How to split a long regular expression into multiple lines in JavaScript?

You could convert it to a string and create the expression by calling new RegExp():

var myRE = new RegExp (['^(([^<>()[\]\\.,;:\\s@\"]+(\\.[^<>(),[\]\\.,;:\\s@\"]+)*)',                        '|(\\".+\\"))@((\\[[0-9]{1,3}\\.[0-9]{1,3}\\.[0-9]{1,3}\\.',                        '[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\\.)+',                        '[a-zA-Z]{2,}))$'].join(''));

Notes:

1. when converting the expression literal to a string you need to escape all backslashes as backslashes are consumed when evaluating a string literal. (See Kayo's comment for more detail.)

2. RegExp accepts modifiers as a second parameter

   /regex/g => new RegExp('regex', 'g')

[Addition ES20xx (tagged template)]

In ES20xx you can use tagged templates. See the snippet.

Note:

• Disadvantage here is that you can't use plain whitespace in the regular expression string (always use \s, \s+, \s{1,x}, \t, \n etc).

(() => {  const createRegExp = (str, opts) =>     new RegExp(str.raw[0].replace(/\s/gm, ""), opts || "");  const yourRE = createRegExp`    ^(([^<>()[\]\\.,;:\s@\"]+(\.[^<>()[\]\\.,;:\s@\"]+)*)|    (\".+\"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|    (([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$`;  console.log(yourRE);  const anotherLongRE = createRegExp`    (\byyyy\b)|(\bm\b)|(\bd\b)|(\bh\b)|(\bmi\b)|(\bs\b)|(\bms\b)|    (\bwd\b)|(\bmm\b)|(\bdd\b)|(\bhh\b)|(\bMI\b)|(\bS\b)|(\bMS\b)|    (\bM\b)|(\bMM\b)|(\bdow\b)|(\bDOW\b)    ${"gi"}`;  console.log(anotherLongRE);})();