We can explain our regular expression within the expression using comments.

Syntax:
(?#Commented Code)

Suppose we have a two strings below:
Hello world
hello World

Regex to match these two lines without comments:

(?i)(hello)\s(world)

Regex to match these two lines with comments:

(?#It is a case insensitive flag)(?i)(?#Will match 'hello')(hello)(?#Matches any whitespace character like space, tab, newline etc)\s(?#Will match 'world')(world)

(?#It is a case insensitive flag) and others with (?#...) are comments in this regular expression and does not affect pattern matching.

You can test these regular expressions in the link : https://regex101.com/