And that depends on if regular expressions are a basic language feature. The proper way to "explain" or "comment" a complex regular expression is not to include a single line comment near it, but to use the /x modifier that allows you to "extend your pattern’s legibility by permitting whitespace and comments". That is, of course, assuming you're using a language where regular expressions are first class types and has syntax to support regular expressions as literals (if you have to include your regular expression in a quoted string, your language doesn't), and uses PCRE.