How to represent a single space character in a square bracket group

You can just use a space.

For the regex structure in the question:

 ([a-zA-Z0-9]| )+

In practice:

[a-zA-Z0-9 ]+

[a-zA-Z0-9 ]

will add a space to the character class. I'm guessing you already know that and you want to make sure that there will be only single spaces in your string. In that case, you need an additional lookahead:

^(?!.*  )[a-zA-Z0-9 ]*$

matches a string that contains alphanumerics and spaces, but only single spaces.