How to replace a space by a comma

expression = "{1 2 3 4 5 6 7 8}";

A few alternatives to PlatoManiac's approach:

ToExpression[
    expression, 
    StandardForm, 
    Function[e, Sequence @@@ Unevaluated[e], HoldAll]
]

ToExpression @ StringReplace[expression, " " -> ","]

{1, 2, 3, 4, 5, 6, 7, 8}

 StringCases[expression, n : NumberString :> ToExpression[n]]

 ToExpression @ StringCases[expression, NumberString]

A possibility with WhitespaceCharacter

ToExpression@
     StringReplace[expression, WhitespaceCharacter -> ","]

(* {1, 2, 3, 4, 5, 6, 7, 8} *)

Another using Interpreter

Interpreter[
   DelimitedSequence["Integer", {"{", " ", "}"}]][expression]

My thought was to drop the brackets using StringTake and then import it using ImportString

"{1 2 3 4 5 6 7 8}"~StringTake~{2, -2}~ImportString~"Table" // First
(* {1, 2, 3, 4, 5, 6, 7, 8} *)

edit I just realized this is very similar to what J.M. suggested in his comment (although I'm playing around with infix notation here).

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