如何在正则表达式中使用变量？

import re

# 定义变量
variable = "example"
pattern = f"{variable}"

# 构建正则表达式
regex = re.compile(pattern)

# 测试正则表达式
test_string = "This is an example string."
match = regex.search(test_string)

if match:
    print("Match found!")
else:
    print("No match found.")

import re

# 定义变量
variable = "example.com"
pattern = f"^{re.escape(variable)}$"

# 构建正则表达式
regex = re.compile(pattern)

# 测试正则表达式
test_string = "example.com"
match = regex.search(test_string)

if match:
    print("Match found!")
else:
    print("No match found.")