Regular expression to remove JavaScript double slash (//) style comments

Question

I'm trying to do remove JavaScript comments via a regular expression in C# and have become stuck. I want to remove any occurrences of double slash // style comments.

My current regex is (?<!:)//[^\r\n]* which will catch all comments and prevent matching of http://. However, the negative lookbehind was lazy and of course bit me back in the following test case:

var XSLPath = "//" + Node;

So I'm looking for a regular expression that will perform a lookbehind to see if an even number of double quotes (") occurs before the match. I'm not sure if this is possible. Or is there maybe a better way to do this?

Always use [Regex Coach](http://weitz.de/regex-coach/) – bobobobo Dec 23 '10 at 21:13 — bobobobo, Dec 23 '10 at 21:13

Steve Wortham · Accepted Answer · 2009-10-15T03:05:05.833

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(Updated based on comments)

It looks like this works pretty well:

(?<=".*".*)//.*$|(?<!".*)//.*$

It appears that the test cases in Regex Hero show that it'll match comments the way I think it should (almost).

For instance, it'll completely ignore this line:

var XSLPath = "//" + Node;

But it's smart enough to match the comment at the end of this line:

var XSLPath = "//"; // stuff to remove

However, it's not smart enough to know how to deal with 3 or more quotation marks before the comment. I'm not entirely sure how to solve that problem without hard-coding it. You need some way to allow an even number of quotes.

edited Oct 15 '09 at 03:05

answered Oct 14 '09 at 22:20

Steve Wortham

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However, that's easily changed via `(?<![":])//.*(?!")$` – Gavin Miller Oct 14 '09 at 22:28
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Fails on something like this: "text // more text" – Joel Oct 14 '09 at 22:30
@LFSR Consulting -- Nicely done, your modification seems to solve that problem. Hope you don't mind but I updated my answer with your fix. – Steve Wortham Oct 14 '09 at 22:37
Joel, it seems you'd need some way to allow an even number of quotes before the slashes, but not an odd number. Hmm... – Steve Wortham Oct 14 '09 at 22:59
I updated my regex again. This will match comments even if two quotes appear before it, but not 1 quote. – Steve Wortham Oct 14 '09 at 23:11
The regex looks good - thanks for all the effort, it's much appreciated! – Gavin Miller Oct 15 '09 at 14:23
You're welcome. It's still not perfect, but I'm glad it works for you. – Steve Wortham Oct 15 '09 at 16:04
If you really want an edge case, try it on this JS snippet: `var regex = /foo\//; //<- that first // is not a comment : )` – sstur May 18 '13 at 18:03

Regular expression to remove JavaScript double slash (//) style comments

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