Question

I am trying to write a regex (in JavaScript) that will match a multi line comment at the beginning of a JS file.

So far, I came up with this: /^(\/\*[^\*\/]*\*\/)/g

It works for a single line comment: https://regex101.com/r/ZS5PVI/1

But my problem is that it does not work for a multi line comment: https://regex101.com/r/ZS5PVI/2

Do you have any ideas how to solve it?

Answer 1

Like HTML, JavaScript cannot be parsed by regular expressions. Attempting to do so correctly is futile.

Instead, you must use a parser that will correctly transform JavaScript source code into an AST, which you may inspect programmatically. Fortunately, there’s libraries that do the parsing for you.

Here’s a working example that outputs the AST of this code:

/* this is a
multi-line
comment */

var test = "this is a string, /* and this is not a comment! */";

// ..but this is

Which gets us:

[
  "toplevel",
  [
    [
      {
        "name": "var",
        "start": {
          "type": "keyword",
          "value": "var",
          "line": 5,
          "col": 4,
          "pos": 57,
          "endpos": 60,
          "nlb": true,
          "comments_before": [
            {
              "type": "comment2",
              "value": " this is a\n    multi-line\n    comment ",
              "line": 1,
              "col": 4,
              "pos": 5,
              "endpos": 47,
              "nlb": true
            }
          ]
        },
        "end": {
          "type": "punc",
          "value": ";",
          "line": 5,
          "col": 67,
          "pos": 120,
          "endpos": 121,
          "nlb": false,
          "comments_before": []
        }
      },
      [
        [
          "test",
          [
            {
              "name": "string",
              "start": {
                "type": "string",
                "value": "this is a string, /* and this is not a comment! */",
                "line": 5,
                "col": 15,
                "pos": 68,
                "endpos": 120,
                "nlb": false,
                "comments_before": []
              },
              "end": {
                "type": "string",
                "value": "this is a string, /* and this is not a comment! */",
                "line": 5,
                "col": 15,
                "pos": 68,
                "endpos": 120,
                "nlb": false,
                "comments_before": []
              }
            },
            "this is a string, /* and this is not a comment! */"
          ]
        ]
      ]
    ]
  ]
]

Now it’s just a matter of looping over the AST and extracting what you need.