/
\A         # start at the beginning of the string
[^\(]+     # match one or more non-( characters
\(\d+\)    # match a (number) expression
(          # start the optional part
  :        #   a single, required colon
  [^\(]+   #   match one or more non-( characters
  \(\d+\)  #   match a (number) expression
)*         # match this subexpression zero or more times
\z         # end at the end of the string
/x;