How to check if the input string is a valid Regular expression?

Question

How do you check, in JavaScript, if a string is a proper regular expression that will compile?

For example, when you execute the following javascript, it produces a

Answer 1

this function could handle the '/' char as a normal char in regex, and also consider escaping when is a common string. it will always return an Regex, null if not a good regex string.

function getRegex(regex) {
    try {
        regex = regex.trim();
        let parts = regex.split('/');
        if(regex[0] !== '/' || parts.length< 3){
          regex = regex.replace(/[.*+\-?^${}()|[\]\\]/g, '\\$&'); //escap common string
          return new RegExp(regex);
        }

        const option =parts[parts.length - 1];
        const lastIndex = regex.lastIndexOf('/');
        regex = regex.substring(1, lastIndex);
        return new RegExp(regex, option);
    } catch (e) {
        return null
    }
}

console.log(getRegex('ab/c'))
let htmlStartTag = getRegex('/<(?!/)(?!br)(.+?)(?<!/)>/mgs');
console.log(htmlStartTag)
let result = `</button><input id="warehouse-search-field-tablet"
class="form-control search-field"
 title="Warehouse Search Field" name="location" type="search" value="">content`.matchAll(htmlStartTag);
 console.log([...result])

Answer 2

The question is solved, but if someone needs to define is the string either valid RegExp or not a RegExp at all.

You can use new Function() and templating inside of the function body with try ... catch and new RegExp() as mentioned earlier.

There is a snippet with the explanations:

const isRegExp = (string) => {
    try {
        return new Function(`
            "use strict";
            try {
                new RegExp(${string});
                return true;
            } catch (e) {
                return false;
            }
        `)();
    } catch(e) {
        return false;
    }
};

// Here the argument 'simplyString' shall be undefined inside of the function
// Function(...) catches the error and returns false
console.log('Is RegExp valid:', isRegExp('simplyString'));

// Here the argument shall cause a syntax error
// isRegExp function catches the error and returns false
console.log('Is RegExp valid:', isRegExp('string which is not a valid regexp'));

// Here the argument is not a valid RegExp, new RegExp(...) throws an error
// Function(...) catches the error and returns false
console.log('Is RegExp valid:', isRegExp('abc ([a-z]+) ([a-z]+))'));

// Valid RegExp, passed as a string
console.log('Is RegExp valid:', isRegExp('/^[^<>()[\]\\.,;:\s@\"]$/'));

// Valid RegExp, passed as a RegExp object
console.log('Is RegExp valid:', isRegExp(/^[^<>()[\]\\.,;:\s@\"]$/));

// Howewer, the code injection is possible here
console.log('Is RegExp valid:', isRegExp(');console.log("This is running inside of the Function(...) as well"'));

Answer 3

Here is a little function that checks the validity of both types of regexes, strings or patterns:

function validateRegex(pattern) {
    var parts = pattern.split('/'),
        regex = pattern,
        options = "";
    if (parts.length > 1) {
        regex = parts[1];
        options = parts[2];
    }
    try {
        new RegExp(regex, options);
        return true;
    }
    catch(e) {
        return false;
    }
}

The user will be able to test both test and /test/g for example. Here is a working fiddle.

Answer 4

function isRegExp(regExp){
          try {
                new RegExp(regExp);
              } catch(e) {
                return false
              }
         return true
    }

ex:
isRegExp(/@(\w+)/g) = true

Answer 5

You can use try/catch and the RegExp constructor:

var isValid = true;
try {
    new RegExp("the_regex_to_test_goes_here");
} catch(e) {
    isValid = false;
}

if(!isValid) alert("Invalid regular expression");