How to use the util.isRegExp function in util

expected: expected,
        message: msg,
        operator: 'throws',
      });
    }

    var keys = Object.keys(expected);
    // Special handle errors to make sure the name and the message are compared
    // as well.
    if (expected instanceof Error) {
      keys.push('name', 'message');
    }
    for (var idx = 0; idx < keys.length; ++idx) {
      var key = keys[idx];
      if (typeof actual[key] === 'string' &&
          util.isRegExp(expected[key]) &&
          expected[key].test(actual[key])) {
        continue;
      }
      strictEqual(actual[key], expected[key], msg);
    }
    return true;
  }
  // Guard instanceof against arrow functions as they don't have a prototype.
  if (expected.prototype !== undefined && actual instanceof expected) {
    return true;
  }
  if (Error.isPrototypeOf(expected)) {
    return false;
  }
  return expected.call({}, actual) === true;
}

for (var i = 0; i < actual.length; i++) {
      if (actual[i] !== expected[i]) return false;
    }

    return true;
  */

  // 7.2. If the expected value is a Date object, the actual value is
  // equivalent if it is also a Date object that refers to the same time.
  } else if (util.isDate(actual) && util.isDate(expected)) {
    return actual.getTime() === expected.getTime();

  // 7.3 If the expected value is a RegExp object, the actual value is
  // equivalent if it is also a RegExp object with the same source and
  // properties (`global`, `multiline`, `lastIndex`, `ignoreCase`).
  } else if (util.isRegExp(actual) && util.isRegExp(expected)) {
    return actual.source === expected.source &&
           actual.global === expected.global &&
           actual.multiline === expected.multiline &&
           actual.lastIndex === expected.lastIndex &&
           actual.ignoreCase === expected.ignoreCase;

  // 7.4. Other pairs that do not both pass typeof value == 'object',
  // equivalence is determined by ==.
  } else if (!util.isObject(actual) && !util.isObject(expected)) {
    return actual == expected;

  // 7.5 For all other Object pairs, including Array objects, equivalence is
  // determined by having the same number of owned properties (as verified
  // with Object.prototype.hasOwnProperty.call), the same set of keys
  // (although not necessarily the same order), equivalent values for every
  // corresponding key, and an identical 'prototype' property. Note: this

for (var i = 0; i < actual.length; i++) {
      if (actual[i] !== expected[i]) return false;
    }

    return true;

  // 7.2. If the expected value is a Date object, the actual value is
  // equivalent if it is also a Date object that refers to the same time.
  } else if (util.isDate(actual) && util.isDate(expected)) {
    return actual.getTime() === expected.getTime();

  // 7.3 If the expected value is a RegExp object, the actual value is
  // equivalent if it is also a RegExp object with the same source and
  // properties (`global`, `multiline`, `lastIndex`, `ignoreCase`).
  } else if (util.isRegExp(actual) && util.isRegExp(expected)) {
    return actual.source === expected.source &&
           actual.global === expected.global &&
           actual.multiline === expected.multiline &&
           actual.lastIndex === expected.lastIndex &&
           actual.ignoreCase === expected.ignoreCase;

  // 7.4. Other pairs that do not both pass typeof value == 'object',
  // equivalence is determined by ==.
  } else if (!util.isObject(actual) && !util.isObject(expected)) {
    return actual == expected;

  // 7.5 For all other Object pairs, including Array objects, equivalence is
  // determined by having the same number of owned properties (as verified
  // with Object.prototype.hasOwnProperty.call), the same set of keys
  // (although not necessarily the same order), equivalent values for every
  // corresponding key, and an identical 'prototype' property. Note: this

for (var i = 0; i < actual.length; i++) {
      if (actual[i] !== expected[i]) return false;
    }

    return true;

    // 7.2. If the expected value is a Date object, the actual value is
    // equivalent if it is also a Date object that refers to the same time.
  } else if (util.isDate(actual) && util.isDate(expected)) {
    return actual.getTime() === expected.getTime();

    // 7.3 If the expected value is a RegExp object, the actual value is
    // equivalent if it is also a RegExp object with the same source and
    // properties (`global`, `multiline`, `lastIndex`, `ignoreCase`).
  } else if (util.isRegExp(actual) && util.isRegExp(expected)) {
    return (
      actual.source === expected.source &&
      actual.global === expected.global &&
      actual.multiline === expected.multiline &&
      actual.lastIndex === expected.lastIndex &&
      actual.ignoreCase === expected.ignoreCase
    );

    // 7.4. Other pairs that do not both pass typeof value == 'object',
    // equivalence is determined by ==.
  } else if (!util.isObject(actual) && !util.isObject(expected)) {
    return actual == expected;

