JavaScript RegExp from a string then running .test() doesn't seem to work

Question

I thought that by instantiating a pattern = new RegExp() and passing a string then using .test(value) would be the same as using /^...$/.test(value), but they don't appear to be equal as using RegExp and passing a string fails each time. Is this not correct? On MDN it seems to say that this should work.

SHOULD ALL FAIL AND THEY DO

var str = "7D>";
var res = /^[A-Za-z0-9\d=!\-@._*]*$/.test(str);
console.log(res); // false which is correct

var patt = /^[A-Za-z0-9\d=!\-@._*]*$/;
var res = patt.test(str);
console.log(res); // false which is correct

var patt = new RegExp("/^[A-Za-z0-9\d=!\-@._*]*$/");
var res = patt.test(str);
console.log(res); // false which is correct, but suspicious based on follow results

SHOULD ALL PASS AND THEY DON'T

var str = "7D";
var res = /^[A-Za-z0-9\d=!\-@._*]*$/.test(str);
console.log(res); // true which is correct

var patt = /^[A-Za-z0-9\d=!\-@._*]*$/;
var res = patt.test(str);
console.log(res); // true which is correct

BUT BOTH THESE ATTEMPTS FAIL WHEN THEY SHOULD PASS

var patt = new RegExp("/^[A-Za-z0-9\d=!\-@._*]*$/");
var res = patt.test(str);
console.log(res); // false which is NOT correct

var patt = new RegExp("^[A-Za-z0-9\d=!\-@._*]*$");
var res = patt.test(str);
console.log(res); // ALSO false which is NOT correct

You have to escape the \- in the string you pass to new RegExp as \\-. Otherwise, put the - at the end of the character set. — user663031
– user663031, Commented Sep 21, 2015 at 3:48
@torazaburo: Beginning, not end. Edit: Hey, interesting, both work. Useful to know. At least one rex engine I used (not JavaScript) took - at the end to mean no upper bound on the character class. — T.J. Crowder
– T.J. Crowder, Commented Sep 21, 2015 at 3:49

T.J. Crowder · Accepted Answer · 2015-09-21 03:47:41Z

5

Two things:

You don't include the / when using a string
You do have to escape backslashes (as always, in a string literal)

So this:

var patt = new RegExp("/^[A-Za-z0-9\d=!\-@._*]*$/");

should be

var patt = new RegExp("^[A-Za-z0-9\\d=!\\-@._*]*$");
//                    ^           ^^   ^^        ^

If you had flags, you'd include them as a second string argument:

var patt = new RegExp("^[a-z\\d=!\\-@._*]*$", "i");

Side note:

\d means "digit" which is defined in the spec as 0-9, so having 0-9 and \d in the character class is redundant

answered Sep 21, 2015 at 3:47

T.J. Crowder

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