To replace a string before another string using regex, you can use the re.sub() function in Python's re module. Here's an example:
import re
text = "Hello, world! I am learning regex."
new_text = re.sub(r'Hello, world', 'Hi', text)
print(new_text)
In this example, we are replacing the string "Hello, world" with "Hi" in the original text using the re.sub() function.