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 an error.
var regex = new RegExp('abc ([a-z]+) ([a-z]+))');
// produces:
// Uncaught SyntaxError: Invalid regular expression: /abc ([a-z]+) ([a-z]+))/: Unmatched ')'
How does one determine if a string will be a valid regex or not?
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");
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.
None of the answers here satisfied my need for checking that a string is a valid regex for other languages (Mostly php), because they either ignore the flags, the delimiters or escaped special characters, so I made my own function
function isValidRegex(s) {
try {
const m = s.match(/^([/~@;%#'])(.*?)\1([gimsuy]*)$/);
return m ? !!new RegExp(m[2],m[3])
: false;
} catch (e) {
return false
}
}
console.log(isValidRegex('abc')) //False
console.log(isValidRegex('/abc/')) //True
console.log(isValidRegex('/ab#\/[c]/ig')) //True
console.log(isValidRegex('@ab#\/[c]@ig')) //Special delimiters: True
console.log(isValidRegex('/ab\/[c/ig')) //False
console.log(isValidRegex('/abc/gig')) //FalseYou can also derive this function to transform a string into a RegExp object
function stringToRegex(s) {
const m = s.match(/^([/~@;%#'])(.*?)\1([gimsuy]*)$/);
return m ? new RegExp(m[2], m[3]) : new RegExp(s);
}
console.log(stringToRegex('abc'))
console.log(stringToRegex('/abc/'))
console.log(stringToRegex('/ab#\/[c]/ig'))
console.log(stringToRegex('@ab#\/[c]@ig'))
try {
console.log(stringToRegex('/ab#\/[c/ig'))
} catch (e) {
console.log('Not a valid regex')
}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])
function isRegExp(regExp){
try {
new RegExp(regExp);
} catch(e) {
return false
}
return true
}
ex:
isRegExp(/@(\w+)/g) = true
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"'));
