Eslint rule to forbid writing non-ASCII characters

non-ascii.js

/**
 * @fileoverview Rule to forbid writing non-ASCII characters.
 * @author amagitakayosi
 */

"use strict";

/**
 * ASCII characters.
 */
var ASCII_REGEXP = new RegExp('([^\x00-\x7F]+)', 'g');

/**
 * Allowed types of token for non-ASCII characters.
 */
var ALLOWED_TOKENS = ['RegularExpression', 'String'];

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

module.exports = function (context) {
    var configuration = context.options[0] || {};

    return {
        Program: function checkForForbiddenCharacters(node) {
            var allowedChars = configuration.allowedChars || '';
            var allowedCharsRegExp = new RegExp('[' + allowedChars + ']+', 'g');

            var errors = [];

            var sourceCode = context.getSourceCode();
            var tokens = sourceCode.getTokens(node);
            tokens.forEach(function (token) {
                if (ALLOWED_TOKENS.indexOf(token.type) !== -1) {
                    return;
                }
                var value = token.value;
                var matches = value.replace(allowedCharsRegExp, '').match(ASCII_REGEXP);
                if (matches) {
                    errors.push({
                        line: token.loc.start.line,
                        column: token.loc.start.column + value.indexOf(matches[0]),
                        char: matches[0]
                    });
                }
            });

            errors.forEach(function (error) {
                context.report(node, {
                    line: error.line,
                    column: error.column
                }, 'Non-ASCII character "' + error.char + '" found.');
            });
        }
    };
};

module.exports.schema = [{
    type: 'object',
    properties: {
        allowedChars: {
            type: 'string'
        }
    },
    additionalProperties: false
}];

@mitchellh can we allow only emoji?