text = "Room 101, Floor 2"
result = re.findall(r'[^0-9]', text)
print(result)  # ['R', 'o', 'o', 'm', ' ', ',', ' ', 'F', 'l', 'o', 'o', 'r', ' ']
# All characters except digits

Example 3: Exclude specific punctuation

python

text = "Hello! How are you? I'm fine, thanks."
result = re.findall(r'[^!?,]', text)
print(''.join(result))  # "Hello How are you I'm fine thanks."
# All characters except !, ?, and ,

Example 4: Exclude specific letters

python

text = "abcxyz123"
result = re.findall(r'[^abc]', text)
print(result)  # ['x', 'y', 'z', '1', '2', '3']
# All characters except a, b, and c

3. [a-z] – Matches any character in the range a to z

Description: Matches any lowercase letter from a to z

Example 1: Find all lowercase letters

python

text = "Hello World 123"
result = re.findall(r'[a-z]', text)
print(result)  # ['e', 'l', 'l', 'o', 'o', 'r', 'l', 'd']
# All lowercase letters

Example 2: Find lowercase words

python

text = "Python is AWESOME for coding"
result = re.findall(r'[a-z]+', text)
print(result)  # ['ython', 'is', 'for', 'coding']
# Lowercase words (note: 'Python' becomes 'ython' because P is uppercase)

Example 3: Validate username (lowercase only)

python

username = "john_doe123"
is_valid = bool(re.fullmatch(r'[a-z0-9_]+', username))
print(is_valid)  # True
# Checks if username contains only lowercase letters, digits, and underscores

Example 4: Extract file extensions (lowercase)

python

files = "image.JPG document.txt script.PY readme.md"
result = re.findall(r'\.([a-z]+)', files)
print(result)  # ['txt', 'md']
# Only lowercase file extensions

4. [A-Z] – Matches any character in the range A to Z

Description: Matches any uppercase letter from A to Z

Example 1: Find all uppercase letters

python

text = "Hello World from PYTHON"
result = re.findall(r'[A-Z]', text)
print(result)  # ['H', 'W', 'P', 'Y', 'T', 'H', 'O', 'N']
# All uppercase letters

Example 2: Find acronyms and abbreviations

python

text = "The USA and UK are countries. NASA is a space agency."
result = re.findall(r'[A-Z]{2,}', text)
print(result)  # ['USA', 'UK', 'NASA']
# Words with 2 or more uppercase letters

Example 3: Find sentence starters

python

text = "hello. World! how are you? I am fine."
result = re.findall(r'[A-Z][a-z]*', text)
print(result)  # ['World', 'I']
# Words that start with uppercase letter (likely sentence starters)

Example 4: Validate product codes

python

codes = ["AB123", "CD456", "ef789", "GH-001"]
valid_codes = [code for code in codes if re.fullmatch(r'[A-Z]{2}\d{3}', code)]
print(valid_codes)  # ['AB123', 'CD456']
# Codes with exactly 2 uppercase letters followed by 3 digits

5. [0-9] – Matches any digit

Description: Matches any digit from 0 to 9

Example 1: Extract all digits

python

text = "Phone: 123-456-7890, Age: 25"
result = re.findall(r'[0-9]', text)
print(result)  # ['1', '2', '3', '4', '5', '6', '7', '8', '9', '0', '2', '5']
# All individual digits

Example 2: Find numbers with specific digits

python

text = "Scores: 95, 87, 73, 64, 59, 100"
result = re.findall(r'[5-9][0-9]', text)
print(result)  # ['95', '87', '73', '64', '59']
# Numbers between 50-99

Example 3: Validate PIN code

python

pin = "1234"
is_valid = bool(re.fullmatch(r'[0-9]{4}', pin))
print(is_valid)  # True
# 4-digit PIN code validation

Example 4: Extract years from text

python

text = "Events in 1999, 2005, 2010, and 2020"
result = re.findall(r'[12][0-9]{3}', text)
print(result)  # ['1999', '2005', '2010', '2020']
# Years between 1000-2999

