How to make part of regex optional.

[Rob Coops]

Hi all,

I am having some trouble matching the following string:

"Some text+...:...:...:...:...+some more text"

The trick is there are two dilimiters in this string the + and the : are
used to separate the string, the + signifies a part of the string ended and
the : signifies a sub part of the string has ended.

Now I need to match this string according to the following rules.

   - Part 1: 0 to 15 alphanumeric characters (Mandatory)
   - Part 2: (Mandatory)
      - Sub part 1 0 to 70 alphanumeric characters (Optional)
      - Sub part 2 0 to 70 alphanumeric characters (Optional)
      - Sub part 3 0 to 70 alphanumeric characters (Optional)
      - Sub part 4 0 to 70 alphanumeric characters (Optional)
      - Sub part 5 0 to 70 alphanumeric characters (Optional)
   - Part 3: 0 to 3 alphanumeric characters (Optional)

I am 100% certain to get all possible variations that these rules allow, so
how do I match this correctly using a single regex?

Thanks for any help provided,

Rob