regex check for white space in middle of string

"[A-Za-z0-9\s]*"

matches alphanumeric characters and whitespace. If you want a word that can contain whitespace but want to ensure it starts and ends with an alphanumeric character you could try

"[A-Za-z0-9][A-Za-z0-9\s]*[A-Za-z0-9]|[A-Za-z0-9]"

Exactly two words with single space:

Regex aNum = Regex("[a-zA-Z0-9]+[\s][a-zA-Z0-9]+");

OR any number of words having any number of spaces:

Regex aNum = Regex("[a-zA-Z0-9\s]");

[A-Za-z0-9\s]{1,} should work for you. It matches any string which contains alphanumeric or whitespace characters and is at least one char long. If you accept underscores, too you shorten it to [\w\s]{1,}.

You should add ^ and $ to verify the whole string matches and not only a part of the string:

^[A-Za-z0-9\s]{1,}$ or ^[\w\s]{1,}$.