Your string ":: ::: ::::" has white spaces which I suspect are causing you grief. Remove the white spaces between the various groups of ":" that you have in your string and then try something like this.
| eval _raw = replace (_raw," +","=")
This worked for me when I had to remove an unknown quantity of white spaces, but only when grouped at 4 or more white spaces. So for you a double eval may work best but might not be the most optimal solution if your regex skills are advanced enough.
| eval _raw = replace (_raw,": :","::")
| eval _raw = replace (_raw,"::+",":")
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