    // 7.5 For all other Object pairs, including Array objects, equivalence is
    // determined by having the same number of owned properties (as verified
    // with Object.prototype.hasOwnProperty.call), the same set of keys

for (var i = 0; i < actual.length; i++) {
      if (actual[i] !== expected[i]) return false;
    }

    return true;

  // 7.2. If the expected value is a Date object, the actual value is
  // equivalent if it is also a Date object that refers to the same time.
  } else if (util.isDate(actual) && util.isDate(expected)) {
    return actual.getTime() === expected.getTime();

  // 7.3 If the expected value is a RegExp object, the actual value is
  // equivalent if it is also a RegExp object with the same source and
  // properties (`global`, `multiline`, `lastIndex`, `ignoreCase`).
  } else if (util.isRegExp(actual) && util.isRegExp(expected)) {
    return actual.source === expected.source &&
           actual.global === expected.global &&
           actual.multiline === expected.multiline &&
           actual.lastIndex === expected.lastIndex &&
           actual.ignoreCase === expected.ignoreCase;

  // 7.4. Other pairs that do not both pass typeof value == 'object',
  // equivalence is determined by ==.
  } else if (!util.isObject(actual) && !util.isObject(expected)) {
    return actual == expected;

  // 7.5 For all other Object pairs, including Array objects, equivalence is
  // determined by having the same number of owned properties (as verified
  // with Object.prototype.hasOwnProperty.call), the same set of keys
  // (although not necessarily the same order), equivalent values for every
  // corresponding key, and an identical 'prototype' property. Note: this

throw "You cannot mix operators and normal fields";
    }

    // queryValue is an object of this form: { $comparisonOperator1: value1, ... }
    if (dollarFirstChars.length > 0) {
      for (i = 0; i < keys.length; i += 1) {
        if (!comparisonFunctions[keys[i]]) { throw "Unknown comparison function " + keys[i]; }

        if (!comparisonFunctions[keys[i]](objValue, queryValue[keys[i]])) { return false; }
      }
      return true;
    }
  }

  // Using regular expressions with basic querying
  if (util.isRegExp(queryValue)) { return comparisonFunctions.$regex(objValue, queryValue); }

  // queryValue is either a native value or a normal object
  // Basic matching is possible
  if (!areThingsEqual(objValue, queryValue)) { return false; }

  return true;
}

keys = Object.keys(queryValue);      
      for (i = 0; i < keys.length; i += 1) {
        if (arrayComparisonFunctions[keys[i]]) { return matchQueryPart(obj, queryKey, queryValue, true); }
      }
    }

    // If not, treat it as an array of { obj, query } where there needs to be at least one match
    for (i = 0; i < objValue.length; i += 1) {
      if (matchQueryPart({ k: objValue[i] }, 'k', queryValue)) { return true; }   // k here could be any string
    }
    return false;
  }

  // queryValue is an actual object. Determine whether it contains comparison operators
  // or only normal fields. Mixed objects are not allowed
  if (queryValue !== null && typeof queryValue === 'object' && !util.isRegExp(queryValue)) {
    keys = Object.keys(queryValue);
    firstChars = utilities.map(keys, function (item) { return item[0]; });
    dollarFirstChars = utilities.filter(firstChars, function (c) { return c === '$'; });

    if (dollarFirstChars.length !== 0 && dollarFirstChars.length !== firstChars.length) {
      throw "You cannot mix operators and normal fields";
    }

    // queryValue is an object of this form: { $comparisonOperator1: value1, ... }
    if (dollarFirstChars.length > 0) {
      for (i = 0; i < keys.length; i += 1) {
        if (!comparisonFunctions[keys[i]]) { throw "Unknown comparison function " + keys[i]; }

        if (!comparisonFunctions[keys[i]](objValue, queryValue[keys[i]])) { return false; }
      }
      return true;

comparisonFunctions.$regex = function(a, b) {
  if (!util.isRegExp(b)) {
    throw new Error('$regex operator called with non regular expression');
  }

  if (typeof a !== 'string') {
    return false;
  } else {
    return b.test(a);
  }
};

exports.buildRegExp = function (stringSplitByComma) {
    if (util.isRegExp(stringSplitByComma)) {
        return stringSplitByComma;
    }

    if (!stringSplitByComma) {
        return /.*/i;
    }

    var array = String(stringSplitByComma).replace(/[^a-z\d_,\s]/gi, '\\$&').split(/\s*,\s*/);
    var reg = array.length === 1 ? array[0] : '(' + array.join('|') + ')';
    return new RegExp('\\.' + reg + '$', 'i');
};