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To substitute any hostname within a URL using regex, you can use the following method:

1. Identify the hostname pattern in the URL using regular expressions. This pattern can vary depending on the specific URL you are working with, but generally, it will include the domain name, such as "www.example.com".

2. Use a regular expression replacement function to replace the hostname with the desired replacement hostname. This function can be built into many programming languages, such as Python or JavaScript.

3. Apply the replacement function to the URL string to substitute the original hostname with the new one.

For example, in Python, the regex pattern to match a typical hostname in a URL may be:

import re

url = "https://www.example.com/path/to/file.html"
new_hostname = "new.example.com"

pattern = re.compile(r"(https?://)(www\.)?example\.com")

new_url = pattern.sub(r"\g<1>" + new_hostname, url)

print(new_url)

This code will output the new URL string:

https://new.example.com/path/to/file.html

Here, the regular expression pattern matches the scheme (http or https), and optional "www" subdomain of the original hostname, and the domain name itself ("example.com"). The replacement function then substitutes these matched portions with the new hostname passed as an argument.