Question: I am trying to do a basic string replace using a regex expression, but the answers I have found do not seem to help - they are directly answering each persons unique requirement with little or no explanation.

I am using `str = str.replace(/[^a-z0-9+]/g, '');` at the moment. But what I would like to do is allow all alphanumeric characters (a-z and 0-9) and also the '-' character.

Could you please answer this and explain how you concatenate expressions.

Answer: 
This should work :

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

```

Everything between the indicates what your are looking for

1. `/` is here to delimit your pattern so you have one to start and one to end
2. `[]` indicates the pattern your are looking for on one specific character
3. `^` indicates that you want every character NOT corresponding to what follows
4. `a-z` matches any character between 'a' and 'z' included
5. `0-9` matches any digit between '0' and '9' included (meaning any digit)
6. `-` the '-' character
7. `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".
Your character class (the part in the square brackets) is saying that you want to match anything except 0-9 and a-z and +. You aren't explicit about how many a-z or 0-9 you want to match, but I assume the + means you want to replace strings of at least one alphanumeric character. It should read instead:

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

```

Also, if you need to match upper-case letters along with lower case, you should use:

```
str = str.replace(/[^-a-zA-Z0-9]+/g, "");

```