The snippet

string = string.replace(/[^a-zA-Z0-9]/g, '');

Uses RegEx (Regular Expression) to find a paten and then replace it.

This is how it works:

1. The .replace function replaces a specific character or patern in a string (first argument) with another (second argument)
2. The /[^a-zA-Z0-9]/g is the regex. a-z means lower case characters from a-z, A-Z means upper case characters from a-z and 0-9 means number characters from 0-9. The ^ means not, so, not a letter, uppercase letter or number. The g at the end stands for global meaning it will not just find one match for the patern but all the matches
3. The empty string is what to replace the patern with. So if it is not a upper/lower case letter or number character it will be replaced with nothing

Read about RegEx here