str = str.replace(/[^a-z0-9-]/g, '');

/* Everything between the indicates what your are looking for

    / is here to delimit your pattern so you have one to start and one to end
    [] indicates the pattern your are looking for on one specific character
    ^ indicates that you want every character NOT corresponding to what follows
    a-z matches any character between 'a' and 'z' included
    0-9 matches any digit between '0' and '9' included (meaning any digit)
    - the '-' character
    g at the end is a special parameter saying that you do not want you regex to stop on the first character matching your pattern but to continue on the whole string

Then your expression is delimited by / before and after. So here you say "every character not being a letter, a digit or a '-' will be removed from the string". */