Are there any common practises for commenting the regular expressions: inline comments referring different part of RegEx or general comment for all expression?
Commenting regular expressions
documentation;coding style;comments
In my view, a good practice is to concisely state in comments what the general idea of the regular expression is.  This saves other developers (or sometimes yourself) the hassle of copy-pasting the regex in a parser like RegExr, only to understand what it does.