6. [a-zA-Z] – Matches any letter (lowercase or uppercase)

Description: Matches any letter regardless of case

Example 1: Extract all letters

python

text = "Hello World 123! @#$"
result = re.findall(r'[a-zA-Z]', text)
print(result)  # ['H', 'e', 'l', 'l', 'o', 'W', 'o', 'r', 'l', 'd']
# All letters, ignoring case

Example 2: Find words (letters only)

python

text = "Python3 is better than Java8!"
result = re.findall(r'[a-zA-Z]+', text)
print(result)  # ['Python', 'is', 'better', 'than', 'Java']
# Words consisting only of letters

Example 3: Validate name field

python

name = "John Doe"
is_valid = bool(re.fullmatch(r'[a-zA-Z ]+', name))
print(is_valid)  # True
# Name containing only letters and spaces

Example 4: Extract capitalized words

python

text = "The Quick Brown Fox jumps Over The Lazy Dog"
result = re.findall(r'[A-Z][a-z]*', text)
print(result)  # ['The', 'Quick', 'Brown', 'Fox', 'Over', 'The', 'Lazy', 'Dog']
# Words that start with uppercase letter

7. [a-z0-9] – Matches any lowercase letter or digit

Description: Matches lowercase letters or digits

Example 1: Extract alphanumeric characters (lowercase)

python

text = "User123: john_doe, Email: test@example.com"
result = re.findall(r'[a-z0-9]', text)
print(result)  # ['s', 'e', 'r', '1', '2', '3', 'j', 'o', 'h', 'n', 'd', 'o', 'e', 'e', 's', 't', 'e', 'x', 'a', 'm', 'p', 'l', 'e', 'c', 'o', 'm']
# Lowercase letters and digits only

Example 2: Validate username (lowercase alphanumeric)

python

username = "john123"
is_valid = bool(re.fullmatch(r'[a-z0-9]+', username))
print(is_valid)  # True
# Username with only lowercase letters and digits

Example 3: Extract hashtags (lowercase)

python

text = "#Python #coding #PROGRAMMING #java123"
result = re.findall(r'#[a-z0-9]+', text)
print(result)  # ['#coding', '#java123']
# Lowercase hashtags with numbers

Example 4: Find version numbers

python

text = "Versions: v1.2.3, V2.5, v10.0.1"
result = re.findall(r'v([a-z0-9.]+)', text, re.IGNORECASE)
print(result)  # ['1.2.3', '2.5', '10.0.1']
# Version numbers after 'v' (case-insensitive)

8. [A-Z0-9] – Matches any uppercase letter or digit

Description: Matches uppercase letters or digits

Example 1: Extract uppercase alphanumeric characters

python

text = "Product Code: AB123, Serial: XYZ456-789"
result = re.findall(r'[A-Z0-9]', text)
print(result)  # ['P', 'C', 'A', 'B', '1', '2', '3', 'S', 'X', 'Y', 'Z', '4', '5', '6', '7', '8', '9']
# Uppercase letters and digits only

Example 2: Validate license plate format

python

plate = "AB123CD"
is_valid = bool(re.fullmatch(r'[A-Z0-9]+', plate))
print(is_valid)  # True
# License plate with uppercase letters and digits

Example 3: Extract stock symbols

python

text = "Stocks: AAPL, GOOGL, MSFT, TSLA123"
result = re.findall(r'[A-Z0-9]{2,}', text)
print(result)  # ['AAPL', 'GOOGL', 'MSFT', 'TSLA123']
# Uppercase stock symbols with possible numbers

Example 4: Find hexadecimal numbers (uppercase)

python

text = "Colors: #FF0000, #00FF00, #0000FF, #abcdef"
result = re.findall(r'#[A-F0-9]{6}', text)
print(result)  # ['#FF0000', '#00FF00', '#0000FF']
# Uppercase hexadecimal color codes

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