Commenting regular expressions

Question

Are there any common practises for commenting the regular expressions: inline comments referring different part of RegEx or general comment for all expression?

Answer 1

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.

Answer 2

Typically, I will write a regex and not explain the individual pieces of the regex, but rather what it's purpose is. That is that what and why. This is a bit like asking "What should my comments look like?" to which one would say "Don't write what the code is doing, write why the code is doing what it does"

// Strip the leading "?" and remove the query parameters "offset=<integer>" & "count=<integer> so we have a pattern of the request"          
var search = location.search.substring(1).replace(/offset=[0-9]+?&/g, "").replace(/count=[0-9]+?&/g, "");

Unless you are trying to teach someone about regexes via comments in code, I don't think explaining what each individual piece will do. When working with other programmers, you can safely assume that one would know something as global regular expressions.

Answer 3

I guess it really depends on how you're putting the regex together. Generally speaking I think it would be a bad idea to put comments within the actual regex string itself (not possible in most scenarios, as far as I know). If you really need to comment specific portions of a regular expression (are you trying to teach someone?), then break each chunk into separate strings on their own lines, and comment each line using the normal commenting process for your programming language. Otherwise, pleinolijf's answer is pretty good.

example:

string myregex = "\s" // Match any whitespace once
+ "\n"  // Match one newline character
+ "[a-zA-Z]";  // Match any letter