initialize a rational from a decimal string

[edA-qa mort-ora-y]

I'm looking for a way to initialize a rational from a decimal string. I
thought it might be a common task but I don't see any function which
would do this. Is there such a function?

Examples:
0.97 => 97/100
1.5 => 3/2
3.89e100 => 389/1

What I do now is parse the three parts of the decimal (XXX.YYYeZZZ) and
then manually assemble the bits. This involves a lot of calculations
however, in particular that fractional part as I need to loop over the
numbers multiplying by 10. Is there a better way?

-- 
edA-qa mort-ora-y
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