Regular expression to find and remove comments in CSS

Question

I'm writing my own minifying tool for practice (regular expresssions practice), but after a few tutorials I'm still not getting it.

For example I'm trying to find and remove all comments from my CSS file and that includes:

Single line comments as in

/** single line comment ****/ or

/****single line comment */ and
Multi line comments as in

/**** start of comment

.myCssClass

{
```
font:13pt Arial;
```
}

********* end of comment **/

So far I'm using an expression which can only deal with single line comments as follows

(\/\*.*\*\/)

But what I'm trying to understand about regular expressions is how do I tell the regex engine to span lines as well. I did try this:

(\/\*[.\n]*\*\/)

which doesn't work at all.

Anyone know where I'm going wrong?

Thanks, Jacques

Typically when you read in a file line-by-line you don't use a regex to span multiple lines. For this you would regex the start of a comment and keep reading in lines to omit until you reach the end comment regex. — user17753
– user17753, Commented May 11, 2012 at 15:45

Community · Accepted Answer · 2017-05-23 10:29:14Z

3

If you're running the match in C#, have you tried RegexOptions?

Match m = Regex.Match(word, pattern, RegexOptions.Multiline);

"Multiline mode. Changes the meaning of ^ and $ so they match at the beginning and end, respectively, of any line, and not just the beginning and end of the entire string."

Also see Strip out C Style Multi-line Comments

EDIT:

OK..looks like an issue w/ the regex. Here is a working example using the regex pattern from http://ostermiller.org/findcomment.html. This guy does a good job deriving the regex, and demonstrating the pitfalls and deficiencies of various approaches. Note: RegexOptions.Multiline/RegexOptions.Singleline does not appear to affect the result.

string input = @"this is some stuff right here
    /* blah blah blah 
    blah blah blah 
    blah blah blah */ and this is more stuff /* blah */
    right here.";

string pattern = @"(/\*([^*]|[\r\n]|(\*+([^*/]|[\r\n])))*\*+/)";
string output = Regex.Replace(input, pattern, string.Empty, RegexOptions.Singleline);

edited May 23, 2017 at 10:29

CommunityBot

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answered May 11, 2012 at 15:44

ltiong_dbl

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1 Comment

Jacques Over a year ago

thanks for the input. I'm using the multiline option yes, which has certainly helped for one or two other items.

Kaz · Accepted Answer · 2012-05-11 17:03:14Z

2

A regular expression which matches C-style comments (which begin with /*, end with */ and do not nest) is:

[/][*]([^*]|[*]*[^*/])*[*]+[/]

(I have a little write up about the derivation of this. See: www.nongnu.org/txr/txr-manpage.html Look for "Appendix A" in the table of contents, and there is a link to "Example: Matching C Language Comments".)

C-style comments can include the sequence /* in the interior, such that /*/**/ is a valid comment. The closest */ terminates the comment so that /* */aaa/* */ is two comments with aaa in between, not one comment. This "non-greedy" behavior complicates the matching in a regex language which has no non-greedy operator.

answered May 11, 2012 at 17:03

Kaz

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