Change first occurrence of character x in a string - how?

[Chris Green]

What's the easiest way to change the first occurrence of a specified
character in a string?

E.g. I want to change linux-raid.vger.kernel.org to
linux-raid at vger.kernel.org, it's a fairly general requirement of
needing to change '.' to '@'.

Alternatively is there an RE 'match' function that would test if
linux-raid at vger.kernel.org matches linux-raid.vger.kernel.org? I don't
really care if the '.' are all regarded as wild cards, the match will
be accurate enough.

-- 
Chris Green
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