I haven't tested this enough to be sure, but it should work:

create function dbo.NumberClean

(@String_in nvarchar(100))

returns bigint

as

begin

declare @NumberStr nvarchar(100), @Number_out bigint

;with Chars (Seq, Chr) as

(select number, substring(@string_in, number, 1)

from dbo.Numbers

where number between 0 and len(@string_in))

select @numberstr = coalesce(@numberstr + chr, chr)

from chars

where chr like '[0-9]'

select @number_out = @numberstr

return @number_out

end