Python教程：如何判断字符串是否为纯英文 | 编程技巧

def is_english_alpha(input_str):
    """检查字符串是否只包含英文字母"""
    return input_str.isalpha()

# 测试示例
print(is_english_alpha("Hello"))     # True
print(is_english_alpha("Python3"))   # False（包含数字）
print(is_english_alpha("你好"))      # False（中文）
print(is_english_alpha("Hello World")) # False（包含空格）

import re

def is_english_regex(input_str, allow_spaces=True, allow_punctuation=False):
    """使用正则表达式检查字符串是否为纯英文"""
    pattern = r'^[A-Za-z'
    
    if allow_spaces:
        pattern += r' '
    if allow_punctuation:
        pattern += r'.,;:!?\'"'
        
    pattern += r']+$'
    
    return bool(re.match(pattern, input_str))

# 测试示例
print(is_english_regex("Hello"))           # True
print(is_english_regex("Hello World"))     # True（允许空格）
print(is_english_regex("Hello, World!"))   # False（默认不允许标点）
print(is_english_regex("Hello, World!", allow_punctuation=True))  # True

def is_english_ascii(input_str, allow_spaces=True):
    """通过ASCII值检查字符串是否为纯英文"""
    for char in input_str:
        ascii_val = ord(char)
        # 检查是否在A-Z或a-z范围内
        is_upper = 65 <= ascii_val <= 90
        is_lower = 97 <= ascii_val <= 122
        
        # 如果允许空格，检查空格字符（ASCII 32）
        if allow_spaces and ascii_val == 32:
            continue
            
        if not (is_upper or is_lower):
            return False
    return True

# 测试示例
print(is_english_ascii("Python"))         # True
print(is_english_ascii("Python Coding"))  # True
print(is_english_ascii("Python编程"))     # False
print(is_english_ascii("123"))            # False

def validate_username(username):
    return username.isalpha() and 3 <= len(username) <= 20

def filter_english_texts(texts):
    return [text for text in texts 
            if is_english_regex(text, allow_spaces=True)]

def check_file_english(file_path):
    with open(file_path, 'r', encoding='utf-8') as file:
        for line in file:
            if not is_english_ascii(line.strip()):
                return False
    return True