Javascript RegExp.prototype.test - how to regex values coming from an array

Question

I want to check each values from my array if it matches the string, however it doesn't work when i'm using console.log(/^reg_arr[i]/.test(str1)); but when I use console.log(/^table/.test(str1)); it works

var str1 = 'table football';
var reg_arr = ["table","football"];
        
for(var i = 0, reg_arr_length = reg_arr.length; i < reg_arr_length; i++)
{
   console.log(/^reg_arr[i]/.test(str1)); /* doesnt work */
}

console.log(/^table/.test(str1)); /* this works */

Is there a problem with my syntax,

Thank you,

I wrote a long answer about how you can create regular expressions from strings, but if you can also change each element in your array into a regular expression by surrounding it with slashes instead of quotes, then get rid of the slashes when testing it. reg_arr = [/table/]; reg_arr[i].test(str1) — Matthias
– Matthias, Commented Mar 15, 2018 at 4:39
Also, the caret (^) at the beginning of your regular expression means match only at the beginning of the string, so /^football/.test("table football") would fail, because it's looking for the start of the string, then "football". I don't know if that's intentional. — Matthias
– Matthias, Commented Mar 15, 2018 at 4:45

Kevin Qian · Accepted Answer · 2018-03-15 03:25:21Z

1

Use new RegExp(source: string[, flags: string]) (simply RegExp(source: string[, flags: string]) is also working)

var str1 = 'table football';
var reg_arr = ["table","football"];
        
for(var i = 0, reg_arr_length = reg_arr.length; i < reg_arr_length; i++)
{
   console.log(new RegExp(`^${reg_arr[i]}`).test(str1));
}

edited Mar 15, 2018 at 3:25

answered Mar 15, 2018 at 3:06

Kevin Qian

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1 Comment

Kevin Qian Over a year ago

@apelidoko just concat a '^' at the beginning, since we are dealing with string as source now. I modified my code and used a ES6 string interpolation to achieve that, but you can just '^' + reg_arr[i]

Matthias · Accepted Answer · 2018-03-15 04:43:01Z

Using slashes to define a regular expression doesn't allow you to enter variables into the regular expression, that is, in the following example, str1 is checked to contain "test", not the value of the variable test.

var str1 = "hello";
var str2 = "Hello world";
console.log(/str1/i.test(str2))

To solve this issue, you need to create a new regular expression for each value in the array, something that can be done with new RegExp(str), instead of slashes.

var str1 = "hello";
var str2 = "hello world";
console.log(new RegExp(str1, "i").test(str2))

However, this method has a catch, characters from that string are not escaped. So new RegExp("test.hello") will match "test hello", because the dot is interpreted as a regular expression wildcard character and not a period. To fix that, we first have to manually escape any special characters in our search string (unless the things reg_arr are actually regular expressions). I took the code to do that from the middle of this MDN page.

var str1 = "hello"
var str2 = "hello world"

str1.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
console.log(new RegExp(str, "i").test(str2))

So without changing anything else, your final console.log could look like this:

console.log(new RegExp(reg_arr[i].replace(/[.*+?^${}()|[\]\\]/g, '\\$&')).test(str1));

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