Regular expressions match the characters in parentheses, not including parentheses

(? <=\ () \s+ (? =\))

(? <=exp) is a string that begins with exp, but does not contain itself.
(? =exp) matches the string that ends with exp, but does not contain itself.

(? <=\ () is the beginning of parentheses, but not the parentheses.

(?=\)) is to end with parentheses.

\s matches any non-whitespace character. equivalent to [^ \f\n\r\t\v].
+ indicates that there is at least one character.

(? <=\ () \s+ (? =\)) is a string that matches at least one non-whitespace character at the end of the parenthesis (beginning,), but does not include the beginning (and end)

Regular expressions match the characters in parentheses, not including parentheses