Comprehensive RegExp to remove JavaScript comments

Question

I need to dependably remove all JavaScript comments with a single Regular Expression.

I have searched StackOverflow, and other sites, but none take into account alternating quotes, multi-line comments, comments within strings, regular expressions, etc.

Is there any Regular expressions that can remove the comments from this:

var test = [
    "// Code",
    '// Code',
    "'// Code",
    '"// Code',
    //" Comment",
    //' Comment',
    /* Comment */
    // Comment /* Comment
    /* Comment
     Comment // */ "Code",
    "Code",
    "/* Code */",
    "/* Code",
    "Code */",
    '/* Code */',
    '/* Code',
    'Code */',
    /* Comment
    "Comment",
    Comment */ "Code",
    /Code\/*/,
    "Code */"
]

Here's a jsbin or jsfiddle to test it.

Answer 1

I like challenges :)

Here's my working solution:

/((["'])(?:\\[\s\S]|.)*?\2|\/(?![*\/])(?:\\.|\[(?:\\.|.)\]|.)*?\/)|\/\/.*?$|\/\*[\s\S]*?\*\//gm

Replace that with $1.

Fiddle here: http://jsfiddle.net/LucasTrz/DtGq8/6/

Of course, as it has been pointed out countless times, a proper parser would probably be better, but still...

NB: I used a regex literal in the fiddle insted of a regex string, too much escaping can destroy your brain.

---

Breakdown

((["'])(?:\\[\s\S]|.)*?\2|\/(?![*\/])(?:\\.|\[(?:\\.|.)\]|.)*?\/) <-- the part to keep
|\/\/.*?$                                                         <-- line comments
|\/\*[\s\S]*?\*\/                                                 <-- inline comments

The part to keep

(["'])(?:\\[\s\S]|.)*?\2                   <-- strings
\/(?![*\/])(?:\\.|\[(?:\\.|.)\]|.)*?\/     <-- regex literals

Strings

    ["']              match a quote and capture it
    (?:\\[\s\S]|.)*?  match escaped characters or unescpaed characters, don't capture
    \2                match the same type of quote as the one that opened the string

Regex literals

    \/                          match a forward slash
    (?![*\/])                   ... not followed by a * or / (that would start a comment)
    (?:\\.|\[(?:\\.|.)\]|.)*?   match any sequence of escaped/unescaped text, or a regex character class
    \/                          ... until the closing slash

The part to remove

|\/\/.*?$              <-- line comments
|\/\*[\s\S]*?\*\/      <-- inline comments

Line comments

    \/\/         match two forward slashes
    .*?$         then everything until the end of the line

Inline comments

    \/\*         match /*
    [\s\S]*?     then as few as possible of anything, see note below
    \*\/         match */

I had to use [\s\S] instead of . because unfortunately JavaScript doesn't support the regex s modifier (singleline - this one allows . to match newlines as well)

This regex will work in the following corner cases:

• Regex patterns containing / in character classes: /[/]/
• Escaped newlines in string literals

---

Final boss fight

And just for the fun of it... here's the eye-bleeding hardcore version:

/((["'])(?:\\[\s\S]|.)*?\2|(?:[^\w\s]|^)\s*\/(?![*\/])(?:\\.|\[(?:\\.|.)\]|.)*?\/(?=[gmiy]{0,4}\s*(?![*\/])(?:\W|$)))|\/\/.*?$|\/\*[\s\S]*?\*\//gm

This adds the following twisted edge case (fiddle, regex101):

Code = /* Comment */ /Code regex/g  ; // Comment
Code = Code / Code /* Comment */ /g  ; // Comment    
Code = /Code regex/g /* Comment */  ; // Comment

This is highly heuristical code, you probably shouldn't use it (even less so than the previous regex) and just let that edge case blow.

Answer 2

First off, I suggest doing this with a proper JavaScript parser instead. Checkout this previous Q&A: JavaScript parser in JavaScript

For the input you've provided¹, here is a solution that might work:

Match the pattern:

/("(?:[^\r\n\\"]|\\.)*"|'(?:[^\r\n\\']|\\.)*'|\/[^*\/]([^\\\/]|\\.)*\/[gm]*)|\/\/[^\r\n]*|\/\*[\s\S]*?\*\//g

Here's a break down of the pattern:

/
  (                                     # start match group 1
      "(?:[^\r\n\\"]|\\.)*"             #   match a double quoted string
    | '(?:[^\r\n\\']|\\.)*'             #   match a single quoted string
    | \/[^*\/]([^\\\/]|\\.)*\/[gm]*     #   match a regex literal
  )                                     # end match group 1
  | \/\/[^\r\n]*                        # match a single line break
  | \/\*[\s\S]*?\*\/                    # match a multi-line break
/g

and replace it with $1 (match group 1). The trick here is that anything besides a comment is matched in group 1, which get replaced with itself again but comments get replaced with an empty string.

Here's a regexr demo that shows the following replacement:

  var test = [
      "// Code",
      '// Code',
      "'// Code",
      '"// Code',




       "Code",
      "Code",
      "/* Code */",
      "/* Code",
      "Code */",
      '/* Code */',
      '/* Code',
      'Code */',
       "Code",
      /Code\/*/,
      "Code */"
  ]

¹ Again, a parser is the way to go since regex literals might be confused with the division operator. If you have an assignment like var x = a / b / g; in your source, the solution above will break!

Answer 3

I suggest you look at parsing JavaScript using a JavaScript parser of itself and then leverage the parser API to strip out what you don't want. I have not personally done this, but regular expressions should be limited to regular content, which I doubt JS falls into.

Here are some good places to look.

JavaScript parser in JavaScript

Answer 4

Is there any Regular expressions that can remove the comments

No. You cannot build a regex that will match a comment (so that you simply can replace the match with the empty string), because without lookbehind it is impossible to determine whether //" is a comment or the end of a string literal.

You could use a regex as a tokenizer (you "only" need to take care of string literals, regex literals, and the two types of comments), but I'd recommend to use a full-blown JavaScript parser, they are freely available.

Answer 5

test.replace(/(/*([\s\S]?)*/)|(//(.)$)/gm, '');