How do i perform a string replace in DSQL in Firebird 2.1?

I mean something like this:

Original string: 'This is some text'
Replace s with _ and get this: 'Thi_ i_ _ome text'

I can accept a function that only replaces the first occurrence if only
such exist.

Thanks,
Kjell
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Kjell Rilbe
DataDIA AB
E-post: kjell@...
Telefon: 08-761 06 55
Mobil: 0733-44 24 64