eslint/lib/rules/no-empty-class.js

/**
 * @fileoverview Rule to flag the use of empty character classes in regular expressions
 * @author Ian Christian Myers
 */

//------------------------------------------------------------------------------
// Rule Definition
//------------------------------------------------------------------------------

module.exports = function(context) {

    "use strict";

    return {

        "Literal": function(node) {
            var tokens = context.getTokens(node);
            tokens.forEach(function (token) {
                /*
                plain-English description of the following regexp:
                0. `^` fix the match at the beginning of the string
                1. `\/`: the `/` that begins the regexp
                2. `([^\\[]|\\.|\[([^\\\]]|\\.)+\])*`: regexp contents; 0 or more of the following
                  2.0. `[^\\[]`: any character that's not a `\` or a `[` (anything but escape sequences and character classes)
                  2.1. `\\.`: an escape sequence
                  2.2. `\[([^\\\]]|\\.)+\]`: a character class that isn't empty
                3. `\/` the `/` that ends the regexp
                4. `[gimy]*`: optional regexp flags
                5. `$`: fix the match at the end of the string
                */
                if (token.type === "RegularExpression" &&
                !/^\/([^\\[]|\\.|\[([^\\\]]|\\.)+\])*\/[gimy]*$/.test(token.value)) {
                    context.report(node, "Empty class.");
                }
            });
        }

    };

};