Custom cite-prefix

[giglio robbo' d'acciaio]

I use the standard message-cite-prefix-regexp

,----[ C-h v message-cite-prefix-regexp RET ]
| "\\([         ]*:[-_.[:word:]]+>+\\|[         ]*[]>|}+]\\)+"
`----

I'd like to customizie it in order to match rows starting with `:' in
this way

: blah blah (should match)
:: blah blah (should match)
http:// (should not match)

I've made some tries with the re-builder but I didn't got what I want.

Any clues?
Thanks in advance.
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